The Hormonal Correlates of Male Chimpanzee Social Behavior.

Abstract

Male chimpanzees are well known for their aggressive behavior. In this dissertation I investigate the hormonal correlates of three types of male chimpanzee aggression: within-group male-male aggression, between-group territorial aggression, and between species predatory aggression. Specifically, I examined how testosterone and cortisol, two steroid hormones, mediate these types of aggression. Within groups, male chimpanzees compete with each other to obtain matings with females. While engaged in this form of reproductive aggression, males displayed increased testosterone levels, but only when competing for specific females. Rates of male aggression were elevated when they competed for parous, estrous females. In contrast, levels of aggression were relatively low in the presence of nulliparous females, who represented less attractive mating partners. Consequently, male testosterone concentrations were higher in the presence of parous, estrous females compared to their baseline levels. The presence of nulliparous, estrous females had no effect on male testosterone concentrations. These results are consistent with the Challenge Hypothesis, which proposes that testosterone correlates with aggression only when the latter enhances fitness. In a novel test of the Challenge Hypothesis, I found that male chimpanzee testosterone concentrations increased during and shortly after territorial boundary patrols but not while they hunted. In addition, male chimpanzees displayed an anticipatory rise in testosterone before they engaged in territorial behavior. Further analyses revealed that male testosterone levels were significantly lower after hunting, perhaps due to the tolerance associated with meat sharing. Males that shared and received meat at hunts exhibited decrements in testosterone, while males who failed to obtain meat at hunts showed no change. In a third study, I examined all three types of aggression as they relate to stress. Male chimpanzees displayed acute stress responses and relatively high cortisol levels when they engaged in within-group reproductive aggression, between-group territorial aggression, and between species predatory aggression. As seen with testosterone, male chimpanzees showed an intriguing rise in cortisol before they began to participate in territorial and hunting behaviors. Thus, wild chimpanzees appear to be able to anticipate conflict situations. The cues that they use to anticipate these events, however, remain unknown and require further study.