Nature's Epic Journeys (2016–…): Season 1, Episode 2 - Caribou - full transcript

We're about to follow the world's
greatest migrations

and reveal their secrets in a way
that's never been done before.

All over the world,
animals are on the move.

Embarking on vast journeys
they depend upon for survival -

to find food, to give birth,
and to escape danger.

Travelling hundreds of miles
through some of the world's

most breathtaking wildernesses...

...that can turn against them
at any moment.

That rapid's really picking up,

they're just disappearing
under the water.

Ground-breaking technology
allows our team to follow these



migrations more closely
than ever before,

and understand them
in unprecedented depth.

We can track this ele's movement
in real time.

Using the latest
satellite-tracking technology,

we can monitor individual animals

and witness first-hand
their struggles for survival.

We've got a drama down here.

We got him, we've got the wolf,
we've got the wolf.

This time, we're following caribou
as they migrate through Canada

and Alaska,
joining them for the final

and most dangerous stage
of their journey,

as they race to get to their
calving grounds before giving birth.

For the first time, we can stay
with them as they brave

ice-strewn rivers,
challenging mountain passes



and starving predators.

This is the setting for the
longest land-mammal migration

anywhere on the planet. Over
100,000 caribou will come pouring

through these mountains
on an extraordinary journey,

and we're here to follow them
every step of the way.

Ivvavik National Park,
in the Western Arctic.

A wilderness so remote,
it has fewer visitors each year

than the summit of Everest.

Our home will be Sheep Creek -

an abandoned
gold-mine-turned-research-station.

It's taken four days
of solid travel to reach it.

It's so good to be here.

It was touch-and-go for a while,
with the low cloud cover,

but we're finally here.

'Using this as base camp,

'a hand-picked team
of specialist biologists

'and camera crews will join
local experts to study

'the caribou migration
as it happens.'

Every spring,

these distant coastal plains
are the goal of almost 100,000

pregnant caribou
and their offspring,

as they walk 500km to give birth.

Incredibly,
at the same time every year,

all the females give birth
within days of each other.

This is what they come for.
Cotton grass -

the perfect nutrient-rich
food for nursing mothers.

And the timing is critical.

The grass is at its best
for just a matter of days.

But first the caribou must come

together from across
their winter range.

We're waiting for them

at a spot that's 350km
from their calving grounds.

Here, as they enter the treacherous
valleys of Ivvavik National Park,

the landscape should funnel them
into one vast group.

They now have just 22 days
until peak calving.

Thanks to the satellite collars,

we can see exactly where the herd is
at any one time.

'Caribou biologist Mike Suitor
is helping me to interpret the data

'as it comes into base camp.'

We're starting to learn lots of
new things with this new technology,

but we have a small sample -
you know, 50 satellite GPS collars

on a herd that's numbering in
the ballpark of 200,000 caribou.

You know, that's one of the really
interesting things

about the satellite GPS collars is,

the more we have out,
the more we are going to learn.

Earlier in the year,
wildlife cameraman Max Hug Williams

joined the team collaring our
caribou in their wintering grounds.

He was accompanied by scientist
Jason Caikoski,

who needs to get as close as
he possibly can with his net gun.

But caribou have a top speed
of nearly 80km an hour.

The whole operation requires
an enormous amount of skill.

Our team of scientists believe
that this is the fastest way

to do the job, stressing
the animal as little as possible.

Netting may appear intense, but
without the use of tranquilisers,

the caribou can rejoin their herd
as soon as the collar is on.

I've just gotta pin her down while
you put the collar on?

Yeah, and just keep your leg up,
and she can't kick back then.

It's incredible being this close
to a caribou.

Even through a glove you can feel
she's got such thick fur.

You can see they're perfectly
adapted for these crazy temperatures

which, in winter, drop to minus 50.

When you look at the hooves,
they're bigger than my hand.

It's almost like a snow shoe.

Yeah, these animals were built
for this.

The collar will provide
Jason's team

with information on their exact
movements for the next four years,

giving them detailed insight
into the animal's birth-rate,

behaviour and survival.

She seems really chilled out now,

she's just looking
for the rest of the herd.

We're going to try and keep up
with the herd on foot. On foot?

No chance!

Max's plan to follow in their
footsteps is incredibly ambitious.

Caribou are the Arctic's most
specialised species of deer,

able to withstand temperatures
down to minus 60 degrees Celsius

and survive on vegetation
that no other mammal can.

