A group of researchers has authored a study showing that male ducks who post up with other dude ducks get longer penises.

The duck penis seems to be the source of much scientific fascination. The female duck vagina has co-evolved with the corkscrew-like male duck penis to prevent forced copulation, which is widespread across duck species. Patricia Brennan, ornithologist at Mount Holyoke College and co-author of the study, told Motherboard about this co-evolution,

“This co-evolution is actually the result of an arms race over the control of reproduction, because male ducks force females to copulate and females don’t like it at all.”

For a visual of the corkscrew dude duck dick watch this video. It’s uh, well, just watch it.

But now these researchers have scientific data to show that duck penises grow when males are around each other. The study, published in The Auk: Ornithological Advances, outlines their findings. From the study:

“We found experimental evidence that in a male-biased social environment, consisting of several males and fewer females, the penis in Lesser Scaup (Aythya affinis) grew longer in 2 separate years, than in males housed in pairs, as predicted if male–male competition influences penis morphology. In Ruddy Ducks (Oxyura jamaicensis), males instead showed evidence of reproductive delays that were explained both by a male’s size and his social environment: most males in social groups exhibited shorter penises, variable onset and duration of genital maturation, and faster penis growth rate.”

So why is this happening? Well, it’s not because females particularly enjoy the larger duck penis, but that male ducks are in sexual and evolutionary competition with each other. A Yale University press release on the study described this competition,

“It appears that longer penis size has little to do with female preference, but is the result of competition between paired males and unpaired males that attempt to fertilize females by force.”

Sounds kind of sinister… Patricia Brennan told Motherboard about this evolutionary phenomenon, saying that because of the threat of duck rape, it’s unlikely that females favor longer penises,

“Maybe the females love big penises because they indicate something about the males. But I already sort of knew that that wasn’t going to be likely, because of these forced copulations.”

Contrarily, it’s competition among male ducks that has driven the evolution of the duck penis, Brennan said, “Sperm competition and male-male competition do favor a longer penis.”

Richard Prum, professor of evolutionary biology at Yale University and co-author of the study (and yes, there is paternal relation to this writer) said the results of the study demonstrate the intersections of social interaction, sexual conflict and autonomy, and evolution in the natural world,

“This study illustrates how social forces can actually shape individual anatomy, but it also suggests how sexual conflict and sexual autonomy shapes social behavior.”

The federally-funded study has come under fire from conservatives for pointless government spending blah blah, but Prum said the study helps us understand evolution and sexual choice, “This research has helped us understand the importance of sexual autonomy, that freedom of choice matters in nature.”

It remains to be seen if results are the same in humans.