The relationship between testosterone and long-distance calling in wild male chimpanzees

Abstract: Long-distance calling is a common behaviour in animals, which has various important social functions. At a physiological level, calling is often mediated by gonadal hormones such as testosterone (T), particularly when its function is linked to intra-sexual competition for mates or territory. T also plays an important role in the development of vocal characteristics associated with dominance in humans. However, the few available studies of T and vocal behaviour in non-human primates suggest that in primates, T has less influence on call production than in other animals. We tested this hypothesis by studying the relationship between T concentrations and pant-hooting in wild male chimpanzees (Pan troglodytes schweinfurthii) of the Kanyawara community in the Kibale National Park, Uganda. We found three kinds of correlation. Hourly T averages were positively associated with hourly rates of pant-hooting. Monthly T levels were likewise correlated with monthly rates of pant-hooting after controlling for other influences such as fission-fusion rates. Finally, males with high T levels had higher peak frequency at the start of the call climax. These results suggest that T affects the production of pant-hoots in chimpanzees. This implies that the pant-hoot call plays a role in male-male competition. We propose that even in cognitively sophisticated species, endocrine mechanisms can contribute to regulating vocal production. Significance statement: Many animals produce long-distance calls. The production of these calls is often modulated by gonadal hormones such as testosterone, especially if the calls are involved in competition between males for mates or territory. However, comparatively little is known about the influence of testosterone over the vocal behaviour of non-human primates, especially among great apes. In this study, we examined the relationship between testosterone and pant-hooting in wild male chimpanzees. We found that testosterone levels were associated with pant-hoot rates and one acoustic feature of the call. More specifically, males pant-hooted more often and produced pant-hoots with higher peak frequencies during periods of elevated testosterone levels. These results imply that gonadal hormones are involved in regulating vocal behaviour in chimpanzees and support the view that pant-hoots play a role in male-male competition.