0:00:03 > 0:00:07We're about to follow the world's greatest migrations
0:00:07 > 0:00:10and reveal their secrets in a way that's never been done before.
0:00:13 > 0:00:16All over the world, animals are on the move.
0:00:18 > 0:00:22Embarking on vast journeys they depend upon for survival -
0:00:22 > 0:00:26to find food, to give birth, and to escape danger.
0:00:29 > 0:00:32Travelling hundreds of miles through some of the world's
0:00:32 > 0:00:34most breathtaking wildernesses...
0:00:36 > 0:00:39..that can turn against them at any moment.
0:00:39 > 0:00:41That rapid's really picking up,
0:00:41 > 0:00:44they're just disappearing under the water.
0:00:44 > 0:00:47Ground-breaking technology allows our team to follow these
0:00:47 > 0:00:51migrations more closely than ever before,
0:00:51 > 0:00:55and understand them in unprecedented depth.
0:00:55 > 0:00:59We can track this ele's movement in real time.
0:00:59 > 0:01:02Using the latest satellite-tracking technology,
0:01:02 > 0:01:04we can monitor individual animals
0:01:04 > 0:01:08and witness first-hand their struggles for survival.
0:01:08 > 0:01:10We've got a drama down here.
0:01:10 > 0:01:12We got him, we've got the wolf, we've got the wolf.
0:01:14 > 0:01:18This time, we're following caribou as they migrate through Canada
0:01:18 > 0:01:20and Alaska, joining them for the final
0:01:20 > 0:01:23and most dangerous stage of their journey,
0:01:23 > 0:01:27as they race to get to their calving grounds before giving birth.
0:01:29 > 0:01:32For the first time, we can stay with them as they brave
0:01:32 > 0:01:36ice-strewn rivers, challenging mountain passes
0:01:36 > 0:01:38and starving predators.
0:01:39 > 0:01:43This is the setting for the longest land-mammal migration
0:01:43 > 0:01:49anywhere on the planet. Over 100,000 caribou will come pouring
0:01:49 > 0:01:52through these mountains on an extraordinary journey,
0:01:52 > 0:01:55and we're here to follow them every step of the way.
0:02:05 > 0:02:09Ivvavik National Park, in the Western Arctic.
0:02:13 > 0:02:17A wilderness so remote, it has fewer visitors each year
0:02:17 > 0:02:18than the summit of Everest.
0:02:27 > 0:02:29Our home will be Sheep Creek -
0:02:29 > 0:02:32an abandoned gold-mine-turned-research-station.
0:02:36 > 0:02:39It's taken four days of solid travel to reach it.
0:02:43 > 0:02:45It's so good to be here.
0:02:45 > 0:02:47It was touch-and-go for a while, with the low cloud cover,
0:02:47 > 0:02:49but we're finally here.
0:02:52 > 0:02:53'Using this as base camp,
0:02:53 > 0:02:56'a hand-picked team of specialist biologists
0:02:56 > 0:02:59'and camera crews will join local experts to study
0:02:59 > 0:03:02'the caribou migration as it happens.'
0:03:08 > 0:03:09Every spring,
0:03:09 > 0:03:13these distant coastal plains are the goal of almost 100,000
0:03:13 > 0:03:16pregnant caribou and their offspring,
0:03:16 > 0:03:19as they walk 500km to give birth.
0:03:20 > 0:03:23Incredibly, at the same time every year,
0:03:23 > 0:03:27all the females give birth within days of each other.
0:03:27 > 0:03:30This is what they come for. Cotton grass -
0:03:30 > 0:03:35the perfect nutrient-rich food for nursing mothers.
0:03:35 > 0:03:37And the timing is critical.
0:03:37 > 0:03:40The grass is at its best for just a matter of days.
0:03:42 > 0:03:44But first the caribou must come
0:03:44 > 0:03:46together from across their winter range.
0:03:46 > 0:03:48We're waiting for them
0:03:48 > 0:03:52at a spot that's 350km from their calving grounds.
0:03:55 > 0:03:59Here, as they enter the treacherous valleys of Ivvavik National Park,
0:03:59 > 0:04:02the landscape should funnel them into one vast group.
0:04:03 > 0:04:07They now have just 22 days until peak calving.
0:04:09 > 0:04:11Thanks to the satellite collars,
0:04:11 > 0:04:14we can see exactly where the herd is at any one time.
0:04:17 > 0:04:22'Caribou biologist Mike Suitor is helping me to interpret the data
0:04:22 > 0:04:25'as it comes into base camp.'
0:04:25 > 0:04:28We're starting to learn lots of new things with this new technology,
0:04:28 > 0:04:32but we have a small sample - you know, 50 satellite GPS collars
0:04:32 > 0:04:36on a herd that's numbering in the ballpark of 200,000 caribou.
0:04:36 > 0:04:38You know, that's one of the really interesting things
0:04:38 > 0:04:40about the satellite GPS collars is,
0:04:40 > 0:04:43the more we have out, the more we are going to learn.
0:04:46 > 0:04:49Earlier in the year, wildlife cameraman Max Hug Williams
0:04:49 > 0:04:53joined the team collaring our caribou in their wintering grounds.
0:04:56 > 0:04:58He was accompanied by scientist Jason Caikoski,
0:04:58 > 0:05:02who needs to get as close as he possibly can with his net gun.
0:05:04 > 0:05:07But caribou have a top speed of nearly 80km an hour.
0:05:21 > 0:05:24The whole operation requires an enormous amount of skill.
0:05:27 > 0:05:31Our team of scientists believe that this is the fastest way
0:05:31 > 0:05:35to do the job, stressing the animal as little as possible.
0:05:35 > 0:05:38Netting may appear intense, but without the use of tranquilisers,
0:05:38 > 0:05:42the caribou can rejoin their herd as soon as the collar is on.
0:05:42 > 0:05:45I've just gotta pin her down while you put the collar on?
0:05:45 > 0:05:49Yeah, and just keep your leg up, and she can't kick back then.
0:05:49 > 0:05:52It's incredible being this close to a caribou.
0:05:52 > 0:05:56Even through a glove you can feel she's got such thick fur.
0:05:56 > 0:06:00You can see they're perfectly adapted for these crazy temperatures
0:06:00 > 0:06:03which, in winter, drop to minus 50.
0:06:03 > 0:06:06When you look at the hooves, they're bigger than my hand.
0:06:06 > 0:06:07It's almost like a snow shoe.
0:06:07 > 0:06:09Yeah, these animals were built for this.
0:06:11 > 0:06:13The collar will provide Jason's team
0:06:13 > 0:06:18with information on their exact movements for the next four years,
0:06:18 > 0:06:21giving them detailed insight into the animal's birth-rate,
0:06:21 > 0:06:23behaviour and survival.
