Olivier Pascalis

Fabrice Damon

Kun Guo

David Méary

It takes one to know one: Do human and nonhuman primates share similar face processing?

Keywords: Face, non human primates, human, development

It takes one to know one: Do human and nonhuman primates share similar face processing?

comparative studies suggest a common evolutionary route for this important ability [START_REF] Leopold | A comparative view of face perception[END_REF]. Non-human primates have often been used as a model of human face processing. Fujita (e.g., 1987) has been interested in nonhuman primate models for many years, and has continued to publish influential work on this topic. In this chapter we will document some similarities in face recognition but also differences that make this cognitive system well adapted for each species.

Faces are omnipresent in our environment, and are crucial for our everyday social life. It has been suggested that face recognition might somehow be «special» [START_REF] Farah | Is face recognition 'special'? Evidence from neuropsychology[END_REF] compared to recognition of other objects. Indeed, faces provide an early channel of communication, operating prior to the onset of language between infant and caretaker. It has been shown that infants preferentially orient to faces during the first week of life [START_REF] Goren | Visual following and pattern discrimination of face-like stimuli by newborn infants[END_REF][START_REF] Johnson | Newborns' preferential tracking of face-like stimuli and its subsequent decline[END_REF][START_REF] Valenza | Face preference at birth[END_REF]. By the time they reach adulthood they can be considered as face experts. [START_REF] Diamond | Why faces are and are not special: an effect of expertise[END_REF] suggested that we learn to process faces differently than other visual stimuli. Information on a face can be construed as hierarchically organized with the facedness information at the top (e.g., faces vs. objects), followed by species information (e.g., human vs. monkey), then by subordinate information such as gender and race, and at the lowest level of individual identity information; individuating information in turn can be further divided into featural information (e.g., the shape or size of the eyes), configural information (e.g., the distance between the eyes), and holistic information (the face gestalt that binds featural and configural information into an unbreakable whole) [START_REF] Leder | When inverted faces are recognized: the role of configural information in face recognition[END_REF]. Despite the role of early visual experience on the development of configural and holistic processing, some researchers speculate that both are adult-like at a later age relative to featural face processing. This speculation was first proposed in the late 1970s with the introduction of the controversial encoding switch hypothesis (Carey & Diamond, 1977;Diamond & Carey, 1977). The encoding switch hypothesis proposes that young children encode faces in a piecemeal fashion (i.e., individual facial features) whereas older childrenbeginning at around the age of 10encode the spatial relations between individual facial features. This hypothesis was based on findings regarding the effects of paraphernalia and inverting faces on children's recognition. This configural processing is responsible for the striking inversion effect [START_REF] Yin | Looking at upside-down faces[END_REF]: inverted faces are less rapidly and accurately processed than upright faces (for reviews see [START_REF] Rakover | Explaining the face-inversion effect: The face-scheme incompatibility (FSI) model[END_REF][START_REF] Rossion | Picture-plane inversion leads to qualitative changes of face perception[END_REF]. [START_REF] Carey | From piecemeal to configurational representation of faces[END_REF] used inversion as a measure of children's use of spatial relations among features (i.e., involved in both configural and holistic processing). They found that the recognition of faces in 10-year-olds was disproportionately more impaired by inversion than was the recognition of houses, whereas 6-and 8-year-olds showed similar impairment for both categories. Recent data are however suggesting that from a young age, infants are sensitive to configuration [START_REF] Anzures | Development of face processing ability in childhood[END_REF]. [START_REF] Diamond | Why faces are and are not special: an effect of expertise[END_REF] showed that this processing mode is a consequence of experience, and can be extended to other categories in which subjects are expert.

How do other species process faces?

