Scientists Explain How Male Birds Lost Their Penises

For years scientists have been mystified by the evolutionary loss of penises in some of the bird species.  Chickens grow a penis in the early embryo and once they hatch it withers away.

A new study conducted by researchers at the University of Florida, provides some explanation for this phenomenon. This research was conducted by Martian Cohn, a developmental biologist at the University of Florida and also the senior author of the study.  The researchers looked into the phenomenon of the loss of the genitalia in most bird species and how only a miniscule number has retained the organ

 For this study, the researchers analyzed land fowl such as chicken as they develop rudimentary penises as adults. They also analyzed the waterfowl such as ducks and geese. The other animal species tested were the emu- the large flightless bird and alligators.

The birds were observed through an electron microscope right from the embryo development stage. They noticed that the gene Bmp4 had a very important role to play in this process. In the development of the chicken, the gene Bmp4 is triggered leading to the withering of the male genitals. Whereas in ducks and emus, Bmp4 remains switched off and that is the reason they are able to retain their penis, as the organ growth is not stopped.

Due to the lack of the penis, the land fowl have found a new way to fertilize that does not depend on external reproductive organs. They have a tiny opening known as cloaco that is responsible for functions like waste excretion and copulation. On rubbing the cloacae together the male ejaculates into the female reproductive tract and this process is termed as 'cloacal kiss' by scientists.

The scientists don't have a definite answer as to why the gene Bmp4 suppresses the growth of the penis. They think that it maybe prevents genital birth defects, or it might be also an evolutionary process to convert a few bird species into more aerodynamic species.

Ana Herrera, also from the University of Florida, and a part of the research, says that the loss of the penis gives hens better control over their reproductive lives, reports BBC.

Dr Cohn was quoted in IBTimes UK: "Genitalia are affected by birth defects more than almost any other organ. Dissecting the molecular basis of the naturally occurring variation generated by evolution can lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected. This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations."

The study was published in the journal Current Biology.