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Morpheus offers the pills in the MatrixThis is the second part of a series about the complex biological realities of sex. Though the posts build on one another, each can be understood alone.

When feminists get to talking about sexism, heterosexism, trans-antagonism, and the gender binary, people in the conversation like to bring up “biological facts” in order to oppress women and gender-variant people. These folks are awfully sure that “biological facts” justify misgendering trans women, violent behavior in men, even dressing women in pink. But the biological facts are stranger and more beautiful than any of us can imagine.

I’m writing this series of posts about the biology of sex not because I think it has that much to do with why women are oppressed or our lived experience as sexual beings, but rather because there is so much people take for granted about sex, and what science has to say about it, that I want to show what you think is simple might in fact be immensely complicated. I have written before about how “biological sex” in humans is socially constructed, and why we should take the red pill and learn the complex truth behind our simplified notions of sex, but that’s just a small part of the picture. Most people don’t even know what sex is.

What's that naked wrestling thing you and Daddy do every Thursday night? / Training for war.

What is sex?

If you see the word “sex” in a biology textbook or research article, it is almost certainly not referring to sex as humans usually define it. When an English speaker says the word “sex” in casual conversation she is either referring to what biologists would call “sexual reproduction” or “sexual behavior,” with the second more likely than the first. Sexual reproduction is the combination of genes from two individuals to create a new individual. That can be as impersonal as tree pollen being carried to a flower on the wind, or as sweet and awkward as Juno and Bleeker’s fateful first night of love.

Sexual behavior is related, but not the same. Sexual reproduction is critical to the survival of many (but not all) organisms, and some of them have evolved powerful physical and mental systems that direct behavior toward sexual reproduction. But just as a tongue evolved for tasting and eating is also used for grooming by cats, a system of erotic feeling evolved for sexual reproduction can also be used for other sexual behaviors that have little to do with reproduction, like what I do with all the items in my nightstand that go buzz.

There’s another meaning of the word “sex,” which refers to different reproductive types that can pool their genes together to make offspring, but not with individuals of the same type. In animals, those reproductive types are called male and female. But they are not the only possible sexes. Some fungi have sexes called plus and minus, while others have thousands of sexes. It makes our gender binary seem positively unimaginative.

Why sex?

For those of you who enjoy sex, the answer to this may seem obvious: because it feels good. That certainly is the motivation for most humans when they engage in sexual behavior. But it doesn’t answer the questions of why sexual reproduction, different sexes, and sexual behavior exist in the first place.

Sexual reproduction exists for the same reason that people shuffle decks in card games. Some living things get dealt a bad hand when it comes to their genes – or, more likely, a good hand that becomes bad when the circumstances change. Organisms that clone themselves can only hope to change from a bad hand to a good one when a gene randomly mutates, which in many cases just doesn’t happen quickly enough. When living things combine their genes with others’ when they make their offspring, it’s like shuffling the deck: new genes get introduced into the mix, which improves the next generation’s chances of having the right “card” to play at the right time.

When living things sexually reproduce, they shuffle their own “deck” of genes and cut it in half. This half-deck, which will combine with another to form a full deck of genes, is called a gamete. Which brings us to the question of why there are different sexes. The problem with not having different sexes is that any gamete can combine with any other gamete, and gametes are most likely to encounter each other within the body of the organism they came from. Basically, if there were only one sex, then it would be as if the eggs in your ovaries could fertilize each other, and you could get pregnant spontaneously with your own baby. That would defeat the whole point of shuffling the deck. When there are different sexes, gametes of the same sex can’t combine with each other, and they have to go out into the wide world to find a gamete of a different sex to combine with.

From that perspective, it is at first completely baffling that there are any species with only two sexes. After all, when there are only two sexes, then half the species is ruled out as potential parents to your offspring. Those fungi with thousands of sexes can’t sexually reproduce with themselves, and their gametes are extremely unlikely to encounter other gametes of the same sex, so they can combine their genes with just about anyone who comes along. Having just two sexes, male and female, seems like a bad strategy in comparison. So why are these two sexes so common?

Why male and female?

To understand why male and female exist, we must first consider what male and female even mean. In biology, “male” refers to individuals that make small gametes (what we call sperm) and “female” refers to individuals that make large gametes (what we call eggs.) That is all. It feels like pulling aside the curtain and discovering the wonderful wizard of Oz is just an old man in a light and sound booth, doesn’t it?

The man behind the curtain

Pay no attention to the biology behind the curtain

It’s easiest to consider the conundrum of male and female from the point of view of the first organisms to develop these two sexes. We mammals have a direct delivery system when it comes to sexual reproduction. We send our gametes very close to the source, as it were. But a pollen grain released by grass, or an egg broadcasted by an oyster into a river, doesn’t get a postal service straight to another compatible gamete. It’s adrift in a vast world where it may never encounter a gamete of the right species.

So imagine you’re lost in the woods without a cell phone, and you can’t find your companions. You have two basic strategies to deal with this situation. You can stay put, send up a flare, dig into your rations, and hope your companions get together a search party to find you. Or you can muster up a big search party, send them into the woods, and hope your companions stayed put so your search party can find them quickly. Each strategy works fine, but depends on the other for its success.

In this analogy, producing eggs is like the hiker who stays put and sends up a flare, and producing sperm is like the hiker who sends out a search party. Because sperm are small, males can make a lot of them, and by sheer numbers they’re more likely to find an egg. But because they’re small, they can only contribute genes to the offspring, and none of the other vital cellular components it needs to survive. So eggs, the big gametes, specialize in providing nutrients and cellular machinery to the offspring, and sperm, the small gametes, specialize in transporting their genes to where the eggs are.

But what about all the other aspects of male and female? Surely there’s more to them than that, especially since creatures like mammals don’t need to worry overmuch about sperm getting lost on their way to the egg? It’s true that all kinds of other characteristics have become associated with male and female over the course of evolution. But those characteristics are so different between species that they can’t be considered fundamental to what male and female are. The two sexes are kind of like your arm bones that way. Your arm bones are very similar, structurally, to the bones in a whale flipper and a bat wing, but they’ve been co-opted to a completely different purpose. Male and female, too, have been built up into many shapes, like different flesh over the same bones.

It is common in our society to think of male and female as fundamentally different kinds of organism that transcend the boundaries of species. But they are, at their core, reproductive strategies that depend on each other for success, sort of like the social strategy of pack hunting that wolves depend on their packmates to share, and they are not the only reproductive strategies possible. Different species deploy these strategies in their own ways, which results in all the amazing sexual diversity we observe in our own species and the natural world at large.


Next up in the series: the different shapes male and female can take, and what determines whether a critter is male, female, both, or neither.