0:00:37 > 0:00:42Nearly all mammals have to go down to the water to drink.
0:00:42 > 0:00:47And even the most unlikely of them can swim.
0:00:47 > 0:00:52You might not think that an elephant would willingly go out of its depth,
0:00:52 > 0:00:55but many do so quite regularly.
0:00:55 > 0:01:02Some scientists even believe that the elephants' ancestors once spent much of their time in water,
0:01:02 > 0:01:08and that their trunk first evolved as a device to help them breathe there - as a snorkel.
0:01:10 > 0:01:16It's certainly true that elephants even now are very fond of bathing
0:01:16 > 0:01:19and can swim across deep channels.
0:01:25 > 0:01:33But there are some mammals that swim so frequently that water has become their true home.
0:01:37 > 0:01:40Fresh water contains all kinds of food,
0:01:40 > 0:01:48both animal and vegetable, and mammals of many kinds have ventured there in search of it.
0:01:48 > 0:01:56The desman belongs to that ancient group, the insect eaters, that were around in the time of the dinosaurs.
0:01:56 > 0:01:58Like its relations the shrews,
0:01:58 > 0:02:02it lives on worms and molluscs as well as insects.
0:02:02 > 0:02:09Most shrews look for such things on land, but the desman is more adventurous.
0:02:10 > 0:02:14And it's got special underwater gear -
0:02:14 > 0:02:19a snorkel. A miniature version of the elephant's trunk.
0:02:29 > 0:02:35It's also got long, dense fur that keeps it warm in the water.
0:02:41 > 0:02:46These two modifications make it a very effective swimmer,
0:02:46 > 0:02:54and its snorkel also serves as a sensitive probe to help it discover things to eat on the riverbed.
0:02:54 > 0:03:01Even so, its body is very buoyant and keeping below the surface is hard work,
0:03:01 > 0:03:05so it seldom dives for more than a few minutes at a time.
0:03:13 > 0:03:18And having caught something, it has to come back to land to eat it.
0:03:27 > 0:03:34But hunger has led other mammals to swim in much bigger and more hazardous waters -
0:03:34 > 0:03:36the seas.
0:03:38 > 0:03:43The oceans that cover two-thirds of the planet are full of food.
0:03:43 > 0:03:48So it's hardly surprising that some mammals have gone there to find it.
0:03:48 > 0:03:55These behind me spend their lives at sea. And these particular ones spend most of their time feeding.
0:03:55 > 0:04:02In fact, in proportion to their size, they probably have the biggest appetite of any mammal.
0:04:02 > 0:04:05They are sea otters.
0:04:05 > 0:04:11The ancestors of otters were weasel-like creatures -
0:04:11 > 0:04:18land-living carnivores that scampered around on four feet, had warm blood and breathed air.
0:04:21 > 0:04:27Each one of these characteristics poses a problem for any mammal that tries to take up swimming.
0:04:27 > 0:04:32I can solve them by putting flippers on my feet,
0:04:32 > 0:04:36by wearing an insulated suit to keep me warm
0:04:36 > 0:04:41and putting a snorkel in my mouth so that I can breathe underwater.
0:04:52 > 0:05:00The otter has developed webs between its toes and in that way converted them to paddles.
0:05:00 > 0:05:05Even the best human scuba diver - and that's certainly not me -
0:05:05 > 0:05:09can't match the sinuous agility of a sea otter.
0:05:17 > 0:05:23Much of the food in the Californian waters is packed up in hard shells.
0:05:23 > 0:05:28To deal with that, the sea otter collects a stone from the sea floor.
0:05:28 > 0:05:35Back on the surface, it puts the stone on its stomach and uses it as an anvil.
0:05:44 > 0:05:49Sea otters are so good at this and so energetic
0:05:49 > 0:05:57that one can crack open and eat a quarter of its own weight in shellfish in a day.
0:06:11 > 0:06:17River otters leave the water to mate, but sea otters are so at home at sea
0:06:17 > 0:06:23that they mate here, bringing a new meaning to the concept of synchronised swimming.
