Vocal greeting behaviour in wild chimpanzee females

Chimpanzees, Pan troglodytes, are unusual among primates in that they express their social position with a unique vocal signal, the pant-grunt. The call is only produced when encountering a higher-ranking group member and has thus been interpreted as a ‘greeting’ signal. We monitored the calling behaviour of nine adult females in a group of free-ranging chimpanzees, the Sonso community of Budongo Forest, Uganda, when encountering higher-ranking adult males. We found that call production was by no means rigid, but that calls were given only if certain social conditions were met. Although all adult males received pant-grunts from females, the alpha male received a significantly larger proportion of calls. The number of pant-grunts given to males was not correlated with their hierarchical position or with the level of anticipated aggression. Instead, females were significantly more likely to vocalize to other males if the alpha male was absent, suggesting that their calling behaviour was moderated by social inhibition. The presence of the alpha female had a similar yet weaker inhibitory effect. Social inhibition was further increased with increasing numbers of bystanders, especially males. Our results thus demonstrate that chimpanzees use their ‘greeting’ signals flexibly by taking into account the social fabric of their community.