Multiple Motivations: Agonistic Coalitions and Interventions in Blue Monkeys

ABSTRACT Primates are known for forming agonistic coalitions, but most data come from species in which agonism occurs frequently and rank predicts fitness. We analyzed coalitions and interventions in wild blue monkeys ( Cercopithecus mitis ), in which both agonism and third‐party involvement are relatively rare, and in which rank does not predict fitness. Data came from a long‐term study in the Kakamega Forest, Kenya, spanning 12 years and 12 groups. Intervening animals both supported winners and defended losers, and coalition partners nearly always prevailed over their opponent. Adult females were joiners and juveniles were coalition‐recipients disproportionately, while opponents were disproportionately adults, especially males. A multivariate analysis confirmed these patterns and also showed that joiners were most likely to support the original contestant who was winning (vs. unclear outcome or losing) and the one to whom they were more closely related. A subset of the data showed higher odds of joining the higher‐ versus lower‐ranking original opponent. In high‐risk interventions (coalition recipient losing, joiner smaller than opponent), the preference for more related opponents was magnified. Blue monkeys intervening in agonistic disputes appear to take sides in ways that minimize costs by supporting the winner, while maximizing inclusive fitness benefits by preferring the more closely related contestant, especially when intervention is risky. Their additional tendencies to support young individuals versus older ones, all else equal, suggest an additional motivation to protect vulnerable group‐mates. Coalitions of smaller‐bodied groupmates may contribute to the social peripheralization of the group's adult male.