0:06:26 > 0:06:28She seems really chilled out now,
0:06:28 > 0:06:31she's just looking for the rest of the herd.
0:06:34 > 0:06:37- We're going to try and keep up with the herd on foot.- On foot?
0:06:37 > 0:06:39No chance!
0:06:45 > 0:06:49Max's plan to follow in their footsteps is incredibly ambitious.
0:06:51 > 0:06:55Caribou are the Arctic's most specialised species of deer,
0:06:55 > 0:06:59able to withstand temperatures down to minus 60 degrees Celsius
0:06:59 > 0:07:02and survive on vegetation that no other mammal can.
0:07:04 > 0:07:08The Porcupine herd, named after the Porcupine River,
0:07:08 > 0:07:10spend their lives roaming over an area
0:07:10 > 0:07:16the size of Great Britain - feeding, breeding and evading predators.
0:07:16 > 0:07:20Key to their success is getting the timing of the spring migration
0:07:20 > 0:07:22just right.
0:07:22 > 0:07:24But they CAN get it wrong.
0:07:25 > 0:07:28The last time they were late to their calving grounds,
0:07:28 > 0:07:3020,000 newborns died -
0:07:30 > 0:07:33their exhausted mothers too weak to feed them.
0:07:36 > 0:07:40Today, the data from their collars suggests they could be late again.
0:07:41 > 0:07:43I'm not sure it's the best night for camping.
0:07:43 > 0:07:47Max and biologist Peter Sinkins are trying to find out
0:07:47 > 0:07:49why they are moving so slowly.
0:08:01 > 0:08:04- Look how deep it is! - Yeah, it's deep.
0:08:04 > 0:08:07It's really crusty on the top as well.
0:08:07 > 0:08:09No wonder the caribou are not coming this way yet.
0:08:10 > 0:08:14Trudging through this snow saps energy and takes time,
0:08:14 > 0:08:16for caribou and humans alike.
0:08:19 > 0:08:23But exhaustion and starvation aren't the only killers here.
0:08:25 > 0:08:29We've got...a grizzly bear, and it's quite a big male.
0:08:30 > 0:08:32It is following the river...
0:08:34 > 0:08:35..towards us now.
0:08:36 > 0:08:40Yeah, I wouldn't be surprised if he's over 600lb, that guy.
0:08:43 > 0:08:47I'm not sure whether he will have smelled us yet, cos we're downwind.
0:08:47 > 0:08:50But he's clearly just come out of hibernation
0:08:50 > 0:08:53and he's probably pretty hungry.
0:08:55 > 0:08:58Having just woken from six months of hibernation,
0:08:58 > 0:09:00the grizzlies here are starving.
0:09:03 > 0:09:06They are badly in need of protein to supplement
0:09:06 > 0:09:08their main diet of roots and berries.
0:09:09 > 0:09:11When the caribou pass through,
0:09:11 > 0:09:14they must make the most of this short-lived but vital food source.
0:09:16 > 0:09:20Right now, during the spring migration, these mountains are home
0:09:20 > 0:09:24to one of the highest concentrations of grizzlies in the world.
0:09:27 > 0:09:29He's definitely spotted us,
0:09:29 > 0:09:32he's now eye-balling me straight down the lens.
0:09:32 > 0:09:35But bears aren't the only predators the caribou need to avoid.
0:09:42 > 0:09:46Wolves are formidable pack hunters, but this Arctic environment
0:09:46 > 0:09:50supports only small packs, so they often have to hunt alone.
0:09:53 > 0:09:56Their lives are desperately hard. Most wolves won't live
0:09:56 > 0:10:01beyond four years, and nearly 80% of pups die before they are a year old.
0:10:03 > 0:10:06If they miss the caribou migration, they will pay dearly for it.
0:10:06 > 0:10:10Starving wolves have even been known to eat their young.
0:10:16 > 0:10:20Using the satellite data and observations on the ground,
0:10:20 > 0:10:24we'll see exactly how the herd cope with the threat of predators.
0:10:25 > 0:10:28'But first we need to find our caribou.'
0:10:28 > 0:10:30We'll look at the last week's worth of movement
0:10:30 > 0:10:33and this will give us a sense of how fast they're moving.
0:10:33 > 0:10:36So they've continued a little farther to the east.
0:10:36 > 0:10:39Yes, yes, I mean, they're tantalisingly close.
0:10:39 > 0:10:41I can't believe how excited I am
0:10:41 > 0:10:44at just coloured dots on a satellite image!
0:10:48 > 0:10:51With just 13 days to get to the coastal plains,
0:10:51 > 0:10:53the caribou still have 300km to go.
0:10:55 > 0:10:59Delayed by unseasonally heavy snow, they're finally drawing close
0:10:59 > 0:11:02to the spot where Max and the team have been waiting.
0:11:02 > 0:11:08- Max, it's Liz, do you copy? Over. - Hey, Liz, how you doing?
0:11:08 > 0:11:11I've got some really good news for you.
0:11:11 > 0:11:14There are basically three lines of caribou
0:11:14 > 0:11:17on a trajectory to your camp.
0:11:19 > 0:11:21Look! We've got our first group!
0:11:21 > 0:11:23- We finally got the caribou. - Whereabouts?
0:11:23 > 0:11:26- On the edge of the mountains. - They're here!
0:11:32 > 0:11:34Come over here.
0:11:36 > 0:11:39- If you look at the ice straight in front of us...- Oooh, my gosh!
0:11:39 > 0:11:42And there are LOADS of them!
0:11:46 > 0:11:49At last, after a long, hard winter,
0:11:49 > 0:11:52the landscape comes alive with caribou.
0:12:05 > 0:12:07Our Inuvialuit host, Lee-John,
0:12:07 > 0:12:10who was born on the edge of the calving grounds,
0:12:10 > 0:12:14already had an idea that the caribou were about to arrive.
0:12:14 > 0:12:17Lee-John actually spotted them. What was it...?
0:12:17 > 0:12:20When we see the ravens flying by, I would say...
0:12:20 > 0:12:23HE SPEAKS HIS OWN LANGUAGE
0:12:23 > 0:12:26..meaning, "Raven, Raven, where are the caribou?"
0:12:26 > 0:12:29And then when they flew over and we started scanning the area,
0:12:29 > 0:12:32we spotted them.
0:12:32 > 0:12:34I'm so delighted - you've no idea.
0:12:35 > 0:12:38The stage is set for one of nature's greatest events.
