Vocal communication in chimpanzees: a tool for social cohesion
Authors:   , Bouchard Alice 
Published: January 2024

In some animal species, individuals develop strong and enduring social bonds, which promote cooperation and usually offer significant fitness advantages. In these highly social species, individuals need to be able to coordinate and negotiate their complex social relationships but how do they do so? Does high sociality promote the evolution of specific communicative signals to manage these relationships? These issues becomes even more problematic for species with fluid social structures, where cohesion between group members varies constantly. Chimpanzees are an ideal model species to study these questions, as they present a high degree of fission-fusion dynamics. In this species, males are more gregarious than females and form highly strong and stable bonds with unrelated individuals of the same sex. These bonds, which can manifest in various contexts, such as grooming and intragroup conflicts, play a crucial role in establishing dominance relations between males and have a significant impact on their reproductive success. However, how do males choose their social partners and how do they maintain their relationships with their preferred partners? By empirically investigating the vocal communication and social cohesion of male chimpanzees in various contexts, this thesis aimed to elucidate how males use vocalisations to manage their social relationships. I first explored specific contexts where social bonds are under strain, i.e., when the social composition of the subgroup is likely to change. Hence, I studied the vocal production of short and long distance calls (i.e., ‘pant hoots’ and ‘rough grunts’) by male chimpanzees upon their arrival at food trees, an socially challenging event, as it is particularly prone to outbreaks of aggression but also offers opportunities for cooperation. I showed that vocal communication in this context appeared to have a dual function to mediate both cooperative and competitive interactions, with males producing ‘pant hoots’ to cooperatively inform absent social partners about the presence of food whereas ‘rough grunts’ seemed to be produced as part of competitive interactions, to avoid aggression. Secondly, I investigated the vocal production of ‘rest hoos’, a context-specific vocalisation given while resting, another context during which the subgroup composition is threatened due to forthcoming travel. My findings provided evidence that ‘rest hoos’ were produced intentionally to prolong resting bouts with desired partners, and that their function would thus be to help manage social cohesion. Furthermore, I discovered that low-ranking males who did not groom often produced ‘rest hoos’ more frequently than other males, hence suggesting that vocal communication could serve as an alternative cohesion strategy to tactile-based bonding. Finally, I described the patterns of dyadic short-term associations between males (i.e., periods of time during which males stay in each other’s visual range) and showed that these associations were mostly determined by dominance relations, as high-ranking males were more effective in prolonging associations when producing a ‘hoo’ and were also more likely to terminate the associations. I also specifically investigated whether males used vocal signals (i.e., production of ‘hoo’ vocalisations) or tactile behaviour (i.e., grooming) to manage these associations and found that 'hoo' vocalisations but not grooming had a significant impact on association patterns, suggesting not only that males produced these calls strategically to ensure social cohesion but also that vocalisations may be a more reliable predictor of short-term affiliations than grooming. Overall, these findings provide evidence that vocalisations have various social functions, including mediating social cohesion and managing both cooperative and competitive interactions, and are therefore playing an essential role in chimpanzee fission-fusion societies. The results presented in this thesis will help better understand chimpanzee vocal communication and socio-cognitive abilities as well as further our comprehension of how complex social systems shaped the evolution of animal communication.