Arianna De Marco
email: ariannadem@yahoo.it

Roberto Cozzolino

Bernard Thierry

Coping with mortality: responses of monkeys and great apes to collapsed, inanimate and dead conspecifics

Keywords: death, emotion, distress, empathy, mental representation, epimeletic behaviour, primate. ORCID

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Review

Coping with mortality: responses of monkeys and great apes to collapsed, inanimate and dead conspecifics It was long assumed that only humans can distinguish the living from the dead. Renewed interest in this question over the last decade has led several authors to assert that non-human primates are also aware of death. We investigate this issue by comparing the behaviours of monkeys and great apes toward helpless conspecifics, basing our analysis on published reports. We first examine the behaviours of mothers towards the body of their dead offspring. They may carry the corpse for days or more before abandoning it. They groom, inspect and protect it, sometimes allowing group members to explore it, and rare cases of cannibalism have been reported. No significant difference is observed in the way that monkeys and great apes treat the bodies of infants. We then examine responses to collapsed (still able to move and react) and inanimate (unresponsive or dead) conspecifics. Monkeys and great apes guard, care for and inspect their helpless partners, and also manipulate and mobilise them. Through these actions, individuals may inform themselves about the state of their partners, test their responsiveness and/or attempt to rouse them. It appears that only chimpanzees and gorillas show violent action such as display behaviours and the rough treatment of bodies. They can also make distress calls, and periods of "stunned silence" sometimes occur in chimpanzees, indicating that they are experiencing intense emotion. Finally, we argue that while both monkeys and great apes detect body dysfunction through the victims' inability to wake up and move, only great apes can understand that something serious has happened. The signs of emotional disturbance reported in them indicate that they may believe that inanimate conspecifics have entered a state of "dormancy", meaning that they are unlikely to regain wakefulness. However, there is no evidence that any non-human primates are aware of mortality.

INTRODUCTION

Speculation about the attitude of animals to death is a time-honoured exercise. Rousseau (1755, p. 36-37) wrote that "an animal will never know what it is to die, and the knowledge of death and its terrors is one of the first acquisitions that man has made in moving away from the animal condition". Voltaire (1771, p. 96) added that such knowledge is learned: "The human species is unique in knowing that it must die, and it knows this only by experience. A child brought up alone, and transported to a desert island, would not realize it any more than a plant and a cat". Awareness of death is often considered to be a defining attribute of our species, and when paleoanthropologists find signs of burial sites, they are quite sure that they are dealing with the remains of human beings (Homo sapiens, Homo neanderthalensis) [START_REF] Pearson | The archeology of death and burial[END_REF][START_REF] Pettitt | The palaeolithic origins of human burial[END_REF][START_REF] Renfrew | Death rituals, social order and the archaeology of immortality in the ancient world[END_REF][START_REF] Stiner | Love and death in the stone age: what constitutes first evidence of mortuary treatment of the human body?[END_REF].

Early scientific observations of responses to death in non-human primates (hereafter referred to as primates) have been interpreted in the same way. [START_REF] Yerkes | Maternal instinct in a monkey[END_REF] described the carrying of a stillborn child for weeks by a female macaque (Macaca sp.) as the result of maternal instinct. [START_REF] Zuckerman | The social life of monkeys and apes[END_REF] also witnessed several instances of retention of dead infants in hamadryas baboons (Papio hamadryas), and noted that primates usually defend corpses and oppose their removal by human beings, which led him to conclude that they fail to distinguish between the dead and the living: "Monkeys and apes do not recognize death, for they react to their companions as if the latter were alive but passive" (1932, p. 305). The first records of response to death in chimpanzees (Pan troglodytes) had led observers to suggest that they may feel grief (Brown 1879) and know what death is [START_REF] Garner | Apes and monkeys: their life and language[END_REF]. A review of these cases by [START_REF] Yerkes | The great apes: a study in anthropoid life[END_REF] highlighted the emotional reactions of individuals to the loss of a companion and their subsequent grief and/or depression, but did not go so far as to suggest that they could appreciate the significance of death.

Zuckerman's opinion prevailed in the field of primatology throughout the 20th century. This may explain the lack of interest in the prolonged transport of dead infants by their mother, despite the common occurrence of this behaviour in groups of primates [START_REF] Rajpurohit | Why do mothers carry the corpses of their infants in Hanuman langurs -Presbytis entellus?[END_REF][START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF], and the poor documentation regarding responses of individuals to the bodies of dead conspecifics (for notable exceptions see : Teleki 1973a;[START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Fossey | Gorillas in the mist[END_REF][START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Gupta | Behaviour of Phayre's langur (Trachypithecus phayrei) on the death of one adult female[END_REF]. A turning point was reached with a publication in 2010 that described the events surrounding the death of an old female in a captive group of chimpanzees; based on the record of the behaviours of group mates towards this female, the authors considered their actions to be indicative of anticipatory grief, and wrote: "Are humans uniquely aware of mortality? We propose that chimpanzees' awareness of death has been underestimated" (Anderson et al. 2010). Over the next decade, this paper triggered a wave of reports describing the behaviours of animals towards dead individuals, as well as multiple reviews [START_REF] Anderson | A primatological perspective on death[END_REF][START_REF] Anderson | Comparative evolutionary thanatology of grief, with special reference to nonhuman primates[END_REF][START_REF] Anderson | Chimpanzees and death[END_REF](Anderson , 2020a;;[START_REF] King | How animals grieve[END_REF][START_REF] Piel | Non-human animal responses towards the dead and death: a comparative approach to understanding the evolution of human mortuary practices[END_REF][START_REF] Gonçalves | Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Brosnan | Nonhuman primate responses to death[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF] and two special issues of journals [START_REF] Anderson | Evolutionary thanatology: impacts of the dead on the living in humans and other animals[END_REF]Anderson 2020b). Several authors have come to assert that not only great apes but also monkeys show compassionate care for the dying [START_REF] King | How animals grieve[END_REF][START_REF] Bezerra | Responses towards a dying adult group member in a wild New World monkey[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF] and have some understanding of death [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF][START_REF] Monsó | Death is common, so is understanding it: the concept of death in other species[END_REF]. The most thorough review to date concludes that primates "have an implicit awareness of death wherein the dead individual ceases its agency" [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. However, this statement could apply to many animal species, as authors do not specify what is meant by "implicit", or which components of agency are recognized. They also suggest that primates are capable of understanding the causation and irreversibility of death [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF], but it is unclear what relationship there may be between awareness of the cessation of agency and awareness of the causation or irreversibility of death.

It is often pointed out that the study of great apes has provided the most circumstantial cases of understanding of death-related events in primates [START_REF] Anderson | Chimpanzees and death[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]Porter et al. 2019;Watts 2020). Most of the information on the topic comes from observations in chimpanzees and gorillas (Gorilla sp.), and it is possible that the current scientific literature be somewhat biased, as monkey deaths may have received less attention. As a consequence, no systematic attempt has been made to date to investigate possible contrasts between monkeys and great apes. A comparison of their behaviours may be instructive with regard to the recognition of the states associated with dying and death. There are grounds for this expectation. Great apes generally outperform monkeys in cognitive tasks involving inferential reasoning, planning for the future and attributing perceptual or mental states to others [START_REF] Call | Inferences about the location of food in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus)[END_REF][START_REF] Mulcahy | Apes save tools for future use[END_REF][START_REF] Call | Does the chimpanzee have a theory of mind? 30 years later[END_REF][START_REF] Shettleworth | Cognition, evolution and behavior[END_REF][START_REF] Bourjade | Are monkeys able to plan for future exchange[END_REF][START_REF] Crockford | Wild chimpanzees inform ignorant group members of danger[END_REF][START_REF] Anderson | Mirror self-recognition: a review and critique of attempts to promote and engineer self-recognition in primates[END_REF][START_REF] Costes-Thiré | Evidence that monkeys (Macaca tonkeana and Sapajus apella) read moves, but no evidence that they read goals[END_REF][START_REF] Marsh | Inference by exclusion in lion-tailed macaques (Macaca silenus), a hamadryas baboon (Papio hamadryas), capuchins (Sapajus apella), and squirrel monkeys (Saimiri sciureus)[END_REF][START_REF] Petit | Inferences about food location in three cercopithecine species: an insight into the socioecological cognition of primates[END_REF]. As a corollary, great apes appear to be more sensitive to the attention and needs of others, and show more empathy in the form of consolation, targeted helping, and concern for injured partners [START_REF] Cheney | How monkeys see the world[END_REF], 2007;[START_REF] De Waal | Good natured: the origins of right and wrong in humans and other animals[END_REF][START_REF] De Waal | Anthropomorphism and anthropodenial: consistency in our thinking about humans and other animals[END_REF][START_REF] De Waal | Consolation, reconciliation, and a possible cognitive difference between macaques and chimpanzees[END_REF][START_REF] Silk | The strategic dynamics of cooperation in primate groups[END_REF][START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Pérez-Manrique | The comparative study of empathy: sympathetic concern and empathic perspective-taking in non-human animals[END_REF][START_REF] Sato | Spontaneous attention and psycho-physiological responses to others' injury in chimpanzees[END_REF].

To investigate what primates may understand about death, we will examine the behavioural events surrounding helpless individuals. We will not consider responses to sick, injured or disabled individuals who can still stand upright and/or communicate, nor the delayed effects of partner loss, as such occurrences cannot be equated with the behaviours induced by the fainting or death of a conspecific (for review see [START_REF] Zeller | The grieving process in non-human primates[END_REF][START_REF] De Waal | Good natured: the origins of right and wrong in humans and other animals[END_REF][START_REF] King | How animals grieve[END_REF][START_REF] Anderson | Chimpanzees and death[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Sato | Spontaneous attention and psycho-physiological responses to others' injury in chimpanzees[END_REF]. No conclusive evidence is available regarding the possible adaptive function of death-related behaviours [START_REF] Piel | Non-human animal responses towards the dead and death: a comparative approach to understanding the evolution of human mortuary practices[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]; they may not be adaptive, and could reflect the importance of social bonding in primate evolution [START_REF] Bercovitch | A comparative perspective on the evolution of mammalian reactions to dead conspecifics[END_REF]. The present review will focus on proximate mechanisms, that is, the individual motives underlying responses to the deceased. We will compare monkeys and great apes for three issues: (1) how mothers and other individuals behave towards the body of dead infants before and during decomposition of the corpse, (2) how subjects behave towards collapsed conspecifics that are still able to move, and others that are inanimate and possibly dead, and (3) whether subjects discriminate dead individuals and have some degree of awareness about mortality.

BEHAVIOUR TOWARDS DEAD INFANTS

The carrying of dead infants by their mother is the most-frequently observed response to death in primates [START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Fernández-Fueyo | Why do some primate mothers carry their infant's corpse? A cross-species comparative study[END_REF]). Such behaviour is generally considered independently from other responses to dead individuals on the grounds that the former would be more amply documented than the latter, and that the mother-infant bond is particularly strong [START_REF] Piel | Non-human animal responses towards the dead and death: a comparative approach to understanding the evolution of human mortuary practices[END_REF][START_REF] Anderson | Comparative evolutionary thanatology of grief, with special reference to nonhuman primates[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Brosnan | Nonhuman primate responses to death[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. There are profound differences between the two types of events that justify their separate treatment. As mothers can carry their deceased infant, the care of the body may last several days or weeks. One day after death, the corpse is cold and begins to smell and change colour, and decomposition progresses at a variable rate depending on the climatic conditions. With respect to the other categories of inanimate or dead conspecifics, most accounts concern responses that last only minutes or hours after death, when the bodies still retain their normal appearance. The two phenomena thus differ both in terms of the physical state of the bodies and the time interval since death, which may entail different psychological effects in the individuals who deal with the bodies.

Multiple hypotheses have been proposed to explain why females display postmortem carrying and care, including: (1) unawareness of death: the mother does not understand that her offspring is no longer alive and therefore maintains her care; (2) post-partum hormonal status: the mother's hormonal levels following parturition drive the persistence of care; (3) emotional relief: the mother copes with the distress caused by her offspring's unresponsiveness by taking care of the body; (4) infantile cues: the morphological characteristics of the offspring attract the mother's interest and trigger care; (5) maternal bond: the mother's attachment to her offspring promotes care [START_REF] Hrdy | Mother nature: a history of mothers, infants and natural selection[END_REF][START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF][START_REF] Li | Maternal responses to dead infants in Yunnan snub-nosed monkey (Rhinopithecus bieti) in the Baimaxueshan Nature Reserve, Yunnan, China[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]Watts 2020). These hypotheses are not mutually exclusive and none has been excluded or deemed sufficient to account for all the mothers' behaviours [START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF][START_REF] Fernández-Fueyo | Why do some primate mothers carry their infant's corpse? A cross-species comparative study[END_REF]. We have no reason to expect differences between monkeys and great apes in the response to dead infants for any of the last four hypotheses. On the other hand, the role of cognitive abilities in the unawareness of death hypothesis raises the question of possible differences between these two categories of species that could be due to a different assessment of the infant's state.

Body carrying by mothers

Primate mothers are distinguished from other mammalian females and other classes of primate individuals by their propensity to carry their dead infant. Three main physical characteristics contribute to the occurrence of this behaviour: primates have hands, in many species the mother gives birth to a single offspring, and the relatively small body size of the infants makes them easy for an adult individual to transport. Monkeys and apes appear to carry the corpse in similar ways. Because carrying impedes locomotion, many mothers walk on three limbs and hold the corpse in one hand, whilst others carry it in their mouth or on their back, or drag it by an arm or a leg (Fig. 1A) [START_REF] Schaller | The mountain gorilla: ecology and behavior[END_REF][START_REF] Van Lawick-Goodall | The behaviour of free-living chimpanzees in the Gombe Stream Reserve[END_REF][START_REF] Green | Variation of vocal pattern with social situation in the Japanese monkey (Macaca fuscata): a field study[END_REF][START_REF] Lü | Prolonged carrying of a dead infant among the golden monkey Rhinopithecus roxellana in the Qinling Mountains, China[END_REF][START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF][START_REF] Li | Maternal responses to dead infants in Yunnan snub-nosed monkey (Rhinopithecus bieti) in the Baimaxueshan Nature Reserve, Yunnan, China[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF][START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]Watts 2020). As noted by [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF], the fact that mothers may carry the corpse in awkward positions has been used as an argument that they know their infant is dead. However, live infants are able to grasp the fur and position themselves correctly, and may also protest if carried in an uncomfortable position, so the infant's lack of feedback is sufficient to explain careless transport by the mother. In wild chimpanzees, for instance, [START_REF] Goodall | Through a window: my thirty years with the chimpanzees of Gombe[END_REF] relates a case in which an infant was suffering from pneumonia. Breathing with difficulty, his eyes remained closed and he was too weak to cling on. His mother "carried him as though he were already dead, slung over her shoulder and held in place with her chin", and he even fell several times before eventually dying.

