Nearly all mammals have to go down to the water to drink.
And even the most unlikely of them can swim.
You might not think that an elephant would willingly go out of its depth,
but many do so quite regularly.
Some scientists even believe that the elephants' ancestors once spent much of their time in water,
and that their trunk first evolved as a device to help them breathe there - as a snorkel.
It's certainly true that elephants even now are very fond of bathing
and can swim across deep channels.
But there are some mammals that swim so frequently that water has become their true home.
Fresh water contains all kinds of food,
both animal and vegetable, and mammals of many kinds have ventured there in search of it.
The desman belongs to that ancient group, the insect eaters, that were around in the time of the dinosaurs.
Like its relations the shrews,
it lives on worms and molluscs as well as insects.
Most shrews look for such things on land, but the desman is more adventurous.
And it's got special underwater gear -
a snorkel. A miniature version of the elephant's trunk.
It's also got long, dense fur that keeps it warm in the water.
These two modifications make it a very effective swimmer,
and its snorkel also serves as a sensitive probe to help it discover things to eat on the riverbed.
Even so, its body is very buoyant and keeping below the surface is hard work,
so it seldom dives for more than a few minutes at a time.
And having caught something, it has to come back to land to eat it.
But hunger has led other mammals to swim in much bigger and more hazardous waters -
the seas.
The oceans that cover two-thirds of the planet are full of food.
So it's hardly surprising that some mammals have gone there to find it.
These behind me spend their lives at sea. And these particular ones spend most of their time feeding.
In fact, in proportion to their size, they probably have the biggest appetite of any mammal.
They are sea otters.
The ancestors of otters were weasel-like creatures -
land-living carnivores that scampered around on four feet, had warm blood and breathed air.
Each one of these characteristics poses a problem for any mammal that tries to take up swimming.
I can solve them by putting flippers on my feet,
by wearing an insulated suit to keep me warm
and putting a snorkel in my mouth so that I can breathe underwater.
The otter has developed webs between its toes and in that way converted them to paddles.
Even the best human scuba diver - and that's certainly not me -
can't match the sinuous agility of a sea otter.
Much of the food in the Californian waters is packed up in hard shells.
To deal with that, the sea otter collects a stone from the sea floor.
Back on the surface, it puts the stone on its stomach and uses it as an anvil.
Sea otters are so good at this and so energetic
that one can crack open and eat a quarter of its own weight in shellfish in a day.
River otters leave the water to mate, but sea otters are so at home at sea
that they mate here, bringing a new meaning to the concept of synchronised swimming.
They don't even go back to land to sleep.
And how do they prevent themselves from being carried away by the current?
They wrap themselves in kelp, like this one has done.
You might think that it wouldn't matter very much to an otter
if it did drift a bit while it dozed.
But these kelp forests are rich feeding grounds, and sea otters are territorial
and they don't want to leave their family hunting grounds undefended.
And how does a sea otter deal with the problem - so crucial for mammals everywhere - of staying warm?
My dry suit gives me very good insulation,
but the sea otters' fur is superb.
It has more hairs in one square centimetre of its body
than any human being has on their head.
In fact, sea otter fur is the densest fur in the whole of the animal kingdom.
It takes a lot of looking after. Its efficiency as an insulator depends on having air trapped in it.
To make sure that it is at its most effective, sea otters spend a lot of time blowing into their dense fur.
When an otter dives, some air, inevitably, is squeezed from its fur.
But even so, enough remains to keep the otter warm and snug.
Few animals look more at ease on the surface of the sea than a sea otter.
Their furry wet suit is even efficient enough to keep them warm in the frozen waters of Alaska.
But that superb fur was nearly their downfall.
Human beings prized it so greatly that they hunted the sea otter close to extinction. Now, hunting's banned.
There are other sea-going mammals that fish along these Pacific coasts of North America -
sea lions.
They may have taken to the water even before the sea otters,
for their limbs are now even more extremely adapted to swimming.
