0:00:04 > 0:00:08Every year, spectacular seasons transform our planet.
0:00:11 > 0:00:14They're the driving force of all life on Earth...
0:00:16 > 0:00:20..bringing opportunities and huge challenges.
0:00:23 > 0:00:25Spring bursts with new life.
0:00:27 > 0:00:30But it's a race to grab fleeting chances.
0:00:32 > 0:00:36Summer is all about glorious abundance,
0:00:36 > 0:00:38but the heat can push animals to their limits.
0:00:41 > 0:00:43Autumn is nature's great gold rush.
0:00:47 > 0:00:49But competition is fierce.
0:00:52 > 0:00:55Winter creates a frozen wonderland...
0:00:57 > 0:00:59..and only the most resourceful will survive.
0:01:08 > 0:01:10In every corner of the planet,
0:01:10 > 0:01:15animals rise to overcome the seemingly impossible,
0:01:15 > 0:01:18and use extraordinary tricks
0:01:18 > 0:01:20to thrive against the odds...
0:01:21 > 0:01:25..as the seasons create the greatest shows on Earth.
0:01:37 > 0:01:39Winter.
0:01:39 > 0:01:41The sun's power is weakening.
0:01:41 > 0:01:44It's getting colder and darker.
0:01:46 > 0:01:49The land is going into suspended animation.
0:01:51 > 0:01:56In this magical world, animals are faced with formidable challenges.
0:01:57 > 0:02:02And when temperatures plummet, the first big challenge
0:02:02 > 0:02:03is keeping out the cold.
0:02:13 > 0:02:16This extreme season needs inventive solutions.
0:02:17 > 0:02:18In the Highlands of Scotland...
0:02:20 > 0:02:23..a ptarmigan has to cope with temperatures
0:02:23 > 0:02:25approaching -30 degrees centigrade,
0:02:25 > 0:02:28so it grows more feathers to keep the heat in.
0:02:29 > 0:02:31It even has feathers on its feet...
0:02:33 > 0:02:34..and its eyelids.
0:02:41 > 0:02:43In North America,
0:02:43 > 0:02:44a red fox keeps warm
0:02:44 > 0:02:48by wrapping himself up in his extra-thick tail.
0:02:55 > 0:02:57Arctic foxes grow one of the thickest coats
0:02:57 > 0:02:59in the animal kingdom.
0:03:02 > 0:03:05Her tiny ears and stubby legs reduce heat loss...
0:03:08 > 0:03:09..and even the pads of her feet
0:03:09 > 0:03:11have a covering of fur.
0:03:14 > 0:03:16Even when it drops to -70...
0:03:17 > 0:03:19..she's quite comfortable.
0:03:23 > 0:03:25But other animals have more inventive ways
0:03:25 > 0:03:27to stave off the cold.
0:03:29 > 0:03:31Finland, on the Arctic Circle.
0:03:32 > 0:03:35Temperatures here can fall to -45 degrees centigrade.
0:03:37 > 0:03:41The snow is so heavy, it can bend the sturdiest of trees.
0:03:47 > 0:03:52But among the drifts lives one of the world's smallest predators,
0:03:52 > 0:03:55with a ruthless approach to keeping warm.
0:04:04 > 0:04:06A least weasel.
0:04:06 > 0:04:09She's tiny, barely 20cm long.
0:04:10 > 0:04:12She's grown a new white coat for winter.
0:04:15 > 0:04:18It keeps her almost invisible in the snow.
0:04:20 > 0:04:22Great for hiding from predators,
0:04:22 > 0:04:25but it's not thick enough to protect her from the cold.
0:04:27 > 0:04:30Even shivering can't generate enough heat.
0:04:33 > 0:04:36But this ingenious weasel has a plan.
0:04:45 > 0:04:47She starts by diving under the snow.
0:04:52 > 0:04:56Snow is 95% trapped air,
0:04:56 > 0:04:58so it keeps the heat in, like a giant duvet.
0:05:07 > 0:05:09There's a network of tunnels...
0:05:11 > 0:05:12..but they're not hers.
0:05:14 > 0:05:16They belong to voles.
0:05:20 > 0:05:23Voles spend the winter in their snow tunnels,
0:05:23 > 0:05:25where they think they're safe.
0:05:30 > 0:05:33But with her skinny, flexible body,
0:05:33 > 0:05:35the weasel can easily navigate the maze.
0:05:40 > 0:05:42There's nowhere for a vole to hide.
0:05:58 > 0:05:59Spotted.
0:06:17 > 0:06:18The vole is fast.
0:06:22 > 0:06:24But the weasel is faster.
