Female -female competition and male sexual coercion in Kanyawara chimpanzees
Historically, it had been supposed that female-female contest competition was unimportant in chimpanzees (Pan troglodytes) because females are semi-solitary and have weakly defined same-sex dominance relationships. However, recent research has challenged this view, showing that female reproductive success increases with dominance rank. Fitness variance has been indirectly linked to differential feeding success, but exactly how dominant females gain feeding advantages over subordinates remains unclear. In this study, I demonstrate that within the Kanyawara chimpanzee community in Kibale National Park, Uganda, female contest feeding competition occurs on two levels. First, using long-term data on dominance interactions, I assign ranks to females and show that high-ranking females reside in areas of the home range containing a higher abundance of preferred foods. Together with evidence of female-female aggression during periods of presumed female range adjustment, this result implies that females compete over foraging areas. Second, in a 17-month field study, I found that high female rank is associated with priority of access to higher-quality feeding sites in fruit trees. I use relative feeding heights as a gauge of feeding site quality, since previous work showed that higher feeding sites contain more and possibly higher-quality fruit.
An additional aim of this study was to shed light on the functional significance of male aggression against females, a behavior that occurs regularly at Kanyawara and elsewhere and is known to inflict costs on females. Using 10 years of data, I test the widely held notion that one function of male-to-female aggression is sexual coercion. In support of this hypothesis, I show that female chimpanzees receive more aggression from males when they are likely to conceive (i.e. when estrous and parous) and males have higher mating success with females they are aggressive towards. Also, I report that high-ranking males use coercion more frequently.
Results presented in these three studies elucidate formerly puzzling aspects of chimpanzee behavioral biology by providing mechanisms for female contest feeding competition and determining one function of male-to-female aggression. These findings are relevant for refining our understanding of the ecology of female social relationships and sexual conflict in primates and other animals.