Genitalia can be considered one of nature's most important treasures - so vital we've taken to calling some of ours 'jewels.' Birds, however, seemed to have gotten screwed out of their share. Sure, some species like ducks are equipped with body-length corkscrews, but many birds lack a penis. A new study, however, may have figured out the mystery behind the Case of the Missing Bird Penis.
Most fowl, around 97 percent, simply don't have a penis. The external genitalia does begin growing in the embryo stage, but it looks like it's cut short by the expression of a gene, which plays an important role in other avian characteristics as well.
"Our results indicate that evolutionary reduction of the intromittent phallus in galliform birds occurred not by disruption of outgrowth signals but by de novo activation of cell death by Bmp4 in the genital tubercle," reads the study, published in Current Biology.
"These findings, together with discoveries implicating Bmps in evolution of beak shape, feathers, and toothlessness, suggest that modulation of Bmp gene regulation played a major role in the evolution of avian morphology."
The team of researchers looked at the Bmp4 gene in two species of birds: Galliformes, which lives on land and lacks a penis, and Anseriformes, which is a waterfowl and sports a quite well-developed tool. To have a litmus test to compare to, they also looked at emus and alligators.
Despite their lack of external genitalia, birds like the chicken have found a way to copulate - with an organ known as the cloacal. All the two birds have to do is touch their cloacals and a rush of sperm is triggered. Not exactly 'birds and the bees' material, but it seems to get the job done.
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