Great Migrations

They are born to move.

To move...or die.

Forging ahead, against terrifying odds.

Navigating the first of millions of miles.

Spanning continents on fledgling wings.

Clattering ever onward.

Right now our planet is on the march...

on the wing...

on the run.

And these are the tales of its creatures...

born into the most moving stories on Earth...

the stories of the great migrations.

Life on Earth is hard.

Perhaps nowhere harder than here...and now.

Every year, a timeless drama plays out on the Mara River.

The elders have been through this many times.

They must remember what awaits them.

Their young know only the pervasive smell of fear...

and the instinct to stay close.

But they must press on.

What on Earth could drive this headlong rush to slaughter?

Only Earth herself, with her insistent clocks.

This story begins in the Great Rift Valley--

where the continent is tearing itself apart...

and being reborn.

The plains in the shadow of the rift-- the Serengeti--

are threaded with the paths of the millions on the move...

in a constant burning search for fresh grass.

Every year, more than a million wildebeest and 200,000 zebras

must chase the seasonal rains,

in a 300-mile loop around Tanzania and Kenya.

But here in the Serengeti, they pause...

for an avalanche of young...

the most migration-ready newborns on Earth.

The wildebeest have a scant three weeks

to deliver half a million calves...

who have only minutes to get up on their feet.

Because the predators-- and the scavengers--

have the migration clock etched in their genes as well.

For them, this is a weak and moveable feast.

For the newborns, it's get going...

or get eaten.

Apprentice killers wait for their first taste of the chase.

But they're about to learn...

that hell hath no fury like a zebra protecting her foal.

Older cheetahs know the value of patience.

Soon the herd will move on.

Hungry eyes turn to one of the last-born wildebeest,

just learning to kick up its heels.

Serengeti vultures have timed their breeding cycles

so their young can share this bounty.

Five out of six wildebeest calves won't make it through their first year.

Only the fit--and the lucky-- will inherit the relay race of their lives.

lt is a race that is increasingly endangered,

like many of the world's great migrations.

Climate change disrupts weather patterns...

and exploding human populations encroach on their ancient paths.

But today they take up their relentless legacy.

To move...chasing the rains.

Rain.

Wherever it falls, life waits for it, yearns for it--

and above all, moves according to its whims.

Water from the skies is a reminder to all land-dwelling species

that their origins were in the seas.

And nowhere is this more remarkably played out

than on a tiny island in the lndian Ocean

called Christmas lsland.

Deep in the forests--

far from the shores where any sensible crustacean should be--

red crabs the size of dinner plates hear the call of the monsoon.

lt's time to return to the seas of their birth.

With the humidity soaring, their moisture-dependent lungs

can now function beyond the damp confines of the forest.

The males go first.

Like generals at the head of a red tank division,

they begin their clicking, yearly march to breed.

Millions of generations have engineered these crabs for life on land--

but their young must hatch in the ocean.

lt's a brutal, month-long migration-- up to five miles--

an ultra-marathon for a crab.

The cruel sun makes its appearance, threatening them with dehydration.

And they face a gauntlet of roads crisscrossing the island.

They stop only to nourish themselves wherever--

and however--they can.

For those who have made it this far, a more horrible fate awaits.

Yellow crazy ants.

Aliens accidentally introduced by cargo ships.

Now super-colonies of billions terrorize the migrating crabs.

The ants attack...spraying acid into their mouths and eyes.

Blinding them...

leaving them helpless.

ln the 1990s, the crazy ants slashed the crab population by a third,

from 80 million to 50 million.

Beyond the ants lies a dizzying descent.

40 feet straight down to the beach.

Now the males face the waters of their birth.

They dip into the ocean to restore their bodies with salts and moisture.

But these land crabs are air-breathers...

and the lndian Ocean plays no favorites

with her prodigal sons.

Ravenous triggerfish seize the luckless.

An unceremonious end to an epic quest.

For the surviving males, the odyssey has just begun.

