The development of chicken penises is cut short by signals that prompt cell death, a finding that could help to explain why 97% of bird species have little or no phallus despite reproducing by internal fertilization.

Researchers led by Martin Cohn at the University of Florida in Gainesville cut tiny windows into eggs to compare developing chickens, which lack phalluses, with ducks, whose penises can be half as long as their bodies.

Chicken embryos began to form penises, but these shrank midway through development. The researchers pinned the cause on elevated levels of a protein called Bmp4, which promotes cell death, at the tip of the organ.

The loss of penises may have been a by-product of the evolution of other features, such as beak shape, which are also influenced by Bmp proteins, the authors suggest.

Curr. Biol. http://dx.doi.org/10.1016/j.cub.2013.04.062 (2013)