Winter Earth's Seasonal Secrets

Every year, spectacular seasons transform our planet.

They're the driving force of all life on Earth...

..bringing opportunities and huge challenges.

Spring bursts with new life.

But it's a race to grab fleeting chances.

Summer is all about glorious abundance,

but the heat can push animals to their limits.

Autumn is nature's great gold rush.

But competition is fierce.

Winter creates a frozen wonderland...

..and only the most resourceful will survive.

In every corner of the planet,

animals rise to overcome the seemingly impossible,

and use extraordinary tricks

to thrive against the odds...

..as the seasons create the greatest shows on Earth.

Winter.

The sun's power is weakening.

It's getting colder and darker.

The land is going into suspended animation.

In this magical world, animals are faced with formidable challenges.

And when temperatures plummet, the first big challenge

is keeping out the cold.

This extreme season needs inventive solutions.

In the Highlands of Scotland...

..a ptarmigan has to cope with temperatures

approaching -30 degrees centigrade,

so it grows more feathers to keep the heat in.

It even has feathers on its feet...

..and its eyelids.

In North America,

a red fox keeps warm

by wrapping himself up in his extra-thick tail.

Arctic foxes grow one of the thickest coats

in the animal kingdom.

Her tiny ears and stubby legs reduce heat loss...

..and even the pads of her feet

have a covering of fur.

Even when it drops to -70...

..she's quite comfortable.

But other animals have more inventive ways

to stave off the cold.

Finland, on the Arctic Circle.

Temperatures here can fall to -45 degrees centigrade.

The snow is so heavy, it can bend the sturdiest of trees.

But among the drifts lives one of the world's smallest predators,

with a ruthless approach to keeping warm.

A least weasel.

She's tiny, barely 20cm long.

She's grown a new white coat for winter.

It keeps her almost invisible in the snow.

Great for hiding from predators,

but it's not thick enough to protect her from the cold.

Even shivering can't generate enough heat.

But this ingenious weasel has a plan.

She starts by diving under the snow.

Snow is 95% trapped air,

so it keeps the heat in, like a giant duvet.

There's a network of tunnels...

..but they're not hers.

They belong to voles.

Voles spend the winter in their snow tunnels,

where they think they're safe.

But with her skinny, flexible body,

the weasel can easily navigate the maze.

There's nowhere for a vole to hide.

Spotted.

The vole is fast.

But the weasel is faster.

But there's more to this than just a meal.

Once the weasel has eaten the hapless vole,

she delicately plucks its hair.

Then she weaves it into her nest, building herself a furry cocoon.

Bed...and breakfast...

..in one convenient package.

Many animals must rely on their wits to survive the bitter cold.

In the mountains of central Japan, winters can be brutal...

..with some of the deepest falls of snow in the world.

But there's a surprising animal eking out a living here.

A monkey.

Aside from people,

these Japanese macaques live further north

than any other primate on Earth.

Their coats are unusually thick to protect them

from the freezing temperatures.

Even when it's -20 degrees centigrade,

a mum and her baby can cope.

But as the temperatures fall,

the monkeys need to find extra calories to stay warm.

These are some of the cleverest monkeys on the planet,

and they are so resourceful, they'll never go hungry,

no matter how cold it gets.

They can eat almost anything.

And, at this time of year, nothing goes to waste.

As a last resort, when all other food is gone,

they'll even eat the bark of trees.

This is smarter than it seems.

Just beneath the woody layer is the sap wood,

where the tree stores its precious nutrients.

It gives them just enough calories to keep them going.

But this is not their only clever trick to get them through winter.

There's warmth to be found here.

Japan is a land of volcanoes.

Below the ground,

this volcanic activity brings hot water

bubbling to the surface.

Even in the depths of winter, it's a thermal spring,

and these ingenious monkeys have learned how to make the most of it.

Snow monkeys have become

spa monkeys.

In their bath, it's a comfortable 41 degrees centigrade.

With constant warm water to bask in,

these monkeys live a life of leisure.

And the youngsters have plenty of time to play.

Thanks to their resourcefulness, these clever monkeys will survive,

no matter what winter throws at them.

Some overcome the cold with their wits,

but others survive through sheer brute force.

Bison.

They've lived just like this since the last Ice Age.

They're throwbacks from a time

when the world was much, much colder.

They can live through some of North America's most bitter winters.

Temperatures here in Yellowstone can drop below -50 centigrade.

They use their giant heads and necks like a plough to sweep the snow away

and reach the grass beneath.

And they'll just carry on grazing, regardless of the weather.

Thanks to his Ice Age ancestors,

this bison is perfectly evolved to deal with the cold.

His body is huge, weighing in at nearly a tonne,

and the bigger and rounder he is,

the better his body retains the heat...

