Laughter, play faces and mimicry in animals: evolution and social functions

Human laughter and laugh faces show similarities in morphology and function with animal playful expressions. To better understand primordial uses and effects of human laughter and laugh faces, it is important to examine these posi tive expressions in animals from both homologous and analogous systems. Phylogenetic research on hominids provided empirical evidence on shared ancestry across these emotional expressions, including human laughter and laugh faces. In addition, playful expressions of animals, in general, arguably have a key role in the development of social cognitive skills, a role that may help explain their polyphyletic history. The present work examines the evol ution and function of playful expressions in primates and other animals. As part of this effort, we also coded for muscle activations of six carnivore taxa with regard to their open-mouth faces of play; our findings provide evidence that these carnivore expressions are homologues of primate open-mouth faces of play. Furthermore, our work discusses how the expressions of animal play may communicate positive emotions to conspecifics and how the motor reson ance of these expressions increases affiliation and bonding between the subjects, resembling in a number of ways the important social–emotional effects that laughter and laugh faces have in humans

Chimpanzees (Pan troglodytes) produce the same types of ‘laugh faces’ when they emit laughter and when they are silent

The ability to flexibly produce facial expressions and vocalizations has a strong impact on the way humans communicate, as it promotes more explicit and versatile forms of communication. Whereas facial expressions and vocalizations are unarguably closely linked in primates, the extent to which these expressions can be produced independently in nonhuman primates is unknown. The present work, thus, examined if chimpanzees produce the same types of facial expressions with and without accompanying vocalizations, as do humans. Forty-six chimpanzees (Pan troglodytes) were video-recorded during spontaneous play with conspecifics at the Chimfunshi Wildlife Orphanage. ChimpFACS was applied, a standardized coding system to measure chimpanzee facial movements, based on FACS developed for humans. Data showed that the chimpanzees produced the same 14 configurations of open-mouth faces when laugh sounds were present and when they were absent. Chimpanzees, thus, produce these facial expressions flexibly without being morphologically constrained by the accompanying vocalizations. Furthermore, the data indicated that the facial expression plus vocalization and the facial expression alone were used differently in social play, i.e., when in physical contact with the playmates and when matching the playmates' open-mouth faces. These findings provide empirical evidence that chimpanzees produce distinctive facial expressions independently from a vocalization, and that their multimodal use affects communicative meaning, important traits for a more explicit and versatile way of communication. As it is still uncertain how human laugh faces evolved, the ChimpFACS data were also used to empirically examine the evolutionary relation between open-mouth faces with laugh sounds of chimpanzees and laugh faces of humans. The ChimpFACS results revealed that laugh faces of humans must have gradually emerged from laughing open-mouth faces of ancestral apes. This work examines the main evolutionary changes of laugh faces since the last common ancestor of chimpanzees and humans

Multimodal communication development in semiwild chimpanzees

Human language is characterized by the integration of multiple signal modalities, including speech, facial and gestural signals. While language likely has deep evolutionary roots that are shared with some of our closest living relatives, studies of great ape communication have largely focused on each modality separately, thus hindering insights into the origins of its multimodal nature. Studying when multimodal signals emerge during great ape ontogeny can inform about both the proximate and ultimate mechanisms underlying their communication systems, shedding light on potential evolutionary continuity between humans and other apes. To this end, the current study investigated developmental patterns of multimodal signal production by 28 semiwild chimpanzees, Pan troglodytes, ranging in age from infancy to early adolescence. We examined the production of facial expressions, gestures and vocalizations across a range of behavioural contexts, both when produced separately and as part of multimodal signal combinations (henceforth multimodal). Overall, we found that while unimodal signals were produced consistently more often than multimodal combinations across all ages and contexts, the frequency of multimodal combinations increased significantly in older individuals and most within the aggression and play contexts, where the costs of signalling ambiguity may be higher. Furthermore, older individuals were more likely to produce a multimodal than a unimodal signal and, again, especially in aggressive contexts. Variation in production of individual signal modalities across ages and contexts are also presented and discussed. Overall, evidence that multimodality increases with age in chimpanzees is consistent with patterns of developing communicative complexity in human infancy, revealing apparent evolutionary continuity. Findings from this study contribute novel insights into the evolution and development of multimodality and highlight the importance of adopting a multimodal approach in the comparative study of primate communication

