Theoretical models about female relationships within primate social groups hypothesise that food abundance and distribution are important factors determining the variation of patterns observed among species and populations. Despite some common premises, models formulated by van Schaik (1989) and Sterck et al. (1997) and by Isbell (1991) differ with respect to the importance of predation risk, the co-variation of contest and scramble competition and causes of female dispersal. In this study, data from a population of Cebus apella nigritus from Brazilian Atlantic Forest are analysed using predictions from these models. Competition among females, both within and between groups, is strong and related to food abundance and distribution. Females can transfer between groups, as well as males. Female dispersal is related to a significant reduction in per capita energy intake by group foragers during fruit scarcity periods. The data from this study are not conclusive about the importance of predation in causing variation of female relationships but favour the assumption from van Schaik and Sterck et al. that contest and scramble competition within and between groups can vary independently; and also favour the formulation from Isbell & Van Vuren (1996) on female dispersal. The exact pattern of female social relationships is not sufficiently explained by ecological causes alone. Social benefits provided by the dominant male also seem to be important.
Socioecology considers that the features of food sources affect female social relationships in group-living species. Among primates, the tests of socioecological models are largely focused on Old World species and do not evaluate if the use of feeding tools affects the competitive regime over food and females’ relations in wild populations. We studied female social relationships among a wild population of bearded capuchins monkeys (Sapajus libidinosus) that use percussive tools (stones) to crack encased foods, in a semi-arid habitat in Brazil. Females fed mainly on clumped, high quality resources, indicating that the habitat provides a high quality diet year-round. Females experienced contest competition within and between-groups. As predicted by socioecological models, females’ social relationships were characterized by philopatry, linear dominance hierarchies, coalitions, and tolerance in feeding bouts. Females spent a small proportion of their feeding time using tools. Nevertheless, tool sites generated high rates of contest competition and lower indices of tolerance among females. Although the social structure of our study population did not differ significantly from the pattern observed in wild populations of Sapajus that do not use tools, tool use increased within-group contest competition and apparently contributed to the linearity of the dominance hierarchies established among females. We predict that when tool use results in usurpable food resources, it will increase contest competition within group-living species.