In 1896, controversy broke out when French entomologist Henri Gadeau de Kerville published the first sketch of two male cockchafers, a species of scarab beetles, copulating.
More than a century later, in 2012, researchers at the Tring Natural History Museum in the U.K. rediscovered a four-page pamphlet originally published in 1915 by the English naturalist George Levick. It seems Levick had observed same-sex sexual behaviour (SSB) in Adélie penguins.
Since then, scientists have reported SSB in pigeons, swans, albatrosses, lions, dolphins, bats, elephants, bonobos, gorillas, monkeys, lizards, tortoises, dragonflies, fruit flies, and bed bugs.
Animals that engage in SSB have been considered a ‘Darwinian paradox’: if reproduction is critical to evolution, then SSB – which is non-reproductive – should have ceased to exist. The supposed paradox is also fed by a longstanding belief among biologists that SSB could be evolutionarily “costly” to species because it leads to fewer offspring, thus reducing the chances of evolution mediated by natural selection.
Now, in a study published in July in Nature Ecology & Evolution, researchers at the Imperial College London have challenged the premise of this paradox. The team, led by postdoctoral fellow Jackson Clive, has reported that at least in rhesus macaques, male SSB is remarkably common and “is not [evolutionarily] costly”.
Volker Sommer, a professor of evolutionary anthropology at University College London, said the study is the first to find evidence that SSB has a “heritable component” in the case of animals. He wasn’t involved in the study.
Island of the monkeys
For their study, Dr. Clive and his team observed rhesus macaques, a common monkey model, in Cayo Santiago, an island east of Puerto Rico.
In 1938, researchers from Columbia University and the University of Puerto Rico shipped hundreds of monkeys from India to the island to establish a “disease-free breeding colony of monkeys in order to provide animals for research on tropical diseases.”
Today, the island is home to more than 1,700 descendants of the original population. The monkeys are surveyed every day. Every year, researchers study newborns to determine their paternal and maternal lineages.
For Dr. Clive & co., the island offered a large population of free-ranging monkeys as well as comprehensive data to answer their question: Is SSB heritable?
The heritability question
For three years, the researchers documented the frequency of same-sex and different-sex mounting by rhesus monkeys under observation. Mounting, according to a 2021 paper, is “sexual behaviour without a reproductive function”, making it an apt “sociosexual behaviour”.
Dr. Clive’s team observed 236 male macaques and found that “72% … engaged in same-sex mounting, in comparison with 46% for different-sex mounting,” per their paper.
Having discovered the high frequency of SSB in their study population, the authors turned their attention to its heritability – an idea mired in controversy.
In 1993, American geneticist Dean Hamer found that the human X-chromosome has a region whose transmission through the members of a family corresponded to “same-sex orientation” of male individuals in that family. Dr. Hamer concluded that “at least one subtype of male sexual orientation is genetically influenced.”
But then anotherstudy, published in 2019 in Science, examined more than half a million human genomes and found that while five “spots” in the genome were potentially related to same-sex behaviour, none of them had the power to predict one’s sexuality.
Instead of attempting to find one or many genes that could determine a monkey’s proclivity for SSB, the Imperial College researchers used two statistical measures: repeatability and heritability.
According to Dr. Clive, repeatability measures the amount by which “differences between individuals” determines variation in a certain behaviour. Heritability, on the other hand, measures the amount by which such variation could be attributed to “relatedness between individuals (i.e. genetics)”.
The researchers found that male SSB was at least partly heritable.
They also calculated a different statistical measure called “evolvability”, which gives an estimate of a behaviour’s potential to evolve over time due to natural selection.
Their calculations suggest that male SSB in rhesus monkeys is “polygenic”: influenced by more than one gene. The estimates indicated that male SSB was “strongly influenced” by non-genetic factors, including a monkey’s interaction with others in the population and the environment.
The results agree with those from genetic studies on human SSB: that there is no single “gay gene” – a deleterious idea that was deflated by the large 2019 study – in monkeys or humans.
As science communicator Nsikan Akpan wrote then, “polygenic traits can be strongly influenced by the environment, meaning there’s no clear winner in this ‘nature versus nurture’ debate.”
Clive et al.’s conclusions also challenge the idea that genetics alone determines one’s sexual behaviour – i.e. the idea of genetic determinism. Instead, the modest role of genetics appears to complement that of an organism’s interaction with others and its environment.
No trade-off with reproductive fitness
The idea that SSB might be evolutionarily expensive hinges on the idea that animals that engage in it more often have fewer opportunities for sexual behaviour involving partners of different sexes, especially unto reproduction. The latter is called different-sex sexual behaviour (DSB).
In the new study, the researchers found three reasons to question this assumption.
First: they observed that all the sexually active males in their cohort barring one were “behaviourally bisexual”, so they had or will have an “opportunity to pass on their genes”.
Second: an individual monkey’s proclivity for SSB wasn’t found to be correlated to that towards DSB. Just because a monkey engaged in more SSB didn’t mean it wasn’t engaging in enough DSB.
Third: the team counted the number of offspring fathered by each monkey in the cohort. “In all cases, we didn’t find that more SSB activity predicted an individual had fewer offspring,” Dr. Clive said.
According to him, this is the “strongest evidence” that SSB doesn’t exact an evolutionary cost, at least from male rhesus macaques.
In fact, the team found that pairs of monkeys engaging in SSB also formed “coalitions” against a common enemy. These bonds, according to Dr. Clive, could have an evolutionary benefit.
Aaron Sandel, an assistant professor at the University of Texas-Austin who studies social relationships in chimpanzees, affirmed the study’s observations.
“Mounting may be a way to communicate and to co-regulate emotions before a risky or stressful situation, like chasing after another monkey,” he said.
A note of caution
Dr. Clive believes future work must investigate female SSB to understand whether their findings might apply to both sexes.
The study he led couldn’t do this because, contrary to his expectations, female monkeys in the cohort weren’t engaging in “much SSB at all”.
He also said that similar studies on heritability, evolvability, and the evolutionary pros and cons of male SSB will need to be conducted with other monkey species before scientists can acquire a “macroevolutionary understanding of such behaviours.”
Dr. Clive and Dr. Sandel both also noted that scientists must be cautious to not extend the findings to humans without further evidence. Dr. Sandel in particular said that while humans and monkeys might exhibit similar behaviours, human behaviour is also influenced by cultural and social norms.
Sayantan Datta (they/them) are a queer-trans freelance science writer, communicator and journalist. They currently work with the feminist multimedia science collective TheLifeofScience.com and tweet at @queersprings.
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