Berg, M. R., Heagerty, A., Coleman, K. 2019. Oxytocin and pair compatibility in adult male rhesus macaques (Macaca mulatta). American Journal of Primatology 81(8), e23031.

Pair housing is considered one of the best ways of promoting psychological wellbeing for caged macaques. However, incompatible partnerships can result in stress or aggression. Though previous studies have analyzed the role of variables such as age, weight, gender, and temperament on pair compatibility, few have examined the relationship between physiological parameters and pair compatibility. Oxytocin is known to promote prosocial nonsexual behavior in various primate species and may serve as an indicator of pair compatibility. In this study, we examined the association between peripheral oxytocin levels and prosocial behaviors in isosexual pairs of male rhesus macaques. We hypothesized that animals that demonstrated high levels of prosocial behaviors would have higher oxytocin levels than those showing low levels of the behavior. In addition, to elucidate the relationship between oxytocin and compatibility, we compared peripheral oxytocin between the highly affiliative animals and single‐housed males identified as having multiple unsuccessful pair attempts with multiple partners. We collected plasma oxytocin on 40 pairs of monkeys that had lived together for at least 1 month and 20 single‐housed animals. Further, we simultaneously collected behavioral data on the pairs, recording prosocial interactions (e.g., groom, play). Oxytocin varied among individuals, but was highly correlated between members of a pair (r= 0.58, p< .001). Additionally, prosocial behavior was positively correlated with plasma oxytocin (r= 0.38, p< .02). However, contrary to our expectations, oxytocin did not differ between single and highly affiliative pair‐housed animals (F(1,38)= 0.71, p= .40). Our results suggest that oxytocin may be associated with the quality of isosexual pairs of male macaques. More work is needed to determine the nature of this relationship.