Research conducted by an Australian scientist indicates that mountain gorillas, particularly females, engage in homosexual behavior.
Dr. Cyril Grueter, Associate Professor and primate expert at the University of Western Australia, was studying the feeding habits of gorillas in Rwanda when he observed sexual activity between some of the females.
Grueter tested three hypotheses to try to explain the behavior, according to Phys.org: the assertion of dominance based on social rank, the strengthening of social bonds, or the easing of relations following an altercation.
"None of the three hypotheses received any consistent support," Grueter said. "So a more prosaic explanation was considered – that homosexual behavior reflects elevated arousal, as there was evidence that homosexual behavior was more frequent at times when females also engaged in heterosexual copulations."
Over the course of two years, Grueter observed that the female gorillas would engage one another sexually when rejected by males.
"They were obviously deriving sexual pleasure from each other," he told the Daily Mail, adding, "I never expected to see something like this."
Of the 22 female gorillas included in Grueter's study from 2008 to 2010, 18 engaged in homosexual activity. This included "genital rubbing," "genital closeness" and mating calls.
"It usually happens that two females get on top of each other, rubbing their bellies and genitals together – that's the most frequent expression [of sexual intimacy]," Grueter said.
He added that, since the observations were made in wild gorillas, and not gorillas held in captivity, it is clear that same-sex mating is natural behavior, according to Phys.org. In this sense they are similar to human beings.
"One thing female gorillas have in common with human women, that has been well documented, is that they can shift from homosexual to heterosexual sex," Grueter said, Phys.org reports.
Over a quarter of the homosexual activity witnessed in the study featured at least one female that had sexual relations with a male the day before or after, according to the Daily Mail.
Grueter's research was published in PLOS ONE journal.