Their front legs have become paddles, their back - broad flippers.
And they have developed an additional means of insulation.
As well as fur, they have a specially thick layer of fat beneath the skin - blubber.
They do, however, still retain their external ears.
And it's this that identifies them as sea lions rather than seals.
Even though all four limbs are flippers,
the front legs are stout enough to act as props
and the hind can still be pointed forward to help them walk.
But they still have to come to land to give birth to their pups.
Beaches, to be suitable for a sea-lion nursery,
must have a gentle seaward approach so they aren't battered by waves
and to be on islands or sheltered coves that are difficult for land predators to reach.
Such places are not common,
so they're usually crowded, like this one in New Zealand.
Each patch is dominated by a big male, a beach master,
who will claim any female who lands on his patch, and mate with her as soon as she's given birth.
He keeps a lookout for any other male who might have the same idea.
HE GROWLS
The females need to get back to the sea in order to feed,
so they rear their babies as quickly as possible and provide them with rich milk - it's about 30% fat.
The baby consumes such quantities at such speed that the growth of its bones and muscles can't keep pace.
So, the baby converts some of the fatty milk into baby fat, blubber, which takes hardly any time.
But inevitably, this pampered life will soon come to an end.
It's going to get much tougher.
After a mere three weeks or so,
a mother leads her baby down for its first swim.
To reach open water, they have to get through the swirling, entangling beds of kelp.
Made it at last.
South of New Zealand, in the Antarctic,
it's so cold that the sea freezes over.
These are seals, not sea lions.
Both groups seem to be descended from an early carnivore - something between a weasel and a bear.
But seals have taken their swimming adaptation farther than sea lions.
They have completely lost their small external ears, which sea lions have retained.
Consequently, their heads are just that much better streamlined.
And their hind legs have become so shortened that they can no longer be pointed forwards to help in walking.
All a seal can do to get around out of water is to hump its whole body, or simply slide.
With the surface of the sea frozen, an expectant mother seal can haul herself out of the water anywhere.
So, a male can't lord it over harems
and the females are left to produce their young in comparative peace.
Seal pups here have a comparatively safe childhood.
The frozen seas around Antarctica
are so far from other continents that there are no terrestrial hunters here to threaten the seals.
That is a privilege, and one that is denied to seal pups elsewhere.
I'm now at the other end of the Earth - the north, the Arctic.
It may look very much the same as the Antarctic, with snowfields and icebergs,
but as far as seals are concerned, it's crucially different
because land extends north into the Arctic and there are land predators that can get out onto the sea ice.
There are tracks of them all here.
These...are the footprints...
..of an Arctic fox.
And foxes prey on new-born seals.
I'm out on the frozen surface of the sea.
Here, mother seals come up through holes in the ice
and dig snow caves as a nursery.
The fox whose tracks I'm following has found it and burrowed into it.
But did it catch the pup?
This...is the surface of the sea ice.
Over there is the hole through which the female seal came
in order to burrow out this lair.
Here, snug, away from the blizzards and gales of the Arctic,
she gave birth.
And here, it seems, that the pup did escape from that fox
for there is no sign of any blood on the ice.
But there are bigger predators here than foxes.
Polar bears.
They, too, are on the lookout for pups.
Ringed seals, at this time of the year, are their staple diet.
Ringed seal pups can't swim until they are several weeks old.
Their survival depends on them remaining undetected in their nurseries under the snow.
The adults are relatively safe beneath the ice.
They can stay submerged for 20 minutes, but visit the breathing holes to prevent them freezing over.
Polar bears have an extraordinarily sensitive sense of smell.
They can detect the breath of a seal drifting up from the snow from over half a mile away.
That could lead them to a pup.
That pounce smashed the roof of the nursery den and could have killed the pup outright.
But this pup was already dead.
Its little body is stiff and frozen.
Maybe its mother failed to keep the entrance hole free of ice
and couldn't get back to feed him.
Several kinds of seals in the Arctic breed away from land, out on the ice.
