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How Chickens Lost Their Penises (And Ducks Kept Theirs)

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.

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.

Credit: Herrera et al, 2013. Current Biology
Credit: 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.

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.
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 http://dx.doi.org/10.1016/j.cub.2013.04.062

More on animal penises:

16 thoughts on “How Chickens Lost Their Penises (And Ducks Kept Theirs)

  1. Barnacles still win the contest! Now, how long do you suppose it will be before medical researchers begin trying to manipulate Bmp proteins in human males…

  2. Further studies of the effect of Bmp4 on birds have been hampered by the paucity of suitable subjects. Although investigators have had to turn away chickens who are anxious to participate in experiments, very few ducks have been willing to volunteer.

  3. I have the strange feeling that all the commenters above are male. And that the bottom just fell out of their world.

    If the female selection theory is true then it serves as a valuable lesson. Guys, you need to stop the douches or we’ll all end up worse off.

  4. Well among humans, female selection would result in more and more “perfect penises.” Similar to the way that all humans are becoming sexier due to mate selection. There will come a point when most humans are so similar, there is little point in choosing a mate for their looks.

  5. We will see if biologists can come up with equally good reasons to study the female form when they come to that. It is often down-prioritized (which is a medical problem), but we know at least half of the public want to see boobs!

    @Kudzu: You can turn that around and note that humans should have been good at stopping douches in comparison with other primates.

  6. It obviously points to who is the older lineage of birds: the anseriformes, not the galliformes. It also implies that dinosaurs had phalluses as well.

  7. According to this article: http://wfcb.ucdavis.edu/www/faculty/John/wfc111/handouts/gill/Anseriformes.pdf, screamers don’t have a penis either. It says: “The screamers are loud-voiced birds of the
    marshlands. They have long, slender toes with only
    rudimentary webs. They have stout spurs at the
    bend of the wings. Unusual among birds, screamers
    lack uncinate processes. The males also lack a penis.
    The most peculiar feature of the screamers is a skin
    filled with small bubbles of air about a quarter of an
    inch thick, which produce a crackling sound when
    pressed; the function of this layer of air bubbles is

  8. According to a tree published in 2011, which placed falcons in the same branch of the bird family tree as parrots, chickens and ducks are each other’s closest relatives. So, it is possible that the chicken may have lost the penis that the common ancestor of ducks and chickens has. According to the same tree, the other modern birds are more derived than ducks and chickens, and they all lack the penis. The Paleognaths (ostrich, rhea, cassowary, tinamous) is basal to all other living birds, and they have a penis. Therefore it is possible that the last common ancestor of all modern birds (exclusive of the paleognaths, galliformes and anseriformes) somehow lost the penis before it diversified into the other modern orders. That would be a simpler explanation that numerous multiple independent losses. That means the penis may have been lost just 3 times, in some galliform birds, in the kiwi and in the common ancestor of the Neoaves. Of course, we must await more data before we can figure out what most likely happened in the past.

  9. The ones that kept them seem to be waterfowl, the ones that lost them didn’t. Maybe mating in water is easier for a bird for some reason than it is on land, since they don’t have forelegs to hold themselves steady and keep a grip like land mammals do.

    1. Julio – Well there are quite a few aquatic birds who haven’t developed them (to be expected) but also a lot of waterfowl mating that happens on land, especially in those species where females are oft pursued by a (or several) males and where forcible copulation is common. Maybe it’s just a group trait?

  10. Julio, I think your theory is dead on. Kudzu makes a valid point that a number of the aquatic birds can and do mate on land, but my observations suggest that is not their preference. They will always wait and attempt to mate in water first or at least when there is a reasonable expectation they can access water. I think your on to something for sure!

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