0:06:27 > 0:06:30But there's more to this than just a meal.
0:06:32 > 0:06:34Once the weasel has eaten the hapless vole,
0:06:34 > 0:06:36she delicately plucks its hair.
0:06:41 > 0:06:45Then she weaves it into her nest, building herself a furry cocoon.
0:06:47 > 0:06:50Bed...and breakfast...
0:06:52 > 0:06:54..in one convenient package.
0:07:06 > 0:07:10Many animals must rely on their wits to survive the bitter cold.
0:07:13 > 0:07:17In the mountains of central Japan, winters can be brutal...
0:07:19 > 0:07:22..with some of the deepest falls of snow in the world.
0:07:24 > 0:07:27But there's a surprising animal eking out a living here.
0:07:30 > 0:07:31A monkey.
0:07:33 > 0:07:35Aside from people,
0:07:35 > 0:07:37these Japanese macaques live further north
0:07:37 > 0:07:40than any other primate on Earth.
0:07:42 > 0:07:45Their coats are unusually thick to protect them
0:07:45 > 0:07:46from the freezing temperatures.
0:07:49 > 0:07:51Even when it's -20 degrees centigrade,
0:07:51 > 0:07:53a mum and her baby can cope.
0:08:01 > 0:08:03But as the temperatures fall,
0:08:03 > 0:08:06the monkeys need to find extra calories to stay warm.
0:08:10 > 0:08:13These are some of the cleverest monkeys on the planet,
0:08:13 > 0:08:17and they are so resourceful, they'll never go hungry,
0:08:17 > 0:08:19no matter how cold it gets.
0:08:23 > 0:08:25They can eat almost anything.
0:08:25 > 0:08:28And, at this time of year, nothing goes to waste.
0:08:31 > 0:08:35As a last resort, when all other food is gone,
0:08:35 > 0:08:36they'll even eat the bark of trees.
0:08:38 > 0:08:39This is smarter than it seems.
0:08:41 > 0:08:43Just beneath the woody layer is the sap wood,
0:08:43 > 0:08:46where the tree stores its precious nutrients.
0:08:47 > 0:08:50It gives them just enough calories to keep them going.
0:09:00 > 0:09:03But this is not their only clever trick to get them through winter.
0:09:03 > 0:09:05There's warmth to be found here.
0:09:08 > 0:09:11Japan is a land of volcanoes.
0:09:15 > 0:09:17Below the ground,
0:09:17 > 0:09:19this volcanic activity brings hot water
0:09:19 > 0:09:21bubbling to the surface.
0:09:24 > 0:09:28Even in the depths of winter, it's a thermal spring,
0:09:28 > 0:09:31and these ingenious monkeys have learned how to make the most of it.
0:09:34 > 0:09:36Snow monkeys have become
0:09:36 > 0:09:38spa monkeys.
0:09:42 > 0:09:46In their bath, it's a comfortable 41 degrees centigrade.
0:09:49 > 0:09:52With constant warm water to bask in,
0:09:52 > 0:09:55these monkeys live a life of leisure.
0:10:03 > 0:10:06And the youngsters have plenty of time to play.
0:10:22 > 0:10:26Thanks to their resourcefulness, these clever monkeys will survive,
0:10:26 > 0:10:29no matter what winter throws at them.
0:10:38 > 0:10:41Some overcome the cold with their wits,
0:10:41 > 0:10:44but others survive through sheer brute force.
0:10:49 > 0:10:50Bison.
0:10:50 > 0:10:53They've lived just like this since the last Ice Age.
0:10:56 > 0:10:58They're throwbacks from a time
0:10:58 > 0:11:00when the world was much, much colder.
0:11:04 > 0:11:07They can live through some of North America's most bitter winters.
0:11:08 > 0:11:12Temperatures here in Yellowstone can drop below -50 centigrade.
0:11:16 > 0:11:21They use their giant heads and necks like a plough to sweep the snow away
0:11:21 > 0:11:22and reach the grass beneath.
0:11:25 > 0:11:29And they'll just carry on grazing, regardless of the weather.
0:11:40 > 0:11:42Thanks to his Ice Age ancestors,
0:11:42 > 0:11:45this bison is perfectly evolved to deal with the cold.
0:11:48 > 0:11:53His body is huge, weighing in at nearly a tonne,
0:11:53 > 0:11:55and the bigger and rounder he is,
0:11:55 > 0:11:57the better his body retains the heat...
0:11:59 > 0:12:03..meaning he can maintain his core temperature with minimal effort.
0:12:06 > 0:12:10And his thick, double-layered fur coat keeps the heat in.
