Original subtitles

[narrator] Earth is a restless place.

Its seasons driven by the orbit around our sun.

An epic journey that sets the rhythms of all life.

Humans honor this passage through ritual and celebration.

Nature marks the cycle with its own patterns.

Over the course of a year,

the planet's most spectacular events of migration,

rebirth and transformation unfold.

[squawking]

[bellowing]

Moment by moment...

..month by month...

throughout a wild year on Earth.

It's November, and the Earth has almost completed

its annual orbit around the sun.

As the planet comes full circle,

many of the life stories on earth

also reach their climax.

In the lush forests of Michoacan, Central Mexico,

one of the greatest migrations on the planet

is coming to an end.

A delicate creature has flown almost 3,000 miles

to spend the winter here, in the warmer southern lands.

By the time the monarch butterflies reach Mexico,

millions of them have converged at their ancestral home.

No one butterfly makes the entire migration.

Rather, it's an intergenerational journey

that was started by their great-great-grandparent

the previous spring.

Using just an internal biological clock,

and the position of the sun,

they've managed to make it to a place

they've never been before.

They'll overwinter here for the next four months,

huddled together to keep warm,

occasionally stretching their wings.

These last sacred groves of fir trees

provide refuge for the fragile, fluttering icon

in a rapidly changing world.

For centuries, Purepecha Indians

have recorded the arrival of the long, flowing cloud

of orange-winged butterflies

at precisely the same time each year.

The butterflies are believed to be

the souls of departed ancestors,

returning to Earth for their annual visit.

Their arrival coincides with Dia de los Muertos,

'The Day of the Dead.'

It's a festival as colorful as the monarch.

On this night, millions of Mexicans will light candles

to help guide the spirits of loved ones back home.

Many Mexicans do not believe death is an ending.

Rather, they embrace it as a part of a natural cycle.

Through its metamorphosis

from caterpillar to ethereal winged creature,

the monarch represents resurrection.

Life after death.

Across cultures,

many creatures have been regarded as spirits of the dead.

In Madagascar, there are also ghosts in the forest.

The name 'lemur' is derived from a Latin word

referring to the spirits of people left unburied

who return by night from the dead to haunt the living.

Perhaps an unfair comparison, for such an appealing creature.

November is baby boom time for lemur parents.

Babies share a tight bond with their families.

The troupe stick together, for there is safety in numbers.

[bird screeching]

In the distance, dinner calls.

[cicadas singing]

Cicadas are the world's noisiest insect.

They may be easy to hear, but they are not so easily caught.

As they take to the wing,

the insects pee honey dew in the face of their pursuer.

Craving the crunch of a cicada,

the primates are undeterred.

While the adults are engrossed with feasting,

the young one has wandered off.

It's a dangerous thing to do.

Predators not only soar. They also slither.

A boa constrictor like this

would make an easy meal of the youngster.

Alone, the baby lemur instinctually cries out.

[lemur screeching]

Luckily, Mum hears his call over the cicada cacophony.

He's safe once again.

And after a bit of comfort grooming,

the lemur troupe are back on the move.

Lemur babies have been born to benefit from the flush of food

that comes with the wet season.

Across the Mozambique Channel on the continent of Africa,

the rains are yet to reach land-locked Zambia.

It's been dry for seven straight months.

What little water remains becomes a beacon

to those searching for sustenance.

Quelea are the most numerous bird on the planet.

Dubbed 'the locust bird' a super-flock like this

can number 30 million at a time.

Constantly on the move as they migrate across the continent,

these waterholes

provide a vital place for them to quench their thirst.

But it's not without its dangers.

Crocodiles lie in ambush.

But they're not an easy catch.

The quelea may be birds, but they're no bird-brains.

They exhibit a super intelligence,

emerging from flock behavior.

No single individual is in charge.

Rather, the flock is governed by simple rules,

mimicking their neighbors' movement

and maintaining equal distance.

The flock becomes more than the sum of its parts.

They take on the characteristics of a super-organism

millions of birds acting as one.

As Zambia's dry season comes to an end,

these living clouds are replaced by darker ones on the horizon.

It's a sign for them to move on

and return to the start of their migration,

before the wet season truly begins.

In a matter of days,

rainfall completely alters the landscape.

Each valley becomes a lush green paradise.

The Emerald season has arrived.

The new season brings with it new enterprise.

Above the safety of a river,

master architects carry out some impressive aerial engineering.

Male weaver birds

rely on the abundance of fresh, soft grass to build new nests.

The tree is filled with males busy building,

hoping their bachelor pad will be the one

that attracts a female's attention.

Nest-weaving is a skill that takes time to master.

Each builder does a bit better than he did the year before.

Yet some are clearly novices.

If a neighbor's house is looking a little too good,

it may attract the wrong kind of attention.

Weaver birds are a jealous bunch.

