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Why Do Animals Like to Live in Groups

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

Information technology was a late summer afternoon in 2000, and the 28-year-old Colombian biologist was stalking her study animal, a squat gray-blue bird chosen the corking tinamou in the dense Costa Rican rainforest. As always, the forest floor was nighttime and shadowy, the sunlight swallowed upward by the upper awning. Information technology was stiflingly humid; she was sweating through her protective gear. "Y'all could dice in that wood, and there would exist no trace of yous in just a few months," she recalls. "You would disappear completely."

That's when she heard it: 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 upward to him, backed away, so chased him again. Finally she crouched down with her tail in the air, inviting him to mountain. Every bit Brennan watched through her binoculars, the male person clambered awfully onto her back. Brennan volition never forget what happened next.

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

Only this time, the pair began waddling around, glued together. The male person 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."

Then 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 2022. "This is my happy place," she says. Rachel Due east. Gross

Birds, she thought at the time, didn't take penises. In her ii years studying them at Cornell University, a globe leader in avian research, she'd never once heard her colleagues mention a bird penis. And anyhow, this certainly didn't look like whatever penis she had ever seen—it was ghostly white, curled upwards similar a corkscrew, thin equally a piece of cooked spaghetti. Why would such an organ have evolved, merely to take been lost in most all birds? That would take been "the weirdest evolutionary thing," she says.

When she returned to Cornell, she decided to learn everything in that location was to know nigh bird penises—which turned out to be non then much. Ninety-seven percent of all bird species accept 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 ane burst, and engorged with lymphatic fluid rather than blood. Sperm traveled down spiraling grooves along the outside.

Brennan had been the first to observe a penetrative penis in this species of tinamou. Only after would she enquire the question that would distinguish her from all her peers: If this was the penis, then what were the vaginas doing? "Plainly you tin can't take something similar that without some place to put it in," she would later on tell the New York Times . "You lot demand a garage to park the car." For the first time, she wondered about the size, shape, and office 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. After realizing that "at that place is a huge gaping hole in our knowledge of this very fundamental role of bird biology," she had pivoted her research 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 petty in the way of outer ballocks. Side by side, she opened upwardly a male person duck from a nearby farm, and gasped.

The tinamou'south penis had been sparse, like spaghetti. This one was thick and massive, but with the aforementioned recognizable spiral shape. Whoa, she idea. Wait a minute—where is this thing gonna go?

No one seemed to accept an answer. The problem was, the typical bird-autopsy technique focused almost entirely on the male. When researchers did dissect a female duck, they sliced all the way up through the sides of the vagina to get at the sperm-storage tubules near the uterus (in birds, it's called the vanquish gland), distorting their true beefcake. They tossed the balance out, unexamined. When she asked Birkhead what the inside of a female duck's reproductive tract looked like, she recalls, he causeless it was the aforementioned as any other bird: a simple tube.

Just she knew in that location was no way an appendage as circuitous and unusual as the duck penis would have evolved on its own. If the penis were a long corkscrew, the vagina ought to exist an equally complex structure.

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

When she showed Birkhead, they both did a double-have. He had never seen anything like it. He called a colleague in France, a world expert on duck reproductive anatomy, and asked him if he'd e'er heard of these structures. He hadn't. The colleague went to examine 1 of his own female specimens, and reported back the same thing: an "extraordinary vagina."

Duck Vagina and Penis Photo — A dissected duck vagina (left) and respective penis (right), spiraling in opposite 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 opposite direction of the male'southward. In other words, this vagina seemed to have evolved non to accommodate the penis, merely to evade it. "I couldn't wrap my head effectually 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 explicate their complexity.

That'south when she began thinking nearly disharmonize. Duck sex, she knew, could be notoriously violent. Ducks tended to mate for at least a season. Nevertheless, actress males lurked in the wings, gear up to harass and mount any paired female person they could get their hands on. This oftentimes leads to a violent struggle, in which males injure or even drown the female. In some species, up to 40 percent of all matings are forced. The tension is thought to stem from the two sexes' competing goals: The male duck wants to sire every bit many offspring as possible, while the female duck wants to choose the begetter of her children.

This story of conflict, Brennan suspected, might likewise shape duck genitalia. "That was the role where I was like: holy moo-cow," she says. "If that'south really going on, this is nuts." She started contacting scientists across North and South America to collect more specimens. 1 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—flash-flash, nudge-nudge—but hadn't bothered to actually examine the female.

