Ecological determinants of the behavior and social structure of Japanese monkeys: A synthesis

A review is presented of the results of the various studies in this volume and an attempt is made to establish connections among several features of the ecology, behavior, and social structure of Japanese monkeys. Several studies in this volume suggest that intergroup direct feeding competition has been much more severe in Yakushima, in the warm-temperate region, than in Kinkazan, in the cool-temperate region of Japan. This result is consistent with the predictions that moderate abundance and clumped distribution of food incur more severe intergroup direct competition. However, the number of adult females within a group in Yakushima was smaller than that in Kinkazan even though severe intergroup direct competition should favor large groups. This contradiction can be mainly explained by the less severe intergroup indirect competition in Kinkazan than in Yakushima. By contrast, some studies in this volume also indicate that adult male to female ratio within a group has been higher in Yakushima than in Kinkazan. This result can be explained in two ways: the females in Yakushima might have recruited more males to increase the competitive ability of the group under conditions of severe intergroup direct feeding competition; alternatively, it might be profitable that the males in Yakushima defend females cooperatively as group males against the males in other groups at a moderate density of females. Some studies in this volume suggest that grooming frequency was higher in Yakushima than in Kinkazan. The higher grooming frequency in Yakushima might have been partly due to a constant increase in engaging in social behavior from a decrease in feeding time. Another reason might be that there is a stronger effect of grooming on promotion of formation of coalitions among adults under conditions of severe intergroup direct and intragroup direct competition.