Nearly three quarters of our planet lies underwater.
Now, venture beneath the waves, and enter an astonishing world.
Life began in the ocean, and, over millions of years,
many animals returned from the land to become creatures of the sea.
This the kingdom of the oceans.
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Say, "hello," to a monster.
The marine iguana looks like a dinosaur,
but it's something relatively new in nature.
These are the only lizards that dive in the sea.
They live on the rocky shores of the Galápagos islands.
Their ancestors were land iguanas,
that probably floated over from the South American mainland,
nearly 1000 kilometres away.
To survive on these barren chunks of lava, they needed a new strategy.
They turned to the sea,
the source of all life on Earth.
But, they had to adapt to life in the water.
Marine iguanas are vegetarians,
Here in the Galápagos, they eat the algae that grows along the shore, ...
but there's a lot more algae growing offshore, under the waves.
They just have to get to it.
An iguana's flat snout and sharp teeth
make it easy to scrape algae off the rocks.
It may not seem like much of a meal, but algae is nutritious.
To conserve heat when he dives,
an iguana's blood moves away from the surface of his body,
and his heart rate slows way down,
from 100 beats a minute, to just 30.
A marine iguana can stay submerged for more than half an hour, ...
but only the biggest can retain enough body heat to undertake these long dives.
To recover from the chill of the sea,
they spend a lot of their time warming up on shore.
Marine iguanas are not the only algae eaters, here.
These Galápagos residents are called Sally Lightfoot crabs.
The Sally Lightfoot crabs like to hang out with marine iguanas
while they bask on the rocks, and the iguanas don't mind their company.
The crabs help their neighbours by picking ticks off their skin.
It's a procedure that demands a light touch.
In the Galápagos, creatures have found unusual ways to survive
on the margin between sea and land.
That happens in other places, too.
Even some fish can't seem to decide which world they prefer.
Fortunately for the mudskipper, it has options.
Mudskippers can live in either salt water or fresh,
and they can even leave the water behind, and venture onto land.
They're like a living lesson in evolutionary history.
This is how vertebrates emerged from the ocean,
more than 300 million years ago.
A mudskipper is a marvel of adaptation.
It doesn't have legs,
but it uses its large pectoral fins to drag itself on shore,
and, then, crawl around, to feed on algae or crustaceans growing in the mud.
Mudskippers use gills to breathe in the water, like any other fish.
On land, though, they seal up their gill chambers,
and breathe through their skin, like amphibians,
but they have to stay wet to survive,
so their time on land is limited.
Of course, if the weather cooperates, they can stay a little longer.
Most fish are content to stay in the ocean,
but they don't alway have the place entirely to themselves.
The Galápagos cormorant traded in its ability to fly,
and became an expert diver.
The cormorant uses its powerful webbed feet as flippers
to propel itself through the water,
while its wings no longer do much of anything.
Poking around in the rocks on the bottom,
it hunts for eels and octopus, as well as fish.
Like marine iguanas, cormorants adopted an aquatic lifestyle.
Another land animal returned to the sea long ago,
and completely transformed itself:
The whales' earliest ancestors were much smaller animals that walked on land.
They began to change into sea creatures around the time the dinosaurs disappeared
16 million years ago.
Their legs turned into flukes and flippers. Hair disappeared.
Their breathing and hearing were radically altered.
The ocean freed them from the constraints of gravity,
and allowed them to grow to enormous proportions.
And, they took to their new world, with uncommon grace.
Over time, whales made themselves right at home in the sea,
but they're still mammals, and need to breathe in the open air.
But, whales differ from their ancestors in their choice of food.
Those ancient creatures were plant eaters.
Whales are the ocean's biggest carnivores.
Among the great whales,
the sperm whale looms large in our imaginations.
For centuries, whalers prized them, and feared them.
The most famous sperm whale in literature is Moby Dick,
the fictional nemesis of Captain Ahab.
These whales are true giants.
They can stretch more than 20 metres,
and they also have the biggest brains on the planet.
But, their lives remain, largely, a mystery.
They spend much of their time hidden at great depths.
Sperm whales can dive to 1000 metres, ...
and stay submerged for an hour, on the hunt for squid,
their favorite prey.
This young sperm whale can't make a deep dive, yet,
but he can follow the action from a distance.
Far below, his elders click and buzz,
as they use echolocation to find their prey in the darkness.
Sperm whales also click near the surface, to communicate with each other.