And are not above a little sabotage.

Once the males have put the finishing touches to their pad,

they are ready to advertise.

Shaking feathers and showing off their plumage.

It's a flashy invitation for the females.

But when it comes to real estate, it's a buyer's market.

She won't be rushed.

Once she's decided,

she'll take care of the interior design...

..and renovate,

immediately changing all the soft furnishings.

This weaver's house has just become a home.

As November showers

bring domestic bliss to Africa's weaver birds

in Thailand, months of rain are finally coming to an end.

The wet season has brought with it

a bounty of rice, and rivers.

The twelfth full moon of the year signals Loi Krathong.

The Festival of Light gives thanks to the water goddess

for her rain and harvest.

Prayers are set afloat down the river.

While paper lanterns carry wishes towards the heavens.

It's a practice that has taken place in Thailand

for thousands of years.

As November's full moon starts to wane,

it sends a signal to colonies of creatures

far away in tropical Australia.

It will initiate the most climactic annual breeding event

on the Great Barrier Reef.

The world's longest and largest coral reef system,

the Great Barrier Reef is so vast, it can be seen from space.

Fringing the shore like lacework,

generations of coral have built it, layer upon layer

for almost 10,000 years.

Around 600 coral species exist on the Great Barrier Reef

of all shapes and sizes.

Their colonies create multi-story citadels

for millions of marine creatures.

Unlike the citizens they host, corals cannot move.

But like them, they need to reproduce.

So what does coral do when it's time for sex?

Magic.

Four nights after November's full moon,

the waning light spills blue into the inky black.

This color is one cue the corals have been waiting for.

The slackening current is another.

But most importantly,

summer's arrival warms the water,

ripening bundles of sperm and eggs.

For coral, that's the ultimate turn-on.

The sexual tension of billions of organisms suddenly erupts.

Across hundreds of miles, corals synchronize

their release in this one moment.

It's the largest reproductive event on the planet.

The sheer volume overwhelms predators

and dazzles onlookers.

Other aquatic beings seize the moment

and join the midnight orgy.

Like confetti, the egg and sperm bundles scatter.

With luck, those of the same species collide

in the liquid dark and fertilize.

By dawn the next day, tiny coral embryos

can be many miles from their birth site,

seeking their own spot on the seafloor to settle.

If successful, a new coral colony begins.

One minuscule bundle at a time,

this annual event has built the reef

into the largest living structure on Earth.

By late November, the Top End of Australia

is at the tail of the long dry season.

What life there is has retreated to the rivers and billabongs...

..or is hiding in the shadows.

For these little red flying foxes,

hot temperatures are a recipe for dehydration and heat stress.

Flapping their wings like leathery fans gives some relief.

But flying foxes have another trick for cooling off.

Skimming the surface of the water,

they soak their bellies.

A quick dip lets them lick thirst-quenching water,

and sodden fur provides cool comfort.

But drinking has never been so dangerous.

Freshwater crocodiles

are sensitive to the tiniest of vibrations.

They lay hidden just below the surface, ready to strike.

Some bats are fortunate...

..others, not so.

But for the colony and its tens of thousands of members,

their luck is about to change.

Water will soon come to them.

[thunder crackling]

As the Australian continent heats,

sea breezes join moisture-laden air

from the Indian Ocean to the shore.

Mountainous clouds billow, creating the perfect conditions

for one of nature's greatest shows.

Huge electrical storms spanning thousands of miles

signal the arrival of the monsoon to Australia.

Each storm cell

unleashes a force equivalent to an atomic bomb.

Other forces arrive unbridled.

Tropical cyclones now rampage along the coast.

No longer fed by the heat of the sea,

they release their energy over land.

As global temperatures rise,

cyclones like these are likely to become even more ferocious.

On Christmas Island, North of Australia in the Indian Ocean,

the monsoon has triggered the movement

of one of its most famous inhabitants.

Red crabs have evolved to live in the inland forest,

safe from predators and the harsh sun.

Yet this female faces a problem.

Once a year, she must travel to the ocean to reproduce.

She's usually confined to the center of the island...

..but now the monsoon provides a solution.

Moist air enables her to breathe in the open

and protects her from drying out on her quest towards the sea.

Soon, others join her on the march.

Hundreds become thousands.

Thousands become millions.

Robber crabs lie in wait.

By their sheer size,

they make an easy meal of their red intruders.

To make it past this giant, she'll have to tiptoe.

Within sight of shore, the males stop to dig courtship burrows.

Mating is a quick transaction,

with the male passing his sperm to her undercarriage.

Having mated,

he now leaves the shoreline and returns to the forest.

The female spends the next two weeks in the burrow,

brooding her eggs, which can number 100,000.

As the moon reaches its last quarter,

she joins millions of other fertilized females

at the water's edge.

She enters the waves

and performs a moon dance to release her eggs

with the receding tide.

