Monkey group exhibits ‘surprising’ bisexual behavior. It’s likely beneficial, study says

While observing a group of rhesus monkeys on a tiny Caribbean island, researchers were shocked by what they saw: widespread homosexual exploits.

“I was expecting to see same-sex behavior … but the level that we saw was surprising indeed,” Vincent Savolainen, professor of organismic biology at Imperial College London, told McClatchy News.

Savolainen and his colleagues determined that same-sex behavior within the monkey population was extremely common and likely beneficial, according to a study published July 10 in the journal Nature Ecology and Evolution.

The rhesus macaque colony, composed of 1,700 animals, lives freely on Cayo Santiago, a 34-acre island off the coast of Puerto Rico. The monkeys, shipped over from India decades ago, are one of the most studied free-ranging primate groups in the world.

Over the course of three years, researchers spent nearly a dozen 12-day stints on the island, where they closely observed the sexual proclivities of 236 males.

What they found was that same-sex behavior was not only common, but it eclipsed different-sex behavior by a wide margin.

Seventy-two percent of the males took part in mounting other males, while only 46% mounted females, researchers said.

“From what we observed, 5% of males engaged exclusively with females, and 31% exclusively with males. Forty-one percent engaged with both (and 23% were not sexually active),” Jackson Clive, one of the study’s authors, told McClatchy News.

Why exactly so many males copulate with both sexes is not clear, but the behavior may give them a reproductive advantage. In fact, males that engaged in same-sex behavior had slightly more offspring than their counterparts interested exclusively in females, researchers said.

“This is because we also found that the males’ sex with each other strengthens social bonds between them, and they are then more likely to support each other in conflicts against other individuals,” Clive said. “This in turn may mean that they can be more successful and more likely to have more children.”

These “coalitions” of males might be compared to the Sacred Band of Thebes, an ancient Greek fighting force composed of male lovers, Savolainen said.

The same-sex behavior was also shown to be heritable, meaning it is able to evolve and be passed down, researchers said. By comparing DNA samples with the monkeys’ sexual activities, they determined genetic factors could explain 6.4% of the same-sex behavior. But the exact genes responsible for the behavior are not yet known.

Savolainen said their findings are significant because they indicate same-sex behavior among rhesus monkeys is “not rare and deviant; it’s actually common, widespread and probably beneficial.”

Same-sex behavior has been documented in a wide array of animals, including insects, birds, bats and other primates, though it’s typically been thought of as rare, researchers said.

Additionally, because rhesus macaques and humans shared a common ancestor about 25 million years ago, the study results could be relevant to understanding human bisexual and homosexual behavior.

“While our results might help combat prejudice against non-heterosexuality,” Clive said, “we also emphasize that society’s obligation for sexual inclusiveness should not rely on any observation of phenomena in the natural world.”

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