Chickens' Small Penis a Result of Genetic Cold Shower

As the result of some strange evolutionary circumstances, most birds don't have penises. But new research on the development of the male chicken and its lack of the appendage reveals new information about the evolution of the cocks.

While still developing as embryos, chickens show the signs of normally developing penises but by the time the fowls reach adulthood, they have little to show for it. They are not alone; about 97 percent of all avian species have little or nothing to show for a penis, according to Nature.    

Martin Cohn, a developmental biologist from the University of Florida led a recent study, which found that male chicken embryos go through the equivalent of a genetic cold shower during their developmental stages that stops the penis from developing into more than a rudimentary phallic nub.

"Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell death in a new location, the tip of the emerging penis," Cohn said in an interview with the BBC.

The team compared chicken embryos with duck embryos. Ducks, along with emus, are among the few well-endowed birds, equipped with spectacular penises that are coiled like corkscrews.

In chickens, the team discovered that a gene called Bmp4 turns on about a week into development, retarding growth of the chicken's penis. The duck embryo, on the other hand, keeps the Bmp4 gene turned off, allowing the bird's organ to continue to grow.

So how do male chickens procreate with such little equipment? They deliver their sperm to the female through an organ called the cloaca, which is present in both sexes of the bird. When the cloaca touch, the male's sperm is transferred to the female's reproductive tract in an act known as the "cloacal kiss."

The research provides great insight into how the chicken's small penis came to be, but it does little to answer why, which opens the door to scientific speculation. Cohn suggests the chicken penis may have been lost as a secondary consequence to the evolution of other body parts also affected by Bmp4, Nature reports.

But others, including Bob Montgomerie, an evolutionary biologist at Queen's University in Ontario, Canada, have different ideas.  Montgomerie told Nature that because the chickens' "cloacal kiss" requires two willing partners, female chickens and other birds may have selected mates with smaller penises to escape forced copulation, which would over time would have influenced the shape the male birds' genital evolution. 

The research was published in the journal Current Biology