A few years ago, at my long-term primate field site in Costa Rica, I was revising our ethogram, the behavioral blueprint we use to systematically record what the monkeys are doing. Since the project began in 2016, I’ve returned to this document again and again, adding new subtypes of play, sharpening our definitions of aggression, and reworking our foraging categories, all to better reflect the specific socioecology of these capuchins’ lives.
This time, as I read through the section on sexual behavior, I noticed something I hadn’t caught before.
We had behavioral codes for all kinds of capuchin sexual behaviors, from mounting to sexual solicits, but it was the definitions of those behaviors that caught my eye. Mounting was defined as “when a male mounts a female.” Copulation required intromission and was, again, implicitly heterosexual. Even our sexual solicitation codes–things like “duck face,” when capuchins purse their lips together in a flirty invitation to mate–were defined as behaviors directed toward a member of the opposite sex. Our ethogram, arguably the most important document in our field methods, simply did not allow room for any kind of sexual behavior that was not between a male and a female. Same-sex sexual interactions, if they were recorded at all, would have been pushed into the ad libitum notes, a space reserved for rare behaviors, or folded into a vague “other” category.
That small realization reflects a much bigger problem. It is not just that same-sex sexual behavior (SSB) in animals has been misunderstood; it has often been left out altogether. We have fostered scientific theories and built methods that make this omission easy. Our frameworks define certain interactions as “sexual” and file others–especially those that do not fit a heterosexual script– under other labels like bonding, dominance, or play. The result is a scientific record in which heterosexual behavior is carefully counted and analyzed, while many same-sex sexual interactions never make it onto the datasheet in the first place. In effect, our science end up with a quiet policy of “don’t ask, don’t tell”: if we can’t find publications on same-sex sexual behavior and omit it from our methods, the data will keep confirming that it is not there.
We are far from the only field site dealing with this problem. A recent 2024 study in PLOS ONE asked scientists about their experiences with same-sex sexual behavior in the species they study (Anderson et al., 2024). Most respondents reported having seen SSB in their animals, yet less than half had ever systematically collected data on it, and just 18% had published those observations. When asked why, respondents reported that their lack of reporting was due to the perception that SSB in their species were rare and therefore not a research priority.
And yet, when same-sex sexual behavior is actually measured, as has begun to happen in recent years, it turns out to be anything but rare. Same-sex sexual behavior has now been documented in over 1,500 species (Monk et al., 2019). Among mammals, primates stand out: SSB has been reported in at least 50 primate species and appears particularly prevalent in Old World monkeys and the great apes (Gómez et al., 2023; Sommer & Vasey, 2006; Vasey, 1995).
In many of the species where it has been studied systematically, SSB accounts for a far larger share of sexual activity than earlier anecdotal reports suggested. In macaques, same-sex mounting is common and can rival or exceed different-sex mounting in both males and females (Clive et al., 2023; Vasey & Jiskoot, 2010). In bonobos, perhaps the best-known case, female genito-genital “GG” rubbing makes up more than half of all observed sexual events and is tightly linked to social bonding, oxytocin release, and coalitionary support among females (Hohmann & Fruth, 2000; Moscovice et al., 2019). Even in mountain gorillas and chimpanzees, species long thought to show little same-sex sexual behavior, researchers now report frequent same-sex mounts and genital contacts as a regular part of their sexual repertoire (Grueter & Stoinski, 2016; Sandel & Reddy, 2021; Sommer & Vasey, 2006). When researchers design studies that actually attend to SSB, quantify it, and report it, it no longer sits at the margins of primate sexual behavior. In many systems, it is right at the center.
So why does the narrative persist that these behaviors are rare? Part of the answer is cultural. In his pioneering book Biological Exuberance (1999), Bruce Bagemihl devotes an entire section to how omission and misinformation about animal homosexuality have shaped journals, textbooks, and scientific discourse. In some cases, references to SSB have been explicitly removed. For example, when a 1979 report on killer whale behavior was republished as a government document for the U.S. Marine Mammal Commission, the only parts of the report that were eliminated were those mentioning homosexual behavior (Bagemihl, 1999). More often, though, the omission is less nefarious. These behaviors are overlooked, not because they are genuinely rare, but because they are already assumed to be. And because science grows on itself, those early omissions echo forward. If the literature suggests that same-sex sexual behavior is uncommon, later researchers are less likely to build projects dedicated to studying it. The result is a self-reinforcing loop: there is little or no published data on same-sex behavior, so we assume it is rare, so we do not report or study it, so there continues to be little or no published data.
But part of the answer is also methodological, and this is where the ethogram comes back in. When Jeanne Altmann formalized systematic sampling methods for primate behavior in the 1970s, she changed the game (Altmann, 1974). The combination of a carefully defined ethogram and focal animal sampling became the building blocks of a new ethology that was supposed to check observational bias at the door and let the data tell the story. Ethograms replaced anecdotes, and statistics replaced speculation.
That promise of neutrality, however, only holds if the behavioral categories and definitions themselves are neutral. Ethograms do not fall from the sky fully formed; they are written by humans with particular questions, assumptions, and blind spots. And because ethograms are passed from generation to generation, those blind spots can persist. If an ethogram devotes half a page to different forms of male–female courtship and a single catch-all code for “other,” then even the most rigorous data collection will faithfully reproduce that bias. The data will look objective, but entire classes of same-sex sexual behavior will have been coded out of existence before the first observation is ever recorded. And when SSB behavior is recorded, it is often relegated to a brief anecdote, a mode of reporting reserved for unusual events, which again feeds the illusion that these behaviors are rare.
