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Do you think animals can be divided into two biological sexes: female and male? Well, it’s way more complicated that that! 

Hosted by: Hank Green
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Hank: We tend to think of biological sex as having one of two distinct genetic codes: If your sex chromosomes – that is, the 23rd pair in your cells – are X and Y, then you’re male, if you have X and X, you’re female.

But, like so many things in biology, and the world, it’s not that simple. Humans and other animals that have sex characteristics – that is, things like sex hormones, gonads, and genitals – that don’t fit the male/female binary are considered intersex. These conditions can also be called differences of sex development, or DSDs.

And there are a lot of various DSDs out there, which can affect physical appearance, or even the behavior of an entire group of animals. In humans, intersex conditions are mostly caused by genetics, like variations in chromosomes or in a single gene. Some kinds of intersex conditions are known as chromosomal DSDs, and in humans they’re linked to an unusual combination of X and Y sex chromosomes.

If an XY individual has an extra X chromosome, for example, so they’re XXY, or even XXXY – the result is a condition known as Klinefelter syndrome. That extra X chromosome can affect how the person’s testes work, including how much testosterone they secrete. Lower levels of testosterone can affect how secondary sex characteristics develop, like body hair, or breast tissue.

And some people with Klinefelter syndrome might also be unusually tall, possibly because of a height-related gene on the X chromosome. By contrast, there’s also Turner syndrome, in which a person has just one X chromosome, and no Y. In this case, a person might start developing female sex characteristics, like someone with two X chromosomes.

But without a second copy of the X chromosome genes, their ovaries might not develop completely, which could result in the inability to secrete sex hormones and develop secondary sex characteristics. Some people with Turner syndrome might also be shorter, because they’re missing a copy of that height gene. And others might experience more serious health problems like heart defects, diabetes, and low levels of thyroid hormone.

But sex characteristics aren’t just dependent on whether you’re XX, XY or neither. There are also hormonal DSDs, which interfere with how sex hormones are made and used, usually because of individual genes. Alterations in these genes can affect how your body processes androgens, the hormones that generally help develop male sex characteristics, or estrogens and progesterone, which help with female characteristics.

Mutations in the androgen receptor gene, for example, mean that no matter how many androgens are floating around in your bloodstream, your cells can’t respond to them. In XY individuals, this can lead to the development of some female secondary sex characteristics – a condition called androgen insensitivity syndrome. On the other hand, XX individuals might experience a condition called congenital adrenal hyperplasia.

This is when the body doesn’t produce enough of an enzyme known as 21-hydroxylase, which results in the over production of androgens, and therefore the development of male secondary sex traits. But of course, intersex conditions aren’t only found in humans. Plenty of other animals can reproduce sexually, but have more than just males and females.

Some animals don’t actually have two separate sexes, and are hermaphroditic – like snails, slugs, and worms – which have both male and female reproductive organs. Other animals just have one set of reproductive organs, but they can change. Clownfish, for example, live in social groups that consist of one breeding pair with a female and male, plus a group of other not-completely-developed males.

If the female dies, or is taken away for some reason, the breeding male’s hormone production changes so it loses its male reproductive organs and develops female ones. This male-to-female change is called protandry, and is basically a way to make sure there’s always a mating pair without as much competition. Knowing this makes Finding Nemo seem... a little inaccurate. Because his dad would actually become his new mom.

Other species of fish can change from female to male – called protogyny – or change sex bidirectionally, depending on the pressure to breed. Scientists have also discovered that a lot of animals that we think have distinct male and female sexes, also experience intersex conditions.

Researchers have found variations in the reproductive anatomy in species like black bears, spotted hyenas, and leopard geckos, which suggests that they have chromosomal or hormonal DSDs too. And this is just some of the stuff that we’ve noticed. Who knows what other sex variations might be out there in the animal kingdom?

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