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What complicated, unsettling, contentious times these are for gender and sex, and thinking about the relationship of biology to either one. Traditionally, sex—the biology of female and male, usually defined by a person’s gametes (sperm or ova)—has been distinguished from gender, the cloak woven by culture, psychology, and society that shapes roles and behavior seen as appropriate for women and men. But the distinction is blurring. What is a woman? What is a man? Most transgender people claim an inherent, fixed gender identity, but how does that reconcile with the argument that gender identity is infinitely variable, malleable, and nonbinary?

With the rise of second-wave feminism in the 1960s, women in the social and physical sciences had their work cut out for them, distinguishing gender from sex to expose the male biases that interpreted all gender differences as being biologically determined—a historically convenient justification for sexist practices and discrimination. Some differences, studied in all seriousness, were in such skills as “finger dexterity,” said to explain why women were better at typing and cooking, and women’s “raging hormones,” said to make them unfit for serious work or political office because, who knows, they might start a war. Such “sex differences” were trampled to death in the crush of women entering professional occupations, once discrimination became illegal.

Understandably, most egalitarian-minded social scientists were reluctant to let biology in the door. When the honored feminist sociologist Alice Rossi proposed her “biopsychosocial” theory of mothering in the 1970s, arguing that maternal behavior was not solely learned but partly rooted in inborn sex differences, she brought the wrath of goddess on her head. Many young women were furious—they wanted the “bio” out of there. In those days, the rising fields of evolutionary biology and psychology were regarded with special alarm, and no wonder. Ever since Darwin, evolutionary biologists and their best-selling popularizers had been telling us that male promiscuity and female monogamy are hard-wired reproductive strategies. As E.O. Wilson said, “It pays for males to be aggressive, fickle, and undiscriminating. In theory, it is more profitable for females to be coy, to hold back until they can identify males with the best genes…. Human beings obey this biological principle faithfully.” In all my years of writing about the science of gender differences, that is one of the most enduring I’ve found: Men really, really, like this theory more than women do.

To understand why the coy female is a myth, stop reading this essay and dive into Frans de Waal’s magnum opus, Different: Gender Through the Eyes of a Primatologist, and zoologist Lucy Cooke’s ill-titled but equally brilliant Bitch: A Revolutionary Guide to Sex, Evolution, and the Female Animal. With wit and erudition, and an array of descriptions of animal behavior from insects to fish to birds to our nearest ape relatives to us, these two authors explore how evolutionary biologists have been demolishing Darwin’s views of the “coy female” who waits quietly while the males duke it out to win her favor.

Discoveries of the existence of “sexually adventurous” (to replace the laden word “promiscuous”) behavior in female birds and other species set the stage for what the pioneering biologist Patricia Gowaty called “Darwinian feminism.” The term was needed, says de Waal, because so many feminists regarded evolutionary science and genetics as being “unfriendly to their cause.” (Hostile to their cause, more like.) Yet, as women have done in every scientific field and profession they have entered, they identified male biases in what was studied, how it was studied, and what conclusions were drawn. “There is no conspiracy here,” says Cooke, “just blinkered science.” Cooke and de Waal remove the blinkers, and their readers will never see the world the same way again. The revelations produced by Darwinian feminists—men among them—have transformed what we know about primates, what we know about evolution, and what we know about females. The focus has shifted from the penis to the clitoris, from the passive female to the female who actively chooses her partners. Even the ovum selects the genetically compatible sperm she will admit entry, and it’s not always the first guy who gets there—or even her romantic partner’s.

