If one were to look under a chicken to look for his reproductive organs, you’d be disappointed to see just tangled feathers and scaly legs. This is because, roosters and the rest of the 97 percent of all avian species have small, cork-like nubs in place of an actual penis.
Most male bird species are equipped with what scientist call a cloaca – a tiny opening that is responsible for all the region’s usual functions, which includes waste excretion and copulation. For example, when pair of birds mate, they will rub their cloacae until the male bird ejaculates into the female. Scientists call this act of bird lovemaking as “cloacal kiss.”
Still the question remains, where is the penis?
A developmental biologist at the University of Florida named Martin Cohn has the answer. In this study published in Current Biology, Cohn observed chicken and duck eggs by cutting tiny windows into them too see the development of the embryos. Unlike ducks, who actually have phalluses, a chicken’s penis all of sudden stops budding during the development process.
It was found out that although chickens also have penises similar to that of a duck, but on the ninth day of the embryo’s development, the genital tubercle stops growing and actually starts shrinking.
Yes, the penis is shrinking! Blame it on the gene called Bmp4, which is somewhat related to cell death. Once the chicken enters the ninth day, the gene comes to life, and this result to the penis’ growth to start regressing.
Cohn and his team of researchers are still unsure on why Bmp4 is stifling avian penis growth. This could be an evolution tactic making some bird species more aerodynamic, or could merely be a special biological consideration to avoid genital birth defects. An expert in evolutionary biologist from Queen's University in Ontario, Bob Montgomerie believes that cloaca may have actually been an evolutionary form that allows female chickens to avoid forced copulation.
