Association by male chimpanzees: a social tactic?

Whether and with whom to associate are possibly some of the most fundamental of choices faced by a chimpanzee, choices constrained by and dependent upon similar decisions made by potential association partners. Although much work has focused on the impact of ecological factors on the size of the temporary associations, or parties, it seems that chimpanzees, especially the males, form parties to gain social benefits, and that these benefits are dependent the particular composition of these parties. This paper examines the social determinates of the association patterns of male chimpanzees, members of the Sonso community in the Budongo Forest Reserve, western Uganda. Male chimpanzees showed clear preferences for association partners, and these partners had a strong tendency to associate in small parties. Strength of association within dyads accounted for a significant fraction of the variance in party size [all parties containing males: r(2) = 0.30; male parties only: r(2) = 10.24 (all dyads), r(2) = 0.50 (dyads with positive associations only)]. These results supported the hypothesis that association was the result of tactical decisions, and refuted predictions of alternate hypotheses: random and passive (independent attraction to the same locations, such as fruiting trees) association. Further support for the tactical association hypothesis was provided by observations that association between males was flexible and that association patterns resolved into alternative association strategies, with some males shifting between strategies over time. The presence of cycling females influenced party size, which increased with the number present. The number of adult males in a party was also greater when cycling females were present, although remained similar whether one, two, three or four such females were in the party. Samples sizes for parties containing more than two cycling females were low however. it is suggested that, for a Machiavellian primate in fission- fusion social system, the need to make decisions concerning association partners is cognitively demanding and may be an important, frequently used, function of the intelligence demonstrated by captive chimpanzees.