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 proposed that this behavior has existed since the dawn of the animal kingdom. But the authors of a new study of thousands of mammalian species paint a different picture, arguing that same-sex sexual behavior evolved when mammals started living in social groups. Although the behavior does not produce offspring to carry on the animals’ genes, it could offer other evolutionary advantages, such as smoothing over conflicts, the researchers proposed.

“It may contribute to establishing and maintaining positive social relationships,” said José Gomez, an evolutionary biologist at the Experimental Station of Arid Zones in Almeria, Spain, and an author of the new study. But Dr. Gomez cautioned that the study, published on Tuesday in the journal Nature Communications, could not shed much light on sexual orientation in humans. “The type of same-sex sexual behavior we have used in our analysis is so different from that observed in humans that our study is unable to provide an explanation for its expression today,” he said. Previous studies of same-sex sexual behavior have typically involved careful observations of a single species, or a small group of them. Dr. Gomez instead looked for the big evolutionary patterns that gave rise to the behavior in some species but not others.

The researchers surveyed the 6,649 species of living mammals that arose from reptile-like ancestors starting roughly 250 million years ago. Looking over the scientific literature, they noted which of them had been seen carrying out same-sex sexual behaviors — defined as anything from courtships and mating to forming long-term bonds.

The researchers ended up with a list of 261 species, or about 4 percent of all mammalian species, that exhibited these same-sex behaviors. Males and females were about equally likely to be observed carrying out same-sex sexual behavior, the analysis showed. In some species, only one sex did. But in still 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. Looking at an evolutionary tree, they found that species engaging in it were scattered across the tree’s branches, suggesting that the behavior independently arose in each lineage. “With the current data available, it seems that it has evolved multiple times,” Dr. Gomez said. The researchers concluded that the earliest members of major groups of living mammals, such as primates or cats, probably didn’t engage in same-sex sexual behavior. As new lineages evolved, some of them started exhibiting the behavior.

Apes, for example, branched off from other primates about 25 million years ago. Since then, they evolved a much higher rate of same-sex sexual behavior than species on older branches of primates, such as lemurs. Dr. Gomez and his colleagues then looked for traits that these same-sex branches had in common. A statistical analysis of the evolutionary tree revealed that they tended to be social species instead of solitary ones.