When Brennan called him up, he was more than happy to assist her collect more than specimens. Today, he admits that perhaps the reason he hadn't considered looking at the female side of things was a result of his ain male person bias. "It was fitting that a woman followed this up," he says. "We didn't need a human being to practice information technology."

By 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 group. There was a lot going on inside those vaginas. The main purpose, it appeared, was to brand the male person's job harder: It was like a medieval chastity belt, built to thwart the male's explosive aim. In some cases, the female person genital tract prevented the penis from fully inflating, and was full of pockets where sperm went to die. In others, muscles surrounding the cloaca could block an unwanted male person, or dilate to allow entry to a preferred suitor.

Duck Vagina and Penis Illustration — When she carefully dissected out a female duck'south reproductive tract, Brennan found an elaborate spring-shape that spiraled in the reverse direction as the male's fellow member. Catherine Delphia

Any the females were doing, they were succeeding. In ducks, merely 2 to 5 percent of offspring are the event of forced encounters. The more than aggressive and better endowed the male, the longer and more circuitous the female person reproductive tract became to evade it. "When you dissected one of the birds, information technology was really easy to predict what the other sex activity was going to look like," Brennan told the New York Times. It was a struggle for reproductive control, not actual autonomy: Although a female couldn't avoid physical harm, her beefcake could help her proceeds control over the genes of her offspring later on a forced mating.

The duck vagina, Brennan realized, was hardly the passive, simple structure that biologists had made it out to be. In fact, it was an expertly rigged penis-rejection automobile. Just what about in other animal groups?

A world opened upwards before Brennan's eyes: the vast variety of animal vaginas, wonderfully varied and woefully unexplored. For centuries, biologists had praised the penis, fawning over its length, girth, and weaponry. Brennan'south contribution, simple as information technology may seem, was to wait at both halves of the genital equation. Vaginas, she would learn, were far more circuitous and variable than anyone thought. Frequently, they play active roles in deciding whether to permit intruders in, what to do with sperm, and whether to help a male along in his quest to inseminate. The vagina is a remarkable organ in its own right, "total of glands and full of muscles and collagen, and irresolute constantly and fighting pathogens all the time," she says. "It'southward merely a really amazing structure."

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

Just conflict, it turned out, was hardly the only force shaping genitals.

For decades, biologists had noted a strange feature plant in the reproductive tracts of marine mammals like dolphins, whales and porpoises: a series of fleshy lids, similar a stack of funnels, leading upward to the cervix. In the literature, they were known as "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 role didn't explicate the variation she was finding. After a chance pairing brought her together with Brennan in 2015, she brought her collection of frozen vaginas to Brennan'due south lab to investigate.

What they found at get-go 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 neck. Porpoise penises, in plow, ended in a fleshy projection, like a finger, that seemed to have evolved to poke through the folds and accomplish the cervix. Just as in ducks, information technology seemed that males and females were both evolving specialized features in club to gain the evolutionary advantage during sex.

Bottlenose Dolphins Breaching — Bottlenose dolphins leap out of the water in the Caribbean. Female dolphins accept been seen masturbating by rubbing their clitorises against other dolphins. David Tipling / Education Images / Universal Images Grouping 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 peel. While the human clitoris has long been cast (erroneously) every bit small and hard to detect, this i was about impossible to miss. When fully dissected out, it was larger than a tennis ball. "It was enormous," Brennan says.

That dolphins would take 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 by rubbing their clitorises confronting sand, other dolphins' snouts and objects on the sea floor. Yet while other scientists had guessed that the dolphin clitoris might exist functional, no one had actually tried to effigy out how it 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 sat simply at the opening of the vagina—hands accessible to a penis, snout or fin. It was fabricated up of two types of erectile tissue, both spongy and porous, allowing information technology to swell with arousal. These erectile bodies also grew and changed shape during puberty, suggesting they played an important role during adult sexual life. Strikingly large nerves, up to half a millimeter in bore, ended in a spider web of sensitive nerve endings just beneath the skin.

In short, the dolphin clitoris looked a whole lot similar the human clitoris, they reported in a newspaper published in January. And it probably worked like one, likewise. Brennan tin can't say for certain that dolphins accept orgasms, "But I'm pretty darn sure that sexual practice feels good to them. Or at least that rubbing of the clitoris feels proficient," she says.

