Loïc Pougnault

Florence Levréro

Maël Leroux

Julien Paulet

Pablo Bombani

Fabrice Dentressangle

Laure Deruti

Baptiste Mulot

Alban Lemasson
email: alban.lemasson@univ-rennes1.fr

Social pressure drives "conversational rules" in great apes

Keywords: communication evolution, vocal communication, social communication, social structure, social organization, vocal exchange, conversation, turn-taking, Primates, Hominidae

In the last decade, two hypotheses, one on the evolution of animal vocal communication in general and the other on the origins of human language, have gained ground. The first hypothesis argues that the complexity of communication co-evolved with the complexity of sociality. Species forming larger groups with complex social networks have more elaborate vocal repertoires. The second hypothesis posits that the core of communication is represented not only by what can be expressed by an isolated caller, but also by the way that vocal interactions are structured, language being above all a social act. Primitive forms of conversational rules based on a vocal turn-taking principle are thought to exist in primates. To support and bring together these hypotheses, more comparative studies of socially diverse species at different levels of the primate phylogeny are needed. However, the majority of available studies focus on monkeys, primates that are distant from the human lineage. Great apes represent excellent candidates for such comparative studies because of their phylogenetic proximity to humans and their varied social lives. We propose that studying vocal turn-taking in apes could address several major gaps regarding the social relevance of vocal turn-taking and the evolutionary trajectory of this behaviour among anthropoids. Indeed, how the social structure of a species may influence the vocal interaction patterns observed among group members remains an open question. We gathered data from the literature as well as original unpublished data (where absent in the literature) on four great ape species: chimpanzees Pan troglodytes, bonobos Pan paniscus, western lowland gorillas Gorilla gorilla gorilla and Bornean orang-utans Pongo pygmaeus. We found no clear-cut relationship between classical social complexity metrics (e.g. number of group members, interaction rates) and vocal complexity parameters (e.g. repertoire size, call rates). Nevertheless, the nature of the society (i.e. group composition, diversity and valence of social bonds) and the type of vocal interaction patterns (isolated calling, call overlap, turn-taking-based vocal exchanges) do appear to be related. Isolated calling is the main vocal pattern found in the species with the smallest social networks (orang-utan), while the other species show vocal interactions that are structured according to temporal rules. A high proportion of overlapping vocalisations is found in the most competitive species (chimpanzee), while vocal turn-taking predominates in more tolerant bonobos and gorillas. Also, preferentially interacting individuals and call types used to interact are not randomly distributed. Vocal overlap ('chorusing') and vocal exchange ('conversing') appear as possible social strategies used to advertise/strengthen social bonds.

Our analyses highlight that: (1) vocal turn-taking is also observed in non-human great apes, revealing universal rules for conversing that may be deeply rooted in the primate lineage; (2) vocal interaction patterns match the species' social lifestyle; (3) although limited to four species here, adopting a targeted comparative approach could help to identify the multiple and subtle factors underlying social and vocal complexity. We believe that vocal interaction patterns form the basis of a promising field of investigation that may ultimately improve our understanding of the socially driven evolution of communication.

I. Introduction

I. INTRODUCTION (1) Vocal communication organisation as a mirror of social complexity

Possibly because communication is widespread across the animal kingdom and occurs in a wide diversity of social contexts [START_REF] Waser | Individual Recognition, Intragroup Cohesion and Intergroup Spacing: Evidence From Sound Playback to Forest Monkeys[END_REF][START_REF] Mccomb | Female choice for high roaring rates in red deer, Cervus elaphus[END_REF][START_REF] Geissmann | Duet songs of the siamang, Hylobates syndactylus: II. Testing the pair-bonding hypothesis during a partner exchange[END_REF][START_REF] Arlet | Grooming-at-a-distance by exchanging calls in non-human primates[END_REF], the evolutionary forces driving communication are still intensively debated.

Communication is indeed influenced by habitat quality, predation and life-history traits (e.g. [START_REF] Lemasson | What can forest guenons « tell » us about the origin of language? In Primate Communication and Human Language: Vocalisation, gestures, imitation and deixis in humans and non-humans[END_REF]Freeberg, Dunbar & Ord, 2012;[START_REF] Mouterde | Acoustic communication and sound degradation: how do the individual signatures of male and female zebra finch calls transmit over distance?[END_REF]Fischer et al., 2017a;[START_REF] Hammerschmidt | Baboon vocal repertoires and the evolution of primate vocal diversity[END_REF][START_REF] Pisanski | Voice Modulation: A Window into the Origins of Human Vocal Control?[END_REF]. The hypothesis that social complexity has been one of the driving forces shaping vocal complexity (or even the main driving force for primates), as suggested by Darwin (1877), has recently received renewed interest (Freeberg et al., 2012;[START_REF] Bouchet | Social complexity parallels vocal HEIDELBERG)[END_REF][START_REF] Sewall | Social complexity as a driver of communication and cognition[END_REF]. One of the biggest challenges of this 'social complexity hypothesis' for communication is to define and measure 'complexity' properly in terms of both social and communicative aspects (Freeberg et al., 2012(Freeberg et al., , 2019;;[START_REF] Manser | Vocal complexity in meerkats and other mongoose species[END_REF]Fischer, Wadewitz & Hammerschmidt, 2017b;[START_REF] Rebout | Measuring complexity in organisms and organizations[END_REF].

Several metrics have been used successfully as proxies for social complexity (Dunbar, 1998;[START_REF] Mccomb | Coevolution of vocal communication and sociality in primates[END_REF]. The group's social structure (e.g. size, density and age-sex composition) and social organisation (e.g. number of roles and variation in social bond strength) are common key metrics used to quantify the degree of complexity of a social system [START_REF] Lemasson | What can forest guenons « tell » us about the origin of language? In Primate Communication and Human Language: Vocalisation, gestures, imitation and deixis in humans and non-humans[END_REF]Freeberg et al., 2012;[START_REF] Pollard | Evolving communicative complexity: Insights from rodents and beyond[END_REF]Freeberg & Krams, 2015;Fischer et al., 2017a). The 'social brain hypothesis' proposes that the complex social world of primates is especially cognitively demanding, and that this imposed an intense selection pressure for increasingly large brains (Dunbar, 1998). It thus predicts that a greater number of individuals and thus greater diversity of personalities and relationships requires greater signalling complexity for effective manipulation of the behaviours of others (Freeberg et al., 2019). A key metric of social complexity is the number of differentiated relationships that individuals have [START_REF] Bergman | Measuring social complexity[END_REF]. In baboons, for instance, the diversity of male-female social bonds may be responsible for the emergence of derived call types in the vocal repertoires of some species [START_REF] Gustison | Derived vocalizations of geladas (Theropithecus gelada) and the evolution of vocal complexity in primates[END_REF]). An additional important metric is group stability. Fission-fusion [i.e. group members regularly join (fusion) or separate from (fission) one another] dynamics, for example, may constrain the regulation of social relationships, introducing uncertainty in interactions between group members [START_REF] Ramos-Fernandez | Quantifying uncertanity due to fission-fusion dynamics as a component of social complexity[END_REF].

Communicative complexity within a signalling system is commonly measured as the number of structurally and functionally distinct elements (e.g. [START_REF] Jansen | Segmental concatenation of individual signatures and context cues in banded mongoose (Mungos mungo) close calls[END_REF] and/or the amount of bits of information in a call (Freeberg et al., 2012). Interestingly, the different call types of a given vocal repertoire have different potential for information coding.

