Oxytocin increases during fur-rubbing regardless of level of social contact in tufted capuchin monkeys.

Social interactions induce oxytocin release in many social species, suggesting that oxytocin is a critical part of social bonding among individuals. However, oxytocin also increases as a result of physical contact and stimulation, making it unclear which features of affiliative behaviors (for instance, social interaction or physical contact with a conspecific) drive the oxytocin increase observed after engaging in these behaviors. We attempted to tease this apart by studying the differential effect of social interaction, visual coordination with a conspecific, and physical stimulation during the fur-rubbing behavior of tufted capuchin monkeys (Sapajus [Cebus] apella), which often involves social contact with groupmates but is in some cases performed alone. We induced fur-rubbing by providing onions under three conditions: when capuchins had physical access to their social group and fur-rubbed in contact with groupmates (social condition), when capuchins were separated from their social group but could still see them fur-rub (visual coordination), and when capuchins were physically and visually separated from their groupmates (physical stimulation only). We assessed urinary oxytocin in these three conditions and compared them to a control condition in which apples were provided and no fur-rubbing was observed. Capuchins fur-rubbed for less time when they could not see their groupmates, but fur-rubbing increased urinary oxytocin above the control condition in all three fur-rubbing conditions equally, suggesting that the physical stimulation derived from fur-rubbing was the most important driver of oxytocin increase. These results support a model in which physical stimulation is an important factor in the relationship between oxytocin and at least some behaviors, suggesting that oxytocin increase alone is not necessarily indicative of a social influence on behavior. Future work is needed to determine the contexts in which social factors do impact oxytocin, and whether the downstream behaviors are the same for socially and nonsocially induced oxytocin release.