0:06:47 > 0:06:51They don't even go back to land to sleep.
0:06:51 > 0:06:57And how do they prevent themselves from being carried away by the current?
0:06:57 > 0:07:02They wrap themselves in kelp, like this one has done.
0:07:02 > 0:07:07You might think that it wouldn't matter very much to an otter
0:07:07 > 0:07:09if it did drift a bit while it dozed.
0:07:09 > 0:07:16But these kelp forests are rich feeding grounds, and sea otters are territorial
0:07:16 > 0:07:22and they don't want to leave their family hunting grounds undefended.
0:07:26 > 0:07:32And how does a sea otter deal with the problem - so crucial for mammals everywhere - of staying warm?
0:07:32 > 0:07:36My dry suit gives me very good insulation,
0:07:36 > 0:07:39but the sea otters' fur is superb.
0:07:39 > 0:07:45It has more hairs in one square centimetre of its body
0:07:45 > 0:07:50than any human being has on their head.
0:07:50 > 0:07:56In fact, sea otter fur is the densest fur in the whole of the animal kingdom.
0:07:56 > 0:08:03It takes a lot of looking after. Its efficiency as an insulator depends on having air trapped in it.
0:08:03 > 0:08:10To make sure that it is at its most effective, sea otters spend a lot of time blowing into their dense fur.
0:08:17 > 0:08:22When an otter dives, some air, inevitably, is squeezed from its fur.
0:08:22 > 0:08:27But even so, enough remains to keep the otter warm and snug.
0:08:31 > 0:08:36Few animals look more at ease on the surface of the sea than a sea otter.
0:08:36 > 0:08:43Their furry wet suit is even efficient enough to keep them warm in the frozen waters of Alaska.
0:08:44 > 0:08:48But that superb fur was nearly their downfall.
0:08:50 > 0:08:57Human beings prized it so greatly that they hunted the sea otter close to extinction. Now, hunting's banned.
0:08:57 > 0:09:05There are other sea-going mammals that fish along these Pacific coasts of North America -
0:09:05 > 0:09:07sea lions.
0:09:07 > 0:09:11They may have taken to the water even before the sea otters,
0:09:11 > 0:09:17for their limbs are now even more extremely adapted to swimming.
0:09:17 > 0:09:22Their front legs have become paddles, their back - broad flippers.
0:09:22 > 0:09:27And they have developed an additional means of insulation.
0:09:27 > 0:09:34As well as fur, they have a specially thick layer of fat beneath the skin - blubber.
0:09:34 > 0:09:39They do, however, still retain their external ears.
0:09:39 > 0:09:45And it's this that identifies them as sea lions rather than seals.
0:09:47 > 0:09:51Even though all four limbs are flippers,
0:09:51 > 0:09:54the front legs are stout enough to act as props
0:09:54 > 0:10:00and the hind can still be pointed forward to help them walk.
0:10:04 > 0:10:09But they still have to come to land to give birth to their pups.
0:10:15 > 0:10:19Beaches, to be suitable for a sea-lion nursery,
0:10:19 > 0:10:24must have a gentle seaward approach so they aren't battered by waves
0:10:24 > 0:10:31and to be on islands or sheltered coves that are difficult for land predators to reach.
0:10:31 > 0:10:34Such places are not common,
0:10:34 > 0:10:39so they're usually crowded, like this one in New Zealand.
0:10:43 > 0:10:48Each patch is dominated by a big male, a beach master,
0:10:48 > 0:10:55who will claim any female who lands on his patch, and mate with her as soon as she's given birth.
0:10:55 > 0:11:00He keeps a lookout for any other male who might have the same idea.
0:11:06 > 0:11:09HE GROWLS
0:11:23 > 0:11:28The females need to get back to the sea in order to feed,
0:11:28 > 0:11:35so they rear their babies as quickly as possible and provide them with rich milk - it's about 30% fat.
0:11:35 > 0:11:42The baby consumes such quantities at such speed that the growth of its bones and muscles can't keep pace.
