Patricia Brennan never intended to become a champion of the vagina. Her journey, in fact, began with a penis.

It was a late summer afternoon in 2000, and the 28-year-old Colombian biologist was stalking her study animal, a squat gray-blue bird called the great tinamou in the dumbo Costa Rican rainforest. As always, the woods floor was dark and shadowy, the sunlight swallowed upwardly by the upper canopy. It was stiflingly humid; she was sweating through her protective gear. "You could dice in that wood, and there would be no trace of yous in just a few months," she recalls. "You lot would disappear completely."

That'due south when she heard information technology: a pure, whistling tone, with an undertone of sadness. A male tinamou, calling for a mate. As she held her breath, a female appeared from the dense underbrush. She ran up to him, backed away, then chased him again. Finally she crouched down with her tail in the air, inviting him to mount. As Brennan watched through her binoculars, the male clambered awfully onto her back. Brennan will never forget what happened adjacent.

For most birds, mating is an artless affair. That's because they don't have external genitalia, just a multipurpose opening nether the tail used to expel waste matter, lay eggs and have sex. (Biologists usually phone call this orifice a cloaca, which ways "sewer" or "drain" in Latin. Brennan simply refers to it as the vagina, since it performs nevertheless functions and then some.) They briefly rub genitals together in an act known equally a "cloacal kiss," in which the male transfers sperm into the female person. The whole event takes seconds.

But this time, the pair began waddling around, glued together. The male started thrusting. When he finally discrete, she saw something dangling off him—something long, white and curly.

"What the hell is that thing?" she remembers thinking. "Oh, God, he's got worms."

And so she had some other thought: "Man, is that a penis?"

Patricia Brennan Working
Brennan dissects a female coachwhip snake in her lab at Mt. Holyoke College in Massachusetts in September 2021. "This is my happy identify," she says. Rachel East. Gross

Birds, she idea at the time, didn't have penises. In her ii years studying them at Cornell University, a earth leader in avian enquiry, she'd never once heard her colleagues mention a bird penis. And anyhow, this certainly didn't look like any penis she had always seen—it was ghostly white, curled up like a corkscrew, thin as a slice of cooked spaghetti. Why would such an organ have evolved, only to have been lost in well-nigh all birds? That would take been "the weirdest evolutionary thing," she says.

When she returned to Cornell, she decided to learn everything at that place was to know about bird penises—which turned out to be not so much. 90-7 percent of all bird species have no phallus. Those that did, including ostriches, emus and kiwis, sported organs quite dissimilar from the mammalian variety. Corkscrew-shaped, they exploded out into the female in one burst, and engorged with lymphatic fluid rather than claret. Sperm traveled down spiraling grooves along the exterior.

Brennan had been the first to notice a penetrative penis in this species of tinamou. Only later would she ask the question that would distinguish her from all her peers: If this was the penis, and then what were the vaginas doing? "Obviously you tin can't accept something similar that without some place to put it in," she would after tell the New York Times. "You need a garage to park the machine." For the offset time, she wondered about the size, shape, and role of that … er … garage.

In 2005, before she turned her lens to vaginas, the pursuit of penises led Brennan to the University of Sheffield in the English countryside. Subsequently realizing that "there is a huge gaping hole in our knowledge of this very key function of bird biology," she had pivoted her enquiry and was now focusing on bird-penis evolution. She was here to learn the art of dissecting bird ballocks from Tim Birkhead, an evolutionary ornithologist. She got to work dissecting quail and finches, which had little in the way of outer genitalia. Next, she opened up a male duck from a nearby farm, and gasped.

The tinamou'southward penis had been sparse, like spaghetti. This one was thick and massive, merely with the same recognizable spiral shape. Whoa, she thought. Expect a minute—where is this matter gonna go?

No one seemed to accept an answer. The problem was, the typical bird-dissection technique focused well-nigh entirely on the male. When researchers did dissect a female person duck, they sliced all the fashion upward through the sides of the vagina to get at the sperm-storage tubules near the uterus (in birds, information technology's called the shell gland), distorting their truthful anatomy. They tossed the rest out, unexamined. When she asked Birkhead what the within of a female person duck's reproductive tract looked like, she recalls, he assumed it was the same equally whatever other bird: a uncomplicated tube.

But she knew at that place was no way an appendage as complex and unusual as the duck penis would have evolved on its ain. If the penis were a long corkscrew, the vagina ought to be an as complex structure.

The first step was to find some female person ducks. Brennan and her husband drove out to one of the surrounding farms and purchased ii Pekin ducks, which she euthanized without ceremony on a bale of hay. (Brennan's husband is used to these kind of excursions: "He brings me roadkill every bit a nuptial souvenir," she says.) Instead of slicing the reproductive tract up the sides, she spent hours carefully peeling abroad the tissues, layer by layer, "like unwrapping a present." Eventually, a complex shape emerged: twisted and mazelike, with blind alleys and hidden compartments.

