The discovery of homosexual Japanese monkeys is providing a fundamental and uncomfortable challenge to one of Charles Darwin's basic evolutionary theories, a Canadian researcher said Monday.
According to the theory of sexual selection, males should compete among themselves for access to mates, or compete for the favors of choosy females, said psychologist Paul Vasey of the University of Lethbridge in Alberta.
Speaking at the annual meeting of the American Association for the Advancement of Science, Vasey and other researchers said although sexual selection has been used to explain the evolution of such unusual characteristics as the brilliant tails of the male peacock and the massive antlers of the elk, the new findings suggest the theory can be applied only to a portion of the animal kingdom.
"There are too many exceptions for the theory to hold," said ecologist Joan Roughgarden of Stanford University in Palo Alto, Calif. Roughgarden, author of six books on evolutionary topics, called for evolutionists to throw out the idea and find other ways to explain how sexual behavior and characteristics could have evolved.
Roughgarden said it is the first time scholars of different disciplines have challenged defenders of the theory of sexual selection, but she acknowledged her hard-line stand is "definitely a minority view at the moment."
Vasey said the Japanese macaques -- monkeys slightly smaller than chimpanzees -- provide some of the most solid evidence Darwin's idea is flawed. He described a colony of about 120 wild macaques in the mountains near Kyoto that shows enormous diversity in sexual behavior, including female-female partnerships.
Contrary to Darwin's idea, females frequently will reject pursuing males for the favors of their same-sex partner. In fact, Vasey told United Press International, if a male tries to break up a pair of females -- essentially giving one the choice between him or her partner -- the female will choose the other female 92.5 percent of the time.
"If females are choosing female sexual partners over male reproductive partners," he said, "that suggests a pretty fundamental revision of sexual selection theory."
Homosexual behavior in female macaques is not all, Vasey added. "I'm out there in the forest and there's this incredible amount of sexual diversity taking place," he said, "and traditional evolutionary theory is not able to account for that."
Among the macaques, he continued, "We've got females that are competing for males with other females, we've got males that are being choosy, males that are sexually coercing females ... we've got females sexually harassing males that don't want to copulate with them, we've got females that have sex with each other, we've got females that are competing with males for other females, we have females that are mounting males."
Vasey said it is clear the females are deriving sexual pleasure when they mount other females. In some positions, he said, a female will rub her clitoris against her partner's back, while in others, "it's common for females to masturbate with their tails" where there is no direct genital contact.
"The traditional evolutionary theory says you do things in order to reproduce," he said, "so why would you do all this non-reproductive sex? To me, that's a really compelling evolutionary puzzle."
The researchers emphasized the twin pillars of Darwin's theory of evolution remain sound. Beginning in 1859, with the publication of his "On the Origin of Species," the English naturalist argued the vast variety of life can be explained by two facts:
-- Offspring are similar but not identical to their parents, and
-- The differences will, over time, favor the traits of offspring that have better-adapted to their environment.
"Darwin got a hell of a lot right," said University of Georgia genetics and ecology Professor Patricia Adair Gowaty, but researchers have been accepting his theory of sexual selection without good evidence.
"What's wrong is our failure to test the theory adequately," she said.
Roughgarden said the problem with sexual selection is it has "piggy-backed" on the success of the other two ideas but cannot stand on its own merits.