Caribou Nature's Epic Journeys

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.)

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.

Grizzlies mostly scavenge kills from other predators,

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.