When she showed Birkhead, they both did a double-take. He had never seen anything similar information technology. He chosen a colleague in France, a world expert on duck reproductive anatomy, and asked him if he'd always heard of these structures. He hadn't. The colleague went to examine one of his ain female specimens, and reported back the same affair: an "extraordinary vagina."

Preview thumbnail for 'Vagina Obscura: An Anatomical Voyage

Duck Vagina and Penis Photo
A dissected duck vagina (left) and corresponding penis (correct), spiraling in contrary directions. Courtesy of Patricia Brennan

To Brennan, it seemed that females were responding in some way to males, and vice versa. But there was something odd going on: the vagina twisted in the reverse direction of the male's. In other words, this vagina seemed to have evolved not to arrange the penis, but to evade information technology. "I couldn't wrap my head around it. I only couldn't," Brennan says. She preserved the structures in jars of formaldehyde and spent days turning them over, trying to figure out what could explain their complexity.

That'due south when she began thinking about conflict. Duck sexual activity, she knew, could be notoriously violent. Ducks tended to mate for at to the lowest degree a season. However, extra males lurked in the wings, prepare to harass and mountain whatsoever paired female they could go their hands on. This often leads to a tearing struggle, in which males injure or fifty-fifty drown the female. In some species, upward to twoscore percent of all matings are forced. The tension is thought to stem from the two sexes' competing goals: The male person duck wants to sire as many offspring equally possible, while the female duck wants to choose the father of her children.

This story of disharmonize, Brennan suspected, might also shape duck ballocks. "That was the part where I was similar: holy cow," she says. "If that'due south actually going on, this is nuts." She started contacting scientists across Northward and South America to collect more specimens. Ane was Kevin McCracken, a geneticist at the University of Alaska who, while out on a wintry jaunt, had discovered the longest known bird phallus on the Argentine lake duck, which unraveled to a stunning 17 inches. He suggested that perhaps the male was responding to female preference—wink-wink, nudge-nudge—simply hadn't bothered to actually examine the female.

When Brennan called him up, he was more than happy to help her collect more specimens. Today, he admits that possibly the reason he hadn't considered looking at the female person side of things was a upshot of his own male bias. "Information technology was fitting that a woman followed this upwardly," he says. "We didn't need a homo to do it."

Past carefully dissecting the genitals of 16 species of waterfowl, Brennan and her colleagues found that ducks showed unparalleled vaginal variety compared to any known bird grouping. There was a lot going on inside those vaginas. The primary purpose, it appeared, was to brand the male person's job harder: Information technology was like a medieval chastity belt, congenital to thwart the male's explosive aim. In some cases, the female genital tract prevented the penis from fully inflating, and was full of pockets where sperm went to dice. In others, muscles surrounding the cloaca could block an unwanted male, or amplify to allow entry to a preferred suitor.

Duck Vagina and Penis Illustration
When she carefully dissected out a female person duck's reproductive tract, Brennan constitute an elaborate bound-shape that spiraled in the opposite direction as the male's fellow member. Catherine Delphia

Whatsoever the females were doing, they were succeeding. In ducks, only 2 to v percent of offspring are the result of forced encounters. The more ambitious and improve endowed the male, the longer and more than circuitous the female reproductive tract became to evade information technology. "When yous dissected one of the birds, it was really piece of cake to predict what the other sexual practice was going to expect like," Brennan told the New York Times. It was a struggle for reproductive control, not bodily autonomy: Although a female person couldn't avert physical impairment, her anatomy could assist her gain control over the genes of her offspring after a forced mating.

The duck vagina, Brennan realized, was inappreciably the passive, simple structure that biologists had made information technology out to be. In fact, information technology was an expertly rigged penis-rejection motorcar. Just what almost in other animate being groups?

A world opened upwards before Brennan's optics: the vast variety of creature vaginas, wonderfully varied and woefully unexplored. For centuries, biologists had praised the penis, fawning over its length, girth, and weaponry. Brennan's contribution, simple every bit it may seem, was to wait at both halves of the genital equation. Vaginas, she would learn, were far more than complex and variable than anyone thought. Often, they play active roles in deciding whether to permit intruders in, what to do with sperm, and whether to help a male person along in his quest to inseminate. The vagina is a remarkable organ in its own right, "full of glands and full of muscles and collagen, and changing constantly and fighting pathogens all the time," she says. "It's just a actually amazing construction."

To centre females in genitalia studies, she knew she would demand to go beyond ducks and start to open up "the copulatory black box" of female genitalia more broadly. And, as she explored genitals, from the tiny, ii-pronged snake penis to the spiraling bat vagina, she kept finding the aforementioned story: Males and females seemed to be co-evolving in a sexual arms race, resulting in elaborate sexual organs on both sides.

