'The sun.
'15 million degrees Celsius at its core.
'Too close,
'and you burn.
'Too far...
'..you freeze.
'But at just the right distance,
'you get this.
'A living, breathing world.
'No other planet in the known universe
'experiences seasons like our planet, Earth.'
By the beginning of March,
we in the north are turning towards the sun...
'..and winter snow begins to melt.
'Just a little at first.
'Then more and more.'
Until the flow of water becomes truly staggering.
Every year, the returning sun
melts over ten million square miles of snow
in the Northern Hemisphere.
'Allowing the land to reawaken...
'..and bringing new life.
'While in the Southern Hemisphere, life braces itself
'for far tougher times ahead.
'In more than 60 locations across the globe,
'we follow the world's most spectacular animals...
'..and reveal the struggles...
'..and opportunities
'of spring.
'The most magical part of our journey around the sun.
'This is going to be an extraordinary day.
'No matter where you are in the world...
'..there will be 12 hours of daylight...
'..and 12 hours of darkness.
'The spring equinox has arrived.
'Incredibly, there is one animal
'that can detect this unique celestial moment.
'A leatherback turtle.
'She has a translucent spot on her head
'that acts like a skylight,
'letting light shine through her skull,
'directly onto her brain.
'And now, after two decades of wandering the oceans,
'it compels her to return to the beach where she hatched...
'..and lay eggs of her own.
'As graceful as they are in water,
'leatherbacks are poorly-adapted for moving on land.
'The largest turtles in the world,
'over a half a tonne,
'and as a big as a two-seater sofa.
'Every inch is won by extreme labour.
'It's going to be a tough night.
'Her eggs are very sensitive to temperature.
'To prevent them from overheating, she must dig deep.
'Finally, after more than an hour, her nest is ready.
'She lays around 80 eggs...
'..each slightly bigger than a golf ball.
'Now safely buried,
'they will hatch in two months.
'But she won't be here to see her babies emerge.
'Exhausted, she must recoup her energy
'by going to feed in the richest part of the ocean...
'..which now lies thousands of miles north.
'As the sun moves further over the Northern Hemisphere,
'days now become longer than nights.
'And light fuels growth.
'The land bursts with colour.
'And surprisingly,
'some of the most spectacular spring blooms of all
'are found right in the middle of the desert.
'Since December,
'day length here has increased by two hours.
'The solar energy reaching the ground has almost doubled.
'And with the Earth still moist from winter rains,
'conditions for growth are perfect.
'And now these nectar-laden flowers
'attract new life from miles around.
'Including one of the most beautiful birds in the desert.
'A male Costa's hummingbird...
'..preparing for his busiest time of the year.
'Without nectar, he would die of starvation within a few hours.
'Hummingbirds have the fastest metabolisms of any animal.
'His tiny heart beats more than ten times a second.
'And just a few yards away
'is someone that can make it beat faster still.
'A female Costa's hummingbird.
'She may look a little drab...
'..but she is the object of his desire.
''The only problem is,
'despite his fancy feathers, she doesn't appear to fancy him.'
'Thankfully, he has a trick up his sleeve.
'One of nature's most extraordinary courtship displays.
'Best admired with a full frontal.
'Positioned at just the right angle to the sun...
'..with feathers splayed...
'..he is absolutely dazzling.
'Having a purple octopus waved in your face
'might not be your thing,
'but the perfection of his plumage shows his quality.
'And it works for her.'
'In just over a month,
'her chicks will have hatched, grown up and flown the nest.
'Spring comes early in the desert, and it doesn't last long.
'But further up in the Northern Hemisphere,
'it's only just begun.
'The greatest baby boom on Earth is still to come.
'In the north, spring is now taking hold.
'The strengthening sun
'fuels billions of tonnes of new plant growth.
'It sends a pulse of energy coursing through the food chain,
'triggering an enormous baby boom.
'A surge of easy-to-catch prey
'makes this the ideal time
'for hunters to raise their families, too.
'But closer to the equator,
'predators aren't having it so easy.
'East Africa,
'home to the planet's greatest population
'of large carnivores.
'Five months ago,
'this landscape was a predator's paradise.
'A million wildebeest.
'300,000 zebras.
'And 200,000 gazelles.
'Nearly all gone.