0:12:14 > 0:12:17Next to his skin, he has a dense, woolly undercoat,
0:12:17 > 0:12:21with more than 2,000 hairs per square centimetre.
0:12:23 > 0:12:28His hair is so insulating that even when snow falls on his back,
0:12:28 > 0:12:29it doesn't melt.
0:12:35 > 0:12:38Bison can keep warm down to -30 without even trying.
0:12:40 > 0:12:41Only when it drops below this
0:12:41 > 0:12:44will they need to use extra energy to keep warm.
0:12:50 > 0:12:53These hard-core grazers stay all winter
0:12:53 > 0:12:55when others are forced to leave.
0:12:56 > 0:12:59So they have the meadows all to themselves.
0:13:07 > 0:13:10But when it comes to battling the elements,
0:13:10 > 0:13:12this is the ultimate place to do it.
0:13:14 > 0:13:15Antarctica.
0:13:17 > 0:13:19The coldest place on earth.
0:13:24 > 0:13:28Temperatures can plummet to -94 degrees centigrade.
0:13:30 > 0:13:33You have to be a true specialist to survive a winter here.
0:13:38 > 0:13:40Emperor penguins.
0:13:40 > 0:13:42At over a metre tall,
0:13:42 > 0:13:44they're the largest penguins in the world.
0:13:49 > 0:13:51They've been out at sea, fishing...
0:13:52 > 0:13:56..building up an insulating layer of fat, nearly 3cm thick.
0:14:05 > 0:14:08And now they're about to face the toughest winter...
0:14:09 > 0:14:11..on Earth.
0:14:18 > 0:14:21Because penguins take so long to raise their chicks,
0:14:21 > 0:14:23they must start early.
0:14:28 > 0:14:32And that means incubating their eggs through the planet's deepest winter.
0:14:35 > 0:14:38When she's laid her single egg, the female must return
0:14:38 > 0:14:39to the sea to feed.
0:14:42 > 0:14:45So it's over to the father to incubate it...
0:14:47 > 0:14:48..all through winter.
0:14:52 > 0:14:56He protects it in a little pocket of skin just above his feet.
0:14:59 > 0:15:02He is now responsible for keeping it safe and warm
0:15:02 > 0:15:04for nearly four months...
0:15:06 > 0:15:09..just as the world's coldest weather is closing in.
0:15:11 > 0:15:15Antarctica is entering six months of darkness.
0:15:16 > 0:15:18Winter here is merciless.
0:15:20 > 0:15:23The winds can top 120mph.
0:15:30 > 0:15:34These are the darkest, coldest, windiest conditions
0:15:34 > 0:15:36faced by any creature on earth.
0:15:42 > 0:15:45But through it all, the emperor penguin dad battles on.
0:15:45 > 0:15:50He's still incubating his precious egg,
0:15:50 > 0:15:53perched on his feet, just above the ice.
0:15:55 > 0:15:58He and all the other fathers have found an inventive way
0:15:58 > 0:16:01to make it through when times get really bad.
0:16:14 > 0:16:18They come together for the world's biggest huddle.
0:16:37 > 0:16:39Penguins don't normally enjoy being this close...
0:16:41 > 0:16:43..but they have no choice.
0:16:48 > 0:16:53By crowding together as a team, they can cut heat loss by half.
0:16:55 > 0:16:57After his stint on the edge of the flock,
0:16:57 > 0:17:00each penguin patiently queues for his turn on the inside.
0:17:02 > 0:17:06It's recently been discovered that their huddle is so effective,
0:17:06 > 0:17:11the heat inside can build up to around 40 degrees centigrade.
0:17:11 > 0:17:15That's nearly 80 degrees warmer than the temperature on the outside.
0:17:15 > 0:17:19So every now and then, they must break formation
0:17:19 > 0:17:21to cool down.
0:17:26 > 0:17:29Throughout the male penguins' four-month tour of duty,
0:17:29 > 0:17:32the sun never rises once.
0:17:37 > 0:17:41But just occasionally, there's a glimmer in the darkness.
0:17:45 > 0:17:48Charged particles from the sun crash into the atmosphere
0:17:48 > 0:17:49above the South Pole...
0:17:56 > 0:17:58..and blaze across the sky.
0:17:59 > 0:18:01It's the Southern Lights.
0:18:03 > 0:18:06The penguins barely seem to notice,
0:18:06 > 0:18:10but in a brutal season, it's a moment of magic.
0:18:15 > 0:18:18Winter is full of spectacle.