A variety of nonhuman social species possesses remarkable face discrimination abilities. [START_REF] Tibbetts | Visual signals of individual identity in the wasp Polistes fuscatus[END_REF] reported that wasps, primarily reliant on chemical signatures for communication and identification of conspecifics, are also capable of hierarchical categorization (e.g., "worker") based on facial visual patterns only. [START_REF] Dyer | Honeybee (Apis mellifera) vision can discriminate between and recognise images of human faces[END_REF] found that honeybees can learn the picture of a human face and recognize it when it is paired with a novel face (for similar findings with wasps see also [START_REF] Avarguès-Weber | Recognition of human face images by the free flying wasp Vespula vulgaris[END_REF]. This result is consistent with what we already know about the visual ability of bees. However, [START_REF] Pascalis | What can bees really tell us about the face processing system in humans[END_REF] pointed out that in the study by [START_REF] Dyer | Honeybee (Apis mellifera) vision can discriminate between and recognise images of human faces[END_REF], recognition could have been performed on the basis of very simple pattern processing, and might not be related to face processing at all. Thus, for now there is no compelling evidence of human-like face processing in insects. Recently, sheep (Ovis aries) have been found to present advanced face-recognition skills, including the ability to recognize and discriminate individual human faces [START_REF] Knolle | Sheep recognize familiar and unfamiliar human faces from two-dimensional images[END_REF]. However, the claim that sheep also possess human-like face processing seems too strong (for a discussion see [START_REF] Kendrick | Response to 'Sheep recognize familiar and unfamiliar human faces from two-dimensional images[END_REF][START_REF] Peirce | Do sheep really recognize faces? Evidence from previous studies. Response to "Sheep recognize familiar and unfamiliar human faces from two-dimensional images[END_REF][START_REF] Towler | Are face recognition abilities in humans and sheep really "comparable[END_REF]. Sheep and human performances were, for instance, not compared on equivalent tasks, as face-recognition tasks typically require that humans learn new face identities in few seconds and in one trial whereas sheep are trained during several consecutive days and throughout many trials [START_REF] Towler | Are face recognition abilities in humans and sheep really "comparable[END_REF]. In addition, the strong advantage in processing familiar over unfamiliar faces thought of as a key feature of human face processing [START_REF] Young | Are we face experts?[END_REF] has not been found in sheep, suggesting qualitatively different mechanisms between two species [START_REF] Towler | Are face recognition abilities in humans and sheep really "comparable[END_REF]. It worth noting however that sheep are obviously better to process sheep faces than human facesas humans are to process own-vs. other-species faceshence, for a true comparison to be made, both human and sheep faces should be used in future comparative studies [START_REF] Kendrick | Response to 'Sheep recognize familiar and unfamiliar human faces from two-dimensional images[END_REF] to determine to what extent the face recognition mechanisms overlap between the species.

The situation is quite different for nonhuman primates, and comparative studies on face recognition in various primate species have burgeoned during the last 20 years. The adult face processing system of nonhuman primates shares several similarities with that of humans: eye scanning, region of interest, individual recognition of face pictures, sensitivity to face inversion (see [START_REF] Pascalis | Picture perception in primates: the case of face perception[END_REF] for a review). [START_REF] Humphrey | Species and individuals in the perceptual world of monkeys[END_REF] used a habituation task to show that rhesus monkeys can distinguish different individuals from their own speciesbut this performance did not extend to individuals from other, nonprimate species. Using a classic habituation paradigm, [START_REF] Dahl | Individuation and holistic processing of faces in rhesus monkeys[END_REF] found that individual recognition by macaques was better for conspecific faces than for other-species faces, and that, like humans, macaques show holistic processing. [START_REF] Gothard | Multiple perceptual strategies used by macaque monkeys for face recognition[END_REF] found the same result, and their detailed analysis of the eye scanning of the faces showed that whereas monkeys use both configural and feature-based processing to recognize the faces of conspecifics, they use primarily feature-based strategies to recognize human faces. These studies suggest the use of different strategies while processing other-species faces.

Another study by [START_REF] Dahl | Humans and macaques employ similar face-processing strategies[END_REF] compared scanning strategies by humans and macaques when looking at conspecific or heterospecific faces. Both species displayed the same pattern of scanning for their own-versus other-species faces: more attention was directed to the eye region for their own species than for other species. Findings of better performance in face recognition tasks with own-versus other-species faces extend to other primates species (e.g., brown capuchin monkeys and Tonkean macaques, [START_REF] Dufour | Face processing limitation to own species in primates: A comparative study in brown capuchins, Tonkean macaques and humans[END_REF]chimpanzees, Martin-Malivel & Okada, 2007;cotton-top tamarins, Neiworth, Hassett, and Sylvester, 2007; Japanese monkeys, [START_REF] Sugita | Face perception in monkeys reared with no exposure to faces[END_REF].

