Group fission among wild toque macaques as a consequence of female resource competition and environmental stress

Abstract

At Polonnaruwa, Sri Lanka, four out of 29, groups of toque macaques, Macaca sinica, divided in a period of 16 years. Temporary peripheral subgroups of varying sizes and compositions preceded fission by 9–40 months. Fission crystallized within a month through an increase and stabilization of subgroup membership and permanent division. All members in the newly seceded groups had been frequent participants in pre-fission subgroups, and belonged to subordinate matrilineages. Subgroups, and hence group divisions, were initiated by cores of mutually loyal females and occurred mostly along kinship lines. In the year of fissions, the rate of change in female dominance relations was significantly greater among groups that divided than among those that did not. It is hypothesized that low-ranking females secede to form new groups when the costs, especially of intragroup competition for food resources, outweigh the benefits of group membership. Such seceding females were easily available and familiar mates for group males that had recently lost rank. Final division, therefore, resulted from a coalition of subordinate females and males acting according to their respective interests. It was triggered in this population by rapid growth of some groups to large size and by environmental stress (the reduction and fragmentation of food resources caused by drought and a cyclone), which accentuated the costs of resource competition. Male aggression, such as infanticide, which negatively affects female fitness, might also have contributed to one group fission.