Variation in intergroup encounters among two provisioned free-ranging populations of Japanese macaques Macaca fuscata

Abstract

Abstract Typically, Japanese macaques are thought to avoid encountering other groups wherever possible. Intergroup relations between macaques on Shodoshima Island, however, appear exceptional. We show that neighboring groups of Shodoshima monkeys spent 32.8% of their active time in proximity to (<100 m) and even foraged simultaneously at the same provisioning site with another group. The average duration and rate of intergroup encounters at Shodoshima (59.8 min, 0.33 times/hour, n=269) were approximately ten times longer and 16.5 times more frequent than those at Jigokudani (6.1 min, 0.02 times/hour, n=14). Since both populations have similar provisioning and ecological conditions, such variation cannot be explained by the socioecology model alone. Compared with other populations of Japanese macaques, intergroup relations of Shodoshima monkeys are also characterized by more frequent neutral encounters, less frequent agonistic encounters, more frequent unsuccessful displacement, a lower intensity of aggression, and more frequent counter-aggression between groups. These characteristics suggest that intergroup relationships on Shodoshima Island are more tolerant than those in other Japanese macaque populations. This study reveals considerable differences in intergroup encounters within local populations of Japanese macaques living in similar environments, and emphasizes the role of social factors in such intra-specific variation.