0:18:19 > 0:18:23When the temperature drops below -6 centigrade,
0:18:23 > 0:18:26water vapour in the air turns directly to ice.
0:18:27 > 0:18:32These microscopic crystals attract more and more water droplets...
0:18:33 > 0:18:36..and they grow into snowflakes.
0:18:39 > 0:18:41In just one second,
0:18:41 > 0:18:43a million billion are made by the clouds.
0:18:45 > 0:18:50In ten minutes, that's enough snow for everyone on the planet
0:18:50 > 0:18:51to build their own snowman.
0:18:54 > 0:18:59Each flake is tiny, but together, they can blanket the landscape...
0:19:01 > 0:19:03..turning it into a wonderland.
0:19:11 > 0:19:14It's a complete transformation,
0:19:14 > 0:19:17and even the animals seem to enjoy the novelty.
0:19:48 > 0:19:50But for some, there is a problem.
0:19:52 > 0:19:55A thick blanket of snow will hide their food.
0:20:02 > 0:20:04So if they're going to find anything to eat,
0:20:04 > 0:20:07they have to come up with some new tactics.
0:20:14 > 0:20:19In North America, a red fox relies on its ears
0:20:19 > 0:20:23to listen for mice scuttling around out of sight.
0:20:29 > 0:20:32In China, pandas have come up with their own special technique
0:20:32 > 0:20:35for clearing snow off a bamboo branch.
0:20:37 > 0:20:39Before they eat it, they spin it round their muzzles
0:20:39 > 0:20:41to brush it clean.
0:20:47 > 0:20:50But some have switched their diet altogether.
0:20:54 > 0:20:56In the mountains of south-west China,
0:20:56 > 0:21:00where winter snow lies a metre deep,
0:21:00 > 0:21:04the Yunnan snub-nosed monkey eats the lichen that drapes the trees.
0:21:07 > 0:21:10The lichen can survive the freezing cold
0:21:10 > 0:21:12of these high mountains.
0:21:14 > 0:21:16This surprisingly nutritious food
0:21:16 > 0:21:21means the monkeys can stay in these mountains all year round.
0:21:32 > 0:21:34But in some parts of the world,
0:21:34 > 0:21:37the heavy snows of winter create an extreme challenge.
0:21:39 > 0:21:42And survival takes a bit of cunning.
0:21:48 > 0:21:50In Yellowstone in North America,
0:21:50 > 0:21:55an otter family has woken up to discover their home river is buried.
0:21:59 > 0:22:02They don't have anywhere to fish.
0:22:02 > 0:22:05They're going to have to relocate for winter.
0:22:11 > 0:22:15In the same neighbourhood, a coyote is having similar difficulties.
0:22:16 > 0:22:19Finding a meal round here isn't going to be easy.
0:22:35 > 0:22:37The otters have struck lucky.
0:22:38 > 0:22:41There's an opening in an icy lake...
0:22:44 > 0:22:46..and there are still fish here.
0:22:49 > 0:22:50It's fine for them...
0:22:52 > 0:22:55..but for the coyote, life is trickier.
0:22:57 > 0:22:59He's no fisherman,
0:22:59 > 0:23:02but he is a master thief.
0:23:05 > 0:23:07The otters are suspicious.
0:23:07 > 0:23:11The coyote would steal their fish in an instant,
0:23:11 > 0:23:13so they look for somewhere to hide it.
0:23:16 > 0:23:19The coyote might not be able to see the otter
0:23:19 > 0:23:20but he can hear him.
0:23:25 > 0:23:27He knows there's a fish here somewhere.
0:23:28 > 0:23:31Stashing it under the snow won't fool a coyote.
0:23:35 > 0:23:37He can smell it.
0:23:57 > 0:23:59Bingo.
0:23:59 > 0:24:02They're not called wily for nothing.
0:24:09 > 0:24:11As snow changes their world,
0:24:11 > 0:24:14animals need to use every trick in the book to find food.
0:24:19 > 0:24:21But there is one animal that uses it to gain an advantage.
0:24:26 > 0:24:27Wolves.
0:24:29 > 0:24:32These Canadian wolves are some of the biggest in the world.
0:24:35 > 0:24:39They are opportunists and they'll travel long distances
0:24:39 > 0:24:41in the search for food.
0:24:44 > 0:24:47Deep snow usually makes moving difficult,
0:24:47 > 0:24:51but sometimes, it creates an opportunity.
0:24:54 > 0:24:56This wolf pair is looking for a bison.
0:25:01 > 0:25:03But they're hard to catch.
0:25:20 > 0:25:23And finding them in this huge wilderness is tough.