'And the great migration won't return until summer.
'Now, only the locals remain.
'Experienced and wary.
'Spring can be a tough time for a cheetah mum.
'Her two six-month-old cubs need to be fed every day
'or they'll begin to starve.
'But she must pick her moment...
'..conserve her energy.
'The cubs have a different agenda.
'Playing is how they learn which animals are impossible to catch.
'And which ones fight back.
'With practice, they will be able to accelerate
'faster than most sports cars...
'..and reach speeds of up to 65mph.
'But not yet.
'Right now, they're just being a pain in the...
'..tail.
'Enough with the play.
'It's time to go to work.
'As far as we know,
'cheetahs are the fastest land animals to have ever existed.
'They can execute turns
'at speeds that would break the legs of other animals.
'But like any high-performance athlete,
'success or failure is separated by the tiniest of margins.
'The hunter's dinner is never guaranteed...
'..especially at this time of year.
'In many ways, the prey have it easier.
'Their food grows everywhere.
'But it is dependent on one crucial resource.
'Water.
'Since winter, the tropical rain band
'has been steadily moving up the continent...
'..forming wherever the sun shines hottest on the Earth's surface.
'It brings life to the dry lands of Africa.
'But as the rains move north...
'..further south...
'..conditions are becoming critical.
'The water holes are shrinking...
'..getting crowded...
'..and more dangerous.
'Even the elephants are vulnerable.
'Especially the youngsters.
'The old matriarch leader of this group
'knows it's time to move on.
'She has made many journeys in search of water
'in her 50 years,
'and can even remember routes made
'during extreme droughts decades ago.
'And now this four-month-old baby
'must learn the way as he begins his first migration.
'Indeed, much of an elephant's behaviour is learned.
'From birth to adulthood, their brains almost triple in size,
'the greatest increase of any animal apart from us.
'The good times are over.
'Ahead, only a thirsty, gruelling march.'
'The red-billed quelea are also heading off.
'They follow the rains and the pulse of fresh grass seed.
'And they do so in their millions.
'The most abundant wild bird in the world.
'The key to their success is a super-fast breeding cycle,
'little more than three weeks from laying eggs to chicks flying.
'Ready to leave when conditions get tough.
'Not everyone has that option.
'In the far south, the darkness is creeping in.
'The sun sinking lower.
'The land growing colder.
'This four-month-old king penguin chick
'is stuck here.
'She won't be ready to leave the island
'for another eight months.
'So she must prepare for the coming cold.
'Since hatching, she's grown from a tiny, naked chick,
'weighing less than 250g,
'into this plump, downy youngster of nearly 12 kilos.
'Just as well - it's the coldest day of the year so far.
'And something new and strange is happening.'
'Curiosity gets the better of her.
'The nasty side of town.
'The source of all this flying fluff?
'Not snow.
'Manky, moulting macaroni penguins.
'And there's something else here.
'Monsters.
'These are giant petrels.
'Three feet tall, and as heavy as a Christmas turkey.
'The stress of moulting makes some macaronis weak.
'But if you show weakness here...
'..there is no mercy.'
'Giant petrels don't miss any opportunity.
'Against these relentless killers, a chick doesn't stand a chance.
'And she's just been spotted.
'When a giant petrel sizes you up like this,
'it's time to get out of here.
'Her nemesis is coming for her.'
'But the giant petrel has underestimated the kings.
'Twice as big as the macaronis, they can stand up to this bully.
'It's all too much for the giant petrel.
'Finally, the chick can relax.
'But not too much.
'Soon, her parents will be gone for weeks,
'even months, on end.
'As the water gets colder,
'the ocean currents shift
'and feeding areas move further away.
'She will grow hungrier, weaker...
'..and she'll be alone.
'While conditions in the south grow harsher...
'..the Northern Hemisphere is in the full flush of spring.
'And here, in this seasonal rainforest,
'the fresh growth feeds all kinds of animals.
'Including one of the most striking monkeys on the planet.
'Red-shanked doucs.
'Timed with the burst of fresh spring leaves and flowers,
'these doucs are having a baby boom of their own.
'Only four years old,
'this is the youngest mum in the community.
'And this is her first baby.
'Just days old.
'Right now, it's a worrying time for Mum.
