Variation in intergroup encounters in two populations of Japanese macaques

The nature of intergroup encounters differed between two populations of wil d Japanese macaques (Macaca fuscata): the Yakushima and Kinkazan population s. In the Yakushima population, intergroup encounters were more likely to r esult in the displacement of one group, intergroup agonistic interaction wa s common, and intergroup dominance was usually distinct When displacement o ccurred at Yakushima, larger groups tended to dominate smaller ones. Conver sely, in the Kinkazan population, intergroup encounters rarely resulted in displacement, intergroup agonistic interaction was rare, and intergroup dom inance was usually unclear. Thus, monkeys in Yakushima appear to defend res ources actively during encounters, while those in Kinkazan usually did not defend resources. The frequency of encounters was significantly higher in Y akushima than in Kinkazan. The two populations had very different group den sities and traveling speeds, both of which directly influence the chance of encounters. Taking these differences into account we compared the observed frequency with those predicted by the ideal gas model. The observed freque ncies in both populations were about one-third of the number expected with the model, which suggests that the differences in encounter frequency were caused by differences in group density and traveling speed. We discuss this intraspecific variation in light of economic defendability in connection t o habitat differences and the evolutionary significance of resource defense behavior.