0:25:26 > 0:25:29This is where the heavy snowfall offers a clue.
0:25:36 > 0:25:40The bison have left deep tracks -
0:25:40 > 0:25:42tracks the wolves can follow.
0:25:47 > 0:25:50These two are working alone, without the help of a pack.
0:26:02 > 0:26:04And they're after very dangerous prey.
0:26:06 > 0:26:10Bison have sharp horns and weigh a tonne,
0:26:10 > 0:26:12ten times the weight of a wolf.
0:26:14 > 0:26:16But the wolves have no choice.
0:26:17 > 0:26:20A bison would keep them fed for a week,
0:26:20 > 0:26:23and in winter, you have to take a chance.
0:26:36 > 0:26:39Finally, they pick up fresh scent.
0:26:39 > 0:26:41The herd must be close.
0:26:41 > 0:26:44The hunt is on.
0:27:18 > 0:27:19The wolves move up a gear.
0:27:21 > 0:27:23They can maintain this speed for miles.
0:27:27 > 0:27:28The bison are tiring.
0:27:34 > 0:27:36The weaker ones fall back...
0:27:40 > 0:27:42..and the wolf couple are getting closer.
0:27:48 > 0:27:50A trip is all it takes.
0:27:55 > 0:27:56Thanks to the snow...
0:27:57 > 0:28:00..the wolves are now a step ahead of starvation.
0:28:04 > 0:28:07And the rest of the herd live to fight another winter battle.
0:28:14 > 0:28:16Dealing with the challenge of deep snow
0:28:16 > 0:28:19forces animals to raise their game.
0:28:19 > 0:28:21But winter has another weapon in its arsenal.
0:28:27 > 0:28:30As days become shorter than nights,
0:28:30 > 0:28:32the temperature on the ground begins to drop.
0:28:36 > 0:28:39At zero degrees centigrade, water turns to ice.
0:28:49 > 0:28:53Little by little, whole worlds transform.
0:29:04 > 0:29:08At the poles, even the sea freezes.
0:29:12 > 0:29:16To survive the ice, you need to be resourceful.
0:29:26 > 0:29:29In North America, a freezing river is a challenging place
0:29:29 > 0:29:31for a small bird.
0:29:31 > 0:29:35But this tough little dipper has extra-thick waterproof feathers.
0:29:36 > 0:29:38It will happily dive right under the ice
0:29:38 > 0:29:41if it thinks there's something worth eating.
0:29:48 > 0:29:50In a frozen Russian river...
0:29:50 > 0:29:56sockeye salmon creep under a ceiling of ice to lay their eggs.
0:29:57 > 0:30:00It might look crowded, but here,
0:30:00 > 0:30:03they're safe from attack by birds of prey.
0:30:10 > 0:30:12In northern Canada,
0:30:12 > 0:30:15the winter freeze has created a surprising opportunity.
0:30:17 > 0:30:20This is a land of lakes and bogs...
0:30:21 > 0:30:27..but in winter, it's encased in a sheet of ice two metres thick.
0:30:33 > 0:30:36And there is one animal that has managed to turn this
0:30:36 > 0:30:37to its advantage.
0:30:42 > 0:30:43Caribou.
0:30:48 > 0:30:51Food is hard to come by,
0:30:51 > 0:30:54so they must spend their winter constantly on the move.
0:30:57 > 0:31:02Despite their warm coats, deep snow is a struggle for them.
0:31:02 > 0:31:04Trudging through it saps their energy.
0:31:09 > 0:31:11But the caribou have discovered
0:31:11 > 0:31:13that snow doesn't build up on the ice
0:31:13 > 0:31:16because powerful winds blow it away.
0:31:17 > 0:31:24So now, the lakes and rivers become nature's very own ice roads.
0:31:24 > 0:31:26It means they can travel more quickly,
0:31:26 > 0:31:28saving precious energy.
0:31:31 > 0:31:33On the other side of the world...
0:31:35 > 0:31:37..winter ice is also bringing opportunities for some.
0:31:44 > 0:31:45Siberia.
0:31:48 > 0:31:52For seven months of winter, this is a bitter place.
0:31:53 > 0:31:57Temperatures can drop to -67 degrees centigrade.
0:31:59 > 0:32:01It's enough to push some animals to the edge.
0:32:06 > 0:32:07A sika deer.
0:32:11 > 0:32:15When the trees are bare, there's very little for them to eat.
0:32:18 > 0:32:22The weaker ones often don't make it,
0:32:22 > 0:32:26But these casualties are just what another animal is looking for.
0:32:27 > 0:32:29An Amur leopard,
0:32:29 > 0:32:32the world's rarest big cat.