'Newborns make easy targets...
'..for all kinds of predators.
'But too much protection can be stifling.
'He needs to learn key skills,
'like how to move through the forest.
'Doucs make some of the longest leaps of all monkeys,
'frequently exceeding ten metres.
'You can't do this without a lot of practice.
'Sometimes, a mum has to let go.
'We see spring as a time of relief after a long winter.
'Yet for many animals, this is a time of great stress.
'Never will their babies be more needy and more vulnerable.
'And never will parents have to be more vigilant.
'But all a mum can do is take it one day at a time.
'At least she's bringing up baby
'in the relative comfort of the tropics.
'A little further north, just beyond the Himalayas,
'it's a different story.
'The Tibetan Plateau.
'Covering nearly a million square miles,
'and averaging over 4,000 metres high,
'it's the largest and highest plateau in the world.
'With little more than half the oxygen at sea level,
'the air leaves you gasping.
'Winter temperatures can plummet to minus 20 degrees Celsius.
'Not many animals have adapted to life up here.
'This is one of the few.
'A Tibetan fox.
'A mere 2.5 million years
'set it apart from our red fox.
'Yet it's enough to have evolved an extraordinarily wide gape.
'An adaptation for a diet consisting almost entirely of these animals.
'Thanks to a surge of spring breeding,
'plateau pika are everywhere.
'But they have hundreds of boltholes.
'To catch one, you have to be...
'..patient.'
'A lot of work for such a small snack.
'And she's not even going to eat it.
'It's for her three eight-week-old cubs.
'In fox families,
'fathers usually stick around to help.
'But Dad hasn't been seen for weeks.
'Up here, conditions are so harsh, lives are often cut short.
'Keeping the family fed is now a real struggle.
'By the time the cubs are up...
'..Mum's already out hunting.
'All this play is hungry work.
'But for some reason,
'breakfast is late.
'Constantly sprinting after prey
'over potholed ground
'has taken its toll.
'An injury as serious as this could stop her hunting.
'And that means starvation for the whole family.'
'Though clearly in pain, Mum is off hunting again.
'With cubs to feed, she has no choice.
'But sometimes, adversity brings out the best in you.
'She's found her A-game.
'Perhaps her luck is returning.
'Thanks to Mum's extraordinary determination,
'the cubs should keep growing strong.
'With so many mouths to feed,
'spring can be the most challenging time of year.
'This cheetah mum...
'..desperately needs a change in fortune.
'No matter how hard she tries...
'..it always ends in frustration.
'Mum is exhausted.
'She can't afford any more failures.
'Hyenas will kill cheetah cubs.
'They're too big to beat in a fight.
'The whole family has to be very careful.
'All she can do
'is stay by her cubs and hope the danger passes.
'The threat from lions and hyenas is constant.
'But she can't hide forever.
'Her family needs to eat.
'A male impala.
'Cheetahs usually avoid such large, heavily-armed prey.
'The risk of injury is just too high.
'But not for this mum.'
'For the first time, her cubs actually help.
'Around 80% of cheetah cubs here
'succumb to predators or starvation
'before the reach their first birthday.
'Her cubs still have a long way to go...
'..but with every kill,
'every passing day,
'their chances improve.
'Spring in the north is drawing to an end.
'But here, on this beach,
'it still has one final wonder to offer.
'For eight weeks...
'..the leatherback eggs
'have been gently incubated by the sun.
'Now the baby turtles are beginning their own journey.
'But everything will depend on the next few minutes.
'Black vultures have been waiting for this moment.
'Their mum's sacrifice,
'her toil on the beach,
'may come to nothing.
'But if this hatchling can make it to the sea,
'the odds of reaching adulthood rise dramatically.
'He's passed his first trial.
'With luck, he may go on to live for over 50 years...
'..and cover tens of thousands of miles.'
Within a month,
some of the leatherback turtles that nested in Trinidad
could have come as far north as here,
in Iceland, to feed.
'Soon, the sun will deliver more energy here
'than the hottest part of the tropics.'
We're about to begin summer.
The hottest, most intense,
and most vibrant part
of our journey around the sun.
'Right across the globe...
'..animals will face new challenges...
'..and unique opportunities...
'..during the most extreme season
'on planet Earth.'
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