But conflict, it turned out, was inappreciably the only strength shaping genitals.

For decades, biologists had noted a strange characteristic establish in the reproductive tracts of marine mammals like dolphins, whales and porpoises: a series of fleshy lids, like a stack of funnels, leading upwardly to the neck. In the literature, they were known every bit "vaginal folds," and were thought to have evolved to keep sperm-killing seawater out of the uterus. But to Dara Orbach, a Canadian PhD student who was studying the sexual anatomy of dolphins, that office didn't explain the variation she was finding. After a gamble pairing brought her together with Brennan in 2015, she brought her collection of frozen vaginas to Brennan'southward lab to investigate.

What they found at first reminded them strongly of the duck story. In the harbor porpoise, for instance, the vagina spiraled like a corkscrew and had several folds blocking the path to the cervix. Porpoise penises, in plough, ended in a fleshy projection, like a finger, that seemed to have evolved to poke through the folds and reach the cervix. Just as in ducks, it seemed that males and females were both evolving specialized features in order to gain the evolutionary advantage during sex.

Bottlenose Dolphins Breaching
Bottlenose dolphins leap out of the water in the Caribbean. Female person dolphins have been seen masturbating past rubbing their clitorises against other dolphins. David Tipling / Education Images / Universal Images Group via Getty Images

Then, in the middle of their dolphin vagina dissections, the scientists stumbled across something else: a massive clitoris, partly enfolded in a wrinkled hood of skin. While the human clitoris has long been cast (erroneously) every bit small and hard to find, this one was most impossible to miss. When fully dissected out, it was larger than a tennis ball. "It was enormous," Brennan says.

That dolphins would have a well-developed clitoris was no surprise. Brennan and Orbach both knew that these charismatic creatures engage in frequent sexual behavior for reasons like pleasure and social bonding. Females have been seen masturbating past rubbing their clitorises against sand, other dolphins' snouts and objects on the body of water floor. Yet while other scientists had guessed that the dolphin clitoris might be functional, no 1 had really tried to figure out how information technology worked.

Past dissecting 11 dolphin clitorises and running the samples through a micro CT scanner, the researchers uncovered a roughly triangular complex of tissues that sabbatum only at the opening of the vagina—easily accessible to a penis, snout or fin. It was fabricated upward of ii types of erectile tissue, both spongy and porous, allowing it to swell with arousal. These erectile bodies besides grew and changed shape during puberty, suggesting they played an important role during adult sexual life. Strikingly large fretfulness, up to half a millimeter in bore, concluded in a web of sensitive nerve endings merely beneath the peel.

In short, the dolphin clitoris looked a whole lot like the homo clitoris, they reported in a paper published in January. And information technology probably worked similar one, likewise. Brennan can't say for sure that dolphins take orgasms, "But I'm pretty darn sure that sex feels skilful to them. Or at to the lowest degree that rubbing of the clitoris feels good," she says.

Marine Mammal Vaginas
In 2018, Orbach and Brennan made molds of xiv species of sea mammals to discover that cetaceans displayed unprecedented vaginal diversity. Oft, Orbach found she could place a species simply by its vaginal morphology. Pictured here are the vaginas of cetaceans, pinnipeds and manatees. Courtesy of Dara Orbach and Patricia Brennan

Before dolphins, even Brennan had not given much thought to part that non-reproductive sexual behavior might play in the evolution of genitals. In general, she subscribed to the tenets of archetype Darwinian evolutionary thinking: "In my mind, everything ultimately has got to be reproductive," she says. Possibly, she thought, these behaviors might encourage future reproductive sex, eventually leading to more than offspring. Or, a male's ability to stimulate the clitoris might influence a female'south selection of mate.

Even so when it came to genital evolution, Darwin left much to be desired. The father of evolution more often than not eschewed talking about genitals, because their chief function to be fitting together mechanically, equally a lock fits into a key. Moreover, he characterized female animals almost universally as chaste, small and about devoid of sexual urges. In his lesser known writings, he described a world in which females honored their "husbands" and kept "marriage-vows." Although he observed a few counter-examples—i.e. females with several "husbands" or those that seemed to pursue sex for pleasure—he steered articulate of them, likely out of a sense of Victorian propriety.

To Darwin, males were the ones with the driving urge to appoint in sexual behavior. The role of females, past contrast, was primarily to choose between competing males. "The males are almost always the wooers; and they alone are armed with special weapons for fighting with their rivals," he wrote in his 1871 book Descent of Man, and Pick in Relation to Sex. "They are by and large stronger and larger than the females, and are endowed with the requisite qualities of courage and pugnacity."

