Eversion in air: from blogs.discovermagazine.com/loom from Carl Zimmer on Vimeo.

That, ladies and gentlemen, is a duck penis. Science blogger Ed Yong has a great article up today about these freaky, corkscrew behemoths and the equally freaky, labyrinthine duck vaginas. A researcher from Yale has been studying both, and thinks these rather baroque naughty bits evolved in competition with one another, as female ducks tried to evade rape (or, rather, impregnation by a rapist) and male ducks tried to get around those barriers.

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.