For instance, in some species 'social' calls (such as contact calls) are more individually variable than alarm calls (e.g. [START_REF] Lemasson | Acoustic variability and social significance of calls in female Campbell's monkeys (Cercopithecus campbelli campbelli)[END_REF][START_REF] Bouchet | Social complexity parallels vocal HEIDELBERG)[END_REF]. Nonetheless, it is not always easy to quantify communicative complexity using these features.

Very often, signals exist in a graded form, leading to difficulties in interpreting their functions [START_REF] Keenan | Graded or discrete? A quantitative analysis of Campbell's monkey alarm calls[END_REF]Fischer et al., 2017b;[START_REF] Keenan | The reliability of individual vocal signature varies across the bonobo's graded repertoire[END_REF]. On the one hand, the existence of gradation may allow coding of information with greater subtlety, including emotional states, but on the other hand it increases the risk of message confusion due to physical constraints on acoustic structure during sound propagation in forested habitats.

Recently, [START_REF] Rebout | Tolerant and intolerant macaques show different levels of structural complexity in their vocal communication[END_REF] studied two components of vocal complexity: diversity (i.e. number of call categories) and flexibility (i.e. degree of gradation between categories), and compared the acoustic structure of vocal signals in groups of macaques belonging to four species with varying levels of uncertainty in social tolerance (i.e. the higher the degree of tolerance, the higher the degree of uncertainty). Tolerant macaques displayed higher levels of diversity and flexibility than did intolerant macaques in situations with a greater number of options and consequences (i.e. agonistic and affiliative contexts). Beyond the level of the call structure, other relevant measures of communication complexity can be found at the level of the vocal sequence (i.e. more-or less-complex strings of sounds) [START_REF] Kershenbaum | Acoustic sequences in non-human animals: a tutorial review and prospectus[END_REF].

In male gelada baboons Theropithecus gelada, sequence complexity and size function to counteract the challenges of living in a large group (overcoming conspecific noise and crowding) to maintain bonds with females [START_REF] Gustison | The social functions of complex vocal sequences in wild geladas[END_REF].

The limits of these communication measures also come from the fact that they are emitter-centred, while communication is two-sided (Freeberg et al., 2019). First, focusing on what a receiver perceives from the observed acoustic variations of a vocal signal is necessary to assess how much meaningful information is encoded within that signal (Freeberg et al., 2012;[START_REF] Manser | Vocal complexity in meerkats and other mongoose species[END_REF]Levréro et al., 2015;[START_REF] Garcia | Evolution of communication signals and information during species radiation[END_REF]. Second, communicating and socialising are above all acts of interaction. To explore the link between the requirements/characteristics of social life and the expression/structure of vocal interactions is another way to assess the social complexity hypothesis for communication. All the aforementioned studies focused on vocal repertoire sizes and variations in acoustic structure, thus with a focus on individuals as if they were isolated callers. However, the social core of communication may also be found in the patterns and rules of vocal interactions among two or more individuals [START_REF] Lemasson | Socially meaningful vocal plasticity in adult Campbell's monkeys (Cercopithecus campbelli)[END_REF][START_REF] Lemasson | What can forest guenons « tell » us about the origin of language? In Primate Communication and Human Language: Vocalisation, gestures, imitation and deixis in humans and non-humans[END_REF]Fedurek et al., 2013a;[START_REF] Arlet | Grooming-at-a-distance by exchanging calls in non-human primates[END_REF][START_REF] Henry | Social coordination in animal vocal interactions. Is there any evidence of turn-taking? The starling as an animal model[END_REF][START_REF] Pika | Taking turns: bridging the gap between human and animal communication[END_REF]. Focusing on the coevolution of social lifestyle and vocal interaction patterns, as we do here, offers a promising new field of investigation.

(2) Vocal interactions in primates: a remarkable diversity of temporal calling patterns A vocal interaction is primarily defined by its temporal pattern, with comparisons among species attracting interest due to the extensive diversity of existing temporal patterns.

When focusing on the temporal rules guiding call production in primates, three main types of temporal calling patterns are typically found (Pougnault, Levréro & Lemasson, 2020a): (i) isolated calls (when only one individual produces a single call); (ii) a series of consecutive calls (where a given individual repeats a call several times in a row); and (iii) vocal interactions (when at least two individuals temporally coordinate their vocal production). The frequency of use of these different calling patterns may depend on the context and call type, but also on the identity (i.e. age, sex) and social characteristics of the caller (Pougnault et al., 2020a). But what motivates and structures the organisation of vocal interactions still remains an open question.

Researchers distinguish several types of vocal interactions with more or less predictable temporal patterns: (1) disorganised phonoresponses, occurring when one individual produces a call, typically an alarm call that triggers calls in an apparently chaotic way from other group members (e.g. blue monkey Cercopithecus mitis stuhlmanni ;[START_REF] Papworth | Male blue monkeys alarm call in response to danger experienced by others[END_REF]; (2) choruses, which result from simultaneous production of the same call type by two or more individuals, implying causal interactions between individuals joining the chorus in a synchronised way [START_REF] Yoshida | Evolution of turntaking: a bio-cognitive perspective[END_REF][START_REF] Ravignani | Chorusing, synchrony and the evolutionary functions of rhythm[END_REF]; (3) duets, when two individuals synchronise or overlap long series of calls or songs with a stereotyped temporal association (e.g. gibbon Hylobates syndactylus ;[START_REF] Geissmann | The relationship between duet songs and pair bonds in siamangs, Hylobates syndactylus[END_REF], and (4) vocal exchanges involving individuals who alternate their calling and vocally 'respond' to one another.

Among the different patterns of vocal interactions, vocal exchanges have recently attracted a great deal of attention because of their complex organisation, possible prosocial function, and implications for our understanding of language evolution (Chow, Mitchell & Miller, 2015). Vocal exchanges are based on a turn-taking principle, and have been documented in all main primate branches [e.g. Catarrhini: Campbell's monkey Cercopithecus campbelli (Lemasson et al., 2011a), Japanese macaque Macaca fuscata (Sugiura, 1993); Platyrrhini: squirrel monkey Saïmiri sciureus [START_REF] Masataka | Temporal Rules Regulat ning Affiliative Vocal Exchanges of Squirrel Monkeys[END_REF], pygmy marmoset Cebuella pygmaea [START_REF] Snowdon | Pygmy marmosets modify call structure when paired[END_REF], spider monkey Ateles geoffroyi (Briseño-Jaramillo et al., 2018); for a review, see Pougnault et al. (2020a)]. Turn-taking is defined as the orderly exchange of communicative signals between individuals adjusting the timing of their vocal responses by respecting a minimum silence gap (preventing call overlap), and a maximum silence gap (allowing call coordination) (for a review see [START_REF] Pika | Taking turns: bridging the gap between human and animal communication[END_REF]. They are typically composed of one to three turns with individuals alternating their calls (e.g. Lemasson, Gandon & Hausberger, 2010a;[START_REF] Levréro | Social bonding drives vocal exchanges in Bonobos[END_REF], and thus are characterised by short vocal sequences. The common response delay is rarely shorter than the average call duration [e.g. Diana's monkey Cercopithecus diana (Candiotti, Zuberbuhler & Lemasson, 2012); squirrel monkey [START_REF] Masataka | Temporal Rules Regulat ning Affiliative Vocal Exchanges of Squirrel Monkeys[END_REF]; Japanese macaque (Sugiura, 1993); spider monkey (Briseño-Jaramillo et al., 2018); western lowland gorilla Gorilla gorilla gorilla [START_REF] Lemasson | Social basis of vocal interactions in western lowland gorillas (Gorilla g. gorilla)[END_REF]; bonobo Pan paniscus [START_REF] Levréro | Social bonding drives vocal exchanges in Bonobos[END_REF].

