How did the chicken lose its penis? By killing off the growing appendage in the egg.
That's the finding of a new study, which reveals how most birds evolved to lose their external genitalia. Turns out, a particular protein released during the development of chickens, quail and most other birds nips penis development in the bud, according to the new research, published today (June 6) in the journal Current Biology.
The findings have implications for genital development in general, which is important because birth defects in the external genitalia are among the most common congenital defects in humans, said study researcher Martin Cohn, a developmental biologist at the Howard Hughes Medical Institute and the University of Florida.
"Comparative evolutionary studies of development allow us to understand not only how evolution works, but also gives us new insights into the possible causes of malformations," Cohn told LiveScience.
About 97 percent of birds lack penises entirely. The exceptions are real odd ducks — literally. Some waterfowl have coiling penises that can exceed the length of the rest of their bodies. [Whoa! The 9 Weirdest Animal Penises]
The most primitive group of birds, paleognaths, which include emus, kiwis and ostriches, have well-developed phalluses, as well. Along the evolutionary line, two newer groups diverged: anseriformes, which include penis-wielding ducks, swans and geese, and galliformes, which make up most land-loving birds and lack penises.
To understand how this genital gap diverges in development, Cohn, along with research assistant Ana Herrera and their colleagues, grew embryos from chickens (galliformes) and ducks (anseriformes) and tracked their penis growth.
"It's pretty surprising, actually," Cohn said. "Chickens and ducks start to develop their genitalia in such a similar manner that they're almost indistinguishable."
A few days after a primitive penile swelling appears on chick embryos, however, development abruptly halts, and then regresses. By the time they're born, chickens and their galliforme relatives are left with only an opening called the cloaca, rather than an external penis. In duck embryos, the penis continues to grow.