Pant hoot chorusing and social bonds in male chimpanzees

Vocal interactions, such as call exchanges or chorusing, are common behaviours in many animal species and their function has often been attributed to social bonding. However, few studies have investigated the effectiveness of vocalizations as bonding signals in comparison to other affiliative behaviours. We tested the hypothesis that male chimpanzee, Pan troglodytes schweinfurthii, pant hoot chorusing, a common behaviour in these primates, is a reliable but also flexible signal of affiliative relationships. The results of our study, conducted on the Kanyawara community of chimpanzees in Uganda, show that males were more likely to join in with the pant hoot of preferred long-term social partners to form a chorus. This supports the hypothesis that this behaviour is a good indicator of strong or long-term social bonds between male chimpanzees. However, our results also show that pant hoot chorusing reliably reflects short-term affiliations between males. For instance, male dyads were more likely to be involved in affiliative behaviours, such as reciprocated grooming, joint nonvocal displays and coalitions, on days when they chorused together, compared to days when they did not. This pattern applied to both preferred and neutral social partners. Moreover, on a short-term basis chorusing was a better indicator of other affiliative behaviours than grooming. We suggest that in male chimpanzees pant hoot choruses are efficient signals of short-term affiliative relationships. We conclude that potentially low-cost bonding behaviours such as coordinated vocal displays might be especially adaptive in highly fluid fission–fusion societies where grouping patterns are often unpredictable.