We observed 584 instances of sociosexual behaviour in chimpanzees in 3 years; all ages and sexes engaged in sociosexual behaviour; most sociosexual behaviour was between adult males

Sociosexual behaviour in wild chimpanzees occurs in variable contexts and is frequent between same-sex partners. Aaron A. Sandel & Rachna B. Reddy. Behaviour, Feb 2 2021, https://doi.org/10.1163/1568539X-bja10062

Rolf Degen's take: Sociosexual behavior, including that between same-sex pairs, is a standard component of chimpanzee behavior

Abstract: Many animals engage in sociosexual behaviour, including that between same-sex pairs. Bonobos (Pan paniscus) are famous for their sociosexual behaviour, but chimpanzees (Pan troglodytes) apparently do not engage in sociosexual behaviour frequently. However, sociosexual behaviour in chimpanzees may have been overlooked. We observed 584 instances of sociosexual behaviour in chimpanzees at Ngogo, Kibale National Park, Uganda during three years of study. All ages and sexes engaged in sociosexual behaviour, which included mounting, touching of genitals, and pressing genitals together. Most sociosexual behaviour was between adult males. Sociosexual behaviour was often during tense contexts, such as subgroup reunions and during territorial behaviour. Among males, grooming and dominance rank relationships do not explain patterns of sociosexual behaviour. Although sociosexual behaviour may be less frequent in chimpanzees than in bonobos, and bonobos remain distinct in their genito-genital rubbing, our findings suggest that sociosexual behaviour is a regular part of chimpanzee behaviour.

Keywords: genital contact; Pan troglodytes; mounting; same-sex sexual behaviour

Lay comments: Same-Sex Sexual Behavior in Chimpanzees Challenge Our Gendered Biases About Evolution. Michelle Rodrigues, 2021. https://thisviewoflife.com/same-sex-sexual-behavior-in-chimpanzees-challenge-our-gendered-biases-about-evolution/

