Intergroup social dynamics of the Cayo Santiago rhesus (Macaca mulatta) with special reference to changes in group membership by males

Observations were made on the rhesus monkeys,Macaca mulatta living on the island of Cayo Santiago over a two-year period. Agonistic behaviors between members of different groups were the most common type of inter-group interaction, typically occurring in the vicinity of the feeders. Instances of social grooming and copulation between members of different groups were observed very infrequently.Changes in groups membership were made almost exclusively by males, particularly three-and four-year-old animals, and the changes occurred predominantly during the autumn breeding season. Peripheral all-male subgroups were preferred by males changing groups, perhaps because siblings or peer-group associates were to be found in those subgroups. The process of joining a new group by forming an affectional relationship with another male is described briefly. It is suggested that the phenomenon of group membership change by adult males is not an artifact of the island colony but its high frequency may be a consequence of the tremendously overcrowded conditions on the island.