0:11:42 > 0:11:50So, the baby converts some of the fatty milk into baby fat, blubber, which takes hardly any time.
0:11:50 > 0:11:55But inevitably, this pampered life will soon come to an end.
0:11:55 > 0:11:58It's going to get much tougher.
0:12:09 > 0:12:12After a mere three weeks or so,
0:12:12 > 0:12:16a mother leads her baby down for its first swim.
0:12:21 > 0:12:28To reach open water, they have to get through the swirling, entangling beds of kelp.
0:13:42 > 0:13:45Made it at last.
0:13:57 > 0:14:01South of New Zealand, in the Antarctic,
0:14:01 > 0:14:05it's so cold that the sea freezes over.
0:14:08 > 0:14:11These are seals, not sea lions.
0:14:11 > 0:14:18Both groups seem to be descended from an early carnivore - something between a weasel and a bear.
0:14:18 > 0:14:24But seals have taken their swimming adaptation farther than sea lions.
0:14:24 > 0:14:30They have completely lost their small external ears, which sea lions have retained.
0:14:30 > 0:14:35Consequently, their heads are just that much better streamlined.
0:14:36 > 0:14:44And their hind legs have become so shortened that they can no longer be pointed forwards to help in walking.
0:14:44 > 0:14:51All a seal can do to get around out of water is to hump its whole body, or simply slide.
0:15:00 > 0:15:07With the surface of the sea frozen, an expectant mother seal can haul herself out of the water anywhere.
0:15:07 > 0:15:10So, a male can't lord it over harems
0:15:10 > 0:15:16and the females are left to produce their young in comparative peace.
0:15:19 > 0:15:24Seal pups here have a comparatively safe childhood.
0:15:24 > 0:15:26The frozen seas around Antarctica
0:15:26 > 0:15:33are so far from other continents that there are no terrestrial hunters here to threaten the seals.
0:15:33 > 0:15:38That is a privilege, and one that is denied to seal pups elsewhere.
0:16:01 > 0:16:06I'm now at the other end of the Earth - the north, the Arctic.
0:16:06 > 0:16:13It may look very much the same as the Antarctic, with snowfields and icebergs,
0:16:13 > 0:16:18but as far as seals are concerned, it's crucially different
0:16:18 > 0:16:26because land extends north into the Arctic and there are land predators that can get out onto the sea ice.
0:16:26 > 0:16:29There are tracks of them all here.
0:16:35 > 0:16:39These...are the footprints...
0:16:41 > 0:16:44..of an Arctic fox.
0:16:46 > 0:16:49And foxes prey on new-born seals.
0:16:51 > 0:16:55I'm out on the frozen surface of the sea.
0:16:55 > 0:17:00Here, mother seals come up through holes in the ice
0:17:00 > 0:17:03and dig snow caves as a nursery.
0:17:03 > 0:17:08The fox whose tracks I'm following has found it and burrowed into it.
0:17:08 > 0:17:10But did it catch the pup?
0:17:41 > 0:17:44This...is the surface of the sea ice.
0:17:44 > 0:17:50Over there is the hole through which the female seal came
0:17:50 > 0:17:53in order to burrow out this lair.
0:17:53 > 0:17:58Here, snug, away from the blizzards and gales of the Arctic,
0:17:58 > 0:18:01she gave birth.
0:18:01 > 0:18:06And here, it seems, that the pup did escape from that fox
0:18:06 > 0:18:11for there is no sign of any blood on the ice.
0:18:11 > 0:18:17But there are bigger predators here than foxes.
0:18:21 > 0:18:24Polar bears.
0:18:24 > 0:18:27They, too, are on the lookout for pups.
0:18:27 > 0:18:32Ringed seals, at this time of the year, are their staple diet.
0:18:41 > 0:18:46Ringed seal pups can't swim until they are several weeks old.
0:18:46 > 0:18:52Their survival depends on them remaining undetected in their nurseries under the snow.
0:18:59 > 0:19:03The adults are relatively safe beneath the ice.
0:19:03 > 0:19:10They can stay submerged for 20 minutes, but visit the breathing holes to prevent them freezing over.
