A pixelated penis and vagina face off wearing boxing gloves
Illustration by Hunter French

These Vaginas Evolved to Fight the Penis, Not Accommodate It

The study of genitals has historically ignored female parts. Now that that's changing, it's revealing an evolutionary competition between penis and vagina.

Evolutionary biologist Patty Brennan had watched a lot of birds have sex. But in 2002, in Costa Rica, she saw something she never had before: a bird penis.

Most male birds don't have penises. They mate using an opening called a cloaca—derived from the Latin word for sewer. It's a cavity inside a bird's anus that's a one-stop shop for the digestive, urinary, and reproductive tracts. When birds mate, the male and female cloaca touch. The male releases sperm, and it enters the female’s body. It's referred to, somewhat romantically, as a "cloacal kiss."


Brennan was observing a pair of Great tinamous—brown, chicken-like birds with small heads that live in the Costa Rican forest. Instead of just the subtle and brief cloacal kiss, the male bird grabbed the female by the neck. Then, the two birds started walking around still attached, as if they were fused together. When they separated, she saw a white, tentacle-looking organ hanging from his body.

"This was unlike anything I had ever seen," she said. "I was like, is this a penis?" (According to biologist Richard Prum, the tinamous penis had been seen and described by Victorian anatomists, but the appendage was forgotten to science. “Her sighting was probably the first-ever observation of the tinamou penis in action," he wrote in a 2017 book.)

That unexpected bird penis launched Brennan, now an assistant professor of biological sciences at Mount Holyoke College, into a career of studying the weird and wonderful variations of genitalia in the natural world. But unlike many scientists before her who had noticed the dizzying variety of penises out there, Brennan began to ask: What about the vaginas? A long-standing misconception in evolutionary biology was that penises were incredibly diverse, but vaginas were not. In the past two decades, biologists, like Brennan, have been finding otherwise.

While doing so, they've been uncovering how gender biases might have played a role in obscuring vaginal variety, and how excluding vaginas from the study of genital evolution led to gaping holes in our understanding of why genitals look and behave the way they do. Only by examining how male and female parts evolve together can we see how sometimes, strange genitals are a result of sexual conflict—each sex trying to get the upper hand to control the reproductive act to best suit their needs. That's what Brennan learned, not through bird penises alone, but also through the vaginas with which they interact.


After her encounter in Costa Rica, Brennan wanted to continue studying bird genitalia. She shifted her focus to ducks, a more accessible subject than tinamous. At a duck farm in California's Central Valley in 2009, she captured some duck penises in action. (These ducks had been trained to ejaculate into small glass bottles for artificial insemination.)

You might remember what she and Prum discovered because, for a short while, the duck penis went viral online. Brennan found that the penises unfurl out of a duck's body at lengths of around 5 to 7 inches—some duck penises can be almost as long as the male's body. And they were spiraled, like a fleshy cavatappi pasta noodle. Male ducks forced these long corkscrew penises onto females. The internet was horrified, and also, enthralled.

In the history of people (and scientists) marveling at genitalia in nature, this is where it often stops: Look at this weird penis! In 1979, Science published a paper on the penis of the damselfly. As Dutch evolutionary biologist, Menno Schilthuizen, wrote in his book, Nature's Nether Regions, “this minuscule penis carried a miniature spoon that, during mating, cleaned out the female’s vagina, scooping out any remaining sperm from previous males. It was an eye-opener as well as a sperm-scooper.”

This finding opened biologists' eyes to the fact that even tiny creatures had strange penises. The chicken flea's penis is rolled up in its body like a coiled spring. Other insects have musical penises, where males rub them against ribbed parts of their bodies to emit loud noises. Black widow spiders have penis tips that break off to block other male’s sperm from entering a female. It was somewhat of an evolutionary mystery: Why were penises so different from one another if they had the same evolutionary purpose—to deliver sperm to a female's eggs?


The lock and key theory was one potential explanation, proposed in the mid 19th century. It said that male genitals were like a key, and for each key there needed to be a corresponding lock (the vagina). If the key doesn’t fit into the lock, mating couldn't take place. Essentially, penises varied to keep different species from mating with one another. Another guess was sexual selection—that females detected some particular feature of the male genitalia and used it to choose a mate, pushing the male's penis evolution in bizarre off-shoots.

Still, the focus remained on male genitalia and how it was changing and evolving, even in more recent texts on genital evolution, like important work from scientist William Eberhard on sexual selection. Brennan wrote in a 2016 paper that while Eberhard noted female choice was important in shaping male genital features, he “concluded that female genitalia are relatively uniform while male genitalia are diverse.”

"It created this idea, from my reading of the literature later on, that the females were somehow boring," Brennan said. "We need to look at the males, because that’s where all the action is.

As a result, most of the research on genital evolution has focused on males. Nearly two times as many studies have looked at male genitals compared to females. In 2014, evolutionary biologist and gender researcher Malin Ah-King and her colleagues looked at 364 studies published over the last two decades, and found that 49 percent of them only looked at male genitals, compared to 8 percent that looked only at females, and 44 percent that looked at both.


