Homosexual behavior emerged to reduce conflicts – 10/04/2023 – Science

In more than 1,500 animal species, from crickets and sea urchins to bottlenose dolphins and bonobos, scientists have observed sexual encounters between members of the same sex.

Some researchers have suggested that this behavior has existed since the beginning of the animal kingdom. But the authors of a new study of thousands of mammal species propose another hypothesis, arguing that same-sex sexual behavior developed when mammals began living in social groups. Although the behavior does not produce offspring to pass on the animals’ genes, it may offer other evolutionary advantages, such as conflict resolution, the researchers proposed.

“This may contribute to establishing and maintaining positive social relationships,” said José Gómez, an evolutionary biologist at the Estação Experimental de Zonas Aridas in Almería, Spain, and author of the new study.

But Gómez warned that the study, published this Tuesday (3) in the journal Nature Communications, could not shed much light on sexual orientation in humans. “The type of same-sex sexual behavior we used in our analysis is so different from that observed in humans that our study cannot provide an explanation for its current expression.”

Previous studies of same-sex sexual behavior generally involved careful observations of a single species, or a small group of them. Gómez and his colleagues instead looked for the broad evolutionary patterns that gave rise to behavior in some species but not others.

Researchers examined the 6,649 species of living mammals that arose from reptile-like ancestors, starting approximately 250 million years ago. When reviewing the scientific literature, they noted which of them were seen performing sexual behaviors between individuals of the same sex — from courtship and mating to the formation of long-term bonds.

The researchers ended up with a list of 261 species, or about 4% of all mammal species, that exhibited these same-sex behaviors.

The analysis showed that males and females were equally likely to be observed performing same-sex sexual behavior. In some species, only males or only females did this. But in others, including cheetahs and white-tailed deer, both males and females engaged in same-sex sexual behavior.

The researchers then investigated how the behavior arose in mammals. When they looked at the evolutionary tree, they found that the species that engage in it were spread across the branches of the tree, suggesting that the behavior arose independently in each lineage.

“With the current data available, it appears that it has evolved several times,” Gómez said.

The researchers concluded that the earliest members of major groups of living mammals, such as primates or felids, probably did not engage in same-sex sexual behavior. As new lineages evolved, some of them began to exhibit this behavior.

Great apes, for example, branched off from other primates about 25 million years ago. They have since evolved to exhibit a much higher rate of same-sex sexual behavior than species in older primate branches, such as lemurs.

Gómez and his colleagues then looked for characteristics that these same-sex branches had in common. A statistical analysis of the evolutionary tree revealed that they tended to be social rather than solitary species.

Paul Vasey, a primatologist at the University of Lethbridge in Canada who was not involved in the study, said that several researchers who have studied same-sex sexual behavior have hypothesized that the evolution of social groups favored it. But they were looking at individual species rather than the entire tree of life.

“For anyone familiar with the literature, I don’t think it’s a big surprise to see that same-sex sexual behavior is related to sociability,” Vasey said. “It’s good to see this conclusion supported by the methods used by the authors.”

Living in a social group offers many benefits to mammals, such as better protection from predators. But it also creates new challenges. Mammalian societies can form hierarchies, for example, in which higher-ranking animals keep lower-ranking animals under control with violence. However, these conflicts can cause a group to split, which is bad for everyone.

Gómez said same-sex sexual behavior may be one of the ways mammals can cope with their unstable social worlds. It may be a way for mammals to form bonds and alliances, reconcile after a fight, or divert aggression into courtship.

Dieter Lukas, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in the new study, remains skeptical of this conclusion. “All things considered, the results of this study do not convince me that there is a single explanation for why same-sex behavior occurs.”

Their skepticism came in part from the data the scientists based their study on. The challenges of observing animals in the wild can mean that homosexual behavior in some species goes unnoticed. “It will be much easier to observe whether the behavior occurs if individuals are in open terrain and active during the day,” Lukas said.

Marlene Zuk, an evolutionary biologist at the University of Minnesota who was not involved in the study, praised the researchers for focusing their study just on mammals rather than the entire animal kingdom. “We’re trying to avoid a single explanation to rule them all.”

In April, Zuk and Jon Richardson, a postdoctoral researcher in his lab, came up with a different explanation for homosexual behavior based on an experiment with crickets. They showed that male crickets sometimes produce courtship songs and attempt to mate with other males and with juveniles.

Since crickets do not live in social groups, this cannot explain the behavior documented by Zuk and Richardson. Instead, crickets and perhaps many other species may engage in same-sex sexual behavior as part of a strategy to take advantage of as many mating opportunities as possible.

Zuk compared the strategy to a smoke detector. “You want a smoke detector that is sensitive enough to detect all fires,” she said. “And if it does, it will occasionally go off when you burn your toast.”