Oxytocin increases after affiliative interactions in male Barbary macaques

Abstract

Mammals living in stable social groups often mitigate the costs of group living through the formation of social bonds and cooperative relationships. The neuropeptide hormone oxytocin (OT) has been proposed to promote both bonding and cooperation although only a limited number of studies have investigated this under natural conditions. Our aim was to assess the role of OT in bonding and cooperation in male Barbary macaques (Macaca sylvanus). First we tested for an effect of affiliation - grooming and triadic male-infant-male interactions - with bond and non-bond partners on urinary OT levels. Secondly we aimed to test whether grooming interactions (and thus increased OT levels) increase a males general propensity to cooperate in polyadic conflicts. We collected behavioral data via full-day focal animal protocols on 14 adult males and measured endogenous OT levels from 139 urine samples collected after affiliation and non-social control periods. Urinary OT levels were higher after grooming with any partner. By contrast, OT levels after male-infant-male interactions with any partner or with bond partners were not different from controls but were higher after interactions with non-bond partners. Previous grooming did not increase the likelihood of males to support others in conflicts. Collectively, our results support research indicating that OT is involved in the regulation of adult social bonds, including in non-reproductive contexts. However, our male-infant-male interaction results go against previous studies suggesting that it is affiliation with bond rather than non-bond partners that trigger the release of OT. Alternatively, OT levels may have been elevated prior to male-infant-male interactions thus facilitating interaction between non-bond partners. The lack of an association of grooming (and by extension increased OT levels) and subsequent support speaks against an OT linked increase in the general propensity to cooperate, yet further studies are needed for a more direct test including the possibility of partner-specific contingent cooperation.