It is not rare for mothers to carry the body of their stillborn or deceased infant for several days before abandoning it. Since most evidence on infant carrying is anecdotal, no measures of the prevalence and duration of transport are available in most primate species. Only three studies have provided quantitative data. One study of chimpanzees showed that more than half of the dead infants in the survey population (individuals under 5 years old) were carried by their mother, with a mean duration of 3.2 days (range: 30 min-16 days) [START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF], calculated from data presented in Table 2). Another study conducted on Japanese macaques (Macaca fuscata) showed that 10% of the dead infants (under 1 year old) were carried by their mother, with a mean duration of 3.3 days (range: 1-17 days) and a carrying rate that increased to 28.7% for infants that died within 30 days of birth [START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF]. A third study on vervet monkeys (Chlorocebus pygerythrus) found that at least 14.4% of dead infants (under 1 year old) were carried by their mother, for a duration ranging from less than 1 day to 14 days or more [START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF]. It seems that the younger the infant at the time of death, the longer the carrying duration [START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF][START_REF] Fernández-Fueyo | Why do some primate mothers carry their infant's corpse? A cross-species comparative study[END_REF]; but see: [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. The mother's attention to the corpse diminishes over time, sometimes leaving the body for a while and returning to retrieve it; the duration of these intervals increases with time until the mother eventually relinquishes the body [START_REF] Jay | Aspects of maternal behavior among langurs[END_REF][START_REF] Green | Variation of vocal pattern with social situation in the Japanese monkey (Macaca fuscata): a field study[END_REF][START_REF] Altmann | Baboon mothers and infants[END_REF][START_REF] Strum | Almost human: a journey into the world of baboons[END_REF][START_REF] Cheney | Baboon metaphysics: the evolution of a social mind[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF]Watts 2020). Some visibly hesitate before leaving the body, going back and forth between their departing companions and the corpse, before abandoning it definitively (Altmann & Altmann 1970;[START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF].

If females generally discard the decomposing body of their infant within 1 week of the death [START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF], some continue to carry the corpse for 2 to 4 weeks (Hanuman langur, Semnopithecus entellus: [START_REF] Sharma | Dead infant carrying in the Hanuman langur (Semnopithecus entellus) around Jodhpur (Rajasthan)[END_REF]Zanzibar red colobus, Piliocolobus kirkii: Georgiev et al. 2020;vervet: Botting & van de Waal 2020;gelada, Theropithecus gelada: Bernstein 1975;[START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF]Japanese macaque: Green 1975;[START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF][START_REF] Watson | Two cases of dead-infant carrying followed by mother-infant cannibalism in captive socially housed Japanese macaques[END_REF]rhesus macaque, Macaca mulatta: Chai et al. 2013;Tonkean macaque, Macaca tonkeana: De Marco et al. 2018;chimpanzee: Biro et al. 2010; mountain gorilla, Gorilla beringei: [START_REF] Warren | Transport of dead infant mountain gorillas by mothers and unrelated females[END_REF]. The longest carrying durations range to up 48 days in geladas [START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF]) and 111 days in chimpanzees [START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF]. Most cases exceeding 2 weeks of transport involve mummified bodies that had dried out under dry and/or cold climatic conditions. Post-mortem change can be seen as a competition between decay and desiccation, with humidity and temperature determining much of the outcome [START_REF] Micozzi | Frozen environments and soft tissue preservation[END_REF]. By slowing the decomposition of corpses, desiccation partially preserves the shape of the body for an extended period of time, which may explain the longer transport duration [START_REF] Biro | Chimpanzee mothers carry the mummified remains of their dead infants: three case reports from Bossou[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF][START_REF] Nishida | Chimpanzees of the lakeshore: natural history and culture at Mahale[END_REF][START_REF] Hanamura | Diseases and deaths: variety and impact on social life[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF]. In some instances, however, mothers continue to carry a disintegrating carcass that no longer resembles a baby (Fig. 1B) [START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF], and three quantitative studies failed to find a significant effect of temperature, season or climate on the duration of transport [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF][START_REF] Fernández-Fueyo | Why do some primate mothers carry their infant's corpse? A cross-species comparative study[END_REF]). Duration appears to vary considerably within each species. Its measurement is insufficient to draw conclusions about possible differences between monkeys and great apes in the attitude of mothers towards their dead offspring.

Behaviours of mothers

Many authors have noted that mothers care for, groom, inspect and protect their dead infants as if they were still alive [START_REF] Zuckerman | The social life of monkeys and apes[END_REF][START_REF] Cheney | Baboon metaphysics: the evolution of a social mind[END_REF][START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF]. This is understandable in the first few days after death, when the appearance of the body is still close to normal. Several days after death, however, the mother may have learned that the infant does not react to her actions, and the rotting corpse no longer looks like a live baby. The female can be then expected to adjust her behaviour towards her deceased offspring. This is indeed what happens when a mother abandons the corpse, but it may be asked whether signs of change are detectable in mothers who continue to carry their infant over a long period of time.

To know whether the mothers' attitudes change over time, we need to compare their behaviour in the days after the death of their offspring with their behaviour in the following period. To this end, we compiled data from the scientific literature on monkeys and great apes. A limited number of studies have systematically described the behaviour of females towards their dead infant [START_REF] Fowler | Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Trapanese | Prolonged care and cannibalism of infant corpse by relatives in semi-free-ranging capuchin monkeys[END_REF]. Most authors have focused on the duration of transport, and only a few behaviours were reported [START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF]. The available information is shown in Table 1, where a distinction is made between the behaviours observed in the first 2 days after death, i.e. when the dead body retains an appearance that is close to normal, and those observed in a second period from the 5th day onwards, i.e. when the body is heavily deformed by decay. The 3rd and 4th days, which can be considered as a transition period, have been excluded to limit the influence of climatic variations on the delineation of the two stages.

Maternal care for the infant's body includes preventing access (i.e. threatening or avoiding individuals who attempt to approach or touch the infant's body), swatting flies, grooming (Fig. 1C), licking, handling, inspecting (i.e., peering, sniffing, touching, fingering), and directing attention to the eyes (Fig. 1D, Table 1). Examination of Table 1 indicates that these behaviour patterns can occur in both the first and second periods, with the noteworthy exception of eye care, which was never reported in the second period. A number of authors have noted the particular interest mothers show for the infant's eye area, whether they are looking, touching or fingering (Table 1). The eyes are particularly informative of the individual's physical condition; not only is the gaze fixed after death, but the cornea dries out and thus gradually looses its transparency, becoming white on the 2nd day post mortem. Visual communication is particularly important in primates [START_REF] Liebal | Primate communication: a multimodal approach[END_REF][START_REF] Strier | Primate behavioral ecology[END_REF], so the absence of eye movement and loss of eye contact through mutual gaze may alarm the mother. About 2 days after death, lysis of tissues through the putrefaction process causes the eyeballs to liquefy, which means that the eyes collapse. This may explain why even females that continue to care for their infant's corpse for a prolonged period of time cease to pay selective attention to eyes once they have lost their specific shape, and thus all communicative significance. Table 1 shows that female monkeys and great apes display similar caregiving behaviours towards their deceased offspring. Although additional data are required to reach definitive conclusions, it seems that motivational factors -whether social, emotional and/or physiological -outweigh the effects of any possible differences between both categories of species in the mother's cognitive assessment of the state of the offspring. This does not mean, however, that cognitive processes are not involved. Some females emit distress or warning calls (Table 1) [START_REF] Green | Variation of vocal pattern with social situation in the Japanese monkey (Macaca fuscata): a field study[END_REF][START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF][START_REF] Dellatore | Two cases of mother-infant cannibalism in orangutans[END_REF], which may indicate a degree of awareness of their infant's helplessness, but these calls are mostly triggered by the loss or abandonment of the infant's body (Altmann & Altmann 1970;[START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF][START_REF] Li | Maternal responses to dead infants in Yunnan snub-nosed monkey (Rhinopithecus bieti) in the Baimaxueshan Nature Reserve, Yunnan, China[END_REF][START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF]. Several unusual events are revealing. In one case in Tonkean macaques, a female appeared particularly agitated during the first 2 days after the death of her daughter, repeatedly staring and screaming at her own reflection in a plastic door that had never produced such behaviour in any individual of the group (De [START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF]. The same female was also observed on the 2nd day inserting her fingers or tongue into the infant's mouth; this behaviour has also been reported towards live offspring in Tonkean macaques, and the mother may have been testing her infant's reactions by attempting to induce a suckling response [START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF]. There are two other reported cases of a female who appeared to test the responsiveness of her dead offspring: a chimpanzee mother who took her infant's hand and shook it [START_REF] Krief | Chimpanzés, mes frères de la forêt [Chimpanzees, my forest brothers[END_REF]; and a mountain gorilla who had left her infant's body on the ground before returning to the body, looking at it, then taking hold of a foot and pulling it gently several times then release her grip, looking at the infant for a long time, and finally walking away (Watts 2020: case 9). Some females may also try to compensate for their infant's dysfunction. In Hanuman langurs, a mother attempted to nurse her dead infant by manually pumping her breast milk into his mouth [START_REF] Mckenna | The evolution of primate societies, reproduction, and parenting[END_REF]. In chacma baboons (Papio ursinus), a mother cleaned debris from inside the dead infant's mouth [START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF], while in whitefaced capuchins (Cebus capucinus) another was seen licking the anal region and sucking oozing body fluids from the corpse [START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF].

In view of the above observations, it is questionable to assert that females treat their dead infant as if she/he was alive. In the initial period, they can take care of the body and inspect it, possibly testing reactions and trying to make eye contact. The same process is found in monkeys and great apes. Mothers may need a transition phase to distance themselves from their infant, both physically and psychologically [START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF]. It is likely that many of them are confused. Most abandon the body after a few hours or days, while those who keep holding the body for longer periods probably become accustomed to the unresponsiveness of the body, and continue to care for it. In one instance in bonnet macaques (Macaca radiata), humans forcibly removed the dead infant of a female for burial, and the mother then made regular visits to the burial spot for at least 2 days [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF]. In olive baboons (Papio anubis) in Gilgil (Kenya), [START_REF] Smuts | Sex and friendship in baboons[END_REF] tells the story of a female named Zandra, who lost the corpse of her infant Zephyr in a bushy area. About a week later, as the troop passed by the same place, "Zandra became extremely agitated. She rushed about as if looking for Zephyr, and then she climbed a tree. When she got to the top, she looked all around her and began calling. (. . .) She repeated her agitated looking and calling each time the baboons passed through this area for the next few weeks".

Cannibalism is another behaviour that remains largely misunderstood. Several cases have been described in which females have consumed flesh from the body of their infant. This usually occurs in the 1st days after death (Table 1), but dried pieces may also be ingested after more than 1 week (Japanese macaque: [START_REF] Watson | Two cases of dead-infant carrying followed by mother-infant cannibalism in captive socially housed Japanese macaques[END_REF]Tonkean macaque: De Marco et al. 2018;orang-utan, Pongo abelii: Dellatore et al. 2009;chimpanzee: Hanamura et al. 2015). These acts must be distinguished from misdirected placentophagia, where the female chews part of the body of the infant, dead or alive, following parturition, as observed in other mammalian species [START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF][START_REF] Tian | Evidence of placentophagia and mother-infant cannibalism in free-ranging Macaca mulatta tcheliensis in Mount Taihangshan, Jiyuan, China[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF]. A striking feature of filial cannibalism in both monkeys and great apes is that mothers continue to protect, carry and care for the remains of the infant's body when they have already begun to cannibalize it, and they display no signs of distress during this period. Several authors note that maternal consumption of the corpse remains partial and provides little nutritional benefit [START_REF] Fowler | Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale[END_REF][START_REF] Anderson | Chimpanzees and death[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF]Fedurek et al. 2020;Watts 2020). As [START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF] point out, if there is a gradual weakening of the mother-infant bond from treating the infant body as if it was alive to treating it as food, cannibalism should represent the end of the process; quite to the contrary, there is an overlap between cannibalism and care, which may reflect contradictory motives that produce confusion in females. Cannibalism is not necessarily a unitary phenomenon and mothers' behaviour could be underpinned by different motives. For example, it is not impossible that eating small bits of skin during grooming represents a form of care.

Behaviours of group members

In general, only a few young individuals or adult females are attracted to dead infants [START_REF] Bernstein | Social roles in a rhesus monkey group[END_REF]Altmann & Altmann 1970;[START_REF] Nash | Parturition in a feral baboon (Papio anubis)[END_REF][START_REF] Ciani | A case of infanticide in a free-ranging group of rhesus monkeys (Macaca mulatta) in the Jackoo Forest, Simla, India[END_REF][START_REF] Balasubramanian | An observation on a mother carrying dead infant in the bonnet macaque, Macaca radiata[END_REF][START_REF] Rajpurohit | Why do mothers carry the corpses of their infants in Hanuman langurs -Presbytis entellus?[END_REF][START_REF] Gupta | Behaviour of Phayre's langur (Trachypithecus phayrei) on the death of one adult female[END_REF][START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF][START_REF] Fowler | Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF][START_REF] Nishida | Chimpanzees of the lakeshore: natural history and culture at Mahale[END_REF][START_REF] Sharma | Dead infant carrying in the Hanuman langur (Semnopithecus entellus) around Jodhpur (Rajasthan)[END_REF][START_REF] Li | Maternal responses to dead infants in Yunnan snub-nosed monkey (Rhinopithecus bieti) in the Baimaxueshan Nature Reserve, Yunnan, China[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF]Watts 2020). However, several studies in chimpanzees report that a fairly large proportion of individuals of both sexes expressed interest in dead infants [START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF][START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF], and the same has been observed in a group of common marmosets (Callithrix jacchus) [START_REF] Thompson | Callitrichid responses to dead and dying infants: the effects of paternal bonding and cause of death[END_REF]. The mother initially limits contact with her dead infant, with some tolerance accorded to her other offspring [START_REF] Van Lawick-Goodall | the shadow of man[END_REF][START_REF] Kano | The last ape: pygmy chimpanzee behavior and ecology[END_REF][START_REF] Muller | A note on scavenging by wild chimpanzees[END_REF][START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF][START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF][START_REF] Wrangham | Teddy's death and the aftermath[END_REF][START_REF] Li | Maternal responses to dead infants in Yunnan snub-nosed monkey (Rhinopithecus bieti) in the Baimaxueshan Nature Reserve, Yunnan, China[END_REF][START_REF] Krief | Chimpanzés, mes frères de la forêt [Chimpanzees, my forest brothers[END_REF][START_REF] Botting | Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. However, other individuals may gain access as she begins to detach herself from the body.

