01x04 - Cold

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The Arctic.

It is intensely cold

yet, nonetheless, mammals live here.

This icy territory is ruled by polar bears.

And for this young, inquisitive bear,

these frozen seas are both its home

and its playground.

This might appear to be a game with a walrus...

..but, if so, it's the walrus who's winning.

The young bear still has much to learn,

as it starts to find its way around

in this freezing world.

From frozen seas to snowcapped mountains,

specialised and hardy mammals

have found many ways of flourishing

in the coldest places on Earth.

Many polar bears spend most of their time on the coast,

hunting seals...

..but this young female

is heading inland.

Guided by her keen sense of smell,

she's making for the mountains.

It's 20 degrees below zero...

..but she's well adapted to these temperatures.

She has two layers of fur

and, beneath them, insulating fat,

so she loses little body heat.

After walking for hours...

..she at last finds what she has come for.

Svalbard reindeer.

The smallest reindeer in the world.

Inland, they rarely encounter polar bears...

..but now, they are about to do so.

Polar bears are not built for high-speed pursuits.

She has been outrun.

It's unusual for a polar bear

to chase prey for so long...

..but even so, she doesn't give up.

Her progress is hardly graceful.

Soon, she's back on their trail...

..and she catches them up.

This time, she chases the reindeer

towards deeper snow.

Perhaps this will slow them down.

The group splits.

For a moment, she is unsure which way to go.

Both prey and predator are now exhausted.

But she's not giving up.

The two reindeer separate.

She follows one downhill...

..back into the deep snow.

Here, her great strength and longer legs

help her to plough through it.

This astonishing behaviour is rarely seen.

However, in this part of the Arctic,

several bears have been seen hunting in this way.

But it's too early to know whether, as the world warms,

a diet of reindeer alone

could be enough to sustain them.

Mammals like the polar bear

have had millennia to find ways to survive in the cold.

Since the extinction of the dinosaur,

there have been over 50 glacial periods.

During the coldest, ice sheets over half a mile thick

lay where Edinburgh and New York now stand.

Today, Alaska's northern wildernesses

are still much as they were during those colder times.

One particularly hardy and resourceful mammal

has survived here for tens of thousands of years.

A wolverine.

Like all mammals, he's warm-blooded,

and to maintain his body temperature,

he must eat a lot.

But it's not easy to find food

in this vast, empty wilderness.

Wolverines have to travel

up to 50 miles

a day in search of it.

But his feet, like snowshoes, are broad

and help him cross snowfields without sinking in.

And he has an extremely sensitive nose.

It's so sensitive,

he can locate a carcass

even though it's frozen solid.

A caribou that has been k*lled by the cold.

The wolverine's powerful jaws

enable him to strip off

the frozen flesh.

He can even crunch

through solid bone.

But he doesn't finish his meal.

Some of it, he carries away.

Surprisingly, perhaps, this male drops his food

in the middle of nowhere and then leaves.

And this is why.

A female wolverine.

The food is a gift for his mate.

It will stave off her hunger for several days.

Wolverines are very secretive animals,

and very few people have ever seen this caring behaviour.

Several days later, it becomes clear how important

these deliveries of food have been.

The female emerges from her den,

beneath the snow.

And...

..another head appears.

Their three-month-old kit.

It was born and has been nurtured

two metres below the surface.

This may well be the first time

it has seen the world above the snow.

The kit is strong enough to accompany its mother,

and they look for a meal together.



take little or no part in raising their young...

..but fatherly devotion helps wolverines

to raise a family during this,

the coldest time of the year.

Not all mammals have to travel huge distances

to find their food, if they know where to look.

Winter in Canada's northerly forests.

It's minus 25 degrees.

Most rivers are frozen over...

..but this one is kept ice-free

by constant springs

bubbling up from its bed...

..and that allows chum salmon

to spawn late in the season.

And they attract hungry brown bears.

While most other bears

are already hibernating,

these few that know about

this special place continue

to fatten up deep into winter.

And these fish, each containing

over 4,000 calories,

are well worth staying awake for.

But not all of the bears

are equally skilled at fishing.

Some do little more than put

their head under the water

and hope for the best.

But their underwater vision

is much like our own,

and it's clearly a struggle.

