Convergence and divergence in gesture repertoires as an adaptive mechanism for social bonding in primates.

A key challenge for primates living in large, stable social groups is managing social relationships. Chimpanzee gestures may act as a time-efficient social bonding mechanism, and the presence (homogeneity) and absence (heterogeneity) of overlap in repertoires in particular may play an important role in social bonding. However, how homogeneity and heterogeneity in the gestural repertoire of primates relate to social interaction is poorly understood. We used social network analysis and generalized linear mixed modelling to examine this question in wild chimpanzees. The repertoire size of both homogeneous and heterogeneous visual, tactile and auditory gestures was associated with the duration of time spent in social bonding behaviour, centrality in the social bonding network and demography. The audience size of partners who displayed similar or different characteristics to the signaller (e.g. same or opposite age or sex category) also influenced the use of homogeneous and heterogeneous gestures. Homogeneous and heterogeneous gestures were differentially associated with the presence of emotional reactions in response to the gesture and the presence of a change in the recipient's behaviour. Homogeneity and heterogeneity of gestural communication play a key role in maintaining a differentiated set of strong and weak social relationships in complex, multilevel societies.