The Porcupine herd,
named after the Porcupine River,

spend their lives roaming
over an area

the size of Great Britain - feeding,
breeding and evading predators.

Key to their success is getting
the timing of the spring migration

just right.

But they CAN get it wrong.

The last time they were late
to their calving grounds,

20,000 newborns died -

their exhausted mothers
too weak to feed them.

Today, the data from their collars
suggests they could be late again.

I'm not sure it's the best night
for camping.

Max and biologist Peter Sinkins
are trying to find out

why they are moving so slowly.

Look how deep it is!
Yeah, it's deep.

It's really crusty on the top
as well.

No wonder the caribou
are not coming this way yet.

Trudging through this snow
saps energy and takes time,

for caribou and humans alike.

But exhaustion and starvation
aren't the only killers here.

We've got... a grizzly bear,
and it's quite a big male.

It is following the river...

...towards us now.

Yeah, I wouldn't be surprised
if he's over 600lb, that guy.

I'm not sure whether he will have
smelled us yet, cos we're downwind.

But he's clearly just come
out of hibernation

and he's probably pretty hungry.

Having just woken from six months
of hibernation,

the grizzlies here are starving.

They are badly in need of protein
to supplement

their main diet
of roots and berries.

When the caribou pass through,

they must make the most of this
short-lived but vital food source.

Right now, during the spring
migration, these mountains are home

to one of the highest concentrations
of grizzlies in the world.

He's definitely spotted us,

he's now eye-balling me
straight down the lens.

But bears aren't the only predators
the caribou need to avoid.

Wolves are formidable pack hunters,
but this Arctic environment

supports only small packs,
so they often have to hunt alone.

Their lives are desperately hard.
Most wolves won't live

beyond four years, and nearly 80% of
pups die before they are a year old.

If they miss the caribou migration,
they will pay dearly for it.

Starving wolves have even been known
to eat their young.

Using the satellite data
and observations on the ground,

we'll see exactly how the herd cope
with the threat of predators.

'But first
we need to find our caribou.'

We'll look at the last week's
worth of movement

and this will give us a sense
of how fast they're moving.

So they've continued
a little farther to the east.

Yes, yes,
I mean, they're tantalisingly close.

I can't believe how excited I am

at just coloured dots
on a satellite image!

With just 13 days to get
to the coastal plains,

the caribou still have 300km to go.

Delayed by unseasonally heavy snow,
they're finally drawing close

to the spot where Max and the team
have been waiting.

Max, it's Liz, do you copy? Over.
Hey, Liz, how you doing?

I've got some really good news
for you.

There are basically three lines
of caribou

on a trajectory to your camp.

Look! We've got our first group!

We finally got the caribou.
Whereabouts?

On the edge of the mountains.
They're here!

Come over here.

If you look at the ice straight
in front of us... Oooh, my gosh!

And there are LOADS of them!

At last, after a long, hard winter,

the landscape comes alive
with caribou.

Our Inuvialuit host, Lee-John,

who was born on the edge
of the calving grounds,

already had an idea that the caribou
were about to arrive.

Lee-John actually spotted them.
What was it...?

When we see the ravens flying by,
I would say...

HE SPEAKS HIS OWN LANGUAGE

...meaning, "Raven, Raven,
where are the caribou?"

And then when they flew over
and we started scanning the area,

we spotted them.

I'm so delighted - you've no idea.

The stage is set for
one of nature's greatest events.

Using a feed from the helicopter's
camera,

I can see just how many caribou
have arrived,

and they're all heading north
to the coastal plains.

There are lines of caribou
descending hillsides,

lines of them
crossing the waterways.

Something triggers this co-ordinated
movement that we still

don't fully understand.

It really is so thrilling
to watch them.

Now that they have all been
funnelled onto the same route,

we can join them
and study them in their entirety.

Max will now try to stick with them
to the calving grounds

as they embark on the most
dangerous leg of the migration.

(The first one's
just coming out now.

(It's amazing, we've just caught the
first front runners. They're making

(a beeline down the valley, and
everyone is following that leader.)

The caribou are so determined
to push on that they completely

ignore Max as they pass through
in huge numbers.

But the calm atmosphere
doesn't last for long.

(Something just spooked them.

(It may be a bear that spooked them,

(and now they're together in a tight
herd, and are just looking around.)

(and now they're together in a tight
herd, and are just looking around.)