And that is how I found myself, a queer, early-career researcher, staring that bias in the face at my own field station. I had inherited our ethogram from earlier studies, based it on “best practices” in animal behavior research, and done all the things I had been taught to do: list all relevant behaviors for capuchins, define those behaviors carefully, and collect data systematically and quantitatively. Yet built into these standardized tools and protocols was the quiet assumption that only some kinds of behaviors really counted as sexual. This was not a deliberate act of erasure. It was simply a reflection of inherited assumptions and decades of unchecked bias about what counted as “real sex” in monkeys. On paper, our monkeys were “straighter” than they really are.
Once we revised our ethogram to explicitly redefine all sexual behaviors as between two individuals, of any age or sex, our data began to reflect the degree of sexual flexibility in our monkeys. It became clear that capuchins engage equally, if not more, in same-sex sexual behavior than they do in different-sex sexual behavior. These behaviors persisted into adulthood and were not simply a byproduct of juvenile “play,” as is so often assumed. Some individuals engaged significantly more in same-sex sexual behavior than other sexual behavior, and there was substantial variation in who did so, with whom, and in what contexts. Omitting these behaviors from our coding scheme meant we were completely missing a crucial component of the social and sexual lives of these primates.
In my own work more broadly, we have started tackling some of these biases head-on (Cunningham and Benítez, 2024). We are currently conducting a systematic review of same-sex sexual behavior in primates to document and quantify how these biases have impacted the production of our scientific knowledge regarding primate sexual behavior (Cunningham et al., 2023). We have found that, although publications on same-sex sexual behavior in primates have increased over the decades, they still represent only a tiny fraction of the literature on primate sexual behavior. And even now, in 2025, these behaviors are more likely to be tucked into an anecdote or a brief footnote than to be the explicit focus of a study.
When I look back at that moment with our ethogram, what stands out is how a simple change–editing a few lines of a spreadsheet–could have such a profound impact on my science. In some ways, that is the hopeful part. Many of the shifts we need, whether in methods and theory or in how we talk about sex and sexuality, do not require throwing out evolutionary theory and starting over. They are about widening our lens: naming what has always been there and letting our science catch up to the diversity of the world it seeks to describe.
For those of us who are queer and work in this space, that widening is also personal. Paul Vasey wrote in 1995 that although reports of homosexual behavior in primates had existed for more than 75 years, “virtually every major introductory text in primatology” failed to even mention it. I often wonder what it would have meant if that had not been the case. What would it have been like to open a primatology textbook as a young queer student and see, plainly stated, that same-sex sexual behavior is a routine part of many primates’ lives? What kinds of questions would I have asked if I had known I was allowed to do so?
When our methods finally make room for those behaviors, we are not just correcting a technical oversight. We are changing which stories are possible. And when we peel back the deeply seated misconception that these behaviors are rare and teach the next generation that variation in sexuality is a normal, expected part of animal lives, the baseline question shifts. Instead of asking whether queer behavior exists in nature, they can start from the assumption that it does and ask what it does, for whom, in what ways, and under what conditions. In doing so, we begin to acknowledge, from within the most basic infrastructure of our science, that sexual diversity is not a deviation from evolution’s plan. It is one of evolution’s greatest strengths.
References:
Altmann, J. (1974). Observational study of behavior: Sampling methods. Behaviour, 49(3-4), 227–266.
Anderson, K. A., Teichroeb, J. A., Ramsay, M. S., Bădescu, I., López-Torres, S., & Gibb, J. K. (2024). Same-sex sexual behaviour among mammals is widely observed, yet seldomly reported: Evidence from an online expert survey. PloS One, 19(6), e0304885.
Bagemihl, B. (1999). Biological exuberance: Animal homosexuality and natural diversity. St. Martin’s Press.
Clive, J., Flintham, E., & Savolainen, V. (2023). Same-sex sociosexual behaviour is widespread and heritable in male rhesus macaques. Nature Ecology & Evolution, 7(8), 1287–1301.
Cunningham, E., Voyt, R., Tripp, H. C., Sandel, A., Reddy, R., & Benítez, M. (2023). A systematic review of same-sex sexual behaviors across primates. OSF Registries. https://doi.org/10.17605/OSF.IO/F9A24
Gómez, J. M., Gónzalez-Megías, A., & Verdú, M. (2023). The evolution of same-sex sexual behaviour in mammals. Nature Communications, 14(1), 5719.
Grueter, C. C., & Stoinski, T. S. (2016). Homosexual behavior in female mountain gorillas: Reflection of dominance, affiliation, reconciliation or arousal? PloS One, 11(5), e0154185.
Hohmann, G., & Fruth, B. (2000). Use and function of genital contacts among female bonobos. Animal Behaviour, 60(1), 107–120.
Monk, J. D., Giglio, E., Kamath, A., Lambert, M. R., & McDonough, C. E. (2019). An alternative hypothesis for the evolution of same-sex sexual behaviour in animals. Nature Ecology & Evolution, 3(12), 1622–1631.
Moscovice, L. R., Surbeck, M., Fruth, B., Hohmann, G., Jaeggi, A. V., & Deschner, T. (2019). The cooperative sex: Sexual interactions among female bonobos are linked to increases in oxytocin, proximity and coalitions. Hormones and Behavior, 116, 104581.
Sandel, A. A., & Reddy, R. B. (2021). Sociosexual behaviour in wild chimpanzees occurs in variable contexts and is frequent between same-sex partners. Behaviour, 158(3-4), 249–276.
Sommer, V., & Vasey, P. L. (2006). Homosexual Behaviour in Animals: An Evolutionary Perspective. Cambridge University Press.
Vasey, P. L. (1995). Homosexual behavior in primates: A review of evidence and theory. International Journal of Primatology, 16(2), 173–204.
Vasey, P. L., & Jiskoot, H. (2010). The biogeography and evolution of female homosexual behavior in Japanese macaques. Archives of Sexual Behavior, 39(6), 1439–1441.
