Intergroup competition enhances chimpanzee (Pan troglodytes verus) in-group cohesion

In-group cohesion is an essential component of successful intergroup competition inboth human and nonhuman animals, likely facilitating group members access topotential benefits. However, when benefits are equally shared among group members,group defense becomes a collective action problem, which might subvert cohesiveparticipation during intergroup competition. There is a lack of consensus across studiesand species with regard to the link between in-group cohesion and intergroup competition, likely as a result of species differences in managing the collective actionproblem. Here, we examine this link in a species with a striking example of collectiveaction during intergroup competition, chimpanzees (Pan troglodytes verus). Using twoyears of focal-follow data on males and females in two groups at the Taï Forest, Côted’Ivoire, we investigated the immediate and long-term effects of intergroup competition(border patrols and intergroup encounters) on measures of in-group cohesion, namelymodularity, party size, and intergroup aggression. We found that groups’ associationpatterns were less modular (more cohesive) in months in which they engaged in moreborder patrols and intergroup encounters. We found that current and greater priorengagement in intergroup competition predicted larger party sizes. Furthermore, current, but not prior engagement in intergroup competition, predicted reduced intragroupaggression by males but not by females. Increased in-group cohesion in chimpanzeeslikely reduces potential costs of intergroup competition engagement, by facilitatingjoint participation in current and future intergroup conflicts, overcoming the collectiveaction problem