An elevated view might help...

..but it didn't.

Upstream, in a secluded part of the river,

a mother is fishing with her two young cubs.

They both will rely on her milk during hibernation,

so she puts on as much weight

as she can.

Up to 2kg a day.

In the months to come, the fat she gains now

will keep her alive during her long winter sleep.

By passing on her knowledge of this special place...

..and teaching her young cubs

the skills they need to survive,

their mother has given them a good start in this icy world.

Full of fish, the family

is now ready to hibernate...

..and so avoid the hardships of winter.

Old disused mines are excellent winter shelters for mammals.

By sleeping down here,

they can avoid the worst of the winter.

And some use their time here in unexpected ways.

A colony of both little

and big brown bats.

The smallest weighs just 6g,

and what little body fat they have

will last them six months.

To do so, they change their bodily functions

in a remarkable way.

Their heartrate drops to just

ten beats a minute

and, at times, they take only

two breaths an hour.

Thermal images reveal

that their body temperatures in midwinter

are only just above freezing.

They are now using only 2% of the energy they would do

were they awake.

The whole colony has put life on hold.

Except for some solitary males.

This one is shivering...

..not from the cold...

..but to wake up.

Vibrating his muscles rewarms his blood...

..and his heart is now b*ating ten times faster than it was.

This awakening consumes as much fat

as he would need for the next two months

were he to continue to hibernate.

So this is a risky thing to do.

But there's a reason.

Before hibernation, some males failed to mate.

Now they have a chance to try again.

Every step costs precious body heat.

He's searching for a small cluster of females.

He must be careful not to wake them

or they too would expend the precious calories

they will need if they are to survive the winter.

He mates with one...

..and then quietly leaves

and goes back to sleep.

Five months later, the females wake.

When they mated in the autumn,

the sperm they acquired was not used, but stored.

Only now will it fertilise an egg.

So a male who mates during hibernation

still has the chance of being a father.

But bringing babies into a cold world of snow and ice

is not easy.

Each February in the Arctic,

thousands of female harp seals

haul out onto unstable pack ice

and give birth.

A newborn pup doesn't have enough fat

to survive in the cold water...

..so mothers have to feed them

as quickly and intensively as they can.

And harp seals have

a special way of doing that.

The females produce

supercharged milk.

It contains ten times more fat than human milk.

This enables each mother to rear her young faster than

almost any other mammal - just 12 days.

For the young that appeared

early in the season,

the value of doing so is already visible.

This one is fattening up nicely...

..but it's still very demanding...

..and its mother sometimes

slips away.

As the pups accumulate fat...

..they need to be fed less often...

..and this allows a mother

to spend longer under the ice,

in a more tranquil world.

As the ice melts, it drifts -

sometimes, over six miles in a day.

So, after a lengthy swim,

it's not always easy for a female to locate her pup.

She flares her nostrils,

in an attempt to pick up its scent.

No, not that one.

Definitely not.

A sniff...

..and a kiss confirms this pup is hers.

But, sadly, not all mothers get it right.

Some never find their pups again.

Alone and hungry, an abandoned pup

understandably seeks comfort.

But this nearby mother

won't share her milk.

Her priority is her own pup.

The nursery continues to drift...

..but most mothers do eventually find their own pups again.

During their 12 short days together,

each mother tries to provide her pup

with nearly 15kg of fat,

which is enough to keep it warm,

no matter how cold the water.

So the mammalian ability to produce rich milk

enables harp seals to breed on the sea ice.

High above the sea,

there are other icy worlds.

These are the Kluane mountains of North America...

..and they carry the world's largest ice field,

other than those around the Poles.

Summer here is shorter than almost anywhere else on Earth.

But it has been colonised by diminutive relatives

of rabbits - pikas.

This young male has been awake all winter,

living on the food reserves

he built up last year.

For him, summer

can't come soon enough.

As the sun's warmth returns,

for just a few weeks,

this harsh world is transformed

into a rich high-altitude meadow.

It's what all the pikas here have been waiting for,

for almost ten months.

There's no time to waste.

Ready, steady...

Now they must collect as much food

as quickly as they possibly can.

But some of them seem in less of a hurry.

Clearly biding their time.

The others

are gathering leaves nonstop.