0:19:27 > 0:19:32Polar bears have an extraordinarily sensitive sense of smell.
0:19:32 > 0:19:39They can detect the breath of a seal drifting up from the snow from over half a mile away.
0:19:39 > 0:19:42That could lead them to a pup.
0:20:05 > 0:20:13That pounce smashed the roof of the nursery den and could have killed the pup outright.
0:20:20 > 0:20:24But this pup was already dead.
0:20:24 > 0:20:27Its little body is stiff and frozen.
0:20:27 > 0:20:32Maybe its mother failed to keep the entrance hole free of ice
0:20:32 > 0:20:35and couldn't get back to feed him.
0:20:56 > 0:21:02Several kinds of seals in the Arctic breed away from land, out on the ice.
0:21:07 > 0:21:11Female seals mate soon after giving birth.
0:21:11 > 0:21:16That means they only have to leave the sea once a year, not twice.
0:21:16 > 0:21:22Here, the males have no chance of assembling a harem, as they can on a beach.
0:21:22 > 0:21:28Instead, each one waits for a female to become sexually receptive again.
0:21:28 > 0:21:33These hooded seals have their own way of impressing rivals.
0:21:34 > 0:21:38It blows up its hood - a cavity beneath the head skin -
0:21:38 > 0:21:44and then inflates a scarlet membrane that balloons out of its nostrils.
0:21:59 > 0:22:06If displays like this aren't enough to settle a dispute, the males have to resort to physical violence.
0:22:38 > 0:22:43Male harbour seals have an even stranger courtship ritual,
0:22:43 > 0:22:50and one that has been discovered so recently that its mechanisms are still not fully understood.
0:22:50 > 0:22:55They go in for competitive choral singing.
0:22:55 > 0:23:00One big male, just off the breeding beach, begins to vocalise.
0:23:00 > 0:23:03LOW RUMBLINGS
0:23:10 > 0:23:15Others - probably younger ones - then join him.
0:23:16 > 0:23:20THEY ALL MAKE SOUNDS
0:23:22 > 0:23:29Eventually, half a dozen may be singing, holding their heads together like a barber-shop group.
0:23:34 > 0:23:41When a female does appear, the one who started the performance swims away with her
0:23:41 > 0:23:45while the rest obligingly wait behind.
0:24:05 > 0:24:09Otters, seals and sea lions
0:24:09 > 0:24:16are all descended from an ancient group of hunting mammals that were tempted into the water to fish.
0:24:16 > 0:24:23But they've retained the character of their ancestors - they're fierce and aggressive.
0:24:23 > 0:24:26But what about the early plant-eating mammals?
0:24:26 > 0:24:31They, too, went into the water about 35 million years ago,
0:24:31 > 0:24:36because there are many water plants, particularly in shallow fresh water.
0:24:36 > 0:24:42They, too, have retained the character of their ancestors.
0:24:42 > 0:24:44They are gentle grazers.
0:24:49 > 0:24:55And here, in the warm, clear waters of the Florida creeks,
0:24:55 > 0:24:58they still are. Manatee.
0:24:58 > 0:25:04They're so completely at home in water...
0:25:04 > 0:25:06that they never leave it.
0:25:06 > 0:25:09Oh, dear...
0:25:12 > 0:25:17I suppose a little halitosis is what you'd expect from all these leaves,
0:25:17 > 0:25:20but - phew! - that's a bit strong.
0:25:23 > 0:25:26But what were those vegetarian ancestors?
0:25:26 > 0:25:32No-one knows. Some characteristics, like teeth, link manatees to elephants.
0:25:32 > 0:25:37These, like those of elephants, are flat, grinding molars.
0:25:37 > 0:25:42As they're worn down by the coarse grass, they're replaced by new ones
0:25:42 > 0:25:47that erupt at the back of the jaw and slowly move forward.
0:25:53 > 0:25:58Manatees are so big that nothing much attacks them.
0:25:58 > 0:26:05With plenty of vegetation there for the taking, there's no need for them to be swift swimmers.
