Thesis Abstract

Cooperative behavior such as territory defense and pack hunting is well-documented in many social animals, including humans and other primates. Individuals that work together through collective action face the problem of “free-riding” members who do not participate, but still benefit. When free-riders consume the common goods gained through cooperation, individuals who did the work gain less net benefit. The maintenance of long-term social relations with group-mates may build trust and reduce the probability of free-riding during collective action. Social bonds should therefore become more important as the cost of free-riding increases. For example, in species that compete to defend territories as groups, social cohesion is predicted to increase when competition with other social groups increases, and this increased group togetherness should in turn discourage free-riding during group conflict. I investigated the relationship between within-group social cohesion and between-group conflict in several groups of blue monkeys (Cercopithecus mitis) living in Kakamega Forest, Kenya. These group-living monkeys compete cooperatively to maintain access to territories that provide all group members with access to food sources. I quantified group cohesion among adult females, the main participants in intergroup contests, with several measures. In particular, I used 10 measures derived from social networks, based on social contact time (grooming plus sitting in contact) as a measure of affiliation between pairs of monkeys. Blue monkeys living in large groups had significantly increased mean node degree during 3-month periods when their group participated in intergroup contests more frequently, but smaller groups did not show this effect. The effect of group size suggests that blue monkeys living in larger social groups may be able to flexibly change their behavior to reinforce social bonds needed in collective action, while those living in smaller groups are already connected to all potential social partners. Between-group differences in social cohesion did not contribute significant information to models predicting the contest winner. Overall, these results suggest that blue monkey social cohesion is sometimes related to collective action in group competition, but the size of social groups is a more important factor in both within-group cohesion and the outcome of group contests.