Interestingly, speech overlap avoidance and turn-taking also represent the social core of human vocal interactions and are found in all human cultures [START_REF] Stivers | Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation[END_REF][START_REF] Levinson | Turn-taking in Human Communication -Origins and Implications for Language Processing[END_REF].

In sum, turn-taking-based vocal exchanges represent one of a variety of methods that allow group members to interact with one another. Vocal exchanges in some non-human primate species appear highly similar to temporally and socially organised vocal interactions in humans. How social structure influences the evolution of vocal turn-taking across the primate phylogeny remains a focus of research interest.

(3) Turn-taking-based vocal exchange in primates: a social function Beyond temporal aspects, turn-taking-based vocal exchanges in non-human primates differ from other types of vocal interactions in several ways. First, they involve a diversity of recurrent vocal partners [differing from duets [START_REF] Mackinnon | The behavior of wild spectral tarsiers[END_REF][START_REF] Geissmann | The relationship between duet songs and pair bonds in siamangs, Hylobates syndactylus[END_REF]]. Second, vocal exchange interlocutors can be of any age or either sex (differing from choruses; [START_REF] Haimoff | Convergence in the duetting of monogamous Old World primates[END_REF]. Third, vocal exchanges are not restricted to a specific context, time of day, or season, which differs from long-distance collective communication associated with mate attraction, territory protection and environmental disturbance (Gros-Louis & [START_REF] Gros-Louis | Chorusing and call convergence in chimpanzees: Tests of three hypotheses[END_REF][START_REF] Schel | Dawn chorusing in guereza colobus monkeys[END_REF]Briseño-Jaramillo et al., 2017). Most commonly, vocal exchanges are observed daily in short-distance communication, occur throughout the day, and consist mostly of soft calls exchanged in peaceful and relaxed contexts (Pougnault et al., 2020a).

Due to their role in within-group socio-spatial coordination (Pougnault et al., 2020a), turn-taking-based vocal exchanges represent an interesting vocal interaction metric to test the proposed links between sociality and vocal complexity. The 'grooming-at-distance' hypothesis proposed by Dunbar (1996) highlighted the possible role played by vocal exchanges during the evolution of primates. This hypothesis suggested that, in large social groups and in groups with long-lasting bonds, the function of gestural grooming was partially transferred to vocal exchanges. Gestural grooming plays an important role in forming new bonds and strengthening existing ones in all primate species (Dunbar, 1991). As group size and social network increased during our ancestral history, vocal exchanges may have developed, as it ultimately would have become impossible to allocate sufficient time to grooming all partners physically.

Vocal exchanges do indeed occur mostly between interlocutors that are non-randomly chosen, depending on their affiliative bonds [e.g. bonobo [START_REF] Levréro | Social bonding drives vocal exchanges in Bonobos[END_REF], spider monkey Ateles geoffroyi (Briseño-Jaramillo et al., 2018), Japanese macaque [START_REF] Arlet | Grooming-at-a-distance by exchanging calls in non-human primates[END_REF], pygmy marmoset [START_REF] Snowdon | Conversations" among pygmy marmosets[END_REF]] or ages [common marmoset Callithrix jacchus (Chen, Kaplan & Rogers, 2009), Campbell's monkey (Lemasson et al., 2010a); for a review see Pougnault et al. (2020a)]. The temporal [squirrel monkey [START_REF] Biben | Temporal and Structural Analysis of Affiliative Vocal Exchanges in Squirrel Monkeys (Saimiri sciureus)[END_REF][START_REF] Masataka | Temporal Rules Regulat ning Affiliative Vocal Exchanges of Squirrel Monkeys[END_REF]] and acoustic [Japanese macaque (Sugiura & Masataka, 1995); Diana monkey (Candiotti et al., 2012); spider monkey (Briseño-Jaramillo et al., 2018)] organisation of vocal exchanges in monkeys is socially guided. These vocal exchanges represent a significant cognitive demand, implying cooperation (Takahashi, Narayanan & Ghazanfar 2013; [START_REF] Levinson | The origin of human multi-modal communication[END_REF][START_REF] Levinson | Turn-taking in Human Communication -Origins and Implications for Language Processing[END_REF].

However, despite growing interest, many knowledge gaps remain to be filled to comprehend fully the social relevance of vocal exchanges in non-human primates and the evolutionary trajectory of this type of vocal interaction. One of the major missing elements is comparative studies that are necessary to characterise the vocal exchange phenotype of a wide diversity of primate species [START_REF] Pika | Taking turns: bridging the gap between human and animal communication[END_REF].

(4) Selection for vocal turn-taking systems from monkeys to humans: an open question

The growing interest in turn-taking vocal exchanges in animals is partly due to their many parallels with human conversations [START_REF] Snowdon | Conversations" among pygmy marmosets[END_REF]Sugiura & Masataka, 1995;Lemasson et al., 2011b;[START_REF] Henry | Social coordination in animal vocal interactions. Is there any evidence of turn-taking? The starling as an animal model[END_REF]. Overlap avoidance, turn-taking, social selection of interlocutors, varied contexts beyond a strict message-transmission function, and acoustic convergence are all found in both non-human primate vocal exchanges and human conversations. Researchers have thus questioned the role played by social factors during both the development and evolution of vocal exchanges.

During development or ontogeny, the extent to which the abilities to converse in primates are socially acquired or learned remains debated. The few available studies on monkeys show that juveniles break conversational rules more often than do adults [howler monkey Alouatta pigra (Briseño Jaramillo et al., 2017); Campbell's monkey (Lemasson et al., 2011a)]. A recent study conducted on common marmosets (Callithrix jacchus) suggested that, rather than parental influence (Chow et al., 2015), the ability to participate in vocal turntaking might be due to self-monitoring that increases with age (Takahashi, Fenley & Ghazanfar, 2016). Despite the lack of empirical evidence to answer this question, results from playback experiments using a violation-of-expectation paradigm showed that adult monkeys clearly discriminated between conversationally appropriate and inappropriate vocal-exchange patterns, whereas socially inexperienced juveniles did not [Japanese macaque (Bouchet, Koda, & Lemasson, 2017); Campbell's monkey (Lemasson et al., 2011a)]. These findings show that the social rules in vocal interactions are not a simple neurobiologically determined behaviour, but rather, may represent an awareness of social rules.

