Doing it.

The Horizontal Hula. Knocking Boots.

Or as award-winning writer and  my fellow Crash Course host John   Green so eloquently put it, skoodilypooping. Today we’re talking about the birds and the bees. That’s right, it’s the reproduction episode.

We’re going to explore the ways  animals make more of themselves,   which runs the gamut from sea anemones, who can reproduce by dividing themselves in half,   to banana slugs, which use a penis they extend out of a pore on their heads to do the slug shimmy. And then of course there’s my personal favorite,   the praying mantis, which sometimes  eats its mates after doing the deed. Which gives a whole new meaning  to looking like a snack.

Now, I know talking about  sex can feel embarrassing,   especially if you happen to be sitting in  a classroom full of your friends right now,   all trying not to giggle every time I say penis. But the ability to reproduce is one of,   if not the most important aspects of  life cuz no organisms live forever. Studying reproduction can give us insights into things like how populations grow over time.

And the better we understand “the deed” the better we understand life itself. Hi! I’m Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. I wonder….has anyone studied  how theme music reproduces? [THEME MUSIC] OK, so there are two main  methods animals use to reproduce. Most vertebrates, or animals with  backbones, get the job done in pairs   by bringing together a sperm and an egg cell to  make brand-new, genetically unique offspring.

This is called sexual reproduction. And then there’s asexual reproduction,  where animals make genetic copies of   their bodies all on their own without the use  of reproductive cells like sperm and eggs. They’re usually invertebrates, like  insects or mollusks, but not always.

And since this is life we’re talking about,  of course, there are those outliers that   blend the two styles and just totally throw off the curve – we’ll get to them in a bit. But for now, let’s peep the wild  world of asexual reproduction. One common way asexual reproduction  occurs is through budding.

Basically, that means an animal  grows a baby version of itself   right out of its body — like if a human  started growing out of your shoulder. Don’t think too hard about that. The underwater invertebrate Hydra vulgaris,  for example, reproduces using this method.

A bud forms on its surface, sort of  like the world’s weirdest pimple,   except it keeps growing until it  becomes a fully-formed adult hydra. At which point, it breaks off and floats away. Probably to look for the person that  named it Hydra vulgaris, poor things.

Budding works for hydras,  and a lot of other animals,   but there's more than one  way to reproduce asexually. Like jellyfish, which can use  fission to split in half and   create two animals of approximately the same size. And then there are animals like sea stars,   which can use a method called  fragmentation and regeneration.

Let’s learn more about it,  over in the Thought Bubble… Welcome to Pearls Before Brine Oyster Farm. Don’t look now but those sea stars that followed   you in are treating the place  like an all-you-can-eat buffet. The owners are less than thrilled about  this, so every time they come across one,   they pull it out, cut it in half, and  toss the remains back into the water.

Harsh, I know, but it’s been known to happen. And, you’ll see who gets the last laugh. Because a couple of weeks  later, there are more sea stars!

Like, twice as many. And they’re all just chowing  down on the precious oysters. So, uh, what the heck happened here?

Well, like all organisms, sea stars have  special cells called stem cells that are   like a blank canvas; they can transform into  any cell type — muscle, nerve, you name it. Normally, once these cells have  specialized into one thing,   they can’t go back – but sea stars are different. See, the cells at the edges of their broken bodies  can de-specialize, becoming those blank-canvas   stem cells again, capable of growing into whatever  they need to be—including a whole new organism.

So, every time a split sea star gets tossed back,   their cells de-specialize and regrow  the missing side of their bodies. And suddenly, this is less of an oyster farm,  more five-star sea-side sea star resort. Thanks, Thought Bubble!

Now, just like any biological process,  asexual reproduction has its pros and cons. For the animals that use it,  it can be a straightforward way to keep their population going. For one, you don’t have to deal with the  sometimes-messy business of finding a mate.

Those stubby little star arms  weren’t built to swipe right. Another perk? If you’re crushing the whole survival thing  and are well-suited to your environment,   your offspring will be, too!

Then again, that’s a double-edged sword. Because if the environment changes, you might find   neither you nor your offspring are  well-equipped to survive anymore. In that case, your offspring might’ve been better  off if its genes hadn’t matched yours exactly.

Of course, that’s not to say that asexual  reproduction means no evolution ever. Mutations and variations in genetic  code still happen in these organisms,   and natural selection still occurs  – that “identicalness” just means   that substantive change in a  population takes a lot longer. So, yeah, the main perk of   sexual reproduction—biologically  speaking—is genetic diversity.

Since you need a sperm cell from one  parent and an egg cell from the other,   offspring get half their DNA from each parent. Which is why kids won’t ever  be identical to their parents. And that can mean survival.

Like, if the environment warms and/or a  new disease rolls in, genetic diversity   means some of the organisms could have  traits that make them likelier to survive. Many animals rely on sexual reproduction,   either instead of or in addition  to, asexual reproduction. In fact, some 99.9 percent of  multicellular organisms reproduce sexually.

