Sexual Conflict: A Twisted Screw

Patricia Brennan has been interested in the weirdness of duck genitals for years. Why are drake penises corkscrew shaped? Why are they so long? Discovery’s Carl Zimmer reports:

As Brennan dissected duck penises, she began to wonder what the female sexual anatomy looked like. If you have a car like this, she said, what kind of garage do you park it in?

Brennan discovered that female ducks have equally weird reproductive tracts (called oviducts). In many species, they are ornamented with lots of outpockets. And like duck penises, duck oviducts are corkscrew-shaped. But while male duck penises twist clockwise, the female oviduct twists counterclockwise.

Brennan speculated that all this bizarre anatomy is the result of a peculiar form of evolution known as sexual conflict. A strategy that allows females to reproduce the most offspring may not be so good for males, and vice versa. For example, male fruit flies inject their mates with lots of chemicals during sex, and those chemicals make her less receptive to other males, thereby boosting his chances of fathering her eggs. But those chemicals are harsh and will make female flies sick. Females, in turn, have evolved defenses against those chemicals, blunting their effects.

With many examples of sexual conflict in nature, Brennan wondered if sexual conflict between male and female ducks was giving rise to their weird genitals. Female ducks pair off with male partners for the breeding season, but they also get harrassed by other males, sometimes being forced to have sex (and sometimes dying from the attacks). A third of all duck matings are forced.

And yet only 3 percent of the ducklings that female ducks produce come from such forced matings. Brennan speculated that the female ducks can block forced copulations with their mismatched spirals. And they might also be controlling which drake got to fertilize their eggs by socking away the sperm of different mates in different pockets. And the extravagant penises of males might be the result of an evolution around those defenses.

Now we have video showing how it all works, too. (And it’s not really safe for work!)

Eversions in barriers / from blogs.discovermagazine.com/loom from Carl Zimmer on Vimeo.

Image by AV Flox. Information from Discovery, via Gizmodo.