A study by Spanish researchers suggests same-sex behavior among mammals − especially primates − is not only common and adaptive but supports social relationships and reduces conflict.

Their findings represent the broadest systematic documentation yet of same-sex behavior in mammals, several researchers not involved in the work said.

"Homosexuality is present, widespread and eternal," said Joan Roughgarden, an emeritus professor of evolutionary biology at Stanford University in California. "If you somehow managed to exterminate it from one species it would re-evolve because it's adaptive."

Same-sex behavior, similar to activities like grooming, nuzzling and sleeping together, promotes cooperation, she said, which helps the survival of species. "Mutual exchange of pleasure is the bonding mechanism that underlies the cooperation."

The findings, which appeared Tuesday in the journal Nature Communications, are the result of a study conducted to better understand why same-sex sexual behavior appears in so many species even though it seemingly offers no reproductive advantage.

The study cataloged more same-sex behavior among species that were social, like bonobos, lions and bison. Species that are closely related on the evolutionary tree don't necessarily have same-sex behavior in common, while distantly related species do, suggesting it evolved separately, multiple times, the study found. It looked at same-sex behavior in the 261 mammalian species, tracking the frequency of mounting and/or genital contact (67%), courtship (27%) and pair bonding (24%).

Although the research was solely in nonhuman animals, it does have implications for humans, researchers said.

"It is still common for people to argue against homosexual behavior (or the entire LGBTQ+ community) on the basis that heterosexual sex is the only approved and natural kind of sex," Frans de Waal, a primatologist and professor emeritus at Emory University in Atlanta, said via email. "This review tells us that this is utter nonsense. Humans are by no means exceptional in the animal kingdom."

But while scientific observations of same-sex behavior among animals have been ongoing for decades, they haven't always been welcomed, striking at the heart of religious arguments that claim same-sex behavior is aberrant and not found in what they see as divine creation.

“What studies like this do is remind us of the really vast range of social arrangements that appear in nature,” said Nathaniel Frank, director of the What We Know Now project, a research initiative that tracks scholarly research on LGBTQ+ issues at Cornell University in New York. “Studies that reveal same-sex behavior in nature show that these natural variations exist without harming anyone and that this diversity can, in fact, reflect real benefits to society.”

Same-sex sexual behavior has been observed in more than 1,500 animal species, from insects to fish to reptiles, birds and mammals. It's particularly common among primates, observed in at least 51 species from lemurs and bonobos to chimpanzees and apes.

Why so many animals practice same-sex behaviors

Over the years, scientists have set out to find out why sexual behavior that seems to lead to an evolutionary dead end might be so common.

They've landed on a variety of explanations for why animals seek out same-sex sexual interactions:

◾ Mistaken identity.

◾ Limited availability of members of the opposite sex.

◾ Sexual frustration among individuals refused by members of the opposite sex.

◾ Indiscriminate sexual behavior simply being the ancestral condition for sexually reproducing animals.

When the Spanish researchers looked at examples of same-sex behavior across multiple species, they determined same-sex sexual behavior is more common in species that live in social groups than in species that are mostly solitary. They found sexual behavior helps animals establish and maintain positive social relationships, create bonds and alliances and reconcile after conflicts. This is very common in female bonobos and female Japanese macaques.

Could same-sex behavior in animals reduce aggression?

The researchers' second finding is that same-sex behavior is more common among males in species where adults routinely kill other adults. The hypothesis is that same-sex sexual behavior helps lower the amount of aggression and conflict between individuals and aids in bonding. It does this by making social status clear, establishing dominance and possibly diverting aggressive behavior toward something more like courtship behavior.

In both sexes, same-sex sexual behavior allows individuals and groups to live more harmoniously and potentially provide more reproductive success.

Claims persist that same-sex relationships are 'abnormal'

Despite the gains in recent decades by the LGBTQ+ movement, beliefs that gay and lesbian relationships are unnatural still persist. In Texas, the Republican Party’s state party platform last year described homosexuality as “an abnormal lifestyle,” drawing the ire of LGBTQ+ conservatives.

Such notions once undergirded the legal framework behind once-widespread sodomy laws in the U.S. that prohibited consensual sex among same-sex adults, said Mary Bernstein, a sociology professor at the University of Connecticut in Storrs.  

“Many state laws called this a ‘crime against nature’ or the “abominable and detestable crime against nature,’” she said.

What can we learn from animal behavior?

The evidence in animal species does not universally resolve these disputes about human behavior. Relying on studies about animals to defend human rights is risky, Bernstein and others said, because nature is full of examples of animals doing things that people might consider odd, or even horrifying.

“Some animals routinely kill their young,” she said. “So it’s not clear that because animals do or do not do something, that humans should or should not do these same things.”

What does the study tell us about humans?

Relying on scientific arguments about LGBTQ+ behaviors and animals could, in some cases, hurt the gay rights movement, said Marc Robert Stein, a professor at San Francisco State University, who has written extensively about urban gay and lesbian history.

"That’s not a very proud or affirming way to think about homosexuality. Which is not to say that these types of scientific studies aren’t interesting and valuable; it’s just to say that we should be careful about the meanings we attach to these scientific 'discoveries,'" Stein said.

José María Gómez, the lead author of the new study, said the findings likely won't support or refute any social or political position.

"But what does seem to be the case is that same-sex sexual behavior (in animals) is not a permanent sexual preference. In most cases, individuals displaying same-sex sexual behavior also mate with individuals of different sex," he said. "For this reason, we believe that similarity with how humans display sexual behavior is very limited."

He likewise cautioned against the notion of equating behaviors found in nature to what ought to be among humans.

"We are a moral species," Gómez said. "We choose freely how to behave and what is considered good or wrong."

Contributing: Karen Weintraub