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

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.

Stable and fluctuating social preferences and implications for cooperation among female bonobos at LuiKotale, Salonga National Park, DRC.

OBJECTIVES: Female bonobos (Pan paniscus) are characterized as highly affiliative and cooperative, but few studies have quantified the strength and stability of female intra-sexual relationships or explored how variation in social relationships influences cooperation. We measure female social preferences, identify causes of variation in preferences, and test whether variation in social preferences predicts food sharing or coalitionary support. METHODS: Data were collected over 3 years from females in the Bompusa community at LuiKotale, DRC. We measured genetic relatedness and constructed social preference indices for party association, proximity, grooming, GG-rubbing and aggression. We identified preferred social partners based on permutation tests and measured stability using Mantel tests. We used factor analysis to identify inter-relationships between preference indices and used LMMs to test whether variation in social preferences was explained by relatedness, rank differences, having dependent young or co-residency time. We used GLMMs to test whether variation in social preferences predicted food sharing or coalitionary support. RESULTS: All females had preferred non-kin partners for proximity, grooming or GG-rubbing, but only grooming preferences were stable across years. Association indices were higher among lactating females, and aggression was lower among females with longer co-residency times. The factor analysis identified one factor, representing proximity and GG-rubbing preferences, labeled behavioral coordination. Dyads with higher levels of behavioral coordination were more likely to share food. CONCLUSIONS: Female bonobos exhibit stable, differentiated grooming relationships outside of kinship and philopatry. Females also exhibit flexible proximity and GG-rubbing preferences that may facilitate cooperation with a wider range of social partners.

Partner choice in genito-genital rubbing among female bonobos (Pan paniscus) is highly dependent on physical proximity.

Sociosexual interactions of non-human primates have multiple functions, and information on partner choice could help us to determine the major purpose of these behaviors. Female bonobos (Pan paniscus) frequently engage in genito-genital (GG) rubbing, which is categorized as a sociosexual behavior. The functions of GG rubbing may vary across allopatric bonobo populations, especially in relation to its use in social bonding. Thus, we aimed to examine the use of GG rubbing in the formation and maintenance of social bonds by examining partner choice in this context in the habituated bonobo population at Wamba, Democratic Republic of the Congo. We examined the effect of female age (and correlated dominance rank) on the proportion of solicited GG rubbing, and the effects of age difference, proximity index, and grooming index on the successful GG rubbing occurrences. Our results showed that female age significantly affected the proportion of solicited GG rubbing, indicating that older and higher-ranking females solicited this activity more frequently. Individuals of female-female dyads who were close in age and dominance rank frequently engaged in GG rubbing. The more the females in a dyad were in physical proximity, the more they engaged in GG rubbing. No correlation was observed between grooming and GG rubbing. These results indicate that partner choice in GG rubbing is highly dependent on physical proximity, and suggest that characteristics of female gregariousness might be important with respect to this choice among bonobos.