Considered "one of the most puzzling events in evolution," male land fowl have reduced or absent external genitalia as adults.
The researchers examined the embryonic development of birds with penises, including ducks, and birds without penises such as chickens.
It turns out that land fowl, which have only rudimentary penises as adults, have normally developing penises as early embryos. Later in development a gene called Bmp4 switches on, causing the bird's budding genitals to shrink away.
In birds with phalluses, that gene remains switched off, allowing full development. In the case of ducks, full development sometimes means phalluses longer than the bird's body. Water fowl are believed to retain their penises to keep water from washing sperm away.
"Genitalia are one of the fastest-evolving organs in animals, from mollusks to mammals," said Martin Cohn of the Howard Hughes Medical Institute and University of Florida, Gainesville. "It is also the case that genitalia are affected by birth defects more than almost any other organ."
Despite losing their genitalia, the males still manage to fertilize female birds through internal insemination. Chickens, for example, perform a "cloacal kiss," in which the male and female must cooperate to rub their opposing cloacae together.
It's not entirely clear why land fowl would lose their penises, said graduate student Ana Herrera, lead author of the study. She says it may be that the loss leaves hens with greater control over reproduction.
The new research, published today in the journal Current Biology, could "lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected," Cohn said. "This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations."