Why have most birds, which reproduce by internal fertilization (within the female) like humans, have lost their penis in the course of evolution? Scientists from the University of Florida (USA) have managed to solve the mystery by showing that, although during development the phallus of embryos of gallinaceous birds begins to grow, from a certain moment a gene called Bmp4 activates a process of programmed cell death that causes limb growth to stop , shrink and eventually disappear before the chicken hatches. In ducks and emus, however, the Bmp4 gene is not activated and the penis continues to grow and remains well developed in adult individuals, as the researchers explain in the journal Current Biology .
That there are so many birds without a penis could be due to the fact that the females have selected males with this characteristic, because copulation with males without a phallus requires the cooperation of the female , and thus she can have greater control of her reproductive life.
Researchers suspect that the process by which a structure begins to develop that then shrinks through cell death is quite common in living things. Another example would be the loss of limbs in snakes.
