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 an elephant would willingly go out of its depth,
but many do so quite regularly.
Some scientists even believe that the elephant's 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 will swim across deep channels if they really want to.
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, every now and then during their long history,
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.
Whereas most shrews look for such things on land,
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 particularly 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 among the pebbles and gravel of the riverbed.
Even so, its body is very buoyant
and keeping below the surface is quite hard work,
so it seldom dives for more than a few minutes at a time.
Having caught something, it has to come back to land to eat it.
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 try and find it.
These behind me spend all 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 into 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 these Californian waters
is packed up inside hard shells.
To deal with that, the sea otter first 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 of shellfish in a day.
River otters leave the water to mate,
but these creatures 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.
How do they prevent themselves from being carried away by the current?
They wrap themselves up in the kelp, as this one has done.
You might think 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 still retain one habit from their land-living past:
they're still territorial and don't want to leave their family hunting grounds undefended.
How does a sea otter deal with the problem
that's so crucial for all mammals everywhere, of staying warm?
My dry suit gives me very good insulation,
but the sea otter's fur is superb insulation.
It has more hairs in one square centimetre of its body
than any human being has on their head.
Sea otter fur is the densest fur in the whole of the animal kingdom.
And it takes a lot of looking after.
Its efficiency as an insulator depends on having air trapped within it.
To make sure it is at its most effective,
they spend a lot of time blowing into their dense under-fur.
When an otter dives, some air inevitably is squeezed from its fur,
but 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 wetsuit is even efficient enough
to keep them warm in the freezing 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, happily, hunting is 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 more extremely adapted to swimming.
Their front legs have become paddles, their back, broad flippers.
And they've 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.
The hind can still be pointed forward to help them walk.
They still have to come to land to give birth to their pups.
Beaches, to be suitable for a sea lion nursery,
need to have a gently shallowing seaward approach,
so that they aren't battered by heavy waves,
and to be on islands or sheltered coves
that are difficult for land predators to reach.
Such places are not common,
so they are usually very crowded, like this one in New Zealand.
Each patch is dominated by a big male, a beachmaster,
who will claim any female who lands on his patch
and mate with her as soon as she's given birth.
He keeps a look-out for any other male who might have the same idea.
(HARSH THROATY GROWL)
The females need to get back to the sea in order to feed,
so they rear their babies quickly
and provide them with extremely rich milk.
and 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 at all.
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.
(MOTHER CALLS - PUP REPLIES)
Made it at last!
South of New Zealand, in the Antarctic,
it is 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.
Seals have taken their swimming adaptations slightly 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.
More importantly, their hind legs have become so shortened
they no longer point 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 whole surface of the sea frozen,
an expectant mother seal can haul herself out of the water almost anywhere.
So a male can't lord it over harems,
and 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 any other continent
that there are no terrestrial hunters here to threaten a mother and her young.
That is a very valuable privilege that is denied to seal pups elsewhere.
I am now at the other end of the earth, in the north, the Arctic.
It may look very much the same as the Antarctic, with snow fields 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 around here.
These are the footprints...
of an Arctic fox.
And foxes prey on newborn seals.
I'm out on the frozen surface of the sea.
Here, mother seals come up through holes in the ice
and excavate a little cave in the snow as a nursery.
The fox whose tracks I'm following has detected such a den
and burrowed into it, but did it catch the pup?
This is the surface of the sea ice,
and 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.
Here it seems that 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're 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 twenty minutes or so,
but they have to regularly visit their 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, and 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 her baby.
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.
Out here, the males have no chance of assembling a harem
as they can on a crowded nursery beach.
Instead, each sits alongside a female,
waiting for her to become sexually receptive again.
Hooded seals have their own particular way of impressing rivals.
It blows up its hood, a cavity beneath the skin on the top of its head,
then inflates the scarlet membrane that balloons out of one of its nostrils.
If displays like this aren't enough to settle a dispute,
then the males have to resort to physical violence.
(RASPING GROWL)
Male harbour seals have an even stranger courtship ritual,
one that's 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.
Others, probably younger ones, then join him.
(''SINGING'')
Eventually, half a dozen may be singing,
holding their heads together like a barbershop 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 become fishermen,
but they've retained very much the character of their ancestors.
They're fierce and aggressive.
What about the early plant-eating mammals?
They too went into the water about 35 million years ago,
because there's quite a lot of water plants, particularly in shallow, fresh water.
