Ballistic penises and corkscrew vaginas – the sexual battles of ducks

By Ed Yong | December 22, 2009 7:10 pm

Patricia Brennan from Yale University is trying to encourage male Muscovy ducks to launch their ballistic penises into test tubes.

Normally, the duck keeps its penis inside-out within a sac in its body. When the time for mating arrives, the penis explodes outwards to a fully-erect 20cm, around a quarter of the animal’s total body length. The whole process takes just a third of a second and Brennan captures it all on high-speed camera. This isn’t just bizarre voyeurism. Duck penises are a wonderful example of the strange things that happen when sexual conflict shapes the evolution of animal bodies.

Childhood… violated… Innocence… lost… SCIENCE!Many ducks form bonds between males and females that last for a whole mating season. But rival males often violently force themselves onto females. To gain the edge in these conflicts, drakes have evolved large corkscrew phalluses, lined with ridges and backward-pointing spines, which allow them to deposit their sperm further into a female than their rivals. These extreme penises are even more unusual when you consider that 97% of bird species lack any penises whatsoever.

But female ducks have developed countermeasures. Their vaginas are equally long and twisting, lined with dead-end pockets and spirals that curve in the opposite direction. They are organic chastity belts, evolved to limit the effectiveness of the males’ lengthy genitals. Two years ago, Brennan showed that duck species whose males have the longest penises tend to have females with the most elaborate vaginas. Now, she has found further evidence that these complex genitals are the result of a long-lasting war of the sexes.

Duck penises (and those other birds) work in a different way to human ones. For a start, they are flexible when erect. Human penises stiffen thanks to an onrush of blood; for ducks, it’s lymph that does the job. Finally, human males get erections before having sex but drakes evert their penis directly into the female’s vagina.

That presents a problem. To really study the mechanics of a duck penis, you can’t just fluff the animal. You need to give it something to have sex with. That’s where the glass tubes come in. Brennan used four different shapes: a straight tube; a counterclockwise helix that matches the spiral of the male’s penis; a clockwise spiral going in the opposite direction; and a tube with a sharp 135 degree kink, that mimics the position of the first cul-de-sac pouch in the female’s vagina.

Brennan took these tubes to a commercial duck farm in California, where Muscovy drakes provide semen for artificial insemination. She watched as the handlers placed a female in the same cage as a randy drake, and waited until the male mounted her. Just as he was getting ready to mate, Brennan intercepted him with one of the four oiled tubes, touching it to a pair of sensitive spots that immediately trigger the explosive erection and ejaculation.

Her videos revealed the process of duck erection in painstaking detail, including how long it takes to evert the different parts of the penis, how fast the ejaculate is and how long it takes to retract the appendage (around 2 minutes). They showed that the drakes easily launched their penises into the straight tubes and the ones that matched the direction of their spiral. The tubes that spiralled in the wrong direction or bent at a sharp angle proved to be more challenging, and mostly stopped the males from achieving a full erection.

So the shape of the female duck’s vagina is a physical barrier that prevents the male from launching forth his ballistic penis to its fullest extent. It won’t stop a drake from ejaculating (and those in Brennan’s trials always did), but it does limit how far the semen is deposited along the vaginal tract.

Not all males are hit equally hard by these defences. Those that the female actually wants to mate with have an easier time. If she’s into a male, she strikes a pose that signals her receptiveness, keeping her body level and lifting her tail feathers high. She repeatedly contracts the walls of her genital tract, relaxing them for long enough for favoured suitors to achieve full penetration.

Males who try to force themselves upon her receive no such help and have to cope with vigorous struggling. The female may not be able to resist such advances, but her convoluted vagina gives her ultimate control over where the sperm of her current partner ends up. The fact that only 3% of duck offspring are born of forced matings suggests that females are indeed winning this battle of the sexes. Clearly, it’s not size that matters, but what she lets you do with it that counts.

Reference: Brennan, P., Clark, C., & Prum, R. (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2139

Update: Check out Carl Zimmer’s excellent take on this story

More on animal penises and sexual conflict:

Comments (28)

  1. Nathan Myers

    I imagine they make a noise like “sploink!”

  2. Joel

    I’ll never look at Donald and Daisy the same again

  3. Wow, this is what I call heart warming science. Please keep us updated.

  4. Fascinating. That really is a brilliant illustration of an evolutionary arms race. I will have to make use of it somewhere.
    “Evert” is now my word of the day.

  5. Ed Yong

    I’ll never look at Donald and Daisy the same again

    It does help to explain why Donald has no pants, doesn’t it?

  6. NJ

    …launch their ballistic penises…

    Now, why does that make me think of this story?

  7. Physicalist

    I’m not quite sure; is this relevant?

  8. Excellent! And if it is not topologically fancy penises, then it is sperm motility enhancing foam! Meanwhile, humans pay for things like ExtenZe® and lubricants and get UTIs . . .

  9. This is Rule 34 material….

  10. Daniel J. Andrews

    You learn something new every day. I’ve been popping cloacas to sex ducks for years. Good thing I never tried it with a Muscovy. You’d think they warn incipient biologists about this–it’s all fun and games until someone loses an eye. :)

  11. Rahne

    @9 — Oh NOW you’ve done it. Soon they’ll be porn of Donald and Daffy with corkscrew penises.

  12. Briana

    Hey Ed, not sure if you know this already but your article was featured on Digg.

  13. Jay

    And the worst part? They didn’t even bother to wear gloves.
    To be perfectly honest, I don’t won’t to meet the type of person who is that okay with getting duck ejaculate on their person.

  14. “his isn’t just bizarre voyeurism.”
    jajajaja
    ke no dice!
    haciendole un pajote al pato donald niño jajajajajajjaja
    eso supèra cualquier tratado de Freud!!!
    xDD :S

  15. George

    I loved the Donald and Daisy comment. I’m glad you did this because for some reason when it comes to the penis and vaginas of animals there isn’t much information out there…so when a kid asks “what’s that huge tumor on the back of his [male chimpanzee] behind?” To which the mother answers, “I dunno”. Then to look it up and realize there’s nothing available. Knowledge is power and you just gave the world more power! Kudos – beautifully done.

  16. tobes

    why the hell is the guy with the ruler not wearing gloves???

  17. mfumbesi

    This all sounds painful for the unwelcome drake.

  18. This is a wonderful post! It’s such an unusual subject, but what a great way to introduce crazy facts and important topics like evolution. We need more of this sort of thing!

  19. How can I add a hit counter to my blogger blog? Detailed instructions would be appreciated. I searched counter on google and i can get this html code but i dont know what to do with it. Thanks!.

  20. How did the genes that code for a more twisty vagina increase their proportion in the population? Is it that females who can be choosier about their mates have more reproductive success?

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