Intra-sexual competition and vocal counter-strategies in wild female chimpanzees (Pan troglodytes schweinfurthii)

Abstract

A growing body of behavioural data is beginning to show that, like their male counterparts, female chimpanzees can be competitive and aggressive, particularly when resources come under pressure. These observations are especially surprising because, for a long time, females were simply considered passive pawns of male social manoeuvrings. While we are beginning to understand the complexities surrounding female chimpanzee group living, exactly how females manage these social pressures is unclear. In this thesis I address this by focusing on female competition in wild chimpanzees and the importance of vocal counter-strategies. I examined two commonly produced female vocalisations: copulation calls and victim screams from chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda. My results regarding the production and acoustic structure of copulation calls suggests that these vocalisations play a crucial role in the lives of female chimpanzees, dissipating the risks associated with female competition. During aggression, chimpanzee females commonly produce victim screams and these calls have been shown to vary systematically with the severity of aggression experienced. A playback experiment showed that victim screams are meaningful to females and that listeners do not just respond to the acoustically most salient signals in their environment. Females may use this information to keep track of out-of-sight agonistic interactions and make appropriate social decisions regarding whether to avoid an ensuing attack. Taken together, I propose that vocalisations may represent important behavioural counter-strategies, enabling females to navigate successfully through their socially intricate world.