Available data on the responses of group members to the infant's body do not allow comparison of the very first days after death with the following period. The behaviour of individuals appears mainly investigative (approaching, looking, sniffing, touching, handling or pulling the corpse), and also involves grooming [START_REF] Bernstein | The lutong of Kuala Selangor[END_REF]Altmann & Altmann 1970;[START_REF] Struhsaker | Social behaviour of mother and infant vervet monkeys (Cercopithecus aethiops)[END_REF][START_REF] Van Lawick-Goodall | the shadow of man[END_REF][START_REF] Ciani | A case of infanticide in a free-ranging group of rhesus monkeys (Macaca mulatta) in the Jackoo Forest, Simla, India[END_REF][START_REF] Izawa | Social changes within a group of wild black-capped capuchins (Cebus apella) III. Field Stud New World Monkeys La Macarena Colombia[END_REF][START_REF] Balasubramanian | An observation on a mother carrying dead infant in the bonnet macaque, Macaca radiata[END_REF][START_REF] Kooriyama | The death of a newborn chimpanzee at Mahale: reactions of its mother and other individuals to the body[END_REF][START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Fowler | Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale[END_REF][START_REF] Cronin | Behavioral response of a chimpanzee mother toward her dead infant[END_REF][START_REF] Sharma | Dead infant carrying in the Hanuman langur (Semnopithecus entellus) around Jodhpur (Rajasthan)[END_REF][START_REF] Wrangham | Teddy's death and the aftermath[END_REF][START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF][START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF][START_REF] Negrey | Corpse-directed play parenting by a sterile adult female chimpanzee[END_REF]Watts 2020). The body can be picked up and carried for some time by young females [START_REF] Mckenna | Postmortem infant caregiving behavior among captive Indian langurs[END_REF][START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF][START_REF] Kooriyama | The death of a newborn chimpanzee at Mahale: reactions of its mother and other individuals to the body[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF], and more rarely by males or adult females [START_REF] Bernstein | Activity patterns in a gelada monkey group[END_REF][START_REF] Taylor | Social interactions between adult male and infant rhesus monkeys in Nepal[END_REF][START_REF] Kano | The last ape: pygmy chimpanzee behavior and ecology[END_REF][START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Warren | Transport of dead infant mountain gorillas by mothers and unrelated females[END_REF][START_REF] Bercovitch | A comparative perspective on the evolution of mammalian reactions to dead conspecifics[END_REF][START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF][START_REF] Negrey | Corpse-directed play parenting by a sterile adult female chimpanzee[END_REF]. Carrying by males appears more likely in callitrichid monkeys and Barbary macaques (Macaca sylvanus), where males commonly carry infants and establish strong social bonds with them [START_REF] Merz | Male-male interactions with dead infants in Macaca sylvanus[END_REF][START_REF] Thompson | Callitrichid responses to dead and dying infants: the effects of paternal bonding and cause of death[END_REF].

Play involving juvenile individuals is regularly observed in chimpanzees: an individual was once observed trying to invite a dead infant to play (van Lawick-Goodall 1971); other reports describe the use of corpses in play sessions [START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF], in which they can be dragged, dropped, thrown or trampled [START_REF] Matsuzawa | The death of an infant chimpanzee at Bossou, Guinea[END_REF][START_REF] Kooriyama | The death of a newborn chimpanzee at Mahale: reactions of its mother and other individuals to the body[END_REF][START_REF] Biro | Chimpanzee mothers at Bossou, Guinea, carry the mummified remains of their dead infants[END_REF][START_REF] Wrangham | Teddy's death and the aftermath[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. A play sequence involving such rough behaviour was once recorded in a juvenile monkey (brown capuchin, Sapajus apella: [START_REF] Trapanese | Prolonged care and cannibalism of infant corpse by relatives in semi-free-ranging capuchin monkeys[END_REF]. In another instance in Tonkean macaques, a juvenile seized the remains of an infant carried by the mother and engaged in rough-and-tumble play with the mother while both were holding the remains (De [START_REF] Marco | Prolonged transport and cannibalism of mummified infant remains by a Tonkean macaque mother[END_REF]. Chimpanzees have been reported to mount a corpse or rub their anogenital region against it [START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. Other rare behaviours that have been described include inserting fingers in the corpse's mouth (bonnet macaque: [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF], cleaning the inside of its mouth (chacma baboon: [START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF], repeatedly rubbing its thorax (yellow baboon, Papio cynocephalus: Altmann & Altmann 1970), threatening the corpse (white-faced capuchin: Perry & Manson 2008), using it as a tool in a charging display (chimpanzee: [START_REF] Matsuzawa | The death of an infant chimpanzee at Bossou, Guinea[END_REF] or using it to promote social contact (Barbary macaque: [START_REF] Merz | Male-male interactions with dead infants in Macaca sylvanus[END_REF].

Two cases of partial cannibalism by group members have been recorded in chimpanzees [START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. In other cases, partial cannibalism by the mother was followed by the ingestion of pieces of skin or flesh by a group member in brown capuchins [START_REF] Trapanese | Prolonged care and cannibalism of infant corpse by relatives in semi-free-ranging capuchin monkeys[END_REF]) and mountain gorillas (Watts 2020), while several individuals consumed the carcass with the mother in groups of chimpanzees (Fedurek et al. 2020) and bonobos (Pan paniscus) [START_REF] Fowler | Cannibalism in wild bonobos (Pan paniscus) at Lui Kotale[END_REF][START_REF] Tokuyama | Cases of maternal cannibalism in wild bonobos (Pan paniscus) from two different field sites, Wamba and Kokolopori, Democratic Republic of the Congo[END_REF]). It may be worth adding that although infanticide, sometimes including cannibalism, has been documented in many primate species (Hiraiwa-Hasegawa 1992; van Schaik & Janson 2000), we will not consider it here, as hostile motivations of infanticidal individuals may continue to affect their behaviour after killing (see [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Lowe | Intracommunity infanticide in wild, eastern chimpanzees: a 24-year review[END_REF][START_REF] Thompson | Callitrichid responses to dead and dying infants: the effects of paternal bonding and cause of death[END_REF].

According to [START_REF] Cheney | Baboon metaphysics: the evolution of a social mind[END_REF], an infant's status seems to change soon after death. Group members would cease treating it as an infant. For example, they note that chacma baboons rarely grunt when they approach an infant's corpse, as they would with live infants. Yet they appear to recognize that the corpse still belongs to the mother, grunting to her when she moves away from the body. As any primatologist has experienced, group members fiercely defend corpses from human approach, just as they would with live infants [START_REF] Zuckerman | The social life of monkeys and apes[END_REF][START_REF] Cheney | Baboon metaphysics: the evolution of a social mind[END_REF]. As observed in mothers, we find no significant differences between monkeys and great apes in the way group members treat the bodies of dead infants. Maternal dispositions and social bonding seem to prevail over other aspects, either in the first days after death, or the period thereafter. Although we obtain little insight into the validity of the unawareness of death hypothesis, there is multiple evidence that primates detect that dead infants are no longer responsive.

BEHAVIOUR TOWARDS COLLAPSED AND INANIMATE CONSPECIFICS OTHER THAN DEAD INFANTS

Several states may precede and/or resemble the state of death. To examine how primates react to death, we first need to clarify which states of impairment or unconsciousness the observer can recognize. When a critical event occurs, it is sometimes difficult to determine at the outset whether an individual is dead or still alive. This may be illustrated by an event that happened at the Orangerie Zoo of Strasbourg (France) in a group of Tonkean macaques that one of us was observing (Table 2). The dominant male of the group suddenly attacked the second-ranking male, named Clide, putting his arms around him from the front and holding him firmly. As the latter did not try to defend himself, the dominant male bit him at the base of the neck in a long hold. Then the dominant male walked away while the other stood still and blood flowed onto the ground, rapidly forming a large puddle. Clide walked 3 m to a rock and slumped against it, sliding down to the ground where he remained facedown, motionless and apparently unconscious. Over the next minutes, the members of the group glanced at him and some approached him, but the only notable behaviour came from the dominant male, who approached Clide until he touched him with his chest and gazed at him for about a dozen seconds before leaving. At this stage, the amount of blood at the site of the attack led the observer to wonder whether a vital blood vessel had been hit, and whether Clide was dying or already dead. However, after lying down for approximately 15 min, he stood up. Zoo staff anesthetised him a few hours later to suture the deep wound that had lacerated the muscles between his neck and shoulder. He may have been shocked by the injury and the blood loss at the time, but he recovered without sequelae.

Other authors have also noted that human observers may be uncertain for some time about what actually happened in the field (Teleki 1973a;[START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Masi | Reaction to allospecific death and to an unanimated gorilla infant in wild western gorillas: insights into death recognition and prolonged maternal carrying[END_REF]. The main point here is that animals may be in the same situation. This is what really occurred in a party of chimpanzees when one of them, named Léo, started vomiting and then fell out of a tree, dying shortly afterwards: "The three chimpanzees who were with Léo did not seem to realize what had happened and continued to rest around him. Within 10 min, flies started to swarm over the body and Bob, another male, groomed Léo but only for a few seconds and then started to chase the flies away and left rapidly" [START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF]). Time duration is not a minor parameter when it comes to assessing the state of an affected individual, yet it is difficult to account for. Not only does it often take a minimal amount of time for group members to become aware of the event, but a short fainting episode followed by recovery will not produce the same levels of response from group mates as a long period of unconsciousness. In the absence of clinical examination and medical devices, only signs of activity, movements and reactions are available to determine whether an individual is still alive, which requires special attention and even the inspection of the body.

In order to address the attitude of monkeys and great apes to death, we will compare their responses to collapsed and inanimate conspecifics based on available reports (Table 2). By the word "collapsed", we mean that an individual is lying down, unable to stand, sit or perform coordinated gestures, but can emit sounds or make some movements, including responses to the actions of partners; the subject appears to be alive. By "inanimate", we mean that an individual is lying down, silent, and unable to move or respond to the actions of partners; the subject thus appears to be dead, but can in fact be dead or alive -e.g. still breathing imperceptibly. Humans adjust their behaviour towards apparently dead individuals. If other primates are able to distinguish between collapsed and inanimate conspecifics, they should behave differently towards them, and the effect could be stronger in great apes than in monkeys. They may exhibit different categories of behaviour: guarding, inspection, handling, violence, affiliation, signs of anxiety and distress, and visitation.

Guarding

Although rarely quantified, it appears that group members in both monkeys and great apes often remain close to collapsed or inanimate partners for several minutes or hours (Table 2). Individuals prevent access to their bodies by repelling or threatening others, particularly infants and juveniles (common marmoset: Bezerra et [START_REF] Gupta | Behaviour of Phayre's langur (Trachypithecus phayrei) on the death of one adult female[END_REF]. Fly swatting regularly occurs around corpses (Table 2), but it does not always have a protective purpose; in one case in brown capuchins, for example, individuals consumed flies after catching them (De [START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]).

Several cases have been observed where an individual has moved the body of a conspecific away from a possible danger. In one instance in wild Barbary macaques, people removed the body of a juvenile killed by the impact of a vehicle from the road, but put it on the ground when the monkeys began screaming; an adult male then took the corpse up a tree and held it for over half an hour, before eventually dropping it to the ground [START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF]). We ourselves recorded three episodes in captive Tonkean macaques, where individuals collapsed to the ground following either an epileptic seizure (one case) or a shock caused by climbing an electrified fence (two cases), then presented myoclonic jerks for 2-4 min before recovering (Table 2). In all three cases the group members threatened the approaching observer, and in two of the cases a partner also pulled the collapsed individual away from the observer: in one instance a juvenile female having a seizure was dragged away by an adult male, and in another a shocked infant male was dragged away by a juvenile male (Table 2).

These behaviours show that primates tend to guard and defend both collapsed and inanimate group mates. Furthermore, not only does it take time to obtain information about their state, but the attachment between individuals with strong social relationships can make it difficult to abandon them [START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF][START_REF] Bercovitch | A comparative perspective on the evolution of mammalian reactions to dead conspecifics[END_REF]. There is evidence that individuals with closer bonds to the deceased are most often the last to leave (common marmoset: [START_REF] Bezerra | Responses towards a dying adult group member in a wild New World monkey[END_REF]brown capuchin: De Marco et al. 2020; Sichuan golden monkey, Rhinopithecus roxellana: [START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF]olive baboon: Strum 1987;chimpanzee: Goodall 1990;[START_REF] Reynolds | The chimpanzees of the Budongo Forest: ecology, behaviour, and conservation[END_REF]Anderson et al. 2010;[START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] De Waal | Mama's last hug: animal emotions and what they tell us about ourselves[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF]mountain gorilla: Porter et al. 2019). There are reports of exceptional cases in which an individual remained near to the corpse for 2 or more days (mantled howler, Alouatta palliata [START_REF] Mittermeier | Group activity and population dynamics of the howler monkey on Barro Colorado Island[END_REF]gelada: Fashing et al. 2011;chacma baboon: Smuts 1985; Japanese macaque: Iida 1999). However, it is worth recalling that primates are nomadic animals that constantly need to forage for food. When sick or injured subjects become unable to follow their group, they are usually left behind, in which case they may die alone [START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF]Porter et al. 2019).

Inspection

Individuals behave towards both collapsed and inanimate conspecifics in a way that can be described as inspection. They frequently peer at the body (Fig. 2A) and touch it through simple physical contact, tapping, poking or fingering (Table 2). They may sniff the body (Table 2), and some individuals sniff or lick their hand after touching a corpse [START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]Porter et al. 2019;[START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]. The different parts of the body may be examined. The face attracts particular attention, especially the eyes and mouth [START_REF] Bertrand | The behavioral repertoire of the stumptail macaque: a descriptive and comparative study[END_REF][START_REF] Fossey | Gorillas in the mist[END_REF]Anderson et al. 2010;[START_REF] Buhl | Response of rhesus macaques (Macaca mulatta) to the body of a group member that died from a fatal attack[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF][START_REF] De Waal | Mama's last hug: animal emotions and what they tell us about ourselves[END_REF][START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF], as does the anogenital region [START_REF] Fossey | Gorillas in the mist[END_REF][START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]Porter et al. 2019;[START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF].

