Four years ago, as the country was wrestling with a federal-budget crisis, conservative news outlets turned their attention, once again, to the topic of wasteful government spending. That March, a reporter with CNS News, a Web site devoted to countering “liberal bias” in the media, came across what seemed to be the quintessential example of such waste—a National Science Foundation grant to Yale University for a study of duck penises. Within days, the story had made its way to Fox News. “It’s part of President Obama’s stimulus plan, and it’s just one example of the kind of spending decisions that have added up to massive debt and deficits,” Shannon Bream told viewers. The following week, Sean Hannity piled on. “Don’t we really need to know about duck genitalia, Tucker Carlson?” he asked. To which Carlson responded, with a smirk, “I know more than I want to know already!” The controversy, dubbed Duckpenisgate by Mother Jones, roared back to life some months later, when Senator Tom Coburn, of Oklahoma, included the N.S.F. grant in his “Wastebook 2013.” At $384,949, it accounted for only a thousandth of one per cent of all the spending that Coburn had tallied up, but it made headlines again. Clearly, the combination of money, sex, and power—your money, ducks’ sex, and Ivy League power—was irresistible to the graying male demographic for conservative news.

I followed Duckpenisgate with particular trepidation, since I was one of the co-investigators on the maligned study. For the past decade, in collaboration with Patricia Brennan, of Mount Holyoke College, and other colleagues, I have explored the sexual behavior and genital evolution of waterfowl. Contrary to what Carlson thinks, it is a fascinating business. It can also be shockingly brutal. In the wintry months before breeding begins, male ducks flaunt their plumage, putting on dramatic courtship displays in an effort to entrance a mate. The females can be choosy, often picking a male only after extensive deliberation. (Their preferences tend to coalesce, like a genetic fashion trend, around a shared ideal of male beauty, with each species evolving off in its own distinct aesthetic direction.) When spring arrives, the pairs migrate together to the breeding grounds. But, as the nest-building and egg-laying season approaches, unpaired males start causing trouble. Many attempt to force copulation with paired females, sometimes even ganging up on them in groups. The female ducks resist strenuously; often they are injured, or even killed, in the process.

The males’ sexual attacks are made possible by the fact that, unlike most birds, ducks still have a penis. It is not, however, an organ that most humans would recognize, being shaped like a counterclockwise corkscrew and possessing a ribbed or spiky surface. Ducks’ erections are driven by lymphatic, not vascular, pressure, which means that their penises never become stiff. Rather, they erect flexibly, but explosively, into the female’s body in less than half a second. Ejaculation takes place immediately. And duck penises can be long—really long. A breeding male mallard in your typical city park has a five-inch penis. In the case of the diminutive Argentine lake duck, the penis is longer than the duck itself—more than sixteen inches.

What, exactly, is the function of these bizarre organs? To find out, Brennan dissected the genitalia of fourteen species of waterfowl. By comparing the results, we discovered that, as males have evolved longer penises with more heavily armed surfaces, females have coevolved increasingly complex vaginal structures—dead ends, cul-de-sac side pockets, clockwise spirals. We hypothesized that these twists and turns create a mechanical barrier to the penis, frustrating forced intercourse and lowering the likelihood of a female duck being fertilized against her will. Our subsequent experiments—high-speed videos of duck penises erecting into glass tubes of various shapes—suggested we were right. (Our observations also revealed that when a female duck solicits sex with a chosen mate, her cloacal muscles dilate to allow uninhibited entry.) The result is that, even for species in which nearly forty per cent of all copulations are violently coerced, only between two and five per cent of ducklings come from extra-pair matings. As a method of contraception, ducks’ vaginal barriers can be ninety-eight-per-cent effective—a level of reliability that the U.S. Food and Drug Administration would readily approve.

A female duck’s vaginal barriers cannot shield her from physical harm. On an evolutionary level, though, they protect her in another way—by allowing her to choose the father of her offspring. If she has ducklings with her chosen mate, then they will inherit the fancy plumage that she and other females prefer. But, if she is fertilized by force, then her offspring will inherit either random display traits or traits that she has specifically rejected as less attractive. These extra-pair offspring will, on average, be less attractive to their peers, which could mean fewer grand-ducklings for the mother duck—and fewer of her genes passed on to posterity. By using her vaginal barriers, she is able to maintain her sexual autonomy in the face of sexual violence. Freedom of choice, in other words, matters to animals; even if they lack the capacity to conceptualize it, there is an evolutionary difference between having what they want and not having it. Unfortunately for female ducks, though, evolving complex vaginal structures doesn’t solve the scourge of sexual violence; it exacerbates it. Each advance results in males with longer, spikier penises, and the coevolutionary arms race continues.

Although many duck species are trapped in costly and unproductive sexual battles, other birds have pursued different evolutionary paths toward male disarmament. In bowerbirds, for instance, females have used mate choice to transform male behavior in ways that have advanced their own sexual autonomy. Male bowerbirds build elaborate seduction theatres, called bowers, out of sticks, which they decorate with gathered artifacts such as feathers, fruits, and flowers. When the time comes to breed, females visit a number of prospective mates, choosing one based on the attractiveness of the male, his bower, and his ornaments. As a result, the architecture of the bowers is shaped by females’ aesthetic preferences. Males work from a blueprint that actually prevents them from successfully coercing copulations. A so-called avenue bower, for example, features two parallel walls of sticks. The female sits cozily between them while the male does his dance at a safe remove. To copulate with her, he must go around the walls and mount her from behind, which gives her a chance to pop out the front, if she prefers, with her freedom of choice intact.

A male bowerbird in his seduction theatre. PHOTOGRAPH BY BELINDA WRIGHT / NATIONAL GEOGRAPHIC / GETTY PHOTOGRAPH BY BELINDA WRIGHT / NATIONAL GEOGRAPHIC / GETTY

Scientists admonish one another, often with good reason, to avoid anthropomorphizing animals. But they themselves regularly redraw the line between good science and anthropomorphism as a way of policing scientific discourse and favoring particular ideas. Most of us, for example, learned a strictly adaptationist version of Charles Darwin’s theory of evolution; we were told that almost every feature of the biotic world, no matter how tiny, could be explained by how it contributed to an organism’s ability to survive and reproduce. In fact, though, Darwin also proposed a theory of sexual selection, in which animals may choose their mates according to aesthetic standards—their own subjective desires. This view has frequently been rejected as too anthropomorphic precisely because it implies that sexual selection can act independently of natural selection—an unsettling thought for the typical adaptationist.

When it comes to the sexual politics of birds and people, there are, of course, enormous differences. Birds don’t have elaborate social cultures, money, or any notion of their own histories. Humans do. But, in seeking to understand the complexities of human evolution and sexuality, we can learn a lot by examining the diversity of life on Earth and acknowledging the parallels where they exist.