For a long time, the runner Caster Semenya made for a terrific and compelling true-life underdog. In a world that was increasingly cognisant of the oppressive interplay of race, gender and class, here was a woman, black and gender non-conforming — a 2009 New Yorker article described her as “breathtakingly butch” — battling against a racist, sexist system that wanted to keep her down. Her trajectory from an impoverished running club in rural South Africa to the winner’s podium at the Olympics was Hollywood movie material, and the repeated indignities she suffered at the hands of officials and fellow competitors who questioned her right to be there were an outrage: Semenya was dogged by rumours that she was a man in disguise, and was routinely required to prove her sex by submitting to humiliating inspections of her genitals. It all seemed like the work of a snivelling patriarchy conspiring to take down an exceptional woman.

But the truth was more complicated: Semenya is a woman — by her own definition, and in virtually every social context that matters — but she is also not exactly female. Testing eventually revealed her to be intersex, possessing an XY chromosomal makeup and a pair of internal, undescended testes, which among other advantages meant she could produce performance-enhancing testosterone at levels that an ordinary female could not.

Athletic governing bodies struggled to come up with a policy that would allow someone like Semenya to continue competing: when she declined to remove her internal testes, they required her to take drugs to suppress her testosterone production. The solution satisfied no one, including Semenya, who found hormone therapy intolerable. In an essay adapted from her new memoir, The Race to Be Myself, she writes: “If I had internal testicles or whatever they were saying I had, they were mine. I was healthy, I loved my body, and it had made me a champion. Why must I go and mutilate it to conform to someone else’s rules?”

In one sense, it is only fair that someone who moves through the world as a woman be allowed to compete alongside women. “I didn’t care then, and I don’t care now what the medical findings are,” Semenya writes. “I was born a girl and raised as a girl.” But socialisation is only one piece of the puzzle when it comes to defining womanhood in sports; science matters, too. And the concept of protected classes in sport also stems from a desire to be fair. Women, disabled people, the very old or very young: all of them compete on playing fields that only admit people who share certain characteristics, which is to say, certain limitations. The boundaries around these fields have always been human-drawn and hence imperfect, but they are also necessary; the alternative, as the International Paralympic Committee notes, is “one-sided and predictable competition, in which the least impaired athlete always wins”.

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Semenya, in her book, puts scare quotes around the words “protected class” as if to mock the idea that it could apply to women, but the argument doesn’t land — not least because her presence in women’s sports has more often than not given rise to exactly the sort of one-sided, predictable outcome the IPC works to avoid. Semenya mops the floor with her competitors, and the women who have raced against her have noted that it doesn’t feel like a fair fight. And while nobody likes to think that half the planet can be reasonably classified as impaired compared with the other half — that womanhood itself is something akin to disability — the truth is: the acceptance that women are weaker is the essential foundation of women’s sports. Men jump higher, run faster, kick and punch harder. And if women’s sport is a ghetto of sorts, it is a necessary one, not just for fairness or safety’s sake but for the sake of sport. Nobody wants a one-sided and predictable competition; we want drama and excitement. This is why we play sport, and why we watch other people doing it. We love to see them fight; we love a win against the odds.

There’s something viscerally distasteful about the inverse: somebody with too much strength clobbering someone who has too little. An able-bodied athlete dunking on a guy in a wheelchair; a grown man winning a sprint against a bunch of 8-year-olds. A ringer parachuted into an amateur competition. We watch in pursuit of the kind of fairness that makes for a good game.

This doesn’t mean that the line where an attribute becomes an unfair advantage is always easy to discern. Years ago, the Olympics governing body became embroiled in a debate over whether Oscar Pistorius’s bionic legs gave him an unfair advantage, leading to months of rigorous study that culminated in the following remarkable conclusion, published in The Journal of Applied Physiology: Pistorius, the scientists agreed, was “physiologically similar but mechanically dissimilar” to an athlete with normal human legs. (Pistorius was allowed to run.) But to the general public, fairness in sport is like obscenity as famously defined by Supreme Court Justice Potter Stewart: we know it when we see it.

This is what makes Semenya’s case so complicated. The notion of her being some sort of cheater is flawed. When she balked at suppressing her hormone levels, it wasn’t because she wanted to win; it was because it felt like lying: “I wasn’t going to take on an identity that did not fit my soul because some doctors had taken my blood and images of my organs. I was not a hermaphrodite or anything other than a woman.”

In her memoir, Semenya is largely describing events from over a decade ago. But the book has been published into a world where a related, ongoing battle is raging, over the place of trans women in sports. These issues are not identical, of course: Semenya never imagined she was anything other than biologically female, while trans women are seeking admission to a category they know full well they were not born into. But both have been plagued by allegations that they’re trying to game the system, that they’re cheaters. Often, trans women are presented as just a bunch of grasping, avaricious men who’ve found the easiest possible pathway to a gold medal. When ESPN aired a special about trans swimmer Lia Thomas, her former competitor Riley Gaines slammed both the network and the athlete as complicit in the destruction of women’s sports: “Lia Thomas is not a brave, courageous woman who earned a national title. He is an arrogant, cheat who stole a national title from a hardworking, deserving woman.”