0:26:05 > 0:26:10Their forelimbs have become short flippers that can be used as paddles
0:26:10 > 0:26:13or to gently punt along the bottom.
0:26:13 > 0:26:19They still carry nails - vestiges of their terrestrial past.
0:26:19 > 0:26:26Their hind legs have disappeared altogether, and they propel themselves on their cruises
0:26:26 > 0:26:32with slow, powerful sweeps of their huge tails.
0:26:39 > 0:26:47Their bristly upper lip is so well muscled that they can use it to grasp leaves,
0:26:47 > 0:26:51rip them up and push them into their mouth.
0:26:51 > 0:26:56They have gentle lives trundling across shallow submarine pastures.
0:26:56 > 0:27:01Their other name is "sea cow", and very appropriate it is, too.
0:27:01 > 0:27:05Manatees live in clear, sunlit waters.
0:27:05 > 0:27:13The plants they feed on only grow in light, so they have little difficulty in finding their food.
0:27:13 > 0:27:17But other swimming mammals have a harder time of it.
0:27:17 > 0:27:20India, the Ganges.
0:27:20 > 0:27:26There are water-living mammals here, though they're rare and hard to spot.
0:27:26 > 0:27:28There's one.
0:27:30 > 0:27:33It's a river dolphin.
0:27:37 > 0:27:42The trouble with rivers in general - and the Ganges in particular -
0:27:42 > 0:27:47is that they're full of sediment and very cloudy.
0:27:47 > 0:27:52Below the surface, it's impossible to see more than a few inches ahead.
0:27:52 > 0:27:56The water is opaque. Eyes are no use at all.
0:27:56 > 0:28:02And the river dolphin has lost the use of them. It's completely blind.
0:28:02 > 0:28:06How, then, does it find the fish it feeds on?
0:28:06 > 0:28:09It uses sound - electronically.
0:28:09 > 0:28:15WE can make a sound and use a system known as "sonar".
0:28:15 > 0:28:20We can send out very high-pitched sounds from this.
0:28:20 > 0:28:25And if that hits their body, it causes an echo which we will receive
0:28:25 > 0:28:27on this monitor.
0:28:27 > 0:28:29Let's try.
0:28:35 > 0:28:38BEEPING
0:28:38 > 0:28:41There they are.
0:28:42 > 0:28:47Shoals of fish, somewhere out there in the murky water.
0:28:47 > 0:28:52River dolphins use sound in exactly the same way.
0:28:52 > 0:28:59If I lower an underwater microphone, we can hear the sounds THEY are making to locate their prey.
0:29:04 > 0:29:07BUZZING
0:29:09 > 0:29:12GURGLING
0:29:15 > 0:29:19All dolphins exploit sound when hunting.
0:29:19 > 0:29:26But here, on the south-eastern coast of the United States, in Georgia and the Carolinas,
0:29:26 > 0:29:31there are dolphins that've invented their own special way of hunting
0:29:31 > 0:29:36that seems to be used by no other dolphins anywhere in the world.
0:29:36 > 0:29:42It's daring and it's complicated, but the birds can predict it.
0:29:42 > 0:29:48They're assembling over there, so that's where we should point our cameras.
0:29:48 > 0:29:54And sure enough, there in the water in front of them are the dolphins.
0:29:54 > 0:30:02They're swimming slowly back and forth, edging a shoal of fish closer to the river bank.
0:30:02 > 0:30:06And now their tactics are about to change.
0:30:15 > 0:30:22Several dozen little fish were swept up onto the mud, and the dolphins are now snapping them up.
0:30:22 > 0:30:26The birds are getting quite a lot, too.
0:30:26 > 0:30:30Now, the dolphins have to wriggle back to water.
0:30:37 > 0:30:44Off they go upriver to find the next suitable place for doing the same thing all over again.
0:30:46 > 0:30:51Once more, the birds show us where that is likely to be.
0:30:51 > 0:30:54But have they got it right?
0:31:23 > 0:31:28This daring strategy depends on a number of things.