..meaning he can maintain his core temperature with minimal effort.

And his thick, double-layered fur coat keeps the heat in.

Next to his skin, he has a dense, woolly undercoat,

with more than 2,000 hairs per square centimetre.

His hair is so insulating that even when snow falls on his back,

it doesn't melt.

Bison can keep warm down to -30 without even trying.

Only when it drops below this

will they need to use extra energy to keep warm.

These hard-core grazers stay all winter

when others are forced to leave.

So they have the meadows all to themselves.

But when it comes to battling the elements,

this is the ultimate place to do it.

Antarctica.

The coldest place on earth.

Temperatures can plummet to -94 degrees centigrade.

You have to be a true specialist to survive a winter here.

Emperor penguins.

At over a metre tall,

they're the largest penguins in the world.

They've been out at sea, fishing...

..building up an insulating layer of fat, nearly 3cm thick.

And now they're about to face the toughest winter...

..on Earth.

Because penguins take so long to raise their chicks,

they must start early.

And that means incubating their eggs through the planet's deepest winter.

When she's laid her single egg, the female must return

to the sea to feed.

So it's over to the father to incubate it...

..all through winter.

He protects it in a little pocket of skin just above his feet.

He is now responsible for keeping it safe and warm

for nearly four months...

..just as the world's coldest weather is closing in.

Antarctica is entering six months of darkness.

Winter here is merciless.

The winds can top 120mph.

These are the darkest, coldest, windiest conditions

faced by any creature on earth.

But through it all, the emperor penguin dad battles on.

He's still incubating his precious egg,

perched on his feet, just above the ice.

He and all the other fathers have found an inventive way

to make it through when times get really bad.

They come together for the world's biggest huddle.

Penguins don't normally enjoy being this close...

..but they have no choice.

By crowding together as a team, they can cut heat loss by half.

After his stint on the edge of the flock,

each penguin patiently queues for his turn on the inside.

It's recently been discovered that their huddle is so effective,

the heat inside can build up to around 40 degrees centigrade.

That's nearly 80 degrees warmer than the temperature on the outside.

So every now and then, they must break formation

to cool down.

Throughout the male penguins' four-month tour of duty,

the sun never rises once.

But just occasionally, there's a glimmer in the darkness.

Charged particles from the sun crash into the atmosphere

above the South Pole...

..and blaze across the sky.

It's the Southern Lights.

The penguins barely seem to notice,

but in a brutal season, it's a moment of magic.

Winter is full of spectacle.

When the temperature drops below -6 centigrade,

water vapour in the air turns directly to ice.

These microscopic crystals attract more and more water droplets...

..and they grow into snowflakes.

In just one second,

a million billion are made by the clouds.

In ten minutes, that's enough snow for everyone on the planet

to build their own snowman.

Each flake is tiny, but together, they can blanket the landscape...

..turning it into a wonderland.

It's a complete transformation,

and even the animals seem to enjoy the novelty.

But for some, there is a problem.

A thick blanket of snow will hide their food.

So if they're going to find anything to eat,

they have to come up with some new tactics.

In North America, a red fox relies on its ears

to listen for mice scuttling around out of sight.

In China, pandas have come up with their own special technique

for clearing snow off a bamboo branch.

Before they eat it, they spin it round their muzzles

to brush it clean.

But some have switched their diet altogether.

In the mountains of south-west China,

where winter snow lies a metre deep,

the Yunnan snub-nosed monkey eats the lichen that drapes the trees.

The lichen can survive the freezing cold

of these high mountains.

This surprisingly nutritious food

means the monkeys can stay in these mountains all year round.

But in some parts of the world,

the heavy snows of winter create an extreme challenge.

And survival takes a bit of cunning.

In Yellowstone in North America,

an otter family has woken up to discover their home river is buried.

They don't have anywhere to fish.

They're going to have to relocate for winter.

In the same neighbourhood, a coyote is having similar difficulties.

Finding a meal round here isn't going to be easy.

The otters have struck lucky.

There's an opening in an icy lake...

..and there are still fish here.

It's fine for them...

..but for the coyote, life is trickier.

He's no fisherman,

but he is a master thief.

The otters are suspicious.

The coyote would steal their fish in an instant,

so they look for somewhere to hide it.

The coyote might not be able to see the otter

but he can hear him.

He knows there's a fish here somewhere.

Stashing it under the snow won't fool a coyote.

He can smell it.

Bingo.

They're not called wily for nothing.

As snow changes their world,

animals need to use every trick in the book to find food.

But there is one animal that uses it to gain an advantage.

Wolves.

These Canadian wolves are some of the biggest in the world.

They are opportunists and they'll travel long distances

in the search for food.

Deep snow usually makes moving difficult,

but sometimes, it creates an opportunity.

