Same-Sex Sexual Behaviors Are Common and Influenced by Genetics, Macaque Study Shows

For decades, same-sex sexual behavior among animals presented a “paradox” of sorts to biologists: It doesn’t obviously fit into the evolutionary drive to survive and reproduce. Despite same-sex sexual behavior being observed in over 1,500 species of animal, the prevailing view was that such behaviors are rare and costly aberrations that don’t fit into evolutionary theory.

But the reality is much more complicated than that old way of thinking suggests: Now, scientists have found same-sex sexual behavior is actually more common than different-sex sexual behavior among male macaque monkeys that live on a small island off the coast of Puerto Rico.

The findings were published on Monday in Nature Ecology and Evolution. Far from being costly, the study hints that these behaviors might actually improve a male’s fitness — the word biologists use to describe reproductive success — in this population, and provides the clearest evidence yet that these behaviors among animals can stem in part from genetics.

“Same-sex sexual behaviors are not rare and are not deviant,” said study author Vincent Savolainen, an evolutionary geneticist at the Georgina Mace Centre for the Living Planet at Imperial College London. In this population, “it’s actually natural, common, and useful,” he added. 

In other words, not paradoxical at all.

Scientists have been quietly documenting these behaviors for over a hundred years, noticing same-sex directed courtship, mounting, genital licking and more in species as diverse as flying foxes and field crickets. But many of these observations were seen as curiosities, consigned to so many scribbled entries in field notebooks while scientists were studying something else.

The haphazard nature of this observational record has made understanding these behaviors challenging, Savolainen said. One of the biggest questions is whether same-sex sexual behavior has a strong genetic basis, a necessary prerequisite for evolution. Understanding that would require investigating a large but manageable population that exhibits same-sex sexual behavior and whose genetic history is well known. Enter the rhesus macaques that live off the eastern coast of Puerto Rico on Cayo Santiago.

Since 1956, scientists have taken a daily census of the monkeys on the island. In 1992, researchers started collecting genetic samples, allowing them to reconstruct the family tree for about 1,700 macaques. Knowing that family tree allows researchers to estimate whether same-sex sexual behavior is influenced at all by genetic differences, Savolainen said. 

“It’s the only place in the world you can do this,” he said.

For three years Savolainen and his colleagues recorded mounting behaviors in 236 male monkeys. Overall, same-sex mounting was more common than different-sex mounting (1,017 observations, compared to 722 observations), and 76 percent of observed males engaged in same-sex mounting. Most of these mountings were accompanied by evidence of arousal, like an erection or repeated thrusting.

Further analysis revealed that about 6.4 percent of the variation across individuals in same-sex sexual behavior can be explained by genetic differences, on par with other behaviors like grooming. “Obviously there are other components besides genetics that affect the behavior,” said Savolainen, but genes do matter.

The researchers also found that males who engaged in more same-sex sexual behavior didn’t have fewer offspring than males who engaged in less. In fact, more same-sex sexual behavior was associated with more offspring, though this relationship wasn’t statistically significant. “We found no cost for same-sex sexual behaviors,” said Savolainen, directly countering traditional assumptions.

Same-sex mounting seemed to help males form closer social bonds that prove beneficial in other contexts, like fights. “The idea is that through having sex, in effect, [males] strengthen their bonds,” he said. In other studies these bonds have been shown to improve reproductive success in macaques, suggesting that these behaviors are evolutionarily beneficial in this population.

Savolainen cautions against making direct comparisons between macaques and humans, but hopes the study “refreshes the conversation,” around same-sex sexual behavior, and shifts it away from the over-simplistic “nature versus nurture” debate.

The research bolsters a growing movement among some biologists to reframe the study of same-sex sexual behavior as a behavior that might be as old as sex itself. From that perspective, the spectacular diversity of sexual behaviors across all animals might stem from a long evolutionary history of less discriminate mating, the particulars of which could be shaped by idiosyncratic forces along each branch of the tree of life. 

Such a view predicts these behaviors “to be both common and not very costly in terms of fitness,” Ambika Kamath, an evolutionary biologist at the University of Colorado Boulder who wasn’t involved in the study. “This paper finds both those to be true in this population of rhesus macaques.”