Billions of crab larvae are now swept out to sea.

The circle of life now complete,

she'll face the same journey back to the forest,

until next year's migration.

The intimate relationship between oceans and land

is more than tidal.

Along the Arabian coast,

December's shorter days means falling temperatures.

As cool night air over the Arabian Sea

meets the heat and humidity on land,

fogs form, shrouding the city of Dubai.

December mornings see the fog build,

sometimes rising as far as 60 stories in the air.

It's a sign that winter days have finally arrived.

Half the world away, on the Californian coast

winter's mist signals it's time for the arrival of a giant.

Elephant seals have swum thousands of miles

to haul out for December's breeding season.

A titanic male rules this stretch of sand

and all the females on it.

Blubbery brute force controls the harem.

His overwhelming sex drive

means he'll copulate up to ten times a day,

pinning each female beneath his 2-ton bulk.

But like all tyrants, his days are numbered.

Another lusty male is ready to challenge the beach master.

There's little skill in their bloody battle.

Weight and aggression wins.

Even as the challenger retreats to the ocean,

there is no guarantee of escape.

The beach master must maintain total authority.

Due to the supreme dominance of so few,

as many as 80 percent of males will never get to mate.

But each year,

new challengers will attempt to topple the hierarchy,

as the annual ritual of sex and violence

plays out on these beaches.

The changes that nature's cycles bring are not always brutal.

More often, they're beautiful.

In the higher latitudes of the globe,

the plummeting temperatures trigger a silent transformation.

As December deepens, the cold creeps over Europe.

Vegetation stiffens as plants grow coats of icicles.

Hoar frost signals winter is coming.

Tiny ice crystals in the air amass to become snowflakes.

Soon, they become heavy enough

to fall to the ground in a silent flutter.

These silver forests

are the territory of the secretive Eurasian lynx.

In winter, he is in his element.

His large paws are like snowshoes

enabling him to move easily through the deep powder

without a sound.

The name of his game is stealth.

Hares turn white in winter,

changing the color of their coat

to match the snow around them.

But camouflage can't fool a keen eye.

The hare, small and nimble, is too fast.

The lynx will seldom succeed at a long chase.

Only ambush ensures a kill.

Which means the lynx must practise patience,

throughout the barren winter.

Unlike solitary predators

haunting the forests of Northern Europe,

others band together

in the Rocky Mountains of North America.

In December,

Yellowstone National Park shimmers in the winter sun.

The beauty of this land belies the harshness of the conditions.

For creatures here, winter is to be endured,

if it is to be survived.

Except for one animal.

[wolves howling]

The gray wolf.

The park is home to packs of wolves,

reintroduced here 25 years ago

after being eradicated by hunters.

Once again, they are the top predator.

And this is their territory.

Wolves thrive in winter,

adapted for speed and agility in the snow.

Their dense, double fur coats

insulate against sub-zero temperatures.

[wolves howling]

The wolves can smell elk up to 2 miles away

before they can see them.

Winter's larder is scant for the elk.

They are weak. Vulnerable.

The pack picks up the pace.

Their strategy is simple.

Separate and slaughter.

Unlike the soft footfall

of the wolf's wide round paws on deep snow,

the elk's hoofs break through the crust and slow them down.

But the elk has found an advantage too.

It seeks safety in a river

too cold and too deep for the wolves.

And its antlers are now at the perfect height

to impale an approaching threat.

[wolves whimper]

A stag drowned in the river

suggests this tactic doesn't always succeed.

[howling]

But today, the pack leader surrenders and retreats.

Wolves need to make a kill every few days.

The clock is ticking.

Downwind, female elk have unwittingly come too close.

Although they have snow on their side

the pack still work together to create chaos.

The kill is shared among the pack.

Together, these apex predators reign over Yellowstone...

[wolves howling]

..keeping the entire park in balance.

By the 21st of December,

the North Pole is at its maximum tilt away from the sun,

creating the longest night of the year

in the Northern Hemisphere.

The Pagan festival known as Yule marks the winter solstice.

Germanic people have observed Yuletide

since the Middle Ages,

before it transformed into Christmas

as it's celebrated today.

The final dark days of the year

are illuminated by the cheer of Christmas decorations.

Further north, in the skies above the Arctic Circle,

the Aurora Borealis pulsates.

A solar flare has ejected

a billion tonnes of radioactive particles towards Earth,

hitting the planet's magnetic field.

The inky night makes it easier to see

Earth's invisible shield

as it's briefly illuminated by an electric sea,

dancing across the Arctic night.

This is our planet protecting all life on her surface.

Without it, our world, our home...

...would be just one more barren rock in space.

Instead, it is a gem,

an oasis of life...

..filled with infinite wonders...

..whose rhythms and movements are driven by the seasons,

as each year,

our planet completes its orbit around the sun.