ducks on a rock
September 21, 2017

Sexual rivals can influence the size of a duck’s penis

Odds are, most people haven’t spent a lot of time pondering the specifics of duck penises, but a researcher from Mount Holyoke College in South Hadley, Massachusetts has spent more than a decade investigating their genitalia and has published a new study detailing her findings.

As study author Patricia Brennan, a visiting lecturer of biological sciences at the institution, told National Geographic earlier this week, she didn’t even realize until near the end of her graduate school work that birds could even have penises. In fact, 97% of them do not, she explained.

Male ducks are one of the exceptions, and unlike most species, they grow a new one each year. Most of the time, they are hidden, but you can convince a duck into showing you his by turning him over onto his back and applying pressure to his belly, Brennan noted. “If you know exactly where to press, you can pop the penis out. They’re quite cooperative.”

The topic of her latest research, however, involves investigating the factors into what determines the size of a duck’s penis – which interestingly enough, is corkscrew shaped, Brennan explained. Specifically, she wanted to see whether or not competition with other males would cause a duck to grow a larger penis than those facing less – ahem – stiff competition from rivals.

Trying to ‘sneak in some copulations’ before the group leader

As Brennan and her colleagues reported in the journal The Auk: Ornithological Advances, they gathered two different types of ducks with two vastly different mating systems (ruddy ducks and lesser scaups) and split them into either pairs or groups of five females and eight males.

Ruddy ducks, she explained, are highly promiscuous, usually copulate by force, never naturally form pair bonds and tend to have larger penises, the researchers explained in a statement. Lesser scaups, on the other hand, do tend to form seasonal pair bonds, typically have small penises and are far less likely to try to force themselves onto females.

All of the ducks were kept in outdoor aviaries during the breeding season over a two year span, and the study authors found that, as expected, lesser scaups tended to have longer average penis length when they were housed in larger groups containing multiple other males.

However, things were a little more complex with the ruddy ducks, as a significant number of males failed to reach sexual maturity until year two of the experiment. Once they did, those in groups grew their penis more quickly, but the timing of penis growth varied from duck to duck and only one alpha male maintained his reproductive organ for an extended period of time.

“Everybody else grows a penis very quickly,” Brennan told Nat Geo, “trying to sneak in some copulations before the [dominant] male starts beating them up.” Once they did so, their penises quickly returned to a non-reproductive state, making the ruddy duck one of the rare species that modifies its genitalia in response to its social environment.

-----

Image credit: Image: Kurt Bauschardt/Flickr