In chimpanzees and mountain gorillas, in several cases when the members of a group knew an individual was lying on the ground, a large proportion of them approached and investigated the body (Teleki 1973a;[START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF]). The number of inspections seems to be lower under similar circumstances in groups of monkeys [START_REF] Buhl | Response of rhesus macaques (Macaca mulatta) to the body of a group member that died from a fatal attack[END_REF][START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF][START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]; but see [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF]). However, most inspection behaviours were similar in both monkeys and great apes (Table 2). They can provide group members with visual, olfactory and tactile information on the state of the victim. Note that while the first two modes are purely perceptive, the tactile mode can also represent an attempt to elicit a response, and thus test responsiveness or even rouse the partner [START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. There have also been observations of chimpanzees in the Taï Forest (Ivory Coast) cutting leafy branches and dropping them on dead bodies, possibly to test their reactions from a distance [START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF].

Handling

Apart from situations in which the body of the victim is carried or dragged away from potential danger, individuals have also been observed to spontaneously act on it. Their actions can be classified into two main categories: manipulating and mobilising. In manipulating actions, they push, pull, shake or nibble the body, which may move a body part such as the head, the tail or a limb, but the body itself does not move appreciably (Fig. 2B). Such actions have been reported in brown capuchins, Sichuan golden monkeys, Phayre's leaf monkeys, rhesus macaques and chimpanzees (Table 2). Manipulations also include opening the victim's mouth, as documented in chimpanzees (Anderson et al. 2010;[START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]. In mobilising actions, individuals similarly grasp then push or pull the body, but with sufficient force to either drag the body for some distance or lift the body or its upper part, thereby moving the body significantly. Such actions have been reported in brown capuchins, rhesus macaques, chimpanzees and mountain gorillas (Table 2).

By manipulating and mobilising the body, individuals may seek to test the responsiveness of their companion or rouse him/her [START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF]Anderson et al. 2010;[START_REF] King | How animals grieve[END_REF][START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]. In a captive group of brown capuchins, for example, a subadult male was seen repeatedly manipulating the corpse of an adult female by lifting or pulling the head, the tail or a limb, and he dragged the body for a distance of some centimetres (De [START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]. In wild mountain gorillas, a juvenile male was observed lying and sitting on his mother's corpse, "staring at her face, and he gently tried to move her head with his hands" (Porter et al. 2019). In wild chimpanzees, two adult males approached the dead body of an adolescent female and "softly shook one of her hands and legs, looking at her face. It looked as if they were looking for some kind of reaction", and her infant brother also "pulled her hand many times, looking at her" [START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF]. Interestingly, an early account about two young captive chimpanzees already mentioned that at the demise of one of them, his companion "made repeated efforts to arouse her, lifting up her head and hands, pushing her violently and rolling her over" (Brown 1879). It must be added that some mobilising actions may sometimes aim to raise the body to an upright position [START_REF] King | How animals grieve[END_REF]. In mountain gorillas, for example, an adolescent male was observed lifting a dying infant into a sitting position that the infant could not maintain, apparently in an attempt to return him to an active state [START_REF] Fossey | Gorillas in the mist[END_REF]Watts 2020: case 1).

If primates differentiate between collapsed and inanimate conspecifics, we would expect their manipulating actions, presumably intended to test their responsiveness and/or rouse them, to concern inanimate conspecifics more frequently than collapsed ones; the inspection of partners that are still able to perform some movements or emit sounds would be sufficient to provide evidence of their ability to react. Such a contrast between collapsed and inanimate conspecifics would not be expected for mobilising actions, as some may be designed to lift individuals up regardless of their state. From Table 2, it appears that although mobilising is observed in both collapsed and inanimate conspecifics, manipulating is indeed primarily directed at inanimate conspecifics. Two cases deviate from this pattern in monkeys, however. In a group of Sichuan snub-nosed monkeys, members of the group gently pulled the hand of an injured female both before and after death [START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF]. In another case registered in Indian rhesus macaques, a subadult male likely fell from a post after receiving an electric shock from the overhead lines of a railway station (Table 2 and Supplemental Material; Anonymous 2014). He remained inanimate on the ground and, over the following minutes, another subadult male performed numerous handling actions, including pushing, pulling, shaking and nibbling, but also repeated mobilising actions seeking to lift the victim's body with his hands and teeth, and also seemingly to place him in a sitting position. After this first period, the victim began to open his eyes and make some movements, but his partner continued his handling and mobilising actions for several minutes until the victim was able to sit upright. Although this event is exceptional in the current literature on animal responses to affected companions, it is possible that such behaviour is more frequent than we believe. The observations made by one of the authors of this paper (B. Thierry) in a group of Tonkean macaques studied under semi-free ranging conditions [START_REF] Thierry | Tonkean macaque behaviour from the perspective of the evolution of Sulawesi macaques[END_REF] showed that on three occasions when individuals were found dead they presented puzzling small postmortem skin lesions around the eyes (2 out of 3 cases), on the top of the head (1 out of 3), the rump (1 out of 3) or the hands and feet (2 out of 3). The example of the two rhesus macaques suggests that these wounds may have been caused by the group members' efforts to stimulate, mobilise and/or lift the bodies with their teeth.

Violence

In three cases where an adult male was likely killed by his group mates (rhesus macaques: [START_REF] Buhl | Response of rhesus macaques (Macaca mulatta) to the body of a group member that died from a fatal attack[END_REF]chimpanzees: Nishida 1996chimpanzees: Nishida , 2011;;[START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF], the bodies of the victims were then attacked by individuals who slapped, hit, bit, dragged or trampled them; display behaviours were also observed in chimpanzees, and in one case body parts were even cannibalized (Table 2; see also [START_REF] Boesch | The real chimpanzee: sex strategies in the forest[END_REF]. In these circumstances, several of the individuals involved in the attacks were probably responsible for the death of the victim, and the attitude of group members towards the dead bodies can be explained by the degree of antagonistic excitement attained in the group during the initial aggression [START_REF] Buhl | Response of rhesus macaques (Macaca mulatta) to the body of a group member that died from a fatal attack[END_REF]. Such events should therefore not be confused with situations where violent behaviours are directed at helpless conspecifics that had not been the object of hostility prior to death.

Two main types of violent behaviours can be directed to collapsed and inanimate conspecifics, namely rough treatments and display behaviours. These behaviours regularly occur in chimpanzees and gorillas (Table 2; see also [START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF][START_REF] Case | The response of sanctuary chimpanzee (Pan troglodytes) groups before, during and after the death of a groupmate[END_REF]. In contrast, rough treatments have never been reported in monkeys, and they rarely perform display behaviours (Table 2). Rough treatments consist of slapping, hitting, grabbing, dragging, trampling, running over or even jumping on their bodies, and sometimes throwing stones or branches at them. Display behaviours include charging, buttress drumming and dragging vegetation in chimpanzees, and chest-beating in gorillas. Rough treatment and display behaviours are often associated in the same behavioural sequences. They are usually directed at dead bodies, but two instances have been described in chimpanzees where collapsed partners were brutalized (Table 2). Violent behaviours are primarily exhibited by adult males, but they can also be shown by immature males (Teleki 1973a;[START_REF] Fossey | Gorillas in the mist[END_REF] Violent behaviours differ from handling actions in terms of the arousal and even frenzy of performers. It must be emphasized that they do not involve threatening signals. In addition, the subjects who express violence may also show affiliative behaviour towards deceased partners, and some may even be their offspring [START_REF] Fossey | Gorillas in the mist[END_REF][START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF]Anderson et al. 2010;[START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF]Porter et al. 2019). In one case, an inanimate female gorilla was hit on the head with a projectile thrown by males: she then moved and got to her feet, and males immediately stopped their violent behaviour (Prince-Hughes 2001). These facts lead observers to consider rough treatments and display behaviours not as aggression, but as attempts to provoke a response from partners and rouse them, and possibly also as a sign of anger due to frustration at their lack of reaction [START_REF] Fossey | Gorillas in the mist[END_REF][START_REF] Prince-Hughes | Gorillas among us: a primate ethnographer's book of days[END_REF]Anderson et al. 2010;[START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] De Waal | The bonobo and the atheist: in search of humanism among the primates[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF]). It should be noted we have observed two episodes of the threatening of a dead partner for a few seconds in brown capuchin monkeys; these occurrences were also interpreted as an expression of frustration caused by the absence of response [START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]; see also [START_REF] Perry | Manipulative monkeys: the capuchins of Lomas Barbudal[END_REF].

Affiliation

Various affiliative behaviours can be directed towards collapsed and inanimate conspecifics. Grooming is frequently reported (Table 2). Other behaviours are occasionally observed. Monkeys sometimes clasp and mount bodies lying on the ground, and facial expressions and presentation of the hindquarters towards corpses have been recorded in macaques (Table 2). In a group of Barbary macaques, one female was seen approaching a dead group mate on four separate occasions over 1 hr, each time prompting the approach of another female, both of whom then engaged in teethchattering towards the body and immediately clasped and/or groomed the body (Table 2; A. De Marco unpublished data). In long-tailed macaques, two juveniles engaged in teeth-chattering and bared-teeth displays towards a dead companion [START_REF] Riley | Observation of a fatal dog attack on a juvenile long-tailed macaque in a human-modified environment in Singapore[END_REF]. Scent-marking (common marmoset [START_REF] Box | Primate behavior and social ecology[END_REF] and licking a dead body (mountain gorilla: Porter et al. 2019) have each been observed on one occasion. Rare instances of care of a dead companion were also reported in great apes, such as placing a favourite food in her hands (lowland gorilla, Gorilla gorilla: King 2013), cleaning her face with paper towels (chimpanzee: Westoll 2011), cleaning his teeth using a stem of grass (chimpanzee: van Leeuwen et al. 2016), or removing straw from her body (chimpanzee: Anderson et al. 2010).

By directing affiliative behaviours towards their helpless peers, individuals can achieve different goals that are not mutually exclusive. Certain behaviours may have a primary caretaking role (Anderson et al. 2010;[START_REF] Bezerra | Responses towards a dying adult group member in a wild New World monkey[END_REF][START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF], while others may serve to communicate or attempt to communicate, appease and/or express attachment to closely bonded partners [START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF]. It is intriguing that communicative displays (facial expression, hindquarter presentation, scentmarking) and expressive behaviours (clasping, mounting) directed towards victims are quite often reported in monkeys, whereas none have been described in great apes (Table 2).

Signs of anxiety and distress

The circumstances surrounding events such as injuries or accidents can induce strong emotional reactions in the individuals who witness them, leading to confounding effects on the anxiety and distress that may be specifically caused by a companion's helplessness. To separate these effects, it is therefore necessary to rule out cases where sources of danger -predators, humans or hostile conspecificsmay still be present in the immediate environment (e.g. [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Gupta | Behaviour of Phayre's langur (Trachypithecus phayrei) on the death of one adult female[END_REF][START_REF] Buhl | Response of rhesus macaques (Macaca mulatta) to the body of a group member that died from a fatal attack[END_REF][START_REF] Riley | Observation of a fatal dog attack on a juvenile long-tailed macaque in a human-modified environment in Singapore[END_REF][START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]). Another issue is that anxiety and distress are emotional states, which means that they can only be assessed by indirect means. Activity rates and frequencies of yawning and selfdirected behaviours can be used to measure anxiety levels, but these data are generally not available for unpredictable events such as injuries, accidents and fainting. It is likely that an anxious state leads to the occurrence of reassurance behaviours, resulting in increased rates of mounting, embracing, grooming or playing among some group members (Teleki 1973a;[START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF]Anderson et al. 2010;Porter et al. 2019), but the evidence remains weak in the absence of a baseline comparison. In four cases, the offspring of deceased females were observed trying to hold their mother's teat in their mouth or suckle (Hanuman langur: Mohnot 1980; chimpanzee [START_REF] Goodall | Through a window: my thirty years with the chimpanzees of Gombe[END_REF]mountain gorilla: Fossey 1983;Porter et al. 2019): three were infants, but the fourth was a juvenile, and thus weaned, supporting the assumption of a comfort nursing function (Porter et al. 2019). The occurrence of vocalisations can also be used to assess the distress possibly experienced by individuals. Warning and distress calls were heard in about one third of documented cases of response to collapsed and inanimate partners (Table 2; see also [START_REF] Hosaka | Reactions to dead bodies of conspecifics by wild chimpanzees in the Mahale Mountains, Tanzania[END_REF][START_REF] Engel | Wild health: lessons in natural wellness from the animal kingdom[END_REF]). These calls appear to be uncommon in gorillas (Table 2), but can occur [START_REF] King | How animals grieve[END_REF].

A phenomenon reported exclusively in chimpanzees is what can be referred to as "stunned silence". There have been numerous reports of periods of quiet attendance interspersed with periods of excitement around dead bodies, both in the wild and in captivity (Teleki 1973a;[START_REF] Boesch | The chimpanzees of the Taï Forest: behavioural ecology and evolution[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] De Waal | Mama's last hug: animal emotions and what they tell us about ourselves[END_REF], but on other occasions group members appeared stunned, sitting passively and silently. Upon the demise of an adult female named Pansy in a safari park, Anderson et al. (2010) write that "The next day the three surviving chimpanzees were profoundly subdued. From the day area they watched silently as two keepers lowered Pansy from the platform, carried her into the exit corridor, placed her in a body bag, and loaded her into a vehicle that was then driven away. They remained subdued the following day as the night area was cleaned". In another case, witnessed by Valérie Dufour (pers. comm.) in 2006 at the Biomedical Primate Research Centre (Rijswijk, Netherlands), an adult male named Bob was found inanimate in the morning when the keepers opened the indoor quarters; the chimpanzees then remained completely silent despite the fact that the arrival of the keepers meant that they were going to be fed. Under normal circumstances, the distribution of food or the handling of a group mate cause a lot of tumult among chimpanzees. Reports of three additional cases of death recorded in captive chimpanzees described the same sustained silence in the colony, even at feeding time or when the body was removed by keepers [START_REF] De Waal | The brutal elimination of a rival among captive male chimpanzees[END_REF][START_REF] De Waal | Good natured: the origins of right and wrong in humans and other animals[END_REF][START_REF] De Waal | The bonobo and the atheist: in search of humanism among the primates[END_REF]. It is known in humans that acute stress disorders resulting from psychological trauma may involve withdrawal and lack of reaction [START_REF] O'brien | Traumatic events and mental health[END_REF][START_REF] Tull | Emotion in posttraumatic stress disorder: etiology, assessment, neurobiology, and treatment[END_REF]. By analogy with the human case, we suggest that stunned silence in chimpanzees indicates that they are experiencing deep emotional shock.

