Sex differences and social organization in free-ranging spider monkeys (Ateles geoffroyi)

Abstract

Several aspects of the social system of spider monkeys remain poorly understood in spite of previous studies of their behavior. Our work investigates sex differences of adultAteles geoffroyi to develop a better understanding of their social organization. A six-month field study of this species in Guatemala showed that adult males were both more aggressive and more socially cohesive than females, as well as more territorial. Adult females were more vocal, more submissive, more nonsocial, and more dispersed than adult males. Males were more likely to associate affinitively with other males than with females, and to direct their aggressive behaviors at females rather than males. Spider monkey society was found to be sex-segregated; males traveling and interacting in all-male subgroups, while females travel alone or with offspring. These findings are used, in conjunction with other evidence, to draw inferences about the dynamics of theAteles social system, and to derive an explanation for the evolution of spider monkey social organization. The frugivorous diet ofAteles is linked to the dispersion females and to the cohesion of related adult males, who form cooperative territorial groups, in which the low level of male-male competition is related to the absence of sexual dimorphism. Spider monkeys provide an illuminating contrast to the general primate model, derived from Old World monkeys, which links sexual dimorphism in size to sex differences in behavior, and ultimately to sexual selection.