4. Discussion Sociosexual behaviour, including same-sex sexual behaviour, occurred in every sex- and age-class combination among chimpanzees at Ngogo. The most common type of sociosexual behaviour was mounting. Chimpanzees would also reach their hand to touch the rump or genitals of others and on rare occasions touch rumps or engage in genito-genital touching. Sociosexual behaviour occurred in multiple contexts. In nearly all cases for which we recorded the context, the context represented cases of tension, often due to aggression or the threat of aggression, including during food sharing, upon hearing neighbouring groups of chimpanzees, or prior to or after joining another subgroup within the community (Table 2). Sociosexual behaviour was variable among the Ngogo chimpanzees. It appeared to be most common between adult males. This may be due to the cooperative nature of male chimpanzees, and the tension involved during certain activities, such as reunions that involve the reestablishment of dominance relationships. Adolescent and young adult male chimpanzees were the A.A. Sandel, R.B. Reddy / Behaviour (2021) 19 focus of our research, so they may be overrepresented in the sample. Indeed, we have considerably more observation time on them than any other agesex class. However we frequently observed sociosexual behaviour between adult male pairs that were not the focus of our research. Despite our focus on adolescent and adult males, we recorded individuals in all possible age-sex class pairings engage in sociosexual behaviour including between sexually mature adult females (Table 1). Nevertheless, given our sampling procedure, females and infants are likely underrepresented. In fact, we likely underestimated the frequency of sociosexual behaviour in general, as such behaviours often occurred quickly and during somewhat chaotic events, including during aggression or just prior to the chimpanzees running toward a neighbouring group of chimpanzees, making it difficult to record. Some individuals appeared to engage in sociosexual behaviour more than others. Several adult males accounted for a large proportion of sociosexual behaviours. One young adult male, somewhat low-ranking but rising in the male hierarchy (Evans), mounted others 30 times, and only twice was mounted by others. Another adult male, middle-aged and relatively lowranking (Mulligan) was mounted nine times, and never mounted others. In fact, these two males engaged in one of the instances of genito-genital rubbing (Video 1 at 10.6084/m9.figshare.13546526). One of these males, Mulligan, was also involved in an unusual case of mounting in which he presented his rump to another male, and while the other male (Dexter) mounted him, Mulligan crouched down on the ground in a posture resembling that of females during a copulation. Other males that were over-represented in our sample included high-ranking adult males. It is possible that status and personality play a role in the frequency of sociosexual behaviour. High-ranking males may encounter tense situations frequently — competing for mating opportunities, sharing meat, participating in border patrols — instigating sociosexual behaviour as well as other forms of relationship regulation, such as grooming. In addition, some males may be more ‘nervous’ in general, and thus seek out reassurance from others. With regard to the prevalence of sociosexual behaviour in chimpanzees, it is possible that it is more common at Ngogo than other chimpanzee sites. There are more males at Ngogo than any other community that has been studied. At Ngogo, chimpanzees, especially adult males, engage in frequent cooperative behaviour, group hunts, and border patrols (Mitani et al., 2000; Mitani & Watts, 2001; Watts & Mitani, 2001; Mitani, 2009b; Langergraber et al., 2017). Being surrounded on all sides by other chimpanzee communities, they regularly have intergroup encounters. As a result, there may be heightened need for regulating tension. However, we consider it unlikely that Ngogo stands out with regard to the nature and prevalence of sociosexual behaviour as similar behaviours have been described at multiple other sites (van Lawick-Goodall, 1968; Bygott, 1974; Nishida et al., 1999). For example, a study at Gombe found that 50% of all touching between male chimpanzees (N = 194 total touches) involved touching of genitals, and 70% of all touching between female chimpanzees (N = 56 total touches) involved touching genitals (Bygott, 1974). There also appeared to be variation among individuals in the frequency and patterning of sociosexual behaviour, including with some individuals being distinct in their sociosexual behaviour. Bygott (1974, p. 63) noted that “the adult male Faben often presented to other males, and when they held out a hand to him he would bounce his scrotum up and down against their hand. No other male was seen to do this.” Further research is required to test hypotheses for the function of sociosexual behaviour in wild chimpanzees. Our findings suggest that sociosexual behaviour is unlikely related to dominance given that combinations of all age and sex classes engaged in these behaviours. Importantly, in cases of mounting between males, the mounter was not higher ranking; if anything, the mounter tended to be lower ranking than the mountee, although mounters were also frequently higher ranking than the mountee (Figure 5). Sociosexual behaviour may relate to other behaviours, such as affiliative bonds, but grooming was not a strong predictor of sociosexual behaviour (Figure 4). Some pairs that groomed frequently also engaged in sociosexual behaviour more than did other pairs, but sociosexual behaviour also occurred between males who associated less frequently because they occupied different social neighbourhoods within the community. Thus, at least for males, sociosexual behaviour may be a way to reduce tension among pairs who meet infrequently. The function of sociosexual behaviour likely varies by age and sex. For example, we observed infant females rubbing their genitals on others, and this may have been for a pleasurable sensation alone (Vasey & Duckworth, 2006). Further study of sociosexual behaviour is required in chimpanzees, particularly focused on females, and how the function of sociosexual behaviour changes with development, which will in turn inform its possible functions. Although we focused on genital contact, much of the behaviour that we observed resembled what other researchers have A.A. Sandel, R.B. Reddy / Behaviour (2021) 21 considered ‘reassurance’ behaviour (Goodall, 1986; Nishida et al., 1999). Prior studies seem to have lumped sociosexual behaviour within reassurance behaviour, and indeed, they may serve the same function. Future studies should analyse sociosexual behaviour along with other forms of touch, including mouth-to-mouth, hand-to-mouth, hand-to-hand, hand-to-body, and embraces without genital contact. Given that much of sociosexual behaviour across mammals has been linked to cooperation and tension reduction in fission-fusion species, it is no surprise that chimpanzees do it. Indeed, this finding is not new. Some of the earliest studies of chimpanzees reported behaviours involving mounting and genital contact, both in captivity (Crawford, 1942) and the wild (Nishida, 1968; van Lawick-Goodall, 1968; Sugiyama, 1969). However, the ensuing decades have focused on such behaviours in bonobos, framing them as key for tolerance and female power (Hohmann & Fruth, 2000; Hohmann et al., 2009), and this seems to have downplayed its significance in chimpanzee life. For example, Hohmann et al. (2009) considered genital contact to be ‘habitual’ in bonobos, but not in other apes, and Grueter & Stoinski (2016) considered same-sex sexual behaviour “rare or absent among chimpanzees” in their study of such behaviours in wild gorillas. Similarly, sociosexual behaviour is considered “weak or infrequent” in East African chimpanzees (Pan troglodytes schweinfurthii) compared to hunting, which is “strong or highly frequent,” and infanticide, which is “moderate or frequent” (Gruber & Clay, 2016). In our study, sociosexual behaviour occurred considerably more frequently than did infanticides and was closer to the frequency of hunts, thus we would not consider it as “weak or infrequent”. Although we were not able to calculate a rate of sociosexual behaviour for most of our subjects, sociosexual behaviour was only one third as frequent as mating behaviour. And adolescent and young adult males tended to exhibit sociosexual behaviour approximately once every two months, which is likely an underestimate. It may have been that the study by Gruber and Clay (2016) considered a more limited array of behaviours as sociosexual, which could also account for its apparent absence between young chimpanzees compared to bonobos in an experimental feeding context (Woods & Hare, 2011). When analysing chimpanzee behaviour, scientists seem to classify mounts or genital contact within other functional behaviours, such as gestures or reassurance. We do not dispute that genital contact may serve conciliatory or communicative purposes in chimpanzees, but we also suggest that it deserves attention in studies investigating sociosexual behaviour explicitly. Doing so will allow proper comparisons to bonobos, and may elucidate the evolution and function of sociosexual behaviour in primates more generally. Although we found chimpanzees to engage in a range of sociosexual behaviours, bonobos remain distinct. Bonobos exhibit frequent genito-genital rubbing, especially between adult females (Hohmann & Fruth, 2000), whereas, we recorded only three instances of face-to-face, genito-genital rubbing in chimpanzees, and it was only observed between adult males. However, different species of primates manifest different forms of sociosexual behaviour, such as ‘ritualized’ touching of the penis in some baboons species (Smuts & Watanabe, 1990; Dal Pesco & Fischer, 2018). For chimpanzees, mounting and touching genitals may be their species-typical manifestation of sociosexual behaviour. That behaviours involving gential contact have not been conceptualized as ‘sociosexual’ in chimpanzees may also be due to cultural biases against homosexuality, as has been seen in the literature on other primates (Vasey, 1995). “Never, however, have we seen anything that could be regarded as homosexuality in chimpanzees,” Jane Goodall writes in her 1971 book, In the Shadow of Man. “Admittedly a male may mount another in moments of stress or excitement, clasping the other around the waist, and he may even make thrusting movements of the pelvis, but there is no intromission. It is true, also, that a male may try to calm himself or another male by reaching out to touch or pat the other’s genitals; while we still have much to learn about this type of behaviour, it certainly does not imply homosexuality. He only does this in moments of stress, and he will touch or pat a female on her genitals in exactly the same contexts” (Goodall, 2010, pp. 183–184). We agree that this does not indicate ‘homosexuality’ in the sense of a sexual orientation, but it does potentially represent sexual behaviour broadly defined, including that between members of the same sex. Our findings reveal that sociosexual behaviour is a standard component of chimpanzee behaviour. Although sociosexual behaviour is, no doubt, more salient and frequent in bonobos, especially with their characteristic side-toside genital rubbing, the range and type of sociosexual behaviour is similar in chimpanzees. Thus, there should not be such a distinction made between the two species. In addition, given the evidence of sociosexual behaviour in gorillas and a range of monkey species (e.g., Yamagiwa, 1987; Grueter & Stoinski, 2016), sociosexual behaviour is likely a common trait to haplorrhine primates. In some primate species, sociosexual behaviour has become key to negotiating relationships, as is the case in bonobos, some baboon A.A. Sandel, R.B. Reddy / Behaviour (2021) 23 species, and some populations of spider monkeys. Given its prevalence across taxa, sociosexual behaviour, including between members of the same sex, may be an important component of relationships that arose early on in primate evolution.