Female seals mate soon after giving birth.
That means they only have to leave the sea once a year, not twice.
Here, the males have no chance of assembling a harem, as they can on a beach.
Instead, each one waits for a female to become sexually receptive again.
These hooded seals have their own way of impressing rivals.
It blows up its hood - a cavity beneath the head skin -
and then inflates a scarlet membrane that balloons out of its nostrils.
If displays like this aren't enough to settle a dispute, the males have to resort to physical violence.
Male harbour seals have an even stranger courtship ritual,
and one that has been discovered so recently that its mechanisms are still not fully understood.
They go in for competitive choral singing.
One big male, just off the breeding beach, begins to vocalise.
LOW RUMBLINGS
Others - probably younger ones - then join him.
THEY ALL MAKE SOUNDS
Eventually, half a dozen may be singing, holding their heads together like a barber-shop group.
When a female does appear, the one who started the performance swims away with her
while the rest obligingly wait behind.
Otters, seals and sea lions
are all descended from an ancient group of hunting mammals that were tempted into the water to fish.
But they've retained the character of their ancestors - they're fierce and aggressive.
But what about the early plant-eating mammals?
They, too, went into the water about 35 million years ago,
because there are many water plants, particularly in shallow fresh water.
They, too, have retained the character of their ancestors.
They are gentle grazers.
And here, in the warm, clear waters of the Florida creeks,
they still are. Manatee.
They're so completely at home in water...
that they never leave it.
Oh, dear...
I suppose a little halitosis is what you'd expect from all these leaves,
but - phew! - that's a bit strong.
But what were those vegetarian ancestors?
No-one knows. Some characteristics, like teeth, link manatees to elephants.
These, like those of elephants, are flat, grinding molars.
As they're worn down by the coarse grass, they're replaced by new ones
that erupt at the back of the jaw and slowly move forward.
Manatees are so big that nothing much attacks them.
With plenty of vegetation there for the taking, there's no need for them to be swift swimmers.
Their forelimbs have become short flippers that can be used as paddles
or to gently punt along the bottom.
They still carry nails - vestiges of their terrestrial past.
Their hind legs have disappeared altogether, and they propel themselves on their cruises
with slow, powerful sweeps of their huge tails.
Their bristly upper lip is so well muscled that they can use it to grasp leaves,
rip them up and push them into their mouth.
They have gentle lives trundling across shallow submarine pastures.
Their other name is "sea cow", and very appropriate it is, too.
Manatees live in clear, sunlit waters.
The plants they feed on only grow in light, so they have little difficulty in finding their food.
But other swimming mammals have a harder time of it.
India, the Ganges.
There are water-living mammals here, though they're rare and hard to spot.
There's one.
It's a river dolphin.
The trouble with rivers in general - and the Ganges in particular -
is that they're full of sediment and very cloudy.
Below the surface, it's impossible to see more than a few inches ahead.
The water is opaque. Eyes are no use at all.
And the river dolphin has lost the use of them. It's completely blind.
How, then, does it find the fish it feeds on?
It uses sound - electronically.
WE can make a sound and use a system known as "sonar".
We can send out very high-pitched sounds from this.
And if that hits their body, it causes an echo which we will receive
on this monitor.
Let's try.
BEEPING
There they are.
Shoals of fish, somewhere out there in the murky water.
River dolphins use sound in exactly the same way.
If I lower an underwater microphone, we can hear the sounds THEY are making to locate their prey.
BUZZING
GURGLING
All dolphins exploit sound when hunting.
But here, on the south-eastern coast of the United States, in Georgia and the Carolinas,
there are dolphins that've invented their own special way of hunting
that seems to be used by no other dolphins anywhere in the world.
It's daring and it's complicated, but the birds can predict it.
They're assembling over there, so that's where we should point our cameras.
And sure enough, there in the water in front of them are the dolphins.
They're swimming slowly back and forth, edging a shoal of fish closer to the river bank.
