Many people and societies think of sex as split  across two tidy boxes: “male” and “female.” But life, in all of its glorious  complexity, tells another story.

Like, there are these fish called Australian  gobies that all start out as females. But later, some start producing sperm, while  still having cells that can become eggs.

So in one lifetime, they can be female…then  male and female…then female again. It’s not a static, either/or thing. And that’s just the start of  life’s binary-busting rainbow.

There’s a plant called tropical ginger that  switches between male and female every afternoon. Many kangaroo rats have both a vagina and a penis. And forget male or female, the split gill  fungus has over twenty thousand sexes.

And the evidence for sex as a continuum isn’t  just found in fish and fungi – but in us, too. Hi, I’m Dr. Sammy, your friendly neighborhood  entomologist, and this is Crash Course Biology.

And this is a singular theme  song that also defies categories! HIT IT!! [THEME MUSIC] This episode, we’re talking  about sex, gender, and sexuality. And the first thing you need to know  is… they’re not all the same thing.

Gender, for example, is totally unique to humans. It refers to a person’s inner  sense of being a man, a woman,   some other gender, or no gender at all. It’s about a person’s identity and how it’s   expressed in relation to  social and cultural norms.

And different cultures have  different ideas about what it   means to be feminine or masculine,  or how many genders there are. Like, some Native American groups  have long recognized a third gender,   which is sometimes called “two-spirit.” The Bugis people of Sulawesi  recognize five genders,   including one that transcends male or female. And many Western cultures today are widening their   understanding to include people who  are non-binary, trans, and more.

So, yeah, sex and gender: not the same thing. Sex isn’t an identity, but rather a way of  categorizing life-based on a range of traits,   including what reproductive  cells an individual makes. This can seem confusing because we do tend   to assign someone a sex and  a gender when they’re born.

Like when we say, “It’s a boy” or  “It’s a girl” based on outward sex   indicators like genitals, we’re assigning a sex. But we tend to bring with that a suite of  cultural expectations that are part of gender,   like the specific colors and gifts that are  typically brought to “gender reveal parties.” And sexuality describes who someone  is sexually attracted to, if anyone. And just like sex and gender,  sexuality exists on a continuum.

But we’ll get to that later. For now, let’s get into biological sex, which,   like I mentioned, is often tied to  reproductive cells, called gametes. You see, the ability to reproduce  is one of the hallmarks of life.

And life has evolved a lot of ways to do it,  with diverse bodies and behaviors to match. Though for a long time, it seemed  like this was relatively simple. See, there are often two different forms of the  same animal, one has small, simple gametes that   don’t require a lot of energy to produce,  called sperm, and the other has a smaller   number of larger gametes that are comparatively  expensive to produce, which are called eggs.

The individual that produces  the sperm is called the male,   the individual that produces  the egg is the female. This holds true for a fair number of organisms,  but the more you look, the more complex it gets. Like, for example, it is not super rare for  one organism to contain both sperm AND eggs.

But the sperm/egg dichotomy is  common enough among life on earth,   and useful enough, that it’s helped  us classify all sorts of life. Like, take Ginkgo trees for example. When we talk about “male and female ginkgos,” we   don’t call it a male tree because  it’s taller and has more body hair.

That would be an odd tree indeed. But no, we call it a male tree  because it produces pollen,   which is tree sperm, and not  ovules, which are tree eggs. Sometimes we talk about “sex” in terms  of ecology and species-wide attributes,   but it’s also important to consider that,  when you drill down into individuals,   we find, again, that nature  will make any harsh line fuzzy.

Like, if you’re talking about individual humans,   you can’t just boil it down to sperm  and eggs, because lots of people don’t   produce either and there are also a ton  of other important traits beyond gametes. Like, biologists also consider several  physical traits, such as anatomy,   chromosomes, and hormones when identifying sexes. And when biologists study sex differences,   they often look at averages for a sex —  average hormone levels, average anatomy.

But those traits can overlap. Let’s look at chromosomes, for example, more specifically our 23rd pair  of chromosomes, called allosomes. In humans and many other animals,  allosomes come in two versions:   the larger and information-rich  X chromosome or the smaller Y.

Most people inherit one allosome from the  egg that made them, and one from the sperm. And each one holds genetic  instructions for making proteins,   which are like the body’s  clay for building organs. An XX person often has organs called ovaries  that make eggs, plus a vagina, vulva, and uterus.

An XY person often has testes that  make sperm, along with a penis. Ovaries and testes secrete different blends  of hormones: so XX bodies usually have higher   levels of estrogen, while XY bodies usually  have more androgens such as testosterone. These hormones shape other physical  features that we tend to think of as   “female” or “male,” like breasts or hairy chests.

But, I’m saying “people with XX chromosomes,” not  “women,” and “XY people,“ not men,” because “man”   and “woman” refer to a person’s gender which,  as we discussed earlier, is different from sex. Someone with XX chromosomes isn’t always a woman;  someone with XY chromosomes isn’t always a man. Plus, when it comes to sex, what’s going  on in the ‘somes is just the beginning.

You see, some XX or XY people have gene variants  that cause a diverse blend of hormones or anatomy. And this can result in the physical expression   of a different sex than the one  their chromosomes would indicate. And some people inherit three  allosomes: so they’re XXY, XXX,   or XYY, which can lead to a bunch  of different sexual features.

And then some people are X0,  with just the one allosome. But there’s nobody out there who’s Y0, because  we all need at least one X chromosome to live. There are important genes in  there that help the brain develop.

