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

In previous posts in this series, I’ve talked about what sex is, and the many ways it can manifest in human bodies. But what determines whether an individual is male, female, or something else altogether?

I’m talking here about the spectrum of sexed bodies that exist in nature, not gender identity, because only humans can have gender. Gender is created by institutions, culture, and human relationships. As far as we know, non-human animals don’t have cultural constructs of male and female. But they do have an amazing variety of sexes and paths to determine what sex an animal will be.

You may have learned that sex determination in humans comes from the sex chromosomes: two X chromosomes will lead to a human with ovaries and a uterus, and an X and a Y chromosome will lead to a human with testes and a prostate. This is not entirely true, but gonadal sex (whether your body is equipped to make eggs or sperm) is indeed genetic in humans. But in many animals, sex determination is environmental, not genetic.

In many reptiles, and all crocodiles and their relatives, sex is determined by the temperature at which parents incubate their eggs. In many of these species, low temperatures produce male offspring and high temperatures produce female offspring. This is because a chemical process for converting testosterone into estrogen is activated by high temperatures in the developing embryo, feminizing its growing reproductive system. In other species such as snapping turtles, low and high temperatures produce females, while intermediate temperatures produce males. This system of sex determination is thought to exist because males and females reproduce most successfully at different times of year or environmental conditions.

But sex can also be determined by social context. There is a type of marine worm, Bonellia, in which the larvae will, if they can detect a female, become male, enter the female’s body, and fuse with her as little more than a sperm-producing organ. If the larva detects no females, though, it will become female, settle to the ocean floor, and wait for males to come along. Animals which can change sex, known as sequential hermaphrodites, can be more complex. Some can turn female if there are too many males in their environment. Or sex can be tied to social rank. The dominant clownfish in a social group is a female, and all the other clownfish are male. When she dies, the most dominant male gets to become female and take over the group. So in Finding Nemo, when Nemo’s mom died, his dad should have become his mom.

Perhaps the most interesting case of sex determination is in the Hymenoptera, the group of insects that includes ants, bees, and wasps. Most biologists classify these insects into two sexes, male and female, but I would argue that there are three. If we define female as “physically equipped to produce eggs,” then in most species of social wasps, bees, and ants, the workers are not female, even though they are genetically programmed to develop ovaries – because the ovaries don’t actually develop. These worker insects are not equipped to make eggs or sperm, which makes them neuter.

So what determines whether these social insects grow up to be male, female, or neuter? It emerges from a fascinating combination of genetics, nutrition, and social context. Unfertilized eggs of these insects will always hatch into males, which have half as much DNA as females. This means that all male ants, bees, and wasps are virgin births, since their mothers don’t have to mate to produce them. Fertilized eggs are genetically disposed to become female or neuter. So what determines which one they’ll be? This is where it gets interesting.

Sex determination between female and neuter is based on nutrition and social interaction. Larvae hatched from fertilized eggs are either fed just enough food to grow to adulthood, or fed a highly nutritious diet that gives them the extra energy they need to develop ovaries. This sex differentiation is socially reinforced by hormonal signals from the colony’s queen. In most social insects, if the queen dies, the workers are freed from her social control and some of them will develop ovaries and become female. Which just goes to show how many factors can interact to produce the sexed body.

So what does all of this mean for humans? The sexed body, for us, is produced by the interaction of many genetic factors. But all of this does not necessarily lead to a human who identifies with or feels comfortable in the sexed body they have. So what determines what gender a person identifies with?

There is no one answer to this question, and there will never be: gender identity is at least as much a product of culture as anything else, since gender itself is not a category that arises neatly from the body. But given how sex arises from a combination of hormones, genetics, nutrition, and social context, it would not surprise me if gender identity arose from an interaction at least as complex.

Next up in this series: variations in sex behavior, or, why nobody is an “alpha male”