0:32:34 > 0:32:38She is a powerful predator, but in winter,
0:32:38 > 0:32:40she is not above a little scavenging.
0:32:45 > 0:32:49This frozen carcass will keep for a week,
0:32:49 > 0:32:50so it is a real lifesaver.
0:32:52 > 0:32:54But it is not just for her.
0:32:55 > 0:32:57She has a cub.
0:32:58 > 0:33:00Less than a year old -
0:33:00 > 0:33:01and he's hungry.
0:33:23 > 0:33:26By making the most of this frozen food,
0:33:26 > 0:33:29she has also taught her cub a valuable lesson.
0:33:33 > 0:33:35In the depths of winter,
0:33:35 > 0:33:39even predators don't waste energy hunting if they don't have to.
0:33:50 > 0:33:55In Antarctica, the freeze-up has come in spectacular style.
0:33:57 > 0:34:01In winter the continent doubles in size.
0:34:04 > 0:34:06It's now bigger than the USA.
0:34:14 > 0:34:16A Weddell seal.
0:34:16 > 0:34:18SEAL BARKS
0:34:22 > 0:34:25At least she has a retreat from the icy world above.
0:34:30 > 0:34:34The sea down here is a constant -2 degrees centigrade.
0:34:34 > 0:34:38That is over 50 degrees warmer than it is on the surface.
0:34:50 > 0:34:53And it has been like this for 25 million years.
0:34:56 > 0:34:57Beneath the layer of ice,
0:34:57 > 0:35:02this enchanted kingdom is protected from the extreme temperatures above.
0:35:05 > 0:35:09It's a wonderland of the strange and beautiful.
0:35:25 > 0:35:28In this safe, stable world,
0:35:28 > 0:35:30life explodes.
0:35:39 > 0:35:43But even here, winter cold can crash the party.
0:35:49 > 0:35:52As temperatures plummet on the surface,
0:35:52 > 0:35:54it triggers the growth of something sinister.
0:36:01 > 0:36:04This is a brinicle -
0:36:04 > 0:36:07super-cooled water sinking from above,
0:36:07 > 0:36:09forming a pillar of ice.
0:36:23 > 0:36:24For the creatures of the seabed,
0:36:24 > 0:36:27it is a stealthy finger of death.
0:36:50 > 0:36:54A river of ice imprisons everything in its path.
0:37:13 > 0:37:16Even those that think they have beaten the extremes of winter
0:37:16 > 0:37:18can get caught out.
0:37:25 > 0:37:30As winter takes hold, there are even greater tribulations coming.
0:37:31 > 0:37:35Now animals not only have to battle the conditions,
0:37:35 > 0:37:37they also have to battle each other.
0:37:37 > 0:37:43For some, it is over food. For others, it is over mates.
0:37:43 > 0:37:46But winter raises the stakes on everything.
0:37:56 > 0:38:01In Russia, black vultures - the world's largest birds of prey -
0:38:01 > 0:38:03fight over scarce scraps in the snow.
0:38:06 > 0:38:10With their three-metre wingspans, it's all about intimidation.
0:38:14 > 0:38:20In Canada, winter is the time for young male polar bears to tussle,
0:38:20 > 0:38:24sorting out their status without doing each other serious damage.
0:38:27 > 0:38:30But it's in South Georgia in the South Atlantic
0:38:30 > 0:38:33where the biggest winter brawls happen.
0:38:35 > 0:38:37Weighing in at four tonnes,
0:38:37 > 0:38:40these elephant seals are the world's largest.
0:38:41 > 0:38:45It's a battle of the big boys to establish who's in charge.
0:38:53 > 0:38:54But there is one place
0:38:54 > 0:38:57where winter hardships force even the weakest
0:38:57 > 0:39:00to step up and be bold.
0:39:03 > 0:39:05Alaska.
0:39:05 > 0:39:07The Chilkat River.
0:39:07 > 0:39:09While all other rivers are frozen,
0:39:09 > 0:39:14a freak of geography keeps one short stretch of the Chilkat ice-free.
0:39:17 > 0:39:21It attracts the world's biggest gathering of bald eagles.
0:39:29 > 0:39:30And they are here...
0:39:32 > 0:39:33..for salmon.
0:39:38 > 0:39:43Over 3,000 eagles are gathering in the trees.
0:39:43 > 0:39:46They might have travelled 1,000 miles for this feast
0:39:46 > 0:39:48and they are ready to fight for it.
0:39:53 > 0:39:56With more eagles than fishing opportunities,
0:39:56 > 0:39:59the challenge here is keeping hold of your catch.
