An expanded test of the ecological model of primate social evolution: competitive regimes and female bonding in three species of squirrel monkeys (Saimiri oerstedii, S. boliviensis and S. sciureus)

Abstract

Two critical premises underlie prevalent interpretations of the ecological basis of variation among female primate social behavior. The first is that food distribution affects competitive regimes for food experienced by females. This leads, in turn, to the second premise that these competitive regimes generate predictable patterns of female social relationships and residence. Long-term field studies of S. oerstedii at Corcovado, Costa Rica and S. boliviensis at Manu, Peru (Mitchell et al. , 1991) provide what is widely considered as the most powerful support to date for such an ecological model. The data from these two squirrel monkey field studies are entirely consistent with the various incarnations of the van Schaik (1989) and associates' models linking a cascade of predictions on within- and between-sex social bonds and dispersal patterns to the presence or absence of significant within-group contest competition for food. A key premise of these models is that females tolerate the significant costs of within-group food competition because of the advantages group living affords in reduction of predation risk.In the current study, comparable ecological and behavioral data from long-term field observations of known individuals of S. sciureus at Raleighvallen, Suriname are used to expand the test of ecological models to three species. In all three sites female within-group direct competition regimes clearly follow from the distribution of fruit patches. In Suriname, however, S. sciureus females exhibit weak social bonds and rarely form coalitions with other females despite frequent, intense, even vicious within-group direct food competition. Although all males appear to emigrate from their natal troops, some females do as well, probably exceeding about 10% of the female group membership annually. Yet the lack of consistency with the expectations of the ecological model is only superficial. In fact, S. sciureus is the exception that corroborates the general robustness of predictions made in the ecological model. The fruit patches defended by S. sciureus, although small, are usually dense, extremely rewarding and easily defended by the individual with the greatest resource holding potential. Female coalitions to defend fruit patches would not be stable, as one female can ultimately prevail in monopolizing the resource. The lack of reliable foraging benefits to females who form coalitions and the consequent lack of strong female social bonds among female S. sciureus are completely concordant with the logical rationale underlying the ecological model.