If you’ve never seen a duck penis before, have a look at the infamous video above. That long corkscrew belongs to a Muscovy duck, and it’s typical of the group. Some ducks have helical penises that are longer than their entire bodies. But forget the helical shape, the size, and the surprisingly explosive extension—the weirdest thing about a duck’s penis is that it has one.

Most birds don’t. There are almost 10,000 species of birds and only around 3 percent of them have a penis. These include ducks, geese and swans, and large flightless birds like ostriches and emus. But eagles, flamingos, penguins and albatrosses have completely lost their penises. So have wrens, gulls, cranes, owls, pigeons, hummingbirds and woodpeckers. Chickens still have penises, but barely—they’re tiny nubs that are no good for penetrating anything.

In all of these species, males still fertilise a female’s eggs by sending sperm into her body, but without any penetration. Instead, males and females just mush their genital openings together and he transfers sperm into her in a manoeuvre called the “cloacal kiss”. Two dunnocks demonstrate the move in the video below. If you blink at 00:36, you will miss it.

“There are lots of examples of animal groups that evolved penises, but I can think of only a bare handful that subsequently lost them,” says Diane Kelly from the University of Massachusetts in Amherst. “Ornithologists have tied themselves in knots trying to explain why an organ that gives males an obvious selective advantage in so many different animal species disappeared in most birds. But it’s hard to address a question on why something happens when you don’t know much about how it happens.”

That’s where Martin Cohn came in. He wanted to know the how. His team at the University of Florida studies how limbs and genitals develop across the animal kingdom, from the loss of legs in pythons to genital deformities in humans. “In a lab that thinks about genital development, one takes notice when a species that reproduces by internal fertilization lacks a penis,” says graduate student Ana Herrera.

Waves of death

By comparing the embryos of a Pekin duck and a domestic chicken, Herrera and other team members showed that their genitals start developing in the same way. A couple of small swellings fuse together into a stub called the genital tubercle, which gradually gets bigger over the first week or so. (The same process produces a mammal’s penis.)

In ducks, the genital tubercle keeps on growing into a long coiled penis, but in the chicken, it stops around day 9, while it’s still small. Why? Cohn expected to find that chickens are missing some critical molecule. Instead, his team found that all the right penis-growing genes are switched on in the chicken’s tubercle, and its cells are certainly capable of growing.

View Images Photograph by AutoCCD, via Wikipedia

It never develops a full-blown penis because, at a certain point, its cells start committing mass suicide. This type of ‘programmed cell death’ occurs throughout the living world and helps to carve away unwanted body parts—for example, our hands have fingers because the cells between them die when we’re embryos. For the chicken, it means no penis. “It was surprising to learn that outgrowth fails not due to absence of a critical growth factor, but due to presence of a cell death factor,” says Cohn.

His team confirmed this pattern in other species, including an alligator (crocodilians are the closest living relatives of birds). In the greylag goose, emu and alligator, the tubercle continues growing into a penis, with very little cell death. In the quail, a member of the same order as chickens, the tubercle’s cells also experience a wave of death before the organ can get big.

This wave is driven by a protein called Bmp4, which is produced along the entire length of the chicken’s tubercle, but over much less of the duck’s. When Cohn’s team soaked up this protein, the tubercle’s cells stopped dying and carried on growing. So, it’s entirely possible for a chicken to grow a penis; it’s just that Bmp4 stops this from happening. Conversely, adding extra Bmp protein to a duck tubercle could stop it from growing into its full spiralling glory, forever fixing it as a chicken-esque stub.

Bmp proteins help to control the shape and size of many body parts. They’re behind the loss of wings in soldier ants and teeth of birds. Meanwhile, bats blocked these proteins to expand the membranes between their fingers and evolve wings.

They also affect the genitals of many animals. In ducks and geese, they create the urethra, a groove in the penis that sperm travels down (“If you think about it, that’s like having your urethra melt your penis,” says Kelly.) In mice, getting rid of the proteins that keep Bmp in check leads to tiny penises. Conversely, getting rid of the Bmp proteins leads to a grossly enlarged (and almost tumour-like) penis.

View Images Herrera et al, 2013. Current Biology

To lose a penis once might be regarded as misfortune…

Penises have been lost several times in the evolution of birds. Cohn’s team have only compared two groups—the penis-less galliforms (chickens, quails and pheasants) and the penis-equipped anseriforms (swans, ducks and geese). What about the oldest group of birds—the ratites, like ostriches or emus? All of them have penises except for the kiwis, which lost theirs. And what about the largest bird group, the neoaves, which includes the vast majority of bird species? All of them are penis-less.

Maybe, all of these groups lost their penis in different ways. To find out, Herrera is now looking at how genitals develop in the neoaves. Other teams will no doubt follow suit. “The study will now allow us to more deeply explore other instances of penis loss and reduction in birds, to see whether there is more than one way to lose a penis,” says Patricia Brennan from the University of Massachussetts in Amherst, the woman behind the duck penis video at the top.

And in at least one case, what was lost might have been regained. The cracids—an group of obscure South American galliforms—have penises unlike their chicken relatives. It might have been easy for them to re-evolve these body parts, since the galliforms still have all the genetic machinery for making a penis.

View Images Family tree of birds showing retention and loss of penises. All the families you know about but aren’t mentioned are in the “Neoaves”. Credit: Herrera et al, 2013. Current Biology

But why?

We now know how chickens lost their penises, but we don’t know why a male animal that needs to put sperm inside a female would lose the organ that makes this possible. Cohn’s study hints at one possibility—it could just be a side effect of other bodily changes. Bmp4 and other related proteins are involved in the evolution of many bird body parts, including the transition from scales to feathers, the loss of teeth, and variations in beak size. Perhaps one of these transformations changed the way Bmp4 is used in the genitals and led to shrinking penises.

There are many other possible explanations. Maybe a penis-less bird finds it easier to fly, runs a smaller risk of passing on sexually-transmitted infections, or is better at avoiding predators because he mates more quickly (remember the dunnocks?).

Females might even be responsible. Male ducks often force themselves upon their females but birds without an obvious phallus can’t do that. They need the female’s cooperation in order to mate. So perhaps females started preferring males with smaller penises, so that they could exert more choice over whom fathered their chicks. (Indeed, the now-infamous story about the duck’s corkscrew penis is really a story about the duck’s corkscrew vagina.) Combinations of these explanations may be right, and different answers may apply to different groups.

And why study the why? Why would scientists care about how penises evolve (and why do I write about them so much)? Cohn makes a good argument. “Genitalia are one of the fastest-evolving organs in animals,” he explains. Even in the groups with backbones, “one sees a remarkable degree of variation”.

In mammals, sperm passes down a tube that’s fully enclosed within the penis; in birds and reptiles, it goes down an open groove. Some mammals have a bone in the penis, or a coat of spines; others don’t. Snakes, lizards and kangaroos have two-pronged penises, while echidnas have four-pronged ones. Mammals inflate their penises with blood; birds use lymph; alligators have a permanently erect penis connected to a bungee cord. When Darwin spoke of “endless forms most beautiful and most wonderful”, it’s easy to envisage him talking about penises. Or a lack of them.

Reference: Herrera, Shuster, Perriton & Cohn. 2013. Developmental Basis of Phallus Reduction during Bird Evolution. Current Biology