Use of overlap zones among group-living primates: a test of the risk hypothesis

Overlap zones between home ranges of neighboring groups of primates are routinely reported to be under-used. However, little is known about how the size of overlap zones varies, or what factors influence their size. Here we use ranging data on three species of group-living primates to test the hypothesis that overlap zones are smaller or used less in species that are subject to a higher risk of lethal aggression in intergroup encounters. Redtail monkeys (Cercopithecus ascanius) have a low risk of violence; white-faced capuchins (Cebus capucinus) have an intermediate risk; chimpanzees (Pan troglodytes) face a high risk of violent encounters with their neighbors. We calculated two indices of use of the overlap zone. First, we assessed the opportunity for groups to meet each other as the range overlap, i.e., the diameter of the home range in relation to the distance between neighboring ranges. Second, we compared the intensity with which groups used the overlap zone by calculating utilization curves that described how space-use patterns change with distance from a group's center of activity. Neither the overlap potentials nor utilization curves supported the risk hypothesis. There was little evidence of differences among the three species, all of which showed substantial under-use of overlap zones. Our data, which provide the first systematic comparison of overlap zones among primates, thus conform to previous reports suggesting that primate groups tend to have large overlap zones, regardless of the risk of violence. Since such zones are potentially responsible for carrying capacity being lower than expected by an ideal-free distribution, it is an important problem to understand why they are apparently widespread.