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.
Highlights
Same-sex sexual behavior, referring to sexual solicitations, mounting or other genital contact between same-sex partners, occurs across a wide range of animal taxa under free-ranging conditions, though with much variation in form and frequency
In the current study we investigate behavioral and physiological outcomes of same-sex sexual behavior in female bonobos in comparison with outcomes related to sexual interactions with males, the majority of which are unlikely to result in insemination (Douglas et al, 2016)
The relative number of dyads of each sex combination that were observed having sex at least once during the study period differed significantly from expected. This was due to more femalefemale dyads and fewer male-male dyads engaging in sexual interactions than predicted, based on their relative frequencies in the community
Summary
Same-sex sexual behavior, referring to sexual solicitations, mounting or other genital contact between same-sex partners, occurs across a wide range of animal taxa under free-ranging conditions, though with much variation in form and frequency (reviewed in: Bailey and Zuk, 2009). There are other taxa where a subset of the population exhibits same-sex preferences even when viable opposite-sex partners are available (e.g. Japanese macaques (Macaca fuscata), Vasey and Duckworth, 2006). The extent of variation across species in the contexts that elicit same-sex sexual behavior and how widespread it is within a population suggest that its functions may be diverse and species-specific. The adaptive hypotheses that have been proposed to explain same-sex sexual behavior more generally (reviewed in Bailey and Zuk, 2009) include: i) Establishing or maintaining cooperative social
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