Original subtitles

A Perfect Planet.

There is a nuclear reactor 93 million miles away.

We call it the sun.

Its rays, travelling through space,

reach the Earth in just eight minutes...

and give power to life

throughout the planet.

Its daily and yearly rhythms

shape the existence of every creature on Earth...

and has done so for over three billion years.

In the beginning...

there was light.

Surprisingly, perhaps, almost every part of the Earth's surface

receives the same quantity of sunlight each year...

4,38O hours of it.

But it's delivered in varying amounts at different times of the year,

depending on where you are.

It's only here in the tropics, close to the equator,

that there are 12 hours of guaranteed daily sunlight the year round.

And here, that has created a great richness of life -

the tropical forest.

Each leaf is a natural solar panel which collects the sun's energy

and fuels a multitude of plants.

And in Southeast Asia, gibbons live here.

A five-year-old female...

and her lifelong partner are on a mission.

Every morning, they head out across their territory to look for ripe fruit.

There is one kind of tree here that they need more than any other.

The fig tree.

These are the only plants in the entire forest that produce fruit

all year round.

Their flowers grow in dense clusters

inside undeveloped figs,

but these have not yet been pollinated

and are not yet edible.

The gibbons will have to wait.

To produce ripe fruit, the fig still needs two things...

a large dose of sunlight

and some of the smallest creatures in the forest.

Tiny fig wasps.

They're only two millimetres long.

The figs have a unique partnership with these insects.

A female wasp has the space of just a single day

when the fig will allow her to burrow into the undeveloped fruit.

It's such a tight squeeze that her wings are ripped off.

But she's not going to use them again.

Once inside, she makes her way to the tiny, tightly packed internal flowers.

And here within, she lays hundreds of eggs.

She then carefully unpacks fig pollen from her abdomen...

and with it fertilises the tiny flowers.

When she's finished laying,

she dies...

inside the unripe fig

alongside her eggs.

The sunlight now slowly ripens the figs

and helps the young wasps inside to develop.

After just five weeks, the eggs start to hatch.

The first to emerge are the golden, wingless males.

Things now become stranger than fiction.

These males start to mate with their unhatched sisters.

To reach them, they use a telescopic penis

that's twice their body length.

While their now-pregnant sisters are beginning to hatch,

the males themselves burrow their way to the outside world.

The brothers' final act is a chivalrous one.

They sacrifice themselves to marauding ants.

They're serving as decoys

so their sisters can take to the skies.

The young females, loaded with pollen, live for just 48 hours,

in which time they must find another fig tree at just the right stage

into which they themselves can burrow, as their mothers did.

With the sun's helping hand, the figs are now ready to eat.

The gibbons know all the fig trees in their territory.

And each day they travel up to two miles

to find one with fruit that's ready to eat.

Ripe figs at last...

a major part of their diet and available the year round.

Thousands of animals rely on fig trees.

This abundance is only possible in a world without seasons

and constant daily sunlight.

But this 12-hour cycle of light and darkness

only happens close to the equator.

Elsewhere, the amount of sunlight fluctuates across the year.

That is because the Earth does not spin in an upright way.

Its axis is on a tilt.

23.5 degrees.

So as the Earth makes its annual orbit around the sun,

keep your eye on the North Pole.

For the first half of the year, it's angled away from the sun,

bringing darkness and winter to the northern hemisphere.

For the second part of the year,

the North Pole swings towards the sun, bringing summer.

The Earth's tilt gives us the seasons.

And life has adapted to deal with even the most extreme changes in light.

The High Arctic.

After six months of being angled towards the sun,

it begins six months of freezing darkness.

This is Ellesmere Island,

the closest land to the North Pole.

It's winter,

and for the last four long months, the only light here has come from the moon.

Few creatures can survive this sustained darkness.

They have become specialists in living for months without sunlight.

Muskox.

Remnants of the last ice age.

Having grazed all summer, they've built up fat reserves

that will enable them to survive the brutal cold of winter.

And this makes them a prime target for other sub-zero specialists.

Arctic wolves.

The moon provides just enough light for them to see.

This alpha female leads the hunt.

Target in sight.

If her pack is to survive, they need to make a large kill

at least once every three weeks.

The alpha female signals the start of the attack.

They run at the muskox and the herd panics.

The muskox close ranks.

The wolves look for weakness.

Charging risks isolation from the crowd.

And that is just what the wolf pack wants.

The wolves can sense victory.

But the herd comes to the rescue.

And once again, they close ranks.

Most winter hunts end in failure for the wolves.

As the long polar nights drag on,

the hungry wolves turn to their only other source of food...

arctic hares.

They've gathered together in their hundreds for safety.

Trying to catch one hare among hundreds is harder than you might think.

