What Does Studying a Duck's Penis Have to Do with Today's Men?

Ask Dr. Patricia Brennan, a behavioral ecologist and post-doctoral researcher at Yale University and the University of Sheffield. Since becoming aware of a huge duck penis in 1999 while in Costa Rica, she has been regularly looking at duck butts, specifically studying male duck penises and female duck lower oviducts (the equivalent of a vagina). After seeing a couple of ducks mate, her initial question was why that particular male duck had such a huge penis. Her studies since then have been looking at how evolution works to help male and female ducks control their breeding practices.

Ninety-seven percent of all bird species have no penis at all, but inseminate the female through an opening called the cloaca; the semen passes from his cloaca to her cloaca. Many female bird species can store semen for several days before choosing which sexual partner's semen to use to fertilizer her eggs.

After studying and learning to dissect duck penises, Dr. Brennan began to question what the female duck's sexual organs must look like. She and Dr. Kevin McCracken of the University of Alaska gathered 16 different duck and geese species and compared the anatomy of the males and females.

What they found was that if a male had a long penis, the female had a more elaborate lower oviduct. Dr. Brennan described some lower oviducts as having "weird structures, pockets, and spirals." She surmises that this elaborate female duck anatomy has evolved as a countermeasure against aggressive males: "Once they choose a male, they're making the best possible choice, and that's the male they want siring their offspring. They don't want the guy flying in from who knows where. It makes sense that they would develop a defense."

The evolutionary history of the penis is complex, but scientists agree that birds probably all had penises at one time; how they lost them is unclear. However, scientists have found that duck and geese penises seem to grow, disappear, and re-grow. No one knows why, but Dr. Brennan has surmised that "it may be easier to re-grow it than to keep it healthy." It also appears that female birds whose sexual partners lack penises are untroubled by the fact.

One thing is certain: the penis is not essential for survival. And, according to Dr. Brennan's findings, the bird penis is similar, although not identical, to the penis of the mammal.

The long and the short of all this is that this may mean something for human beings. "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," she said.