0:40:07 > 0:40:09As the salmon begin to run out...
0:40:11 > 0:40:12..tempers boil over.
0:41:00 > 0:41:03This youngster is right at the bottom of the pecking order.
0:41:05 > 0:41:07He just can't catch a break.
0:41:18 > 0:41:21Even when he manages to catch a fish,
0:41:21 > 0:41:23he stands no chance of keeping it.
0:41:29 > 0:41:32It only takes one adult to come close
0:41:32 > 0:41:34and he loses concentration.
0:41:35 > 0:41:36There goes his meal.
0:41:39 > 0:41:42To survive, he is going to have to play by their rules...
0:41:44 > 0:41:47..and stand up to his tormentors.
0:41:54 > 0:41:57With bald eagles, attitude is everything.
0:41:59 > 0:42:03Hunger drives him to do the unthinkable and fight back.
0:42:12 > 0:42:14It works.
0:42:34 > 0:42:36He has earned his right to fish this river.
0:42:36 > 0:42:39He'll make it through this winter just fine.
0:42:49 > 0:42:51On the Canadian tundra,
0:42:51 > 0:42:53there is another battle looming.
0:42:56 > 0:42:58And it is winter's ultimate heavyweight fight.
0:43:04 > 0:43:06Female caribou are ready to mate.
0:43:07 > 0:43:12It needs to be now so their young will be born in early spring.
0:43:15 > 0:43:18So the massive caribou bulls must compete with each other
0:43:18 > 0:43:20to father the calves.
0:43:23 > 0:43:25The younger males watch from the sidelines
0:43:25 > 0:43:27as the heavyweights eye each other up.
0:43:42 > 0:43:44Serious fights are rare
0:43:44 > 0:43:47but when they happen, they can be very dangerous.
0:43:51 > 0:43:54But while this old bull has been dealing with a rival,
0:43:54 > 0:43:58the young bucks have been trying their luck with his female.
0:44:03 > 0:44:06He must break off his fight and chase them away.
0:44:20 > 0:44:23The younger males may be lighter on their feet...
0:44:25 > 0:44:28..but his bulk means he can muscle his way past.
0:44:33 > 0:44:36Another rival steps in with a challenge.
0:44:41 > 0:44:44But the old bull easily deals with him.
0:44:55 > 0:45:00As the chase slows down, he can finally impose some order.
0:45:04 > 0:45:08Now he's shown who is boss, a look is all that is needed.
0:45:18 > 0:45:22Only when he is sure he has made his point can he claim his female.
0:45:29 > 0:45:33The caribou may have a bruising battle for their chosen females...
0:45:37 > 0:45:40..but for other animals, competing for the right to mate
0:45:40 > 0:45:42is even more challenging.
0:45:48 > 0:45:53In a forest in Finland, a black grouse is preparing
0:45:53 > 0:45:54to put on a show.
0:46:09 > 0:46:11It's the most important performance of his life.
0:46:14 > 0:46:17It will determine whether or not he gets to breed.
0:46:29 > 0:46:33Only one male will get the chance to mate this year
0:46:33 > 0:46:36and that male will have all the females.
0:46:37 > 0:46:39The others will get nothing.
0:46:41 > 0:46:45With such high stakes, the competition is intense.
0:47:03 > 0:47:08The females begin to arrive and the dance-off moves up a gear.
0:47:17 > 0:47:19More and more males join in.
0:47:27 > 0:47:30These birds can wait years for an opportunity to mate.
0:47:30 > 0:47:32Some will never get the chance at all.
0:47:37 > 0:47:40The females seem to have spotted a potential winner.
0:47:44 > 0:47:45It is half dance...
0:47:46 > 0:47:48..half handbags at dawn.
0:47:56 > 0:47:59One last lunge is all he needs to win the bout.
0:48:06 > 0:48:08It's all over.
0:48:08 > 0:48:09The loser slips away.
0:48:15 > 0:48:17The best performer is the strongest male...
0:48:19 > 0:48:21..and that's just what the females are after.
0:48:23 > 0:48:26His strength will be passed to their chicks,
0:48:26 > 0:48:29giving them the very best start in life.
0:48:36 > 0:48:38As the battles of winter are settled,
0:48:38 > 0:48:42there are tantalising signs that the season is changing.
0:48:48 > 0:48:53For some animals, it's their one chance to meet a partner.
0:49:03 > 0:49:07A male polar bear has been wandering the Arctic ice alone
0:49:07 > 0:49:09all through the dark winter.
0:49:10 > 0:49:14But as the sun returns, it triggers his desire to find a mate.