Even if they do catch one, it's not much of a meal.

Sometimes you just have to admit defeat.

But life is about to get better for the wolves.

For the first time in six months, the sun rises above the horizon.

Its appearance marks the beginning of half a year of continuous light.

The sun's warmth will allow the muskox to give birth,

and that will provide easier hunting for the wolves.

It's early spring,

and much of North America is still locked in ice.

As our tilted planet orbits the sun,

the northern hemisphere receives increasing sunlight...

and wakens an animal with an almost supernatural ability.

It is frozen solid,

like a block of ice,

and has been all winter.

An ordinary-looking frog...

but an extraordinary one.

A wood frog.

Its heart has stopped beating completely.

But as the sun's power increases,

almost miraculously it begins to change.

Its frozen blood is melting

and begins to flow through its veins...

as its heart begins to beat again.

The wood frog is cryogenic.

In just 12 hours,

it thaws and comes back to life, as if by magic.

This defrosting ability

means it's ready for the moment when, at last,

spring arrives.

All across the northern hemisphere,

the sun's warmth is bringing dramatic change.

As the sun rises higher and spring takes hold,

the warming air reaches the ground and the rocks beneath.

Here, thousands of animals begin to stir.

Garter snakes,

one of Canada's most northerly reptiles.

After six months of hibernation,

the males are the first to emerge.

They haven't eaten for months.

Even so, it's not food that is on their minds, it's something else...

females.

But they stand no chance of doing anything about that

until they've charged their batteries.

Snakes are cold-blooded

and they need to absorb the sun's heat before they're able to move quickly.

After a few hours, the males are raring to go.

20,000 snakes,

the largest emergence of reptiles

anywhere on Earth.

Now a few females appear.

They're much bigger than the males...

and warming will take them longer.

She releases a scent, a pheromone,

that attracts the males.

They outnumber her a hundred to one.

To speed her own return to activity, she will absorb heat from them.

Intoxicated by her scent, the males compete for her,

wrapping themselves around her.

She now has hundreds of males on top of her,

making mating near impossible.

But she has a way of weeding out the men from the boys.

She'll make a daring ascent of the nearby cliff.

She barges her way through the crowd to get to the rock face.

As she climbs, only the strongest, fittest males can keep up with her.

She's made it to the top.

Only a few of her suitors have managed to rise to the occasion,

and she may reward them all.

This entire mating jamboree only lasts for one short week

in the year.

And when it's over,

they all move off into the forest.

There, they disperse to lead solitary lives

searching for food

until winter drives them back

to the shelter of the rocks below ground.

It'sjune, halfway through the Earth's annual journey around the sun,

and summer has arrived in the northern hemisphere.

Karak Lake in Canada, north of the Arctic Circle.

The summer will be brief,

but there is sunlight for 24 hours every day...

so the next few months are vital for every living thing here.

And this arctic fox knows it.

Winter has been hard for her,

and now with a new family to support,

she must make the most of the summer's abundance.

Her three-week-old pups have just emerged from the den.

Each of these little bundles of fur

needs nearly 300 calories of food every day...

a tall order for Mum.

But a solution for her problem is just arriving.

Snow geese.

They have flown over a thousand miles from the United States,

timing their arrival to coincide with the end of the snow.

More than half a million will spend the summer here,

nesting and feeding on the grass.

There's fierce competition between couples for the safest nesting sites.

The losers will have to nest on the outskirts of the colony,

the first part to be raided by the foxes.

Mother is after eggs.

But getting them is not going to be easy.

The geese will risk their lives to protect their eggs.

If they lose them, they can't produce another clutch this season.

The fox wins.

She gives the first eggs to her young pups...

and then heads back for more.

Some of the eggs she stashes away.

They will be food to help her through the coming winter.

In just three weeks,

she will steal over 800 eggs from the geese.

But eating a big egg is not easy

if one is rather small.

It's a technique the pups haven't quite yet cracked.

Mum shows how it's done.

Goslings are now hatching all across the colony.

And they will need all the food they can get to build up their fat

before their journey south.

Now that the chicks have hatched, they can all find safety on the water.

In these brief sunlit months, there's time for the cubs to play.

But there is no playtime for Mum.

But the Arctic is warming,

and the timing of the seasons

and the migration of the geese

is becoming unpredictable.

Although the sun stays above the horizon continuously

throughout the Arctic summer,

its rays are comparatively feeble.

Closer to the equator, in our planet's deserts,

cloud cover is rare.

The sun here is not a friend,

but an enemy.

Africa.

The Sahara.

Few creatures can live here.

Get caught out in the open

at the wrong time of day

and it's game over.

By ten in the morning,

the temperature on the surface of the sand

is pushing 6O degrees Celsius.