0:31:28 > 0:31:30First, obviously, teamwork.
0:31:30 > 0:31:37And that requires an ability for the members of the team to communicate with one another,
0:31:37 > 0:31:41which in this murky water must be done by sound.
0:31:41 > 0:31:45But it also requires a high intelligence...
0:31:51 > 0:31:58The high intelligence needed to plan ahead, which was more than I managed to do!
0:31:58 > 0:32:03They obviously knew that they were going to come to a safe place,
0:32:03 > 0:32:08and one of the keys that tells you that it's going to happen
0:32:08 > 0:32:13is when one of the members of the team pokes its head out of the water
0:32:13 > 0:32:17in order to make sure everything is safe on the bank.
0:32:17 > 0:32:20Synchronisation must be perfect
0:32:20 > 0:32:27to create the necessary surge, and that can only be done by underwater communication between the team.
0:32:27 > 0:32:30And they must all turn the same way.
0:32:30 > 0:32:35If two alongside one another were to turn in different directions,
0:32:35 > 0:32:40they would either end up facing one another, competing for the same fish,
0:32:40 > 0:32:46or both turning their backs on those same fish, allowing them to escape.
0:32:54 > 0:32:57Out in the open ocean,
0:32:57 > 0:33:00dolphin teams may number several hundred.
0:33:24 > 0:33:27These are common dolphin,
0:33:27 > 0:33:31and the speed with which they are going - wow! -
0:33:31 > 0:33:38and the determined way in which they're travelling, and the fact that all these birds are soaring
0:33:38 > 0:33:43means that they know there are some fish right over there.
0:33:51 > 0:33:58The whole school stretches out on either side of me for a quarter of a mile or more in either direction.
0:33:58 > 0:34:05They seem to be chasing a shoal of fish ahead of them, just as those dolphins were doing in the river.
0:34:05 > 0:34:08But this is on vastly greater scale.
0:34:08 > 0:34:13They've succeeded in isolating a huge school of sardines,
0:34:13 > 0:34:18and now they're swimming round them, herding the shoal in upon itself,
0:34:18 > 0:34:22forcing it into one gigantic meatball.
0:34:32 > 0:34:38They drive the shoal upwards so that it will be trapped against the surface.
0:34:42 > 0:34:47And now the moment has come to swim straight into the meatball
0:34:47 > 0:34:51and collect the rewards for all this effort.
0:34:54 > 0:34:59As the sardines are forced towards the surface,
0:34:59 > 0:35:03so they come within range of sea birds overhead.
0:35:41 > 0:35:46There's a water-living mammal that feeds in a quite different way.
0:35:46 > 0:35:53Instead of teeth, it uses baleen - horny plates that are hung from its upper jaw
0:35:53 > 0:35:56and fringed with long, coarse hairs.
0:35:56 > 0:36:03It collects krill - little shrimp-like creatures, scarcely bigger than my little finger -
0:36:03 > 0:36:08but it finds them in such quantity that it's become gigantic.
0:36:20 > 0:36:25It takes in a great mouthful of water and krill, then shuts its jaws,
0:36:25 > 0:36:29and up comes its tongue. It's as big as an elephant.
0:36:29 > 0:36:36The tongue pulls back, wipes the krill from the baleen, and the animal swallows it.
0:36:36 > 0:36:42And that krill is so nutritious that this creature, the blue whale,
0:36:42 > 0:36:46is the biggest that has ever existed on this planet -
0:36:46 > 0:36:50almost twice as heavy as the biggest known dinosaur.
0:37:01 > 0:37:04Its vast ribcage houses its lungs.
0:37:04 > 0:37:09They carry 2,000 litres of air -
0:37:09 > 0:37:13that's 500 times the capacity of our lungs.
0:37:15 > 0:37:20The heart is as big as a small family car.
0:37:20 > 0:37:24It only beats five or six times a minute,
0:37:24 > 0:37:31but it drives ten tonnes of blood through a million miles of blood vessels.
