In 2009, duck penises took the Internet by storm. Thanks to a newly published study and an eye-opening video, people learned that while most birds lack penises at all, male ducks have huge, corkscrew-shaped ones. During sex, they extrude these into females at high speed. Since then, duck penises have become a short-hand for the “ain’t nature wacky” genre of science writing, and an unexpected focal point for debates about the value of basic science.

And during that time, one important part of the original study was lost. People forgot that the story of duck penises is really the story of duck vaginas.

Duck sex can be fiercely competitive, and several males will often try to force themselves onto a female. Their extreme penises help them to deposit sperm as far inside her as possible. But duck vaginas are also long and twisting. They’re lined with dead-end pockets and they spiral in the opposite direction to the male’s penis. This shape stops the progress of a male’s ballistic organ—something that Patricia Brennan proved by getting drakes to launch their penises into variously shaped glass tubes.

As I wrote in 2009, a duck’s vagina is an “organic chastity belts that evolved to limit the effectiveness of the males’ lengthy genitals.” If she actually wants to mate, she can change her posture and relax the walls of her genital tract to offer a male easy passage. If not, she makes him insert his key into an inconveniently shaped lock. To a casual observer, the male looks like the one with the power, but the female is actually in control, thanks to her convoluted genitals.

And yet, everyone talks about the penises.

This isn’t just relevant to ducks. It’s a problem that runs through all of zoology. Animal genitals are really interesting—they are extraordinarily diverse in their shape and use, and they tell us fascinating things about evolutionary conflicts. Many scientists have realised this. In the early 1990s, only five studies on genital evolution got published every year. In 2012, there were 40 such studies.

But males have benefited from this rising interest more than females. By analysing 364 studies published in the last 25 years, Malin Ah-King, Andrew Barron and Marie Herberstein found that 49 percent only looked at male genitals, 8 percent only looked at female genitals, and 44 percent looked at both. There’s some variation: people who study spiders, snails and slugs are more likely to pay attention to female genitals. But in general, female sex organs—vaginas, bursas, cloacas, spermathecae and more—get a short shrift.

Why?

The gender of the scientists themselves isn’t a factor. Ah-King, Barron and Herberstein found that both male and female scientists are just as likely to skew towards studying male genitals.

One possibility is that the bias is justified because female genitals vary less than male ones do. But Ah-King, Barron and Herberstein disagree with this argument. They point to many groups of flies, spiders, ducks and even primates, where female genitals vary a lot between species, or even within them.

A more likely explanation is that a tube is much easier to study than a cavity. Male genitals stick out and they’re often rigid, making them easy to observe, measure and manipulate—there’s an entire genre of penis–shaving studies out there. But female genitals are usually concealed. “They have to be dissected out, which changes their shape,” says Michael Jennions from Australian National University. “This is laborious work [and requires] old-school anatomical skills that are declining in modern biology.” (This may explain why the bias is less pronounced in studies of spiders, whose females often have obvious external genitals.)

But Ah-King, Barron and Herberstein note that some groups have used modern medical scanners to get around the problem. Accessibility, while important, can’t be the only factor at work.

Instead, they believe that male genitals still get more attention because of longstanding gender stereotypes that have seeped into evolutionary biology. For a long time, researchers believed that males played a dominant role in sex, while females were more passive—Darwin himself referred to them as “coy”.

These stereotypes are pervasive. In the most cited studies on sexual conflict, authors use active words like ‘intimidation’ and ‘coercion’ to describe males, but passive words like ‘resistance’ and ‘avoidance’ to describe females. More tellingly, males have ‘adaptations’ and females have ‘counter-adaptations’. Males act; females react.

But we know that, as in ducks, females exert a tremendous amount of control during sex. They can store sperm in pouches, expel unwanted sperm, or mate with more males. All of these tricks allow them to reject a male as a father even after having sex with him, and all of them are hard to observe unless you’re actually looking. And perhaps, people don’t look very hard.

Although few modern biologists would see females as passive players in sex, Ah-King, Barron and Herberstein argued that this myth has cast a lingering shadow over the field. It meant that people started studying male-focused topics like sperm competiton—where the sperm of different males compete for fertilisation rights inside a female’s body—long before topics like female choice.

Consider also the notion that males are promiscuous and females are choosy. This concept was stamped into textbook wisdom after a classic 1948 paper from geneticist Angus Bateman, based on experiments with fruit flies. Through breeding experiments, Bateman found that males benefit a lot from mating repeatedly but females do not.

But Bateman’s methods included a serious problem, one that undermined his conclusions and was only uncovered last year. As Eric Michael Johnson writes, “the premiere study on sexual selection—which had been cited by more than 2,000 peer-reviewed papers and textbooks—contained a fatal flaw… The uncomfortable implication is that Bateman’s paradigm was so widely cited because it conformed to assumptions about how female sexuality ought to be.”

Bateman’s principle isn’t universally wrong—in the intervening decades, other scientists found many examples to support it. But in suggesting that sexual success matters more to males, and varies more among them, Ah-King, Barron and Herberstein argue that it guided students towards studying males rather than females. Hence: the explosion of studies on penises and sperm, and the relative dearth of research on vaginas and eggs.

This bias leads to problems. We can seriously misinterpret the sexual lives of animals if we only know about half the partners. For example, the corkscrew penis of a male duck looks like a tool for beating other males in sexual conquests, until you realise that the helical vagina of the female grants her control.

Likewise, the male earwig has a long, brush-tipped organ called a virga, which he supposedly uses to scrape the sperm of previous suitors out of a female. As Ah-King, Barron and Herberstein write, “Too often, the female is assumed to be an invariant container within which all this presumed scooping, hooking, and plunging occurs.” In fact, the female earwig has storage organs for sperm, which lie beyond the reach of the virga. The male can scrape away all he wants; the female decides whether to keep or jettison her sperm.

“Increased emphasis on female genitalia would help in understanding genital evolution,” says William Eberhard, a pioneer in the study of female choice. “The authors aren’t the first to point this out, but it is useful and necessary to repeat the arguments periodically.”

But Diane Kelly from the University of Massachussetts, who has studies a wide range of animal penises, adds, “I think the authors underestimate how difficult it is to study female genitalia.” It’s not just that these organs are hidden. They also work in complicated ways.

Penises are simple. They might have piercing tips or attachments for scraping out a rival’s sperm, but they’re still basically tubes with one basic job: get sperm near eggs. Female genitals, on the other hand, might change shape to interact with male organs. They might contract to improve or reduce the odds of insemination. They might release floods of hormones to control the release of eggs or storage of sperm. They’re much more than just simple cavities.

“Faced with that kind of complexity, is it any wonder that the people studying “lock and key” models opt to study the key end of the system?” says Kelly “It’s a lot easier and faster to shave some spines off a male’s penis and assess whether or not that changes his reproductive success than to work through all the factors that a female could be evaluating before she uses or rejects that male’s sperm. In short, when it comes to female genitals, we still have to figure out what they’re doing during sex.

“That’s probably one reason why half of the animal genitalia studies the authors assessed looked at males and females together. I think that’s a good thing. Studying how female genitalia actually behave during copulation is likely to tell us a lot more about the evolutionary pressures on those tissues than we’d learn from studying them in isolation.”

Reference: King, Barron & Herberstein. 2014. Genital Evolution: Why Are Females Still Understudied? PLOS Biology http://dx.doi.org/10.1371/journal.pbio.1001851