Skin temperature changes in wild chimpanzees upon hearing vocalizations of conspecifics

The authors are grateful to the Royal Zoological Society of Scotland for providing core funding to the Budongo Conservation Field Station. The research was supported by a Fyssen fellowship awarded to GD, funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration (grant agreement no 283871), and the Swiss National Science Foundation (PZ00P3_154741) awarded to CDD.A growing trend of research using infra-red thermography (IRT) has shown that changes in skin temperature, associated with activity of the autonomic nervous system, can be reliably detected in human and non-human animals. A contact-free method, IRT provides the opportunity to uncover emotional states in free-ranging animals during social interactions. Here, we measured nose and ear temperatures of wild chimpanzees of Budongo Forest, Uganda, when exposed to naturally occurring vocalizations of conspecifics. We found a significant temperature decrease over the nose after exposure to conspecifics’ vocalizations, whereas we found a corresponding increase for ear temperature. Our study suggests that IRT can be used in wild animals to quantify changes in emotional states in response to the diversity of vocalizations, their functional significance and acoustical characteristics. We hope that it will contribute to more research on physiological changes associated with social interactions in wild animals.Publisher PDFPeer reviewe

Chimpanzee communities differ in their inter- and intrasexual social relationships

Male and female human social bonding strategies are culturally shaped, in addition to being genetically rooted. Investigating nonhuman primate bonding strategies across sex groups allows researchers to assess whether, as with humans, they are shaped by the social environment or whether they are genetically predisposed. Studies of wild chimpanzees show that in some communities males have strong bonds with other males, whereas in others, females form particularly strong intrasex bonds, potentially indicative of cultural differences across populations. However, excluding genetic or ecological explanations when comparing different wild populations is difficult. Here, we applied social network analysis to examine male and female social bonds in two neighbouring semiwild chimpanzee groups of comparable ecological conditions and subspecies compositions, but that differ in demographic makeup. Results showed differences in bonding strategies across the two groups. While female–female party co-residence patterns were significantly stronger in Group 1 (which had an even distribution of males and females) than in Group 2 (which had a higher proportion of females than males), there were no such differences for male–male or male–female associations. Conversely, female–female grooming bonds were stronger in Group 2 than in Group 1. We also found that, in line with captive studies but contrasting research with wild chimpanzees, maternal kinship strongly predicted proximity and grooming patterns across the groups. Our findings suggest that, as with humans, male and female chimpanzee social bonds are influenced by the specific social group they live in, rather than predisposed sex-based bonding strategies

A machine learning approach to infant distress calls and maternal behaviour of wild chimpanzees

We are grateful to the Royal Zoological Society of Scotland for providing core funding to the Budongo Conservation Field Station. This research was supported by funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration (Grant agreement no 283871), a Fyssen Foundation post-doctoral fellowship awarded to GD, the Swiss National Science Foundation (PZ00P3_154741) and Start up-funding of the Taipei Medical University (108-6402-004-112) awarded to CDD.Distress calls are an acoustically variable group of vocalizations ubiquitous in mammals and other animals. Their presumed function is to recruit help, but there has been much debate on whether the nature of the disturbance can be inferred from the acoustics of distress calls. We used machine learning to analyse episodes of distress calls of wild infant chimpanzees. We extracted exemplars from those distress call episodes and examined them in relation to the external event triggering them and the distance to the mother. In further steps, we tested whether the acoustic variants were associated with particular maternal responses. Our results suggest that, although infant chimpanzee distress calls are highly graded, they can convey information about discrete problems experienced by the infant and about distance to the mother, which in turn may help guide maternal parenting decisions. The extent to which mothers rely on acoustic cues alone (versus integrate other contextual-visual information) to decide upon intervening should be the focus of future research.PostprintPeer reviewe