Concerning their evolution, the debate likewise remains open, mainly because most published research on vocal interactions has focused on Platyrrhini and Catarrhini monkeys or lesser apes [e.g. agile gibbon Hylobates agilis [START_REF] Koda | Possible Role of Mother-Daughter Vocal Interactions on the Development of Species-Specific Song in Gibbons[END_REF]; siamang Hylobates syndactylus [START_REF] Geissmann | The relationship between duet songs and pair bonds in siamangs, Hylobates syndactylus[END_REF]]. Remarkably, researchers have largely neglected vocal interactions among great apes for decades. However, they are also excellent candidates for such research given their well-described complex social structures with longterm bonds and intricate social interactions [e.g. orang-utan Pongo pygmaeus (Delgado & van Schaik, 2000); gorilla [START_REF] Robbins | Social structure and life-history patterns in western gorillas (Gorilla gorilla gorilla)[END_REF]; chimpanzee Pan troglodytes (van Lawick [START_REF] Van Lawick-Goodall | The Behaviour of Free-living Chimpanzees in the Gombe Stream Reserve[END_REF][START_REF] Goodall | The Chimpanzees of Gombe: Patterns of Behvior[END_REF]; bonobo [START_REF] Kano | The Social Group of Pygmy Chimpanzees (Pan paniscus) of Wamba[END_REF][START_REF] Kano | The last ape: Pygmy chimpanzee behavior and ecology[END_REF]]. Moreover, vocal communication plays a central role in their daily lives [e.g. orang-utan [START_REF] Hardus | A description of the orangutan's vocal and sound repertoire, with a focus on geographic variation[END_REF]; gorilla [START_REF] Salmi | Western gorilla vocal repertoire and contextual use of vocalizations[END_REF]; chimpanzee [START_REF] Slocombe | Vocal Communication in Chimpanzees[END_REF]; bonobo (De Waal, 1988[START_REF] Bermejo | Preliminary Vocal Repertoire and Vocal Communication of Wild Bonobos (Pan paniscus) at Lilungu (Democratic Republic of COngo)[END_REF]]. Great apes are also of interest because of their close phylogenetic proximity to humans. As a result, the selective pressures that maintain such vocal behaviours in non-human primates remain unclear, notably because the current state of knowledge still prevents us from concluding in favour of a convergentevolution scenario (turn-taking has been favoured in some species more than others by the requirements of social life) or a shared-inheritance scenario (turn-taking is universal suggesting an ancient mechanism present deep within the primate lineage).

In the following section, we explore further explore how the requirements of a social life may match the use of turn-taking, as well as other types of vocal interaction patterns, in great apes. While comparisons between a limited number of species should be interpreted cautiously, similar studies at the genus level have enabled a better understanding of some aspects of social-vocal coevolution (see the studies cited above on macaques and baboons, for example). The four species of great apes described herein are valuable candidates for a comparative analysis of social and vocal features because they exhibit four very different and well-described social systems.

II. THE SOCIO ECOLOGY OF GREAT APES: SIMILARITIES AND

DIFFERENCES

Current knowledge on the socio-ecology of great ape species, particularly in terms of vocal communication, varies among species. We selected four species/sub-species for which sufficient literature is available to perform a comparative analysis: (i) bonobo, (ii) chimpanzee, (iii) western lowland gorilla, and (iv) Bornean orang-utan. All key references for the phylogenetic and socio-ecological information discussed below are provided in Table 1.

(

1) Origins and habitat

The evolution of vocal communication in great apes is likely to have been influenced by phylogenetics, habitat use and social life-related factors. Genetic evidence indicates that the last common ancestor to Ponginae and Homininae lived about 9-18 million years ago (Mya) (Castellano & Munch, 2020). A second speciation, between ~8.6 and 11.5 Mya, led to the separation of Gorillini and Hominini, and a final differentiation, between ~4.2 and 9 Mya, gave rise to the Panina and Hominina clades. Given their phylogenetic proximity, the Pan genus thus represent our closest living relatives. An evolutionary hypothesis based on phylogeny would thus predict more vocal similarities between chimpanzees and bonobos than between other pairs of these species, as well as more similarities between Pan spp. and gorillas than with orang-utans.

Despite a growing body of evidence for an European origin of great apes (e.g. [START_REF] Begun | Miocene Hominids and the Origins of the African Apes and Humans[END_REF][START_REF] Böhme | A new Miocene ape and locomotion in the ancestor of great apes and humans[END_REF], all currently living non-human great apes are located in tropical and equatorial rain forests of Asia and Africa. The four species considered herein usually occupy habitats with reduced visibility due to the density of the surrounding vegetation, and travel, to some extent at least, in a three-dimensional space. These characteristics are known to be suitable for vocal communication (Bradbury & Vehrencamp, 2011). However, these four species do show differences in habitat preferences and modes of locomotion, which could also influence vocal communication [START_REF] Marler | Primate behavior: Field studies of monkeys and apes[END_REF][START_REF] Lemasson | What can forest guenons « tell » us about the origin of language? In Primate Communication and Human Language: Vocalisation, gestures, imitation and deixis in humans and non-humans[END_REF]. Bornean orangutans live almost exclusively in the canopy of humid forests, while western lowland gorillas are mainly terrestrial and inhabit both humid and terra firma lowland forests (Table 1).

Chimpanzees and bonobos mainly occupy primary and secondary humid forests but can sometimes be found on savannah woodland mosaics. They travel in the trees as well as on the ground according to their age. Therefore, a habitat-based evolutionary hypothesis would again predict more similarities between chimpanzees and bonobos than between any other pair of species.

(2) Social systems

The four great apes considered herein live in four different social systems that differ in group size, group structure, and social organisation (see Table 1 for details and references).

Bornean orang-utans are known as the most solitary great ape species. Males (particularly flanged males) generally live alone whereas females are typically found with their infants. Orang-utans can be described as an individual-based fission-fusion species with a mean party size of less than two independent individuals (usually related females or unflanged males). Temporary groupings are peaceful, typically observed near food sources [START_REF] Van Schaik | Geographic variation in orangutan behavior and biology. Its functional interpretation and its mechanistic basis[END_REF]. Physical interactions are rare and a high level of intolerance is generally reported between adult males for access to females [START_REF] Galdikas | Adult male sociality and reproductive tactics among orangutans at Tanjung Puting[END_REF].

In contrast to orang-utans, which are female-philopatric, chimpanzees and bonobos are male-philopatric species that live in large multi-male multi-female and highly flexible fissionfusion societies (i.e. with subgroups of variable size and composition; [START_REF] Kummer | Primate Societies: Group Techniques of Ecological Adaptation[END_REF]. Subgroups are typically composed of 20-100 individuals. Both species have a high rate of physical interactions, notably very high grooming rates compared to the two other great apes considered herein. Male chimpanzees are highly territorial. Related males form strong social bonds on which they rely to manage inter-and intra-community competition, while females haver weaker social bonds with each other. This high level of competition leads to more aggressive conflicts in chimpanzees and to the highest recorded death rate due to intraspecific conflict among great apes (Table 1). Chimpanzees appear to perform fewer cooperative behaviours than their bonobo relatives [START_REF] Hare | Chimpanzees are more skilful in competitive than in cooperative cognitive tasks[END_REF][START_REF] Jensen | What's in it for me? Self-regard precludes altruism and spite in chimpanzees[END_REF][START_REF] Melis | Engineering cooperation in chimpanzees: tolerance constraints on cooperation[END_REF][START_REF] Hare | Tolerance Allows Bonobos to Outperform Chimpanzees on a Cooperative Task[END_REF][START_REF] Hirata | Chimpanzees (Pan troglodytes) learn to act with other individuals in a cooperative task[END_REF][START_REF] Pelé | Token transfers among great apes (Gorilla gorilla, Pongo pygmaeus, Pan paniscus, and Pan troglodytes): Species Differences, Gestural Requests, and Reciprocal Exchange[END_REF][START_REF] Gruber | A Comparison Between Bonobos and Chimpanzees: A Review and Update[END_REF]Suchak et al., 2016), for whom inter-and intragroup interactions are mostly tolerant. In bonobos, members from different communities regularly mix peacefully to feed, and females have a central role in the community. They establish strong affiliative bonds with other females independently of their kinship and also form bonds with unrelated males.