Even the language that researchers use to describe male and female genitals has differed. A study found that active words like "coercion" are used for males, while more passive words like "avoidance" or "resistance," are provided for females. As Ah-King and her co-authors wrote: “Too often, the female is assumed to be an invariant container within which all this presumed scooping, hooking, and plunging occurs.”

When Brennan first saw the duck penis, though, she immediately considered the duck vagina. “I looked at their penis and next question was, 'wow these penises are so big. So what do the vaginas look like?'” Surprisingly, no one had investigated that before, she told me. To her, it was an obvious question. As she told science writer Carl Zimmer for a New York Times article: “You can’t have something like that without some place to put it in. You need a garage to park the car.”

When she dissected some female ducks, "I could not believe it,” she said. “The differences in the vagina of a duck compared to the vagina of a chicken or a finch or quail was like the difference between night and day.”

What Brennan found was a vagina like a labyrinth. Yes, duck penises were spiraled—but duck vaginas were too, in the opposite direction. Rather than finding a vagina that had evolved to fit this weird penis—a garage that fit the car—the duck vagina indicated a less cooperative history.

Given that duck mating was often forced, Brennan and her colleagues hypothesized that the vagina had co-evolved to actively resist the males. The duck’s vagina is swirled in a clockwise coil, so the males can only completely penetrate her with their counter-clockwise penis if she chooses to relax her vaginal muscles. Even though female ducks can't stop the male ducks from forcing themselves on them, they can control if the male could successfully inseminate—reclaiming some reproductive autonomy.


Brennan and her colleagues looked at other species where the males took part in forced copulation, and then at the corresponding females. In ducks and geese, they found that when male birds forced sex on the females, females also had complicated vaginas. “In species where there’s no forced copulation, then the females have a regular, tube-looking vagina," Brennan said.

It also meant that the duck penis size and shape wasn't solely a result of males competing with other males, or females making a choice between males—it was the female and male ducks' competition driving the evolution.

This is the core tenet of sexual conflict: Males and females don’t always agree about the best way to mate. For males, mating with a large number of females is the ideal way for them to procreate. For females, who are often left with the care of the offspring, as well as giving birth and pregnancy, being selective about reproduction is her best bet for creating progeny that will survive. This creates a conflict, where the males are going for quantity and the females, for quality.

Let’s say a male animal evolves a penis hook, which allows him to latch onto a female. Even if that hook hurts the female, or gives her an infection, if it benefits the male by allowing him to reproduce more, the genes for that hook will be passed to the next generation. That puts the female a step behind, so evolution might next select for females that can defend themselves against the hook, and evolve thicker walls in their vagina. (Something very similar has happened in sharks.) This is a way of understanding the evolution of genitals as a kind of conversation, even if a contentious and competitive one. And this perspective is providing new understanding for a whole host of creatures.


“The males evolve these weird penises and females evolve their convoluted vaginas in response,” Brennan said. “This is a lot more widespread than what we had originally realized. It's just, we have to go out there and look.”

Take the earwig, an insect with a male reproductive organ called a virga. The virga has a fringe-like tip that can brush away sperm from any male that mated with a female before him. Looking at the male genitals only tells one half of the story, because the females have receptacles in their bodies to store sperm that lie just out of reach of the virga. It may seem that the males are controlling the sperm, but the females have the upper hand. As science writer Ed Yong wrote: “The male can scrape away all he wants; the female decides whether to keep or jettison her sperm.”

Dolphins have a complex series of vaginal folds that researchers once assumed were there to keep sea water from getting inside the female reproductive tract. They’re realizing now how intricate their vaginas are, partly by making the effort to look closer at them. In 2017, biologist Dara Orbach made silicone molds of the dolphin vagina "revealing complex folds and spirals," the New Scientist reported. Brennan said it's now thought that those folds are actually barriers to male's penises.

Paying more attention to vaginas can help explain strange mating behavior too: In water striders, bugs that live and walk on water, the females evolved a “genital shield,” which can block any males that try to force them into mating. That led the males to adopt new "courting" techniques. "The males have started tapping the surface of the water while mounted on a female; the resulting ripples attract fish, and since the female is under the male, she's more likely than him to become a meal," according to post on Nature's blog. "Females can avoid this grisly end by giving in to the male's intimidation and mating with him.”


Without knowing that the females have a genital shield, researchers' understanding of such behavior would be incomplete. “It allows us to understand all of these bizarre morphologies and behaviors that we see in the context of, essentially, an arms race,” said Teri Orr, a evolutionary ecologist at The University of Utah.

Spiders are another of Orr’s favorites, because they can have around a dozen different pockets in them for manipulating sperm—some are for receiving sperm, or moving it around. Orr frequently studies bats, and said they will store sperm for a full year in the reproductive tract. Leaf cutter ants can store sperm for around ten years.

