Love is love even in the animal kingdom

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 behaviour.

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 shows the 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.

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 world-view 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.

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?

Mongabay India/TWF