From the air it's clear
that they are reacting to something.

The caribou are sat on the ice over
there and they're bolting this way.

There's a wolf in the distance,
moving over the ice,

and it's coming towards the herd.

It's ridiculous,
it's like a stampede.

Can't see the wolf at the moment.

Where's the radio?

Hey, guys, did you see the wolf?

In the open,
the wolf is easy to spot.

Caribou are a big prize
for a pack with pups to feed,

but weighing up to twice that of
an adult wolf, they are dangerous.

This lone wolf has no choice.
It desperately needs to feed.

Having panicked the herd,

the wolf looks out
for weaker or slower individuals.

As the herd splinters and reforms,

some of the caribou lose
sight of their attacker,

giving the wolf the advantage.

The wolf needs to grab the caribou
by the throat.

But one well-aimed hoof could end
the battle.

This time, the wolf has won.

The game of survival is finely
balanced up here.

But a fresh kill won't be kept
secret for long.

They've just been spooked
by something over there.

Just six minutes after the wolf
made its kill,

grizzly bears are already homing in.

I can't quite believe
what these guys are up against.

A wolf coming from that side, and
now we've got a bear over here.

I've never seen anything like it.

Bears are not as fast as caribou,

so they need to use
the element of surprise,

or scavenge kills
from other predators.

But they are much faster
than humans.

This guy's spotted us
and he's coming straight for us now,

which is a little bit unnerving,
to be quite honest.

What do we do now?
OK, let's back off a little bit.

He's off.

Yeah, he was just curious there.
We were standing still

and as soon as we started moving,
that's when he backed off.

He's not expecting a camera crew,
and so it's very likely

he was just coming to check us out.
"Was that a caribou?"

and backed right off,
so that's good.

Within moments, another bear
has detected the dead caribou.

Grizzlies can smell food
from miles away.

The wolf took a huge risk,
but for very little reward.

Max's bear has also turned
its attention to the carcass.

This bigger grizzly is running in.

He's chasing off this smaller bear.

Hey, Liz, Max here.
Can you hear us?

Yes, I can hear you, Max. Several
bears have been sort of fighting

over this carcass and displacing
each other. We got him, Max.

There's a bear on the carcass

and he's dragging the carcass
at the moment.

While the predators in the area
remain focused on the kill,

the caribou can press on towards
their calving grounds,

leaving the bear to his spoils.

This bear is making light work
of this caribou.

There's hardly anything left.

When this bear is done with it, the
eagles will come in, the gray jays,

the foxes, nothing will be left. And
it just goes to show how important

a source of food the caribou are
for so much of the wildlife here.

All the predator activity
has now pushed large numbers

of caribou
to the west of the Firth River.

But 30km ahead, their path
is blocked by a sheer cliff face.

They'll have to decide

whether to cross the river
or negotiate huge mountain peaks.

But for now they have
more urgent problems to deal with.

There are more bears around now
than at any other time of the year.

Even around our camp.

So you can see there's a track
right there. Is this the back foot?

Yeah, the hind feet are quite a bit
larger than the front foot.

He's a big fella, right?
It's a reasonable-sized bear, yeah.

But could they catch and kill
an adult caribou?

Found an interesting one on the
website - can you see that OK?

Yeah. OK, you've got a grizzly bear
there,

running pretty good, obviously.

That's a bull caribou.

You can see he's got the big
antlers... With the cubs!

That's a female with her cubs,
and she's going for it!

Yes, this female thinks it's worth
the risk of tackling

an adult bull caribou.

It looks like a younger bull.
He's not extremely large.

He's still a fair size. Oh, my gosh.

She is really going for it,
this is extraordinary.

Good Lord!

Grizzly bears are just so powerful.

That was amazing. She was fearless!

So that caribou was going to provide
a lot of food for her,

and will obviously transfer
that to her cubs,

so that is important
for the cubs' survival.

'Given the chance, grizzlies pose
a real threat to our caribou herd.'

With just 12 days to calving,

the caribou push on around the clock

in the 24 hours of daylight.

Max, Max, it's Liz.
Do you copy? Over.

Hey, Liz, morning, morning,
how are you doing?

I've got a data update for you -

the pace has picked up.

The fastest pace
is about 30km a day.

The majority of the herd
are moving north.

I was actually going to say,
we saw a collar last night.

Number 88,
so it would be really interesting

if you could track that female down
and see how far she's gone.

