A recent study published in the journal Nature Communications has traced the evolution of same-sex sexual behaviour in mammals, using phylogenetic analysis, a method that traces evolutionary relationships among biological entities. Such behaviour, which is common in mammals, may have evolved in part “to establish, maintain and strengthen social relationships that may increase bonds and alliance between members of the same group,” the authors write.

“Our study has tested for the first time two adaptive hypotheses on the origin and maintenance of same-sex sexual behaviour using a large group of animals, the class Mammalia,” says José Maria Gómez, an evolutionary biologist at the Experimental Station of Arid Zones in Almería, Spain and an author of the study. “In this sense, our study provides strong evidence that this sexual behaviour is functional and plays an important role, at least in this group of animals.”

In their study, Gómez says the scientists conclude that social behaviour that helped maintain positive social relationships and mitigate intrasexual aggression were two factors shaping the evolution of these behaviours. The former factor did so for both males and females and the latter factor only for such behaviour expressed by males, they found.

Same-sex sexual behaviour, which includes courtship, mounting, genital contact, copulation and pair bonding, was observed in 261 species, which constitutes 4% of all mammal species. Their study also indicates that same-sex sexual behaviour is not randomly distributed across the mammalian phylogeny but tends to be frequent in some clades and rare in others and has been observed in males and females both in captivity and in wild conditions.

Same-sex sexual behaviour in animals is a topic that is seeing increasing interest in scientists of evolutionary biology. This growing field of research has amassed a list of 1,500 animal species exhibiting same-sex sexual behavior.

Two male mallard ducks in Hesse, Germany, showing the behaviour of a duck couple. Photo by Norbert Nagel/Wikimedia Commons.

Not an aberration

“In the early 2000s, same-sex sexual behaviour in animals would often be seen as a ‘zoo problem,’ like it was the animals in captivity that were making the best out of a bad situation,” says Eliot Schrefer, author of Queer Ducks (and Other Animals), a young-adult book that illustrates the diversity of sexual behaviour in animals. “But this kind of science shows the prevalence of said behaviours throughout the animal kingdom, which shows that it’s not some aberration that has been localised, but it is something that is essential,” adds Schrefer, who was not part of the study.

The study suggests that social bonds may have played a role in the evolution of same-sex sexual behaviour and it may be connected to animals’ transition from solitary living to “sociality,” or living in groups, which has evolutionary advantages. “Due to the multiple benefits of sociality, many behavioural strategies have evolved to ensure the cohesion and stability of social groups,” the authors write.

Janet Mann, a behavioural ecologist who was not involved in the study, says, “It makes sense that animals make use of the social behaviour that they have available for them for social bonding.”

However, she finds maintaining social bonds and intrasexual aggression to be the flip sides of the same coin. Social bonding, she explains, includes when animals ally themselves with others and that provides protection. In extreme cases, male chimpanzees form tight alliances with one another, resulting in the whole community bonding to some degree. “They kill males with the neighbouring community, so it’s not like they are having sex with those males,” she says.

While the Nature Communications report is one of the first studies that has provided research on a broader scale rather than sticking to one species, the authors are not hesitant to acknowledge that the data available are limited because interest among scientists and researchers studying same-sex sexual behaviour in animals is very recent.

Mann says this lack of data meant the researchers couldn’t comprehensively address the frequency of same-sex sexual behaviour; rather, the data primarily show presence or absence of behaviours. Therefore, a case in which a behaviour is rare was weighed the same as a case in which it occurs frequently; both were reported as “occurring,” which is a limitation of the study.

Two male dragonflies in the mating position. For a long time, the prevailing notion was that sexual behaviour in the animal kingdom served solely as a tool for procreation, and that sexual behaviour among the same sex in animals was considered either an error — or was labelled “perverted.” Photo by Ludo Dolu/Flickr.

Stigma in past research

This absence of sufficient data stems from intentional erasure by some scientists in the past due to the stigma attached to homosexuality and expected heterosexuality among animals. For a long time, the prevailing notion was that sexual behaviour in the animal kingdom served solely as a tool for procreation, and that sexual behaviour among the same sex in animals was considered either an error — or was labelled “perverted.”

Heterosexual worldview influenced the approach of scientists like Valerius Geist, a mammalogist who, decades ago, refrained from publishing about frequent same-sex behaviour noticed in bighorn sheep because, the Washington Post reports, it made him “cringe … to conceive of those magnificent beasts as ‘queers.’” Years later, he reportedly “admitted that the rams lived in essentially a homosexual society.”