Although both humans and macaques can learn to individuate conspecifics by faces, the former outperform the latter in individual recognition of own-species faces [START_REF] Rossion | What can we learn about human individual face recognition from experimental studies in monkeys?[END_REF], and macaques need extensive operant conditioning to achieve even modest recognition performance. As noted by [START_REF] Parr | The evolution of face processing in primates[END_REF], studies using macaques have reported mixed evidence regarding the extent of human-like face processing (e.g., inversion effect, configural processing, etc.), with results varying depending on the methods used, perhaps suggesting that humans and monkeys have evolved different face processing strategies. By contrast, compared to macaques, chimpanzees have been found to present face processing mechanisms more homologous with those of humans [START_REF] Parr | The evolution of face processing in primates[END_REF][START_REF] Taubert | Robust representations of individual faces in chimpanzees (Pan troglodytes) but not monkeys (Macaca mulatta)[END_REF][START_REF] Wilson | Visual discrimination of primate species based on faces in chimpanzees[END_REF], but see [START_REF] Griffin | Quantifying the face inversion effect in nonhuman primates: a phylogenetic meta-analysis[END_REF]. Although the macaque-chimpanzee difference may be related to the latter's greater phylogenetical proximity to humans, it may also be related to species-specific social structures: chimpanzees and humans share a "fissionfusion" social organization [START_REF] Parr | The evolution of face processing in primates[END_REF], whereas macaque monkeys live in more stable social groups with more or less strict, often linear dominance hierarchies.

In fission-fusion society, group composition is dynamic and subject to frequent fracture and growth over time; hence, robust face processing mechanisms are highly advantageous, and there is a strong social pressure to recognize the faces of conspecifics. Strikingly, it has been reported that spider monkeys (Ateles geoffroyi), a New World species with a fission-fusion social structure, also presents more human-like face processing abilities than macaque monkeys (i.e., holistic processing for high-experienced face categories, [START_REF] Taubert | Evidence of human-like, holistic face processing in spider monkeys (Ateles geoffroyi)[END_REF][START_REF] Taubert | Visual expertise does not predict the composite effect across species: A comparison between spider (Ateles geoffroyi) and rhesus (Macaca mulatta) monkeys[END_REF], further suggesting that socioecological pressures might contribute to the emergence of a specialized face processing system. Beyond these differences, visual biases for own-species seem to be especially common across primates species [START_REF] Scott | The own-species face bias: A review of developmental and comparative data[END_REF], including humans [START_REF] Heron-Delaney | Infants' knowledge of their own species[END_REF]. [START_REF] Fujita | Species recognition by five macaque monkeys[END_REF], using an operant lever-pressing task, explored the extent to which different species showed conspecific preference, by comparing portrait preference in five macaques species (Macaca fuscata, M. mulatta, M. radiata, M. nemestrina and M. arctoides). Each monkey was trained to press a lever to show a picture on a screen. The picture remained on-screen for as long as the subject held the lever down. Using this simple method it was possible to assess the time spent looking at a picture of a conspecific compared to one showing a different species. In this task, it is assumed that habituation will occur more quickly (i.e., reduced looking time) with a picture of a heterospecific, which is less attractive for the subject.

Fujita found that monkeys showed conspecific preference, with the exception of the stumptailed macaque M. arctoides (but see [START_REF] Demaria | Responses to animal stimulus photographs in stumptailed macaques (Macaca arctoides)[END_REF]. Most of the subjects discriminated conspecifics in pictures, and preferred to look at their own species than macaques of a different species [START_REF] Fujita | Development of visual preference for closely related species by infant and juvenile macaques with restricted social experience[END_REF][START_REF] Fujita | Visual preference for closely related species by Sulawesi macaques[END_REF]. Overall, Fujita's studies demonstrated that preference for pictures of own species is common across multiple macaque species [START_REF] Fujita | Species recognition by five macaque monkeys[END_REF][START_REF] Fujita | Species preference by infant macaques with controlled social experience[END_REF](Fujita, , 1993a)). Preferences were diluted when either the head, or head and tail, were removed from the picture [START_REF] Fujita | Role of some physical characteristics in species recognition by pigtail monkeys[END_REF], suggesting that the head is a crucial cue driving the preference. Similarly, the face was found to be important in conspecific discrimination by longtailed macaques (Dittrich, 1994), although

it should be noted that this last study used line drawings, not halftone images of real faces.

