Ruddy Duck Image: Dick Daniels/Wikipedia

Ducks are one of only two species in the world (the other is the acorn barnacle) that can adjust their penis size to fit a given situation — including competition from other males, according to new research.

Duck mating is an uncomfortable subject, to begin with — male ducks have corkscrew-shaped genitalia, often covered in hooks or barbs. These intimidating specimens have developed over time to keep pace with the evolution of the female’s vagina, which also has twists and deceiving pockets meant to prevent fertilization by less-than-top-notch suitors.

In case that’s not weird enough, ducks also grow their penises anew each year — similar to the way deer and elk grow a seasonal set of antlers.

Evolutionary biologist Patricia Brennan studies duck mating rituals, and she found that how large a duck grows his penis each season is dependent on environmental factors, including the female-to-male ratio.

Researchers studied two breeds: A) Lesser scaup and C) Ruddy ducks. Image: Patricia L. R. Brennan, et al/ via The Auk

Brennan studied two breeds of duck: the lesser scaup and the ruddy duck. Lesser scaups are the nice guys of the duck world. They have a short penis with smooth ridges, and they stick with one mate for the season. Ruddy ducks are players; they have some of the largest penises in the duck world (think eight inches) covered in the aforementioned spines. They’re also promiscuous and prone to forcing copulation with females.

Lesser Scaup Image: Ómar Runólfsson/Wikipedia

For the study, Brennan created two groups of each breed. One group had equal numbers of male and female ducks, while the other had more males. When there were more males than females around, all the lesser scaup males grew penises that were larger than those of their equally matched up peers. By contrast, the ruddy ducks, which already have long penises, developed a sort of … um … pecking order.

The dominant male grew his penis to full size, while the other males, grew shorter penises and only kept them for a short window in the mating season.

So now you know lots of stuff about duck penises. You’re welcome?