Social environment matters for duck penis size
(PHYSorg.com Biology)

Most birds lack genitalia, but male ducks are known for their long, spiraling penises, which have evolved through an ongoing cat-and-mouse game with females. A new study from The Auk: Ornithological Advances looks at whether these impressive organs are affected by the social environment-that is, whether male ducks 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.