Like humans, birds reproduce by internal fertilization, which begs the question: Why don’t male members of many land fowl species have penises? Wouldn’t a lack of fully developed external genitalia impede reproduction, making the trait evolutionarily undesirable?

A new report in Current Biology seeks to explain this mystery. Researchers at the University of Florida suggest the gene Bmp4 may in part be to blame. They have found that during chicken development, male birds’ developing genitals wither away as Bmp4 is switched “on.”

“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,” Florida’s Martin Cohn, Ph.D., explained in a statement.

While the researchers are not entirely sure why chickens and other birds have only rudimentary genitalia, they say their study has implications for further, broader research on evolutionary loss.

“Genitalia are one of the fastest-evolving organs in animals, from mollusks to mammals. It is also the case that genitalia are affected by birth defects more than almost any other organ,” Dr. Cohn said.

“Dissecting the molecular basis of the naturally occurring variation generated by evolution can lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected,” he added. “This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations.”

The study, “Developmental basis of phallus reduction during bird evolution,” was published June 6 in Current Biology.

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