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Six decades ago researchers documented same-sex sociosexual behavior between male macaques on Cayo Santiago and described it as almost as common as similar behaviors between two different sexes.

It eventually became clear that these primates and many others commonly engage in same-sex sociosexual behaviors whether humans are around or not.

Males engaging in the same-sex sociosexual behaviors were more likely to back each other up in disputes with other monkeys, which gave them a winning edge.

Savolainen took advantage of the carefully tracked family trees of the Cayo Santiago monkeys and their genetic data to evaluate the role of inheritance in their same-sex sociosexual behaviors.

The animals that engaged in same-sex sociosexual behavior seemed to have slightly better reproductive success.