Developmental biologists have identified a protein which is released in the early stages of avian embryonic development to kill off an already partially-formed penis. 

Just three percent of birds have a penis, despite the species reproducing through internal fertilisation. It's a mystery that has kept avian experts up at night for many years. It seems even more peculiar considering that those birds with a penis have some of the most extraordinary members on the planet. Ducks, for instance (as you will see from this article's accompanying video below) have bizarre corkscrew-shaped members that protrude on cue, and retract neatly when not needed. These penises can grow to about half the length of the birds body. Compare that with the chicken. Both the male and female of the species have a cloaca, an orifice from which they urinate, defecate and procreate. The pair engage in a cloacal kiss when they touch, the moment when sperm is transferred to the female.

With these oddities galore, it's no wonder the avian penis has stirred up a bit of debate. Most recently, Patricia Brennan of the University of Massachusetts was forced to write an editorial in Slate in defence of research into the humble bird penis.

"Genitalia, dear readers, are where the rubber meets the road, evolutionarily," she wrote. "To fully understand why some individuals are more successful than others during reproduction, there may be no better place to look."

And look they did. Writing in an issue of Current Biology a team from the University of Florida explains how it approached the conundrum by engineering tiny windows into the eggs of chickens and quails (non-penis owning birds), and ducks, geese and emus (birds with a penis). Using an electron microscope to spy on the embryos, they discovered that each bird began life by growing a penis. However, by around day nine, the genital tubercle (a kind of precursor penis) stopped growing in chickens and quails and ended up as a retracted nub. It continued growing as normal in the others. Closer observations revealed that the penis cells were actually dying off in the chickens and quails, from their tips back. 

Taking a look at the genetics behind this turnaround, they found that the same genes were switched on in the early days of development in all the birds. However, the quails and chickens began producing a protein known as Bmp4 (bone morphogenetic protein 4) at their tubercle's tips, when the Bmp4 gene switches on.

To test the theory that this gene was responsible for the penis destruction, the team implanted Bmp4 proteins into duck tubercles. The result: a shrivelled member that began to resemble something more akin to a chicken's cloacal.

On the flipside, they used the protein Noggin to stall Bmp production in chickens on one side of their tubercle. That side grew to 6.5 times its normal size and, in returning the penis to the chicken, the Florida team destroyed thousands of years of evolution in one fowl swoop.

Now we know how this penis stunting is occurring; but the mystery of why remains.

The authors behind the study have hypothesised that due to "discoveries implicating Bmps in evolution of beak shape, feathers, and toothlessness", it might be that while the protein was busily being regulated in other parts of the bird's body to produce these features, it was neglected at the penis locale.

Evolutionary speaking, there could be many other factors coming into play here. For one, duck sex is notoriously aggressive, with it suggested they were forced to develop an elaborate corkscrew penis to reach their intended. On the other hand, hens might have more control over who they produce with because it is dependant on them allowing the roosters to get close enough for the "kiss".

This research is not simply a look at the bizarre world of avian genitalia; it is actually an important step in understanding defects in human development. 

"Genitalia are affected by birth defects more than almost any other organ," the BBC reports study coauthor Dr Martin Cohn as saying. "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."

If that weren't enough to convince you of the importance of the avian penis, we'll leave you with these wise words from Patricia Brennan:


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