Female Affiliation and Status in Semi-Free-Ranging Chimpanzees

Sex differences in social behavior and status are pervasive across primates and other mammals. In the wild, chimpanzees (Pan troglodytes) exhibit many sex-typed behaviors: adult female chimpanzees exhibit lower aggression, are subordinate to adult males, and also are generally less social than males. This pattern is thought to partly reflect the energetic constraints (e.g., feeding competition) that wild female chimpanzees face.

We test the drivers of sex differences in chimpanzee behavior by examining a semi-free-ranging African chimpanzee sanctuary group where individuals are provisioned, an environment that should relax ecological constraints on socializing. Using two years of focal observations on a group with 45 chimpanzees (17 males, 28 females), we examined sex differences in social proximity, grooming, aggression, coalition formation, and dominance.

In contrast to patterns in wild chimpanzees, we found that males and females in this population exhibited comparable rates of affiliative behavior. Males engaged in more aggressive behavior overall than females, as in the wild. However, females were equally likely to aggress either sex, and a high proportion of female aggression involved coalitions. Finally, David's scores revealed that a few of the adult females outranked some of the lower-status adult males.

These findings show that sex differences in chimpanzee social behavior are partially flexible, and females may show more affiliation, cooperation, and higher status when ecological conditions are favorable. More generally, some forms of female power can emerge even in a species with strong sex biases in behavior for male affiliation and status.