These books are important reading on two levels: first, for the riveting research they contain. Some of the same stories and studies turn up in both, but they complement each other beautifully. De Waal gives us his insider’s experience as a world-renowned primatologist whose life work has focused on our two closest ape relatives, chimpanzees and bonobos; Cooke, a superb science writer with a Master’s in zoology, covers the spectrum of female sexuality (and many other topics) throughout the animal kingdom, including more about spiders and hyenas than you ever thought you wanted to know. Second, both books offer exemplary lessons about science: how hard it is to overturn dogma and established paradigms; how resistant even leading scientists are to accepting disconfirming evidence. When faced with anomalies, like the blatant promiscuity of females, Darwin’s true believers looked away. They denied what was in front of their eyes; they blocked publication of the disconfirming evidence; they worked hard to squeeze the discrepant evidence into their existing beliefs; some manipulated their data.

Both authors tell the saga of Angus Bateman, the British geneticist and botanist who, in 1948, formulated Bateman’s Principle, which embedded itself in every biology textbook. Bateman, observing fruit flies, determined that females will “be choosy and reticent to make sure that they conceive with the best-quality males.” This was obedient Darwinism: The more partners males have, the greater the number of offspring; females produce the same number of offspring no matter how many partners they have. As Richard Dawkins, who was Cooke’s tutor at Oxford, summed it up: “The word excess has no meaning for a male.” Cooke quotes biologist Zuleyma Tang-Martinez’s gentle riposte: “History has not been kind to this pronouncement.” Try speaking of “excess” to the female lioness and chimpanzee who mate hundreds of times a day during oestrus with multiple males, leaving the poor guys panting on the ground. “A wild female chimpanzee engages in six thousand matings with more than a dozen males during her lifetime,” de Waal writes, yet she produces only five or six offspring. Impossible! What was evolution thinking?

Do female primates enjoy sex and have orgasms? What evolutionary reason for this could there be? Orgasm, as Desmond Morris asserted and many still believe, is unique to human women (supposedly because it bonds us to our promiscuous male partners). But then why does every female mammal, from mouse to elephant, have a clitoris, some larger than penises? “The clitoris likely evolved to turn sex into a pleasant, addictive affair,” de Waal speculates, which would explain why the largest and most prominent clitorises are found in “species marked by multipurpose eroticism”—such as dolphins and bonobos. Bonobos, says de Waal, are “Kama Sutra apes” who “mate in every conceivable posture, including some that we are incapable of, such as hanging upside down by their feet.” Female bonobos get enormous pleasure from their sexual activities, which include masturbation. Some use “handmade ‘French ticklers’ made of twigs,” Cooke reports. Many embarrassed scientists refused to acknowledge that all that genital rubbing and stroking was sexual. Isn’t it “extreme affection?” they’d ask de Waal. “I couldn’t help but point out,” he replied, “that if I were to show this kind of ‘affection’ on a busy street, I’d be in handcuffs within minutes.”

How can it be that females in 93 percent of all species have multiple partners? Doesn’t evolution want them to be monogamous and protect their precious eggs? Didn’t these females read Bateman? The fairy wren didn’t. She chooses a male with the most splendid blue plumage to be her “social partner,” but later she will sneak off to have sex with some neighboring naughty boys. DNA testing shows that more than three-fourths of her chicks will have been sired by different males. (The study of bird sex is a thriving career nowadays. One investigator told Cooke, “We’re basically pornothologists.”) When Patricia Gowaty tested the paternity of the eggs of female bluebirds, her results were welcomed by the male scientific establishment with dead silence. No wonder, says Cooke wryly: She was accusing the bluebird of happiness, a Disney heroine, of being a Jezebel! A male ethology professor told Gowaty that the female bluebirds in her study must have been “raped.” This would be a challenge, seeing as how male songbirds have no penis.

Why would some 450 animal species (and counting) engage in same-sex behavior? Since this activity doesn’t produce offspring, what possible evolutionary reason for it could there be? Therefore, when a male animal mounts another male and ejaculates, traditional Darwinians were forced to conclude that is not “homosexual” or “same-sex” behavior, but pseudo- or sham sexuality, or a sign of dominance. It couldn’t be what it looked like. When primatologist Linda Cooke published a field report of same-sex behavior in monkeys, she was accused of doctoring photos and misinterpreting the animals’ behavior: females were mounting each other “by mistake”!