0:37:37 > 0:37:40And all that is left
0:37:40 > 0:37:45of the hind legs and hip bones are these two isolated fragments,
0:37:45 > 0:37:48buried in a mountain of muscle.
0:38:14 > 0:38:18I can see its tail, just under my boat here.
0:38:18 > 0:38:21It's coming up...
0:38:23 > 0:38:25The blue whale...
0:38:25 > 0:38:28is 100 feet long...
0:38:28 > 0:38:3030 metres.
0:38:30 > 0:38:37Nothing like that can go on land because no bone is strong enough to support such bulk.
0:38:37 > 0:38:43Only in the sea do you get such huge size as that magnificent creature.
0:39:01 > 0:39:03And down it goes...
0:39:03 > 0:39:10The land-living deer-like creatures that were the ancestors of the great whales first entered the water
0:39:10 > 0:39:13around 55 million years ago.
0:39:13 > 0:39:20Since then, their descendents have evolved ways of solving all the problems of life at sea.
0:39:20 > 0:39:27With one blast from its nostrils, a whale discharges 90% of the spent air from its lungs and takes in new.
0:39:27 > 0:39:32Most land-living mammals only manage to void about 15%.
0:39:32 > 0:39:40It's able to store oxygen not just in its blood, but in the tissues of its vast body.
0:39:40 > 0:39:45And so it can stay beneath the surface for half an hour or more.
0:39:45 > 0:39:48It collects food wholesale.
0:39:48 > 0:39:53With one sideways gulp, it takes in a tonne of krill-filled water.
0:39:53 > 0:40:00Their ancestors' coat of hair, so characteristic of all land mammals, has been completely lost.
0:40:00 > 0:40:07Instead, the whale's entire body is swathed by a blanket of fat beneath the skin, in places 20 inches thick,
0:40:07 > 0:40:12which insulates it against the chill of the water, no matter the depth.
0:40:18 > 0:40:22It has a near-perfect hydrodynamic shape,
0:40:22 > 0:40:27uninterrupted by hind limbs, ears or genitals.
0:40:39 > 0:40:43And as it tilts its hundred-tonne body downwards,
0:40:43 > 0:40:48so it can plunge to the black world 500 feet or more below the surface.
0:40:53 > 0:40:57Down in the blue immensities of the oceans,
0:40:57 > 0:41:03where the great whales spend much of their time, they communicate, like dolphins, by sound.
0:41:03 > 0:41:07BOOMING ECHOES HIGH-PITCHED SQUEAKING
0:41:07 > 0:41:10GRUNTING AND RASPING
0:41:11 > 0:41:18Sound travels further and faster in water than it does in air.
0:41:18 > 0:41:22Loud noises can be heard hundreds of miles away,
0:41:22 > 0:41:27so whales may be able to hear the distant waves breaking on the shore
0:41:27 > 0:41:34and use that to find their way around the otherwise featureless expanses of the open oceans.
0:41:34 > 0:41:41Individuals also call to one another and may keep in contact even though they're hundreds of miles apart.
0:41:41 > 0:41:47Humpback whales have developed particularly complex sounds.
0:41:47 > 0:41:52They produce deep notes, almost beyond the range of our hearing.
0:41:52 > 0:41:57If you swim alongside them, these vibrations seem to fill your body
0:41:57 > 0:42:02as the low notes of an organ will throb inside a cathedral.
0:42:10 > 0:42:15The more complex notes are directed to females, inviting them to mate.
0:42:15 > 0:42:21All humpback males in one part of the ocean sing the same sequence of sounds,
0:42:21 > 0:42:29the same song, but each, as he sings, may repeat the phrases within that sequence several times.
0:42:29 > 0:42:33GRUNTING AND TRILLING
0:42:45 > 0:42:49A complete song may last for half an hour.
0:42:49 > 0:42:56Once it's over, the male may repeat it and continue doing so over and over again
0:42:56 > 0:43:00in a performance that may last several days.
0:43:34 > 0:43:39Off the coast of Patagonia, southern right whales are assembling.
0:43:39 > 0:43:45The males announce their arrival by gigantic leaps. 100 tonnes propelled into the air with the flip of a tail.
