Squirrel Monkeys: A Test of Primate Social Evolution Theory

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The 200 species that comprise the Order Primates exhibit extreme diversity in social organization compared to other mammalian orders. Current theory suggests that the extent of competition for food within a primate species is the single best predictor of that species' social organization. With greater food competition, for example, more rigid dominance hierarchies, greater female versus male dominance, closer affiliations among females, and little female dispersal are expected.. Squirrel monkeys, a species distributed throughout the Neotropics, offer a uniquely powerful test of the importance of food competition in the evolution of primate social organization. No other primate taxon has such widely divergent population differences in social organization. Two previous field studies were entirely consistent with predictions. Costa Rican squirrel monkeys, with negligible within-troop food competition, exhibit almost non-existent female social bonds and females emigrate. In contrast, food competition is high among Peruvian squirrel monkeys, females form close social bonds, and females have never been known to emigrate. My preliminary observations in Suriname indicate yet a third form of social organization with strong male dominance of other troop members, and little female affiliation. Yet the extremely intense food competition that exists in this population is in apparent contradiction to theory. If confirmed, the squirrel monkey social system of Suriname provides a powerful test of the hypothesis of close linkage between food competition an social behavior in primates, and provides an important window to our understanding of the mechanisms that affect and structure primate social systems.