What does it mean to be female? A few years ago, this would have been a question with a simple, biological answer. Increasingly, however, it seems to be leaving people stumped — like the shadow minister for women and equalities, who last week claimed when pressed that “female” doesn’t have a clear definition, just that it “depends on the context”. It’s now considered that “female” and “male” refer to something more than grubby, materialist bodies.

In her new book, Bitch, zoologist Lucy Cooke tells us that actually, even the biological side of this issue is not clear-cut. If you thought binary sex could be defined by genitals, think again. Females in “dozens” of species, from spider monkeys to tiny cave-dwelling insects, have genitalia that look more like a typical male’s. Famously, female hyenas are endowed with pendulous enlarged clitorises, referred to by researchers as pseudo-penises, through which they copulate and give birth. Moles go a step further: as well as sporting a “penile clitoris”, outside the breeding season their vaginas actually seal shut, and “testicular tissue” fills their bodies with testosterone. Not so female now, eh?

“You might think the ultimate answer to what makes a female animal is a pair of XX chromosomes”, Cooke continues — but you’d be wrong. Other species have different sex chromosome systems; or do something else entirely, deciding their sex by external factors like the temperature they experience before hatching. Many can switch sex throughout their lives, and occasionally, mutations give rise to individuals whose bodies are split down the middle, with one half female and the other male.

The conclusion Cooke draws from all this is that “sex is no static binary, but a fluid phenomenon”. Sex, she says, is an “anarchy”’; chromosomes are “chaotic”. Nature contains a “smorgasbord of sexual flavours” that defies human categorisation.

But many questions are not answered by this account. Cooke mentions that the “standard biological definition of sex” is based on the type of gametes an organism produces, and that “across the animal kingdom these come in just two sizes: big and small.” Why is gamete size used as the definition of sex though? It’s not just some arbitrary characteristic like eye colour, or indeed the length of a creature’s (pseudo)penis; nor is it a gradient that’s been artificially crammed into two boxes.

The fact that a mole’s vagina closes shop when out of season is neither here nor there

Each gamete contains half an organism’s genetic material, and in sexual reproduction, two halves combine and make a new genome. Half of your genetic material comes from each of your two parents: one small gamete producer and one large. This is true whether you’re a human, a blue whale, or an asparagus — it’s a pretty fundamental binary.

The other side of this coin is that there are two, and only two, routes for a sexually reproducing animal’s genes to reach future generations — via eggs or via sperm. Evolution equips our bodies to succeed at reproducing one way or the other (or, in some species, both), providing us with genitalia to exchange gametes, secondary sexual characteristics to attract mates, and so on. There’s as much natural variation in how this process is navigated as the day is long, but it is built upon the fundamental binary of gamete size.

When sex is considered this way — in terms of function, not just the characteristics superficially associated with it — the fact that a mole’s vagina closes shop when out of season is neither here nor there. It raises the question of why natural selection has found this to be a good way of transmitting genes via the female route in moles. But it doesn’t make the mole any less binarily female.

Cooke’s approach — presenting variability as evidence for the “anarchy of sex”, not explaining that it does fit into an existing binary model — illustrates something I’ve come to think of as “Life Is Diverse” science. Someone will present, say, lots of wacky and wonderful ways that animals reproduce, and imply that all this variety shows it’s pointless trying to draw any general conclusions beyond marvelling at the sheer diversity of nature.

Last year, for instance, I attended a zoom workshop on “Teaching accurate and inclusive sexual selection” run by Cornell University. One of the main messages of this session was to “present diversity first”: to primarily emphasise that there is much diversity in sexual characteristics and behaviour, instead of first explaining general trends and afterwards discussing exceptions.

But one of science’s functions is to make generalisations: to look for the signal in the noise and deduce underlying principles. The scientific method means asking questions about observations that don’t fit neatly into boxes. Why was our model of how the world works wrong in this particular case? Could this somehow be an exception that proves the rule? Or is there a specific way we need to update our model to account for it? This is the opposite of throwing your hands up and saying “well, Life Is Diverse, we can’t say anything more than that”.

Cooke is not the first scientist to suggest that sex is not a binary

In the 1970s, the field of “sociobiology” (now more often called evolutionary psychology) proposed that much of human nature could be explained by evolutionary principles, from the existence of war and racism to behavioural differences between women and men. These sometimes quite sweeping claims have sparked a huge amount of controversy and become a shibboleth for a wider cultural rift, with much of the Left believing on some level that “nature” rather than “nurture” explanations for human behaviour are ideologically suspect. Now, the concept of biological sex has similarly been engulfed into the culture wars, with many seeing it as transphobic to say that sex is fixed (in humans) and binary.

The role of “Life Is Diverse” science in culturally embattled areas like this is to point your ideological opponents towards an exception to their theory, and say “good luck explaining THAT”. There’s a perhaps surprising parallel with religious objections to the theory of evolution, which sometimes cite adaptations like the eye as examples of “irreducible complexity” supposedly too complex to have arisen from natural selection — ignoring the fact that natural selection can and does explain these things.

Cooke is not the first scientist to suggest that sex is not a binary. An article in Scientific American visualises a spectrum from female to male, with an in-between populated by people with various developmental differences in their sex characteristics, and illustrated with a complex colour scheme and network of criss-crossing arrows. But what can it possibly mean for any ova-producing person to be “more female” than any other, as implied by this spectrum? This is never explained.

What all these arguments have in common is that they reach the conclusion “wow, so complex!” and stop there. But our curiosity should extend further. We should ask why, and we should follow the answers where they lead us.