Visitation

Visitation is defined as returning to the place where death occurred or the body was last seen [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. It is not uncommon for monkeys and great apes to leave a corpse, then return and stay nearby, either the same day (Teleki 1973b;[START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF]Watts 2020) or the next day [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Strum | Almost human: a journey into the world of baboons[END_REF][START_REF] Fawcett | Death of a wild chimpanzee community member: possible outcome of intense sexual competition[END_REF][START_REF] Campbell | Behavioral responses to injury and death in wild Barbary macaques (Macaca sylvanus)[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]Porter et al. 2019;[START_REF] Yokoyama | Behavioral responses toward a conspecific corpse of wild bonobos (Pan paniscus) at Wamba[END_REF]) (Table 2). If the body has been removed, individuals may inspect the area by looking or sniffing [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF]Porter et al. 2019;Watts 2020).

Individuals may return to the place of death to check the state of their companion and seek information about the event, but this behaviour may also express their attachment to the deceased [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. There is evidence that the place of death may be associated with this event in their memory.

In two instances in captivity, individuals avoided the site of death for days (chimpanzee: Anderson et al. 2010) or weeks (lowland gorilla: King 2013). In the chimpanzees of Gombe National Park (Tanzania), [START_REF] Goodall | Through a window: my thirty years with the chimpanzees of Gombe[END_REF] relates the case of Flint, an adolescent male who became depressed and ill following the death of his mother: he returned to the site of death 3 weeks later and stayed there for several hours before dying.

RECOGNITION OF DYSFUNCTION AND AWARENESS OF MORTALITY

Since death is the cessation of life, it may seem necessary to know what primates understand about life in order to know what they understand about death. Animals are defined by their animacy, and it has been shown in several animal species, including primates, that individuals are able to distinguish between animate and inanimate as human infants do: they can separate entities that engage in selfgenerated motion from those that do not [START_REF] Opfer | What is the animate-inanimate distinction and why is it important?[END_REF][START_REF] Tsutsumi | Infant monkeys' concept of animacy: the role of eyes and fluffiness[END_REF][START_REF] Abdai | Animacy perception in dogs (Canis familiaris) and humans (Homo sapiens): comparison may be perturbed by inherent differences in looking patterns[END_REF]. Some authors have proposed that animals and humans are endowed with several innate systems of "core knowledge" that are designed to build representations with specific contents. One system would serve to represent (inanimate) objects and their mechanical effects, while another would serve to represent (animate) agents and their actions [START_REF] Spelke | Core knowledge[END_REF][START_REF] Carey | The origin of concepts[END_REF]). On the basis of this theoretical framework, Gonçalves and colleagues [START_REF] Gonçalves | Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF] hypothesize that when a living being dies, its body becomes an object, thus activating both the object and agency core systems in bystanders. This would create a perceptual mismatch in them, resulting in an "animacy detection malfunction".

It is sound to consider that an inanimate body can present contradictory clues that do not meet the expectations of conspecifics, and hinder their ability to recognize its state. However, it is probably too early to assert that the perception of a corpse induces a conflict between an object representation system and something like a life representation system. We are far from knowing what dynamic clues subjects use to assess animacy, and as [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF] acknowledge, agency involves much more than animacy: agents are capable of attentional states, goal-directed movements and interactions with others; primate species probably vary in their ability to identify these different components of agency. Moreover, subjects can also identify living beings like animals using morphological features such as the presence of a face, eyes, limbs or species-specific characteristics, and we do not know how dynamic and morphological clues are integrated at the brain level [START_REF] Opfer | What is the animate-inanimate distinction and why is it important?[END_REF][START_REF] Fisher | Whole-agent selectivity within the macaque face-processing system[END_REF]. We should add to these multiple clues the past interactions and social relationships that a subject may have memorized about a given conspecific, which would also contribute to the subject's representation of this individual.

Rather than starting from theoretical constructs about the recognition of life by primates, it seems more productive for the time being to address what they understand about the state of their inanimate conspecifics. However, discriminating inanimate individuals from perceptual features is not the same as knowing what it means to be dead [START_REF] Allen | Concept attribution in nonhuman animals: theoretical and methodological problems in ascribing complex mental processes[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]. In what follows, we will examine in turn whether monkeys and great apes detect conspecific dysfunction, show signs of disturbance, and exhibit some awareness of mortality.

Detection of dysfunction

Although death is usually regarded as an event, it is more accurate to think of it as a process in which one organic function fails after another. In principle, it would thus be possible to detect the dysfunction of a body using different clues: cardiorespiratory arrest, cessation of eye movements, and the inability to move and react.

As previously reported, monkeys and great apes have been observed inspecting the face and/or mouth of collapsed or inanimate partners, and Anderson et al. (2010) wrote that the companions of a dying chimpanzee "appeared to test for signs of life by closely inspecting her mouth and manipulating her limbs (test for pulse or breath)". However, nobody ever saw a monkey or an ape move their ear close to the mouth of another to listen to his/her breathing; nobody ever saw a monkey or an ape put their hand on the chest of another to feel his/her heartbeat. Not only would the contrary imply expectations about the need for breathing and heartbeats, but it is even hard for humans to be certain that the lungs and heart have stopped, especially in weakened individuals [START_REF] Stewart | Vital signs and resuscitation[END_REF]. Historically, people waited for signs of putrefaction to confirm death. Until a few centuries ago, tests for respiratory ventilation consisted for example of placing a feather under the nose, or holding a mirror before the mouth to detect condensation; it was only with the invention of the stethoscope in the 19th century that medical practitioners were able to determine with a reasonable degree of certainty whether the heart had ceased to function (President's Commission 1981;[START_REF] Powner | Medical diagnosis of death in adults: historical contributions to current controversies[END_REF][START_REF] Whetstine | The history of the definition(s) of death: from the 18th century to the 20th century[END_REF]. Against this background, it seems unlikely that primates other than humans have the ability to use soundless breathing, heart rate or pulse to assess the state of their conspecifics.

Another long-established medical sign of decease has been the fixity of the eyes (President's Commission 1981;[START_REF] Whetstine | The history of the definition(s) of death: from the 18th century to the 20th century[END_REF]. This is understandable given the normal animation of the eye area, which includes gaze orientation, pupillary reflex, and eyelid movements. Experimental studies in humans and other primates have shown that they are disproportionally interested in the eye area; they are highly sensitive to the attention and gaze direction of others, and are able to adjust their behaviour to get a partner to attend to them [START_REF] Emery | The eyes have it: the neuroethology, function and evolution of social gaze[END_REF][START_REF] Bourjade | Olive baboons, Papio anubis, adjust their visual and auditory intentional gestures to the visual attention of others[END_REF][START_REF] Liebal | Primate communication: a multimodal approach[END_REF][START_REF] Tomasello | Thirty years of great ape gestures[END_REF]. Correspondingly, monkeys and great apes often inspect the face and/or eyes of collapsed and recently deceased conspecifics (Tables 12). It is safe to assume that they are able to detect the absence of eye movement. The inability to make eye contact and get attention may warn them that something is going wrong with their companion. We can note that the eyes of inanimate individuals may be open or closed, but this information is not found in scientific reports. Fingering or picking at the eyes or eyelids has sometimes been observed in monkeys [START_REF] Yerkes | Maternal instinct in a monkey[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF]. According to a second-hand account, chimpanzees were once seen opening the eyes of a dead companion [START_REF] Bekoff | The emotional lives of animals: a leading scientist explores animal joy, sorrow, and empathy -and why they matter[END_REF].

Even if animacy is a main attribute of animals, it is common for primates to encounter motionless conspecifics. They regularly experience periods of immobility, which are first and foremost due to sleep phases. It is not the lack of animacy per se that must seem odd to observers of motionless individuals, but rather their inability to regain animacy: they are expected to wake up when someone acts on them, and the problem arises when they fail to do so. Assessing the responsiveness of partners and probing or stimulating them may account for many of the behaviours of primates towards collapsed and inanimate conspecifics. Although observing and sniffing can provide initial information, it is only by touching and above all handling partners that group members can test their reactivity or attempt to rouse them -with rough treatments producing the same effects. Whatever the exact goals of the intervening individuals, their actions can inform them if their partners were previously asleep or if something is going wrong. This is not an issue for collapsed partners, as they may still show some movement; we have indeed found that manipulative actions were mainly reported for inanimate partners, further indicating that monkeys and great apes are able to discriminate between these two conditions (Table 2). Note that we would expect the opposite trend for mobilising actions that specifically seek to lift a partner and restore him/her to an upright position: if the performers are able to distinguish between the two conditions, these actions should be applied to apparently wakefuli.e. collapsed -partners rather than to inanimate ones. As already mentioned, only two unambiguous cases of this type of helping behaviour have been described. In the first instance a mountain gorilla attempted to pull a juvenile into a sitting position while the latter was still reactive (Watts 2020: case 1). In contrast, the second instance concerns a rhesus macaque who tried to lift a partner when the partner was inanimate, and did so again when the partner was beginning to regain consciousness after fainting (see Supplemental Material). There are too few cases to be able to draw firm conclusions.

The termination of breathing, heartbeat, eye movements and any reflex or motor response is referred to by medicine as negative signs of life. They are later followed by positive signs of death: cooling of the body, stiffening, putrid odours and changes in skin colour that result from the decomposition process. These different alterations gradually take the body away from its living appearance, which could contribute to the information -or confusion -of those who inspect it [START_REF] Gonçalves | Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates[END_REF]). However, positive signs of death do not appear until several hours or even more than a day after death, so in many cases group members have assessed the state of the affected partner long before this time.

It may be expected that the more experienced individuals are better at detecting unresponsiveness and other signs of death. Three studies have compared the duration of dead infant carrying in females of different age and maternal experience. This yielded contradictory results that provided little support for the hypothesis that younger and/or first-time mothers would carry their dead infant for longer periods of time than other mothers [START_REF] Sugiyama | Carrying of dead infants by Japanese macaque (Macaca fuscata) mothers[END_REF][START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. Besides, younger chimpanzees tend to exhibit more exploratory behaviours toward corpses than adults (Teleki 1973a;[START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Van Leeuwen | Chimpanzees' responses to the dead body of a 9-year-old group member[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF]), and it is not rare for them to play with the bodies of dead infants (see above). However, little quantitative data are available in primates about the tendency of different age-and-sex categories to examine the body of affected conspecifics (see De [START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF].

Signs of disturbance

In the foregoing, we deliberately used the vague expression "something is going wrong" to account for what primates can infer when faced with a conspecific's inability to respond to their actions. The question now is whether this can cause concern to subjects. Not understanding something is not necessarily a reason for worry. On the contrary, if primates understand that something serious has happened, it can be disturbing to them and can be reflected in their behaviour.

Several authors have claimed that the occurrence of affiliative behaviours towards collapsing or dying conspecifics is a mark of compassion [START_REF] Bezerra | Responses towards a dying adult group member in a wild New World monkey[END_REF][START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. However, the affected individuals in these situations can move and emit sounds, so partners may respond to their signs of distress without the need for a mental representation of their emotional states. If compassion implies an understanding of distress that involves empathic perspectivetaking or the sharing of feelings [START_REF] Cheney | Baboon metaphysics: the evolution of a social mind[END_REF][START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Adriense | Challenges in the comparative study of empathy and related phenomena in animals[END_REF], then responses to the behavioural expression of distress are not proof of compassion. On the other hand, there is converging evidence that great apes such as chimpanzees are capable of targeted helping and empathic perspective-taking, and can thus apprehend the suffering of conspecifics [START_REF] De Waal | Anthropomorphism and anthropodenial: consistency in our thinking about humans and other animals[END_REF][START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Pérez-Manrique | The comparative study of empathy: sympathetic concern and empathic perspective-taking in non-human animals[END_REF][START_REF] Sato | Spontaneous attention and psycho-physiological responses to others' injury in chimpanzees[END_REF]). Nevertheless, when chimpanzees display affiliation towards a dying conspecific, it is difficult to justify any assumption that this a caretaking behaviour anticipating an approaching death (Anderson et al. 2010): an individual who is collapsing or dying is by definition still alive; we have no reason to believe that others can predict she/he is going to die, and they may actually perceive her/him as rescuable. Stringent conditions are necessary to examine whether a companion's physical condition may disturb partners, regardless of the occurrence of any communicative cues. We must rule out reactions to collapsed conspecifics and focus on reactions to inanimate conspecifics. We must also exclude cases where the presence of a danger in the surroundings can elicit distress or warning calls from bystanders (e.g. [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Gupta | Behaviour of Phayre's langur (Trachypithecus phayrei) on the death of one adult female[END_REF]Perry & Manson 2008: Maní/Pobrecito;[START_REF] Riley | Observation of a fatal dog attack on a juvenile long-tailed macaque in a human-modified environment in Singapore[END_REF]Campbell et al. 2016: case 4).

The display behaviours and rough treatments that chimpanzees and gorillas show towards their inanimate conspecifics are among the most dramatic responses reported in animals (Table 2). Males are primarily responsible for these actions, which in some cases were repeated over several hours [START_REF] Fossey | Gorillas in the mist[END_REF][START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF]. Even though rough treatments may contribute to test or awaken the partner, the energy devoted to violent behaviours indicates that the individuals' motivations go beyond these goals. Regardless of whether this behaviour expresses frustration due to incomprehension or a lack of acceptance of the partner's state, it at least informs us that individuals find the situation serious enough to engage in such heated action. The alternation of periods of quiet attendance and periods of excitement is another hallmark of the chimpanzee response. Emotional displays may include warning and distress calls, while stillness may pervade the entire group in cases of stunned silence (see above). Here again, signs of intense emotional disturbance in chimpanzees indicate that they attribute a special meaning to the unresponsiveness of their companion. No comparable periods of stunned silence have been recorded in gorillas, but it is difficult to make inferences about this difference with chimpanzees because gorilla temperament is characterized by high emotional stability [START_REF] Schaeffer | Personality and subjective well-being in captive male western lowland gorillas living in bachelor groups[END_REF][START_REF] Eckhardt | Personality dimensions and their behavioral correlates in wild Virunga mountain gorillas (Gorilla beringei beringei)[END_REF]. No conclusions can be drawn for bonobos and orang-utans due to the lack of documented reports in these species.

