Personality Homophily Affects Male Social Bonding in Wild Assamese Macaques (macaca Assamensis)

Animal social bonds are defined as stable, equitable and strong affiliative and cooperative relationships similar to human friendships. Just as human friendships, social bonds are thought to function as alliances that generate adaptive benefits via support in critical situations. In humans, similarity in many sociodemographic, behavioural and intrapersonal characteristics leads to trust and is predictive of friendships. Specifically, personality homophily, that is the tendency of individuals to form social bonds with others who have a similar personality, may increase predictability and facilitate trust and reciprocity among partners with compatible behavioural tendencies. While evidence for social bonding in nonhumans is accumulating, far less is known about its predictors. Here, personality homophily effects on the formation and maintenance of social bonds are shown in twenty-four wild male Assamese macaques (Macaca assamensis), at Phu Khieo Wildlife Sanctuary, Thailand. Dyadic bond strength increased with increased similarity in the trait Connectedness (i.e. frequent and diverse neighbours in 5m proximity and pronounced social tolerance, as high rates of friendly approaches to and by others). To differentiate whether homophily indeed predicted bond formation or whether bonded males’ personalities became more similar over time, we tested the stability of the connectedness traits in a subset of immigrating males that had to form new bonds. Connectedness in these males remained stable suggesting that males do not adapt their personality to their partner. Our results support the idea of a shared evolutionary origin of homophily as a partner choice strategy in human and non-human animals. The main selective advantage of personality similarity in animal social bonds may result from a more reliable cooperation among individuals with similar cooperative behavioural tendencies.