Yellowstone

Yellowstone,

volcanic wonderland.

Two million acres of wild space ...

right in the heart of North America.

The heat of the summer has unveiled ...

the full extent of the Yellowstone wilderness ...

and, for a few precious months, it has blossomed.

But, now, Yellowstone is changing.

In just a few weeks, the snow and ice of winter will be back.

As the animals of Yellowstone now turn to face ...

perhaps the biggest challenges of their year,

the true value of the world's first national park ...

is about to become clearer than ever.

It's late August and on Yellowstone's peaks ...

there is already a dusting of fresh snow.

Now, a new sound marks the new season.

Fuelled by testosterone,

the bugle call of this male elk is a boast of his strength.

All over Yellowstone,

male elk are challenging each other for dominance.

The sound of Yellowstone's autumn.

They are trying to win the admiration of females ...

and gather them into a harem.

Only then do they stand a chance at mating with them before winter.

But the females are not yet in season,

so they are not really that interested.

And, for now, they have a more practical concern.

Winter will soon be here.

They are eager to head down to lower ground ...

before the snow comes in earnest.

Some will move down into nearby valleys,

whilst others will journey much further,

even beyond the boundaries of Yellowstone itself.

Yellowstone is deep in the Rocky Mountains of North America,

an isolated high plateau defended by rugged peaks.

In the middle is the National Park.

The park and the surrounding mountains form one of the most important ...

and spectacular wilderness areas on Earth.

In just two months, this great plateau will become a deep freeze once more.

Before then, the animals of Yellowstone will have to get ready ...

or get out.

But, for now, below the snow-dusted peaks,

summer still lingers in the heart of Yellowstone.

The sun has revitalised this place ...

and, now, there are more living things here ...

than at any time of the year.

The summer has brought visitors too,

who are enjoying Yellowstone at its most vibrant.

As the sun now starts to get lower in the sky,

the rich colours make this ...

one of the best times to see the geysers.

On the grasslands, the good times are already over.

These bison are making the most of the grazing,

but it is now dry and parched.

From now on, they will have to rely heavily on stored fat ...

to keep them going,

as these meadows become covered in more than four feet of snow.

Others are already thinking of leaving.

Pronghorn evolved to outrun a now-extinct North American cheetah ...

and, so, are the fastest antelope on Earth.

But, unlike bison, their lightweight bodies can't store ...

enough energy to keep them here through the winter.

So, now they must head out.

Their journey will be the longest of all.

But as many are preparing to get out, some have no choice but to stay.

In the remote north east of the Yellowstone wilderness ...

are the Beartooth Mountains.

Here, surviving above 8,000 feet,

a tree now welcomes the change of season,

the whitebark pine.

All summer, these trees have been soaking up the energy of the Sun ...

preparing for this moment.

Now, they offer the animals of Yellowstone ...

a bumper crop of pine cones.

The whitebark pine is gambling on the fact ...

that animals now need all the food they can get before winter,

and is hoping it can entice them to spread its seeds ...

far and wide across Yellowstone.

So, inside the cones, it has put tasty, nutritious pine nuts.

A pine squirrel.

It snacks on a few of the nuts to keep going ...

and then buries them, one by one, in a sheltered hollow beside the tree.

If it hides them well and packs them carefully,

they should last through the winter.

But this is not much good for the tree.

Its seeds have gone nowhere.

A grizzly bear mother and cubs.

It's unusual for a grizzly to have so many cubs.

This mother has found two orphans and adopted them.

But now she has four cubs to fatten up ...

before they go into the den to hibernate this winter.

She's after pine nuts, too.

They are 50% fat.

In a good year, a grizzly bear can put on five pounds a day ...

eating nuts alone.

If squirrels have done the hard work,

it doesn't matter that grizzlies can't climb trees.

The squirrel will just have to start again.

Luckily, this year, the trees are being particularly generous.

A Clark's Nutcracker.

This is what the tree has been waiting for.

With its perfectly-shaped beak, it prises the nuts from the cones ...

and tucks them one by one into a special pouch under its tongue,

up to 150 at a time,

a fifth of its entire body weight.

Then it flies as much as 15 miles away ...

and drills the nuts into the ground in sets of 10,

placing a stone on top of the stash to mark the site.

It goes back ...

for more ...

and more.

Over the autumn, a single bird can bury 30,000 nuts ...

across an area of 100 square miles.

When the winter comes,

it will manage to remember the location of a staggering 70% of these seeds,

even when hidden beneath the snow.

But as the Clark's Nutcracker remembers its way into surviving the winter,

it becomes the whitebark's greatest ally.

Although its feat of memory is extraordinary,

for every 1,000 seeds it buries, it still forgets 300.

From all those missed seeds carried far and wide across Yellowstone,

new whitebark pines will germinate next spring.

It's now September ...

and the elk have made their way down to graze ...

where the grass on the river banks is still green ...

and they can browse the nutritious shoots of young willow trees.

The males are now upping their game.

This bull urinates on himself to increase his masculine appeal ...

and by thrashing his antlers to decorate them,

he hopes to make himself look more impressive.

The females are paying a little more attention now.

The bull has succeeded in gathering a fair-sized harem.

But the females are still not quite ready to mate.

They are now focused on feeding as much as they can ...

before moving lower still.

But the elk are being watched.

Over the summer, wolves have been less mobile because of their young pups ...

but their strength is building again.

The elk get twitchy and head for the cover of trees.

They may be a little safer here ...

but the food in the forest is far less nutritious than on the river banks.

If they want to eat well and avoid wolves this winter,

they'll need to keep on moving.

As the elk move gradually downwards,

they follow the rivers out of Yellowstone's central plateau.

The rivers, in turn, follow the path of glaciers ...

that flowed from this great bowl in the last ice age ...

and carved their way right through the barrier of surrounding mountains.

Today, these valleys are escape routes ...

for animals from the returning ice of winter.

Lower down, the valleys broaden, the rivers slow ...

and a richer variety of trees grows in the alluvial soils,

the perfect home for Yellowstone's most industrious creature.

A beaver can fell a cottonwood tree in just a few hours,

hundreds in a year.

The beaver doesn't chew through the whole trunk,

just enough to make the tree unstable.

It then retreats ...

and lets the wind do the rest.

It cuts branches into more manageable lengths ...

and then swims them down a network of purpose-built canals towards the dam.

The pond gives this beaver protection from predators ...

and the canals allow it to forage far into the forest,

carrying many times its own weight with ease.

Autumn is the busiest time of year for beavers.

It won't be so easy to make repairs when the pond is frozen over.

The sound of running water is their stimulus ...

to shore up gaps with timber and plug leaks with mud.

But the dam not only serves the beavers.

Moose come here from the forests around ...

to feed on weeds that thrive in the beaver's shallow pond.

The weed is rich in vital sodium that the forest can't easily provide.

But now that winter is approaching,

another essential role for the dam is revealed.

These smaller branches are not for fixing the dam,

they're for eating.

The beaver secures them to the mud in the lake bottom.

In just a few weeks, this lake will be frozen ...

and the beavers won't be able to cut and move trees.

But they will be able to swim right under the ice ...

to feed from this underwater larder.

Moose also eat twigs and branches ...

and often try to take advantage of the beaver's hard labour.

This young male is getting a little too close to the beaver's larder.

Autumn is not a time for sharing.

It's mid-September.

As the sun drops further in the sky,

the aspens, cottonwoods and maples ...

start shutting down for the winter.

They now digest the green pigments in their leaves ...

to claw back what nutrients they can into the trunk and roots.

What's left behind make the colours of autumn.