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Twisted Duck Sex

by Bryan on October 30th, 2010

These Elegant Trogons are about to kiss, with the male above and a view toward the female's cloaca below.

Blogger’s Note: Today’s post inaugurates an occasional feature on The Daily Wing called Unnatural Selections, odd events in nature that illuminate evolution.

If you’re looking for kink among birds, and a lesson in evolution, consider a trip to your local duck pond.

Birds do it … with a cloaca. In males and females alike, the cloaca is an opening through which waste and either sperm or eggs leave the body. Birds copulate by joining at the cloaca for but a second or so, a lovely move called the cloacal kiss.

Among ducks, however, there is no kiss. Not even dinner and a movie. Many waterfowl engage in forced copulation. At least that’s what biologists call it. Others might call it rape.

Male ducks (drakes) are among the few birds with a penis. And this is no ordinary phallus. Most of the time it is retracted, inside out, into his cloaca. When erect, a duck penis is shaped like a corkscrew, spiraling up to 20 centimeters in length. Yet before copulation, the male will not have the benefit of an erection. Instead, he will seize upon a female for sex and, in what biologists call eversion, extend his penis – in a counter-clockwise route – into her reproductive tract. The extension happens fast, less than a half second in some ducks. It is a feat biologists at Yale University describe in some wild research as “explosive eversion.” Yeah, it’s twisted. Among these waterfowl, the males have presumably evolved, through natural selection, longer penises as an advantage in the case of these forced matings. Here’s an example of eversion in slow motion.

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But biology is rarely uncomplicated. As it turns out, female ducks, in order to ward off forced copulations from undesirable males, have evolved a response to this screwy sex. Their reproductive tracts are convoluted, spiraling in the opposite direction of the male’s penis and outfitted with various cul-de-sacs. When the female is receptive to a particular male, the contours of her tract present no barrier to fertilization; her posture, with her body prone and her tail lifted high, exposes her cloaca and allows for ready fertilization. But when confronted with a forced copulation, a female duck may assume no such receptive posture. Her countervailing reproductive tract has evolved to prevent unwanted fertilization, a literal and genetic dead end to the male’s aggressive behavior.

But how can a biologist test this theory of twisted duck sex? It’s not as if Masters and Johnson can stand around with clipboards watching ducks do it. So the Yale researchers employed duck sex toys. They encouraged males to, well, um, er, perform explosive eversion into glass tubes twisted into different shapes. (The tubes look like bongs.) And in each case the researchers video-recorded the event in order to measure the “functional response to different mechanical challenges.” That’s science parlance for determining if he can “go the distance.” Here’s some video of eversions along more circuitous routes.

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So what’s going on here (besides some kinky video)? It is an example of co-evolution – males evolving one physical characteristic (phenotype) and females a counter-phenotype that suggests conflict rather than cooperation, even in sex. It is in some ways like birth control, or fertilization control, but only on an evolutionary scale. And it illustrates that reproduction is rarely simple, hardly a fairy tale of courtship, selection then cooperation in order to expand the genome.

In formulating his theories on evolution, Charles Darwin most certainly recognized the role of conflict and “the struggle for existence.” Only now, however, are biologists discovering more about “sexual conflict,” how males and females do not necessarily share equally in the burdens and benefits of sex and how they have evolved accordingly. Forced copulations can result injury or death in a female duck or perhaps abandonment by her preferred mate. So she has good reason, the theory goes, to evolve some sort of defense to unwanted males.

“Our observations support the hypothesis that novelties in waterfowl vaginal morphology can restrict forced intromission, and prevent the deposition of sperm deep within the reproductive tract where it would be more likely to achieve fertilization,” wrote the research team of Patricia L. R. Brennan, Christopher J. Clark and Richard O. Prum (who happens to be a friend of mine).

Among Darwin’s few oversights was that he did not recognize the complexity of copulation, how sexual selection and these kinds of contrivances may be hidden or how females may even find other ways to prevent fertilization after copulation. Darwin did recognize, however, that there is more to sex than blind reproduction. “The courtship of animals,” he wrote in Descent of Man and Selection in Relation to Sex, “is by no means so simple and short an affair as might be thought.”

How true. It can also be screwy and twisted.


Patricia L. R. Brennan, C. J. Clark, R. O. Prum. 2009. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia. Proc. R. Soc. B May 7, 2010 277:1309-1314; published online before print December 23, 2009, doi:10.1098/rspb.2009.2139

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One Comment
  1. Pat Folsom permalink

    Hi Bryan,
    Commenting on your finch report this morning on For the Birds – I had lots of Pine Siskins yesterday (30+). Everyday I have a few. One afternoon I had 2 Evening Grosbeaks in the trees, the following morning 6 at the feeder, the first in several years. Didn’t find the Bohemians in Montpelier yesterday, but Larry Clarfeld saw them near the capitol. Snow buntings have been seen in the Valley as well as other places, still waiting for redpolls, shrikes, and Snowy Owl!
    Happy birding and thanks for all you do,

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