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 with 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 between partners with compatible behavioural tendencies. While evidence for social bonding in animals is accumulating, far less is known about its predictors. Here, personality homophily effects on the formation and maintenance of social bonds are shown in 24 wild male Assamese macaques at Phu Khieo Wildlife Sanctuary, Thailand. Dyadic bond strength increased with increased similarity in the trait Gregariousness (i.e. frequent and diverse neighbours within 5 m and pronounced social tolerance, as shown by 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 Gregariousness traits in a subset of immigrating males that had to form new bonds. Gregariousness 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 nonhuman animals. The main selective advantage of personality similarity in animal social bonds may result from a more reliable cooperation between individuals with similar cooperative behavioural tendencies. (C) 2019 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.