0:13:00 > 0:13:03Using a feed from the helicopter's camera,
0:13:03 > 0:13:06I can see just how many caribou have arrived,
0:13:06 > 0:13:09and they're all heading north to the coastal plains.
0:13:17 > 0:13:20There are lines of caribou descending hillsides,
0:13:20 > 0:13:24lines of them crossing the waterways.
0:13:26 > 0:13:30Something triggers this co-ordinated movement that we still
0:13:30 > 0:13:32don't fully understand.
0:13:32 > 0:13:36It really is so thrilling to watch them.
0:13:42 > 0:13:45Now that they have all been funnelled onto the same route,
0:13:45 > 0:13:48we can join them and study them in their entirety.
0:13:50 > 0:13:53Max will now try to stick with them to the calving grounds
0:13:53 > 0:13:56as they embark on the most dangerous leg of the migration.
0:13:59 > 0:14:01(The first one's just coming out now.
0:14:08 > 0:14:11(It's amazing, we've just caught the first front runners. They're making
0:14:11 > 0:14:15(a beeline down the valley, and everyone is following that leader.)
0:14:16 > 0:14:20The caribou are so determined to push on that they completely
0:14:20 > 0:14:24ignore Max as they pass through in huge numbers.
0:14:26 > 0:14:28But the calm atmosphere doesn't last for long.
0:14:32 > 0:14:34(Something just spooked them.
0:14:36 > 0:14:38(It may be a bear that spooked them,
0:14:38 > 0:14:42(and now they're together in a tight herd, and are just looking around.)
0:14:44 > 0:14:47From the air it's clear that they are reacting to something.
0:14:50 > 0:14:54The caribou are sat on the ice over there and they're bolting this way.
0:14:54 > 0:14:57There's a wolf in the distance, moving over the ice,
0:14:57 > 0:14:58and it's coming towards the herd.
0:15:05 > 0:15:07It's ridiculous, it's like a stampede.
0:15:07 > 0:15:09Can't see the wolf at the moment.
0:15:09 > 0:15:11Where's the radio?
0:15:11 > 0:15:14Hey, guys, did you see the wolf?
0:15:21 > 0:15:24In the open, the wolf is easy to spot.
0:15:27 > 0:15:31Caribou are a big prize for a pack with pups to feed,
0:15:31 > 0:15:35but weighing up to twice that of an adult wolf, they are dangerous.
0:15:35 > 0:15:39This lone wolf has no choice. It desperately needs to feed.
0:16:40 > 0:16:42Having panicked the herd,
0:16:42 > 0:16:46the wolf looks out for weaker or slower individuals.
0:17:03 > 0:17:06As the herd splinters and reforms,
0:17:06 > 0:17:08some of the caribou lose sight of their attacker,
0:17:08 > 0:17:10giving the wolf the advantage.
0:17:27 > 0:17:31The wolf needs to grab the caribou by the throat.
0:17:31 > 0:17:33But one well-aimed hoof could end the battle.
0:18:21 > 0:18:22This time, the wolf has won.
0:18:30 > 0:18:33The game of survival is finely balanced up here.
0:18:34 > 0:18:37But a fresh kill won't be kept secret for long.
0:18:48 > 0:18:51They've just been spooked by something over there.
0:18:51 > 0:18:54Just six minutes after the wolf made its kill,
0:18:54 > 0:18:57grizzly bears are already homing in.
0:18:57 > 0:19:00I can't quite believe what these guys are up against.
0:19:00 > 0:19:04A wolf coming from that side, and now we've got a bear over here.
0:19:04 > 0:19:06I've never seen anything like it.
0:19:07 > 0:19:09Bears are not as fast as caribou,
0:19:09 > 0:19:12so they need to use the element of surprise,
0:19:12 > 0:19:14or scavenge kills from other predators.
0:19:16 > 0:19:18But they are much faster than humans.
0:19:21 > 0:19:25This guy's spotted us and he's coming straight for us now,
0:19:25 > 0:19:28which is a little bit unnerving, to be quite honest.
0:19:33 > 0:19:37- What do we do now? - OK, let's back off a little bit.
0:19:44 > 0:19:45He's off.
0:19:47 > 0:19:50Yeah, he was just curious there. We were standing still
0:19:50 > 0:19:53and as soon as we started moving, that's when he backed off.
0:19:53 > 0:19:58He's not expecting a camera crew, and so it's very likely
0:19:58 > 0:20:01he was just coming to check us out. "Was that a caribou?"
0:20:01 > 0:20:04and backed right off, so that's good.
0:20:06 > 0:20:10Within moments, another bear has detected the dead caribou.
0:20:11 > 0:20:14Grizzlies can smell food from miles away.
0:20:15 > 0:20:19The wolf took a huge risk, but for very little reward.
0:20:23 > 0:20:26Max's bear has also turned its attention to the carcass.
0:20:30 > 0:20:32This bigger grizzly is running in.
0:20:36 > 0:20:38He's chasing off this smaller bear.
0:20:52 > 0:20:55Hey, Liz, Max here. Can you hear us?
0:20:55 > 0:20:59- Yes, I can hear you, Max.- Several bears have been sort of fighting
0:20:59 > 0:21:03- over this carcass and displacing each other.- We got him, Max.
0:21:03 > 0:21:04There's a bear on the carcass
0:21:04 > 0:21:07and he's dragging the carcass at the moment.
0:21:12 > 0:21:16While the predators in the area remain focused on the kill,
0:21:16 > 0:21:19the caribou can press on towards their calving grounds,
0:21:19 > 0:21:22leaving the bear to his spoils.
0:21:23 > 0:21:26This bear is making light work of this caribou.
0:21:26 > 0:21:28There's hardly anything left.
0:21:28 > 0:21:32When this bear is done with it, the eagles will come in, the gray jays,
0:21:32 > 0:21:38the foxes, nothing will be left. And it just goes to show how important
0:21:38 > 0:21:41a source of food the caribou are for so much of the wildlife here.
0:21:45 > 0:21:49All the predator activity has now pushed large numbers
0:21:49 > 0:21:51of caribou to the west of the Firth River.
0:21:53 > 0:21:58But 30km ahead, their path is blocked by a sheer cliff face.
0:21:58 > 0:21:59They'll have to decide
0:21:59 > 0:22:03whether to cross the river or negotiate huge mountain peaks.
0:22:04 > 0:22:08But for now they have more urgent problems to deal with.
0:22:08 > 0:22:13There are more bears around now than at any other time of the year.
0:22:13 > 0:22:14Even around our camp.
0:22:17 > 0:22:20- So you can see there's a track right there.- Is this the back foot?
0:22:20 > 0:22:23Yeah, the hind feet are quite a bit larger than the front foot.
