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
grey-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
<500m 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.