Ducks who hang out in big groups have larger penises

Most birds do not have any genitalia, but ducks are an exception.

Ducks have long, spiralling penises thought to have evolved to give males a bit of an advantage, because a third of all duck mating activities are forced. If the future of your offspring depends on it, it seems size does matter.

Exactly what affects the size of a duck’s strangely long penis is not completely understood, but a new study shows it might be something to do with competition.

The paper, published in The Auk: Ornithological Advances, found ducks who hang out in larger groups tend to have larger penises.

To reach this conclusion, the researchers from Mount Holyoke College studied two species of duck: the Ruddy Duck and the Lesser Scaup. While it may sound like Alan Partridge angry at a waterfowl, a Ruddy Duck is the name of a particularly promiscuous duck species.

Ruddy Ducks don’t form pair bonds, and they have relatively long penises compared to their committed counterparts, the Lesser Scaup, which do form pair bonds.

The researchers kept some of the ducks in large groups, and some in pairs. They found for the Lesser Scaups, those in larger groups, on average, grew larger penises than those in pairs.

For the Ruddy Ducks, being in large groups complicated things a bit more. For some, it accelerated their penis growth and how fast they reached sexual maturity, but for others it delayed it.

“This is an excellent experimental study of penis morphology, looking at the effects of social environment on penis size in two duck species that have different mating systems,” said Bob Montgomerie of Queen’s University, an expert on reproductive strategies who was not involved in the study.

The hardest part of the challenge was not measuring the penises, according to lead author Patricia Brennan.

“Keeping ducks in captivity is expensive,” says Brennan. “We were lucky to partner with the Livingstone Ripley Waterfowl Conservancy in Litchfield, Connecticut, where their expert personnel kept the ducks healthy and in beautiful, naturalistic enclosures year-round.”

There still remain mysteries over duck penis size, but the study shows competition does matter.

“The question now is whether the observed increase in penis size in Lesser Scaup under the threat of sperm competition actually gives males a competitive advantage,” said Montgomerie. “Like all good studies, this one will undoubtedly stimulate more research, as it provides both methodologies and a clear focus on interesting questions.”