0:22:23 > 0:22:26- He's a big fella, right? - It's a reasonable-sized bear, yeah.
0:22:27 > 0:22:31Grizzlies mostly scavenge kills from other predators,
0:22:31 > 0:22:34but could they catch and kill an adult caribou?
0:22:36 > 0:22:38Found an interesting one on the website - can you see that OK?
0:22:38 > 0:22:41- Yeah.- OK, you've got a grizzly bear there,
0:22:41 > 0:22:43running pretty good, obviously.
0:22:43 > 0:22:45That's a bull caribou.
0:22:46 > 0:22:50- You can see he's got the big antlers...- With the cubs!
0:22:50 > 0:22:53That's a female with her cubs, and she's going for it!
0:22:54 > 0:22:57Yes, this female thinks it's worth the risk of tackling
0:22:57 > 0:22:59an adult bull caribou.
0:23:11 > 0:23:14It looks like a younger bull. He's not extremely large.
0:23:15 > 0:23:18He's still a fair size. Oh, my gosh.
0:23:21 > 0:23:24She is really going for it, this is extraordinary.
0:23:28 > 0:23:29Good Lord!
0:23:29 > 0:23:31Grizzly bears are just so powerful.
0:23:40 > 0:23:43That was amazing. She was fearless!
0:23:43 > 0:23:46So that caribou was going to provide a lot of food for her,
0:23:46 > 0:23:48and will obviously transfer that to her cubs,
0:23:48 > 0:23:51so that is important for the cubs' survival.
0:23:52 > 0:23:57'Given the chance, grizzlies pose a real threat to our caribou herd.'
0:24:05 > 0:24:08With just 12 days to calving,
0:24:08 > 0:24:10the caribou push on around the clock
0:24:10 > 0:24:12in the 24 hours of daylight.
0:24:23 > 0:24:26Max, Max, it's Liz. Do you copy? Over.
0:24:26 > 0:24:28Hey, Liz, morning, morning, how are you doing?
0:24:28 > 0:24:30I've got a data update for you -
0:24:30 > 0:24:32the pace has picked up.
0:24:32 > 0:24:36The fastest pace is about 30km a day.
0:24:36 > 0:24:39The majority of the herd are moving north.
0:24:39 > 0:24:42I was actually going to say, we saw a collar last night.
0:24:42 > 0:24:44Number 88, so it would be really interesting
0:24:44 > 0:24:49if you could track that female down and see how far she's gone.
0:24:49 > 0:24:52OK, we'll take a closer look at number 88 from the data,
0:24:52 > 0:24:54and we'll get back to you ASAP.
0:24:59 > 0:25:01The herd is moving fast,
0:25:01 > 0:25:04and Max and the ground team are finding it hard to keep up.
0:25:09 > 0:25:12Max needs to stop, eat and sleep,
0:25:12 > 0:25:16but the caribou's specially adapted body clock allows them
0:25:16 > 0:25:18to keep going, taking short naps along the way.
0:25:19 > 0:25:23Max and his team are in for a gruelling few days.
0:25:23 > 0:25:27The satellite data shows the caribou are hardly stopping at all.
0:25:37 > 0:25:39'Meanwhile, the data provides Mike and I
0:25:39 > 0:25:43'with an insight into the female that Max saw the night before.'
0:25:43 > 0:25:46I was shocked when Max told us it was 88.
0:25:46 > 0:25:48I took a look, and I was blown away
0:25:48 > 0:25:52because she does have quite a bit of history for all of our collars.
0:25:52 > 0:25:56She's an older cow, so she's going to be ten years old this June.
0:25:56 > 0:26:00Caribou 88 has been collared for the whole of her life,
0:26:00 > 0:26:04and her data has revealed something quite remarkable.
0:26:04 > 0:26:08Year after year, she has travelled with the rest of the Porcupine herd.
0:26:10 > 0:26:12But earlier this year, she left,
0:26:12 > 0:26:16and joined a completely separate one - the Central Arctic herd.
0:26:18 > 0:26:23Is it possible that she bred with a member of this different herd?
0:26:23 > 0:26:25It's entirely possible, yeah, she would have been down
0:26:25 > 0:26:28in the mountains here, where the Central Arctic herd
0:26:28 > 0:26:31is during the rut. The one thing we can say with some certainty
0:26:31 > 0:26:33is that it looks like she's going to calve with the Porcupine caribou,
0:26:33 > 0:26:35- not the Central Arctic. - She's back to her own herd.
0:26:35 > 0:26:37She's a Porcupine caribou.
0:26:37 > 0:26:41Exactly why she left is a mystery,
0:26:41 > 0:26:45but the herd are lucky to see the return of one of their older
0:26:45 > 0:26:48and more experienced females.
0:26:48 > 0:26:50She HAD been bringing up the rear.
0:26:50 > 0:26:54Now she's straight to the front and is helping to lead the entire herd.
0:27:00 > 0:27:03This is an incredibly important role,
0:27:03 > 0:27:06especially as this wilderness presents a new challenge
0:27:06 > 0:27:08at every turn.
0:27:10 > 0:27:14Whilst frozen, rivers are safe and easy to cross.
0:27:16 > 0:27:19But as the temperature rises,
0:27:19 > 0:27:22they can quickly turn into raging, deadly torrents.
0:27:38 > 0:27:40This is unbelievable.
0:27:40 > 0:27:45We were up at camp and heard the change in the noise of the river.
0:27:46 > 0:27:51Huge boulders of ice have broken free further upstream
0:27:51 > 0:27:53and are rushing past here.
0:27:55 > 0:27:57The river has gone up by at least a couple of metres.
0:28:00 > 0:28:03There is no way a caribou can get across this right now,
0:28:03 > 0:28:06but this is what they have to deal with.
0:28:09 > 0:28:11Last year, as the Porcupine River broke up,
0:28:11 > 0:28:14locals saw just how dangerous it can be.
0:28:18 > 0:28:21Caribou crossing the river were caught out as the ice broke,
0:28:21 > 0:28:24carrying them unstoppably downstream.
0:28:42 > 0:28:46With just ten days left to calving, our caribou on the western
0:28:46 > 0:28:49side of the river are forced to make a crucial decision.
0:29:00 > 0:29:03(The majority of the herd passed through the hills back there,
0:29:03 > 0:29:07(but there is a small group that has come to this dead-end.
0:29:07 > 0:29:09(There's a huge cliff face here
0:29:09 > 0:29:12(so there is no way they can pass on this side of the river.
