Sociological study on the troop fission of wild

Abstract

During the period from June to July 1983, the Hanyama-A troop of wild non-provisioned Japanese monkeys on Yakushima Island began to show signs of troop fission. Adult females together with their infants and juveniles subdivided into two groups, the Hanyama-K group and Hanyama-M group. After the subdivision, all of the troop males were observed vacillating between these two female groups. During the mating season, non-troop males were also observed moving around the two female groups. After this mating season, one of these non-troop males was found to have entered and become the alpha male in one of the groups, while higher-ranking adult males of the original troop settled into the other group. Each fissioned group was strongly considered to be composed of either high-ranking matrilines or low-ranking matrilines as observed previously in provisioned troops. The dominance relation between the two fissioned groups indicated that dominance rank reversal between these two female kin groups must have occurred during the course of subdivision of the troop. However, different from most previous cases of troop fission, there was no indication that males ever participated in the subdivision of the original female group. This was disrupted not as a result of males' involvement, but only as a result of antagonism among females, which initiated the troop fission. The main factor which appeared to determine when and in which fission group males eventually settled was the competition between the troop males' coalition and non-troop males and their ability to monopolize females. The present process of troop fission suggests a dual strategy between males and females (Wrangham, 1979, 1980) even in the society of Japanese macaques.