Western lowland gorillas live in single-male/multi-female medium-sized groups of 10-20 individuals, and both sexes disperse. The sole adult male in a group develops strong social bonds with adult females. He is the leader of the group, coordinating activities, protecting the group and managing social interactions between females [START_REF] Parnell | Group size and structure in western lowland gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Republic of Congo[END_REF][START_REF] Harcourt | Gorilla society: conflict, compromise, and cooperation between the sexes[END_REF]. Despite the strong social bonds between the male and females, individual gorillas appear to cooperate only rarely [START_REF] Harcourt | Gorilla society: conflict, compromise, and cooperation between the sexes[END_REF][START_REF] Pelé | Token transfers among great apes (Gorilla gorilla, Pongo pygmaeus, Pan paniscus, and Pan troglodytes): Species Differences, Gestural Requests, and Reciprocal Exchange[END_REF].

Living in different society types will obviously require different daily social interactions and it is important to consider how this may influence vocal interaction patterns. Interestingly, all great ape species are known to have comparably high cognitive and social skills (Chalmeau et al., 1997;[START_REF] Pelé | Token transfers among great apes (Gorilla gorilla, Pongo pygmaeus, Pan paniscus, and Pan troglodytes): Species Differences, Gestural Requests, and Reciprocal Exchange[END_REF][START_REF] Pelé | Token transfers among great apes (Gorilla gorilla, Pongo pygmaeus, Pan paniscus, and Pan troglodytes): Species Differences, Gestural Requests, and Reciprocal Exchange[END_REF], suggesting that any differences in vocal communication among these species may be related more to social differences than to habitat or cognitive differences.

III. TESTING THE SOCIAL COMPLEXITY HYPOTHESIS FOR COMMUNICATION IN GREAT APES

(1) Traditional vocal metrics (repertoire size, call rate)

The social complexity hypothesis for communication predicts a higher number of distinct call types in the vocal repertoires of more social species, hence increasing from orangutans to gorillas and then to bonobos and chimpanzees if group sizes and time spent grooming are representative indices of social complexity (e.g. [START_REF] Mccomb | Coevolution of vocal communication and sociality in primates[END_REF][START_REF] Gustison | Derived vocalizations of geladas (Theropithecus gelada) and the evolution of vocal complexity in primates[END_REF]. Table 2 summarises findings from the literature on the vocal behaviour of the four great ape species considered herein, supplemented where necessary with new data (see online Supporting Information, Appendix S1-S3). Previous studies on wild free-ranging animals show that the vocal repertoire size of Bornean orang-utans and chimpanzees is very similar (25 for orang-utans and 13-29 for chimpanzees; Table2). A comparable vocal repertoire size is reported for wild bonobos (15 call types) and gorillas (16 call types). In captivity, the repertoire size of these great ape species contains fewer distinct call types than in wild populations, with all species using only 8-12 distinct call types.

The hourly rate of vocal production is high for all great ape species. Even if the two species forming multi-male multi-female groups appear to have higher call rate than the two others, at least compared to gorillas in the wild, there are strong variations among study populations and environmental contexts (Table 2). The relative richness of the environments encountered by wild animals compared with those in captivity typically result in more call types and higher call rates in free-ranging populations [START_REF] Lameira | Understanding language evolution: beyond pan-centrism[END_REF]. As for vocal repertoire size, any link between hourly vocalisation rate and social complexity appears very weak (Table 2). This is particularly striking when considering the very large vocal repertoire size of orang-utans, which are the most solitary species [START_REF] Hardus | A description of the orangutan's vocal and sound repertoire, with a focus on geographic variation[END_REF]. This questions the use of group size and grooming rate as measures of social complexity in the debated hypotheses [START_REF] Mccomb | Coevolution of vocal communication and sociality in primates[END_REF]Freeberg et al., 2012;[START_REF] Kershenbaum | Estimating vocal repertoire size is like collecting coupons: A theoretical framework with heterogeneity in signal abundance[END_REF]. Some methodological issues involved in the classification of call types, also need to be addressed. Many reports of communicative repertoires reveal intermediate forms between specific signals as well as variations within signal types. These so-called 'graded repertoires' are characteristic of great apes, particularly chimpanzees and bonobos, leading to difficulties in identifying a specific number of stereotypic signals in a species' repertoire [START_REF] Keenan | The reliability of individual vocal signature varies across the bonobo's graded repertoire[END_REF]. Experimental approaches using playback and systematic contextual studies of call emission are still needed to test whether overly simplistic acoustic analyses may lead to overestimation or underestimation of repertoire size, and thus bias interspecific comparisons.

Interestingly, a comparative study on macaques found more gradation of vocal communication in tolerant than in despotic species [START_REF] Rebout | Tolerant and intolerant macaques show different levels of structural complexity in their vocal communication[END_REF]. Thus whether vocal interaction pattern measures are suitable candidates to assess the social complexity hypothesis for communication remains unclear at this stage, and is considered in more detail in Section III.2.

(2) Vocal interaction patterns

Vocal interaction patterns are not randomly distributed across the four great apes studied herein (Table 2). Vocal interactions are most frequent in bonobos (53% of their calls in wild animals; 74% in captivity), followed by gorillas (38% in captive animals; unstudied in the wild) and then chimpanzees (19% in wild animals; 16% in captivity), and they seem to be extremely rare in orang-utans. Vocal interactions, regardless of the species' average group size, typically concern only two or three coordinating partners. This parallels results for monkeys' conversation-like vocal exchanges (Pougnault et al., 2020a). The typical pattern of vocal production in the orang-utan is isolated calling. Chimpanzees also often emit isolated calls, but most of their vocal production consists of series of repeated calls. However, all our data on vocal interaction in orang-utans come from two captive groups, and this has never been investigated in the wild. To draw firm conclusions about this species, future investigations should focus on their call patterns in the wild during fusion periods.

Differences do exist among the three other great ape species. Considering the vocal interactions (namely excluding isolated and repeated calls), most of chimpanzees' vocalizations overlapped (58%) while it occurred in 27.5% in gorillas and 19% in bonobos (Table 2 and Fig. 1c). Arcadi (2000) also identified an intriguing rarity of turn-taking in freeranging chimpanzees. Chimpanzees are known for their structured choruses, mainly based on a superimposed pant-hoots (Gros-Louis & [START_REF] Gros-Louis | Chorusing and call convergence in chimpanzees: Tests of three hypotheses[END_REF]Fedurek, Schel & Slocombe, 2013b;[START_REF] Ravignani | Chorusing, synchrony and the evolutionary functions of rhythm[END_REF], which seem to play a role in territory defence and in regulating group dynamics facilitating fusion and changes in sub-group composition (Fedurek, Donnellan & Slocombe, 2014). A pant-hoot vocalisation is composed of four distinct phases (Gros-Louis & [START_REF] Gros-Louis | Chorusing and call convergence in chimpanzees: Tests of three hypotheses[END_REF][START_REF] Slocombe | Vocal Communication in Chimpanzees[END_REF]. During pant-hoot chorusing, interlocutors adjust the duration of constituent phases, promoting choruses by mirroring the partner's call, and revealing that the temporal structure of this rather stereotyped vocalisation is sensitive to fine details of the vocal behaviour of the audience (Fedurek et al., 2013b).