OK, we'll take a closer look
at number 88 from the data,

and we'll get back to you ASAP.

The herd is moving fast,

and Max and the ground team
are finding it hard to keep up.

Max needs to stop, eat and sleep,

but the caribou's specially adapted
body clock allows them

to keep going,
taking short naps along the way.

Max and his team
are in for a gruelling few days.

The satellite data shows the caribou
are hardly stopping at all.

'Meanwhile, the data provides
Mike and I

'with an insight into the female
that Max saw the night before.'

I was shocked when Max told us
it was 88.

I took a look, and I was blown away

because she does have quite a bit of
history for all of our collars.

She's an older cow, so she's going
to be ten years old this June.

Caribou 88 has been collared
for the whole of her life,

and her data has revealed something
quite remarkable.

Year after year, she has travelled
with the rest of the Porcupine herd.

But earlier this year, she left,

and joined a completely separate
one - the Central Arctic herd.

Is it possible that she bred with
a member of this different herd?

It's entirely possible, yeah,
she would have been down

in the mountains here,
where the Central Arctic herd

is during the rut. The one thing
we can say with some certainty

is that it looks like she's going to
calve with the Porcupine caribou,

not the Central Arctic.
She's back to her own herd.

She's a Porcupine caribou.

Exactly why she left is a mystery,

but the herd are lucky to see
the return of one of their older

and more experienced females.

She HAD been bringing up the rear.

Now she's straight to the front and
is helping to lead the entire herd.

This is an incredibly important
role,

especially as this wilderness
presents a new challenge

at every turn.

Whilst frozen,
rivers are safe and easy to cross.

But as the temperature rises,

they can quickly turn
into raging, deadly torrents.

This is unbelievable.

We were up at camp and heard the
change in the noise of the river.

Huge boulders of ice have broken
free further upstream

and are rushing past here.

The river has gone up by at least
a couple of metres.

There is no way a caribou can
get across this right now,

but this is what
they have to deal with.

Last year, as the Porcupine River
broke up,

locals saw
just how dangerous it can be.

Caribou crossing the river were
caught out as the ice broke,

carrying them unstoppably
downstream.

With just ten days left to calving,
our caribou on the western

side of the river are forced
to make a crucial decision.

(The majority of the herd passed
through the hills back there,

(but there is a small group
that has come to this dead-end.

(There's a huge cliff face here

(so there is no way they can
pass on this side of the river.

(So that lead female, I don't know

(whether
it's an inexperienced leader,

(but now she's got to make
a massive decision -

(either they have got to swim
across this river or head up this

(really steep slope here, and we
just saw a grizzly bear go up there,

(so it's not looking good
either way.)

They are all going for it.

A female enters the water.

The others follow.

That looks like the worst part
of the river.

The force of the meltwater has
created a powerful set of rapids.

The caribou plunge in, regardless.

That rapid is really picking up.

They're just disappearing
under the water.

The power of the water
is simply too much.

One, I saw disappear under
the water - I don't know

if he came back up.

As the drama unfolds,
I take to the air to join Max.

I think they've finally seen sense
and are doing a U-turn.

It just goes to show the experience
you need

to navigate through these mountains.

With nowhere left to turn, the
caribou opt for the mountain route.

Hey, Liz, we've got drama down here.

We've just had a big group who were
stuck in this dead-end valley.

Half of them nearly got swept away,

the other half wouldn't
get in the water

and have taken on this massive peak
in front of us.

OK, Max, there's a bear
in the vicinity as well.

'And it's not just
the bear that spots an opportunity

'as the caribou change course.'

See it moving, there in the trees?
See it moving about halfway up?

Pete's just spotted
a wolf on the hill.

He seems to be going up into the
left, as if he's checking out

these guys, but he's gone
quite a bit up that slope.

We've got him, we've got the wolf.
We've got the wolf.

I mean, we knew there were
going to be predators here,

but I didn't quite imagine
it would be so busy,

and they're all looking to get
their share of the caribou.

The caribou now have no choice
but to come back down to the river.

And from here,
there's only one option.

Despite being further upstream,
the current is still strong.

The more experienced females
lead the way.

Suddenly a yearling starts
drifting away from its mother.

And soon
even the adults are struggling.

The yearling is out of its depth,
separated from the herd

and heading towards the rapids.

But caribou fur is hollow,
to insulate against the cold,

which means the calf floats.

There's no time to rest.