Sadly for the enthusiastic gay community and their allies who love this research, I must pause to tell readers who fondly remember Silo and Roy, the “gay” penguins who hatched an egg and raised baby Tango to healthy penguinhood, that they didn’t stay together. Silo left Roy and took up with a California gal named Scrappy. De Waal shows that there is no such thing as “gay penguins”—their relationships fluctuate so often that they are better considered bisexual. Likewise, Cooke reports that “lesbian” albatrosses will partner with a female for a couple of seasons and then switch to a male. There are no gay bonobos, either. On Kinsey’s scale of 0–6, exclusively homo- or heterosexual at each extreme, all bonobos are a perfect three. “Sex is no big deal for them,” de Waal explains. “It’s such a natural and spontaneous part of their lives that it is hard to detect a borderline between social and sexual affairs.”

One likely reason for female sexual adventurousness and same-sex behavior, obvious the second we hear it, is that sex is not only for reproduction. It’s also for bonding, affection, conflict resolution, and sheer pleasure. A randy giraffe wants to mount; he’s not thinking of siring calves. Only humans make the connection between intercourse and pregnancy. In the animal world, females who have several partners, even while pregnant, ensure the safety of their young. Because male primates have no concept of paternity, says de Waal, “nature may have implanted a simple rule of thumb…. ‘Tolerate and support the offspring of females with whom you have had sex in the recent past’.” Females who have sex with many males thereby reduce the risk that a new male partner will kill offspring that he didn’t sire, and the males provide food and protection as well.

Both authors have chapters on discoveries about animal behavior—such as the prevalence of terrible mothers and doting fathers, of aggressive females and peaceful males—that also threw traditional Darwinians into confusion. Consider the contortions that two male ornithologists went through in search of the “alpha male” among pinyon jays. “This took some doing,” Cooke writes, because “male pinyon jays are committed pacifists and hardly ever fight.” The researchers ended up counting subtle cues, like sideways glances that would get the “submissive” male to leave the feeder. They patiently recorded 2,500 of these “aggressive” encounters, and even then found that only 14 of 200 flock members “qualified for a place in the dominance network.” Nonetheless, they concluded: “There is little doubt that adult males are in aggressive control.” Some birds actually did display plenty of violence beyond annoyed looks—dueling in flight and pecking opponents fiercely; it was just that all the fighters were female. The researchers suggested lamely that the females were suffering from a hormone surge that produced “the avian equivalent of PMS which we call PBS (pre-breeding syndrome).” They made that up. “There is,” says Cooke, “no such thing as avian PBS.”

So much variety; so many evolutionary riddles. Why do the females of some insect species kill their male inseminators and eat them? Why do only human women and female whales have menopause, living years past their reproductive prime, remaining sexually active? As they offer answers, these books dispel many dated beliefs of male and female behavior. “While baboons inspired the myth that patriarchy is natural and that macho males make up the core of society,” says de Waal, “we now know that this isn’t true even for baboons, let alone for most of the other primates to which this idea was generalized.” Neither are males more hierarchical than females. “In almost every social animal, both sexes arrange themselves on a vertical scale. Anyone who has watched female chimps or bonobos “will be quickly disabused of notions about female egalitarianism.” (Both authors remind us that the term pecking order comes from hens, not cocks.) This is not inherently a bad thing; hierarchies are essential to smooth group living in every social species. We can, however, observe the difference between alphas who are obsessed with loyalty and obedience, who terrorize subordinates into submission, and those who are true leaders, neither abusive nor aggressive, who protect the underdog and keep the peace.

