DENVER--Female Japanese macaques go for female sexual partners because the sex is good, a new study suggests. Researchers have been perplexed by rampant female homosexual behavior in some monkeys. Now a team suggests it is a byproduct of an adaptation to derive pleasure from mounting males, by which females arouse a male's interest.
Scientists had noted that Japanese macaque females have sex with other females, mounting each other dozens, hundreds, even thousands of times a liaison. In 1998, evolutionary biologist Paul Vasey, now at the University of Lethbridge in Canada, reported that females even compete with male macaques for female sexual partners. He noted that a male would occasionally try to break up female lovers and get one of them to mate with him instead. In this situation, females chose to stick with lady lovers nine times out of 10. Vasey and others failed to find an evolutionary explanation for this lusty behavior--it doesn't appear to help the females' survival and reproduction.
To try to make sense of the puzzle, Vasey began compiling observations of wild macaques of both sexes and all ages. From preliminary data, he has formed a theory for the evolutionary roots of female-female courtship. His explanation, presented here 17 February at the meeting of the American Association for the Advancement of Science, begins with the play behavior of young male macaques. In their romping, males will often mount other males. In response, the mountee reciprocates by mounting his playmate. Adult females seem to take advantage this reflex, using it to get sexually sluggish or disinterested males to mate with them. Vasey thinks that this would have provided an evolutionary benefit, and that females--who masturbate when they mount either males or females--discovered they could get similar sexual gratification out of mounting females.
Because there's no evolutionary benefit from two females shacking up, the next step is to see if the behavior hurts their reproductive success, says behavioral endocrinologist Kim Wallen of Emory University in Atlanta. Vasey is now investigating whether females will choose female lovers over males when they are ovulating, a choice that would likely dampen a female's chances of passing on her genes.