Abstract
Fight interference data from a group of freeβranging rhesus monkeys at Cayo Santiago were used to test hypotheses about the evolution and importance of altruism. Both females and natal males (those born in the group) aided close relatives more than distant relatives and aided distant relatives more than nonβrelatives. This result is consistent with that predicted by the theory of kin selection. However, it was found that female aid to nonβrelatives involved higher risk and greater success than female aid to relatives. This is not consistent with kin selection or with any strictly genetic explanation for altruism. Natal and nonβnatal males also did not conform completely to patterns suggested by strictly genetic explanations. Overall, the findings suggest that the survival value and thus evolution of interference behavior involves factors acting through both the social and genetic levels of organization. Rhesus monkeys apparently use interference as a means of establishing social bonds and ensuring group membership. For a macaque this membership is critical since the social group is the context for most behavior. Thus, an animal not well integrated might be unable to ever reach the point of mating or successfully raising offspring.