This knee-jerk demonisation has always seemed wrong to me — morally, but also logically. The humiliation, the opprobrium, the physical and mental burden of transitioning: all seem like a wildly disproportionate price to pay for the sake of a college swimming championship. Instead, what has always struck me about Thomas’s case is that elite swimming is one of the few arenas where the women celebrated are broad shouldered, long limbed, and tall. Perhaps swimming represents, to her, a chance to feel not just like a winner, but like she belongs.

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This is the tragedy: that what is fair at the macro level is not fair, as in compassionate, to an individual person. Lia Thomas cannot fairly compete in the sporting category known as “women”, and arguably neither can Semenya, whose memoir presents this as an obscene attempt to deny her identity. No doubt it felt that way to her: to alter her body to compete with women would have amounted, in her mind, to a tacit admission that she wasn’t one.

And as long as women’s sports are seen as having the power to validate or invalidate one’s innate sense of self — for as long as we see the regulatory bodies as not just deciding who can compete as a woman, but who is one — this issue will remain intractable. The best hope is a future where the category of “woman” in sports is treated in much the same way as the IPC treats the category of “disabled”: “If an athlete is not eligible to compete in a sport, this does not question the presence of a genuine impairment,” the committee website assures readers. “It is a sport ruling.”

The truth, unfortunate but inevitable, is this: the most carefully drawn lines, the ones that need to not just include but protect, will always leave someone on the outside.

As Charles Darwin put it, the female, “with the rarest of exceptions, is less eager than the male”. Males work to “sedulously display their charms before the female”, who “requires to be courted”. This distinction between chaste females and indiscriminately horny males became a central principle of evolutionary theory for much of the 20th century.

Females — the sex defined by production of large gametes, and in most animal species the sex that either lays eggs or gives birth to young — have a physical limit on the rate at which they can produce offspring. Once you’re pregnant, you can’t get more pregnant by having sex again. Males, on the other hand, are not limited in the same way: if they manage to find two sexual partners instead of one, the maximum number of offspring they could have is doubled.

This binary is neat, has instinctive appeal, and has been enormously influential in the wider public imagination. Perhaps most overtly nowadays, it remains popular in the “manosphere”, where men lament that “Chads” are swimming in attention from “hypergamous” women while average Joes get ignored.

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Except of course, like most tidy binaries, this one turns out to be an oversimplification. In her latest book, Bitch, zoologist Lucy Cooke addresses misconceptions about the role of females in the animal kingdom. One of the main takeaways is that the idea females are not naturally slutty is due for an update: today’s cougars “are leading sexually liberated lives, for the benefit of themselves and their family, with no shame attached”.

But why would they do this? Isn’t more than one baby daddy surplus to requirements? Though less obvious, there are evolutionary benefits to playing the field for females as well as for males. Having multiple fathers improves the genetic diversity of a female’s offspring, for instance, and provides insurance in case one is infertile. It can help you trade up in mates, too: if you’re “shacked up with Mr Average”, you might pay a visit to your “neighbour, Mr Fabulous, for some superior genes”.

Cooke tells us how anthropologist Sarah Blaffer Hrdy suggested as early as the 1970s that female philandering could serve a different purpose altogether: protecting offspring. One of the bleaker facts of life is that infanticide is a fairly common practice in our primate cousins, and the animal kingdom more widely, since it “forces the bereaved mother into oestrus, making her readily available for fertilisation” months or years ahead of schedule. Hrdy’s idea, inspired by her observations of Indian langur monkeys, was that by “confusing paternity”, mothers might secure protection, or at least benevolent tolerance, from males who think the baby might be theirs.

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The assumption of female monogamy was so strong that for a long time observations to the contrary were written off as anomalies. The idea that female songbirds, for instance — the very symbol of marital bliss — might be sneaking off to find a bit on the side was seen as ludicrous. Perhaps these poor birds were being raped, or else there must be some kind of hormonal malfunction.

Eventually, in the 1990s, these assumptions were overturned, partly thanks to the fact that biologists could now determine paternity through DNA. The conclusion they reached was this: polyandry (that is, when a female mates with multiple males) is much more common, across the whole animal kingdom, than we had previously thought. Many species thought to be monogamous have “affairs” — and often, it is the females that actively seek out these liaisons.

This rewriting of evolutionary theory to include female promiscuity is, of course, catnip to any popular science writer of a feminist bent. As well as in Bitch, this tale features prominently in Angela Saini’s Inferior and Cordelia Fine’s Testosterone Rex — books sold as explicitly feminist reappraisals of scientific consensus. Other topics Cooke covers will appeal to the same audience, from the science of the female orgasm to the lady spiders who dominate in the bedroom by killing and eating their lovers (in response to which, males show counteradaptations “that would make even Christian Grey blush” — oral sex, bondage, and threesomes).

