Detecting an Effect of Group Size on Individual Responses to Neighboring Groups in Gray-Cheeked Mangabeys (Lophocebus albigena)

Evolutionary game theory posits that competitive ability affects the initiation of conflicts. When contests occur among groups, competitive ability is generally measured as the size of the group and larger groups are expected to win against smaller groups. However, in some cases, individual participation during intergroup conflicts appears unaffected by competitive ability. To test whether these instances might be due to an unduly strict definition of participation, I re-evaluate the responses of gray-cheeked mangabeys (Lophocebus albigena) to the calls of real and simulated neighboring groups. In contrast with previous analyses, I consider multiple measures of group size, treat movement responses as a continuous variable, and evaluate individual responses (N = 201 focal follows). Males made stronger approaches toward calling neighbors than females, though both sexes tended to retreat from groups that were <500 m away and to approach more distant neighbors. Individuals in small groups retreated while those in large groups both approached and retreated. There was no evidence of a collective action problem: in fact, approaches were more likely within large groups than small groups, and approaches were stronger when at least one other individual within the focal group made a dramatic approach toward the caller. The absence of a group size effect is attributable to coarse methods in some contexts and to a stronger effect of collective behavior or resource-related motivation in other contexts.