Within the field of behavioral ecology, investigating the range of behavioral and ecological variability within species is crucial for understanding broader patterns of social adaptation while minimizing confounding effects of phylogeny. Variability may be seen by studying the social system of one species in several different habitats. Differences in social systems within the same species may be explained by differences in food availability and distribution across different habitats. I investigated social system variation of spider monkeys ( Ateles spp.) by studying them in a productive, lowland wet forest in Costa Rica .

Spider monkeys are one of only several mammalian species that has a fission-fusion social system, in which the entire group rarely coalesces but instead is split into subgroups with fluid, variable membership. These monkeys specialize on eating ripe fruit, and their social system has been suggested to be an adaptation for reducing feeding competition within the group. How spider monkeys respond to fluctuations in their environment, and how their responses vary across a wide range of habitats, is informative for the broad scope of behavioral ecology. The fluid nature of spider monkeys' fission-fusion social system allows sensitive detection of how such variables as diet, food availability and distribution, and subgroup size and composition interact. No long-term studies exist of spider monkeys living in a large, continuous tract of productive, diverse, lowland wet forest where they also live with a complete suite of potential predators.

The objectives of this project were the following: (1) to test hypotheses relating diet and foraging ecology to the social structure and organization of spider monkeys living in a diverse, wet forest with a decreased dry season, (2) to gain a clearer view of this primate's variation in behavior and ecology by comparing the results of this study with those of published studies of spider monkeys from other sites, and (3) to provide the first investigation of the relationships between subgroup changes and social and ecological variables in a large group of spider monkeys.

Funding for this project was provided by the National Science Foundation (Doctoral Dissertation Improvement Grant #0233248), Wenner-Gren Foundation for Anthropological Research, L. S. B. Leakey Foundation, and the American Society of Primatologists.

Sept 2005