The comparison of monkey behaviour with that of chimpanzees and gorillas is illuminating. No rough treatment of corpses was reported in monkeys. Display behaviours were observed once in Hanuman langurs, but they were not directed at the body of the dead individual [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF]. Monkey responses to inanimate conspecifics are generally unremarkable. In one well-documented case in brown capuchins, for example, group members quietly inspected and handled the body of a recently deceased female in turn, and even her sister and offspring did not exhibit any signs of distress [START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]. In another instance in rhesus macaques, a male made sustained efforts to revive an unconscious partner by mobilising and lifting him, and although the intervening male yawned a few times, indicating a significant degree of tension (see [START_REF] Maestripieri | A modest proposal: displacement activities as an indicator of emotions in primates[END_REF][START_REF] Deputte | Ethological study of yawning in primates. I. Quantitative analysis and study of causation in two species of Old World monkeys (Cercocebus albigena and Macaca fascicularis)[END_REF], he showed no evidence of being alarmed (see Supplemental Material). Likewise, the other reported cases showed no signs of distress (Table 2; see also [START_REF] Takeshita | Changes in social behavior and fecal glucocorticoids in a Japanese macaque (Macaca fuscata) carrying her dead infant[END_REF]. There were two types of situation in which distress vocalisations were heard, but they do not actually contravene the previous statement. In the first case, infants cried and screamed following their mother's death [START_REF] Mohnot | Behavioural changes in Hanuman langur infant after mother's death[END_REF][START_REF] Fashing | Death among geladas (Theropithecus gelada): a broader perspective on mummified infants and primate thanatology[END_REF]; however, these behaviours also occur as part of normal mother-offspring interactions when a female does not attend to her infant. The second situation involves mothers emitting warning, distress or contact calls when they lose the body of their dead infant. Here, the triggering event is not the infant's lack of response, but the absence of the infant's body, as already mentioned (Altmann & Altmann 1970 and5). Although monkeys sometimes display signs of tension or anxiety such as restlessness, scratching and yawning when confronted with the unresponsiveness of a conspecific (Campbell et al. 2016: case 1;[START_REF] Carter | Baboon thanatology: responses of filial and non-filial group members to infants' corpses[END_REF][START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]; Supplemental Material), they do not show any signs of the intense emotional disturbance observed in chimpanzees and gorillas. The communicative signals that only monkeys send to non-responding conspecifics may represent a further consequence of their inability to realize the severity of their state, whereas chimpanzees and gorillas could be aware that any attempt to communicate is pointless.

Awareness of mortality

We came to the conclusion that monkeys and great apes are able to understand that "something is going wrong" when confronted with an inanimate conspecific. We also reached the conclusion that at least great apes such as chimpanzees and gorillas can also understand that "something serious has happened". However, the question remains as to whether they are aware of what it means when a partner remains permanently inert, and whether we can gain any insight into this question. Research studies in developmental psychology have shown that the understanding of death in human children can be defined and measured through several dimensions [START_REF] Speece | Children's understanding of death: a review of three components of a death concept[END_REF][START_REF] Kenyon | Current research in children's conceptions of death: a critical review[END_REF][START_REF] Slaughter | Young children's understanding of death[END_REF][START_REF] Longbottom | Sources of children's knowledge about death and dying[END_REF]. From 4 to 10 years of age, children successively master four main dimensions related to the concept of mortality: (1) Irreversibility: a dead individual cannot return to life, (2) Universality: all living beings must eventually die, (3) Cessation: all bodily and mental functions cease upon death, (4) Causation: the breakdown of bodily functions is due to external and internal causes.

Irreversibility is probably the easiest dimension of mortality to grasp, as it is the first dimension that children successfully master. Several authors have expressed the opinion that primates are capable of understanding the permanence of death (de Waal 2013; Anderson 2018; Gonçalves & Carvalho 2019). de Waal ( 2013) reports an anecdote where a film was projected to a group of captive chimpanzees. The film showed a male from the group who had died a few years earlier, and his appearance on the screen provoked a strong emotional reaction from his former rivals. [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF] argue that their reaction was evidence of a violation of expectation regarding the irreversibility of death. However, this event is by no means conclusive, as it could just as well be used to support the view that chimpanzees accept that dead individuals can return to life, and be touched if this appears to happen. Other researchers claim that the weakening of the mother's attachment to her dead infant and her eventual abandonment of the corpse prove her awareness of the irreversibility of death [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF][START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]). If it were true, however, this explanation would be tantamount to asserting that mothers of any animal species who give up the putrefying body of their offspring are aware of the irreversibility of death. One might just as well reason ad absurdum that chimpanzee females are among the least cognitively gifted primates since more than half of them in the Gombe population carry their dead offspring (see [START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. It is more straightforward to say that mothers have no other choice but to abandon the corpses after some time, and that this behaviour tells us nothing about death awareness in primates. [START_REF] Anderson | A primatological perspective on death[END_REF][START_REF] Anderson | Chimpanzees and death[END_REF] points out that chimpanzees have many opportunities to learn that inanimate individuals generally do not wake up, not only because they are exposed to deceased conspecifics -infant death being the most common event -but also because they regularly capture animals that they may kill and handle for a period of time before consuming them. Indeed, these different experiences are likely to inform individuals that subjects who remain immobile for extended periods of time generally do not recover. It should be noted however that small animals of a wide range of taxa follow death-feigning anti-predator strategies in which they go into tonic immobility when captured [START_REF] Humphreys | A review of thanatosis (death feigning) as an anti-predator behaviour[END_REF]. This means that it is not uncommon for primates to encounter motionless animals, some of which can subsequently resume activity. It should also be remembered that witnessing sleeping and awakening sequences in others is a daily reality for them. Furthermore, individuals may have also seen unconscious partners who later recover, because they were temporarily ill or had passed out. For example, falling from a tree is a continuing risk for tree-dwelling animals (see Table 2); primates cannot avoid occasionally landing on a dead branch, and if they fail to catch hold of a lower branch, they may pass out or die when they hit the ground, depending on the distance they fall and the nature of the ground. Waking up is instantaneous in a sleeping partner, whereas it may seem delayed in an unconscious partner. The consequence is that it is always difficult for primates to predict what will happen to an inanimate conspecific because the possibility that she/he may recuperate can rarely be excluded. The occurrence of visitations points in the same direction. By returning to a body they have previously left, they can check if their companion is still in the same state.

Children grow in a world of cultural symbols and conventions, and the representations embodied in the human language help them to form abstract categories [START_REF] Tomasello | The cultural origins of human cognition[END_REF][START_REF] Carey | The origin of concepts[END_REF]). The words "death" and "dead" are offered to them as tools that are passed on from one generation to the next [START_REF] Renfrew | The unanswered question': investigating early conceptualisations of death[END_REF]). Yet it takes years for them to understand that terms such as "never", "always" or "definitely" can have absolute meaning [START_REF] Kuczaj | On the acquisition of a semantic system[END_REF][START_REF] Hoffner | Children's understanding of adverbs denoting degree of likelihood[END_REF]). In comparison, animals are solipsist thinkers, meaning that they learn by themselves. They receive no support from others that would help them to conceptualize the absolute dimensions of death. While great apes can learn that unresponsive companions may not recover, this is probabilistic knowledge and not the absolute knowledge implied by words such as "irreversibility" and "permanence". Rather than asserting that great apes may be aware of the irreversibility of death, it seems more appropriate to echo [START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF] formulation that chimpanzees may be able to "realize dead individuals do not move or need help anymore, and that they will remain inanimate". This knowledge involves some uncertainty; it may reflect the belief that these individuals have fallen into a state where they seem to remain asleep indefinitely. We propose to call this state "dormancy", which means that the individual is in a prolonged state of immobility that is not normal sleep, with no rule specifying that recovery is excluded. In terms of the conceptualization abilities required, belief in dormancy is a less costly hypothesis than an awareness of the irreversibility of death.

Like irreversibility, the second dimension of mortality, universality, has an absolute meaning. We cannot question primates directly like we do with children, and there is no indication to date that they are able to know that death happens to everyone, whether they are conspecifics or individuals of other species [START_REF] Anderson | Chimpanzees and death[END_REF]. The third and fourth dimensions of mortality are the last to be mastered by children because they involve basic knowledge about the biological properties of living beings and the body functions that maintain life [START_REF] Slaughter | Young children's understanding of death[END_REF]. Some authors assume that primates recognize death as the cessation of bodily functions, basing their reasoning on the fact that mothers handle and defend the body of their dead infant [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF], or on changes in maternal behaviour such as atypical carrying postures, distancing from the corpse and its final abandonment [START_REF] Lonsdorf | Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses[END_REF]. However, the first argument ignores the fact that mothers also handle and defend their live infants [START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF], and the second fails to distinguish between detection of dysfunction and awareness of mortality: mothers may be able to recognize that something is going wrong with their non-responding infant without necessarily understanding what has happened (see above). The fourth main dimension of mortality, causation, is particularly demanding on the cognitive level since it requires subjects to infer causal relations, i.e. formulate hypotheses about the mechanisms involved in the breakdown of the functioning of the organism. It has been suggested that prey killing and lethal attacks on conspecifics may help individuals to gain insight into the causes of death [START_REF] Anderson | Chimpanzees and death[END_REF], and also that the investigation of inanimate bodies would enable the understanding of these causes [START_REF] Das | Deceased-infant carrying in nonhuman anthropoids: insights from systematic analysis and case studies of bonnet macaques (Macaca radiata) and lion-tailed macaques (Macaca silenus)[END_REF]). However, it should be clear that inspecting corpses or even inflicting death on others -as all predators do -is by no means proof that they realize how death occurs. Several authors have argued, based on a limited number of cases, that individuals react differently to deceased partners depending on the cause of death, but the views expressed by these authors are conflicting. According to Anderson and collaborators (Anderson 2011;[START_REF] Yang | Tending a dying adult in a wild multilevel primate society[END_REF][START_REF] Pettitt | Primate thanatology and hominoid mortuary archeology[END_REF], sudden and traumatic deaths resulting from an accident or a killing would trigger more alarm and violent behaviours than deaths due to non-apparent causes such as illness. In contrast, [START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF] proposes that bodies with wounds generate fewer signs of fear and alarm in chimpanzees than those of individuals who die from unexplained causes such as illness. Whatever the case, the accumulation of reports in chimpanzees and other species indicates that none of these expectations have been confirmed, either for dead infants [START_REF] Rajpurohit | Why do mothers carry the corpses of their infants in Hanuman langurs -Presbytis entellus?[END_REF][START_REF] Watson | Behaviour of nonhuman primate mothers toward their dead infants: uncovering mechanisms[END_REF], or other inanimate conspecifics (Table 2).

While the absence of evidence is not evidence of absence, the burden of proof rests on the person who asserts that something exists, and not on the person who denies it. [START_REF] Monsó | How to tell if animals can understand death[END_REF] looked for behaviours that would demonstrate that animals understand death, but she only proposed signs of emotional disturbance and ways to detect the dysfunction of the body. To date, there is no evidence that primates, including great apes, have an absolute concept of death, whatever the dimension considered, be it irreversibility, universality, cessation or causation. At this point, it is important to notice that irreversibility differs from the other three dimensions in two major respects. First, irreversibility can be reduced to a more concrete notion such as dormancy. To bystanders, a dormant conspecific may appear to be a sleeping individual who fails to wake up despite repeated interactions with her/him, and who remains in this state for an indefinite period of time. It is also conceivable that experienced individuals may have learned that it is common for a dormant body to cool down, smell, be surrounded by flies, and gradually lose its shape. Second, the awareness of the mortal condition of all living beings or the biological functions involved in the cessation of life and the causation of death is very abstract knowledge with little emotional content. These three dimensions of mortality cannot account for the strong emotional commitment revealed by the behaviour of great apes such as chimpanzees and gorillas. On the contrary, the ability to conceive of irreversibility or dormancy may explain that they understand something serious has happened. Realizing that the inanimate individual in front of them is unlikely to come out of sleep, unlikely to revert to the way she/he was, and must be left behind, can induce intense emotional disturbance. This may explain the display behaviours and rough treatments that have been recorded in chimpanzees and gorillas, reflecting a lack of comprehension and/or acceptance that their companion remains in an inanimate state. This may also explain the distress calls and episodes of stunned silence reported in chimpanzees. Some authors consider that such behaviours indicate an awareness of the irreversibility of death [START_REF] Anderson | Comparative evolutionary thanatology of grief, with special reference to nonhuman primates[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]. As discussed earlier, we find it more accurate to attribute these responses to the belief that their partner has gone into a dormant state. Belief in dormancy represents a more parsimonious explanation that does not require the involvement of concepts of mortality that may be too abstract to belong to the cognitive realm of great apes.

CONCLUSION

Mortality is a sensitive subject for humans and it is difficult to address it without a certain amount of anthropomorphism (Fig. 3). Using human subjective experiences to formulate hypotheses about the intentions, beliefs and emotions of animals as close to us as primates can provide a heuristic approach [START_REF] Mitchell | Anthropomorphism, anecdotes, and animals[END_REF][START_REF] De Waal | Anthropomorphism and anthropodenial: consistency in our thinking about humans and other animals[END_REF], but we must be prepared to accept that certain events may be beyond their capacity to comprehend [START_REF] Povinelli | Toward a science of other minds: escaping the argument by analogy[END_REF][START_REF] Brosnan | Nonhuman primate responses to death[END_REF]. To date, too much discussion about the concepts of mortality in primates has been based on armchair speculations that lack any reference to the actual behaviour of monkeys and great apes. This has led some authors to assert that primates understand the irreversibility of death and its biological causes despite a lack of evidence (e.g. [START_REF] Monsó | Death is common, so is understanding it: the concept of death in other species[END_REF]. We need more than convictions and presumptions. The scientific process is based on the search for evidence and we must refrain from romanticizing the primates' responses to death by attributing unsubstantiated meanings to their actions.

When an individual falls, passes out or dies, others cannot know from the start what is happening and may be confused. It is therefore not surprising that the majority of behaviours reported in group members may seek to obtain information by any means, by sniffing or looking at the body, with particular attention paid to expressive areas such as the face and eyes, and by carrying out various physical actions. Group members may handle the victim to test her/his reactivity or even to rouse her/him, and the fact that most manipulations are directed towards inanimate individuals and few concern collapsed individuals -i.e. live individuals with no need to be rousedindicates that primates are capable of discriminating between the two states. They can understand that something is going wrong when an individual does not come out of apparent sleep. Exceptional cases have also been reported in which a partner attempted to lift a victim in an upright position. Group members often stay near the victim for some time, ready to protect him/her from harm, and they direct affiliative behaviours such as grooming towards him/her. No significant differences are observed between monkeys and great apes with respect to these various behaviours. The same is true for the special case of the mothers of dead infants: both monkey and great ape mothers inspect, handle and groom the bodies of their offspring; some also carry the decaying corpses for extended periods of time, but all eventually abandon inert bodies.

