Agonistic support, affiliation and kinship: A study of social relationships among adult female and juvenile rhesus macaques

Patterns of agonistic support and affiliation among adult female and juvenile rhesus macaques were studied on the island of Cayo Santiago, Puerto Rico. It was found that, when entering coalitions on behalf of kin, individuals tended to provide support when recipients were most vulnerable and in need of aid. When individuals aided nonkin, different patterns resulted depending on the age/sex class of the opponent: when individuals intervened against adult male opponents, they tended to do so when recipients were most vulnerable and in need of support. However, when individuals intervened against adult female or juvenile opponents, they did so when recipients were least vulnerable and the risk to the supporter was low. Individuals formed coalitions at highest rates and highest risk with close kin, at intermediate rates and intermediate risk with intermediate/distant kin, and at lowest rates and lowest risk with nonkin. Findings suggest that individuals distinguish among several classes of kinship when choosing how frequently, with whom, and at what level of risk to form coalitions; a threshold hypothesis of kinship is therefore rejected in this population in favor of a more complex, graduated pattern of kin relationships. Kin coalitions are likely to have evolved primarily through kin selection; nonkin coalitions probably evolved as a cooperative strategy to reinforce rank and deter male aggression. Reciprocal altruism is unlikely to be a mechanism for the evolution of coalitions. At least two models have been proposed which suggest that females cultivate coalition partners through grooming and other affiliative behaviors, a model of affiliation-for-support, also called the rank-related attractiveness model, proposed by Seyfarth (1976, 1977), and a model based on a similarity principle, proposed by de Waal and Luttrell (1986). Neither model was supported in this population. An alternative model is proposed in which animals primarily maintain close relationships with kin, with whom they groom, contact-sit, remain in proximity, and support in coalitions. High rates of grooming directed towards high-ranking females is explained in terms of appeasement and avoidance of aggression rather than as an attempt to cultivate coalition partners