Some genitals fit, but what of the duck?

"Anatomically the vagina is designed to receive the penis."

This observation by the Kentucky cardiologist - part of his 1991 paper, "Pathophysiology of Male Homosexuality," which was discussed during a Senate hearing on his nomination for surgeon general of the United States - seems reasonable by itself.

Some might wish he had written "evolved" instead of "designed." Biologists who study genitalia, however, say even evolution doesn't always point to the most obvious explanation behind behavior or physiology.

From a scientist's perspective, Holsinger's words would have been spot on if he had written them in 1591, when the highest scientific authorities believed that the vagina was designed for the penis - not the other way around. Earlier that century, the world's leading anatomist, Andreas Vesalius, drew an accurate representation of the penis but a gross distortion of the vagina that made it look like a penis turned inside out. Subsequent anatomists corrected this.

The genitalia are where previous thinkers saw design flaws that they attributed to divine punishment. Eve's transgression made the vagina rather narrow for comfortably delivering babies, and Adam's transgressions, according to St. Augustine, cost men the ability to control their own erections.

Other species were stuck with genitals that are more difficult to explain. The Argentine lake duck, for example, has a phallus that extends more than a foot - as long as the rest of the duck - and is coiled like a corkscrew. How and why such a thing would evolve puzzled Patricia Brennan, a biologist at Yale University.

Other duck species have large and elaborate penises, although they are among only 3 percent of birds that have a penis at all. Long ones tend to evolve in species where different males compete to fertilize the same females; whoever gets the sperm farther down the oviduct wins.

But some ducks were taking this to extremes.

To investigate this phallogical mystery, Brennan looked for clues in duck vaginas of various species. What she found was even more baffling.

"It spirals as well but in the opposite direction as the male spiral," she says. Beyond that, she found duck vaginas often had pockets and cul-de-sacs.

Brennan's hypothesis is that the female's reproductive tract evolved to keep out the penises of undesirable males, which in many species have a habit of trying to force themselves on females who've already chosen a mate. A very long, reverse-spiral vagina would make it difficult for a male to impregnate a female without her full cooperation.

The male members probably evolved to extreme lengths as an adaptation to these long, convoluted vaginas, Brennan says. By comparing 16 different species, she found that as females got longer and more complex vaginas, the males got longer penises.

She proposes a sort of evolutionary arms race, in which both sexes adapted to gain control over their reproduction.

She still can't explain how the males benefit from their corkscrew-shaped members. She's hoping to get clues by coaxing male ducks to mate with a genitally correct female decoy whose vagina is made from a thick glass tube. That way Brennan can see how the twisted penis fits.

While the human system of penis and vagina isn't quite so convoluted, it still seems jury-rigged in various ways, says Swarthmore developmental biologist Scott Gilbert. The most obvious problem goes back to an old joke, he says, about the questionable engineering decision to put sewer lines through our favorite recreational areas.

Scientists point out that species' sexual evolution is intertwined with individuals' sexual development, and so works within a big constraint - a single gene, called SRY, turns an otherwise female fetus into a male. Most of what makes a boy different from a girl depends on hormones.

At six weeks, a human embryo is a sort of hermaphrodite; it has the makings of both male and female genitals. The bit of tissue that becomes a penis if it's a male develops into a clitoris in a female.

Male fetuses secrete what's called anti-Mullerian hormone to prevent development of a female's uterus and fallopian tubes. There's no anti-nipple hormone, however, so those stay with males. (Breast development is triggered by a hormone at puberty.)

The anti-Mullerian hormone dissolves what would be the upper part of the vagina. The lower part becomes something called the prostatic utricle. It is a duct that, in the male, leads nowhere.

Obviously, says Gilbert, "this is not a good piece of design."

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