Except this one.

They build haystacks, and that takes a lot of work.

Each mouthful has to be

carefully placed to allow

the sunlight to dry it.

They also add a few leaves from other plants

that contain chemicals which will help

preserve the hay during the winter.

As many as 11,000 return journeys may be needed...

..to build a reasonably large haystack.

And then it has to be guarded

against opportunistic neighbours.

The weather is showing signs of changing.

It's time to make a move.

This pika specialises

in stealing from its neighbours.

Got away with it, again.

Pika are one of the few mammals

that prepare for winter in this way.

With their food successfully stored,

they're ready for the winter lockdown.

The Qinghai-Tibet Plateau

is around 4,000 metres above sea level.

It's the largest and highest plateau in the world.

The air here is so thin that it hardly retains any heat.

Here, it's almost always very cold.

A snow leopard.

A female. Her dappled coat

making her hardly noticeable against the broken rocks.

The plateau's top predator.

Her ancestors have lived and hunted here

for over two million years.

But recently, people have also come to live up here.

A Tibetan homestead.

The settlers have brought domesticated yak...

..and protect them with fences and dogs.

Tibetan mastiffs.

They were bred originally

to protect the livestock kept by

Buddhist monks, and now

they are guarding the herders' yak.

A yak is tempting prey

for a hungry snow leopard in winter.

But this female has been seen.

The mastiffs outweigh

even the biggest snow leopard.

She's also outnumbered,

so she retreats.

She'll have to change her plan.

This herd is unguarded...

..but they would not be easy prey.

An adult yak weighs over half a ton

and has horns up to a metre long.

Even a calf is about twice the weight of a snow leopard.

The leopard edges closer.

As the calf disappears behind the slope, she follows.

Her impact takes them both careering downwards.

But she holds on.

It was a risk worth taking.

The people here may lose ten yaks a year

to predators like snow leopards.

But the local community aims to live alongside snow leopards

and compensate herders who lose yak...

..so they are not hunted here.

Thanks to these decisions,

this once-persecuted mammal

is now thriving here.

Today, for many mammals,

the cold is not their biggest challenge.

A far greater thr*at

comes from changes in the climate.

Here, along the coast of Canada's Hudson Bay,

there is open water where there should be ice.

An Arctic fox.

At the beginning of each winter,

foxes here form unlikely alliances

with polar bears.

It's late in the year

and both species are waiting for the sea to freeze.

Once it does, polar bears

will once again be able to hunt seals out on the ice...

..and Arctic foxes will scavenge

from their leftovers.

But right now, both are hungry,

and both must wait.

She needs to be careful not to push her luck.

Polar bears are not sociable creatures.

Ten days pass, but there is still no sea ice in sight.

For the foxes, the situation is becoming desperate.

A few edible morsels in the seaweed

are all that she can find.

The wait, for one Arctic fox,

has been too long.

The starving survivors

cannot ignore this opportunity.

Desperation has led to cannibalism.

Something being seen more and more in the Arctic.

Others begin to gather.

As each tries to see off the competition...

..the sound of the quarrel

attracts even more starving foxes.

None are willing to surrender this meal...

..but each is unable to keep the others at bay.

The commotion hasn't gone unnoticed.

A cross fox.

This stronger, more aggressive cousin from the south

has been moving north

as the winters have become steadily milder.

It is now the top dog here.

With the arrival of this newcomer,

life for the long-time residents

will only get harder.

At last, the sea ice has formed.

And for one Arctic fox, with luck,

a meal is not far away.

Mammals living in the coldest parts of the world

have found remarkable ways to succeed

where others could not do so.

But as global temperatures continue to rise...

..only time will tell

if they will be able to survive

in this warming world.

To capture one of the cold's

most elusive mammals, the crew travelled

to the tundra of northern Alaska.

Home to the legendary wolverine.

The ambition was not just to film wolverines

in this vast landscape, but to record

the emergence of their kits for the first time.

The crew joined forces with a cr*ck team

of wolverine scientists and experts.

Everything's working and, um...

..yeah, we just need some wolverines now.

Wolverine biologist Tom Glass

has an early report of activity.

There are some pretty old tracks and then there are

some fresher ones. It looks like a promising one.

