Greetings promote cooperation and reinforce social bonds among spotted hyaenas

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.