The cooperative sex: Sexual interactions among female bonobos are linked to increases in oxytocin, proximity and coalitions

The cooperative sex: Sexual interactions among female bonobos are linked to increases in oxytocin, proximity and coalitions. Liza R. Moscovice, Martin Surbeck, Barbara Fruth, Gottfried Hohmann, Adrian Jaeggi, Tobias Deschner. Hormones and Behavior, September 10 2019, https://doi.org/10.1016/j.yhbeh.2019.104581

Highlights
•    Female bonobos have more frequent sex with female than male partners in feeding contexts.
•    Female dyads are more likely than inter-sexual dyads to remain close after sex.
•    Female urinary oxytocin levels increase following sex with females, but not with males.
•    Dyads that have more sex engage in more joint coalitionary aggression.
•    Sex among female bonobos may facilitate cooperation via oxytocinergic effects.

Abstract: In some species habitual same-sex sexual behavior co-occurs with high levels of intra-sexual alliance formation, suggesting that these behaviors may be linked. We tested for such a link by comparing behavioral and physiological outcomes of sex with unrelated same- and opposite-sex partners in female bonobos (Pan paniscus). We analyzed behavioral outcomes following 971 sexual events involving n = 19 female and n = 8 male adult and sub-adult members of a wild, habituated bonobo community. We additionally collected n = 143 urine samples before and after sexual interactions to non-invasively measure oxytocin (OT), which modulates female sexual behavior and facilitates cooperation in other species. The majority of sexual events (65%) consisted of female same-sex genito-genital rubbing (or GG-rubbing). Female dyads engaged in significantly more sexual interactions than did inter-sexual dyads, and females were more likely to remain within close proximity to their partners following GG-rubbing. Females also exhibited greater increases in urinary OT following GG-rubbing compared with copulations, indicating a physiological basis for increased motivation to cooperate among females. The frequency of coalitionary support among non-kin was positively predicted by the frequency of sexual interactions for female as well opposite-sex dyads, although coalitionary support tended to be more frequent among females. The emergence of habitual same-sex sexual behavior may have been an important step in the evolution of cooperation outside of kinship and pair-bonds in one of our closest phylogenetic relatives