Fellatio among male sanctuary-living chimpanzees during a period of social tension

Fellatio among male sanctuary-living chimpanzees during a period of social tension. Authors: Jake S. Brooker et al. Behaviour, Dec 15 2020. https://doi.org/10.1163/1568539X-bja10053

Abstract: Same-sex sexual behaviour has been documented across the animal kingdom, and is thought to reflect and enhance dyadic cooperation and tolerance. For instance, same-sex fellatio — the reception of a partner’s penis into another’s mouth — has been reported in several mammalian species other than humans. Although same-sex sexual behaviour is observed in our close relatives, the chimpanzees, fellatio appears to be very rare — as yet there are no published reports clearly documenting its occurrence. At Chimfunshi Wildlife Orphanage in Zambia, we observed an instance of fellatio occurring during a post-conflict period between two adult male chimpanzees (born and mother-reared at the sanctuary) where one of the males was the victim. We discuss this event with respect to the putative functions of homosexual behaviour in great apes. Given its rarity in chimpanzees, this fellatio between adult males also highlights the apparent behavioural flexibility present in our close relatives.

Keywords: chimpanzee; fellatio; same-sex sexual behaviour; homosexual behaviour; sanctuary; behavioural flexibility; social tension

4. Discussion

In this report, we described an instance of same-sex fellatio occurring between two sanctuary-living adult male chimpanzees. Although fellatio has been observed in other mammals (Ogawa, 2006; Tan et al., 2009; Sergiel et al., 2014; Sugita, 2016), including captive/semi-captive bonobos (de Waal, 1988; Clay, personal observation) and chimpanzees (Heesen, personal communication) it has not yet been reported among adult chimpanzees during social tension, with two previous reports noting the occurrence of oral-genital contact among play in immature chimpanzees (Savage & Malick, 1977; Savage-Rumbaugh & Wilkerson, 1978). Moreover, while bonobos habitually engage in diverse same-sex sexual behaviour, fellatio appears to be rare and constrained to play contexts involving immatures (de Waal, 1988; Clay, personal observation). To our knowledge, fellatio has not been reported to occur in post-conflict affiliative interactions among either chimpanzees or bonobos. This observation thus provides relevant insights into the potential diversity of its functions in our closest living relatives.

Homosexual interactions, such as those reported here between Max and David, have been hypothesised to reinforce same-sex alliances and increase the propensity to support and cooperate (Kirkpatrick, 2000; Moscovice et al., 2019). Creating such connections may facilitate alliance formation and provide greater opportunities for future cooperation and support during agonism between non-kin. As a low-ranking chimpanzee, it is possible that Max’s request for fellatio in this case may have been driven by a motivation to associate with potential coalition partners, with genital contacts having been shown to facilitate coalitionary support in bonobos, the close relative of chimpanzees (Moscovice et al., 2019). As our closest phylogenetic relatives, studying genital contacts in chimpanzees, including fellatio, in specific contexts may provide clarity on the evolutionary origins of same-sex sexual behaviour within and between the sexes.

Our observation may also reflect fellatio serving a reassurance and alliance testing function by a subordinate male, akin to genital touches and mounting commonly observed between wild male chimpanzees during socially tense periods such as before boundary patrols and intergroup encounters (Wittig et al., 2016; Samuni et al., 2019). It has been proposed that engaging in intimate, risky behaviour such as fellatio can be used to test social relationships and tolerance (Kirkpatrick, 2000).

Chimpanzees are behaviourally innovative and spontaneously develop unique behaviour that may culturally transmit, even if such actions appear to lack adaptive benefits (van Leeuwen et al., 2014). As chimpanzees are not known to habitually engage in same-sex sexual interactions, Max’s repeated attempts to initiate genital contact for reassurance may also represent a behavioural innovation. Observing Max’s group in varying contexts would be necessary to detect whether he possesses a more general tendency for same-sex sexual interactions. Further, longitudinal investigations may show that idiosyncratic sexual behaviour, such as fellatio, is culturally transmitted over time.

Although the functions of fellatio remain to be explored in great apes, its initiation by a lower ranking to a dominant male in a socially tense context makes a novel contribution to the literature. Given the apparent overlaps between this behaviour and genital contacts occurring among the close sister species, the bonobos, systematically comparing sexual behaviours during periods of social tension between Pan would provide greater clarity on how sexual behaviours may be adapted and deployed to fortify and repair social relationships.