Even the toughest will soon have to take cover.

But one creature is waiting underground for the sun to get even hotter.

Temperatures are now so high

that everything else has to shelter.

The desert now belongs to the Earth's greatest solar specialist...

the Saharan silver ant.

At midday,

when the sun is at its fiercest,

they emerge

to look for creatures killed by the scorching heat.

They're one of the fastest insects on Earth, and they need to be,

with just five minutes to find food before the heat kills them.

Getting lost would mean certain death,

so every few seconds they spin round,

taking a bearing from the sun.

When others are frying,

these ants have solar tech

to stop them from overheating...

special glassy hairs and shining bodies

that reflect the sun's lethal rays.

It buys them precious time.

To get their prize back as quickly as possible

to their underground den demands teamwork.

But shelter is a long way away.

Any technique will do.

The push-and-slide.

The spin.

Even dune-surfing.

The nest is still 7O metres away.

Some are already collapsing in the heat.

Reinforcements are needed urgently.

Now the heat whips up violent gusts of hot desert air.

For an ant, these are hurricanes.

They must get their prize underground as soon as possible,

and blown sand has blocked the entrance to their den.

Any longer out here and they will be toast.

One final effort.

The last of the team race home.

Shade at last.

Much of the land on our planet could become as scorched and lifeless as this

if we allow our activities to continue to change the atmosphere.

The Sahara alone has expanded by an area twice the size of France

in just the last hundred years...

part of a global desert invasion that threatens a third of all land.

The sun can certainly be a lethal threat,

but it could also be our saviour.

The solar energy that strikes our planet in just an hour

contains more power than that used by all of humanity

in an entire year.

By October, as the Earth completes its annual journey around the sun,

day lengths in the northern hemisphere are shortening once again.

The sun's power is diminishing.

Trees are beginning to shut down their solar panels.

The green chlorophyll with which they collected the sun's energy

is broken down chemically and reabsorbed.

And the forests turn from emerald to gold.

It's autumn.

Once again, the living world away from the tropics is transformed.

Plants stop growing,

and many animals begin to prepare for tough times ahead.

The forests of central China...

home to the golden snub-nosed monkey.

They will not survive the fast-approaching winter

unless they stock up on calorie-rich food.

Top of their autumn menu - pine cones.

They grew during the summer sunlight

and are rich in fats.

Few are now left,

and time to gather them is running out.

Dominant males patrol the troop,

making sure they get the pick of the crop.

As the number of pine cones dwindles,

tensions between rival families increase.

Mere threats between two males may not be enough to settle disputes.

Conflict is in the air.

There's going to be a fight.

Vital food is at stake,

so every pine cone is worth fighting for.

The victor and his family enjoy the spoils of war.

The losers get no more than a few dead leaves

and will have fewer energy reserves for the coming winter.

The sun's power dims.

Temperatures drop.

Everything slows down.

Ahead lies months of crippling cold.

Most of the life on Earth away from the tropics

has managed to adapt to the changing seasons,

but there are some creatures

that have found a way to avoid the cold of winter altogether.

Snares Island, New Zealand,

deep in the southern hemisphere.

Here, sooty shearwaters have been nesting throughout the long summer

and gorging themselves on fish.

As a result, they've piled on quite a few pounds.

But the sun is now fading,

and the southern winter is approaching.

No time to hang around.

When you're a touch tubby,

launching into the air...

is a leap of faith.

They won't see land again for four weeks.

Setting off across the Pacific,

they're starting on one of the longest journeys made by any living thing.

Their aim - to avoid the consequences

of the Earth's tilt

and follow the sun's warmth as it dwindles in the south

and increases in the north.

For those who survive the 10,000-mile marathon,

there will be a great prize.

The reward for travelling from one end of the planet to the other

is long summer days the year round.

This is their destination...

Alaska's Aleutian Islands.

While the south of our planet is slipping into autumn...

here, summer is just beginning.

Below the waves,

vast clouds of plankton are blooming...

triggered by the sunlight.

These tiny marine organisms

are food for crustaceans - krill.

This is the sun-filled bounty

for which the shearwaters have crossed the planet.

By cheating the tilt,

they're able to enjoy the riches of summer the year round.

But shearwaters are not the only ones to visit the warming seas.

Humpback whales.

They've been breeding in the tropics...

and haven't fed for six months.

Up to 6,000 whales

are now heading for these islands -

the only place where they can find food

in the quantities they need.

The shearwaters begin to dive into a huge shoal of krill.

The humpbacks attack from below.

Each year, a million shearwaters join the whales

in these dramatic feeding frenzies.

It's one of the greatest gatherings of life on Earth.

And it only happens because some whales

and some birds

have found a way to live in a summer that never ends.