How birds lost their penises

Jun 06, 2013
This is an embryonic chicken penis (pseudocolored red) viewed under a scanning electron microscope. Credit: C.L. Perriton and M.J. Cohn

In animals that reproduce by internal fertilization, as humans do, you'd think a penis would be an organ you couldn't really do without, evolutionarily speaking. Surprisingly, though, most birds do exactly that, and now researchers reporting in the Cell Press journal Current Biology on June 6 have figured out where, developmentally speaking, birds' penises have gone.

It turns out that land fowl, which have only rudimentary penises as adults, have normally developing penises as early . Later in development, however, the birds turn on a that leads their budding penises to stop growing and then wither away.

"Regulation of the balance between and cell death is essential for controlled growth and development," said Martin Cohn of the Howard Hughes Medical Institute and University of Florida, Gainesville. "Too much cell division or too little cell death can lead to overgrowth or mis-regulated growth, as in cancer. If the balance is tipped in the other direction, deficient cell division or excess cell death can lead to underdevelopment or even absence of a tissue or organ.

"Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of in a new location, the tip of the emerging penis."

This video is not supported by your browser at this time.
Credit: Cell Press

A critical gene in the process is one called Bmp4. In chicken development, Bmp4 switches on and the birds' developing genitals shrink away. In more well-endowed ducks and , that gene stays off and their penises continue to grow.

It's not entirely clear why chickens and other birds would have lost their penises, said graduate student Ana Herrera, lead author of the study, but it may be that the loss leaves with greater control over their reproductive lives.

The findings could have implications for understanding evolutionary loss more broadly. (Think of snakes and their lost limbs, as another example.) They might yield some answers to medical questions as well.

This is a developing penis (pseudocolored red) of a chick embryo before it regresses, viewed under a scanning electron microscope. Credit: A.M. Herrera and M.J. Cohn, University of Florida

"Genitalia are one of the fastest-evolving organs in animals, from mollusks to mammals," Cohn said. "It is also the case that genitalia are affected by birth defects more than almost any other organ. 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. This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations."

Explore further: Conservation and immunology of wild seabirds: Vaccinating two birds with one shot

More information: Current Biology, Herrera et al.: "Developmental Basis of Phallus Reduction During Bird Evolution." dx.doi.org/10.1016/j.cub.2013.04.062

Related Stories

Guppies and sexual conflict? It's a genital arms race

Jun 03, 2013

(Phys.org) —It's not always easy to tell if a fish is male or female: they look more or less the same. But there are exceptions, such as guppies and, as with humans, guppy genitalia varies in size across ...

Recommended for you

Nature offers video of 10 cutest animals of 2014

16 hours ago

(Phys.org)—The journal Nature has released a video that ventures a bit from its traditional strictly-science approach to technical journalism—it's all about the cutest animal stories of the past year ( ...

Big data and the science of the Christmas tree

20 hours ago

Often called the "Cadillac of Christmas trees," the Fraser Fir has everything a good Christmas tree should have: an even triangular shape, a sweet piney fragrance, and soft needles that (mostly) stay attached ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.