Although own-species preference seems to be widespread in primates, its origin may differ between species. For instance, own-species preference can be spontaneously present at birth, or emerge from social experience with conspecifics during infancy. Cross-fostering studies can be especially informative to disambiguate this issue. Fujita showed that Japanese monkeys raised by rhesus macaque mother presented a preference for rhesus macaque faces [START_REF] Fujita | Species preference by infant macaques with controlled social experience[END_REF](Fujita, , 1993a)), while Japanese monkeys raised by their mother presented own-species preferences. By contrast, rhesus macaques showed ownspecies preferences regardless of their early social experience. Fujita concluded that rhesus have a hard-wired preference for their own species, whereas Japanese macaques have a more flexible preference that is influenced by experience. Related findings were reported in great apes, as adult chimpanzees that had been reared by humans preferred pictures of human faces over chimpanzee faces [START_REF] Tanaka | Visual preference by chimpanzees (Pan troglodytes) for photos of primates measured by a free-choice-order task: Implication for influence of social experience[END_REF][START_REF] Tanaka | Development of the visual preference of chimpanzees (Pan troglodytes) for photographs of primates: Effect of social experience[END_REF]. These biases may, in turn, influence face discrimination processes, as human-reared chimpanzees were found to be better at processing human faces than chimpanzee faces, compared to a group of chimpanzees who grew up in their own species group [START_REF] Lutz | Human and chimpanzee face recognition in chimpanzees (Pan troglodytes): Role of exposure and impact on categorical perception[END_REF].

From a theoretical point of view, preferences for own-species faces may have emerged from some ancient general perceptual bias shared among primates [START_REF] Damon | A review of attractiveness preferences in infancy: From faces to objects[END_REF][START_REF] Rhodes | The evolutionary psychology of facial beauty[END_REF]. This development might be linked to the ease of processing (i.e., perceptual fluency, [START_REF] Whittlesea | Two fluency heuristics (and how to tell them apart)[END_REF], thus facilitating the development of familiarity preferences. Such frameworks extend beyond mere face processing mechanisms, and have been proposed as relevant for a general theory of object representation and recognition [START_REF] Wallis | Toward a unified model of face and object recognition in the human visual system[END_REF][START_REF] Wallis | The prototype effect revisited: Evidence for an abstract feature model of face recognition[END_REF].

Interestingly, Fujita provided indirect evidence for such shared perceptual mechanisms between human and nonhuman primates, in reporting preferences for regular and symmetrical patterns in capuchin and squirrel monkeys [START_REF] Anderson | Are monkeys aesthetists? Rensch (1957) revisited[END_REF]. These findings indicated that monkeys prefer visual stimuli that humans find aesthetically pleasing, as visual symmetry is perceived as attractive in a variety of domains [START_REF] Little | Domain specificity in human symmetry preferences: Symmetry is most pleasant when looking at human faces[END_REF]. Furthermore, when it comes to faces, ratings of rhesus macaque face attractiveness by human judges can predict implicit visual preferences in rhesus macaques [START_REF] Damon | Preference for attractive faces is species-specific[END_REF].

However, mate selection mechanisms probably operate in addition to general perceptual biases for symmetry, as the effect of face attractiveness turned out to be species-specific. When two faces from the same species but differing in attractiveness were presented side-by-side, humans showed a visual preference for human faces (but not macaque faces) rated as attractive, and macaques displayed a visual preference for macaque faces (but not human faces) rated as attractive. Findings of similar preferences in human and nonhuman primates have also been reported for curved contours (humans and chimpanzees: [START_REF] Munar | Common visual preference for curved contours in humans and great apes[END_REF], and for prototypicality (human and macaque infants: Damon, Méary, et al., 2017), further highlighting shared underlying face/object processing mechanisms. Interestingly, face familiarity further shows differential impact on face preference in humans and rhesus macaques [START_REF] Méary | Seeing two faces together: Preference formation in humans and rhesus macaques[END_REF]. When two faces of different species/races were presented side-by-side, Caucasian humans demonstrated clear visual preference for Caucasian faces over Asian faces (own-race bias), and human faces (regardless of races) over nonhuman primate (e.g., chimpanzee, Barbary and rhesus macaque) faces (own-species bias). Rhesus macaques also showed visual preference for nonhuman primate faces over human faces regardless of race, but by contrast displayed preference for chimpanzee or Barbary macaque faces over own-species faces. This pattern suggests that face preference in macaques, unlike humans, is modulated by factors beyond familiarity, such as species.

How does face processing change early in life?

A developmental approach may help in determining what the common processes in different species are before experience influences the face processing system, if the developmental trend is similar across primates, and how flexible the system is for handling other-species faces. Faces represent a highly attractive stimulus for infant primates, including humans [START_REF] Goren | Visual following and pattern discrimination of face-like stimuli by newborn infants[END_REF], pigtailed macaques (Macaca nemestrina, [START_REF] Lutz | Human and chimpanzee face recognition in chimpanzees (Pan troglodytes): Role of exposure and impact on categorical perception[END_REF], gibbons (Hylobates agilis, [START_REF] Myowa-Yamakoshi | Development of face recognition in an infant gibbon (Hylobates agilis)[END_REF], rhesus monkeys (Macaca mulatta, [START_REF] Kuwahata | Development of schematic face preference in macaque monkeys[END_REF], and chimpanzees (Pan troglodytes, [START_REF] Tomonaga | Development of social cognition in infant chimpanzees (Pan troglodytes): Face recognition, smiling, gaze, and the lack of triadic interactions[END_REF]. Some researchers have argued that primates may possess an innate face processing system that is predisposed to respond to conspecifics (e.g., [START_REF] Sackett | Monkeys reared in isolation with pictures as visual input: Evidence for an innate releasing mechanism[END_REF]. Two-month-old pigtailed macaques already demonstrate a strong preference for their own species [START_REF] Kim | Humans all look alike: Cross-species face recognition in infant pigtailed macaque monkeys[END_REF]. The argument is also supported by several studies demonstrating that rhesus macaques raised without any experience of monkeys still prefer to look at faces of conspecifics than other species [START_REF] Fujita | Species recognition by five macaque monkeys[END_REF](Fujita, , 1993a)). This is not true for Japanese macaques, however; as mentioned above, rhesus monkeys may have a hard-wired preference for their own species, while Japanese macaques have a more malleable preference, influenced by experience.

An influential study by [START_REF] Sugita | Face perception in monkeys reared with no exposure to faces[END_REF] illustrated the critical role of experience in the development of face processing in macaques. Infant Japanese macaques separated from their parents at birth and reared by humans in a faceless environment for a minimal period of 6 -24 months, showed a preference for both monkey and human faces over objects, but no preference for either category of faces when tested in a visual preference task. Furthermore, they were able to recognize both monkey and human faces. Following the face deprivation period, half of the monkeys were introduced to macaques, and the other half to humans. When tested following a minimum of one month of exposure to faces, monkeys exposed to human faces displayed a preference for human faces over objects, and also for human over monkey faces. They displayed no preference between monkey faces and objects. Consistent with this pattern of results, monkeys exposed to monkey faces preferred monkey faces over objects and over human faces, but showed no preference when human faces and objects were presented simultaneously. Furthermore, when tested for recognition, monkeys were only able to recognize faces from the category to which they were exposed. These results highlight the crucial role visual experience can play in the specialization of the face system toward own-species faces. Japanese macaques are not displaying hard-wired preference for their own species, but an experience-dependent preference.

As mentioned above, Japanese macaque monkeys looked more at faces than objects regardless of species, even though they were deprived of face exposure. This suggests that a broad face detection mechanism was still functional despite the lack of exposure to faces, as if initial perceptual sensitivities for faces were preserved, and waiting for refinement through individualized experiences (i.e., experience-expectant processes, [START_REF] Nelson | The development of face recognition reflects an experience-expectant and activity-dependent process[END_REF].