0:29:12 > 0:29:13(So that lead female, I don't know
0:29:13 > 0:29:15(whether it's an inexperienced leader,
0:29:15 > 0:29:17(but now she's got to make a massive decision -
0:29:17 > 0:29:21(either they have got to swim across this river or head up this
0:29:21 > 0:29:24(really steep slope here, and we just saw a grizzly bear go up there,
0:29:24 > 0:29:27(so it's not looking good either way.)
0:29:28 > 0:29:31They are all going for it.
0:29:32 > 0:29:34A female enters the water.
0:29:34 > 0:29:36The others follow.
0:29:43 > 0:29:45That looks like the worst part of the river.
0:29:49 > 0:29:53The force of the meltwater has created a powerful set of rapids.
0:29:57 > 0:30:00The caribou plunge in, regardless.
0:30:03 > 0:30:05That rapid is really picking up.
0:30:05 > 0:30:07They're just disappearing under the water.
0:30:12 > 0:30:15The power of the water is simply too much.
0:30:31 > 0:30:34One, I saw disappear under the water - I don't know
0:30:34 > 0:30:36if he came back up.
0:30:38 > 0:30:41As the drama unfolds, I take to the air to join Max.
0:30:48 > 0:30:52I think they've finally seen sense and are doing a U-turn.
0:30:53 > 0:30:55It just goes to show the experience you need
0:30:55 > 0:30:58to navigate through these mountains.
0:30:59 > 0:31:03With nowhere left to turn, the caribou opt for the mountain route.
0:31:05 > 0:31:07Hey, Liz, we've got drama down here.
0:31:07 > 0:31:12We've just had a big group who were stuck in this dead-end valley.
0:31:12 > 0:31:13Half of them nearly got swept away,
0:31:13 > 0:31:16the other half wouldn't get in the water
0:31:16 > 0:31:19and have taken on this massive peak in front of us.
0:31:19 > 0:31:22OK, Max, there's a bear in the vicinity as well.
0:31:23 > 0:31:26'And it's not just the bear that spots an opportunity
0:31:26 > 0:31:28'as the caribou change course.'
0:31:29 > 0:31:33See it moving, there in the trees? See it moving about halfway up?
0:31:34 > 0:31:37Pete's just spotted a wolf on the hill.
0:31:37 > 0:31:40He seems to be going up into the left, as if he's checking out
0:31:40 > 0:31:43these guys, but he's gone quite a bit up that slope.
0:31:43 > 0:31:46We've got him, we've got the wolf. We've got the wolf.
0:31:47 > 0:31:51I mean, we knew there were going to be predators here,
0:31:51 > 0:31:54but I didn't quite imagine it would be so busy,
0:31:54 > 0:31:58and they're all looking to get their share of the caribou.
0:32:05 > 0:32:08The caribou now have no choice but to come back down to the river.
0:32:13 > 0:32:16And from here, there's only one option.
0:32:25 > 0:32:29Despite being further upstream, the current is still strong.
0:32:34 > 0:32:36The more experienced females lead the way.
0:32:45 > 0:32:50Suddenly a yearling starts drifting away from its mother.
0:32:56 > 0:32:58And soon even the adults are struggling.
0:33:15 > 0:33:18The yearling is out of its depth, separated from the herd
0:33:18 > 0:33:20and heading towards the rapids.
0:33:26 > 0:33:29But caribou fur is hollow, to insulate against the cold,
0:33:29 > 0:33:31which means the calf floats.
0:33:42 > 0:33:44There's no time to rest.
0:33:44 > 0:33:46The herd must keep moving
0:33:46 > 0:33:49if they are to reach the coastal plains before calving.
0:33:53 > 0:33:57The more we observe our caribou, the more apparent the challenges
0:33:57 > 0:33:59and dangers they face.
0:33:59 > 0:34:02Their survival hangs in the balance,
0:34:02 > 0:34:04every step of this astonishing journey.
0:34:17 > 0:34:20Although Porcupine herd numbers are currently stable,
0:34:20 > 0:34:25other herds have decreased by as much as 85% in just a few years.
0:34:29 > 0:34:33Our data, backed up by our observations on the ground,
0:34:33 > 0:34:36will contribute to the bigger picture of caribou survival
0:34:36 > 0:34:40across the Arctic and will hopefully provide clues
0:34:40 > 0:34:41to these sudden declines.
0:34:46 > 0:34:49Eight days until calving, and the relentless pace
0:34:49 > 0:34:53of the migration has now left Max and the ground team far behind.
0:34:55 > 0:34:57Max, it's Liz. Do you copy? Over.
0:34:57 > 0:34:59Roger. It's all gone quiet here.
0:34:59 > 0:35:02This is where the bulk of your herd have headed,
0:35:02 > 0:35:05not west along the Firth, as we first thought.
0:35:05 > 0:35:10There are thousands of caribou here, this is where you need to be.
0:35:12 > 0:35:18The caribou are already 25km ahead of Max's position.
0:35:18 > 0:35:19It's time to catch up.
0:35:20 > 0:35:24The plan is to relocate you there as soon as possible.
0:35:24 > 0:35:27Max's mission to keep up with the caribou on foot
0:35:27 > 0:35:28has proved impossible.
0:35:30 > 0:35:33The animals' extraordinary strength and stamina - and their
0:35:33 > 0:35:37relentless urge to push north to the coastal plains - has beaten him.
0:35:42 > 0:35:44There's only one way to catch up.
0:36:01 > 0:36:04The caribou have walked nearly 400km
0:36:04 > 0:36:08from their wintering grounds, and yet have barely eaten.
0:36:10 > 0:36:12Their reserves are running dangerously low.
0:36:18 > 0:36:22Number 88 is still with the other trailbreakers, leading the way.
0:36:24 > 0:36:27She and the others have only 50km to go,
0:36:27 > 0:36:31but they're each burning nearly 6,000 calories a day.
0:36:33 > 0:36:35They need to conserve as much energy as possible.
0:36:37 > 0:36:40You have to remember that these caribou are running on empty now.
0:36:40 > 0:36:42Of course, they are pregnant too,
0:36:42 > 0:36:45and they're running low on all the reserves they stored up
0:36:45 > 0:36:47the previous summer and autumn
0:36:47 > 0:36:50for this extraordinary migration to the coastal plain.
0:36:52 > 0:36:54What's more, the calves they carry
0:36:54 > 0:36:57are now making their greatest demands.
0:36:57 > 0:37:0180% of all foetal growth happens during this final stage.
0:37:03 > 0:37:06And if that wasn't enough, they must now cross
0:37:06 > 0:37:09a vast array of snow-filled plateaus.
0:37:13 > 0:37:17It takes seven times more energy to break a trail through deep snow
0:37:17 > 0:37:19than it does to walk on bare ground.