Highly sociable, they enjoy physical contact, as well as conversation.
Scientists recently discovered that many whales
may be hard-wired for social complexity.
Their brains contain specialised neurons, like those in our brains.
Neurons related to empathy, speech, and social skills.
Some whales take sociability to an extreme.
Pilot whales spend their life in tight-knit pods,
and they're, almost, never alone.
They even sleep together, with half a brain awake,
so they can occasionally rise to the surface for air.
When they're active, pilot whales move as a group
thought to be guided by a leader, their pilot.
But, their cohesion can be their undoing.
If one pilot whale is beached,
the others may stay close by.
The result can be a mass stranding, and mass death.
Whales and dolphins have paid a price for their return to the sea.
Having renounced life on land, they must now avoid land, at all costs,
but, on the coast of Argentina, orcas; killer whales, embrace the risk.
They turn the beach into their hunting ground.
To score a kill, the orcas put themselves into a situation that could kill them.
They're after a young prey,
sea lion pups, that are just learning to swim in the icy surf.
The orcas' strategy is to hit the beach and grab a pup.
This a dangerous game.
Once an orca drags its massive bulk onto the sand,
it takes a lot of effort to get back in the water,
and a beached orca is a dead orca.
These orcas overcome gravity and instinct, ...
and, when the hunt is over, they return to the world that they know best.
Sea lion pups can avoid the danger on the beach by swimming in a safer place.
A rocky cove serves to keep out big predators.
Sea lions adapted to life in the ocean much more recently than whales.
Maybe, 24 million years ago.
But, they seem to revel in the life aquatic.
To find enough to eat,
sea lions must leave the safety of the colony,
and the shelter of the cove, and venture out into the open water,
and that exposes them to dangerous predators,
like the great white shark.
The bigger the shark, the more likely it is to target a marine mammal,
instead of a fish.
With a top speed of more than 40 kilometres an hour,
a great white can easily overtake a sea lion.
Mammals may have found a home in the sea,
but they do not reign supreme.
More than 40 million years ago,
an ancient sea bird gave up flight.
and it became a penguin.
Many penguins not only made the shift from flying to diving;
they adapted to life in the coldest parts of the planet.
These emperor penguins, almost, never leave the sea, at all.
Even now, in midsummer,
they march across frozen ocean, the icy shelf that fringes Antarctica.
They're returning from an extended fishing trip.
They say, "home is where the heart is,"
and these penguins won't be home until they locate their chicks,
in a crowded nursery at the bottom of the world.
The chicks are nearly 5 months old,
still completely dependent on their parents for food.
On the menu today, regurgitated fish.
The chicks' light, downy coats help them survive the bitter cold,
during their first months of life.
Soon, they'll molt - trade their baby feathers -
for the black and white plumage of an adult.
But, even before they molt, they abandon the nursery,
and begin their trek to the sea.
Fortunately, the chicks won't have so far to walk.
By midsummer, the icy shelf starts to break up,
and the ocean comes closer to them.
On the rocks above the ice field,
Adélie penguins have their own colony.
They're much smaller than emperors, and their chicks grow much more quickly,
once they finally hatch.
In this barren landscape, a nest is lined with pebbles,
and a choice pebble is worth fighting over.
Here in Antarctica, a midsummer blizzard is nothing unusual.
The Adélies hunker down, and do their best to shield their chicks,
but, not all will survive.
Emperor penguins are less bothered by bad weather.
Certainly not a summer storm.
After all, they spend the winter exposed to the worst Antarctica can throw at them,
the only animals that winter over on the surface.
As for the young ones,
they still have to master the art of staying upright in a gale-force wind.
Once the storm lets up,
the penguins continue the journey to open water.
They soon cross paths with the little Adélies.
eager to show the emperor chicks who's boss.
The emperors ignore them. They have other fish to fry.
In fact, fish is, just, what they're after.
Penguins may look awkward on land,
but once they hit the ocean, these birds are in their element.
Their streamlined silhouette helps them speed through the water,
and to help them dive, they have solid bones,
instead of the light, hollow skeletons of other birds.
A layer of fat keeps them warm.
Their adaptations make penguins world-class swimmers.
Besides emperor penguins,
Weddell seals are the only warm-blooded marine predators
that live so far south year-round.
To escape the worst of winter, they retreat beneath the sea ice,
where the water temperature never sinks below -2 degrees Celsius.