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But what’s typically missing from these accounts is context. Because, although Cooke is right to refute the patriarchal falsehood that females only want monogamy, this doesn’t mean we should leap to the opposite conclusion that females seek as many mates as possible; or that there are no systematic differences between the sexual proclivities of females and males. Casual readers of Bitch will likely be left with both these impressions, though.

The benefits of polyandry to females are actually limited: one meta-analysis of 35 species found that overall, it had only a modest effect on the number of offspring produced, and no measurable effect on what proportion of offspring survived. Another meta-analysis, this time focusing on insects, suggests that you can have too much of a good thing: having multiple mates increased females’ fertility, but reduced their lifespan, leading the authors to conclude that the optimum number of mates was probably “intermediate”.

Rigid oversimplification though it may be, as a general rule of thumb Darwin was right: females are often “less eager than the male”, and this divergence can be explained by the different reproductive investments required by each sex. Reading Bitch, you’d be forgiven for thinking this theory was completely debunked, but in fact the logic remains sound, even if we’re now more familiar with the exceptions and caveats.

It’s tempting, if you believe men and women should be equal, to argue that this state of affairs is “natural”. But a key lesson of biology is that if you look closely at any blissful-seeming union, you will find an imperfect alliance, with each party driven by their own interests as well as shared goals. Conflict rages between sexual partners over when to mate, over how promiscuous each of them should be, and how much care each should devote to their shared offspring. The size of new-born mice, for instance, seems to result from a compromise between genes: the fathers’ encourage a developing foetus to grow larger by leaching energy from the mothers, while the mothers’ push them in the other direction.

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Another result of conflict between the sexes is that, bluntly, forced and coerced mating is common in the animal kingdom; from insects, to the chickens I’ve spent the last four years studying, to our cousins the orangutans, where one researcher observed almost 90% of copulations to be forced. This is a result of the difference in “choosiness” between the sexes: if on average females are more “choosy” than males, then there will be many situations where they don’t want to mate but a male does.

Perhaps this is why feminist science writers tend to avoid studying conflict between the sexes, even though it has obvious implications for the feminist movement: in this field, there are some inconvenient truths. Popular discussions of sexual conflict and potential implications for human society come almost exclusively from older male biologists, who are predictably (and not always unfairly) accused of insensitivity or suspect motives; a prime example being the spectacularly controversial A Natural History of Rape by Randy Thornhill and Craig Palmer.

In Bitch, we do hear about the nasty and exploitative side of sexual selection, albeit briefly. Cooke’s account of Hrdy’s infanticidal langurs alerts us to the fact that not all female promiscuity follows a “girl power” narrative — it can be the result of something much darker. In her chapter on genital evolution, we learn that the bizarre vaginas of female ducks — which have “helical piping”, “opposing spirals and strange cul-de-sacs” — likely evolved in response to the high rate of forced mating in these birds: their convoluted tracts seem to reduce the chance of fertilisation in such encounters and allow them some control over who fathers their offspring.

But despite the rest of the book being woven with — arguably premised upon — implicit or explicit anthropomorphisation, any such speculations in this case are quickly headed off by a footnote implying that to ask whether rape in human and non-human animals might be related phenomena is to “dangerously” suggest that “a rapist lives inside all human males”. Humans and animals are, in this case, completely different beasts. “This is a very important distinction to make”. It feels as though there’s a lack of curiosity in this kind of popular science book for any aspects of the natural world that don’t fit into the story their audiences want to hear.

It’s no surprise that feminist biologists have little appetite for potentially pessimistic messages from nature. For a start, it’s pretty much the polar opposite of “sex positive”. It’s also at odds with the current trend within feminism to minimise, rather than emphasise, differences between the sexes.

And then, of course, it’s just depressing. Most of us are reluctant to accept fatalistic views of the world, which skate too close for comfort towards potentially disturbing implications for our own species. Readers of popular books on feminism are on average likely to be highly agreeable people, who value pleasantness and see the best in others. They are probably also “low decouplers”, who feel ethically uneasy with “separating is from ought“. According to Cooke, the idea that sexual violence is more than just a man-made aberration is “dangerous” — perhaps even morally repulsive to contemplate.

Feminists are, in general, resistant to the idea that any aspect of sexism could be “natural”, or in male interests to maintain. It’s easier to assume that it’s all a big misunderstanding. But the point of science is to understand the world as it is, not to pick and choose parts to present in a certain light. A blinkered lack of imagination led decades of male biologists to ignore inconvenient evidence and assume females were passively monogamous. Correcting that imbalance shouldn’t mean assembling a contradictory narrative to promote in its place, despite the evidence.