Unfamiliar with duck loving? Here are the basics: Corkscrewed vaginas and long, temporary, lymph-filled penises that uncoil in fractions of a second. Now researchers have found that some males’ members grow longer when they’re fiercely competing for a mate.

The photo we have to illustrate this magnificent mating equipment is so graphic–in a duck kind of way–that we’re putting it below the jump. As Carl Zimmer memorably put it when writing on the kinkiness of duck sex, it may not be “appropriate for ducklings.”

Last week, Yale University’s Patricia Brennan presented another finding on duck phalluses at a meeting of the Animal Behavior Society. When competing for females, it seems males of the scaup species try to out-size one another.

Brennan found this after placing male scaups in two setups. In one, seven to eight males lived with five or six females, while in the other, drakes lived in more equal numbers with females. As Science News reports, males with more competition grew longer penises–usually around 15 percent, but up to 25 percent longer than those living with more females.

She told Science News that this competition may be the reason for the bird’s unusual penis size to begin with.

“It’s really likely that having a longer penis evolved in male-male competition.”

For more about duck mating, Brennan’s research, and … videos, check out Carl Zimmer’s post.

Related content:
The Loom: Kinkiness Beyond Kinky
Discoblog: Mixed-Up, Adopted Ducks Try to Mate With the Wrong Species
Not Exactly Rocket Science: Ballistic penises and corkscrew vaginas – the sexual battles of ducks
Not Exactly Rocket Science: Infants match human words to human faces and monkey calls to monkey faces (but not quacks to duck faces)
80Beats: Mockingbird to Annoying Human: “Hey, I Know You”

Image: Patricia Brennan