Male chimpanzee vocal interactions and social bonds

Vocal interactions, such as call exchanges or choruses, are common behaviours in animals and their function has been often linked to social bonding. In this study, I examined the relationship between two types of vocal behaviour common in chimpanzees, pant hoot chorusing and food calling, and social bonds between males. The results of this study, which was conducted on the Kanyawara community of chimpanzees (Pan troglodytes schweinfurthii) in the Kibale National Park, Uganda, show that both pant hoot choruses and food-associated calls reliably reflect social bonds between males. For example, males were more likely to join in another male’s pant hoot when a well affiliated male was calling. Males, irrespective of the long-term affiliation status between them, were also more likely to be involved in grooming or form coalitions on days when they chorused together, suggesting that chorusing is also a flexible bonding behaviour operating on a short-term basis. Males were also considerably more likely to produce food-associated calls when feeding in close proximity to well affiliated males than to less affiliated ones or females. Importantly, a male feeding partner was more likely to remain with the focal until the end of a feeding bout if the focal food called at the onset of this bout, suggesting that these vocalisations mediate feeding decisions between affiliated males. The results of my study suggest that these two types of vocalisations play important functions in chimpanzee fission-fusion societies. Pant hoot chorusing, for example, might facilitate the occurrence of other affiliative interactions between individuals who are not necessarily long-term preferred social partners. Food-associated calls, on the other hand, might facilitate anticipation of each feeding decision which in turn facilitates individuals remaining in proximity or in the same party. Both these types of vocalisations, therefore, might mitigate the costs of living in unstable societies formed by these primates.