Personality heterophily and friendship as drivers for successful cooperation

Cooperation is widespread and arguably a pivotal evolutionary force in maintaining animal societies. Yet, proximately, what underlying motivators drive individuals to cooperate remains relatively unclear. Since ‘free-riders’ can exploit the benefits by cheating, selecting the right partner is paramount. Such decision rules need not be based on complex calculations and can be driven by cognitively less-demanding mechanisms, like social relationships (e.g., kinship, non-kin friendships, dyadic tolerance), social status (e.g., dominance hierarchies) and personalities (social and non-social traits); however, holistic evidence related to those mechanisms is scarce. Using the classical ‘loose-string paradigm’, we tested cooperative tendencies of a hierarchical primate, the long-tailed macaque (Macaca fascicularis). We studied three groups (n=32) in social settings, allowing free partner choice. We supplemented cooperation with observational and experimental data on social relationships, dominance hierarchies, and personality. Friendship and dissimilarity in a non-social ‘exploration’ personality trait predicted the likelihood of cooperative dyad formation. Furthermore, the magnitude of cooperative success was positively associated with friendship, low rank-distance, and dissimilarity in an ‘activity-sociability’ personality trait. Kinship did not affect cooperation. While some findings align with prior studies, the evidence of (non-social)personality heterophily promoting cooperation may deepen our understanding of the proximate mechanisms and, broadly, the evolution of cooperation.