If you’ve ever been given “The Talk,” you might think you know  what sexual reproduction looks like. But trust me, it’s not always as  straightforward as it might seem. The heart of sexual reproduction is fertilization:   that moment when a sperm and egg  cell successfully come together.

But that doesn’t always require sex. Lots of animals use external  fertilization to reproduce. This is where egg and sperm  cells are released outside   the body and mix together, almost always in water.

It’s the preferred method of lots of aquatic   or semi-aquatic species like fish,  frogs, corals, and even some insects. The biggest challenge here is timing. For fertilization to happen,  the sperm and egg cells need to   run into each other at the same time  and have their own little meet-cute.

And since most animals don’t have group  chats to coordinate their schedules,   they use environmental cues instead. Like, corals on the same reef will release  their egg and sperm cells under a full moon. So… consider that your warning  if you go snorkeling at night.

You might be…interrupting something. Meanwhile, other animals are  more about physical proximity. For instance, male frogs will jump on a female’s  back so that when the female releases its eggs,   the male can fertilize them right away,  without interference from any other frogs.

But for animals who aren’t near a body  of water, there’s internal fertilization. This is when sperm are deposited  in or near the reproductive tract   of the egg-producing organism, and the  egg gets fertilized inside the body. And it usually requires copulation — what we call   sexual intercourse — where  sex organs come together.

But not always! Some amphibians and some insects like  katydids leave little sperm packets   instead, for a female to scoop up  later and stick inside its body. Like Doordash… but for making babies…soooooo  not like Doordash at all actually.

No matter what kind of fertilization an  animal uses, though, the sperm and egg   come together, and a new organism starts to form. And one of the first things that  develops in this new creature is its sex. In other words, how it might eventually reproduce.

Broadly speaking, sexes are the general  reproductive or mating types within a species. For instance, most male animals produce  sperm, and most female animals produce eggs. But male and female are far from the only  sexes available to the animal kingdom.

Instead, they represent points on a  spectrum of potential sex outcomes. There’s a lot of diversity here. For example, in humans, we’re learning that  the distinct, binary categories for sex that   we’ve used for ages are more  complicated than we thought.

We’ll get more into that—and gender—in Episode 48. Not only do sexes look different in different   animals, but how an organism’s  sex is determined also varies. For instance, in some animals, sex is  mainly decided by certain chromosomes.

But in other species, sex is decided by what  the environment is like during development! Like, if you scooped up a bunch of sea  turtle eggs and kept some of them toasty   warm and the others cooler, the toasty-warm  group would all be females, and the others male. The process that follows is  called sexual differentiation,   when an animal develops the body parts  and features it needs for reproduction.

It involves lots of genes and hormones,   and even within one sex of one single  species, there can be a lot of variation. Like, some species are hermaphroditic  — a word for non-human animals that   either have both sperm- and egg-making  systems or that can switch between them. For instance, earthworms are  simultaneous hermaphrodites:   which lets them produce both  eggs and sperm at the same time.

Meanwhile, some other animals  are sequential hermaphrodites,   meaning they might change from  making sperm to making eggs   at some point — maybe because of age,  social status, or some other condition. Clownfish are a great example of this:   When the local female dies, the  dominant male fish changes sex. Which means the beginning of Finding Nemo  really should have gone differently for Marlin.

Or should I say, Mom-lin. And while some animals take on multiple sexes,  others take on multiple reproduction styles. Meaning they can reproduce both sexually  and asexually depending on the conditions.

Like, insects called aphids reproduce asexually  when there are lots of resources, and then switch   to sexual reproduction seasonally, playing  both sides so they can always come out on top. This fluid club includes  lots of different creatures,   from Komodo dragons to California condors. But don’t expect a condor to bud  or split in half any time soon.

They do asexual reproduction a bit differently,   using a very sci-fi-sounding  process called parthenogenesis. It means eggs develop into  embryos without sperm cells. And it comes in handy when there  aren’t males around to mate with.

Some Whiptail lizards, for example, are  uni-sexual, a.k.a., they’re all female. So, they only reproduce using parthenogenesis. Now, even though organisms made this way  come from the DNA of only one parent,   strictly speaking, they’re not clones.

The process can happen  differently in different species,   but often the chromosomes in the parent  egg cells duplicate themselves, and then   development just sort of happens  as if fertilization had occurred. But to repeat: it hasn’t — there’s no sperm! It just goes to show how incredibly diverse,  innovative, and strange animal reproduction is.

Whether an organism is getting  it on with a mate or flying solo,   there are tons of different  ways for them to reproduce. And there are always exceptions and ways to  break the mold, each with its own pros and cons. But no matter how different   an animal’s reproductive strategies  might seem to us, if it works, it works.

Next time, we’ll unpack human  sex, gender, and sexuality. 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.

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