Why Did the Chicken Lose Its Penis?

This is Scientific American's 60-Second Science. I'm Karen Hopkin. This'll just take a minute.
Most birds don't have penises.
Good, I have your attention.
Of course female birds don't have penises, but neither do most males.
Now researchers have unraveled the genetics behind the disappearance of this anatomical apparatus.
Their penetrating insights are in the journal Current Biology.
Only 3 percent of avian species have males with obvious organs of intromission.
Ducks and other water fowl are fairly well endowed.
But most birds have just rudimentary gear, if anything at all.
To get to the root of this penile puzzle, researchers compared the embryos of chickens and ducks.
Both types of birds start to develop a member.
But in chickens, this so-called "genital tubercle" shrinks before the little guys hatch.
And it's because of a gene called Bmp4.
Bmp4 basically tells cells to commit suicide.
This form of cell death keeps birds from sprouting teeth.
And in developing male chickens, it causes the phallus to regress.
Why this happens is still a mystery.
It may allow female birds to be more selective about their mates.
Because males with external equipment tend to impose it where they will.
Whereas those without have to let the ladies rule the roost.
Thanks for the minute, for Scientific American's 60-Second Science. I'm Karen Hopkin.