0:37:21 > 0:37:23But the caribou have an ingenious solution.
0:37:25 > 0:37:27They walk in single file.
0:37:28 > 0:37:33It really is fascinating how precisely they follow each other
0:37:33 > 0:37:36in a single file. It's all about
0:37:36 > 0:37:41conserving energy. Every calorie that you can save by not breaking
0:37:41 > 0:37:46your own trail, by following in the footsteps of another caribou,
0:37:46 > 0:37:49means you've got a bigger chance of succeeding at the other end.
0:37:51 > 0:37:54There's clear evidence that caribou have been walking like this,
0:37:54 > 0:37:58in these valleys, for over 12,000 years.
0:38:00 > 0:38:02Using the very same path for millennia,
0:38:02 > 0:38:07this ancient migration has etched its story on the land.
0:38:19 > 0:38:23Max and the ground team have been following the caribou
0:38:23 > 0:38:26relentlessly, and have hardly slept.
0:38:30 > 0:38:34But the pace is taking its toll on the caribou too.
0:38:34 > 0:38:36These caribou have been going all night.
0:38:36 > 0:38:39They're so focused on their destination at this point,
0:38:39 > 0:38:42they're so close to getting to the north slope,
0:38:42 > 0:38:44that they just keep going.
0:38:44 > 0:38:47Their drive is so strong, they're even prepared
0:38:47 > 0:38:51to leave behind their yearlings to get there on time.
0:38:53 > 0:38:56These guys don't get to sleep. If you look up there,
0:38:56 > 0:38:59you see the snow patch.
0:38:59 > 0:39:02A yearling just looks absolutely knackered, wiped out -
0:39:02 > 0:39:04he's just sort of given up.
0:39:04 > 0:39:07He's lying down, while the rest just moved on.
0:39:07 > 0:39:10I mean, it's just tough love, isn't it?
0:39:10 > 0:39:12Even though his mum is probably in that group,
0:39:12 > 0:39:15I mean, she's not waiting for him.
0:39:15 > 0:39:17It looks like he's given up.
0:39:19 > 0:39:22Oh, he's trying to get up. There's no way he's going to catch them.
0:39:22 > 0:39:25They are on a mission, and he's stumbling along.
0:39:26 > 0:39:30The pregnant females are a week away from giving birth at this point.
0:39:30 > 0:39:34You know, they've started this migration as early as January,
0:39:34 > 0:39:37so they've come a huge distance.
0:39:40 > 0:39:44Well over 5,000 caribou die during each spring migration,
0:39:44 > 0:39:48from sheer exhaustion or predator attacks.
0:39:48 > 0:39:51A bear can't catch a full-grown caribou
0:39:51 > 0:39:53unless they catch them by surprise,
0:39:53 > 0:39:56but that is what they are looking for - stragglers
0:39:56 > 0:40:00on their own. That is just easy pickings for a bear.
0:40:00 > 0:40:03Especially when the yearling's already exhausted.
0:40:10 > 0:40:14One last push, and the lead caribou are finally heading
0:40:14 > 0:40:17out of the mountains and onto the coastal plains.
0:40:20 > 0:40:24After nearly 500km, barely feeding,
0:40:24 > 0:40:27they've arrived just in time for the early growth
0:40:27 > 0:40:29of highly nutritious cotton grass.
0:40:30 > 0:40:32But even now they can't afford to relax.
0:40:35 > 0:40:40We've got a big grizzly bear just startled this group of caribou.
0:40:40 > 0:40:42I mean, they're quicker than him, but it looks like
0:40:42 > 0:40:44he's trying to ambush them,
0:40:44 > 0:40:48and we're just trying to get into position so we can see something.
0:40:53 > 0:40:58We saw him in these bushes. He startled them.
0:40:58 > 0:40:59There's caribou all around us.
0:40:59 > 0:41:03There is a group that are calm here, which hopefully suggest the bear's
0:41:03 > 0:41:08not just in front of us, but that group over there
0:41:08 > 0:41:11got completely spooked. They just ran for it.
0:41:11 > 0:41:14He's somewhere in front of us, probably about 100 metres.
0:41:15 > 0:41:19They are faster than any bear, but if he can play his cards right
0:41:19 > 0:41:24and catch them by surprise, then he'll have a tasty dinner.
0:41:24 > 0:41:27But it looks like they've got away this time.
0:41:31 > 0:41:36Do you see anything? There is nothing down by the river, there.
0:41:36 > 0:41:39I don't see anything, but the willows are thick down there.
0:41:49 > 0:41:50Look!
0:41:50 > 0:41:52He's big.
0:41:58 > 0:42:00These caribou haven't seen him at all.
0:42:02 > 0:42:07They are so fixated on just eating after this long journey,
0:42:07 > 0:42:09that it's like they haven't even seen him.
0:42:09 > 0:42:12He's just wandering up and they're not even responding.
0:42:17 > 0:42:22In such open landscape, the bear has little chance of catching a caribou.
0:42:23 > 0:42:26This leaves Max and Pete at risk.
0:42:26 > 0:42:29He's now closer to us than he is the caribou.
0:42:29 > 0:42:31What's the plan here, then, Pete?
0:42:33 > 0:42:36Staying calm is essential.
0:42:36 > 0:42:39Bear bangers are at hand if they need to startle the bear.
0:42:39 > 0:42:41And he's coming close now.
0:42:41 > 0:42:45And if it charges and gets too close, as a last resort,
0:42:45 > 0:42:46Pete would have to use his gun.
0:42:50 > 0:42:52He's charging.
0:42:52 > 0:42:53OK. Let's not run.
0:42:53 > 0:42:56Running can actually trigger an attack.
0:42:56 > 0:42:59This is not the best terrain for us, is it?
0:42:59 > 0:43:01We don't want to trip up now.
0:43:05 > 0:43:08That was a little bit too close for comfort.
0:43:08 > 0:43:11That bear was clearly going for the caribou,
0:43:11 > 0:43:16but they're a little quicker than us, and as they started to run off,
0:43:16 > 0:43:19he started to change his attentions towards us.
0:43:19 > 0:43:25He gave us a bluff charge, let us know who's boss -
0:43:25 > 0:43:26and that gets your heart rate going.
0:43:26 > 0:43:29They're so quick, they can be on you before you know it,
0:43:29 > 0:43:30if they want to be.
0:43:30 > 0:43:32Yeah, they can run as fast as a racehorse.
0:43:32 > 0:43:35Oh, you didn't tell me that before!
0:43:35 > 0:43:37- He's still coming. - Oh, he's still coming.
0:43:37 > 0:43:39He's coming with purpose now.
