Male Monkeys Have More Sex with Other Males Than with Females in This Well-Studied Group

In 1993 reports that researchers had found a “gay gene” generated a jaw-dropping headline from the U.K.-based tabloid Daily Mail: “Abortion Hope after ‘Gay Genes’ Findings.” The article raised the inflammatory idea of giving a prospective parent the option of abortion if they were afraid that a child might one day seek a partner of the same sex. As a counterpoint to these critiques, researchers who pursued these studies emphasized that a biological cause of same-sex sexual behaviors could offer a defense against persecution arising from culture-based moral claims.

The never-replicated 1993 findings have been controversial, and the quest for genes linked to same-sex behaviors in humans and other primate species continues to this day. In July, 30 years after the publication of the 1993 study, a separate group of researchers at Imperial College London have published findings in Nature Ecology & Evolution that raise the prospect of evolutionary benefits that accrue to an isolated group of macaques who have same-sex encounters.

In the study, Vincent Savolainen, a professor of organismic biology and director of the Georgina Mace Center for the Living Planet at Imperial College London, and his colleagues tracked the social behavior of 236 male rhesus macaques during a three-year period on Cayo Santiago, a 38-acre island off the coast of Puerto Rico. They found that 72 percent of males engaged in same-sex behavior, compared with 46 percent that opted for liaisons with the opposite sex.

The results challenge a so-called Darwinian paradox that expresses a sense of perplexity about the prevalence of same-sex sexual behaviors in many species. The conundrum boils down to the fact that some animals expend energy on nonreproductive sexual behavior that does not seem to contribute to passing along genes to later generations, a concept known as evolutionary fitness. Some bigoted narratives appropriate the “paradox” as an anti-gay rationale.

The latest results pose a challenge to the paradox by suggesting that social benefits accrue from same-sex encounters, including improved evolutionary fitness for animals that engage in it. The monkeys that do so on Cayo Santiago tend to form more well-developed social ties with other males with whom they have sex and also experience greater reproductive success.

Like the “gay gene” reports from 30 years ago, the new study’s findings also raise questions about the wisdom of distilling a complex behavior down to a single trait linked to one or just a few genes. Additionally, the findings are prompting discussion about drawing too many conclusions about the relevance of a nonhuman species’ behaviors to our own species.

Humans are the sole living members of the genus Homo, and researchers often use primate models to get at the evolutionary origins of human behaviors. But how findings for another species meaningfully convey information about humans is unclear.

“I have very mixed feelings” about the study, says Michelle Rodrigues, an assistant professor in the department of social and cultural sciences at Marquette University, who was not involved in the work. “I appreciate seeing it, but I don’t like the paradigm it’s operating from.”

Cayo Santiago’s current generation of macaques, numbering around 1,700 animals, is descended from 409 monkeys that were brought from India in 1938 by a researcher who hoped to monitor their behaviors in a delimited space. Since then scientists have collected abundant multigenerational data about the monkeys, all of whom live in well-established social groups on the island.

Six decades ago researchers documented same-sex sociosexual behavior between male macaques on Cayo Santiago and described it as almost as common as similar behaviors between two different sexes. At the time, researchers had attributed the behaviors to “unnatural” factors related to the constant human presence of the researchers observing the monkeys.

It eventually became clear that these primates and many others commonly engage in same-sex sociosexual behaviors whether humans are around or not. What remained unknown was what evolutionary benefit, if any, they derived from these encounters. Typically, behaviors that give an edge for survival and reproduction and persist across generations have genetic roots. Savolainen took advantage of the carefully tracked family trees of the Cayo Santiago monkeys and their genetic data to evaluate the role of inheritance in their same-sex sociosexual behaviors. The researchers sorted out a small genetic contribution—about 6 percent of the same-sex behavior can be explained by genetics, which is a roughly similar heritability percentage to that of other common complex primate activities, such as grooming.

When it came to whether time spent on these same-sex doings had reproductive or fitness costs, the results suggested the reverse. The animals that engaged in same-sex sociosexual behavior seemed to have slightly better reproductive success. That advantage implies an evolutionary payoff to the activity (although the researchers noted that this trend was nonsignificant).

A previously offered explanation for these same-sex behaviors in primates is that they are a way to establish or maintain a dominance hierarchy. But Savolainen and his colleagues found no link between social status and whether a male tended to be the mounter or the “mountee.” Their analysis flagged the behavior instead as a way to cement “wingman” status among males. Males engaging in the same-sex sociosexual behaviors were more likely to back each other up in disputes with other monkeys, which gave them a winning edge. Membership in strong male coalitions has been linked to greater reproductive success in macaques.

If that hypothesis holds up for the macaques, that does not mean it can be assumed to hold for other primates, including humans. “The setup is that same-sex sexual behavior is a weird puzzle that doesn’t make evolutionary sense, and we have to solve it based on usefulness from a fitness perspective,” Rodrigues says. “That can be reductionist and lead to making assumptions or generalizations about behavior that we can’t begin to understand in other animals because of different cultural constructions.”

Complex social behaviors exist for many reasons and are shaped by environmental inputs. “A lot of primates have their own cultures, and that influences how we see different behaviors manifest in different populations,” she says.

In an e-mail to Scientific American, one researcher, who was not involved in the recent study and requested anonymity, remarks that “mate guarding” might be one explanation for the lower frequency of opposite-sex encounters as compared with same-sex ones in the study. During mate guarding, a male will monopolize any mating that occurs with a female, which limits different-sex encounters for other males.

Further, this researcher added that the recent study’s authors’ conclusion that the same-sex behaviors do not seem to have fitness costs is “fairly premature” because such encounters could promote detrimental parasite or disease transmission. Overall, the conclusions of the study “might very well be correct, but the present shortcomings lead to an equally likely and plausible assessment that they might be incorrect,” the researcher wrote.

Savolainen and his co-authors say that they considered mate guarding and discussed it in their study. “The causes … may include mate guarding,” he says. “Nothing about our analysis precludes this.”

Casting macaques as a potential explanatory model for human behavior requires caution, Rodrigues says. Such studies are “valuable for understanding things like the evolution of social and sexual relationships,” she says. “At the same time, we have to be really careful about what conclusions we are drawing and how that will be applied to understanding humans.”

Savolainen does not disagree. “One thing that we can probably say is that we can learn from related species about our past, but obviously humans have evolved in societies that are very different from the macaques,” he says.

Savolainen and his colleagues anticipated some of the critiques. “Some people might say that whatever you find in animals is irrelevant to what humans might face,” he says. “But what would be interesting to see is: Do people feel better by realizing that it’s something very common in nature? In societies that condemn homosexuality even with the death penalty, they often say that with scientific evidence that it is natural, they wouldn’t be so harsh on them.”

The hope, Savolainen says, is that these or similar findings might limit such extreme reactions. The macaque offers an example of how these behaviors are “actually beneficial,” he adds, “which should be a refreshing take on what’s going on in nature.”

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