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Societies characterized by fission–fusion dynamics consist of subgroups that frequently change in size and composition. Although this flexible lifestyle permits individuals to reduce conflicts of interest, it simultaneously imposes a unique set of challenges on group members that are regularly subject to prolonged separation. Theory predicts animals should evolve ritualized and risky displays to quickly update relationships at reunions. Here we investigated the function of nonconciliatory greetings among adult female spotted hyaenas, Crocuta crocuta, belonging to a single, large female-dominated social group in Kenya. We tested three hypotheses forwarded to explain the occurrence of these multimodal signals: formal submission, tension reduction and social bonding hypotheses. In contrast to predictions of the formal submission hypothesis, rank distance and relative rank were excluded from our best model predicting greetings among adult females. Moreover, directional consistency of greetings was low (0.65) compared to that of submissive behaviours during dyadic agonistic interactions (0.97). Instead, our data revealed that adult females greeted coalition partners and close associates, including kin, most often per opportunity, and they did so in neutral contexts more frequently than in situations characterized by social tension. Although these findings are in direct contrast to the predictions of the tension reduction and formal submission hypotheses, they strongly support the social bonding hypothesis. Importantly, the immediate act of greeting promoted cooperation among allies during intragroup and intergroup coalition formation. Thus, these risky, multimodal signals permit hyaenas to effectively communicate cooperative affiliations within a continuously shifting social milieu.

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.