0:43:39 > 0:43:43The bear is coming towards us. Fire the chopper up quickly
0:43:43 > 0:43:46and get ready to pull us out of here.
0:43:48 > 0:43:50He's coming pretty quick now, guys.
0:43:50 > 0:43:52No running.
0:43:52 > 0:43:54- No.- OK.- OK.
0:43:59 > 0:44:01It's a close shave,
0:44:01 > 0:44:05and a powerful reminder of what the caribou are constantly up against.
0:44:13 > 0:44:16We're finding out so much more about the harsh realities
0:44:16 > 0:44:19of this epic migration - helped, of course,
0:44:19 > 0:44:21by the invaluable satellite data.
0:44:22 > 0:44:25The collars themselves form the basis for all of our sampling
0:44:25 > 0:44:27that we do. But even though we've been studying them
0:44:27 > 0:44:29for as long as we have - and they are
0:44:29 > 0:44:31one of the better-researched herds in the world...
0:44:31 > 0:44:34- And still there's so much you don't know yet.- Absolutely.
0:44:34 > 0:44:37And of all of the behaviours that you have observed,
0:44:37 > 0:44:40what has been the most surprising so far?
0:44:40 > 0:44:44I think the synchrony of movement is what blows me away.
0:44:44 > 0:44:47Here's a great example. This was back at the end of August
0:44:47 > 0:44:50last year, and we had caribou that were spread all across,
0:44:50 > 0:44:52all the way towards Aklavik,
0:44:52 > 0:44:57200km away from each other, and on the same day,
0:44:57 > 0:44:58it was like a switch went off -
0:44:58 > 0:45:01all those caribou started moving.
0:45:01 > 0:45:02And they all went
0:45:02 > 0:45:03to the exact same location.
0:45:03 > 0:45:05It was amazing -
0:45:05 > 0:45:07we're talking hundreds of thousands
0:45:07 > 0:45:08of caribou here.
0:45:09 > 0:45:12And they're all, on the same day, starting to move,
0:45:12 > 0:45:15and they're all going to the exact same location, and then they are
0:45:15 > 0:45:19all moving together as one pretty much cohesive group at that point.
0:45:21 > 0:45:23How did they know?
0:45:23 > 0:45:25Like, you're at one end of the range,
0:45:25 > 0:45:26I'm at the other end of the range.
0:45:26 > 0:45:29How do we both know that that's the day that we're going to move
0:45:29 > 0:45:31and we're all going to go to this one spot?
0:45:33 > 0:45:35Typically, this type of thing might have gone unnoticed in the past,
0:45:35 > 0:45:38that's one of the beautiful things about the satellite GPS collars,
0:45:38 > 0:45:42is that it actually allows us to see this as it occurs.
0:45:45 > 0:45:47The more we study this migration,
0:45:47 > 0:45:50the more we can reveal about these remarkable animals.
0:45:54 > 0:45:56With just five days left to calving,
0:45:56 > 0:45:59the herd is spreading out across the coastal plain.
0:46:05 > 0:46:08There are huge numbers that have already come down to these plains,
0:46:08 > 0:46:10and there's such a difference in landscape
0:46:10 > 0:46:14from those rugged hills to flat, open plains.
0:46:14 > 0:46:16The last leg of the journey.
0:46:16 > 0:46:18Yeah, that's right, home stretch.
0:46:20 > 0:46:22Somewhere in this vast landscape,
0:46:22 > 0:46:25the caribou will give birth.
0:46:25 > 0:46:27Despite the endless challenges and obstacles,
0:46:27 > 0:46:32their relentless pace and stamina have got them here on time.
0:46:33 > 0:46:37The caribou aren't due to calve here for another few days, but we don't
0:46:37 > 0:46:41want to risk distressing them, when they are at their most sensitive.
0:46:41 > 0:46:44Which means that this is Max's last chance to catch up with the herd
0:46:44 > 0:46:47before leaving them alone to give birth.
0:46:55 > 0:46:58Wow, it's so different here, isn't it?
0:47:01 > 0:47:08They seem so much more relaxed here, it's like they know they've made it.
0:47:10 > 0:47:11Look at them coming in now.
0:47:11 > 0:47:13They're curious, they'll do that.
0:47:15 > 0:47:17They're just checking us out here.
0:47:17 > 0:47:20They're just munching away on this grass.
0:47:20 > 0:47:23Yeah, like the little flower on top there.
0:47:23 > 0:47:26As you can see, it's new growth.
0:47:26 > 0:47:28Very critical to the caribou at this time of year.
0:47:28 > 0:47:31- So those fresh shoots have just come out.- Yes, those are fresh shoots.
0:47:31 > 0:47:35This ground is covered with cotton grass.
0:47:35 > 0:47:37So it's that that they've come all this way for.
0:47:37 > 0:47:40Right, yes, it's fresh stuff.
0:47:45 > 0:47:48In the two weeks after the snow melts,
0:47:48 > 0:47:50the cotton grass sprouts and flowers.
0:47:50 > 0:47:53This is when it's at its most nutritious.
0:47:54 > 0:47:58It's this narrow window that the caribou have raced here for.
0:48:01 > 0:48:04Not only can they now replenish their reserves,
0:48:04 > 0:48:07but scientists have shown that eating the grass at this
0:48:07 > 0:48:11specific time helps mothers produce a richer milk
0:48:11 > 0:48:15for their newborn calves, greatly improving their chances of survival.
0:48:19 > 0:48:24And it's earned this place a special name with the Inuvialuit.
0:48:24 > 0:48:28Ivvavik in our Inuvialuktun language means a place of birth,
0:48:28 > 0:48:29a nursery for the young.
0:48:31 > 0:48:36Now that we've followed the caribou from the southern part of the park
0:48:36 > 0:48:38to the northern part,
0:48:38 > 0:48:44now we leave the caribou in peace to have their young ones.
0:48:48 > 0:48:51We may have had to leave the caribou for a while,
0:48:51 > 0:48:55but the satellite data continues to tell their story as they move
0:48:55 > 0:48:57along the plains, into Alaska.
0:48:58 > 0:49:01The pregnant females finally stopped to give birth in the area
0:49:01 > 0:49:06with the most nutritious and abundant cotton grass on the plains.
0:49:06 > 0:49:08And at last, we can rejoin them.
0:49:10 > 0:49:13Here they are, here they are. It's so good to see them again.
0:49:15 > 0:49:17After all their efforts,
0:49:17 > 0:49:20I'm keen to know how successful this year's calving has been.
0:49:22 > 0:49:26I wonder if I'm setting eyes on any we saw
0:49:26 > 0:49:28at the beginning of the migration.