And now their tactics are about to change.
Several dozen little fish were swept up onto the mud, and the dolphins are now snapping them up.
The birds are getting quite a lot, too.
Now, the dolphins have to wriggle back to water.
Off they go upriver to find the next suitable place for doing the same thing all over again.
Once more, the birds show us where that is likely to be.
But have they got it right?
This daring strategy depends on a number of things.
First, obviously, teamwork.
And that requires an ability for the members of the team to communicate with one another,
which in this murky water must be done by sound.
But it also requires a high intelligence...
The high intelligence needed to plan ahead, which was more than I managed to do!
They obviously knew that they were going to come to a safe place,
and one of the keys that tells you that it's going to happen
is when one of the members of the team pokes its head out of the water
in order to make sure everything is safe on the bank.
Synchronisation must be perfect
to create the necessary surge, and that can only be done by underwater communication between the team.
And they must all turn the same way.
If two alongside one another were to turn in different directions,
they would either end up facing one another, competing for the same fish,
or both turning their backs on those same fish, allowing them to escape.
Out in the open ocean,
dolphin teams may number several hundred.
These are common dolphin,
and the speed with which they are going - wow! -
and the determined way in which they're travelling, and the fact that all these birds are soaring
means that they know there are some fish right over there.
The whole school stretches out on either side of me for a quarter of a mile or more in either direction.
They seem to be chasing a shoal of fish ahead of them, just as those dolphins were doing in the river.
But this is on vastly greater scale.
They've succeeded in isolating a huge school of sardines,
and now they're swimming round them, herding the shoal in upon itself,
forcing it into one gigantic meatball.
They drive the shoal upwards so that it will be trapped against the surface.
And now the moment has come to swim straight into the meatball
and collect the rewards for all this effort.
As the sardines are forced towards the surface,
so they come within range of sea birds overhead.
There's a water-living mammal that feeds in a quite different way.
Instead of teeth, it uses baleen - horny plates that are hung from its upper jaw
and fringed with long, coarse hairs.
It collects krill - little shrimp-like creatures, scarcely bigger than my little finger -
but it finds them in such quantity that it's become gigantic.
It takes in a great mouthful of water and krill, then shuts its jaws,
and up comes its tongue. It's as big as an elephant.
The tongue pulls back, wipes the krill from the baleen, and the animal swallows it.
And that krill is so nutritious that this creature, the blue whale,
is the biggest that has ever existed on this planet -
almost twice as heavy as the biggest known dinosaur.
Its vast ribcage houses its lungs.
They carry 2,000 litres of air -
that's 500 times the capacity of our lungs.
The heart is as big as a small family car.
It only beats five or six times a minute,
but it drives ten tonnes of blood through a million miles of blood vessels.
And all that is left
of the hind legs and hip bones are these two isolated fragments,
buried in a mountain of muscle.
I can see its tail, just under my boat here.
It's coming up...
The blue whale...
is 100 feet long...
30 metres.
Nothing like that can go on land because no bone is strong enough to support such bulk.
Only in the sea do you get such huge size as that magnificent creature.
And down it goes...
The land-living deer-like creatures that were the ancestors of the great whales first entered the water
around 55 million years ago.
Since then, their descendents have evolved ways of solving all the problems of life at sea.
With one blast from its nostrils, a whale discharges 90% of the spent air from its lungs and takes in new.
Most land-living mammals only manage to void about 15%.
It's able to store oxygen not just in its blood, but in the tissues of its vast body.
And so it can stay beneath the surface for half an hour or more.
It collects food wholesale.
With one sideways gulp, it takes in a tonne of krill-filled water.
Their ancestors' coat of hair, so characteristic of all land mammals, has been completely lost.
Instead, the whale's entire body is swathed by a blanket of fat beneath the skin, in places 20 inches thick,
which insulates it against the chill of the water, no matter the depth.
It has a near-perfect hydrodynamic shape,
uninterrupted by hind limbs, ears or genitals.
