Testing the effect of oxytocin on social grooming in bonobos

Abstract

Oxytocin has attracted research attention due to its role in promoting social bonding. One notable recent hypothesis is the biobehavioral feedback loop, which posits that the oxytocin system has evolved to support the formation and maintenance of social bonds through a positive feedback loop, where oxytocin promotes social behaviours which then cause oxytocin release themselves. In the two Pan species, humans’ closest relatives, oxytocin is known to be released following key behaviours related to social bonding, such as social grooming in chimpanzees and female-female sexual behaviour in bonobos. However, no experimental evidence has demonstrated that oxytocin promotes such socio-positive behaviours. To test this, we administered nebulized oxytocin or saline placebo to a group of female bonobos and subsequently observed the change in their gross behavior during free interaction. We found that bonobos groomed other group members significantly more frequently in the oxytocin compared to placebo condition. Other behavioural measures did not largely differ between conditions, except for a nonsignificant trend for reduction in abnormal regurgitation/reingestion behaviour. Overall, we found that oxytocin promoted socio-positive interaction in bonobos, providing support for the biobehavioural feedback loop hypothesis of oxytocin in bonobo social evolution.

Parochial cooperation in wild chimpanzees: a model to explain the evolution of parochial altruism

“…Research on the bonobo oxytocinergic system also reveals interesting insights on its implication in sociality and parochialism. In bonobos, captive experiments where oxytocin is intranasally administrated demonstrate that oxytocin promotes pro-social behaviour, with increased levels of grooming [ 179 ]. In wild bonobos, non-invasive endocrinological analyses [ 180 ] showed increases in urinary oxytocin following female–female sexual interactions, but not after inter-sexual interactions, and females that had more sexual interactions were more likely to engage in coalitionary aggression.…”

Oxytocin promotes species-relevant outgroup attention in bonobos and chimpanzees

“…In experiments administering nebulized OT to great apes, two studies found no significant effect of exogenous OT on chimpanzee social behaviour (Hall et al, 2019;Proctor et al, 2016) while one found an effect on social attention in bonobos and chimpanzees (Brooks et al, 2021a) and another found a positive effect of OT on social grooming when administered to whole subgroups of bonobos (Brooks et al, 2021b). Of the null results, one focused on observing natural social behaviour after giving OT or placebo to one individual at a time (Proctor et al, 2016), and the other measured responses in a token exchange task after OT or placebo was given to one or both individuals (Hall et al, 2019).…”