0:49:34 > 0:49:37Seeing them in such huge numbers -
0:49:37 > 0:49:42almost each adult accompanied by a calf - is just...
0:49:44 > 0:49:47Yeah, it's a little bit emotional, actually. It's so lovely to see.
0:49:49 > 0:49:53There must be thousands of caribou.
0:50:03 > 0:50:06This time, our base camp will be in the Alaskan part
0:50:06 > 0:50:08of the Porcupine caribou range.
0:50:11 > 0:50:15'I've been joined by Eric Wald of the US Fish and Wildlife Service,
0:50:15 > 0:50:18'who'll guide us through the area.'
0:50:18 > 0:50:22What direction and distance from our current location are they?
0:50:22 > 0:50:23'Being so remote,
0:50:23 > 0:50:27'we have to get the most recent data by satellite phone.'
0:50:27 > 0:50:30- What did he say? - Well, they're heading our way -
0:50:30 > 0:50:32about 17km to the northwest of us.
0:50:32 > 0:50:35- We saw them from the air as far north-west as there?- Yes.
0:50:36 > 0:50:40'Using this information, Max is moving into position.'
0:50:43 > 0:50:45There's maybe up to 200,000 caribou,
0:50:45 > 0:50:49and they're all about to pass directly in front of us.
0:50:51 > 0:50:54They're so quick, it's ridiculous.
0:50:58 > 0:51:00With new calves alongside,
0:51:00 > 0:51:03the females moult into their thinner summer coats.
0:51:07 > 0:51:11There's satellite collar number 25, he's right in front of us.
0:51:14 > 0:51:21I think back to when we saw these guys for the first time in Alaska
0:51:21 > 0:51:26and put those collars on, and they've gone so far since then.
0:51:26 > 0:51:30To think what that lady's been through - it's mind-blowing.
0:51:34 > 0:51:38Having survived an immense and arduous journey,
0:51:38 > 0:51:40our caribou got the timing just right.
0:51:43 > 0:51:47Over 70,000 pregnant females gave birth
0:51:47 > 0:51:49within five days of each other.
0:51:51 > 0:51:55That's one calf born every eight seconds,
0:51:55 > 0:51:58day and night, for five whole days.
0:51:59 > 0:52:03Nourished by the rich milk from the best cotton grass,
0:52:03 > 0:52:06the calves have doubled in size in just ten days.
0:52:08 > 0:52:10But they've got a lot to learn.
0:52:15 > 0:52:19There's a calf, right in the middle of the herd, calling.
0:52:19 > 0:52:22Clearly lost his mum.
0:52:24 > 0:52:29How you begin to find your mum when there's 200,000 caribou...
0:52:31 > 0:52:33She could well have already moved on.
0:52:36 > 0:52:38Ah, he's found her!
0:52:39 > 0:52:42You could see his relief, he suddenly must have heard his mum
0:52:42 > 0:52:48and he ran forward, straight to her side and tried to get some milk.
0:52:48 > 0:52:50And that's so good to see.
0:52:57 > 0:53:00Despite having only just calved,
0:53:00 > 0:53:04our satellite data shows that the herd are soon on the move again.
0:53:05 > 0:53:09- So May 25th, through to the 28th. - OK, that's when we left them.
0:53:09 > 0:53:12And when was the majority of the calving, then?
0:53:12 > 0:53:15Calving was from June 8th.
0:53:15 > 0:53:19They move very quickly away from the calving location, don't they?
0:53:19 > 0:53:21Yeah, after two, three days,
0:53:21 > 0:53:24the calves are able to keep up with their mom.
0:53:25 > 0:53:26'And keep up they must.'
0:53:29 > 0:53:35Soon, the whole herd comes together and can move as one -
0:53:35 > 0:53:38joined by another 70,000 males and youngsters.
0:53:44 > 0:53:48This time it's not bears or wolves that are driving them together.
0:53:52 > 0:53:54It's a creature a million times smaller
0:53:54 > 0:53:57but a billion times more numerous than the caribou.
0:54:01 > 0:54:02Mosquitoes.
0:54:04 > 0:54:09A single caribou can lose half a pint of blood a day to them.
0:54:12 > 0:54:15They have even been reported to die from asphyxiation
0:54:15 > 0:54:19as thousands of tiny mosquitoes clog up their airways.
0:54:21 > 0:54:25The relentless attack creates waves of panic through the herd.
0:54:34 > 0:54:37Walking constantly, into the wind, gives some relief.
0:54:41 > 0:54:44As does herding together as closely as possible.
0:54:47 > 0:54:51This gives us our best chance to see how they've fared this year,
0:54:51 > 0:54:53and how their numbers are holding up.
0:54:57 > 0:54:58It's good news.
0:55:00 > 0:55:04The Porcupine herd is now bigger than ever recorded.
0:55:06 > 0:55:08And it's an incredible sight.
0:55:37 > 0:55:41During our time here, the satellite data and our team
0:55:41 > 0:55:45on the ground have captured the critical decisions made
0:55:45 > 0:55:50by this herd - when to move, how fast to travel,
0:55:50 > 0:55:54which routes to take, and which rivers to cross.
0:55:54 > 0:55:57We've discovered the importance of experienced females,
0:55:57 > 0:56:02like number 88, leading the herd out of danger.
0:56:02 > 0:56:07And we've seen how sheer stamina, determination and ingenuity
0:56:07 > 0:56:11helped them to time this journey faultlessly.
0:56:11 > 0:56:14In a punishing wilderness where their lives constantly
0:56:14 > 0:56:17hang in the balance, and little else can survive,
0:56:17 > 0:56:21these caribou have adapted perfectly.
0:56:23 > 0:56:28This has been a journey of epic proportions,
0:56:28 > 0:56:34and what's struck me the most is just what caribou are capable of.
0:56:35 > 0:56:38Not only do they carry out the longest land migration
0:56:38 > 0:56:39in all of the animal kingdom,
0:56:39 > 0:56:42but they have found ways to survive
0:56:42 > 0:56:47countless threats and obstacles that are thrown at them at every turn.
0:56:47 > 0:56:52And next year the new calves will have to learn just what it takes
0:56:52 > 0:56:54to make it out here.
0:56:54 > 0:56:56Their mothers will pass down all of their knowledge
0:56:56 > 0:56:58and all of their experience
0:56:58 > 0:57:03as the race to the calving grounds begins all over again.
0:57:15 > 0:57:17Next time, in a world first,
0:57:17 > 0:57:21we join a newly discovered animal migration.
0:57:21 > 0:57:24Thousands of zebra migrate hundreds of miles across
0:57:24 > 0:57:27one of Africa's harshest scrubland.
0:57:27 > 0:57:29Failure could end in disaster.