Social environment matters for duck penis size


Most birds lack genitalia, but male ducks are known for their long, spiralling penises which have evolved through an ongoing cat-and-mouse game with females.

A new study has looked at whether these impressive organs are affected by the social environment – that is, whether drakes that face more competition grow bigger penises. While this appears to be true for some species, in others the relationship between social environment and penis growth is more complex.

Patricia Brennan of Mount Holyoke College, Massachusetts, USA, and her colleagues tested their hypothesis in two species: Ruddy Duck, which is very promiscuous, does not form pair bonds and has a relatively long penis, and Lesser Scaup, which forms seasonal pair bonds and has a relatively short penis.

Drake Lesser Scaup has a relatively small penis in the duck world (Jeff Lack).

Keeping captive ducks in either pairs or groups during the breeding season over two years, they found that Lesser Scaup had longer penises on average when housed in groups with other males, as predicted. For Ruddy Ducks, the effects were more complicated: many males failed to reach sexual maturity until the second year of the experiment, and when they did the smaller Ruddy Duck drakes housed in groups grew their penises faster than males housed in pairs, but grew out of sync with each other and stayed in reproductive condition for only short periods of time.

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Small Ruddy Duck males faced with intense competition may strategically offset their development from each other to reduce the costs of male-male aggression and make the best of a bad situation. Additionally, since Ruddy Ducks already have relatively long penises on average compared to other waterfowl species, their ability to grow even larger based on social cues may be limited. In any case, the study shows that the level of competition that individual male ducks experience can have a big effect on their genitals.

The biggest challenge during the study, says Brennan, wasn’t measuring the ducks, it was simply keeping them housed and fed. “Keeping ducks in captivity is expensive,” said 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.”

“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 Queen’s University’s (Belfast) Bob Montgomerie, an expert on reproductive strategies. “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.”



Brennan, PLR, Gereg, I, Goodman, M, Feng, D, and Prum, R O. 2017. Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl. The Auk 134:882-893.