Conversely, bonobos, and to lesser extent gorillas, tend to avoid overlap when they interact vocally (about 67% and up to 94% of the vocal interactions without overlap in bonobos in captivity and in the wild, respectively (81% on average for both populations), and 72% in gorillas; Table 2 and Fig. 1c). As found in several monkey species and in humans, gorillas and bonobos engage frequently in vocal exchanges that involve a turn-taking rule.

Their vocal exchanges are call-type dependent. For both species, soft contact calls, known to be characteristic of peaceful contexts (grunts/double grunt in gorillas; peeps/peep-yelps in bonobos), are predominantly used. The inter-call duration between two consecutive interlocutors is also similar: a maximum of 3 s in gorillas (0.5 s median; [START_REF] Lemasson | Social basis of vocal interactions in western lowland gorillas (Gorilla g. gorilla)[END_REF] and a maximum between 2.1 and 2.5 s in bonobos [wild bonobos 0.7 s median (Cornec et al., in press); captive bonobos 0.4 s median [START_REF] Levréro | Social bonding drives vocal exchanges in Bonobos[END_REF]. This inter-call duration is consistent with those found in monkeys, i.e. it is rarely shorter than the mean duration of a call, although most published maximum response delays appear slightly shorter in monkeys (around 1 or 1.5 s). Recent playback experiments using violation-of-expectations on gorillas and bonobos revealed that individuals could perceive whether a short silent gap between vocalisations produced by different interlocutors is present (Levréro et al., 2017;[START_REF] Pougnault | Breaking conversational rules matters to captive gorillas: A playback experiment[END_REF]. These results suggest that the presence of a response delay matters to the audience (Levréro et al., 2017;[START_REF] Pougnault | Breaking conversational rules matters to captive gorillas: A playback experiment[END_REF].

Our analysis does not support a strict correlation between group size, grooming rate and vocal exchange rate, as would be expected from Dunbar's grooming-at-distance hypothesis (Dunbar, 1996). However, a subtler link between social structure and type of vocal interaction emerges from our comparative data. Call overlap and turn-taking exchanges may represent two possible vocal alliance strategies to advertise or strengthen social bonds as detailed in Section III.3.

(3) Social structure as a defining factor of vocal interaction patterns

In chimpanzees, pant-hoot choruses are mainly produced by high-ranking males [START_REF] Mitani | Contexts and social correlates of long-distance calling by male chimpanzees[END_REF]Clark & Wrangham, 1994), with the aim of recruiting and maintaining the company of close associates [START_REF] Mitani | Contexts and social correlates of long-distance calling by male chimpanzees[END_REF]. Researchers highlight that choruses occur between allies without consideration of kinship or age [START_REF] Mitani | Contexts and social correlates of long-distance calling by male chimpanzees[END_REF]. Indeed, this vocal behaviour appears to be a strong indicator of affiliation between males (Fedurek et al., 2013a), and during a chorus, allies tend to match vocally the acoustic features of a partner's calls [START_REF] Mitani | Social Factors Influence the Acoustic Variabilty in the Long Distance Calls of Male Chimpanzees[END_REF][START_REF] Gros-Louis | Chorusing and call convergence in chimpanzees: Tests of three hypotheses[END_REF]. Pant-hoot choruses thus might represent a reliable social bonding signal in highly fluid fission-fusion societies (Fedurek et al., 2013a). The prevalence of pant-hoot chorusing in chimpanzee vocal interactions could be explained by the fact that, in a context of highly risky intra-and intercommunity competition, opponents seem able to perceive the number of participants of a pant-hoot chorus and to adapt their behaviour accordingly [START_REF] Wilson | Does participation in intergroup conflict depend on numerical assessment, range location, or rank for wild chimpanzees?[END_REF]. By attesting to social affinities, such loud calls produced in a chorus might deter potential rivals and avoid a costly fight [START_REF] Wilson | Does participation in intergroup conflict depend on numerical assessment, range location, or rank for wild chimpanzees?[END_REF].

In western lowland gorillas, the adult male responds vocally more to adult females with whom he establishes durable social bonds, and less to individuals distant in age such as sub-adults or juveniles [START_REF] Lemasson | Social basis of vocal interactions in western lowland gorillas (Gorilla g. gorilla)[END_REF]. The same is true for bonobos for which vocal turn-taking occurs mainly between interlocutors with strong social affinities, and is not influenced by the age, sex, hierarchical rank or kinship of interlocutors [START_REF] Levréro | Social bonding drives vocal exchanges in Bonobos[END_REF].

Experimental approaches using violation-of-expectation paradigms have been used to test the relevance of social rules previously described in bonobos and western lowland gorillas. These studies compared the attentional response of the audience towards vocal exchanges of individuals with or without strong social affinities in bonobos (Levréro et al., 2017) and close or distant in age in gorillas (Pougnault et al., 2020a). It seems clear that individuals have some expectations about which individuals will interact vocally with them (Pougnault et al., 2020a). Additional playback experiments are now needed to explore the social function of vocal interaction patterns in more detail.

Our focused phylogenetic comparison provides evidence both in support of and against the social complexity hypothesis for communication [START_REF] Blumstein | Does sociality drive the evolution of communicative complexity? A comparative test with ground-dwelling sciurid alarm calls[END_REF]Freeberg et al., 2012;[START_REF] Pollard | Evolving communicative complexity: Insights from rodents and beyond[END_REF][START_REF] Manser | Vocal complexity in meerkats and other mongoose species[END_REF]. If we focus our attention only on the number of different call types composing the vocal repertoire of each great ape species and the nature of social interactions or the number of group members, the results are not convincing. Such a comparison can be criticised for omitting potentially important variables, such as relationship types between group members, or a subjective classification of calls from a graded repertoire (Fischer et al., 2017a,b;[START_REF] Hammerschmidt | Baboon vocal repertoires and the evolution of primate vocal diversity[END_REF]. However, by considering a more complex set with more subtle features, it seems possible to structure a gradation in social-vocal complexity using the three socialvocal patterns (see Fig. 1). Bornean orang-utans form dispersed single/multi-male groups with extended solitary periods and favour the production of isolated calls. In chimpanzee societies, which are mainly despotic with a strong male influence, consecutive calls from single callers [i.e. characteristic of emotional discharge [START_REF] Lemasson | Speed of call delivery is related to context and caller identity in Campbell's monkey male[END_REF][START_REF] Owren | An Affect-Conditionning Model of Non-human Primate Vocal Signaling[END_REF][START_REF] Scherer | Vocal communication of emotion: A review of research paradigms[END_REF]) are particularly common, and vocal interactions occur frequently in the form of choruses of pant-hoots. On the contrary, gorillas and bonobos, who are more social than orang-utans and have more peaceful intra-group interactions than do chimpanzees, show a majority of vocal interactions characterised by overlap avoidance and turn-taking. Interestingly, chimpanzees spend more time in social grooming than the other three species (see Fig. 1). They appear to use dyadic mutual grooming (Fedurek & Dunbar, 2009) in a fluid way to access a large number of grooming partners, changing partners more often than do bonobos [START_REF] Girard-Buttoz | Variable use of polyadic grooming and its effect on access to social partners in wild chimpanzees and bonobos[END_REF]. Moreover, a large part of their time spent in social grooming is allocated to polyadic grooming [16% of total grooming time in chimpanzees [START_REF] Nakamura | Gathering' of social grooming among wild chimpanzees: Implications for evolution of sociality[END_REF] against only 3% in bonobos [START_REF] Sakamaki | Social grooming among wild bonobos (Pan paniscus) at Wamba in the Luo Scientific Reserve, DR Congo, with special reference to the formation of grooming gatherings[END_REF]]. Polyadic grooming also involves a larger number of partners in chimpanzees than in bonobos [START_REF] Girard-Buttoz | Variable use of polyadic grooming and its effect on access to social partners in wild chimpanzees and bonobos[END_REF]. In this way, chimpanzees can reach the same efficiency as human conversations in terms of number of partners accessed simultaneously in polyadic grooming clusters (a mean of 2.72 partners in humans versus 2.18 partners in chimpanzees; [START_REF] Nakamura | Is human conversation more efficient than chimpanzee grooming ? comparison of clique sizes[END_REF]. These clusters occur particularly between males of similar hierarchical rank in chimpanzees, while bonobos engage more often with conspecifics distant in dominance rank [START_REF] Girard-Buttoz | Variable use of polyadic grooming and its effect on access to social partners in wild chimpanzees and bonobos[END_REF]. Thus, at a first glance, the low rate of vocal exchanges in chimpanzees appears counter-intuitive given their high rate of social grooming (see Fig. 1). However, it is possible that the apparent efficiency of polyadic social grooming, its focus on a limited number of close social partners, and high levels of intra-group competition could explain why this species does not need to 'groom-at-distance' using vocal exchanges to maintain their relationships (Dunbar, 1996). By contrast, in species displaying more tolerance among group members, as bonobos and gorillas, vocal exchanges that follow temporal and social rules may be favoured during vocal interactions. This interpretation must be considered preliminary, given the representation of each social system by a single species here, but it does highlight the potential for integrative approaches to investigate diverse complementary social parameters.

