The Structure of Bonobo Copulation Calls During Reproductive and Non-Reproductive Sex

Copulation calls in primates are usually identified as sexually selected signals that promote the reproductive success of the caller. In this study, we investigated the acoustic structure of copulation calls in bonobos (Pan paniscus), a great ape known for its heightened socio-sexuality. Throughout their cycles, females engage in sexual relations with both males and other females and produce copulation calls with both partners. We found that calls produced during sexual interactions with male and female partners could not be reliably distinguished in terms of their acoustic structure, despite major differences in mating behaviour and social context. Call structure was equally unaffected by the size of a females sexual swelling and by the rank of her mating partner. Rank of the partner did affect call delivery although only with male, but not female partners. The only strong effect on call structure was because of caller identity, suggesting that these signals primarily function to broadcast individual identity during sexual interactions. This primarily social use of an evolved reproductive signal is consistent with a broader trend seen in this species, namely a transition of sexual behaviour to social functions.