They too have retained the character of their ancestors.
They are gentle grazers.
Here in the warm, clear waters of the Florida creeks, they still are.
Manatee.
They are so completely at home in water...
- (SNORTS) - Ooh!
..that they never leave it.
Oh, dear!
I suppose a little halitosis is what you'd expect from all those leaves,
but phew, that's a bit strong!
What were those vegetarian ancestors? No one really knows.
Some characteristics link manatees to elephants, in particular their teeth.
These, like those of elephants, are flat, grinding molars.
As they are worn down by the coarse grass,
they are 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,
and, with plenty of vegetation there for the taking,
there is 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 leisurely 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 quiet, gentle lives, trundling across the 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 where there's light,
so they have little difficulty in finding their food.
Other swimming mammals have a harder time of it.
India - the Ganges.
There are water-living mammals here too, though they're rare and difficult to spot.
There's one!
It's a river dolphin.
The trouble with rivers in general, and the Ganges in particular,
is that they are full of sediment and very cloudy.
Below the surface, it's impossible to see more than a few inches ahead.
The water is as opaque as milky coffee. Eyes are no use at all.
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, just as we have learned to do these days, electronically.
We can make the 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 then perceive on this monitor.
Let's try.
(BLEEPING)
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.
- And there it is. - (CLICKING)
(CLICKS AND SQUEAKS)
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 have invented
their own very special way of hunting,
that as far as we know is used by no other dolphins anywhere in the world.
It's daring and complicated, but the birds know how to 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.
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 very murky water must be done by sound.
It also requires the high intelligence needed...
..the high intelligence needed to plan ahead,
which is more than I managed to do!
They obviously knew they were going to come to a safe place,
and one of the keys that tells you it's going to happen
is when one of the team pokes its head out of the water
in order to make sure that 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.
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
and having to compete for the same fish on the mud between them,
or both turning their backs on those same fish and 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 way - the determined way - in which they are travelling,
and the fact that all these birds are following them,
leads one to think they actually know there are fish over there - and they're after them.
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 a 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.
Now the moment has come to swim straight into the meatball
and collect the reward for all this effort.
As the sardines are forced towards the surface,
they come within range of seabirds 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.
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.
All that is left of the hind legs and the 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...it's coming up. There!
The blue whale...is 100 feet long...thirty metres.
Nothing like that can grow on land
because no bone is strong enough to support such bulk.
Only in the sea can you get such huge size as that magnificent creature.
And down it goes.
The land-living, deer-like creatures, ancestors of the great whales,
first entered the water around 55 million years ago.
Since then, their descendants have evolved ways
of solving all the problems of life at sea.
With one blast from its nostrils,
It's able to store oxygen not just in its blood
but in the tissues of its vast body.
It can stay beneath the surface for half an hour or more.
It collects food wholesale.
With one sideways gulp, it takes in tonnes of krill-filled water.
Their ancestor's 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 nearly 20 inches thick,
that insulates it against the chill of the water, no matter what the depth.
It has a near perfect hydrodynamic shape,
uninterrupted by hind limbs, ears or genitals.
As it tilts its 100-ton 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 so much of their time,
they communicate, like dolphins, with sound.
(SQUEAKS, GRUNTS AND GROANS)
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 listen to the distant thunder
of waves breaking on a 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 songs.
They produce deep notes, almost beyond the range of our hearing.
If you swim alongside them, these vibrations seem to fill your entire body
as the lowest notes of a great organ will throb inside a cathedral.
(WHALES ''SING'')
The more complex notes are directed to females,
inviting them to come and mate.
All humpback males in one part of the ocean
sing the same sequence of sounds, the same song.
Each, as he sings, may repeat the phrases within that sequence several times.
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 tons propelled into the air with a flip of a tail.
The sound above water is like a cannon shot.
Below, it must be felt for miles around.
And here, these whales demonstrate their solution
to that crucial 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.
But that is not easy
when a male is as formidably equipped as a right whale.
A slit has opened in a male's underside and a penis protrudes,
twelve 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.
(BELLOWS)
Male right whales have gigantic testes, the largest in the world.
They weigh a ton 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,
even though they 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 ourselves, with the aid of plastic flippers and compressed air bottles,
managed to follow them a few decades ago,
but we still have a great deal to learn about how they organise their lives.
Given the elusive nature of most marine mammals,
it will be many years yet before their mysteries are finally unravelled.