“Science is made by scientists, and [some] scientists who go out in the field have the assumption that only heterosexual behaviour is natural. And so, for a long time, they weren’t bothering to sex the animal they were finding or seeing if it were male or female; they were just assuming when one animal is mounting another.” Consequently, Schrefer says he suspects same-sex sexual behaviours in the animal world are vastly underestimated “because there are very few scientists that are going out and looking.”

These stigmas have not entirely left the scientific community. Mann recognises that societal biases against same-sex sexual behaviour in human societies influence the willingness of researchers to undertake such studies. The stigma creates a barrier, as approaching traditional funding agencies for studies may be met with reluctance or denial. The hierarchical nature of science, mirroring broader societal structures, also imposes limitations on junior researchers, dissuading them from delving into studies that challenge established norms, she says.

Mann’s contribution to the anthology Homosexual Behaviour in Animals: An Evolutionary Perspective has also opened up a window to same-sex sexual behaviour among bottlenose dolphins. Reflecting on the trajectory of her work, she explains, “I wrote that chapter in the book because I was asked to, and they knew I had a lot of data. I had also already started tenure, so when the book came out, I was a full professor. As a full professor, I don’t have to worry; but my more junior colleagues, for them, there is some stigma.” It’s not correct, she says, but “if you are studying same-sex sexual behaviour … people make assumptions about you as a person, that you are homosexual.”

This assumption about researchers’ identities has put skeptical eyes on the sciences that many pursue. Schrefer talks about a primatologist named Linda Wolfe, who wrote about female-female sex among macaques in the 1970s. She was a graduate student when she published about this, and the response was: “Why are you interested in this? Is something wrong with you? Are you a lesbian?” Schrefer says. “And so, if someone is part of the queer community, people would not trust the science that much.”

Melina Packer, an assistant professor of race, gender and sexuality studies at the University of Wisconsin-La Crosse says she believes that while many people push for science to be depoliticized, it is impossible to do that, as everyone is a political being. What can be done is to acknowledge those potential biases. She further explains that those in power have a political position, and to state that some folks are biased because they are different from so-called societal norms is a political statement in itself. Further, she points out, scientists of diverse racial, gender and sexual identities are expected to leave those identities at the laboratory door. “But what about the identity of the dominant being, what about the white man’s identity? Why doesn’t he have to leave his identity at the laboratory door?” she says.

Many times, these biases lead to the dismissal of same-sex sexual behaviours in animals within research. “Because it is infrequent behaviour, there aren’t necessarily studies of just sexual behaviour; there are studies of courtship and mating, but it’s usually in the context of reproduction and the population and how it’s progressing. So, most of the focus is on the things that have obvious adaptive value.” The lack of focus on behaviours with unknown or uncertain adaptive values is another form of bias, Mann adds.

A study published in 2019 by a group of researchers suggests that same-sex behaviour in animals is ancestral, meaning that it did not evolve independently but instead was always there in animals and persisted, as there are very few costs associated with same-sex behaviours. The authors note that it can be advantageous, and that the expression of both different-sex and same-sex behaviours “may be the norm for most animal species.” The authors propose shifting the questions from Why same-sex behaviour? to Why not same-sex behaviour?

To keep such biases from permeating scientific study, Packer refers to feminist science studies, which look at how science is embedded in culture and history. “Scientists are people, too, right? And scientists cannot help from bringing their biases, intentional or not, to the work we do, who we are, what culture we are raised in, how we are socialized, what historical moment we have lived in. All these forces are influencing the scientific process, what we sort of have been socially trained to see when we make an observation, particularly of animals. [If] you are raised in a culture that understands same-sex sexual behaviour or homosexuality or queerness as wrong or abhorrent, you’re more likely to project that view onto the nonhuman animals you are viewing.”

To resolve this issue, Packer suggests more interdisciplinary collaboration with experts outside the sciences from including, but not limited to, arts and humanities. “The way we educate scientists, fundamentally, would shift; ideally, we all would be working together and breaking open those boxes and constraints as well. I think being encouraged and empowered to think differently as a scientist is essential, and ultimately science is supposed to be about asking critical questions and testing hypotheses. But all too often, what happens in science is you just follow behind the scientist who came before you where you take dominant theories for granted and don’t necessarily try to implode them or explode them.”

Stating the relevance of their research for the public, Gómez says, “any behaviour or characteristic, no matter how unique we think it is to human beings, can be studied scientifically in a calm, disinterested and rigorous manner. And … the honest use of a scientific and rational approach can help us much more than other forms of knowledge to understand vital aspects of our life and our way of being.”