The fact that many animals engage in same-sex behavior is well-established, and has been reported in over 1500 species, including invertebrates, birds, reptiles, and mammals. A new study published in Nature Communications found that this behavior among mammals is especially prevalent in social species, and proposes that it evolved in some species to maintain positive social relationships and to diminish aggression and conflict.
It's easy to see how opposite-sex behavior has a reproductive advantage for an individual. But it's harder to see how same-sex behavior could have evolved through natural selection. The authors of the study propose that for social species it helps individuals survive and thrive. A link to the New York Times article about the study is here.
"It may contribute to establishing and maintaining positive social relationships," said José Gómez, an evolutionary biologist at the Experimental Station of Arid Zones in Almería, Spain, and an author of the new study. …
Dr. Gómez said that same-sex sexual behavior might be one of the ways that mammals can manage their unstable social worlds. It may be a way for mammals to form bonds and alliances, to reconcile after a fight or to divert aggression into courtship.
It's long been known that bonobos, a species within the chimpanzee genus, and humanity's closest relative, uses sexual activity — all kinds — to ease social tensions. From a 2006 Scientific American article, "Bonobo Sex and Society" (link here):
The species is best characterized as female-centered and egalitarian and as one that substitutes sex for aggression. Whereas in most other species sexual behavior is a fairly distinct category, in the bonobo it is part and parcel of social relations–and not just between males and females. Bonobos engage in sex in virtually every partner combination (although such contact among close family members may be suppressed).
The authors of the Nature study are careful to note that human same-sex behavior is so different from the type they studied in non-human primates that there may be a wholly different biological reason for it. The New York Times article:
But Dr. Gómez 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.
I always found it interesting that those who denounce homosexuality in our society first condemn it because it's "unnatural." Then if it's pointed out that it's common throughout the animal kingdom, they'd decry it because it is natural, and animalistic.