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

Paternity alone does not predict long-term investment in juveniles by male baboons

Adult male chacma baboons (Papio hamadryas ursinus) form preferential associations, or friendships, with particular lactating females. Males exhibit high levels of affiliative contact with their friends’ infants and defend them from potentially infanticidal attacks (Palombit et al. 1997). Little is known about males’ associations with juveniles once they have passed the period of infanticidal risk. We conducted an observational, experimental, and genetic study of adult male and juvenile chacma baboons in the Moremi Reserve, Botswana. We identified preferential associations between males and juveniles and used behavioral data and a playback experiment to explore whether those associations have potential fitness benefits for juveniles. We determined whether males preferentially invest in care of their own offspring. We also determined how often males invest in care of their former friends’ offspring. The majority of juveniles exhibited preferential associations with one or two males, who had almost always been their mother’s friend during infancy. However, in only a subset of these relationships was the male the actual father, in part because many fathers died or disappeared before their offspring were weaned. Male caretakers intervened on behalf of their juvenile associates in social conflicts more often than they intervened on behalf of unconnected juveniles, and they did not appear to differentiate between genetic offspring and unrelated associates. Playbacks of juveniles’ distress calls elicited a stronger response from their caretakers than from control males. Chacma males may provide care to unrelated offspring of former friends because the costs associated with such care are low compared with the potentially high fitness costs of refusing aid to a juvenile who is a possible offspring

Female chacma baboons form strong, equitable, and enduring social bonds

Analyses of the pattern of associations, social interactions, coalitions, and aggression among chacma baboons (Papio hamadryas ursinus) in the Okavango Delta of Botswana over a 16-year period indicate that adult females form close, equitable, supportive, and enduring social relationships. They show strong and stable preferences for close kin, particularly their own mothers and daughters. Females also form strong attachments to unrelated females who are close to their own age and who are likely to be paternal half-sisters. Although absolute rates of aggression among kin are as high as rates of aggression among nonkin, females are more tolerant of close relatives than they are of others with whom they have comparable amounts of contact. These findings complement previous work which indicates that the strength of social bonds enhances the fitness of females in this population and support findings about the structure and function of social bonds in other primate groups

Results of a GLMM modeling the proportion of solicitations on reunion days that led to socio-sexual interactions.

<p>Solicitations by joiners were almost always accepted, while solicitations by residents were more likely to be rejected (P = 0.002). In addition, solicitations by higher-ranking females were more likely to be accepted (P = 0.014). Box plots as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127305#pone.0127305.g001" target="_blank">Fig 1</a>. Dashed lines indicate the expected values based on the corresponding model.</p

Proportion of non-reunion and reunion days during which aggression, grooming and socio-sexual solicitations occurred between pairs.

<p>Box plots indicate medians and 25–75% inter-quartile ranges. Dashed lines indicate the expected values on non-reunion and reunion days based on the corresponding model.</p

Description of female subjects, the number of urine samples provided and average daily cortisol levels on non-reunion and reunion days.

<p>Description of female subjects, the number of urine samples provided and average daily cortisol levels on non-reunion and reunion days.</p

Results of a GLMM modeling the probability of receiving socio-sexual solicitations on reunion days.

<p>The x-axis is shown on the square root scale. Larger symbols indicate more pairs with the same proportional score. Regression lines indicate expected probabilities of receiving solicitations based on the model. Females were more likely to receive solicitations when they had been separated from the solicitor for longer (P = 0.004). In addition, there was a tendency for joiners to receive more solicitations than residents (P = 0.08). For full model results refer to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127305#pone.0127305.t002" target="_blank">Table 2</a>.</p