The issue of whether animals may attend disabled partners is an ancient topic of discussion. In the "History of animals", 2500 years ago, Aristotle reports the following observation: "A herd of large and small dolphins has been seen together, and two of these having left appeared soon after, supporting and carrying on their back a small dead dolphin, that was ready to sink, as if in pity for it, that it might not be devoured by any other wild creature" (Aristotle 1883, Book IX, Chapter XXXV). Stories of sick or dead individuals that others try to support or lift have long been told about dolphins and elephants. It is only recently that first-hand accounts have confirmed the reality of these behaviours [START_REF] Gonçalves | Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates[END_REF]. Dolphins (e.g. Tursiops truncatus, Stenella coeruleoalba, Sousa chinensis) often stay close to dead conspecifics, and they may carry and keep them at the water surface, presumably to help them breathe (Bearzi et al. 2018;Reggente et al. 2018). The most commonly recorded behaviours in elephants (Loxodonta africana, Elephas maximus) appear to be investigative, but elephants may also push and pull dying or recently deceased individuals, attempting to make them stand up; they sometimes vocalise or mount a corpse, and in rare cases they have been seen to cover it with vegetation (Moss 1988;[START_REF] Payne | Sources of social complexity in three elephant species[END_REF]Douglas-Hamilton et al. 2006;[START_REF] Bercovitch | A comparative perspective on the evolution of mammalian reactions to dead conspecifics[END_REF]Goldenberg & Wittemyer 2020;[START_REF] Sharma | Behavioural responses of free-ranging Asian elephants (Elephas maximus) towards dying and dead conspecifics[END_REF]. Data in other vertebrates remain scarce and physical contact with the dead appear to be more limited. In ungulates, some individuals may stay close to the body and sniff it [START_REF] Bercovitch | Giraffe cow reaction to the death of her newborn calf[END_REF][START_REF] Mendonça | Feral horses' (Equus ferus caballus) behavior toward dying and dead conspecifics[END_REF]), but reported exceptions include pushing and scent-marking the body in a peccary (Pecari tajacu) (de Kort et al. 2018), as well as protecting a carcass and lifting it out of the water in a hippopotamus (Hippopotamus amphibius) [START_REF] Inman | Observations on the response of a pod of hippos to a dead juvenile hippo (Hippopotamus amphibius, Linnaeus 1758)[END_REF]. In corvids, birds may gather around a carcass and call loudly, and some individuals may peck at it or attempt copulation [START_REF] Iglesias | California scrub-jay funerals: cacophonous aggregations in response to dead conspecifics[END_REF][START_REF] Swift | Occurrence and variability of tactile interactions between wild American crows and dead conspecifics[END_REF].

Primates, but also animals such elephants and cetaceans, appear to stand out from many other vertebrate species in terms of their direct actions on the bodies of collapsed or inanimate conspecifics, and their various attempts to mobilise them. Primate mothers are the only mothers who regularly carry their dead offspring for long periods. Great apes such as chimpanzees and gorillas further differ from other species by the occurrence of violent behaviours and marked signs of distress, which led us to conclude that they may be aware that something serious has happened to their inanimate conspecifics. In contrast, monkeys only show signs of tension and anxiety under the same circumstances; they try to communicate with inanimate conspecifics, and there is no evidence that they understand that something serious has happened. Monkeys are probably disoriented by partners who appear to be asleep but do not wake up. For example, the male rhesus macaque who made strenuous efforts to rouse his unconscious companion and lift him into an upright position was able to recognize that his companion needed support (see Supplemental Material). His primary goal was probably to restore the victim to his normal state of wakefulness. It is not necessary to hypothesise he knew that his companion was in a state close to death; it is sufficient to assume that he was unaware that the other was not sleeping to account for his behaviour. Similar ignorance may explain many of the animals' attempts to lift the bodies -dead or alive -of their affected groupmates. [START_REF] Zuckerman | The social life of monkeys and apes[END_REF] assertion that monkeys and great apes do not distinguish between the living and the dead is not accurate. They are able to detect that an inanimate individual does not react as they expect him/her to do. However, this does not mean that they conceive of life and death in the same way that we do. In examining the arguments put forward so far regarding awareness of mortality, we have found no evidence that primates -including chimpanzees and gorillas -are aware of the concepts related to death such as irreversibility, universality, cessation or causation. This is particularly consistent with what we know about the cognitive abilities of great apes. [START_REF] Herrmann | Humans have evolved specialized skills of social cognition: the cultural intelligence hypothesis[END_REF] showed that the performances of mature chimpanzees and orang-utans were comparable to those of 2.5-year-old children in physical cognition tasks (spatial skills, assessing quantity, tool use), and that they were far outstripped by children in social cognition tasks (social learning, attracting attention, attributing intention) (see also [START_REF] Civelek | Inferring unseen causes: developmental and evolutionary origins[END_REF]. Children under the age of 3 years are unaware of the idea of mortality, and tend to see death as sleep or a trip from which one returns [START_REF] Speece | Children's understanding of death: a review of three components of a death concept[END_REF][START_REF] Opfer | What is the animate-inanimate distinction and why is it important?[END_REF]. Nevertheless, great apes appear capable of attributing mental states to others and empathizing with them [START_REF] Call | Does the chimpanzee have a theory of mind? 30 years later[END_REF][START_REF] De Waal | Anthropomorphism and anthropodenial: consistency in our thinking about humans and other animals[END_REF][START_REF] Boesch | Wild cultures: a comparison between chimpanzee and human cultures[END_REF][START_REF] Buttelmann | Great apes infer others' goals based on context[END_REF][START_REF] Pérez-Manrique | The comparative study of empathy: sympathetic concern and empathic perspective-taking in non-human animals[END_REF][START_REF] Kano | Great apes use self-experience to anticipate an agent's action in a false-belief test[END_REF]. The signs of emotional disturbance displayed by chimpanzees and gorillas faced with inanimate companions suggest that previous encounters with similarly incapacitated individuals may have taught them that they are in a serious condition. To account for such concern, we propose that they may regard an inanimate individual as having entered a dormant state, meaning she/he is unlikely to recover to an awake state. While it is more parsimonious to ascribe a belief in dormancy to great apes than a belief in the irreversibility of death, we must acknowledge that neither can be proven beyond doubt.

Behaviours towards victims can vary greatly among individuals of the same species or group. Experience is likely to play an important role, but we lack information about how individual understanding may change with repeated exposures to sick, unconscious, dying or dead conspecifics [START_REF] Stewart | Responses of chimpanzees to a recently dead community member at Gombe National Park, Tanzania[END_REF][START_REF] Anderson | Chimpanzees and death[END_REF][START_REF] Gonçalves | Comparative thanatology, an integrative approach: exploring sensory/cognitive aspects of death recognition in vertebrates and invertebrates[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]). Most of our conclusions are based on indirect evidence, and much remains to be investigated about primate responses to death [START_REF] Pettitt | Primate thanatology and hominoid mortuary archeology[END_REF]. The experimental study of the cognitive abilities of monkeys and great apes will undoubtedly provide valuable insights into their capacity for abstraction and the clues they can use to recognize animacy and agency [START_REF] Brosnan | Nonhuman primate responses to death[END_REF]. However, we also need specific knowledge pertaining to the response to helplessness and death. Several authors have suggested conducting experiments using deceased individuals; a possible approach would be to test reactions to vocalizations, photographs or anesthetized individuals, but this must involve individuals that are known to the subjects in the study to be truly informative, which raises ethical issues [START_REF] Kaplan | Responses of mother squirrel monkeys to dead infants[END_REF][START_REF] Allen | Concept attribution in nonhuman animals: theoretical and methodological problems in ascribing complex mental processes[END_REF][START_REF] Anderson | A primatological perspective on death[END_REF][START_REF] Brosnan | Nonhuman primate responses to death[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF][START_REF] Monsó | How to tell if animals can understand death[END_REF]. This type of experience might cause anxiety or distress to subjects, with the risk of consequences such as great apes inflicting rough treatment on anesthetized individuals.

As imperfect as they may be, opportunistic observations will remain an essential source of information. We must continue to collect cases to better appreciate the full spectrum of primate responses to collapsed, inanimate and dead conspecifics. In particular, this will allow us to see whether the disparities we have discerned in patterns of violence, distress and communication between monkeys and great apes are borne out. To this end, we must avoid a bias in the current literature that tends to privilege the most sensationalistic stories [START_REF] Brosnan | Nonhuman primate responses to death[END_REF][START_REF] Marco | Responses to a dead companion in a captive group of tufted capuchins (Sapajus apella)[END_REF]. Scientific journals do not wish to publish papers finding that nothing significant has happened: when a mother grooms her infant's corpse a little then abandons it some time later, or when members of a group show limited interest in a dead companion, nothing is reported. In addition, the responses of monkeys are generally not spectacular, and as a result little attention has been paid to several accounts published about these species, whilst abundant commentaries have been devoted to the more dramatic events often depicted in chimpanzees (Table 2). As proposed by [START_REF] Brosnan | Nonhuman primate responses to death[END_REF], the creation of a collaborative data repository would permit observers to report the circumstances of any deaths they may have witnessed, whatever the responses of individuals. Such a living database has just been created by Alicia Carter under the name "ThanatoBase" to invite researchers to contribute their observations on primate death [START_REF] Carter | ThanatoBase: a project to understand primates' responses to death and dying[END_REF]. To make progress, future studies will need to clearly separate behaviours directed at inanimate individuals (i.e. inert and/or dead) from those directed at collapsed or dying individuals (i.e. still able to move or react), and record the occurrence of any behaviour that may be indicative of emotion, such as vocalizations and self-directed behaviours. It would also be desirable, whenever possible, to document the responses of primates to corpses in the different periods after death -the first hours, the day after, and the following days -in order to provide information on the clues they may use to recognize the state of their conspecifics.

Finally, it is well known that monkeys and great apes may exhibit stress, depression or grief behaviours after the loss of a significant attachment figure [START_REF] Zeller | The grieving process in non-human primates[END_REF][START_REF] Engh | Behavioural and hormonal responses to predation in female chacma baboons (Papio hamadryas ursinus)[END_REF][START_REF] King | How animals grieve[END_REF][START_REF] Santos | Towards a natural history of grief[END_REF][START_REF] Gonçalves | Death among primates: a critical review of non-human primate interactions towards their dead and dying[END_REF]. In the present review, we focused on the responses of primates to the bodies of their helpless conspecifics. Within this scope, we found signs of intense emotion only in great apes, probably related to the awareness that inanimate individuals are unlikely to recover. In monkeys, on the other hand, no signs of distress were reported; we believe that it is over time that they may experience the absence of their companion -or remember the circumstances of the disappearance -and then show signs of grief as they miss her/him. Needless to say, since we do not have direct access to the thoughts and feelings of primates and other animals, we can expect that their attitude to death will continue to be the subject of much debate for a long time to come, as with the more general question of animal consciousness.

1

SUPPLEMENTAL MATERIAL

Response of a rhesus macaque to a companion that has suffered an electric shock: a review of an anonymous online video

The video footage concerned here has been freely available online since 21st December 2014, in the form of several versions of different durations. The accompanying information states that it depicts an event that is claimed to have taken place on Saturday 20th December at an Indian railway station, where a monkey attended a conspecific that had suffered an electric shock. The source of the footage was anonymous, but it was quickly picked up by many media outlets: Indian channels and newspapers like News Hour India, The Telegraph, Business Standard and Wilderness Films India, as well as non-Indian media like The Independent and National Geographic. This record is exceptional both in view of the actions performed by the companion of the victim, and the small number of scientific reports detailing the response of monkeys to inanimate groupmates.

In a previous paper where we intended to quote this episode, an anonymous reviewer told us that "The citation to a Youtube video is not appropriate in a scientific article", and we had to remove the reference. It is true that most of the footage of animal behaviour found on the web is posted by non-professionals; these recordings lack circumstantial evidence and some of them could even be faked. Science needs observers trained to avoid the pitfalls of anthropomorphism. It should be noted, however, that some of the reports in scientific literature itself are based on fleeting events with no video evidence, thus leaving the door open to observational errors and interpretation bias. In the present case, a fairly long online video is available. It is possible to subject it to critical scrutiny to determine whether the videotape and associated information can be used as a reliable record. As scientists, we believe we should not deprive ourselves of a potential source of information.

To examine the videotape on the electrified monkey, we chose the longest sequence available on the web (https://www.youtube.com/watch?v=g8wXhmHVtXE). It is 6'16" long and has low visual definition, probably because it was recorded using a mobile phone. The online comments associated with the videotape indicate that the event occurred at the railway station of the city of Kanpur, in the state of Uttar Pradesh, India. This is one of the main stations of northern India, with heavy traffic. The overhead lines of the trains are powered by standard 25 kV, 50 Hz alternating current. The animals in the video are three rhesus macaques (Macaca mulatta). Based on their physical appearance, it can be said that the shocked individual and his partner are both subadult males, while the third individual is a juvenile male.

2'51"-2'57" The partner grasps and lifts the victim, who shows the first signs of life: a blink of the eyes and head movements (Fig. S1i). 2'58"-3'3" The partner pushes the victim, who ends up sitting against a pipe: the victim's eyes are open, he moves an arm and his head (Fig. S1j). 3'4"-3'8"

The partner lifts the victim by the armpits and puts him in a sitting position, but the victim cannot sit up (Fig. S1k). 3'11"-3'16" The partner pulls the victim's leg. 3'17"-3'27" The partner looks around and yawns. (cut) 3'28"-3'30" The partner pulls the victim towards him, but the result is that the victim falls off the grid into the water-filled drain. 3'31"-3'35" The partner tries to pull the victim out of the water by the armpits, and then looks around. 3'36"-3'38" The partner tries to pull the victim out of the water by one leg. 3'39"-3'47" The partner attempts to pull the victim out of the water by the hips, and then looks around. 3'48"-3'58" The partner grasps the victim's eyebrows with his teeth to pull the victim out of water (Fig. S1l). (cut) 3'58"-4'1" The partner manages to pull the victim out of water and lays him on the grid (Fig. S1m). (cut) 4'2"-4'27" People on the platform and the passage of trains. (cut) 4'28"-4'38" The animals are hidden by the train and people look underneath to spot the animals: the victim is still lying down (Fig. S1n). (cut) 4'39"-4'44" People on the platform and street scene. (cut) 4'45"-4'50" The victim is able to sit up unaided and the partner yawns. 4'51"-4'54" The partner pushes the victim's back with both hands (Fig. S1o), forcing him to briefly lift his rump. The partner then grooms and grasps the victim for a few seconds. 4'55"-5'

The partner pushes the victim's back with his hands. 5'1"-5'5"

The partner yawns and then looks around. . (cut) 5'6"-5'19" The victim is sitting, still dizzy, and the partner is not paying attention to him. (Fig. S1p). 5'20"-5'25" The partner approaches the victim, and pushes and grasps his back with both hands. (cut) 5'26"-5'30" People on the platform. (cut) 5'31"-6'16" Street scenes and the exterior of the buildings of Kanpur Central Station.

