Little did he know it, but when biology-challenged Congressman Todd Akin recently claimed that women rarely get pregnant from rape, he accidentally stumbled into interesting science. Namely, duck sex. Female ducks actually do have ways to "shut the whole thing down" during forced matings. It's one of the many surprising ways female animals have control over their own reproduction.

Sexual reproduction in the animal kingdom is often a tenuous alliance between male and female. Both parents want (in an evolutionary sense) to pass on their genes, but that's all they really agree on. Females of most species invest more energy in caring for babies, so they are more picky about when and with whom they mate. To get their wishes, males sometimes turn to strategies like forced matings or "traumatic insemination" (read on...), but their female counterparts have quite a few reproductive tricks up their hoo-hahs too.

Clockwise Vaginas v. Counterclockwise Penises

Unlike most birds, male ducks have penises. And not just your garden-variety penises: corkscrew-shaped phalluses that are over a foot long and capable of "explosive eversion" (watch the videos on this Carl Zimmer blog post if the term isn't evocative enough). They often use these large penises to forcibly mate with unwilling females.

Both sides have evolved some pretty bizarre anatomy in this evolutionary battle of the sexes.

Female ducks may not be able to fight off aggressive males, but they do have ways of keeping out their sperm. Unlike nearly any other species on the planet, female ducks have exceedingly complicated vaginas to keep unwanted phalluses out. And while the males penises spiral counterclockwise, the females' vaginas wind clockwise, making it harder for the males to get in a significant distance. What's more, female ducks have anatomical blind alleys in their vaginas: Unwanted males that can't get very far in end up ejaculating into shallow pouches that keep sperm far away from any eggs. A willing female, on the other hand, can relax her muscles to let the male further in, increasing the chances of conception.

Female ducks are doing something right: While a third of duck sex is the forced kind, only 3% of all offspring are born out of these matings.