Bird penises start strong, wither away

Male chickens lose phalluses before hatching

Some ducks have penises longer than their bodies, while chickens make do with a tiny bump. The vast size difference between the two types of fowl is due to a wave of cell death during chicken development, researchers report June 6 in Current Biology.

A false-color micrograph shows the rudimentary penis (pink) of a chick embryo before it begins to shrink. A.M. Herrera and M.J. Cohn, University of Florida

Although the ancestors of birds had penises, 97 percent of bird species have phalluses so small that they can’t insert into the female genitalia. Martin Cohn of the University of Florida and his colleagues found that in chick embryos, a gene turns on in the tip of the developing phallus, causing the cells to die and the tissue to wither away. In duck embryos, the gene does not turn on and the penis keeps on growing.

The researchers were able to kick-start some growth of the embryonic chick penis by blocking the action of the gene. That result suggests that chickens have not entirely lost the genetic pathways that make their waterfowl relatives better endowed.

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