The function of chimpanzee greeting calls is modulated by their acoustic variation

Abstract

Signalling plays an important role in mediating social interactions in many animal species. For example, during approaches certain species produce ‘greetings’, which can take the form of vocal or visual signals, which reduce the probability of aggressive interactions and/or facilitate affiliation when approaching each other. However, in species where greetings comprise both vocal and visual signals, little is known about how the vocal component relates to the visual component or, in species with fission–fusion dynamics, to the time spent together by the dyad in the same subgroup prior to the approach. Similarly, in species with several acoustic variants of greeting calls, it is unclear whether different variants have different functions. We looked at the production of two acoustically distinct greeting call variants, low-fundamental frequency pant grunts and high-fundamental frequency pant barks, during approaches between two individuals in five communities of wild chimpanzees, Pan troglodytes, in Uganda and Ivory Coast. More specifically, we explored the relationship between greeting call production and (1) aggressive and submissive interactions during the approach and (2) preceding and subsequent proximity levels between the involved individuals. Calls were more likely to be produced during aggressive interactions and were associated with postures and gestures linked to submission; these patterns were stronger when the utterance contained a pant bark rather than a pant grunt alone. The production of greeting calls was more likely soon after party fusion and was negatively related to subsequent proximity levels between the caller and receiver. These results expand our knowledge of greeting calls and imply that these calls might be used to re-establish dominance relationships after a period of separation, and that the function of these calls can be modulated by their specific acoustic variants and by visual signals that often accompany them.