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Nothing to crow about: Why did the rooster lose his penis?


Birds may have traded penises for adaptable beaks. Elizabeth Finkel reports. 


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The rooster might have a reputation but it is not well-deserved. Like the males of 97% of bird species, it has no penis to speak of. Insemination is achieved through the “cloacal kiss”. The female lifts her tail; the male curves his under hers. Their cloacal openings pout and the male ejaculates. It’s a precarious manoeuvre that relies on the willingness of the female.

About 3% of the 10,000 bird species do have penises – and utterly remarkable ones at that. It seems to be evolution’s plaything. The duck is famous for its spiral penis, which can exceed the length of the bird itself. On a far branch of the evolutionary tree, emus and ostriches are also well-endowed. This indicates that the ancestor of birds had penises but most descendants lost it.

Why would creatures lose an organ so useful for reproductive success? This compelling question has generated no shortage of hypotheses. One is that the lack of a penis allowed the female to choose the best father for her offspring. Absent a penis, a rooster simply cannot force itself upon a female. Drakes, by contrast, are capable of rape.

Evolution’s plaything turns out to be inordinately easy to sculpt.

Another theory is that losing the penis was the price paid for gaining something else. A recent paper in Current Biology from Martin Cohn at the University of Florida gives weight to this view. He found developing chicken embryos start growing penises just like ducks.

Then a gene called BMP4 switches on. BMP4, a well-known gene to those who study embryo development, sculpts fingers and toes by carving away the chunks of tissue in between. It also gets to work on the embryonic rooster penis, nibbling it down to a nub.

Where is the gain? Darwin’s Galapagos finches, the pre-eminent example of how evolution moulds body parts to their environment, provides an answer: thick beaks evolved for cracking nuts, thin ones for sucking nectar. The beak sculptor is none other than BMP4, with the apparent side-effect of this useful adaptation being loss of the penis.

Evolution’s plaything turns out to be inordinately easy to sculpt. In the well-endowed duck, researchers could nix its developing penis by applying a bead of BMP4. More spectacularly, they applied the Noggin protein, the nemesis of BMP4, to the rooster embryo. Presto! Its nubbin grew.

Mammals also use BMP4 and Noggin to sculpt the penis. What might be the practical application of this work?

“I can imagine applications for infants with genital deformities,” says Melbourne University zoologist Marilyn Renfree, who specialises in mammalian reproduction.

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Elizabeth Finkel is editor-in-chief of Cosmos.
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