And as it tilts its hundred-tonne body downwards,
so it can plunge to the black world 500 feet or more below the surface.
Down in the blue immensities of the oceans,
where the great whales spend much of their time, they communicate, like dolphins, by sound.
BOOMING ECHOES HIGH-PITCHED SQUEAKING
GRUNTING AND RASPING
Sound travels further and faster in water than it does in air.
Loud noises can be heard hundreds of miles away,
so whales may be able to hear the distant waves breaking on the shore
and use that to find their way around the otherwise featureless expanses of the open oceans.
Individuals also call to one another and may keep in contact even though they're hundreds of miles apart.
Humpback whales have developed particularly complex sounds.
They produce deep notes, almost beyond the range of our hearing.
If you swim alongside them, these vibrations seem to fill your body
as the low notes of an organ will throb inside a cathedral.
The more complex notes are directed to females, inviting them to mate.
All humpback males in one part of the ocean sing the same sequence of sounds,
the same song, but each, as he sings, may repeat the phrases within that sequence several times.
GRUNTING AND TRILLING
A complete song may last for half an hour.
Once it's over, the male may repeat it and continue doing so over and over again
in a performance that may last several days.
Off the coast of Patagonia, southern right whales are assembling.
The males announce their arrival by gigantic leaps. 100 tonnes propelled into the air with the flip of a tail.
The sound above water is like a cannon shot.
Below, it must be felt for miles around.
These whales show their solution to the problem for all mammals if they are to live permanently in the sea -
how to breed in water.
This female is surrounded by ardent males.
She's not yet ready to mate,
and rolls over on her back in an attempt to keep her genital region away from her suitors.
That's not easy when a male is as formidably equipped as a right whale.
A slit has opened in the male's underside and a penis protrudes -
12 feet long and highly mobile.
The males barge and jostle one another to reach her and several may succeed - one after the other.
Now, seemingly, the female has changed her mind.
She rights herself and leaves the surface. Now she is ready to receive a male.
Male right whales have gigantic testes, the largest in the world.
They weigh a tonne, and produce gallons of sperm.
One coupling can flush out whatever preceded it,
so it may not be the first male who succeeded in copulating who becomes a father. It will be the last.
So, some mammals who started out with four legs and no fins, with bodies that had to be kept warm
and with an awkward necessity to breathe air
have managed to colonise the waters of the world.
We, with the aid of plastic flippers and compressed air bottles,
managed to follow them a few decades ago,
but we still have lots to learn about how they organise their lives.
Given the elusive nature of marine mammals, it will be many years yet before their mysteries are solved.
You may wonder how I was lucky enough to get alongside that surfacing blue whale.
It was done using the latest technologies -
a radio tagged on the whales sent signals up to a satellite which gives their position.
Then, a low flying aircraft calls a swift launch. That's how I got there.
That gets scientists there, too, of course, and it's their job, in a very short time, to collect data.
To study a mammal that spends the vast majority of its life underwater or far out at sea
presents an enormous challenge for humans.
For marine biologists, each close encounter with a huge whale is ample reward.
I've studied blue whales for years,
but I will never forget the first time I saw one from the air.
I was in a rickety old Cessna, over the Sea of Cortez in Mexico.
We flew over an adult blue whale just as it surfaced.
It looked more like a Boeing 737 than a real, wild, living whale.
It was absolutely huge.
Yet despite its size, the blue whale has remained surprisingly well hidden from humans.
In August 1986, whale biologist John Calambokidis
was doing an aerial survey in a vast area of sea off California
and he made the most incredible discovery.
He saw a giant amongst humpbacks. It was blue with a huge blow
and he recognised it as a blue whale.
Then he saw more and more.
He had discovered the largest known population of blue whales on Earth.
Research shows that there's about 2,000 blue whales off California.
How they could have gone unnoticed for so long I don't know, but it was an extraordinary discovery.
But the key to understanding is to identify and follow individuals.