With a wolverine in the area,

producer Will Lawson leads the team to investigate.

As a wolverine can cover

up to 600 square miles,

they will need to roam far and wide.

To build a picture of the wolverines' movements,

they set cameras along travel corridors

and potential hot spots.

But as the days pass,

it becomes clear it's going to be a challenge.

It's, like, corrupted or something.

The lens has really frosted up.

Disappointing.

After weeks of frustrations,

finally, the team gets a reassuring sign.

What have we got?

Wolverine came and checked our camera here.

Yes!

This is encouraging.

I'm not sure what's happened here,

but I don't think it's worked.

This is the heartbreak of camera traps.

How annoying!

At least we know a wolverine has definitely been here.

Though the camera traps haven't recorded the behaviour,

they have confirmed it's a wolverine hot spot.

So specialist pilot Mark Keech

takes Will up for an aerial view...

..with the hope of locating a potential den.

With thousands of hours and many years

spent surveying wildlife from the air,

Mark knows exactly what to look for.

That's probably a wolverine track there our shadow is just about

to go across, right now.

There, you can see that is

definitely a wolverine track there.

This could be actually a den here.

That would be great. Yeah.

I think there's a very good chance,

cos there is multiple tracks.

So would you say that this site looks like

a promising option? Absolutely. Great.

With this exciting discovery,

the crew decides to set up a remote camp near the den,



Here's where the den is. Oh, I see. This is the route

we took in, so we can connect to that route really easily.

All right, I think we've got a plan.

The film crew heads out over the vast tundra,

deep into the wolverines' territory,

hauling everything they need

to be self-sufficient in the field.

We are exactly



It looks like it's in that drainage we can see ahead,

but on this side of it.

Pretty cool to actually be here, standing so close.

The camp established,

the crew devises a strategy

to watch the potential den

around the clock.

With Will and Matt working the perimeter,

Neil sets up his filming hide,

his home for the foreseeable future.

It's hard to tell from the ground,

one, exactly where the den is,

and, two, how active it's been.

All Neil can do now is wait.

My sandwiches are frozen solid.

Is there a wolverine in the hole?

We've done 200 hours in the hide

without seeing one yet.

That's more of the question.

Oh! There's one, there.

I'm not joking, it's an actual...

Come on, come on, come on.

Beautiful to see.

Finally get to see it.

Thank goodness.

With a female at the den finally confirmed,

Matt and Will head in to resupply Neil.

On the way, they make an exciting discovery.

So we've got these wolverine tracks

coming this way down the valley,

and there's more than one set of tracks.

So they come together right there

and follow the same path.

They look to be about the same age,

which is pretty, pretty cool.

With signs of two adult wolverines in the area,

they meet Neil and find he's already filmed them both.

So, this is what we have.

The blonde one, and then that's the other one.

Oh, wow! There's quite a marked difference in...

Oh, wow! Yeah.

That's the male. Really?

Yeah. Guys!

There's that wolverine.

I mean, I could try and get back in the hide. Shall I?

I think you should. OK, boys.

Neil's managed to make it back to his filming hide.

I'm just hoping that the wolverine didn't see him.

Not only did he make it back,

but Neil manages to film the larger, darker male

delivering an edible gift.

The male brought some food in.

He's a bit bigger, because he actually got his bum stuck.

His wee legs were sticking out of the top of the hole for a minute.

With the mating pair using the den,

the team has high hopes

that the female has kits.

All the signs are pointing

towards her having young.

But as days pass, there's no sign of kits,

and the team's time with the wolverines has run out.

Feeling quite, yeah, quite disappointed.

I don't want to be going, cos it feels so close.

On their final day, under the cover of freezing fog,

Will and Neil take the opportunity

to leave camera traps behind.

It will be their last roll of the dice to get the vital sh*ts

and finish the wolverines' story.

OK.

That's done.

Days after the crew's departure,

the cameras finally start recording...

..and, against the odds,

record the wolverines' kit emerging

for the very first time.

Mammals like the wolverine have colonised

the coldest parts of the planet.

We can only marvel at their ability to survive

in these unforgiving landscapes.

Next time, we journey to the hottest places on Earth

to discover how mammals overcome

challenges and adversity...

..and use heat in the most

surprising and unexpected ways.