This wolf pair is looking for a bison.

But they're hard to catch.

And finding them in this huge wilderness is tough.

This is where the heavy snowfall offers a clue.

The bison have left deep tracks -

tracks the wolves can follow.

These two are working alone, without the help of a pack.

And they're after very dangerous prey.

Bison have sharp horns and weigh a tonne,

ten times the weight of a wolf.

But the wolves have no choice.

A bison would keep them fed for a week,

and in winter, you have to take a chance.

Finally, they pick up fresh scent.

The herd must be close.

The hunt is on.

The wolves move up a gear.

They can maintain this speed for miles.

The bison are tiring.

The weaker ones fall back...

..and the wolf couple are getting closer.

A trip is all it takes.

Thanks to the snow...

..the wolves are now a step ahead of starvation.

And the rest of the herd live to fight another winter battle.

Dealing with the challenge of deep snow

forces animals to raise their game.

But winter has another weapon in its arsenal.

As days become shorter than nights,

the temperature on the ground begins to drop.

At zero degrees centigrade, water turns to ice.

Little by little, whole worlds transform.

At the poles, even the sea freezes.

To survive the ice, you need to be resourceful.

In North America, a freezing river is a challenging place

for a small bird.

But this tough little dipper has extra-thick waterproof feathers.

It will happily dive right under the ice

if it thinks there's something worth eating.

In a frozen Russian river...

sockeye salmon creep under a ceiling of ice to lay their eggs.

It might look crowded, but here,

they're safe from attack by birds of prey.

In northern Canada,

the winter freeze has created a surprising opportunity.

This is a land of lakes and bogs...

..but in winter, it's encased in a sheet of ice two metres thick.

And there is one animal that has managed to turn this

to its advantage.

Caribou.

Food is hard to come by,

so they must spend their winter constantly on the move.

Despite their warm coats, deep snow is a struggle for them.

Trudging through it saps their energy.

But the caribou have discovered

that snow doesn't build up on the ice

because powerful winds blow it away.

So now, the lakes and rivers become nature's very own ice roads.

It means they can travel more quickly,

saving precious energy.

On the other side of the world...

..winter ice is also bringing opportunities for some.

Siberia.

For seven months of winter, this is a bitter place.

Temperatures can drop to -67 degrees centigrade.

It's enough to push some animals to the edge.

A sika deer.

When the trees are bare, there's very little for them to eat.

The weaker ones often don't make it,

But these casualties are just what another animal is looking for.

An Amur leopard,

the world's rarest big cat.

She is a powerful predator, but in winter,

she is not above a little scavenging.

This frozen carcass will keep for a week,

so it is a real lifesaver.

But it is not just for her.

She has a cub.

Less than a year old -

and he's hungry.

By making the most of this frozen food,

she has also taught her cub a valuable lesson.

In the depths of winter,

even predators don't waste energy hunting if they don't have to.

In Antarctica, the freeze-up has come in spectacular style.

In winter the continent doubles in size.

It's now bigger than the USA.

A Weddell seal.

SEAL BARKS

At least she has a retreat from the icy world above.

The sea down here is a constant -2 degrees centigrade.

That is over 50 degrees warmer than it is on the surface.

And it has been like this for 25 million years.

Beneath the layer of ice,

this enchanted kingdom is protected from the extreme temperatures above.

It's a wonderland of the strange and beautiful.

In this safe, stable world,

life explodes.

But even here, winter cold can crash the party.

As temperatures plummet on the surface,

it triggers the growth of something sinister.

This is a brinicle -

super-cooled water sinking from above,

forming a pillar of ice.

For the creatures of the seabed,

it is a stealthy finger of death.

A river of ice imprisons everything in its path.

Even those that think they have beaten the extremes of winter

can get caught out.

As winter takes hold, there are even greater tribulations coming.

Now animals not only have to battle the conditions,

they also have to battle each other.

For some, it is over food. For others, it is over mates.

But winter raises the stakes on everything.

In Russia, black vultures - the world's largest birds of prey -

fight over scarce scraps in the snow.

With their three-metre wingspans, it's all about intimidation.

In Canada, winter is the time for young male polar bears to tussle,

sorting out their status without doing each other serious damage.

But it's in South Georgia in the South Atlantic

where the biggest winter brawls happen.

Weighing in at four tonnes,

these elephant seals are the world's largest.

It's a battle of the big boys to establish who's in charge.

But there is one place

where winter hardships force even the weakest

to step up and be bold.

Alaska.

The Chilkat River.

While all other rivers are frozen,

a freak of geography keeps one short stretch of the Chilkat ice-free.

It attracts the world's biggest gathering of bald eagles.

And they are here...

..for salmon.

Over 3,000 eagles are gathering in the trees.

