[MUSIC] [MUSIC] Hey smart people, Joe here.

When I went to the Amazon rainforest last year, I knew I would see things I'd never seen before, and boy did I.

[MUSIC] But it's also one of the few places on Earth where you can see something now one has seen before.

And that's exactly what happened to us in Peru.

We were walking through the rainforest shooting a completely different video when Pedro, our guide, suddenly points down at the ground, and this is what we saw.

"It's a caterpillar" "But it's moving with it's own house" "This caterpillar is taking it's house with it."

"It won't leave this leaf behind it" Wait a second.

Did you see how that thing was moving?

Most caterpillars move and climb using their legs, either the six front "true legs" or a number of "prolegs" that eventually disappear when it matures into an adult.

This caterpillar does something totally different.

"It's climbing with its mouth!"

"That's the first time I've seen this, actually."

That's really weird.

This caterpillar has to climb with its mouth because it's dragging around a rolled up leaf on its butt.

And when I went to pick one up, I immediately figured out why.

"See?

and it like tucks itself back in, it's like a little shield."

"You're like a little knight.

It's like your suit of armor.

I'll call you Sir Caterpillar, Knight of the Rainforest" That's some awesome camouflage.

I mean, imagine you're a predator.

The rainforest floor is covered in dead leaves.

If we hadn't seen a few of these moving, I bet we would have walked right by them.

Now, our guide Pedro has been in the rainforest most of his life and had never seen one of these, but we decided to take a couple back to camp just to see what our rainforest biologist friends Aaron and Daniel thought.

"Alright, look in the end there, what do you see?"

"Whoa.

What is in there?

Is that?"

"It's a caterpillar!"

"No" "It's like a little camper house, like a trailer."

"Wooooooa!"

"We were calling it like the Winnebago Caterpillar."

"It's like a hermit crab" "Yeah, yeah" We found a caterpillar that thinks it's a hermit crab.

Except a hermit crab moves into a home that another animal already built.

This caterpillar builds its own by cutting and pasting leaves into a caterpillar-sized tube.

"This is a cool strategy of wrapping a leaf around yourself and just crawling around like a hermit crab."

"I've never seen anything like this before."

For the past few months, I've been emailing the world's most knowledgeable butterfly and moth experts, and I don't think anyone's seen anything like this before.

The leaf camouflage and way of moving is a totally new behavior, and this is potentially a totally new species.

We could call it the Winnebago moth, the Pedropillar, Sir Leafsalot, I don't know, Mon Mothma!

...but we're getting ahead of ourselves.

And that's what I want to talk about today.

How do we figure out if a species is new?

Answering this is a lot harder than you think, and it touches at the very core of how we organize nature.

Even the question of what a species even is.

The study of how we classify organisms is called taxonomy.

Like any language, biology needs a standard set of names to call living things, so we know what you call a chinchilla is the same thing I call a chinchilla.

That category is put into another, and that into another, and so on.

This builds a system that lets us organize living things based on how closely related they are, and to study how they've evolved.

But it still doesn't tell us what a species is.

The obvious way to tell two organisms apart is how they differ in size, anatomy, and other physical characteristics.

Lions and cheetahs?

Obviously different species.

But what about a these two?

On looks alone, we'd say the Eastern and Western Meadowlark are the same species.

But scientists noticed Eastern birds bred with Eastern birds, and Western birds with Western birds, so they were classified as two different species.

This is probably the definition you learned in school: A species is "a group of organisms that can breed with one another and produce fertile offspring."

This "biological species concept" makes sense based on our daily experience.

Dogs make dogs, cats make cats, and so on.

Based on that definition, Eastern and Western Meadowlarks are two different species.

Except: later, when scientists looked closer at some places where their populations overlap, it turns out meadowlarks actually can interbreed, and make at least some fertile offspring, just most of the time they just choose not to.

Why?

Because they sing different songs, a mate recognition species concept.

So are they the same species?

Or aren't they?

This is a picture of the moth our caterpillar turned into.

I sent this picture to an entomologist who's probably the world expert in one special family of moths.

He immediately knew that ours was part of that family.

He was even able to narrow it down to this genus thanks to the shape and pattern of the moth.

Unfortunately, tropical moth research doesn't attract as much money as curing male pattern baldness, so that's about all he could tell me.

These moths just haven't really been studied enough to tell us more.

You probably don't know this, but moths have pretty special genitalia.

Male and female insect parts have very specific shapes, like a lock and key.

[SEXY MUSIC] We're filing away some very personal pictures so one day, when scientists have looked at a whole bunch of other moth genitals, we can see how ours matches up.

It's kind of like the world's weirdest fingerprint.

Who gets that job?

The best tool taxonomists have today is genetics.

We can determine how closely two organisms are related by comparing how the letters of their DNA match up, the genotypic species concept.

But to do this you've gotta find some piece of DNA shared by organisms as distant as elephants and oysters.

Luckily, all animals have mitochondria THE POWERHOUSE OF THE CELL!

Which have their own DNA, and that DNA holds a gene that all animals share.

If we look at my mitochondria...

THE POWERHOUSE OF THE CELL!

...are we gonna do that every time?

The sequence of my COI gene will be closer to, say, a chimpanzee's, than a starfish's.

But there's no rule about how big a DNA difference suddenly means you're looking at a new species, so even modern genetics can't totally solve our problem.

We're analyzing this moth's DNA now, and we'll definitely share what we find.

Or what we don't find.

All in all, there's more than a dozen ways of defining a species, by some counts as many as 26.

This is "the species problem."

Here's what you should know: Saying something is its own species isn't like giving it a unique ID code, like something you buy at the store.

It's a hypothesis, a prediction about how we think that organism is related to others.

And then we can test that prediction.

We'll probably never settle on one definition of a species, because different situations make us ask different questions.

And that's the funny thing about species: Sometimes, the closer we look, the harder they are to see.

Basically we don't know if our hermit crab caterpillar is new, which is a bummer since "Cicinnus curiosus" sure has a nice ring to it.

We don't even know if someone else will even ever find them again, because shockingly, it's hard to find one small caterpillar in the big 'ol Amazon rainforest, especially when what makes it special is that it doesn't want be found.

Stay curious.

[MUSIC]