Are rhesus different from Japanese macaques as suggested by Fujita? Recent studies with nursery-reared infant rhesus macaques (with no exposure to adult macaque faces and limited face exposure in general) showed a "coarse to fine" pattern of face detection [START_REF] Simpson | Face detection and the development of own-species bias in infant macaques[END_REF]. Three-week-old rhesus infants with very limited face exposure showed a bias for both own-and other-species faces over objects. Three-, and 6-month-olds, after peer exposure, also showed a similar bias for faces over objects, but also looked faster toward conspecific faces compared to heterospecific faces. Taken together, these results slightly qualify Fujita's earlier findings suggesting hard-wired preference for ownspecies faces in rhesus macaques, and indicate that the face processing system of rhesus monkeys undergoes a transition from experience-expectant to experiencedependent mechanisms. This developmental trajectory is not limited to nonhuman primates, but seems to extend to humans as well. [START_REF] Nelson | The development and neural bases of face recognition[END_REF] hypothesized that in humans the representation of faces at birth is broad and that it develops according to the type of facial input received, tuning toward the predominant faces in the environment. To test whether experience tunes face processing, [START_REF] Pascalis | Is face processing speciesspecific during the first year of life ?[END_REF] investigated the ability of 6-and 9-month-old infants to recognize faces from their own species and another species (rhesus macaques) using a standard recognition paradigm. Infants at both ages showed individual recognition with human faces, looking longer at a new face compared to a previously seen face.

However, when tested with the monkey faces, only the 6-month-old group showed evidence of individual recognition. By contrast, 9-month-olds were unable to recognize which monkey face they had seen before. These findings suggest that the face system becomes 'tuned' to human faces between 6and 9-months of age [START_REF] Pascalis | Is face processing speciesspecific during the first year of life ?[END_REF]. In a follow-up study, [START_REF] Pascalis | Plasticity of face processing in infancy[END_REF] investigated the impact of experience with other-species faces on the development of face processing in relation to this species. They provided 6month-olds' parents with a book containing a selection of monkey faces, which they were instructed to show to their infant following a fixed schedule. Tests showed that this exposure was sufficient to preserve recognition capabilities for monkey faces when the infants returned for testing at 9 months of age. Training experienced near the end of the tuning period appears to be effective for maintaining the ability to discriminate between individuals of other species. Thus, it is possible to alter the development of a cognitive system (for face processing) by providing training and learning via pictorial stimuli. However, it is still unclear how much training is necessary to produce other-species face processing skills at 9 months and for how long long-term training might affect the face system (for a review see [START_REF] Maurer | Perceptual narrowing during infancy: A comparison of language and faces[END_REF].

Overall, the studies reviewed here illustrate that the similarity of the face processing systems of human and nonhuman primates is best found in their plasticity. The experience-expectant/dependent mechanisms progressively adjust the infant primate to its environment, refining the face processing system as the infant accrues experience with the facial characteristics of the individuals from its social group [START_REF] Pascalis | On the links among face processing, language processing, and narrowing during development[END_REF]. Despite the indisputable importance of faces in many primate species, however, it is probably too far-fetched to conclude that cognitive and neural face processing mechanisms are continuous across all primates [START_REF] Rossion | What can we learn about human individual face recognition from experimental studies in monkeys?[END_REF]. Some face-processing mechanisms appeared to be shared among primate species whereas others have turned out to be species-specific, possibly reflecting solutions to evolutionary challenges imposed by particular ecological niches. However, these mechanisms always share a similar purpose: to subserve social cognition.

Acknowledgments

This work was supported by the French "Investissements d'Avenir" program, project ISITE-BFC (contract ANR-15-IDEX-0003) to F.D.

The ability to identify individuals within the group, and to interpret their expressions and intentions is essential for many social animals. In humans, it is well established that face processing abilities underpin such functions, and


Sylvain Pincebourde

Aurélien Salle

Commentary on "SoilTemp: a global database of near-surface temperature" by Lembrechts et al

) are initiating a new global database of near-surface temperatures, especially focusing on above ground (up to 2 m) and below ground soil temperatures. This approach is welcome to assess the amplitude of microclimate change around the world and across biomes.

Here, I extend this approach by highlighting that fine-scale temperature heterogeneity can be large over short times frames (minutes to hours) and short distances (centimeters to meters) in forest covers. This mosaic of microclimates likely influences the response of organisms to climate change. Further, the SoilTemp database could be expanded to the other interfaces with atmosphere.

