Chimpanzees at the Maryland Zoo in Baltimore on June 24. (Marvin Joseph/The Washington Post)
As animals go about their lives -- eating, fighting, having babies -- they don't always follow the script laid out for them by Charles Darwin's theory of evolution. Not every behavior furthers the cause of survival in a way that is obvious to humans.
One mystery that has long puzzled biologists is why animals of the same gender engage in sex that cannot produce offspring.
Now scientists report evidence that in nonhuman primates, same-gender sexual behavior plays an important role in the survival of individuals within certain species, helping them navigate harsh climates, predators, food scarcity and complex social structures.
A study, published last week in the journal Nature Ecology & Evolution, offers new insight into the process of natural selection, the core mechanism behind evolution. The idea is that when random mutations in the genetic code benefit an individual, they persist and are passed on to offspring.
But this raises what scientists have called a Darwinian paradox: Genes linked to same-sex behavior have not vanished from nonhuman primates even though they don't result in offspring.
Vincent Savolainen, one of the authors of the paper and the director of the Georgina Mace Center for the Living Planet at Imperial College London, said young animals might engage in same-sex behavior "to practice having sex," in preparation for mating later in their lives.
Same-sex behavior may also bond individuals in a group of nonhuman primates, leaving them more inclined to trust each other when signaling the presence of predators and to defend one another.
Savolainen, who spent eight years studying rhesus macaques on the Puerto Rican island of Cayo Santiago, said that same-sex behavior may ultimately improve the survival of individuals, allowing them to form the alliances that help them to rise in a group's hierarchy.
The researchers at Imperial College London analyzed data on 491 nonhuman primate species, finding evidence of same-sex behavior in 59 species, 23 of which showed repeated occurrence of the behavior. They found that both males and females engage in same-sex relations.
"One of the aims of this study was to show how widespread" same-sex relations are in nature," Savolainen said. A 2023 study in Nature Communications reported that same-sex sexual behavior has been found in 261 mammal species, about 4 percent of the total. The behavior has been observed in animal species including humpback whales, elephants, giraffes and even in a Seychelles giant tortoise.
Savolainen said the results of his team's study show that same-sex relations must be taken into account to understand how a particular animal society works.
"Feeding, fighting, looking after your offspring, reproductive sex and same-sex sexual behavior is a repertoire of behavior that makes these complex societies function," he said.
The researchers focused their study on mounting and other sexual behaviors that involved the genitals as opposed to practices such as grooming. Their work analyzed data from dozens of other studies of same-sex sexual behavior, as well as data on climate and predation.
The scientists used the data to evaluate three evolutionary theories that attempt to explain the prevalence of same-sex relations in nonhuman primates. The first is that the behavior is influenced by extreme climates, scarcity of resources and predators; the second, that it is more likely to occur among sexually dimorphic species, meaning those with greater differences between males and females in terms of body size, lifespan and other factors; and finally, that it is driven by social complexity.
They found that same-sex relations were more likely in species that live in harsh, dry environments and in those that have distinct differences between males and females in size and appearance. Same-sex relations, they discovered, are also more likely in species that have social hierarchies.
They also looked at conditions that influence opposite-sex sexual relations among species and found that these are less likely in places under environmental stress.
Marlene Zuk, an evolutionary biologist at the University of Minnesota, said the new paper's systematic approach makes an important contribution to a discussion that is anything but new.
"Basically, this is the nature-nurture controversy," Zuk said. "Let's cut to the chase, this is what people really want to know: Is homosexuality caused by your genes or is it caused by the way you grew up, or your environment, or however you want to put it?"
The nature-nurture argument has haunted discussions about same-sex attraction in humans for years, with researchers at one time hunting for a single "gay gene" before determining that one does not exist.
For a while, Zuk said, "I became the go-to person for every zoo that had gay penguins." (Penguin sexuality has struck a nerve with some Americans as evidenced by frequent efforts to ban the award-winning children's book "And Tango Makes Three" about two male penguins raising a baby chick).
Julia Monk, an assistant professor in New York University's department of environmental studies, called the new study an advance that takes a refreshing approach by shifting away from the notion of "same-sex behavior as some conundrum that needs to be solved and really trying to understand the specific environmental and social factors that might drive the prevalence of same-sex behavior in primates."
Julie Teichroeb, an associate professor of anthropology at the University of Toronto at Scarborough in Canada, praised the care and analysis taken by Savolainen and his colleagues and said that research has shown that the "Darwinian paradox" no longer makes sense.
"The idea is that mating behavior is costly in terms of time and energy and then if you're directing that behavior towards same-sex mates, you don't have the potential for offspring," Teichroeb said. "But the fact that that time and energy can go to building really strong alliances with same-sex individuals -- that might allow you to compete better for food or even to defend a group of different-sex mates that you can then mate with.
"It's only an evolutionary paradox if you never engage in different-sex sexual behavior," she said.
Savolainen said his team hopes to build on this study by focusing on macaques and delving into the ways they may benefit later in life from same-sex relations.
"That's a big thing," he said,"because it would definitely be a complete reversal of this Darwinian paradox to show that the more same-sex you do,the more babies you may have in the end."