The females will arrive within days--

urged on by celestial forces,

demanding they reach the males and the sea in time.

Like the crabs, creatures around the globe are moving to the music of the spheres.

They register the sun, the moon, the earth's magnetic field...

and move at their bidding.

Even the navigational feats of the most delicate of creatures

beggar the imagination...

and confound our understanding.

And in tiny slices of endangered Mexican forest--

hundreds of millions of the world's most astounding navigators are stirring.

These monarch butterflies have waited patiently through the winter,

barely moving, for the sun to tell them it's time.

They stream to water to slake a winter-long thirst.

Water that will bring spring's sexual awakening.

Males pluck females from the trees like reluctant flowers.

And they mate.

Then, the brilliant masses take to the wing,

heading to a place they've never been.

Their only map-- imprinted on their genes.

A mysterious interplay of sun on eyes and antennae--

an uncanny sense of magnetic north guides the way.

lt will take five months for three generations

to get the species as far north as Canada...

and a fourth super-generation to get them back to Mexico.

Such a multi-generational migration is incredibly rare.

lt takes the leading generation three weeks

to fly 500 miles into the southern U.S.

These intrepid elders sense that death is approaching fast.

There is urgency now to pass on the migratory baton.

The female, having carried her eggs from Mexico,

now searches for the monarch's life-giving grail:

The milkweed sprouting all around.

Her offspring will be laid on it, hatch on it, nurse on its toxic milk,

and become toxic themselves...

a defense they will flaunt with their singular beauty.

She is dying--

but with luck, her children, grandchildren and great-grandchildren

will carry on the journey as far north as Canada...

and then an astounding 2,000 miles back to Mexico.

For the delicate monarch, it may as well be a trip to the moon.

And yet somehow they do it,

their tiny bodies attuned to a seasonal sundial

and resonating to the invisible signature of the earth's core--

its magnetic field.

Around the planet, animals large and small

are responding to these forces,

as palpable to them as they are indecipherable to us.

And far out to sea, the largest brains on Earth

fix on these mysterious cues.

The male sperm whale-- a creature of superlatives.

lt is the largest toothed predator alive today--

weighing up to 50 tons.

lt dives deeper and faster than any other whale.

And it undertakes some of the greatest journeys.

ln a lifetime, this whale may log a million nautical miles.

The males often circle the Arctic to feast.

But for the species to survive, their paths must intersect

with the females' own astounding journeys--

coursing warmer waters.

Many will meet in the Azores...

nine tiny islands where clashing currents create an explosion of life.

A mother ushers her newborn calf into coastal waters...

guided by sun, sea currents

and perhaps even magnetic sensors in her brain.

She also navigates by sonar,

bouncing her clicks and creaks off the sea floor.

Some of the loudest sounds made by any creature on Earth.

Females travel in family groups, sometimes leading their young

on yearly wanderings of up to 20,000 miles.

Each day, they gather to reaffirm their bonds.

They seem to crave physical contact.

Finally, the mothers disperse-- to feed in shifts.

They leave their calves for up to 40 minutes of every hour.

They may dive up to two miles

for a daily catch of 300 pounds of octopus and squid.

This calf, too young to dive, tracks his mother closely from above,

listening intently for her signature clicks.

Chunks of octopus rise to the surface from the matriarchs hunting below,

like breadcrumbs leading back to safety.

From the depths, mother finally returns.

All of his young life, the calf has been on the move.

Now his mother and aunts seem to be waiting for something big.

Unknown to the calf,

he's about to be welcomed into a world of giants--

a world rarely seen and little understood.

Migrations are journeys of attrition--

Darwinian in the extreme.

A quarter of a million wildebeest will die in their yearly odyssey.

ln the sun burnt Serengeti,

hunger keeps the wildebeests thundering northward,

following the rain.

A collective intelligence keeps the herd on track.

Neighbor leading neighbor along ancestral paths.

Testosterone drives young males

to flaunt their strength, fleetness and stamina.

The plain is awash in hormones...