Cooperation, Competition, and Killing: Reproductive strategies of chimpanzees (Pan troglodytes)

Abstract

Group-living commonly involves tensions between conflict and cooperation. Group members need one another to survive, but also compete for access to key resources such as food and mates. To better understand reproductive strategies in group-territorial species with sex-biased dispersal, I used decades of data from Gombe National Park, Tanzania to test hypotheses regarding how and why chimpanzees (Pan troglodytes) cooperate, compete, and fight. First, I found that male chimpanzees exhibit a consistently high degree of participation in boundary patrols (mean=75%) and that the best predictors of participation in patrols were sighting frequency and participation in hunting bouts, indicating a mutualistic payoff structure for male territorial effort. Second, I found that female chimpanzees produced copulation calls more frequently when they were nulliparous, and in the early days of their swelling. Thus, these calls likely function as an anti-infanticide strategy, inducing otherwise uninterested males to mate and maximizing the pool of potential sires. Females also called less frequently in the presence of higher-ranking females, indicating that intrasexual competition plays a role in call production. Third, I found that males killed by other chimpanzees suffered a higher-than-expected rate of genital wounding, but during non-fatal fighting, only in one of four communities (Kasekela) did males experience a higher-than-expected rate of genital wounds. Females in all four study communities experienced a higher-than-expected rate of genital wounds, indicating that genital wounding is an unlikely alternative to lethal aggression. Finally, I found that lethal aggression was more common in Mitumba than Kasekela. As a smaller community, Mitumba has fewer females and thus less overlap between reproductively active females and a greater opportunity to monopolize mating opportunities. Overall, this work emphasizes the importance of within-group reproductive competition.