Ducks.. it’s complicated

If you have ever seen mallards in spring you know that male mallards (drakes) are actually not as sweet and cuddly as they look. Given the chance they rather rape females, especially when a group of drakes is together.

It was long thought that females (hens) were the helpless victims in this and, given the fact that sometimes they drown during such an endeavor, you can’t blame anyone for that opinion.

The sex of most other birds isn’t all to exciting. The males lack external genitalia and females have a simple, as in straightforward and short, vagina. But not so the mallard and a number of other waterfowl.

It is all due to the battle of the sexes.

And that battle isn’t as unequal as it seems.

Enter the penis. Or actually the pseudo penis because it isn’t made from the same tissue as ours. The length of the pseudo penis of waterfowl is correlated to the frequency of forced copulations. Mallards don’t have the longest penises out there, but it has a considerable length. And it is counterclockwise screwed.

The penis of a drake is an instrument developed to be able to force copulation.

It develops in a yearly cycle and is biggest in the mating season. At rest, the penis resides in the phallic sac the wall of the cloaca, inverted (inside out). It gets erect via the lymphatic system rather than the vascular system in mammals. The lymph goes via two lymphatic cavities at the base of the cloaca into a lymphatic lumen in the phallic sac, erecting the penis outwards. Also unlike mammals, semen isn’t transported via a tube within the penis (which is of course the urethra in mammals, something a bird doesn’t have, at least not that way) but in a grove on the outside of the penis.

The vagina of a hen is the answer.

A hen can’t prevent being raped but she can prevent being fertilized by the drake raping her. By making the vagina so complex that most forced copulations end up being in vain. Where the penis is screwed counterclockwise, the vagina is screwed the other way around. And it has blind pouches where a penis can get stuck in preventing it getting further than a few centimeter. Giving the fact that a drakes penis sort of explodes out of the cloaca and immediately ejects its semen, it can’t just go back and try again. Also, deflating a penis does cost a lot more time than inflating it. But since the penis of a duck remains flexible when fully everted, it apparently can navigate a bit inside the vagina.

The more forced copulations a species of waterfowl has, the more complex the hens vagina of that species is.