They might have travelled 1,000 miles for this feast

and they are ready to fight for it.

With more eagles than fishing opportunities,

the challenge here is keeping hold of your catch.

As the salmon begin to run out...

..tempers boil over.

This youngster is right at the bottom of the pecking order.

He just can't catch a break.

Even when he manages to catch a fish,

he stands no chance of keeping it.

It only takes one adult to come close

and he loses concentration.

There goes his meal.

To survive, he is going to have to play by their rules...

..and stand up to his tormentors.

With bald eagles, attitude is everything.

Hunger drives him to do the unthinkable and fight back.

It works.

He has earned his right to fish this river.

He'll make it through this winter just fine.

On the Canadian tundra,

there is another battle looming.

And it is winter's ultimate heavyweight fight.

Female caribou are ready to mate.

It needs to be now so their young will be born in early spring.

So the massive caribou bulls must compete with each other

to father the calves.

The younger males watch from the sidelines

as the heavyweights eye each other up.

Serious fights are rare

but when they happen, they can be very dangerous.

But while this old bull has been dealing with a rival,

the young bucks have been trying their luck with his female.

He must break off his fight and chase them away.

The younger males may be lighter on their feet...

..but his bulk means he can muscle his way past.

Another rival steps in with a challenge.

But the old bull easily deals with him.

As the chase slows down, he can finally impose some order.

Now he's shown who is boss, a look is all that is needed.

Only when he is sure he has made his point can he claim his female.

The caribou may have a bruising battle for their chosen females...

..but for other animals, competing for the right to mate

is even more challenging.

In a forest in Finland, a black grouse is preparing

to put on a show.

It's the most important performance of his life.

It will determine whether or not he gets to breed.

Only one male will get the chance to mate this year

and that male will have all the females.

The others will get nothing.

With such high stakes, the competition is intense.

The females begin to arrive and the dance-off moves up a gear.

More and more males join in.

These birds can wait years for an opportunity to mate.

Some will never get the chance at all.

The females seem to have spotted a potential winner.

It is half dance...

..half handbags at dawn.

One last lunge is all he needs to win the bout.

It's all over.

The loser slips away.

The best performer is the strongest male...

..and that's just what the females are after.

His strength will be passed to their chicks,

giving them the very best start in life.

As the battles of winter are settled,

there are tantalising signs that the season is changing.

For some animals, it's their one chance to meet a partner.

A male polar bear has been wandering the Arctic ice alone

all through the dark winter.

But as the sun returns, it triggers his desire to find a mate.

It's not an easy search.

In these vast wastes,

potential dates are few and far between.

Eventually, he picks up an unmistakable scent.

A female.

She could be ten miles away...

..so he follows her tracks.

There she is.

She is half his size,

so first-date nerves are understandable.

He needs to win her over gently.

He waits for her to make the first move.

She leads him off to higher ground.

Their relationship is brief.

And when they part company...

..they will probably never meet again.

The wastes of the Arctic might seem a tricky place to find a mate,

but there are places that, at first glance,

seem even more unlikely.

This is the Gobi Desert in Mongolia.

It goes through one of

the greatest seasonal temperature swings on Earth.

In summer, it can reach 50 degrees centigrade,

and in winter, it is -40.

But these are perfect conditions for a Bactrian camel

to practise his seduction techniques.

Freezing winds have come from the north...

..and they have brought snow to the desert.

This male suddenly has a limitless water source -

he can eat the snow.

He is no longer tied to sporadic water holes.

He is free to wander miles across the desert looking for mates.

And when he finds a female,

he has an unusual way of getting her attention.

He urinates on his legs,

then, using his tail, flicks it over his back.

If he plays it right,

he might attract as many as 20 females

before winter is over.

While camels are looking for new mates in the Gobi Desert,

on the other side of the world,

older relationships are about to be rekindled.

In Antarctica, the long, cold winter is coming to an end.

The sun has finally returned.

The male penguin has survived 100 days fighting the cold

to keep his precious egg warm.

He has lost nearly half his body weight.

As he enjoys the first warming rays of sun,

he hears a familiar sound.

Figures are appearing on the horizon.

His mate has come back.

There is a great cacophony of calls

as couples find each other in the throng.

Reunited, he can finally introduce her to a new arrival.

All his effort was worth it.

With enormous commitment,

these parents have given their chicks the very best start in life.

They are ready to join the family firm

as the ultimate winter survivors.

Winter brings huge challenges to animals caught in its path.

Some have to battle freezing temperatures.

Others have to struggle to find food in the snow.

And some have to fight off fierce competition.

But through it all...

..animals find mates...

..and some even raise a family.

By being clever...

..resourceful...

..and brave...

..you can survive, and even prosper,

no matter what winter brings.