Researchers have long wondered why evolution robbed many bird species — like the chicken — of a piece of anatomy considered pretty key in most of the breeding we’re familiar with — the penis. A new study of a wide range of birds has revealed a key gene that stymies penis growth in males and suggests a few reasons that nixing the penis could be evolutionarily advantageous for the animals, though it does make calling a male rooster a cock among the crueler jokes in the history of time.

Rather than a penis, male chickens have, like most birds, a cloaca, a multipurpose orifice used for urinating, defecating, and mating, which is generally performed using a “cloacal kiss” in which the male and female of a species will touch these orifices together long enough for the male to transfer sperm from his cloaca into that of the female and and beginning fertilization. This notably demure sex act has left researchers wondering why some birds, like kiwis, ostriches, some ducks, and their relatives have penises while others, like the chicken, have developed the cloacal kiss instead.

Researchers led by Martin Cohn at the University of Florida asked that question and found a surprising answer. Chickens, it turns out, do have normal penises — or at least, penises that start off developing normally. In their early embryonic stages, chickens develop the bud of a penis. Later, though, as the chick keeps growing, a gene known as Bmp24 kicks into gear and nips penis growth in the bud. By the time they’re hatched, the incipient genitals of the male have withered away to nothing. In birds like ducks and emu, which still have penises they use for mating, Bmp24 remains off, allowing the animals to grow more traditional sex organs.

Some birds, meanwhile, can’t seem to make up their minds on the penis/no penis issue. Both male and female cassowaries, for example, have a penis-like sex organ, but it’s not connected to their reproductive system, and sperm is still introduced in the species via the cloaca. Perhaps this should make us feel more connected with the natural world, though — bird or human, penis or no penis, it seems, mating is an often complicated and occasionally awkward proposition.

Now that they know how birds left their genitals behind, evolutionarily speaking, researchers can move on to the more difficult question of why they did so. Among their early speculation is a theory that a lack of a penis leaves males less sexually aggressive than they might otherwise be. Considering how sexually aggressive — and deeply inappropriate — penis-endowed ducks can be, it’s certainly not a bad notion to start from.

(via PhysOrg, image via flickr)

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