In the sensitive matters of reproduction, birds seal it with a kiss. Not a peck. Not a smooch. Birds instead do it with the cloaca – a rear opening in male and female through which waste, sperm or eggs all must pass. During copulation birds conjoin at their cloacas. Although the union lasts only seconds, it is an elegant maneuver nonetheless. We call it the “cloacal kiss.”
Among ducks, however, there is no kiss. Not even dinner and a movie. Male ducks (drakes) are among the few birds with a penis. This is no ordinary phallus. It’s shaped like a corkscrew, spiraling up to 20 centimeters in length. (Don’t believe me? Have another look above at my photo of a Black-bellied Whistling Duck.) So rather than exchanging a cloacal kiss, male ducks seize upon females in what biologists politely call “forced copulation.” We might call it rape. Before copulation, the male does not have the benefit of an erection. Instead, as he overtakes a female, he extends his penis – in a counter-clockwise route – into her reproductive tract. The extension happens fast, less than a half second in some ducks, a feat biologists describe as “explosive eversion.”
But, as it turns out, females aren’t entirely passive in this screwy situation. In the battle of the sexes, females have evolved a crafty defense to the male’s aggression. And, yeah, it’s twisted. To ward off forced copulations from undesirable males, the female duck’s reproductive tract spirals in the opposite direction of the male’s penis and is outfitted with various cul-de-sacs. When confronted with an unwelcome male, the countervailing tract can prevent fertilization – a literal and genetic dead end for the male. Rejection times two. When she’s receptive to a particular male, however, with her body prone, her tail lifted high and her cloaca exposed, the contours of a relaxed female’s tract present no such barrier to fertilization.
But how can a biologist test this notion of twisted duck sex? It’s not as if Masters and Johnson can stand around with clipboards watching ducks do it. So researches at Yale University (including my pal Rick Prum) 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 – this is a college experiment, after all.) And in each case the researchers video-recorded the event to measure the “functional response to different mechanical challenges.” That’s science parlance for determining if he can “go the distance” along her countervailing routes. Here’s some video of eversions along those winding paths.
So what’s going on here (besides some kinky video)? This is an example of coevolution – 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. These forced encounters may prevent her from going about her business feeding herself and avoiding predation. So she has good reason, the hypothesis goes, to evolve with 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 lead by Patricia L. R. Brennan.
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.”
In other words, “it’s complicated.” It can also be 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