Main text

Our climate is changing globally. The pressing need to identify the ecological consequences of these changes boosted the development of macroecological approaches as well as climatological tools (e.g., Worldclim climate data provider). These approaches revealed some of the complex trends in the response of biodiversity to global changes [START_REF] Scheffers | The broad footprint of climate change from genes to biomes to people[END_REF] and sometimes the mechanisms underlying these trends were identified [START_REF] Sunday | Thermal-safety margins and the necessity of thermoregulatory behavior across latitude and elevation[END_REF]).

Nevertheless, the major pitfall remains that the actual climatic conditions experienced by organisms in their microhabitat and across their home range are largely ignored [START_REF] Pincebourde | Fine-scale microclimatic variation can shape the responses of organisms to global change in both natural and urban environments[END_REF]. The notion of microclimate is part of our daily lives and this concept was also applied to natural systems long ago to detail the abiotic conditions experienced by organisms in their microhabitat. For instance, an aphid lives at the temperature of the leaf it attacks [START_REF] Pincebourde | Narrow safety margin in the phyllosphere during thermal extremes[END_REF] and a mussel experiences a body temperature resulting from the complex interaction of multiple environmental variables in the rocky intertidal [START_REF] Helmuth | Interdidal mussel microclimates: predicting the body temperature of a sessile invertebrate[END_REF]. In both cases, the temperature experienced in fine can deviate from atmospheric temperature, sometimes strongly by >15°C [START_REF] Pincebourde | Fine-scale microclimatic variation can shape the responses of organisms to global change in both natural and urban environments[END_REF]. Therefore, it is crucial to identify these microclimatic deviations for any kind of microhabitat anywhere on Earth. Such a huge task inevitably calls for collaborative work to develop global databases on microclimates.

In this context, [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF] are initiating a new global database of near-surface temperatures, especially focusing on above ground (up to 2 m) and below ground soil temperatures. Mounting such a database of soil temperatures across biomes, continents and microhabitats is the first step towards (i) the empirical quantification of the buffering property of surfaces (i.e., how much of atmospheric temperature variation is buffered at the different soil layers) and (ii) the assessment of the amplitude microclimatic change actually experienced by organisms (plants, animals, fungi and microorganisms) living in or near the ground (i.e., how much of the global/regional warming is transferred into the soil microhabitat over long temporal scales). This initiative of [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF] is more than welcome for climate change biologists and more generally for ecologists. [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF] propose several environmental factors driving the surface temperature deviation to be reported in the database, including horizontal (topography, land cover type, distance to water bodies) and vertical (vegetation characteristics, snow cover, soil properties) features that are operating at the local or landscape scale. This approach is reasonable especially because most microclimatic loggers deployed around the world are operating at these spatial scales. Nevertheless, the SoilTemp database would gain in strength and pertinence by including the micro-scale as well. Local heterogeneity in surface temperatures is important for species maintenance via various processes including behavioral thermoregulation (Woods et al. 2015). We exemplify this point based on unpublished data (from the project CANOPEE funded by Region Centre-Val de Loire, France) illustrating (near) surface temperature heterogeneity at the meter scale or below along both the vertical and horizontal gradients of a broadleaf temperate forest (Fig. 1). Along the vertical gradient of forests, different surfaces from below ground to the canopy top show quite distinct temperature signature with deviations of up to 20°C between the soil 15 cm deep and the air at the canopy surface (Fig. 1A). This gradient illustrates the buffering effect of forest understory (Zellweger et al. 2020). Canopy gaps that allow solar radiation to reach the ground can rapidly and markedly influence soil temperature, while they have little effect on air temperature at 2 m (Fig. 1 A). Therefore, over short times frames, the temperature experienced by organisms living in the soil versus at the different heights in the canopy can differ as much as macroclimate temperature varies across 20° of latitude.

In addition, soil surface temperature can be quite heterogeneous at micro-scale over the horizontal layer. The temperature at the leaf litter surface in early spring, as measured with an infrared camera, can be quite variable with a temperature range of about 16°C across a short distance of less than 1 m (Fig. 1B). This variability results from the interaction of sun angle, leaf litter microtopography, and shading effects by vegetative elements above the ground. This effect is extremely dynamics and depends on the sun course in the sky. Soil surface can quickly reach high temperature when receiving direct solar radiation. This effect influences the temperature within the soil at 5 cm below ground (Fig. 1A,B). Two dataloggers only 40 cm apart can measure soil temperatures differing by as much as 4°C due to this effect.

