Most birds don’t have penises. The genetics behind the disappearance of the bird or chicken penis is written up in the journal Current Biology. [Ana Herrera et al, Developmental Basis of Phallus Reduction During Bird Evolution]
In order to solve this puzzle, researchers compared chicken embryos and duck embryos. Both birds start to develop a member. But in chickens, the ‘genital tubercle’ shrinks before the bird hatches. 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.
Next time you want a humiliation fantasy, just mention BMP4. We’ve got a million ways to insult a regressive chicken phallus.