"The party line is that genitals are used for the exchange of sperm," she says. "But the fact is that among mammals, they are often colored very brightly and are bigger than they need to be."
She believes oversized genitalia, the peacock's tail and perhaps even the enormous human brain evolved as a medium of communication, of body language between members of the same sex, because of this need for social inclusion.
The second part of her theory is that females do not choose males for their genes, as Darwin taught, but to avoid "deadbeat dads." She says females manage male power by selecting for good fathers rather than good sperm. This, she believes, creates a marketplace for reproductive opportunity.
Dominant males have a lot on their plate, maintaining their physical condition, controlling large territories and seeing off challengers. So it is in their interest to sub-contract out the task of finding a mate. The example she gives is the bluegill sunfish of North America, where a dominant male will recruit a smaller, feminine male -- so-called because he sports female colors -- in what looks like a homosexual courtship. They mate with a female in a menage a trois. The conventional view is that the feminine male mimics the female to steal copulations. But Roughgarden says no one has proved the dominant male does not know the feminine male is male. She argues that he is negotiating a reproductive opportunity for both himself and the dominant male, that he may have been "schooled" with the females and therefore brings to the deal his prior rapport with them.
Data show females prefer to enter territories containing dominant and feminine males.
She admits there is no direct evidence to support her hypothesis and has a get-out clause, arguing that as a theoretical ecologist it is her job to explain diversity and the job of experimentalists to gather the proof.
Paul Vasey, a behavioral neurobiologist at the University of Lethbridge in Alberta, Canada, studies Japanese macaques in their habitat of Mount Fuji. He says their lesbian pairings in the breeding season do not promote social cohesion, because -- just as in heterosexual pairs -- the females avoid incest. Dominant females will often also protect their subordinate lesbian partners against higher-ranking aggressors. In his opinion, their motivation is pure sensual pleasure.
Perhaps, in taking on Darwin, Roughgarden only wants to set the ball rolling. In the book she makes clear her personal and political standpoint, warding off criticism with the argument that throughout history those who have upheld Darwinian theory have had an axe to grind -- whether it be to defend male philandering or to propagate the notion of a genetic elite.
She is also careful not to extrapolate her findings to humans, pointing out that patterns of homosexuality vary between species.
"One can't draw parallels with humans other than to say that homosexuality is a regular part of nature and not some pathology," she says. However, she mentions the hijra, an ancient, caste-like group of transgendered people in India, and traces gender-crossing in history from the Cybelean priestesses of the Roman Empire, through the transvestite saints of the middle ages right back to Joan of Arc.
Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People, by Joan Roughgarden, will be published by University of California Press.