Bonobos have gotten a lot of attention for their sexual behavior, particularly their wide range of sociosexual behavior, behaviors involving sexuality that do not involve conceptive sex and occur across ages and partners. Furthermore, we now know that there are a wide range of primates that engage in some level of same-sex sexual behaviors. But what about chimpanzees?
For a long time, we have known that same-sex sexual behaviors among chimpanzees occurred, but they were considered rare. For example, one 2016 paper on observations of homosexual behavior in female gorillas summarized the literature as “Same-sex sexual behavior exists in all great apes: it is common and varied among bonobos, but rare or absent among chimpanzees, and orangutans.” However, recent research conducted by primatologists Aaron Sandel and Rachna Reddy challenges this characterization. They observed chimpanzees for three years at the Ngogo site of Kibale National Park, focusing research on adolescent and young adult chimpanzees. They found that sociosexual behavior, defined as “physical interaction involving contact with the anogenital region except for mating/copulations” was frequent, especially mounting, which occurred in 57% of the 584 instances. Other sociosexual behaviors included hand-to-anogenital touches, rump-to-body contact, and rump-to-rump contact.
Because the research was part of a broader study focused on young males, they were able to calculate the rate of occurrences for those focal subjects. For those adolescent and young adult males, sociosexual behaviors occurred approximately twice a month. However, sociosexual behaviors were observed among all age and sex classes; even though adolescent females were not the focus of the study, they noted that adolescent females engaged in sociosexual behaviors more frequently with adult females than with adolescent or adult males. Overall, most sociosexual behaviors occurred during tense contexts, such as fusion events when members of different foraging subgroups come together or encounters with neighboring chimpanzee communities.
The big question is, how did we manage to miss these behaviors in chimpanzees for so long? Chimpanzees are one of the most well-studied primates, due to our biases toward studying our closest relatives. Bonobos are equally related to us, but chimpanzees are better studied due to their wider geographic distribution, and higher numbers in captivity. The longest-running chimpanzee field sites have been running for over 50 years.
The answer is that we have not missed it entirely, but simply overlooked it as part of a suite of other behaviors. It has been documented by many studies before, but often is reported as something other than “sociosexual behavior,” subsumed under behaviors such as “reassurance” or “reconciliation,” or “gestures.” This likely is related to cultural biases preventing consideration of these behaviors as related to sexuality, particularly the potential for homosexuality. Sandel and Reddy point out that Jane Goodall, as well as other primatologists, have observed such behaviors in chimpanzees. However, in Goodall’s 1971 book, In the Shadow of Man, she expressly distanced it from homosexuality. She wrote:
Never, however, have we seen anything that could be regarded as homosexuality in chimpanzees… Admittedly, a male may mount another in times 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 behavior, 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 context.
Goodall’s characterization, shaped by her own cultural lens, may have in turn influenced how subsequent primatologists interpreted these behaviors.
I am reminded of Ambika Kamath’s description of “Victorian” lizards, where scientists’ early descriptions of animal behavior were grounded in cultural biases about how scientists assumed those animals should act. Conclusions derived from those biases then became entrenched in future scientific research. Biases regarding chimpanzee behavior may be even more pervasive, as much of the behavioral research is grounded in providing theoretical models for human evolution. Because early research shapes the trajectory of future research, these types of behaviors may have been discounted in importance or framed as part of a repertoire of other, non-sexual behaviors. There is value in recognizing how sociosexual behaviors are related to contexts such as tension, reassurance, and reconciliation. However, when those behaviors in bonobos are classified as “sociosexual” whereas in chimpanzees they are classified as ‘’reassurance,” it prevents us from directly comparing and contrasting behaviors between the two sister species. Furthermore, it prevents us from considering ape sexuality, and the potential for homosexual apes, on its own terms.