Female chickens can eject about 80 percent of sperm from undesirable mates. Female guppies can hold onto sperm too—one study found that one in four guppies in Trinidad and Tobago were fathered by males that had been dead for 10 months. By doing so, females could wait to reproduce at favorable times of the year.

“They’re able to keep those sperm until it’s a good time of year for them to become pregnant, and then carry out that pregnancy and have babies when there’s food available for them,” Orr said. “To me, that is absolutely mind-blowing. A lot of it is almost science fiction, what these species are able to do."

It also shows how the female anatomy is anything but passive. Outside of sperm storage, the vagina is awash with muscles that control contractions and movement—it’s as mobile as the digestive tract is, Orr said. These muscles can play a part in moving the sperm where they want it to go. “We didn’t know what until about a decade ago,” she said. “And even then, it’s only in cattle, horses, mice and humans that it’s been studied. That’s such a small part of the diversity that’s out there.”


In 2005, more than 200 scientists met in London at The Royal Society for a meeting titled Sexual conflict: a new paradigm? Brennan said that since then, she feels the field is moving to include vaginas, and that several of the most recent papers on genital evolution acknowledge the fact that female genitals have been overlooked. But Orr said that when she presents her work at conferences, it can still feel like it’s regarded as “out there" or niche. “It hasn’t reached mainstream science yet,” she said. “I think it’s going to take a little while until it’s fully embraced and not just a novelty—but normal biology.”

It's not as if Brennan wants the research to flip and only focus on females—the point is that you need both pieces of the puzzle. “I’ve been very adamant that when you’re looking at genitalia, you can’t just look at the female or the male alone," she said. "You need to look at both because of that mechanical fit. I could commit the opposite sin, in a way. I could just go look at a bunch of females and never look at the males. That’s not going to tell me much.”

She hopes that the field of genital evolution become more well-rounded, and also that the public will recognize its value. When Brennan's work on duck genitalia went public, conservatives latched onto it as a waste of government money (like a lot of academic research, it was partly funded by the National Science Foundation), acquiring the moniker #DuckPenisGate. Fox News put up a poll on their site where readers could vote if the research was a worthwhile use of taxpayer money, and 89 percent voted it was not. Brennan and her co-author Prum had to write articles defending the research.


The thing about basic science, Brennan said, is that you never know when a seemingly obscure discovery in nature is going to lead to an application for humans. So could secrets to our evolutionary past (and sexual conflicts) be hidden in our genital shapes? It's true that humans also have weird penises awash with unsolved questions, Brennan said. They are disproportionately wide given our body size and MRI studies of people having sex show that the shape of the male and female genitalia can change during intercourse, for reasons that are not completely understood.

Humans don't have penile spines, unlike many of our close primate relatives. Humans have also lost the baculum, a little bone inside of the penis of many animals, like bats, rodents, and primates. “We have no idea what it does,” Orr said. “It’s buried in tons of soft tissue and so it’s not interacting with the female, so it’s quite mysterious." Even less understood is the tiny little bone some animals have inside of the clitoris which humans didn't retain.

But more often, translation from basic science will come from where you least expect it. One obvious example is how the immune system of a bacteria was developed into a revolutionary gene editing technique—CRISPR/Cas9.

In the realm of genital evolution: duck penises grow and shrink every season, which means there are probably stem cells in the penis that allow for that growth each year. If researchers could learn what those cells are and how they work, they could have all sorts of medical or cosmetic applications. “Could we actually grow penile cells that might become a treatment someday? It’s perfectly possible," Brennan said.

Many of the stages where pregnancy fails in humans are the same ones where bats are able to intervene and store fertilized eggs or sperm. By looking closer at those processes, it might lead to ideas for aiding issues in human or reproduction or endocrinology, Orr said.

Hypospadias is a birth defect leading to a malformed urethra; one in every 200 boys is born with some type of hypospadias. For people with such developmental problems, or others, like malformation of the uterus, research into genitals that are naturally bifurcated could lead to an understanding of what causes those hiccups, and how to fix them.

Even if those animal-human translations aren't right around the corner, the field of genital evolution has already offered something else: Recognizing the value in seeing how gender biases and language can divert research to ignore crucial elements. Anthropologist Emily Martin's 1991 essay The Egg and the Sperm highlighted how the (often incorrect) descriptions of human sperm and eggs reflected stereotypical male and female roles. It's a reminder that it could happen again, and to examine what social constructs are currently inseminating scientific research.

And Brennan wonders if the response to her research doesn't betray how touchy and judgemental people still are about genitalia, especially vaginas. “It's almost as if there was something a little perverse with that line of questioning or that particular type of research," she said. "I happen to think that we actually need to understand a lot more about sex and sexual interactions than less.”

She views genitals just like any other organs. If you think about our other organs: livers, kidneys, hearts, or brains—there's much less variation and excitement. It's a rare window into what evolution can do. “Genitalia are critical biological organs to be studying,” she said. “I’m still surprised that we know as little as we seem to know. Evolutionarily, this is where the rubber meets the road.”

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