The herd must keep moving

if they are to reach
the coastal plains before calving.

The more we observe our caribou,
the more apparent the challenges

and dangers they face.

Their survival hangs in the balance,

every step of this astonishing
journey.

Although Porcupine herd numbers
are currently stable,

other herds have decreased by as
much as 85% in just a few years.

Our data, backed up by our
observations on the ground,

will contribute to the bigger
picture of caribou survival

across the Arctic and will hopefully
provide clues

to these sudden declines.

Eight days until calving,
and the relentless pace

of the migration has now left Max
and the ground team far behind.

Max, it's Liz. Do you copy? Over.

Roger. It's all gone quiet here.

This is where the bulk
of your herd have headed,

not west along the Firth,
as we first thought.

There are thousands of caribou here,
this is where you need to be.

The caribou are already
25km ahead of Max's position.

It's time to catch up.

The plan is to relocate you there
as soon as possible.

Max's mission to keep up with
the caribou on foot

has proved impossible.

The animals' extraordinary strength
and stamina - and their

relentless urge to push north to the
coastal plains - has beaten him.

There's only one way to catch up.

The caribou have walked
nearly 400km

from their wintering grounds,
and yet have barely eaten.

Their reserves are running
dangerously low.

Number 88 is still with the other
trailbreakers, leading the way.

She and the others have
only 50km to go,

but they're each burning
nearly 6,000 calories a day.

They need to conserve
as much energy as possible.

You have to remember that these
caribou are running on empty now.

Of course, they are pregnant too,

and they're running low on all
the reserves they stored up

the previous summer and autumn

for this extraordinary migration
to the coastal plain.

What's more, the calves they carry

are now making
their greatest demands.

80% of all foetal growth happens
during this final stage.

And if that wasn't enough,
they must now cross

a vast array of snow-filled
plateaus.

It takes seven times more energy
to break a trail through deep snow

than it does to walk
on bare ground.

But the caribou have
an ingenious solution.

They walk in single file.

It really is fascinating how
precisely they follow each other

in a single file. It's all about

conserving energy. Every calorie
that you can save by not breaking

your own trail, by following
in the footsteps of another caribou,

means you've got a bigger chance
of succeeding at the other end.

There's clear evidence that caribou
have been walking like this,

in these valleys,
for over 12,000 years.

Using the very same path
for millennia,

this ancient migration has etched
its story on the land.

Max and the ground team have
been following the caribou

relentlessly, and have hardly slept.

But the pace is taking its toll
on the caribou too.

These caribou have been going
all night.

They're so focused
on their destination at this point,

they're so close to getting
to the north slope,

that they just keep going.

Their drive is so strong,
they're even prepared

to leave behind their yearlings
to get there on time.

These guys don't get to sleep.
If you look up there,

you see the snow patch.

A yearling just looks absolutely
knackered, wiped out -

he's just sort of given up.

He's lying down,
while the rest just moved on.

I mean, it's just tough love,
isn't it?

Even though his mum is probably
in that group,

I mean, she's not waiting for him.

It looks like he's given up.

Oh, he's trying to get up. There's
no way he's going to catch them.

They are on a mission,
and he's stumbling along.

The pregnant females are a week away
from giving birth at this point.

You know, they've started this
migration as early as January,

so they've come a huge distance.

Well over 5,000 caribou die
during each spring migration,

from sheer exhaustion
or predator attacks.

A bear can't catch
a full-grown caribou

unless they catch them by surprise,

but that is what
they are looking for - stragglers

on their own. That is
just easy pickings for a bear.

Especially when the yearling's
already exhausted.

One last push, and the lead
caribou are finally heading

out of the mountains
and onto the coastal plains.

After nearly 500km, barely feeding,

they've arrived just in time
for the early growth

of highly nutritious cotton grass.

But even now
they can't afford to relax.

We've got a big grizzly bear just
startled this group of caribou.

I mean, they're quicker than him,
but it looks like

he's trying to ambush them,

and we're just trying to get into
position so we can see something.

We saw him in these bushes.
He startled them.

There's caribou all around us.

There is a group that are calm here,
which hopefully suggest the bear's

not just in front of us,
but that group over there

got completely spooked.
They just ran for it.

He's somewhere in front of us,
probably about 100 metres.

They are faster than any bear,
but if he can play his cards right

and catch them by surprise,
then he'll have a tasty dinner.

But it looks like they've got away
this time.

Do you see anything? There is
nothing down by the river, there.

