Tuesday, May 1, 2007

Duck penises show "arms race" between sexes


Several species of ducks have evolved complicated genitals in what appears to be an "arms race" between the sexes, researchers reported on Tuesday.

And females may be coming out ahead, said the team of biologists at Yale University in Connecticut and the University of Sheffield in Britain.

Their findings not only open a window into a little-studied area of biology, but could help shed light on how evolution works to help both males and females control their own breeding, the researchers said.

Patricia Brennan of both Yale and Sheffield was trying to figure out why some species of birds have penises and some do not.

"Birds are the only group where it mostly has been lost -- 97 percent of birds do not have phalluses at all," Brennan said in a telephone interview.

"So if it is such a handy tool, why don't they have them any more?" Brennan asked.

Instead, they mate using what biologists call a "cloacal kiss" -- a brief touch of the single opening that birds of both sexes have for disposing of waste and that both eggs and sperm come out of.

Brennan noted that in many species, females choose a mate after he puts on an elaborate courtship display, and breeding pairs are often monogamous.

An exception is ducks -- especially mallards. Although mallards pair off to mate, females are often raped by stray males.

Yet studies show that these rapes do not pay off for the males. "Even in a species where 40 percent of the copulations are forced copulations, the ducklings still are mostly sired by the mates," Brennan said.

"That implies the females may have some kind of mechanism that allows them to keep control of the paternity."

So Brennan's team looked at a lot of duck bottoms.

What they found surprised them -- corkscrew-shaped oviducts, with plenty of potential dead-ends.

"Interestingly, the male phallus is also a spiral, but it twists in the opposite, counterclockwise, direction," said Yale ornithologist Richard Prum in a statement.

"So, the twists in the oviduct appear designed to exclude the opposing twists of the male phallus. It's an exquisite anti-lock-and-key system."

Brennan believes females evolved convoluted oviducts to foil the male rapists.

"You can envision an evolutionary scenario that, as the male phallus increases in size, the female creates more barriers. You get this evolutionary arms race," Brennan said.

Only if the female is relaxed and cooperative can the male's sperm get anywhere near the unfertilized eggs, the researchers suggest.

"What I think is really cool is this does speak a lot about the ability of the female to have these cryptic mechanisms of choice," Brennan said.

And it may mean something for people. "We can expect that these types of antagonistic traits are probably widespread and are likely part of the reproductive interactions of all sorts of animals, including humans," Brennan said.


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