Unraveling the Great Butterfly Migration Mystery
Oh my gosh, the trees are like made of butterflies.
I knew this would be amazing, but I didn't know it would be this amazing.
That was me, a couple of months ago, on a mountain in central Mexico.
Hey smart people, Joe here.
Nearly every monarch butterfly in North America–tens of millions of them–flies here each winter.
Not gonna 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 all those monarch butterflies needed to get there was their wings…
Oh, and also some of the coolest frickin' navigation biology I've ever heard of.
Millions of tiny orange compasses with wings.
Yeah, this is gonna totally change how you look at butterflies.
The monarch butterfly migration is one of a kind.
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 km, 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.
Our journey starts in spring, as monarchs make their way north.
It's a leap-frog 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 4, sometimes 5 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 8 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…
which is good, because they have a heckuva journey ahead of them.
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's Jason Goldman,
And he is literally the perfect person to explain it.
I'm a science journalist on the wildlife and conservation beat
but before I was a journalist, I was a scientist, and I studied animal cognition.
So it's really remarkable that these insects, with like a million neurons, brain the size of a sesame seed,
can get from the northernmost parts of North America, 2000-3000 miles to these forests in Mexico,
relatively accurately and relatively effectively. Absolutely blows my mind.
almost every biological organism has some kind of 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, its 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.
So there's very few animals that we know 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 they have a compass in their minds.
It's a solar compass, that tells time!
Let me explain…
Monarchs' 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?
Remember earlier, how Jason said monarchs have a second internal clock?
Monarch butterflies actually have two internal clocks.
Well I always thought butterfly antennae were just, like long, skinny noses.
But these antennae 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:
So that antenna is telling them what time of day it is.
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, a thousand years, using advanced tools
and mathematics.
And these butterflies are doing it with pinhead brains!
Ok, keep it together Hanson.
Let's break this down how this sun compass works.
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 antennae clock tells you it's late afternoon, the sun should be on your right.
Special cells in their compound eyes can even find the sun on cloudy days using polarized
light.
Pretty genius navigation system.
Scientists have actually tested this by putting monarch butterflies in flight simulators,
and 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 they didn't have, that kept them from aging?
They start making it, and they become reproductively active.
VERY reproductively active.
And they begin their journey north, tracking the sun again.
Their inner sun 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 it 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 generation doesn't need those skills eventually one of them will take advantage
of it.
There's evidence 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 so the caterpillars will
grow up, eating, or 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 and 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.
Give you something to think about maybe something to chew on.
Stay curious.
That was fun, wasn't it.
Studying a migration this massive is hard.
We just figured it out a few years ago, I mean it took like half the 20th century for
scientists to just figure out where they go each year.
Follow me over to the Atlas Obscura YouTube channel, where we tell the story of how scientists
found this place in Mexico, and what it means to the people who live there.
Atlas Obscura is the definitive guide to the world's hidden wonders, and this definitely
qualifies.
And if you're wondering what it was like to be there, well grab a headset or a Google
cardboard or your phone, because we made a 180˚ VR video inside the butterfly forest.
So beautiful.
You're gonna love the rest of this story.
