Variation in Intergroup Encounters in Two Populations of Japanese Macaques

The nature of intergroup encounters differed between two populations of wild Japanese macaques (Macaca fuscata): the Yakushima and Kinkazan populations. In the Yakushima population, intergroup encounters were more likely to result in the displacement of one group, intergroup agonistic interaction was common, and intergroup dominance was usually distinct. When displacement occurred at Yakushima, larger groups tended to dominate smaller ones. Conversely, in the Kinkazan population, intergroup encounters rarely resulted in displacement, intergroup agonistic interaction was rare, and intergroup dominance was usually unclear. Thus, monkeys in Yakushima appear to defend resources actively during encounters, while those in Kinkazan usually did not defend resources. The frequency of encounters was significantly higher in Yakushima than in Kinkazan. The two populations had very different group densities 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 frequencies 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 to habitat differences and the evolutionary significance of resource defense behavior.