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.