It is almost impossible to read these books without seeing ourselves in the animal behavior being described. De Waal himself describes a power play among his colleagues, in which an alpha male senior professor, the silverback gorilla of his department, was undermined by a coalition of junior faculty members who voted him out. His booming voice silenced, he retired within a year. “I had seen it all before,” notes de Waal, “only in another species.” Not that he suggests we should—or can—model ourselves after any other species, no matter how delighted we might be with the sex-loving bonobos or amused by the female chimps who, unlike males, will suddenly burst into screaming at each other for no apparent reason. Other primates hold up a mirror to ourselves, but they are not the same as us, so they only offer a comparison. “We cannot just go around the animal kingdom and pick and choose which species we like the best,” he says. But we cannot overlook the similarities either. De Waal knows that many people resist the comparison of humans with other animals on the grounds that our magnificent brains (and genitals) are unique. Indeed, a colleague told me that her copyeditor cautioned her not to use females as a noun because it feels demeaning, “as though you’re talking about animals.” This attitude is “idiocy” to de Waal: we are animals. “It’s all vanity,” he writes. “As if evolution came to a screeching halt when it reached the human (and only the human) neck, thus leaving our lofty heads alone!”

These books could not have come at a better time, with the debates about “sex” and “gender” reaching deafening levels. Both authors agree that gender and sex are profoundly intertwined and influenced by culture; that gender includes identities that don’t correspond to biological sex; and that “biological sex” is itself complicated by genetic, hormonal, and chromosomal anomalies. But here the authors diverge sharply: just how gender “fluid” are we humans?

De Waal emphasizes that for a great human majority, biological sex and gender identity are congruent. “We come into the world with a large monkey brain and the psychology that it entails, including how we navigate a world of (mainly) two sexes,” he argues, which is why our “gender radar” is always on, exquisitely attuned to height, muscularity, voice, and facial shape and structure. The discomfort that many people have with transgender individuals, or the greater attention a transman gets with a testosterone- lowered voice than he got as a female, “highlights how deeply primate sexual dimorphism sticks in our subconscious.” To overcome this bias, we must understand where it comes from, and that in turn requires us to separate biological science from political ideology: we don’t get to use biology to say that sexual orientation and transgender identity are innate and immutable, and then reject biological contributions to sexual dimorphism in other behavior.

Cooke, however, aims to persuade us that “sex is wildly variable and that gendered ideas based on assumptions of binary sex are nonsense.” She starts at the genetic and cellular level, with new discoveries that move beyond X and Y: “The entire process of sex organ determination involves an orchestra of around sixty genes working in concert.” They don’t sit neatly on either X or Y chromosome; they are scattered across the genome and have more than one function. She describes an astonishingly complex continuum between male and female, a plasticity that has persisted in reptiles, fish, and amphibians for millions of years across diverse species and thereby must have had evolutionary benefit. For most scientists, gonads determine what sex an animal is. But what to do with animals that transition from male to female and back again? Transitioning anemonefish (also called clownfish) have a female brain but male gonads. The fish thinks it is a female and behaves like one, fiercely battling another rival female put in its territory, even though it has testes. So is sex assigned by its gonads or its brain? Trans activists love the clownfish, as gay activists love Silo and Roy.

This article appeared in Skeptic magazine 27.3
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We aren’t bonobos, but we aren’t penguins or anemonefish either. Across culture and history, our gendered sexual behavior has been as fluid as theirs, seeing as how humans can and do have sex in every possible combination, with every possible partner, in every possible way (apart from hanging by our feet, perhaps, and even then…). Will our sexual anatomy prove as fluid? Will the term “pregnant person” one day be a commonly accepted phrase instead of what it means to many today—an infuriating repudiation of female uniqueness, an eradication of biological women? Evolution is a work in progress; stay tuned. These two books, even as they blast a hole in our vanity, inspire awe in the breathtaking diversity of nature and the evolutionary roots of our behavior. Far from calling upon biology to justify the status quo, both authors are optimistic that our biology allows us to create more empathic, more egalitarian societies. Perhaps our human brain, for all its flaws, might help us get there.

This essay originally appeared in the TLS, April 29, 2022.