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Coalitionary support in agonistic interactions represents cooperation because intervening in a fight is potentially costly to the donor of support but benefits the recipient. Here, we first review the characteristics of, and evolutionary forces favoring, intragroup coalitions in 49 species and find that patterns of intragroup coalition formation are remarkably similar between primates and nonprimates. We then test hypotheses suggesting kin selection, reciprocal altruism, and direct benefits as adaptive explanations for coalitionary interventions among adult female spotted hyenas (Crocuta crocuta) belonging to a large social group in Kenya. As predicted by kin selection theory, females supported close kin most often, and the density (connectedness) of cooperation networks increased with genetic relatedness. Nevertheless, kinship failed to protect females from coalitionary attacks. We found no evidence of enduring alliances based on reciprocal support among unrelated adult females. Instead, donors generally minimized costs to themselves, intervening most often during low-intensity fights and when feeding opportunities were unavailable. Females also gained direct benefits from directing coalitionary attacks toward subordinates. Finally, females monitored the number of dominant bystanders in the "audience" at fights and modified their level of cooperation based on this knowledge. Overall, hyenas made flexible decisions regarding whether or not to intervene in fights, modifying their tendency to cooperate based on multiple types of information about their immediate social and ecological environments. Taken together, these findings indicate that the combined evolutionary forces of kin selection and direct benefits derived from reinforcing the status quo drive coalitionary interventions among adult female spotted hyenas. Copyright 2010, Oxford University Press.

When social partners vary in their relative value, individuals should theoretically initiate partnerships with conspecifics of the highest value. Here, we tested this prediction in a wild population of spotted hyenas (Crocuta crocuta). Crocuta live in complex, fission–fusion societies structured by dominance hierarchies in which individuals vary greatly in their value as social companions. Because patterns of association among Crocuta reflect social preferences, we calculated association indices (AIs) to examine how social rank influences intrasexual partner choice among unrelated adults of both sexes. The highest-ranking individuals were generally most gregarious in both sexes. Females associated most often with dominant and adjacent-ranking females. Females joined subgroups based on the presence of particular conspecifics such that subordinates joined focal females at higher rates than did dominants. Dominants benefit from associations with subordinates by enjoying priority of access to resources obtained and defended by multiple group members, but the benefits of these associations to subordinates are unknown. To investigate this, we tested three hypotheses suggesting how subordinates might benefit from rank-related partner choice among unrelated females. We found that subordinates who initiated group formation benefited by gaining social and feeding tolerance from dominants. However, rates at which dominants provided coalitionary support to subordinates did not vary with AIs. Overall, our data resemble those documenting patterns of association among cercopithecine primates. We consider our results in light of optimal reproductive skew theory, Seyfarth’s rank attractiveness model, and biological market theory. Our data are more consistent with the predictions of Seyfarth’s model and of biological market theory than with those of skew theory.