Since the early 1970s, scientists have identified killer whales
using the shape of the dorsal fin.
Similarly, humpback whales have individual patterns on their tail fins.
Blue whales proved more of a challenge, but in the early '80s,
biologist Richard Sears showed that individual blue whales could be identified by their mottled flanks.
Now we can recognise over 1,300 of the whales in the California group.
It's like a human mugshot. Imagine the CIA files for criminals. This is the equivalent for blue whales.
You build up catalogues of an entire population.
This is the bricks and mortar of blue whale research now.
When the whales dive out of camera range,
it's still possible to follow them by listening to their calls.
This, too, is a way to identify individuals and families.
Surprisingly, the end of the Cold War brought the next advance.
The US navy used very sensitive hydrophones to track submarines.
Its operatives had listened in to whales for decades.
The crux point was in 1993 when Chris Clark of Cornell University
was given permission to use this system to eavesdrop on whales.
For whale biologists all over the world, this was like Christmas a thousand times over. It was great.
It meant he could listen to one whale as it crossed an ocean basin.
The first afternoon Chris and his colleagues listened in
with the help of navy analysts,
they actually heard more blue whales than had ever been written about by scientists ever before.
It was mind-boggling stuff.
The latest technology gives even more detail.
A harmless tag sends biological data up to a satellite each time the whale surfaces to breathe.
It provides vital information for whale biologist Bruce Mate.
There's a low population of blue whales worldwide now -
we've lost 92% of them to whaling.
With 8% left, and the Californian coast having 25% of those,
we really need to know where they go to breed and calve in the winter to protect them.
We only know where they are in the summer,
so tagging them here and tracking them in the winter is important.
Our understanding of whales, dolphins and other marine mammals
has increased enormously over recent years thanks to the researches of marine biologists.
It's THEIR expertise that has enabled us to get our camera teams to the right place at the right time.
Nonetheless, filming whales still poses enormous problems.
Sometimes, it's as simple as get on a boat, point the camera.
In that case, you're only seeing them for a small part of their life.
We think that to see the subject properly, you have to go underwater.
Entering the underwater world obviously brings up a whole set of fresh challenges.
For a start, you can't breathe.
A lot of times, marine mammals live in water which is quite cold -
temperate or polar zones - so you have to wear a dry suit rather than a wet suit.
Your extremities, like your hands, tend to get quite cold, so we put hot water into our gloves,
which means our fingers are slower to get cold.
You either use a scuba or a rebreather - which doesn't give out so many bubbles.
But sometimes the best technique is to go completely simple - simply use your snorkel and breath hold.
As a testament, this is one of the most memorable whale images.
It's the sei whale, with a mouth the size of a dustcart, passing right by the lens.
Cameraman Doug Anderson managed to film it holding his breath
and with an amazingly steady camera from just two metres.
Being close to a blue whale is something I'll never forget.
But conveying its sheer size visually requires a whole range of skills
and poses a considerable challenge.
The idea of this sequence was to get over how huge the blue whale is.
Although there's a great shot of David in the boat, there's no sense of scale.
We got the skeleton through the internet - there were images of one that's outside a Californian museum.
We thought it would be a great location, cos there was plenty of access, plenty of space.
To prepare for shooting, we had to put up a drape to hide the car park behind it
and then we lit it with arc lamps on cherry pickers from way above,
and then got David to walk around and through it as we filmed him.
When we arrived on location, we noticed one problem - the inside was supported by a black frame
that would appear in every shot.
We had to paint it electronically.
Back in Bristol, for each shot we created a scene in the 3-D computer,
built models of the organs, lit them, animated them and composited them with the shots of David
to give the final sequence. It's fun AND informative with the organs appearing next to David.
We brought something new to the screen.
Finally, we can show you the essence of the greatest mammal on the planet.
But even now, we've only touched the surface of our understanding.
The more we care for and study these magnificent creatures, the closer our relationship will become.
Subtitles by Red Bee Media Ltd