IV. CONCLUSIONS

(1) Our goal herein was to highlight the necessity to include metrics at the vocal interaction level when investigating the coevolution between social and vocal complexity in primates or other animals. The limited number of species included here, and the limited number of sampled populations per species, make our description of the potential relationships between social bonds and vocal interaction patterns essentially qualitative. Although it does not allow us to provide any evolutionary scenario across primate genera, we believe that this analysis has several major implications for the field and opens interesting perspectives for future studies.

(2) Vocal interaction patterns broadly map onto the quality of social bonds in apes, in agreement with the predictions of a socially driven evolution of communication. A welltargeted comparative approach could help to identity the multiple and probably subtle social factors underlying social and vocal complexity, beyond a consideration of only social structure or organisation. While focusing on four great apes provides only one template for each different social system, we believe that focusing on our nearest living relatives is a necessary step to a better understand of the social factors influencing different vocal interaction patterns.

(3) It will be a difficult task to derive a scenario of the evolution of vocal interaction skills, from the Strepsirhini to the genus Homo, because the emergence of social and vocal complexity is found throughout the primate lineage and probably results from multiple evolutionary pressures. However, some great apes (mainly bonobos and gorillas) and monkeys share some temporal and social rules regulating their vocal exchanges that may indicate similar constraints between primate 'conversational' vocal exchanges and human conversations. It is possible that chimpanzees and orang-utans have been subject to different selection pressures, due respectively to a strong male-despotic hierarchy and an extended solitary life, leading to the appearance of derived traits.

(4) It is somewhat reductive to discuss the evolution of social life and vocal communication without considering other modalities of communication (e.g. see the social complexity hypothesis proposed for visual and chemical signals; Freeberg et al., 2012). Human language is likely to have multimodal origins [START_REF] Prieur | The origins of gestures and language: history, current advances and proposed theories[END_REF]. This is particularly true for some great apes who are known to communicate intensively with gestures, with reported cases of gestural turn-taking (Fröhlich et al., 2016). Further research may extend this comparison to visual and tactile communication and thereby highlight alternative options to vocal interactions for short-range communication, adding even more complexity [START_REF] Smith | A new heuristic for capturing the complexity of multimodal signals[END_REF][START_REF] Peckre | Clarifying and expanding the social complexity hypothesis for communicative complexity[END_REF]. 1 and2 for data and references).

VI. ACKNOWLEDGMENTS

Values are means + SD. ~4.2 to ~9 Mya [START_REF] Patterson | Genetic evidence for complex speciation of humans and chimpanzees[END_REF][START_REF] Hobolth | Incomplete lineage sorting patterns among human, chimpanzee, and orangutan suggest recent orangutan speciation and widespread selection[END_REF][START_REF] Prüfer | The bonobo genome compared with the chimpanzee and human genomes[END_REF][START_REF] Rogers | Comparative primate genomics: Emerging patterns of genome content and dynamics[END_REF]Upham et al., 2019) ~4.2 to ~9 Mya [START_REF] Patterson | Genetic evidence for complex speciation of humans and chimpanzees[END_REF][START_REF] Hobolth | Incomplete lineage sorting patterns among human, chimpanzee, and orangutan suggest recent orangutan speciation and widespread selection[END_REF][START_REF] Prüfer | The bonobo genome compared with the chimpanzee and human genomes[END_REF][START_REF] Rogers | Comparative primate genomics: Emerging patterns of genome content and dynamics[END_REF]Upham et al., 2019)

Geographical distribution

South-east equatorial Asia (Ancrenaz et al., 2018) Western equatorial Africa [START_REF] Maisels | Gorilla gorilla ssp. gorilla (amended version of 2016 assessment)[END_REF] Western, central and eastern Africa [START_REF] Oates | Pan troglodytes: IUCN Red List of Threatened Species[END_REF] Central Africa [START_REF] Kano | The last ape: Pygmy chimpanzee behavior and ecology[END_REF][START_REF] Idani | Changes in the status of bonobos, their habitat, and the situation of humans at Wamba in the Luo Scientific Reserve, Democratic Republic of Congo[END_REF]Fruth et al., 2016)

Main habitat type

Humid forest (Delgado & van Schaik, 2000;Ancrenaz et al., 2018) Humid and terra firma lowland forests [START_REF] Gatti | Population and group structure of western Lowland Gorillas (Gorilla gorilla gorilla) at Lokoué, Republic of Congo[END_REF]Levréro et al., 2006;[START_REF] Maisels | Gorilla gorilla ssp. gorilla (amended version of 2016 assessment)[END_REF][START_REF] Forcina | [END_REF] Humid forest (sometimes savannah woodland) [START_REF] Nishida | The social group of wild chimpanzees in the Mahali Mountains[END_REF][START_REF] Van Lawick-Goodall | The Behaviour of Free-living Chimpanzees in the Gombe Stream Reserve[END_REF][START_REF] Goodall | The Chimpanzees of Gombe: Patterns of Behvior[END_REF][START_REF] Oates | Pan troglodytes: IUCN Red List of Threatened Species[END_REF] Humid forest (sometimes forest-savannah mosaics) [START_REF] Kano | The Social Group of Pygmy Chimpanzees (Pan paniscus) of Wamba[END_REF][START_REF] Narat | Bonobo habituation in a forest-savanna mosaic habitat: influence of ape species, habitat type, and sociocultural context[END_REF]Fruth et al., 2016)

Group size

Small (1-2 adults) (Delgado & van Schaik, 2000;[START_REF] Van Noordwijk | Female philopatry and its social benefits among Bornean orangutans[END_REF] Medium (10, occasionally up to 20) [START_REF] Williamson | Gorilla gorilla Western Gorilla[END_REF] Large (20 to 120) [START_REF] Muller | Conflict and Cooperation in Wild Chimpanzees[END_REF] Large (30 to 80) (Fruth et al., 2013(Fruth et al., , 2016)