These mother seal's canine teeth are adapted for chewing ice.
She needs to maintain access to the water below for food,
and the air above for breath.
It's time for her pup to follow her into this strange new world.
And, with a little gentle persuasion, that's what he does.
Even after he grows accustomed to this new lifestyle,
the pup will face grave danger, ...
because these waters are also home to one of Antarctica's most feared predators:
the leopard seal.
It's the only seal that preys on other marine mammals.
A leopard seal is a deadly hunter, agile, sleek, and big:
4 metres long, and weighing in at more than half a ton.
While it may be a good swimmer, the leopard is not a great diver,
and it can only stay submerged for a few minutes at a time, ...
so, it never ventures far from open water, even when it's hunting.
It can't risk being trapped under the ice without air.
Though it will hunt seals, the leopard's favorite prey is penguin,
including Gentoos.
Gentoo penguins prefer a milder climate than emperors or Adélies,
so, they live farther north, like here,
on the rocky shore of the Antarctic peninsula.
They like to do things together, swimming included.
But, for these Gentoos, playtime is coming to an abrupt end.
A deadly stranger lurks near the colony.
It's a leopard seal.
A leopard seal can't move quickly when it's out of the water,
so the Gentoos are safe, as long as they stay on the rocks.
But, this leopard is a patient hunter.
He'll wait for his prey to come to him.
Like a cat, the leopard seal seems to toy with his prey,
but it's the best way to separate feathers and skin from meat.
A leopard seal can't make a dent in this big colony, ...
but penguins were, once, massacred here by the thousands.
Whalers used the fatty Gentoos like firewood to melt down blubber.
You can still find whale skeletons littering Antarctic coastlines, ...
and there are more on the sea floor.
Today, international whaling treaties
have turned the southern ocean around Antarctica from a killing ground
into a sanctuary.
Since their ancestors first returned to the sea,
more than 50 million years ago,
whales and their relatives have adapted to the most extreme conditions.
From the antarctic to the arctic,
marine mammals thrive in every ocean and every climate.
Of all marine mammals, the narwhal may be among the strangest.
With a single tusk that can stretch more than half its body length,
this unicorn of the arctic sea has inspired myth, and speculation.
The tusk is, actually, an overgrown tooth,
usually the left incisor,
but, until recently, its purpose was hotly debated.
Now, scientist know that these tusks contain millions of nerve endings,
helping the narwhal detect changes in the weather and in the water.
Narwhals hunt in deep, dark water,
diving down 1500 metres beneath the pack ice.
Their tusks help them find food below,
and return to their air holes in the ice above.
The arctic may seem barren, but its waters teem with life.
Algae covers the rocks,
and coats the underside of the ice.
The algae feeds countless tiny crustaceans.
The crustaceans are food for fish, like arctic cod, hiding in the crevices.
The fish, in turn, are a feast for marine mammals,
including the narwhal's cousins: beluga whales.
These talkative creatures have earned the nickname, "sea canaries."
Their squeaks are not, just, for communication.
They are crucial for survival.
To track down their prey,
beluga whales use their voices and their heads.
They transmit calls, and pick up the echo, to pinpoint objects in the water.
That helps belugas find food, and holes in the pack ice, so they can breathe.
Millions of years ago, when mammals embarked on their new lives in the water,
they brought with them something extremely rare in the ocean:
a profound attachment between mother and infant.
It can be seen among all marine mammals, including the walrus.
This calf was born on the ice just a few hours ago.
She takes to the water readily,
but, like all young mammals, she's dependent on her mother,
and their bond is strong.
They will stay together until the calf is weaned; two years or more.
Their life seems idyllic,
and, besides humans, they face few threats,
but the ice is ruled by one predator, above all.
The polar bear's scientific name is Ursus maritimus, the bear of the sea.
He, too, is a marine mammal.
To stalk his prey, a polar bear depends upon a frozen ocean.
His ancestors evolved and adapted to this life on the ice,
the only life he knows.
For marine mammals and the other terrestrial transplants,
the ocean provides an ideal home.
Their's is one of nature's great success stories.
Even the biggest of them all,
once nearly hunted to extinction, is, now, making a comeback.
Today, the ocean's largest mother, and its largest infant, are blue whales.
Their home is a world of possibilities,
habitat, and hunting ground.
A laboratory for evolution, and a showcase
for the infinite variety of life on Earth.
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