How the chicken lost its penis: It sounds like a weird cousin of one of Rudyard Kipling’s “Just So Stories for Little Children” from 1902, which featured “How the Leopard Got His Spots” and “How the Camel Got His Hump.” But weird or not, this month we explore why almost all birds lack that flagpole of masculinity: the penis.

Back when I was a postgraduate student, a biologist in the department where I worked had a quiet word with my adviser: “Your student,” he said, “is obsessed with animal genitalia. There’s something wrong with him.”

My professor laughed it off. “That’s part of his PhD,” he said. “I’d be worried if he wasn’t thinking about sex all the time.”

I’ll happily accept that there may well be something wrong with me. After all, I did spend several years looking at the sperm and the genital morphology of many different kinds of animals. But in my defense I’d say this: If that’s the case then there’s also something wrong with most evolutionary biologists.

The evolution of sex, of male and female genders — and of the organs that accompany the act of procreation — pose fascinating questions for biologists. So they think about it a lot. That’s their excuse, anyway.

I was reminded of that episode last week when I came across a scientific paper reporting on a study of how birds lost their penises. And yes indeed, in 97 percent of bird species, the males don’t have a member. It’s a vexing question for biologists, let alone our feathered friends.

Nonetheless, these individuals are just as randy and strutting as males of other more well-endowed species — though the poor wretches have to make do with a cloaca, which is basically just an opening where you’d expect to find a penis.

Now Martin Cohn and Ana Herrera of the Howard Hughes Medical Institute in Chevy Chase, Maryland, and colleagues, have found that chickens possess normally developing penises as early embryos. But then, as the birds develop, a genetic program swings into action and effectively chops off their little budding appendages. Think of it as a grim reaper just for males — though it’s achieved through the programmed, deliberate dying-off of cells.

“Our discovery,” says Cohn, “shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell-death in a new location, the tip of the emerging penis.”

You may well ask: Why do we care? Well, it’s important to understand how this automatic cell-death mechanism operates because, if it fails to work when needed in the human body, it can lead to excessive or unregulated cell growth — and this can spell cancer.

And there’s another reason: We care because we are “obsessed with animal genitalia” and quite simply want to understand why birds don’t have penises.

Herrera, the grad-student lead author of the study, who probably has a fixation like the one I had, says it’s not clear why chickens and other birds would have lost their penises. It might be down to aerodynamics: A flapping male phallus would not help in graceful flight.

But Herrera has another interesting idea. It may be, she says, that the loss of a penis gives hens greater control over their reproductive lives. The natural evolutionary selection of females of the many species may, in other words, have driven the loss of the male’s pride and joy.

How can that be? To find out, let’s turn to ducks. As alert readers will have noticed — and will likely be troubling over right now — I said that most male birds don’t have a penis. Some do — and boy, do they.

Male ducks have giant, corkscrew-shaped penises. They use them to force females into having sex — you might have seen mallards chasing females of their species and jumping on them — which is what happens when the drake has an organ he can forcibly insert into the female.

There’s not much a female duck can do to avoid a drake’s penetrative intentions, but over evolutionary time they have come up with something very clever to inhibit them.

Hence the female’s reproductive tract is also corkscrew-shaped — but it twists the other way: The penis twists counter-clockwise, the vagina goes clockwise. So the females have evolved some measure of control over exactly where the male can put his penis, even if she can’t stop him inserting it in the first place. That’s because there are pockets and dead-end alleys inside her that prevent the penis growing to its full size. (If you’re a female duck you really don’t want that, as some drakes’ “manhood” can be as long as their body.)

So female ducks have evolved quite elaborate countermeasures in response to the questing, unwanted penis. Yet how much easier if there wasn’t a penis at all! That is the case in the vast majority of bird species.

Herrera and Cohn found that chickens have a gene known as Bmp4 that switches on at a certain stage in embryonic growth and culls the bird’s penis. In ducks and other enpenised avians, this gene stays switched off — as the results of their work, published in the journal Current Biology, show (DOI: dx.doi.org/10.1016/j.cub.2013.04.062)

It appears that Bmp4 operates in mammals, too. What would happen — oh, horror — if it were switched on in the development of a human male?

One can imagine a kind of reverse of Margaret Atwood’s dystopian 1985 Arthur C. Clarke Award-winning novel “The Handmaid’s Tale” — a version in which some crazed ruling class of women would have activated Bmp4 in men, causing them to lose their penises. The plot could center on a group of rebel women genetic engineers who restore men to their natural glory.

I’d enjoy such a book — just as long as it remained in the realms of fiction.

Rowan Hooper (@rowhoop on Twitter) is the News Editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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