0:49:17 > 0:49:19It's not an easy search.
0:49:22 > 0:49:24In these vast wastes,
0:49:24 > 0:49:27potential dates are few and far between.
0:49:35 > 0:49:39Eventually, he picks up an unmistakable scent.
0:49:42 > 0:49:43A female.
0:49:45 > 0:49:47She could be ten miles away...
0:49:50 > 0:49:52..so he follows her tracks.
0:50:10 > 0:50:12There she is.
0:50:15 > 0:50:17She is half his size,
0:50:17 > 0:50:20so first-date nerves are understandable.
0:50:29 > 0:50:31He needs to win her over gently.
0:50:42 > 0:50:44He waits for her to make the first move.
0:50:55 > 0:50:57She leads him off to higher ground.
0:51:11 > 0:51:13Their relationship is brief.
0:51:15 > 0:51:16And when they part company...
0:51:19 > 0:51:21..they will probably never meet again.
0:51:26 > 0:51:30The wastes of the Arctic might seem a tricky place to find a mate,
0:51:30 > 0:51:33but there are places that, at first glance,
0:51:33 > 0:51:34seem even more unlikely.
0:51:37 > 0:51:40This is the Gobi Desert in Mongolia.
0:51:44 > 0:51:46It goes through one of
0:51:46 > 0:51:49the greatest seasonal temperature swings on Earth.
0:51:52 > 0:51:55In summer, it can reach 50 degrees centigrade,
0:51:55 > 0:51:57and in winter, it is -40.
0:51:59 > 0:52:03But these are perfect conditions for a Bactrian camel
0:52:03 > 0:52:06to practise his seduction techniques.
0:52:09 > 0:52:11Freezing winds have come from the north...
0:52:13 > 0:52:16..and they have brought snow to the desert.
0:52:18 > 0:52:21This male suddenly has a limitless water source -
0:52:21 > 0:52:23he can eat the snow.
0:52:26 > 0:52:29He is no longer tied to sporadic water holes.
0:52:29 > 0:52:34He is free to wander miles across the desert looking for mates.
0:52:36 > 0:52:37And when he finds a female,
0:52:37 > 0:52:40he has an unusual way of getting her attention.
0:52:42 > 0:52:45He urinates on his legs,
0:52:45 > 0:52:48then, using his tail, flicks it over his back.
0:52:53 > 0:52:54If he plays it right,
0:52:54 > 0:52:57he might attract as many as 20 females
0:52:57 > 0:52:58before winter is over.
0:53:08 > 0:53:11While camels are looking for new mates in the Gobi Desert,
0:53:11 > 0:53:13on the other side of the world,
0:53:13 > 0:53:17older relationships are about to be rekindled.
0:53:30 > 0:53:34In Antarctica, the long, cold winter is coming to an end.
0:53:36 > 0:53:38The sun has finally returned.
0:53:51 > 0:53:55The male penguin has survived 100 days fighting the cold
0:53:55 > 0:53:57to keep his precious egg warm.
0:53:59 > 0:54:02He has lost nearly half his body weight.
0:54:05 > 0:54:08As he enjoys the first warming rays of sun,
0:54:08 > 0:54:10he hears a familiar sound.
0:54:13 > 0:54:16Figures are appearing on the horizon.
0:54:22 > 0:54:24His mate has come back.
0:54:39 > 0:54:41There is a great cacophony of calls
0:54:41 > 0:54:44as couples find each other in the throng.
0:54:55 > 0:55:00Reunited, he can finally introduce her to a new arrival.
0:55:03 > 0:55:05All his effort was worth it.
0:55:20 > 0:55:22With enormous commitment,
0:55:22 > 0:55:26these parents have given their chicks the very best start in life.
0:55:28 > 0:55:32They are ready to join the family firm
0:55:32 > 0:55:34as the ultimate winter survivors.
0:55:45 > 0:55:48Winter brings huge challenges to animals caught in its path.
0:55:50 > 0:55:52Some have to battle freezing temperatures.
0:55:54 > 0:55:57Others have to struggle to find food in the snow.
0:56:00 > 0:56:03And some have to fight off fierce competition.
0:56:07 > 0:56:08But through it all...
0:56:09 > 0:56:11..animals find mates...
0:56:12 > 0:56:14..and some even raise a family.
0:56:17 > 0:56:18By being clever...
0:56:22 > 0:56:23..resourceful...
0:56:25 > 0:56:26..and brave...
0:56:29 > 0:56:34..you can survive, and even prosper,
0:56:34 > 0:56:36no matter what winter brings.