Assessing the Occurrence of Sexual Segregation in Spider Monkeys (Ateles geoffroyi yucatanensis), Its Mechanisms and Function

Sexual segregation is a recognized dimension of the socioecology of many vertebrates, but it has not been systematically examined in primates. We investigated temporal patterns of sexual segregation in spider monkeys (Ateles geoffroyi yucatanensis) using a test that distinguishes sexual segregation from aggregation and random association between the sexes. We further investigated how sexual segregation varies over time as a function of food availability, and then tested other possible factors that might be causally linked to sexual segregation in spider monkeys. We predicted that male philopatry and cooperative territorial defence leads to sexual dimorphism in behavior, which in turn creates different optimal energetic requirements for males and females as reflected in differing activity budgets and diet. We investigated sexual segregation in a group of 33-35 spider monkeys at Runaway Creek Nature Reserve in Belize over 23 mo in 2008-2009. We used the sex compositions of subgroups recorded in scan samples to test the occurrence of sexual segregation at monthly and biweekly time scales.We found that males and females were significantly segregated in 15 out of the 23 mo of the study, and that periods of nonsegregation coincided with months of low food availability. The sexes differed significantly in activity and diet; males spent more time traveling, and less time resting and feeding than females, and they had a higher proportion of ripe fruits in their diets than did females. We propose that sexual segregation in spider monkeys is primarily a form of social segregation that results from males and females pursuing optimal dietary and behavioral strategies to satisfy sex-specific energetic demands. We further suggest that sexual segregation represents an important constraint on fission-fusion dynamics that should be considered when assessing the potential for variability in subgroup composition.