Social group pattern

Mix of solitary life and temporary ♂ / ♀ grouping (fission-fusion society) (Delgado & van Schaik, 2000;[START_REF] Van Noordwijk | Female philopatry and its social benefits among Bornean orangutans[END_REF][START_REF] Van Schaik | The socioecology of fission-fusion sociality in Orangutans[END_REF] Single ♂/multi ♀ and temporary grouping composed mainly of immature ♂♂) [START_REF] Gatti | Population and group structure of western Lowland Gorillas (Gorilla gorilla gorilla) at Lokoué, Republic of Congo[END_REF][START_REF] Robbins | Social structure and life-history patterns in western gorillas (Gorilla gorilla gorilla)[END_REF]Levréro et al., 2006;[START_REF] Harcourt | Gorilla society: conflict, compromise, and cooperation between the sexes[END_REF] Multi-♂/multi-♀ (fissionfusion society) [START_REF] Wrangham | [END_REF][START_REF] Muller | Conflict and Cooperation in Wild Chimpanzees[END_REF] Multi-♂/multi-♀ (fissionfusion society) [START_REF] Kano | The Social Group of Pygmy Chimpanzees (Pan paniscus) of Wamba[END_REF]van Elsacker et al., 1995;[START_REF] Furuichi | Social bonds ♂-♂ Weak [medium if immature (unflanged)[END_REF] Sex dispersal ♂ Disperse Disperse (Stokes et al., 2003;[START_REF] Forcina | [END_REF] Philopatric [START_REF] Nishida | The social group of wild chimpanzees in the Mahali Mountains[END_REF][START_REF] Wrangham | [END_REF] Philopatric [START_REF] Kano | The Social Group of Pygmy Chimpanzees (Pan paniscus) of Wamba[END_REF]van Elsacker et al., 1995;[START_REF] Furuichi | Social bonds ♂-♂ Weak [medium if immature (unflanged)[END_REF]

( 1 )

 1 Vocal communication organisation as a mirror of social complexity (2) Vocal interactions in primates: a remarkable diversity of temporal calling patterns (3) Turn-taking-based vocal exchange in primates: a social function (4) Selection for vocal turn-taking systems from monkeys to humans: an open question II. The socio ecology of great apes: similarities and differences (1) Origins and habitat (2) Social systems III. Testing the social complexity hypothesis for communication in great apes (1) Traditional vocal metrics (repertoire size, call rate)

  This work was financially supported by a CIFRE grant (#2018_00074) funded by the Association Beauval Nature pour la Conservation et la Recherche, Centre National de la Recherche Scientifique, Rennes 1 University and Lyon/Saint-Etienne University. M.L. was funded by the Swiss National Science Foundation (PP00P3_163850). This research was also financially supported by the University of Lyon/Saint-Etienne, Nature & Découverte foundation and WWF-Belgium. We are deeply grateful to the ZooParc de Beauval, the Ouwehand Dierenpark and the orang-utan caregivers for their valuable assistance. Invaluable help in the field was provided by the field assistants of WWF-DRC, of the NGO Mbou-Mon-Tour and the management and staff of the Budongo Conservation Field Station. 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Fig. 1 .

 1 Fig. 1. Synthesis of the social structure, distribution of calling patterns and interaction

Table 1 .

 1 Socio-ecology of four great ape species. Mya, million years ago. 1138

		Bornean orang-utan (Pongo pygmaeus)	Western lowland gorilla (Gorilla gorilla gorilla)	Chimpanzee (Pan troglodytes)	Bonobo (Pan paniscus)
		~9 Mya to ~18 Mya	~8.6 to ~11.5 Mya
		(Hobolth et al., 2011;	(Upham et al., 2019;
	Last common ancestor with humans	Castellano & Munch, 2020)	Castellano & Munch, 2020)

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VII. SUPPORTING INFORMATION

Additional supporting information may be found online in the Supporting Information section at the end of the article. Appendix S1. Details of new data for wild free-ranging chimpanzees.

Appendix S2. Details of new data for wild free-ranging bonobos.

Appendix S3. Details of new data for captive orang-utans.

Percentage of lethal intra-specific aggression 0.08% [START_REF] Gómez | The phylogenetic roots of human lethal violence[END_REF] 0.14% [START_REF] Gómez | The phylogenetic roots of human lethal violence[END_REF] 4.49% [START_REF] Gómez | The phylogenetic roots of human lethal violence[END_REF] 0.68% [START_REF] Gómez | The phylogenetic roots of human lethal violence[END_REF] Percentage of time spent on social grooming 0% (Dunbar, 1991;[START_REF] Mccomb | Coevolution of vocal communication and sociality in primates[END_REF] 0.09% [START_REF] Lehmann | Group size, grooming and social cohesion in primates[END_REF] 8.3-11.7% [START_REF] Lehmann | Group size, grooming and social cohesion in primates[END_REF] 5.7% [START_REF] Lehmann | Group size, grooming and social cohesion in primates[END_REF] Table 2. Vocal behaviour of four great ape species.

Bornean orangutan (Pongo pygmaeus)

Western lowland gorilla (Gorilla gorilla gorilla)

Chimpanzee (Pan troglodytes)

Bonobo (Pan paniscus)

Vocal repertoire size (number of call types) Wild 25 [START_REF] Hardus | A description of the orangutan's vocal and sound repertoire, with a focus on geographic variation[END_REF] 16 [START_REF] Salmi | Western gorilla vocal repertoire and contextual use of vocalizations[END_REF] 13-29 [START_REF] Marler | Signaling behavior of apes with special reference to vocalizations[END_REF][START_REF] Goodall | The Chimpanzees of Gombe: Patterns of Behvior[END_REF]

Main call type used during vocal interactions

None (new data #3 ) Grunts (turn-taking) [START_REF] Harcourt | Function and Meaning of Wild Gorilla 'Close' Calls II. Correlations with Ranks and Relatedness[END_REF]Hedwig et al., 2015a,b) Pant-hoots (overlap) [START_REF] Mitani | Contexts and social correlates of long-distance calling by male chimpanzees[END_REF][START_REF] Mitani | Social Factors Influence the Acoustic Variabilty in the Long Distance Calls of Male Chimpanzees[END_REF][START_REF] Gros-Louis | Chorusing and call convergence in chimpanzees: Tests of three hypotheses[END_REF]Fedurek et al., 2013aFedurek et al., ,b, 2014;; (Pougnault et al., 2020a): isolated calls occur when one call is emitted in isolation with no other calls is close proximity; repeated calls occur when the same individual calls several times in a row; calls in overlap involve partial (or total) superposition of calls from two or more callers; calls in turn-taking involve an orderly exchange of vocalisations between individuals, adjusting the timing of their vocal responses by including a minimum silence gap (preventing call overlap), and a maximum silence gap (ensuring call coordination). New data: #1 for wild chimpanzees, Budongo Forest Reserve, Uganda, details of these unpublished data are given as online Supporting Information in Appendix S1, N=1 wild chimpanzee group including 35 identified individuals; #2 for wild bonobos, Nkala forest in Bolobo Territory, Democratic Republic of Congo, see Appendix S2, N=1 wild bonobo group including 19 identified individuals; #3 for orang-utans, ZooParc de Beauval (France) and Ouwehand Dierenpark (Netherlands), see Appendix S3, N=2 captive orang-utan groups including 8 identified individuals in each zoo.