Intersex Across the Animal Kingdom


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? Thanks for watching this episode of SciShow,
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