Social Rank or Social Bonds: Which one Facilitates Coalition Formation in Male Tibetan Macaques?

SIMPLE SUMMARY: The social behavior and the evolution of non-human primates have always been a focus of animal behavior. In this study, we determined that male Tibetan macaques (Macaca thibetana) formed a strong social bond, and there were two mechanisms (social bond and social rank) to promote the formation of coalitions. This study provides a new perspective for understanding the maintenance mechanism of social stability in non-human primates under competitive environments. ABSTRACT: Previous studies have shown that fierce competition could promote male coalitions. There are two ways for males to choose their allies in the conflict. The first is that supporters choose high-ranking individuals, and the second is that supporters choose affiliated individuals. It is necessary to clarify the factors for forming a coalition and the process of cooperation in primates with complex relationships and strict hierarchies. Thus, we conducted a study on a group of free-ranging Tibetan macaques (Macaca thibetana) in Anhui, China, and recorded the whole process of male macaques forming coalitions once agonistic support occurred. The results showed that a higher intensity of the social bond between males was associated with more frequent coalitions. Dominance rank also significantly influenced male coalitions, showing that high-ranking individuals allied more frequently. Moreover, males with longer residence times formed more stable coalitions. We suggest that male Tibetan macaques form a stable social relationship, where social bond and social rank both promote the formation of a coalition. This study provided some insights into the mechanism by which social cooperation develops in multi-male and multi-female groups.