I don't see anything, but
the willows are thick down there.

Look!

He's big.

These caribou haven't seen him
at all.

They are so fixated on just eating
after this long journey,

that it's like
they haven't even seen him.

He's just wandering up
and they're not even responding.

In such open landscape, the bear has
little chance of catching a caribou.

This leaves Max and Pete at risk.

He's now closer to us
than he is the caribou.

What's the plan here, then, Pete?

Staying calm is essential.

Bear bangers are at hand
if they need to startle the bear.

And he's coming close now.

And if it charges and gets
too close, as a last resort,

Pete would have to use his gun.

He's charging.

OK. Let's not run.

Running can actually trigger
an attack.

This is not the best terrain for us,
is it?

We don't want to trip up now.

That was a little bit too close
for comfort.

That bear was clearly going
for the caribou,

but they're a little quicker than
us, and as they started to run off,

he started to change his attentions
towards us.

He gave us a bluff charge,
let us know who's boss -

and that gets your heart rate going.

They're so quick, they can be on
you before you know it,

if they want to be.

Yeah, they can run
as fast as a racehorse.

Oh, you didn't tell me that before!

He's still coming.
Oh, he's still coming.

He's coming with purpose now.

The bear is coming towards us.
Fire the chopper up quickly

and get ready to pull us
out of here.

He's coming pretty quick now, guys.

No running.

No. OK. OK.

It's a close shave,

and a powerful reminder of what the
caribou are constantly up against.

We're finding out so much more
about the harsh realities

of this epic migration -
helped, of course,

by the invaluable satellite data.

The collars themselves form the
basis for all of our sampling

that we do. But even though
we've been studying them

for as long as we have -
and they are

one of the better-researched herds
in the world...

And still there's so much
you don't know yet. Absolutely.

And of all of the behaviours
that you have observed,

what has been the most surprising
so far?

I think the synchrony of movement is
what blows me away.

Here's a great example.
This was back at the end of August

last year, and we had caribou
that were spread all across,

all the way towards Aklavik,

200km away from each other,
and on the same day,

it was like a switch went off -

all those caribou started moving.

And they all went

to the exact same location.

It was amazing -

we're talking hundreds of thousands

of caribou here.

And they're all, on the same day,
starting to move,

and they're all going to the exact
same location, and then they are

all moving together as one pretty
much cohesive group at that point.

How did they know?

Like, you're at one end
of the range,

I'm at the other end of the range.

How do we both know that that's
the day that we're going to move

and we're all going to go
to this one spot?

Typically, this type of thing might
have gone unnoticed in the past,

that's one of the beautiful things
about the satellite GPS collars,

is that it actually allows us
to see this as it occurs.

The more we study this migration,

the more we can reveal
about these remarkable animals.

With just five days left to calving,

the herd is spreading out
across the coastal plain.

There are huge numbers that have
already come down to these plains,

and there's such a difference
in landscape

from those rugged hills
to flat, open plains.

The last leg of the journey.

Yeah, that's right, home stretch.

Somewhere in this vast landscape,

the caribou will give birth.

Despite the endless challenges
and obstacles,

their relentless pace and stamina
have got them here on time.

The caribou aren't due to calve here
for another few days, but we don't

want to risk distressing them, when
they are at their most sensitive.

Which means that this is Max's last
chance to catch up with the herd

before leaving them alone
to give birth.

Wow, it's so different here,
isn't it?

They seem so much more relaxed here,
it's like they know they've made it.

Look at them coming in now.

They're curious, they'll do that.

They're just checking us out here.

They're just munching away
on this grass.

Yeah, like the little flower
on top there.

As you can see, it's new growth.

Very critical to the caribou
at this time of year.

So those fresh shoots have just come
out. Yes, those are fresh shoots.

This ground is covered
with cotton grass.

So it's that that they've
come all this way for.

Right, yes, it's fresh stuff.

In the two weeks
after the snow melts,

the cotton grass sprouts
and flowers.

This is when
it's at its most nutritious.

It's this narrow window that
the caribou have raced here for.

Not only can they now replenish
their reserves,

but scientists have shown
that eating the grass at this

specific time helps mothers produce
a richer milk

for their newborn calves, greatly
improving their chances of survival.

And it's earned this place
a special name with the Inuvialuit.

Ivvavik in our Inuvialuktun
language means a place of birth,

a nursery for the young.