Marine Mammal Vaginas — In 2022, Orbach and Brennan made molds of 14 species of sea mammals to find that cetaceans displayed unprecedented vaginal diversity. Often, Orbach found she could place a species simply by its vaginal morphology. Pictured hither are the vaginas of cetaceans, pinnipeds and manatees. Courtesy of Dara Orbach and Patricia Brennan

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

However when it came to genital evolution, Darwin left much to be desired. The father of development generally eschewed talking about genitals, considering their main office to be plumbing equipment together mechanically, as a lock fits into a key. Moreover, he characterized female animals well-nigh universally as celibate, modest and virtually 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.east. females with several "husbands" or those that seemed to pursue sex for pleasure—he steered clear of them, likely out of a sense of Victorian propriety.

To Darwin, males were the ones with the driving urge to engage in sexual beliefs. The role of females, by contrast, was primarily to cull between competing males. "The males are almost ever the wooers; and they lonely are armed with special weapons for fighting with their rivals," he wrote in his 1871 book Descent of Human, and Selection in Relation to Sex. "They are generally stronger and larger than the females, and are endowed with the requisite qualities of courage and pugnacity."

A century and a half after, Darwin's influence still casts a long shadow over the field. In her frank exploration of animate being vaginas, Brennan is offset to claiming some the traces of prudery, male person bias and lack of curiosity most female genitals that Darwin left behind. Still she too had inherited some of that framework: Namely, she still thought about genitals mainly in conjunction with reproductive, heterosexual sex.

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

Today, we know that genitalia do far more than than but fit together mechanically. They can also signal, symbolize and titillate—not merely to a potential mate, but to other members of a group. In humans, dolphins and beyond, sexual beliefs tin 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'due south Rainbow: Diversity, Gender, and Sexuality in Nature and People .

These other uses of sex may be one reason that animal genitalia are so weird and wonderful across your standard vagina/penis combo. Consider the long, pendulous clitorises that dangle from female spider monkeys and are used to distribute scent; the notorious hyena clitoris, which is the same size every bit the male'southward penis and used to urinate, copulate and requite nativity; and the showstopping genitalia that Darwin did briefly highlight in monkeys—the rainbow-hued genitals of vervets, drills and mandrills, and the red swellings of female macaques in estrus—that may connote social status and assist troupes avoid conflict.

These diverse examples of "genital geometry" (Roughgarden's term) serve a multitude of purposes beyond reproduction. "All our organs are multifunctional," she points out. "Why shouldn't the genitals be every bit well?"

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

"Information technology does seem more logistically favorable, allow's say, for the kinds of sex activity they're having," says primatologist Amy Parish, a bonobo expert who was the first to describe bonobo societies as matriarchal. Primatologist Frans de Waal, too, has mused that "the frontal orientation of the bonobo vulva and clitoris strongly suggest that the female genitalia are adapted for this position." Roughgarden has therefore coined this clitoral configuration the "Mark of Sappho." And given that bonobos, similar chimps, are some of our closest evolutionary cousins—they share 98.5 percent of our genes—she wonders why more scientists haven't asked whether the aforementioned forces could be at play in humans.

These are questions that the current framework of sexual selection, with its simple assumptions near ambitious males and choosy females, renders unaskable. Darwin took for granted that the basic unit of measurement of nature was the female-male pairing, and that such pairings ever led to reproduction. Therefore, the theory he came up with—coy females who pick among competing males—simply explained a express slice of sexual beliefs. Those who followed in his footsteps similarly treated heterosexuality as the 1 True Sexuality, with all other configurations as either curiosities or exceptions.

The effects of this pigeonholing go beyond biology. The dismissal of homosexuality in animals, and the treatment of such animals as freaks or exceptions, helps reify negative attitudes toward sexual minorities in humans. Darwin's theories are oft misused today to promote myths most what homo nature should and shouldn't exist. Roughgarden, a transgender adult female who transitioned a few years earlier writing her volume, could run into the damage more clearly than most. Sexual choice theory "denies me my place in nature, squeezes me into a stereotype I tin can't maybe alive with—I've tried," she writes in Evolution'due south Rainbow.

Focusing solely on a few dramatic cases of sexual conflict—the "boxing of the sexes" approach—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 stiff as heterosexual ones. In fact, it appears that the stunning variety of animal genitals are shaped by an equally stunning multifariousness of driving forces: conflict, communication, and the pursuit of pleasance, 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 alive," Roughgarden writes. Rather than chaste Victorian couples marching two by two up the ramp into Noah's nifty and tidy ark, "the living globe is made of rainbows inside rainbows within rainbows, in an countless progression."