How to Lose Your Phallus (If You're a Bird)

STORY BY Artem Kaznatcheev

Published: June 9, 2013

If you've ever peeked inside your spam folder, then you know there is nothing more important than enlarging your penis. At first glance, biologists would agree with this view on the birds and the bees: why would a male that needs to reproduce by internally fertalizing a female ever settle for a reduction in the size of the required organ? Unfortunately, the birds disagree; penises have been lost several times during bird evolution. A June 6th study by University of Florida scientists in the journal Curreny Biology explores how and why the phallus was lost in most avian lineages.

How do we know that birds once had penises? Because the most evolutionary ancient birds, such as the flightless ostriches and emus and closest non-avian relatives such as crocodilians all have penises (except for the Australian Kiwi that lost his). In fact, even some common birds like ducks, geese, and swans still have theirs. Maybe this is why swans are so happy to pair for life? In the case of ducks, the penis can be longer than their entire bodies, helical, and capable of explosive erections (video is NSFW). However, birds like chickens start to develop a penis but stop at tiny nubs that aren't capable of penetrations. The hedge sparrows and all neoaves (the largest clade of modern birds) have no penises at all and just grind their genital openings together to let the male deposit some sperm by a quick cloacal kiss.

What stops a cock from growing a cock? The surprising finding of the study is that chickens didn't lose the machinery to grow a penis but instead actively supress penis growth. For the first week or so, a chicekn develops a penis in much the same way as a duck (or even mammals like us), but on day 9 the Bmp4 protein triggers mass suicide among penis cells resulting in a stunted phallus. When the scientists absorbed away the protein, the cells stopped dying and the chicken penis kept growing. If the Bmp protein was added to a duck's growing genetilia then instead of a grandiose spiral, the ducks grew a microstub.

This protein plays a role in controlling the growth, size, and shape of many body parts across species. If you allow unchecked expression of Bmp in mice, then like the ducks with artifically high Bmp, they develop tiny chicken-sized penises. On the flip-side, if you eliminate Bmp, then the developed penis is grossly enlarged.

In other words, don't be surprised if you start receiving promising offers of Bmp-blocking quack drugs to get you those 4 extra inches in 4 weeks.

For an accessible but more detailed review, read the National Geographic's How Chickens Lost Their Penises (And Ducks Kept Theirs).

Other Stories by Artem Kaznatcheev
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