Now that we've followed the caribou
from the southern part of the park

to the northern part,

now we leave the caribou in peace
to have their young ones.

We may have had to leave
the caribou for a while,

but the satellite data continues
to tell their story as they move

along the plains, into Alaska.

The pregnant females finally stopped
to give birth in the area

with the most nutritious and
abundant cotton grass on the plains.

And at last, we can rejoin them.

Here they are, here they are.
It's so good to see them again.

After all their efforts,

I'm keen to know how successful
this year's calving has been.

I wonder if I'm setting eyes
on any we saw

at the beginning of the migration.

Seeing them in such huge numbers -

almost each adult accompanied
by a calf - is just...

Yeah, it's a little bit emotional,
actually. It's so lovely to see.

There must be thousands of caribou.

This time, our base camp
will be in the Alaskan part

of the Porcupine caribou range.

'I've been joined by Eric Wald of
the US Fish and Wildlife Service,

'who'll guide us through the area.'

What direction and distance from
our current location are they?

'Being so remote,

'we have to get the most recent data
by satellite phone.'

What did he say?
Well, they're heading our way -

about 17km to the northwest of us.

We saw them from the air as far
north-west as there? Yes.

'Using this information,
Max is moving into position.'

There's maybe up to 200,000 caribou,

and they're all about to pass
directly in front of us.

They're so quick, it's ridiculous.

With new calves alongside,

the females moult
into their thinner summer coats.

There's satellite collar number 25,
he's right in front of us.

I think back to when we saw these
guys for the first time in Alaska

and put those collars on,
and they've gone so far since then.

To think what that lady's been
through - it's mind-blowing.

Having survived an immense
and arduous journey,

our caribou got the timing
just right.

Over 70,000 pregnant females
gave birth

within five days of each other.

That's one calf born
every eight seconds,

day and night, for five whole days.

Nourished by the rich milk
from the best cotton grass,

the calves have doubled in size
in just ten days.

But they've got a lot to learn.

There's a calf, right
in the middle of the herd, calling.

Clearly lost his mum.

How you begin to find your mum
when there's 200,000 caribou...

She could well have
already moved on.

Ah, he's found her!

You could see his relief,
he suddenly must have heard his mum

and he ran forward, straight to her
side and tried to get some milk.

And that's so good to see.

Despite having only just calved,

our satellite data shows that
the herd are soon on the move again.

So May 25th, through to the 28th.
OK, that's when we left them.

And when was the majority
of the calving, then?

Calving was from June 8th.

They move very quickly away from
the calving location, don't they?

Yeah, after two, three days,

the calves are able to keep up
with their mom.

'And keep up they must.'

Soon, the whole herd comes together
and can move as one -

joined by another 70,000 males
and youngsters.

This time it's not bears or wolves
that are driving them together.

It's a creature
a million times smaller

but a billion times more numerous
than the caribou.

Mosquitoes.

A single caribou can lose
half a pint of blood a day to them.

They have even been reported
to die from asphyxiation

as thousands of tiny mosquitoes
clog up their airways.

The relentless attack creates
waves of panic through the herd.

Walking constantly, into the wind,
gives some relief.

As does herding together
as closely as possible.

This gives us our best chance
to see how they've fared this year,

and how their numbers
are holding up.

It's good news.

The Porcupine herd
is now bigger than ever recorded.

And it's an incredible sight.

During our time here,
the satellite data and our team

on the ground have captured the
critical decisions made

by this herd -
when to move, how fast to travel,

which routes to take,
and which rivers to cross.

We've discovered the importance
of experienced females,

like number 88,
leading the herd out of danger.

And we've seen how sheer stamina,
determination and ingenuity

helped them to time this journey
faultlessly.

In a punishing wilderness
where their lives constantly

hang in the balance,
and little else can survive,

these caribou
have adapted perfectly.

This has been a journey
of epic proportions,

and what's struck me the most is
just what caribou are capable of.

Not only do they carry out
the longest land migration

in all of the animal kingdom,

but they have found ways to survive

countless threats and obstacles that
are thrown at them at every turn.

And next year the new calves will
have to learn just what it takes

to make it out here.

Their mothers will pass down
all of their knowledge

and all of their experience

as the race to the calving grounds
begins all over again.

Next time, in a world first,

we join a newly discovered
animal migration.

Thousands of zebra migrate
hundreds of miles across

one of Africa's harshest scrubland.

Failure could end in disaster.