Frozen Planet II

In the far north of our planet

lie vast frozen lands...

...of snow-covered forests...

...and open tundra.

During the months to come,

they will be transformed

by the greatest seasonal changes on Earth.

But now, in midwinter,

conditions are so hostile that life seems all but impossible.

But...

...not so.

In this world of extremes, animals must go

to extraordinary lengths to survive.

February, Northern Canada.

The harsh winter

has brought together an extended family of wolves.

But this is no ordinary pack.

There are 25 of them.

A super-pack,

one of the largest ever recorded.

They're ravenously hungry.

They may have walked over 150 miles

since they last had a meal.

But now, at last, the lead male and female

have picked up a trail.

It's been left by the only substantial prey there is here

at this time of year...

...bison.

They're formidable animals,

not easy to attack.

The bulls are ten times the size of a wolf.

And they have horns

with which they could rip any attacker to pieces.

Gathered together and on guard,

they appear invulnerable.

The wolf pack's only chance...

...is to try and break up the group

and isolate one of them.

To do that,

they will have to work as a team

and try to start a stampede.

Even in thick snow, bison can run at up to 35mph.

The lead female wolf leads the chase.

A kick from a bison's hind leg could shatter her jaws.

But the pack must feed, and they take turns to attack.

The bison head for cover.

The deep snow slows down both attacked...

...and attacker.

The herd splits into two.

But the wolves are far from giving up.

Hours pass.

The thicker vegetation has turned

a straightforward chase...

...into a deadly form...

...of hide...

...and seek.

Discovered.

The lead female manages to chase the smaller group

into the open.

But here the bison are able to close ranks once again.

The leading wolves wait

for the rest of the pack to catch up.

The super-pack is once again at full strength.

Suddenly, two of the bison panic.

It's just what the wolves have been waiting for.

One of the bison is tiring.

The lead female gets a grip on its flanks.

The male joins her.

Together, they might be able to bring it down.

The rest of the wolves catch up and help.

After eight long, hard hours, the hunt is coming to an end.

Victory is sounded to summon the rest of the pack.

A single wolf wouldn't stand a chance

of making such a Kill by itself.

The only way they have of surviving the winter

is as a super-pack.

These snow-covered forests encircle the Arctic

from Scandinavia, through Russia,

to Alaska and the rest of North America.

But close to the North Pole,

the winters are too long and too cold

for even the hardiest of trees.

This is the tundra, and it stretches

all the way to the shores of the Arctic Ocean.

Here, conditions are even more extreme than in the forests.

During the two months of winter darkness,

temperatures may fall to minus 50 degrees Celsius.

The ground deep beneath the snow

has been frozen solid since the last ice age.

There is no shelter of any kind.

Very few animals can survive here.

But this is one which does.

An arctic fox.

It has the warmest fur of any mammal.

And it can survive blizzards that may rage for days on end.

It must nonetheless eventually eat something.

But how is that something to be found

when you're all by yourself in this desolation?

Arctic foxes are wanderers.

And they can travel over 2,000 miles,

from Norway to Canada,

in a constant search for food.

There might be something here.

A patch of grass, kept clear of the snow

by the near-constant wind.

The slightest hint of scent

might indicate the presence of prey.

Less than a metre below the surface

may lie a hidden network of tunnels,

shown here using a special filming burrow.

Tiny arctic lemmings.

With a carpet of edible moss beneath

and a thick blanket of snow above...

...the lemmings spend their entire winter

hidden below the surface.

And here they breed.

For a hungry fox,

a group of lemmings could be a life-saving discovery...

...if it can catch one.

Lemming burrows may extend for up to 15 metres.

Pinpointing the position of one of the inhabitants...

...Is not easy.

An acute sense of hearing is essential.

And the arctic fox

has a special trick.

It doesn't always work first time.

And it does require dedication.

With less than 25 lemmings per square mile of tundra...

...you have to persevere.

And at last...

...one plump lemming.

The foxes' tenacity and unique hunting skills

enable them to find prey which few others could catch.

On the tundra's southern fringes,

winter is significantly less severe.

The slight increase in annual sunlight

has enabled a stunted forest to develop.

It's the northern edge of the world's largest woodland...

...the boreal forest.

750 billion trees.

A quarter of all the trees on Earth.

And it encircles the globe.

These forests store twice as much carbon

as a tropical forest,

and they play a crucial part

in regulating the climate of the entire planet.

One of the most remote tracts

stands in the far east of Siberia.

It's cut off from the rest of the world by towering peaks...

...and precipitous valleys.

And it is the last stronghold

of the world's rarest big cat.

The Amur leopard.

Hunting has brought it close to extinction.

Only 120 are left in the wild.

And this is one of the very few ever to be caught on film.

Each must patrol a territory of over 100 square miles

to find enough to eat.

In the cold of March,

prey is at its most scarce.

But leopards are clever opportunists

and watch for clues.

Crows.

They, too, are looking for food.

And their calls tell him that they've found something.

But he is not the only predator here.

On one of the trees, the marks of huge claws,

ten centimetres long.

They've been made by a hunter twice his size...

...a Siberian tiger.

Fewer than 500 remain in the wild.

These two rare big cats are sharing the same territory.

And fixed remote cameras

show that they even use exactly the same trails.

But leopards know they must be careful.

Siberian tigers are the biggest of all the big cats,

and could kill a leopard...

...should they ever meet.

In winter,

many forest animals rely on carrion for food.

And that includes leopards.

But he can't let down his guard.

The crows that led him here...

...may also have alerted the tiger.

The crows are disturbed.

Another mouthful might be risky.

Survival sometimes depends on knowing

when to leave.

Only a few choose to take their chances here

in the depths of winter.

Elsewhere, vast swathes of the forest remain eerily empty.

Above, there is an otherworldly light show...

...the aurora borealis.

High in the sky,

electrically charged solar particles from space

collide with the Earth's atmosphere

and create this ethereal display.

Meanwhile, in Canada,