Comments

The onset of the event does not appear on the video. However, there is no basis to question the fact that an electric shock was the reason for the inanimate state of the individual. One of the post supporting the overhead line can be seen in the video (Fig. S1a,d,h). The inanimate individual has no apparent injuries. He is lying on a grid near the base of the post under the electric cables, making it likely that he had climbed up the post and fallen after touching the cables and suffering an electric shock. The fall to the ground may also have contributed to his condition.

The shots including monkey views last 4'48" in total. They are separated by 13 cuts. At 4'18", for example, a train has stopped in front of the monkeys, making it difficult to see the animals (Fig. S1n). In the next shot, filming is resumed from the opposite platform (Fig. S1o), which means that a significant amount of time has elapsed between the two shots. Given the number of cuts and the absence of the first moments after the accident, it can be assumed that the entire event lasted at least 10 minutes. Online commentary accompanying the videotape states that the entire behavioural event lasted 15-20 minutes. It should be noted that an examination of the locations of animals in the successive sequences showed that shots appear in the correct chronological order in the videotape.

Three periods can be distinguished in the video according to the state of the shocked individual: (1) from the beginning until 2'50", the victim appears inert and does not respond to his partner's actions (Fig. S1a-g); (2) from the sequence starting at 2'51", the victim blinks his eyes and makes head and limb movements (Fig. S1i,j), but remains unable to remain in a sitting position; (3) only after the pause ending at 4'45" is the victim able to sit up unaided and hold his head upright (Fig. S1o,p). The partner displayed two main categories of actions: acting on the body (nibbling, shaking, pushing, pulling horizontally), and lifting the body (pulling upwards with hands and/or teeth). Both categories of actions occurred in the first period (acting on the body: 7 times, lifting the body: 9) and the second period (acting on the body: 2, lifting the body: 5), while acting on the body is seen twice in the third period. The number of actions per period should only be considered as indicative information since the footage does not record the complete course of the event. The three brief yawns made by the partner may reflect a significant level of tension. It may be noted that the third individual stays close to his companions and looks in their direction most of the time.

Conclusion

Rhesus macaques are known for their ability to live in close contact with humans and thrive in Indian cities [START_REF] Southwick | Rhesus commensalism in India: problems and prospects[END_REF][START_REF] Srivastava | City monkeys (Macaca mulatta): a study of human attitudes[END_REF][START_REF] Beisner | Human-wildlife conflict: proximate predictors of aggression between humans and rhesus macaques in India[END_REF]. The presence of males of this species in a train station is therefore not surprising. The animals appear to be free-ranging, with one interacting with the other under exceptional circumstances, so there is no reason to suspect that the behaviour of these individuals could have been influenced by any human training.

One of the main comments by non-professionals on the Web is that a monkey came to the rescue of his fellow monkey who had lost consciousness, and revived him by hitting and biting him, and tipping him into the water. If the electric shock caused cardiopulmonary arrest, then we can consider the entire episode as one of the most aggressive cardiopulmonary resuscitations ever performed. A variety of pressures, namely pinching, hitting and shaking, could have played a role in this, as well as immersing the victim in water. In contrast, other commentators see this behavioural sequence as a violent attack by one individual against another. It is true that the partner's behaviour was particularly rough, and his disordered actions caused the victim to fall into the water twice, putting him at risk of drowning.

From the primatologist's point of view, the partner's behaviour appears to be very unusual. His actions can be described as acting on and lifting the body of a shocked conspecific. This is by no means an aggression, as there was no threat or real biting. Irrespective of any assumptions about the animal's abilities in terms of empathy or awareness of death, his behaviour can be explained by the desire to bring the companion back to his normal state, i.e., awake and upright. In such a perspective, acting on the body mainly aimed to rouse the victim, and lifting actions intended to pull the victim out of the water and/or into a sitting position.

Tales of groupmates trying to lift or support sick or dying individuals have long been told in reports about African elephants (Loxodonta africana) and dolphins (Tursiops truncatus, Stenella coeruleoalba). It is only fairly recently that experts have corroborated these stories (Moss 1988;Douglas-Hamilton et al. 2006;Bearzi et al. 2018;Reggente et al. 2018;Goldenberg & Wittemyer 2020). Similar actions have been observed in chimpanzees (Pan troglodytes) (Brown 1879;Anderson et al. 2010) and mountain gorillas (Gorilla beringei) (Porter et al. 2019;Watts 2020). In this context, the present video footage provides the first evidence of such helping behaviour in a monkey species, and moreover with an intensity that has never been reported before in any animal species. Behaviours that appear to rouse inanimate partners have been reported in other primates (see main text). In yellow baboons (Papio cynocephalus), Altmann and Altmann (1970) observed a case in which an adult male rubbed the thorax of a dead infant for a long time, adding that "If the infant had been in a state of shock, rather than dead, the thoracic massage might have revived it".

The present case in rhesus macaques deserves to be taken up by primatological literature. It is all the more interesting as it concerns rhesus macaques, which are known for their low level of social tolerance compared to other monkey species [START_REF] Thierry | Unity in diversity: lessons from macaque societies[END_REF]. As with other types of evidence, it needs to be confirmed by further observations in macaques and other non-human primates.

Fig. 1 .

 1 Fig. 1. -The mothers' behaviour towards their dead infant: (A) Holding the body by one leg in the Barbary macaque, (B) Carrying mummified remains in the Tonkean macaque, (C) Grooming the body in the Tonkean macaque, (D) Peering at the eyes in the brown capuchin (photos: A. De Marco & B. Thierry).

  sniffs, touches, pushes or fingers the body (except the eye area).

Fig. 2 .

 2 Fig. 2. -Behaviour directed toward to the corpse of an adult female in a group of brown capuchin monkeys (De Marco et al. 2020): (A) An adult and a juvenile males peer at the face of the female, (B) A subadult male handles a leg, (C) A subadult male grooms the eye area (photos: A. De Marco).

Fig. 3 .

 3 Fig. 3. -Mourning in Macaques: the deliberate anthropomorphisation of primates in satirical drawing (etching by Thomas Landseer 1827).

Fig. S1 .

 S1 Fig. S1. -Still images from videotape: (a) Both males are between the tracks, (b) The victim falls into the drain, (c) The partner lifts the victim with his teeth, (d) The post supporting the two overhead cables, (e) The partner nibbles the victim's neck, (f) The partner lifts the victim by catching the eyebrows with his teeth, (g) The partner nibbles the thigh of the victim, (h) The people watch and film the monkeys.

Fig

  Fig. S1 (continued) -(i) The partner lifts the victim, who shows the first signs of life, (j) The victim turns his head, (k) The partner lifts the victim by the armpits, (l) The partner lifts the victim by catching his eyebrows with his teeth, (m) The partner has managed to pull the victim out of the water, (n) People look under the train to see the animals, (o) The victim can sit up and the partner pushes his back, (p) the victim is sitting up unaided but is still dizzy.

Table 1 .

 1 Dead-infant carrying: maternal behaviours in the first 2 days following death, and from the 5th day onwards. The absence of a behaviour does not necessarily mean that it did not occur in the corresponding period, only that it was not reported.

			Authors		Rumbaugh (1965)		Trapanese et al.	(2020)	A. De Marco	(personal	observation); Fig.	1D	Perry & Manson	(2008)		Perry & Manson	(2008)	Jay (1962)	Rajpurohit (1997)	Sharma et al. (2011)	Lü et al. (2007)
	Behaviours	occurring in an	unspecified period				NA		NA				NA			NA		grooms, handles,	attention to eyes	prevents access,	swats flies	grooms, inspects
	Behaviours occurring	in the second period (≥	5 days)				NA 2		NA				NA			NA			prevents access
		Behaviours occurring in the	first period (first 2 days)		prevents access, licks,	attention to eyes 1	grooms, licks, handles,	cannibalism	grooms, licks, inspects 3 ,	attention to eyes			prevents access, grooms,	licks, inspects, warning	call	prevents access, swats flies	and wasps, grooms, licks,	warning call	grooms, inspects, attention	to eyes	grooms, inspects, attention	to eyes
		Number of	infants		1		1		1				1 (Omni)			1 (Pobrecito)		3	12	10	1
		Living	conditions		captive		captive		captive				wild			wild		wild	wild	wild	provisioned
			Species	Monkeys	Saimiri	sciureus	Sapajus apella		Sapajus apella				Cebus	capucinus		Cebus	capucinus	Semnopithecus	entellus	Semnopithecus	entellus	Semnopithecus	entellus	Rhinopithecus	roxellana

Table 2 .

 2 Behaviours directed towards collapsed and inanimate subjects. The occurrence of a behaviour means that it was performed by at least one member of the group.

			Authors		Box (1984)		Bezerra et al.	(2014)				De Marco et al.	(2020)		Mohnot (1980)			Gupta (2000)	Yang et al. (2016)
		Behaviours directed towards the	inanimate individual		sniffs, manipulates, grooms, scent-	marks	no response					swats flies, peers, sniffs, touches,	manipulates, mobilises,	threatens, grooms	stays nearby, peers, sniffs, touches,	display behaviour, warning call,	distress call, visitation	stays nearby, prevents access,	touches, manipulates, grooms,	warning call	touches, manipulates, visitation
	Behaviours directed	towards the collapsed	individual		NA 1		stays nearby, prevents	access, sniffs,	clasps 2 , mounts 3 ,	warning call		NA			NA			NA	stays nearby, prevents	access, peers, sniffs,	touches,	manipulates,	grooms, clasps,	warning call
			Cause		unknown		fall from a tree					unknown			hit by stone thrown by	human & fall from	a tree	electric shock	fall from a tree
			Victim		adult female		adult female	(F1B, last	78 min	before	death)	adult female	(Petula)		adult female			adult female	adult female	(DM)
		Living	conditions		captive		wild					captive			wild			wild	wild
			Species	Monkeys	Callithrix	jacchus	Callithrix	jacchus				Sapajus apella			Semnopithecus	entellus		Trachypithecus	phayrei	Rhinopithecus	roxellana

1 NA = not applicable. 2 Grasps or embraces.

  al. 2014; chimpanzee: Boesch & Boesch-Achermann 2000;[START_REF] Pruetz | Intragroup lethal aggression in West African chimpanzees (Pan troglodytes verus): inferred killing of a former alpha male at Fongoli, Senegal[END_REF][START_REF] Jakucińska | Reaction to the death of the oldest female in a group of chimpanzees at the Municipal Zoological Garden[END_REF] mountain gorilla: Watts 2020). Threats may also be directed against humans (common marmoset[START_REF] Box | Primate behavior and social ecology[END_REF] Hanuman langur: Mohnot 1980; long-tailed macaque, Macaca fascicularis:[START_REF] Riley | Observation of a fatal dog attack on a juvenile long-tailed macaque in a human-modified environment in Singapore[END_REF] Barbary macaque: Campbell et al. 2016). In Phayre's leaf monkeys (Trachypithecus phayrei), individuals interposed themselves between the dead body of an adult female and arriving vultures

  : Shorty; Smuts 1985: Zandra/Zephyr; Perry & Manson 2008: Abby/Omni; Li et al. 2012: case 1; Botting & van de Waal 2020: Nurks; Carter et al. 2020: cases 1

Mounts or makes pelvic thrusts against the body.Coping with mortality in primates

ACKNOWLEDGEMENTS

We are grateful to Francesco Dessì-Fulgheri for inviting us to write this review, to two reviewers for constructive comments, and to Valérie Dufour for fruitful discussion. We dedicate this paper to all the animals we have seen being born, living and dying.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

SUPPLEMENTAL DATA

Supplemental data for this paper can be accessed here.

Video sequences

The videotape is composed of a succession of 14 shots depicting monkeys, interspersed with several sequences showing only people, platforms, trains and buildings. The sounds heard in the recording are people's voices and noise that is typical of a train station.

0"-2"

Arrival of a train and people on the platform. (cut) 3"-8" Two subadult males are between the railway tracks (in the centre of the image), not far from the base of a large post (on the right of the picture) (Fig. S1a): one individual (victim) lies on a grid, another individual (partner) is nearby and looking around. 9"-18"

People on the platform. 19"-24"

The partner pulls the victim towards him, causing him to fall from the grid into a drain with water in the bottom (Fig. S1b). The juvenile individual approaches and looks at them. 25"-31"

The partner grasps the skin on the victim's back with his teeth, tries to lift him up, and shakes him. The juvenile observes the scene. 32"-48"

The partner looks around and then tries to pull the victim out of the water by lifting him up with his teeth. The juvenile observes the scene (Fig. S1c). 49"-55"

The partner looks around then attempts to lift the victim with his teeth. The juvenile observes the scene. (cut) 56"-1'

The post near the monkeys supports two overhead cables (Fig. S1d). 1'1"-1'11" The partner tries to lift the victim with his teeth and shakes him. (cut) 1'12"-1'14" The people on the platform watch and film the two rhesus macaques. (cut) 1'15"-1'35" The partner looks around, then attempts to lift the victim with his teeth while using his hands to pull the victim by the hips, then he looks around again. (cut) 1'36"-1'41" The partner tries to lift the victim with teeth and hands, and then looks around. (cut) 1'42"-2'1" The partner nibbles the victim's neck (Fig. S1e), then looks around. 2'2"-2'8"

The partner roughly pushes and pulls the victim. 2'9"-2'17" The partner lifts the victim by grasping the eyebrows with his teeth and by holding the victim around the shoulders (Fig. S1f). 2'18"-2'23" The partner pushes and pulls the victim. (cut) 2'24"-2'34" The partner nibbles the inner part of the victim's thigh (Fig. S1g), and then looks around. (cut) 2'35"-2'46" The people on the platforms watch and film the event. The juvenile has moved away to a location several meters behind the other two individuals (Fig. S1h). (cut) 2'47"-2'50" The partner is close to the victim and looks around.