These variations along the sex continuum  are called differences in sex development,   or some people prefer the term intersex. And a person is about as likely to have these  gene variations as they are to have red hair;   they’re present in about one  in every one hundred people. So chances are, you’ve probably  met at least somebody with one   of these differences, even if you didn’t know it.

Even if they themselves didn’t know it. There aren’t always outward signs. Like, if an XY person inherits  a variant of the AMH gene,   they may develop organs often found in XX people.

So, they might have a uterus and fallopian  tubes, while also having a penis, testes,   and levels of the hormone testosterone  that would be normal for an XY person. Or if an XY person has a variant of the AR gene,   their body ignores testosterone — a condition  called Androgen Insensitivity Syndrome. Often, they have a vagina  and breasts, and no uterus,   and sometimes a testicle inside their bodies.

So, yeah: sex is way more complex than “male or  female,” it isn’t determined by any single trait,   and it’s not turned on or off by any single gene. It’s an assortment of overlapping interactions  between chromosomes, hormones, and genes. So that’s gender and sex, neither of which  can be put into nice, tidy little boxes.

Now let’s take some time to talk about sexuality,   which, like I mentioned earlier, describes who  someone is sexually attracted to, if anyone. Just like sex and gender, sexuality  doesn’t fit neatly in distinct boxes. Someone can be attracted to the same gender,   another gender, both their own and other  genders, all genders, or none at all.

For some, attraction stays steady for life. For others, it’s more fluid. All of this is normal.

And diverse sexuality is nothing new. Attraction to the same gender, for example,   has been written about in ancient texts  from China, Japan, India, Greece, and more. Across cultures today, it’s experienced  by about five percent of people.

That number varies a bit from place to  place depending on cultural attitudes   and the question that’s being asked in surveys. Like, it’s more common for people  to report they’ve had at least one   same-gender experience than to say that they’re  exclusively attracted to the same gender. But either way, diverse sexuality  is a very normal, very human trait.

And while we don’t need to look to other  animals to justify human sexuality,   life’s diversity does help us understand our own. Let’s head over to the Thought Bubble… It’s 1834. And German schoolteacher August  Kelch can’t believe his eyes.

In a forest in Poland, he has just  stumbled on two male doodlebugs…well… doodling each other. Keep in mind, the word  “homosexual” didn’t exist yet,   in German or in English, so you  can imagine Kelch’s surprise. Especially considering people attracted to the  same sex were considered unnatural at the time.

Nonetheless, Kelch described what he’d seen  and shared it with the scientific community. The reception was…not great. A mix of moral outrage, disbelief, and disgust.

Some tried to explain it away,  arguing one bug was actually female. Or that both bugs were confused. I wouldn’t be surprised if  someone tried to claim they   were just “really good friends” or “roommates.” But other scientists came forward with their own  sightings of male doodlebugs doin’ the doodle.

And then, in the 1890s, another  scientist argued that it was natural,   and the males were choosing to mate  with each other—sparking more criticism. Eventually, the doodlebug debate faded. But then, same-sex activity  popped up again in penguins.

Bighorn sheep. Giraffes. And the same questions simmered each   time—questions that were never  really just about doodlebugs.

If same-sex activity is truly  “unnatural,” as many people claimed,   well, why is it found again and again in nature? Thanks Thought Bubble! Since the doodlebugs, scientists have  observed over one thousand different   species mating and even  parenting with the same sex.

For some species, these are one-off flings. But others show a strong  preference for same-sex partners. Like, about eight percent of male sheep  aren’t interested in sex with females,   and exclusively prefer other males.

And all of this raises interesting  questions about evolution. Like, if the evolutionary point  of sex is making more life,   why would same-sex mating be a thing at all? A common thing, even — in whales,  koalas, geese, even beetles!

And the answer is…we don’t exactly know yet. But there are a few different ideas. One idea is that organisms who’ll  mate with anyone mate more overall,   increasing their chances of having offspring.

So, maybe there's an evolutionary advantage  to just being... a little thirsty. Another idea is that same-sex mating reduces   competition and helps social  animals form close bonds. Best way to not have haters?

Make ‘em your lovers. Like, male dolphins often pair off together,   having sex with each other multiple  times a day and inviting the occasional   female to join in – I’m thinking they  deleted that scene from Shark Tale. And then there are bonobos,  humanity's closest relatives.

Among them, same-sex mating keeps  the vibe of the whole group peaceful. In fact, female bonobos have more sex  with each other than they do with males. For some species, same-sex sexuality  might actually help survival.

And for the rest, it’s  definitely not hurting them. In the end, sexuality is just one aspect of  the incredible diversity built into life;   diversity that underpins  healthy, robust populations. After all, like we said way back in episode one,   life is the most fascinating thing  to ever happen on this planet.

Biology sometimes gives us broad  categories that are useful overall,   but when it’s something as complex and gigantic  as life it’s not going to be that simple. And when you start talking about human  life and our societies and cultures,   well, then it’s going to get really fun. We are like light through a  prism – each of us a fractal,   beautiful in our own right, but bound  together in life’s kaleidoscope of color.

We know so much about ourselves,  and yet we are all still learning,   but here’s one thing we like to say  around here: imagine others complexly. And that means looking deep, thinking hard,   being kind, and recognizing the reality of  the beautiful variety of human experience. Next time, we’ll take a deep dive into  animal behavior, I’ll see you then!

Peace! This series was produced in  collaboration with HHMI BioInteractive. If you’re an educator, visit  BioInteractive.org/Crashcourse for   classroom resources and professional development  related to the topics covered in this course.

Thanks for watching this episode of Crash  Course Biology, which was filmed at our   studio in Indianapolis, Indiana, and was  made with the help of all these nice people. If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.