Same-sex behaviour among wild female gorillas has been documented for the first time and the Australian behind the study believes it is motivated by sexual arousal.
Associate Professor Dr Cyril Grueter, a primate expert from the University of Western Australia, stumbled on the behaviour when studying the feeding ecology of mountain gorillas in Rwanda,
It was after a miserable period of rain in the jungle and female gorillas enjoying the sunshine were trying to court the male but, Dr Grueter said, he had no interest in them.
That was when he first noticed the females engage in activity with each other.
Working with the Dian Fossey Gorilla Fund International while on an 18-month field trip in 2010, Dr Grueter turned his attention to studying what was behind this behaviour.
"We decided to delve a little deeper, because it reminded me of human homosexual behaviour," Dr Grueter said.
"Gorillas are closely related to humans and we thought by looking at this behaviour we could learn a little more about our own evolution."
The study found female gorilla sexuality was very "flexible," which is consistent with research that has found human women are more fluid in their sexuality than men, he said.
"The female gorillas are quite flexible when it comes to sexuality, they can easily switch from a preference for males to a preference for females," Dr Grueter said.
"When the male is not available they try to entice another female to mate with them."
Out of 22 female gorillas across two separate groups studied, 18 were found to engage in sexual activity with other females, including engaging in genital rubbing.
Although it was not a frequent behaviour, it was something nearly all of the females did.
"We believe it is simply a function of sexual arousal, it doesn't have another function," Dr Grueter said.
Other theories for the behaviour, including asserting dominance, bonding, or reconciliation after a fight, had all been tested but lacked evidence, he said.
The behaviour has been known to happen among captive gorillas.
It has also likely been happening for a long time within wild gorilla populations and would have been observed in the past but there are almost no records of it and it had not been studied, Dr Grueter said.
"No one really tried to systematically document the behaviour and to think about the underlying reason for it, this is the first time for us to do that," Dr Grueter said.
He said unlike the bonobo – a relative of the chimpanzee - that is well known to engage in homosexual acts, very little was known about the homosexual behaviour of other Great Apes.
The study will be published in the PLOS ONE Journal.
