Hold on.

Oh, my gosh.

The trees are, like, made of butterflies.

[MUSIC PLAYING] I knew this would be amazing, but I didn't know it would be this amazing.

JOE: That was me a couple of months ago on a mountain in Central Mexico.

This is incredible.

Hey, smart people, Joe here.

Nearly every monarch butterfly in North America, tens of millions of them, flies to these mountain forests each winter.

I'm not going to lie, one of the most amazing sights I've ever witnessed.

And it was not easy to get there.

I had to fly on a plane, drive in a car, ride a horse, and hike to get up there.

It's like 10,000 feet above sea level.

But the only things those monarch butterflies needed to get there was their wings, and some of the coolest frickin' navigation biology I've ever heard of-- millions of tiny orange compasses with wings.

Yeah, this is totally going to change how you look at butterflies.

[MUSIC PLAYING] The monarch butterfly migration is one of a kind.

It's totally unmatched in the insect world.

It's more like the migration of some birds.

I mean, we're talking distances of up to 4,500 kilometers each way, north and south.

But there's one big thing that makes this migration different from what birds and other animals do.

It actually takes multiple generations of butterflies to do it.

And as you're about to learn, that's what makes it so amazing.

[MUSIC PLAYING] Our journey starts in spring, as monarchs make their way north.

It's a leap-frog kind of journey.

Each generation flies, mates, lays their eggs, and ultimately dies, as it passes the baton to the next generation.

A typical adult monarch only lives for two to six weeks.

So it takes four, sometimes five generations to make the trip north.

The fading light of summer marks the end of their northern migration.

Shorter days and cooler temperatures prompt female butterflies to lay a special generation of eggs.

When they hatch, the caterpillars that emerge are very different from their parents.

They'll grow up to be part of a super generation.

These super generation butterflies will fly all the way to Mexico in a single generation.

Now, most things about a monarch's life-- metamorphosis, migrating, mating-- are controlled by hormones, chemicals in their bodies that signal different activities.

Super generation monarchs make less of one special hormone.

And this essentially prevents them from aging.

They live about eight times longer than other monarchs.

I mean, think about that-- same species, totally different lifespan, like one of us living past 400.

This super generation also develops differently as adults.

They're bigger.

They can fly farther.

And they can't reproduce.

OK, but here's the thing.

How does a bug with a brain the size of a sesame seed know it's supposed to go to a tree in Mexico thousands of kilometers away?

And how does it find its way to a place it's never been?

Luckily my friend Jason was in Mexico with me.

My name is Jason Goldman.

And he is literally the perfect person to explain it.

I'm a science journalist on a wildlife and conservation beat.

But before I started doing journalism, I was a scientist.

And I studied animal cognition.

So it's really remarkable that these insects, with like a million neurons in a brain the size of a sesame seed, can get from the northernmost parts of North America, 2,000, 3,000 miles to these forests in Mexico, relatively accurately and relatively effectively.

It absolutely blows my mind.

Almost every biological organism has some kind of an internal clock.

In humans, our roughly 24-hour cycle tells us when to wake up, when to go to sleep, when to eat.

Monarch butterflies actually have two internal clocks.

One clock inside their brains is called the circannual clock.

It keeps track of annual cycles.

It's what tells them it's time to pack their bags and head south for the winter.

The other clock is the key to their navigation.

There's very few animals that we know of that really have a true mental map.

Monarch butterflies, they don't have a route to follow, but they do have a heading.

What we do know is that they have a compass in their minds.

It's a solar compass that tells time.

Let me explain.

Monarch's main navigation trick is reading the horizontal position of the sun.

But the sun moves from east to south to west throughout the day.

So to keep pointing yourself in one direction relative to the sun, you also have to know what time it is.

Only, butterflies don't wear wristwatches, so how do they do it?

Well, remember earlier how Jason said that monarchs have a second internal clock?

Monarch butterflies actually have two internal clocks.

Well, I always thought that butterfly antenna were just like long skinny noses.

But these antenna do way more than just sniff.

My buddy Phil Torres was down there with me.

He's a butterfly expert.

And he blew my mind when he told me this.

PHIL TORRES: So that antenna is telling them what time of day it is.

And then they use the information from their eyes and the location of the sun to then orient to the right direction.

Are you kidding me?

That's incredible.

That's how humans navigated at sea for like 1,000 years, using advanced tools and mathematics.

And these butterflies are doing it with pinhead brains.

OK, keep it together, Hanson.

Let's break down how this sun compass works.

[MUSIC PLAYING] Say your internal clock tells you it's mid-morning.

If you're supposed to be heading south-southwest, then the sun should be on your left.

If your antenna clock tells you that it's late afternoon, the sun should be on your right.

Special cells in your compound eyes can even find the sun on cloudy days using polarized light.

It's a pretty genius navigation system.

Scientists have actually tested this by putting monarch butterflies in flight simulators, and then watching how they orient themselves.

These monarchs spend all winter here in Mexico, basically hibernating, living off stored energy.

But as spring arrives in these mountain forests, their internal seasonal clock tells them it's time for the super generation to leave.

And something changes in their bodies.

That hormone that they didn't have that kept them from aging, they start making it.

And they become reproductively active-- very reproductively active.

[MUSIC PLAYING] They begin their journey north, tracking the sun again.

Their inner compass somehow flips direction.

Sadly, in a few short weeks, every one of these super generation butterflies will be dead, but not before the females lay the eggs that will become the next generation to carry on this great migration.

What makes this so incredible to me is that the butterflies that journey south are reading a map passed down from great-great-grandparents who died half a year before they were born.

It seems like it's almost magic, but the truth is that these skills are written into their genetics.

And since the genetics get passed down generation to generation, of course, these behaviors get passed down with it.

Even if each individual generation doesn't need those skills, eventually, one of them will take advantage of it.

JOE: There's evidence that monarch butterflies have been making this journey for millions of years.

It's an instinct, a behavior that's built into their body.

Their sun compass, their antenna clock, are written in their genes, an unbroken chain of DNA stretching back millions of generations.

But there are still more mysteries to solve.

It's not enough for the butterflies to know where they're going.

They also need to know where they are.

And we don't know what makes them stop here in this Mexican forest.

And if we want to solve that mystery, we need to make sure the butterflies keep coming back.

We're on the lookout for a very special plant.

Where is it?

This is it.

This is it right here.

This is milkweed, a native milkweed.

This is the plant that the butterflies need to lay their eggs on.

This is what the caterpillars will grow up eating to make the next generation.

And if we're lucky-- yes, a monarch has been here and laid an egg.

Preserving the monarch migration is about more than just preserving that forest in Mexico.

It's about preserving plants like this in places like this, flowers that give them nectar to fuel that migration.

It's about protecting more than just a behavior.

It's about protecting a habitat that stretches across an entire continent.

And that's something that we can all play a part in.

It gives you something to think about and maybe something to chew on.

Stay curious.

[MUSIC PLAYING]