Besides the biases from early research and its impact in structuring subsequent research, there are other cultural factors that might affect recording these behaviors as well. One is the presence of queer scientists in the field. As we have more queer scientists in the field, that shapes the questions they ask, particularly in questioning some of the gendered and heteronormative assumptions that shape primate research. Furthermore, our changing cultural acceptance of gender and sexual diversity may allow us a better perspective to recognize sociosexual behaviors and consider their role in evolutionary theories. For example, more recent research on same-sex sexual behavior across animal species suggests we may have been overemphasizing heterosexual behavior all along.
However, there are also cultural factors that may hinder studying and reporting sociosexual behaviors. Ngogo is located in Uganda, where there has been a series of controversial laws that criminalized homosexuality, with penalties for “promoting” or “failing to “report” homosexuality. There are several other countries with similar laws where chimpanzee field sites are located. This can potentially affect research in two ways: first, queer primatologists may not feel safe conducting fieldwork there. Second, researchers and field assistants might be more hesitant to characterize same-sex behaviors in a way that could imply homosexuality because of cultural and legal risks.
There is an additional potential cultural factor: the chimpanzees’ own culture. Chimpanzees are cultural animals, and culture shapes the expression of social behaviors and their variants. Sandel and Reddy suggest it may be possible that these behaviors are more frequent at Ngogo, potentially due to the large number of males at the site, and the heightened tension due to neighboring chimpanzee communities on all sides. However, they note that it is unlikely that Ngogo is an outlier for these behaviors because they are described at other sites. Nonetheless, including sociosexual behaviors in further studies of cultural behavior of chimpanzees across sites may help clarify whether the Ngogo chimpanzees engage in them more frequently, or if there may be cultural components to their expression.
The occurrence of same-sex sociosexual behaviors in primates is not the same thing as homosexuality in humans. In humans, gender identity and sexual orientations have cultural components. Thus, we cannot classify primates that engage in same-sex sociosexual behaviors as “gay” or “bi” because these are human cultural terms. Nonetheless, one thing I find interesting is that there was a great deal of inter-individual variation, and several adult males were involved in a large proportion of these observations. Sandel and Reddy note that rank and personality may play a role. There is likely variation in interest or inclination to engage in same-sex sociosexual behaviors that may have some parallels to human sexual orientations, and this dynamic is one that deserves further inquiry.
However, just as we recognize that primate sexuality is multifaceted, and may differ from our own, the greater recognition of sexuality variation among primates has implications for understanding our own evolution. It has implications for how we understand our own cultural constructs and often challenges cultural assertions of what constitutes “natural” sexual behavior. As our own cultural perspectives change, it becomes clear that our cultural perspectives also shape how we view and interpret other species’ behaviors. That’s why having researchers from a variety of cultures, gender identities, and sexual identities may strengthen our ability to understand our closest relatives’ behaviors.
References
Bezanson, M., & McNamara, A. (2019). “The What and Where of Primate Field Research May Be Failing Primate Conservation.” Evolutionary Anthropology: Issues, News, and Reviews, no. November 2018: evan.21790. https://doi.org/10.1002/evan.21790
Goodall, J. (2010). In the Shadow of Man. Mariner Books/Houghton Mifflin Harcourt, Boston, MA/New York, NY.
Grueter, C., & Stoinski, T.S. (2016). Homosexual behavior in female mountain gorillas: Reflection of dominance, affiliation, reconciliation, or arousal? PLoS One11(5): e0154185. doi: 10.1371/journal.pone.0154185
Meredith, S.L., & Schmitt, C.A. (2019). The outliers are in: Queer perspectives on investigating variation in biological anthropology. American Anthropologist 121 (2): 487-479. https://doi.org/10.1111/aman.13223
Monk, J.D., Giglio, E., Kamath, A., Lambert, M.R., & McDonough, C.E. (2019). An alternative hypothesis for the evolution of same-sex sexual behavior in animals. Nature Ecology and Evolution 3, 1622-1631. https://doi.org/10.1038/s41559-019-1019-7
Sandel, A.A., & Reddy, R.B. (2021). Sociosexual behavior in wild chimpanzees occurs in variable contets as is frequent between same-sex partners. Behaviour 158 (3-4):249-276. https://doi.org/10.1163/1568539X-bja10062
