Not Exactly Rocket Science

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:

• South African wildlife - Wait, that's not a trunk...
• Traumatic insemination - male spider pierces female's underside with needle-sharp penis
• Horrific beetle sex - why the most successful males have the spikiest penises
• Holy fellatio, Batman! Fruit bats use oral sex to prolong actual sex
• Male chimps trade meat for sex
• Frigid echidna sex - competition drives males to mate with hibernating females