About 97% of birds either have a severe reduction in size in the, or a complete loss of ... a penis. In terms of animal reproduction, this is clearly counter-intuitive. For animals that fertilize an egg internally, like birds, having a penis to deliver sperm efficiently and effectively makes sense. So why did birds lose this organ?
The genitals of animals that make use of internal fertilization evolve surprisingly quickly with respect to size, shape, detachability, and even number. Yes, you read that right, number. Snakes actually have two penises. In the bird population a few species have extraordinarily complicated penises (like the duck, with its corkscrew penis) while the majority are lacking in that department. Birds without external genitalia have what's called a cloaca, an opening for the urinary, intestinal and reproductive tract.
In a paper published this week in Current Biology, scientists have identified the gene responsible for the loss of the penis in birds. Martin Cohn and his lab showed that the gene, Bmp4, when expressed in the growing fetal penis causes the cells to die faster than they are made.
Evolution is driven by the random mutation of a gene, as was seen with Bmp4 in birds, but the second part of evolution is selection of that gene. That means that mutation has to provide a better situation than was previously available. So what's so great about not having a penis?
The anatomical differences between the avian Haves and Have Nots lead to them to approach sex completely differently. While the well-endowed birds can physically manipulate and forcibly copulate with a female, those without penises have to play nice. In what biologists term the "cloacal kiss" (left to your interpretation), the male must convince the female she wants to mate. When she is interested, she will display her cloaca for the male to eject sperm into.
This consent allows the female the power of parental choice. She can now select genetically better mates, which will allow her offspring an improved chance of survival. Because she chose to mate with a more genetically fit bird and the offspring were more likely to survive, the Bmp4 gene is passed on to the next generation.
In this case, evolution seems to confirm what we already know: Consent is sexy.