A century and a half later, Darwin's influence still casts a long shadow over the field. In her frank exploration of fauna vaginas, Brennan is showtime to claiming some the traces of prudery, male bias and lack of marvel about female genitals that Darwin left behind. Yet she also had inherited some of that framework: Namely, she still idea about genitals mainly in conjunction with reproductive, heterosexual sex.

What she institute in dolphins gave her intermission. The substantial clitoris before her was a hint at something that seems obvious, but ofttimes isn't: sex isn't just for reproduction.

Today, we know that genitalia do far more than simply fit together mechanically. They can besides signal, symbolize and titillate—not just to a potential mate, but to other members of a group. In humans, dolphins and beyond, sexual behavior can be used to strengthen friendships and alliances, make gestures of dominance and submission, and as part of social negotiations like reconciliation and peacemaking, points out evolutionary biologist Joan Roughgarden, author of the 2004 book Development'southward Rainbow: Diversity, Gender, and Sexuality in Nature and People.

These other uses of sexual practice may be one reason that animal genitalia are so weird and wonderful beyond your standard vagina/penis philharmonic. Consider the long, pendulous clitorises that dangle from female person spider monkeys and are used to distribute scent; the notorious hyena clitoris, which is the same size as the male's penis and used to urinate, copulate and requite birth; and the showstopping genitalia that Darwin did briefly highlight in monkeys—the rainbow-hued genitals of vervets, drills and mandrills, and the ruby swellings of female macaques in estrus—that may connote social status and help troupes avoid conflict.

These diverse examples of "genital geometry" (Roughgarden'due south term) serve a multitude of purposes across reproduction. "All our organs are multifunctional," she points out. "Why shouldn't the genitals exist besides?"

Across the animal kingdom, same-sex activity behavior is widespread. In female-dominated species similar bonobos, for instance, aforementioned-sex activity matings are at least equally common as between-sex activity matings. Notably, female bonobos have massive, cantaloupe-sized labial swellings and prominent clitorises that can attain two and a half inches when cock. Some primatologists have gone so far as to suggest that the position of this remarkable clitoris—it's in a frontal position, as in humans, and unlike in pigs and sheep, which have clitorises inside their vaginas—might have developed to facilitate same-sex genital rubbing.

"Information technology does seem more logistically favorable, let'southward say, for the kinds of sex they're having," says primatologist Amy Parish, a bonobo skilful who was the get-go to describe bonobo societies as matriarchal. Primatologist Frans de Waal, also, has mused that "the frontal orientation of the bonobo vulva and clitoris strongly suggest that the female ballocks are adapted for this position." Roughgarden has therefore coined this clitoral configuration the "Marking of Sappho." And given that bonobos, like chimps, are some of our closest evolutionary cousins—they share 98.5 percent of our genes—she wonders why more than scientists haven't asked whether the same forces could be at play in humans.

These are questions that the current framework of sexual selection, with its simple assumptions nearly aggressive males and finicky females, renders unaskable. Darwin took for granted that the basic unit of nature was the female-male person pairing, and that such pairings e'er led to reproduction. Therefore, the theory he came up with—coy females who option among competing males—only explained a limited piece of sexual behavior. Those who followed in his footsteps similarly treated heterosexuality equally the Ane True Sexuality, with all other configurations equally either curiosities or exceptions.

The effects of this pigeonholing get across biology. The dismissal of homosexuality in animals, and the treatment of such animals equally freaks or exceptions, helps reify negative attitudes toward sexual minorities in humans. Darwin's theories are frequently misused today to promote myths near what human nature should and shouldn't be. Roughgarden, a transgender woman who transitioned a few years before writing her book, could see the impairment more than clearly than most. Sexual selection theory "denies me my place in nature, squeezes me into a stereotype I can't possibly live with—I've tried," she writes in Evolution'southward Rainbow.

Focusing solely on a few dramatic cases of sexual conflict—the "battle of the sexes" arroyo—obscures some of the other powerful forces that shape genitals. Doing so risks leaving out species in which the sexes cooperate and negotiate, including monogamous seabirds like albatrosses and penguins, and those in which homosexual bonds are as strong as heterosexual ones. In fact, it appears that the stunning diverseness of animal genitals are shaped by an equally stunning variety of driving forces: disharmonize, communication, and the pursuit of pleasure, to name a few.

And that, to both Brennan and Roughgarden, is freeing. "Biology need not limit our potential. Nature offers a smorgasbord of possibilities for how to live," Roughgarden writes. Rather than chaste Victorian couples marching two by two upwards the ramp into Noah's neat and tidy ark, "the living globe is fabricated of rainbows inside rainbows inside rainbows, in an endless progression."

Adapted fromVagina Obscura: An Anatomical Voyage. Copyright © 2022 past Rachel E. Gross. Used with permission of the publisher, Westward. W. Norton & Company, Inc. All rights reserved.

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