Most male birds are missing a little something down there. In 97% of living birds, the male doesn't have a penis; instead, he secretes sperm out of an opening called a cloaca, which is also used for excretion of urine and feces. Scientists comparing embryos of the Pekin duck, Anas platyrhynchos, which has an external penis, and the domestic chicken, Gallus gallus domesticus, which lacks one, have now figured out why this is. At first, the chickens' genitals develop the same as the ducks', with a phallus precursor (red in above image) appearing in the first few days. But 8 or 9 days into the embryos' development, the chickens start to look drastically different: The cells in the rudimentary penis begin to die, eventually leaving only a slight bulge. When the researchers looked at the patterns of gene activity in the embryos, they found that the cell death was linked to the switching on of a gene called Bmp4. Turning on Bmp4 in duck embryos made their penises stop growing during development as well, the team reports online today in Current Biology. The findings likely mean that a change to Bmp4 is what led, in evolutionary history, to some bird lineages lacking a penis, although the evolutionary pressures that may have led to this are still under debate. Because other Bmp genes exist, and they're found across the tree of life, the researchers say that the mechanism discovered here could also reveal how other organisms have lost body parts throughout evolution.

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