Obviously, deploying a single temperature logger within this thermal mosaic will sample only a fraction of the local temperature range, and therefore temporal series from logger networks are better candidates to estimate these levels of thermal heterogeneity at micro-scale.

Determining the amplitude of the buffer ability of near-ground surfaces requires high temporal resolution when measuring temperatures. [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF] are judiciously calling for "time series spanning one month or more, with temperature measurements a maximum of 4 hours apart". In the context of meta-analysis, getting series with a temporal resolution of few hours will certainly add up to the bunch of temperature data obtained at a finer temporal resolution, thereby enriching the analysis. Nevertheless, soil temperature recorded with a time step of > 1 hour may not provide an accurate estimate of maximal temperature experienced by some soil organisms (Fig. 1B: the duration of the temporal series is 4 hours). Acute thermal events of about 1 hour matter for ectotherms by promoting their thermal tolerance threshold (heat hardening) or by inducing irreversible physiological damage [START_REF] Ma | Survive a warming climate: insect responses to extreme high temperatures[END_REF]. A high temporal resolution may be especially important for organisms with a weak dispersal ability such as Isotoma Collembola which disperse by only 5 cm per week [START_REF] Ojala | Dispersal of microarthropods in forest soil[END_REF] and which may not be able to escape from acute exposure.

The SoilTemp database focuses on temperatures near soil surfaces, including air temperature 2 m above ground and below ground temperatures [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF]). We see this important call for data as a first step towards a more global near-surface temperature database.

In the future, the call could be expanded to other types of interfaces with atmosphere including plant organ temperature (e.g., leaf surfaces and plant inner tissues such as stems, woody tissues or galls) and temperature of different layers in water bodies (ponds, rivers, wetlands). Indeed, the forest canopy is with soil one of the two key forest 'compartments' supporting tremendous forest biodiversity (e.g., [START_REF] Parmain | Forest decline differentially affects trophic guilds of canopy-dwelling beetles[END_REF]) and aquatic thermal environments are still missing for global analysis (van Klink et al. 2020). Moreover, the SoilTemp database would also integrate near-surface temperatures for urban areas to analyze how the urban heat island influences temperature patterns near the ground in green areas.

Urban areas can be seen as simulators of climate warming and urban surface temperature data may help to anticipate the thermal behavior of surfaces in natural areas in the future.

Given the accumulating evidence that fine-scale microclimates can play a crucial role in the fate of organisms thriving at the interface with a changing atmosphere, strong collaborative work among multiple disciplines (incl. computer and atmospheric sciences, ecology and physiology) is likely to become the streamline towards a global assessment of microclimate buffering. This empirical approach should be combined with mechanistic modelling [START_REF] Maclean | Predicting future climate at high spatial and temporal resolution[END_REF] in particular to generate microclimatic data for places and/or periods that were not covered by dataloggers. indicate when an infrared image of the leaf litter surface was taken (using an infrared camera FLIR B335). The position of the three dataloggers in these thermal images is shown by the color dots (their color corresponds to the color of the lines). In general, the thermal heterogeneity at the litter surface generates temperature deviations among the three dataloggers. The entire peak of soil temperature illustrated in this graph was linked to incoming solar radiation directly reaching the ground via a canopy gap.

Figure legend

Fig. 1 .

 1 Fig. 1. Near-surface temperature heterogeneity across fine-scale vertical (A) and horizontal (B) gradients in a broadleaf temperate forest (Vierzon, France). (A) Daily dynamics of temperature (on August 26, 2019) in the soil (at 15 and 5 cm deep) and for air at 2 m and 25 m (canopy top). The three dotted lines indicate the moment at which solar radiation directly reached the soil surface. The dataloggers (Hobo Tidbit v2) were set to record temperatures every 10 minutes. The loggers measuring air temperature were shielded (Hobo solar shield RS3). (B) Temperature dynamics during a sunny day (on July 14, 2019) at -5cm underground,

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