0:43:45 > 0:43:50The sound above water is like a cannon shot.
0:43:50 > 0:43:53Below, it must be felt for miles around.
0:44:00 > 0:44:08These whales show their solution to the problem for all mammals if they are to live permanently in the sea -
0:44:08 > 0:44:11how to breed in water.
0:44:11 > 0:44:16This female is surrounded by ardent males.
0:44:16 > 0:44:19She's not yet ready to mate,
0:44:19 > 0:44:26and rolls over on her back in an attempt to keep her genital region away from her suitors.
0:44:34 > 0:44:39That's not easy when a male is as formidably equipped as a right whale.
0:44:44 > 0:44:49A slit has opened in the male's underside and a penis protrudes -
0:44:49 > 0:44:5112 feet long and highly mobile.
0:45:12 > 0:45:19The males barge and jostle one another to reach her and several may succeed - one after the other.
0:46:03 > 0:46:08Now, seemingly, the female has changed her mind.
0:46:08 > 0:46:14She rights herself and leaves the surface. Now she is ready to receive a male.
0:47:00 > 0:47:05Male right whales have gigantic testes, the largest in the world.
0:47:05 > 0:47:10They weigh a tonne, and produce gallons of sperm.
0:47:10 > 0:47:14One coupling can flush out whatever preceded it,
0:47:14 > 0:47:21so it may not be the first male who succeeded in copulating who becomes a father. It will be the last.
0:47:40 > 0:47:47So, some mammals who started out with four legs and no fins, with bodies that had to be kept warm
0:47:47 > 0:47:51and with an awkward necessity to breathe air
0:47:51 > 0:47:55have managed to colonise the waters of the world.
0:47:55 > 0:48:00We, with the aid of plastic flippers and compressed air bottles,
0:48:00 > 0:48:04managed to follow them a few decades ago,
0:48:04 > 0:48:09but we still have lots to learn about how they organise their lives.
0:48:09 > 0:48:17Given the elusive nature of marine mammals, it will be many years yet before their mysteries are solved.
0:48:20 > 0:48:27You may wonder how I was lucky enough to get alongside that surfacing blue whale.
0:48:27 > 0:48:31It was done using the latest technologies -
0:48:31 > 0:48:38a radio tagged on the whales sent signals up to a satellite which gives their position.
0:48:38 > 0:48:44Then, a low flying aircraft calls a swift launch. That's how I got there.
0:48:44 > 0:48:51That gets scientists there, too, of course, and it's their job, in a very short time, to collect data.
0:48:53 > 0:48:59To study a mammal that spends the vast majority of its life underwater or far out at sea
0:48:59 > 0:49:03presents an enormous challenge for humans.
0:49:03 > 0:49:10For marine biologists, each close encounter with a huge whale is ample reward.
0:49:10 > 0:49:13I've studied blue whales for years,
0:49:13 > 0:49:18but I will never forget the first time I saw one from the air.
0:49:18 > 0:49:22I was in a rickety old Cessna, over the Sea of Cortez in Mexico.
0:49:22 > 0:49:27We flew over an adult blue whale just as it surfaced.
0:49:27 > 0:49:32It looked more like a Boeing 737 than a real, wild, living whale.
0:49:32 > 0:49:35It was absolutely huge.
0:49:35 > 0:49:42Yet despite its size, the blue whale has remained surprisingly well hidden from humans.
0:49:42 > 0:49:46In August 1986, whale biologist John Calambokidis
0:49:46 > 0:49:52was doing an aerial survey in a vast area of sea off California
0:49:52 > 0:49:56and he made the most incredible discovery.
0:49:56 > 0:50:00He saw a giant amongst humpbacks. It was blue with a huge blow
0:50:00 > 0:50:04and he recognised it as a blue whale.
0:50:04 > 0:50:07Then he saw more and more.
0:50:07 > 0:50:13He had discovered the largest known population of blue whales on Earth.
0:50:13 > 0:50:19Research shows that there's about 2,000 blue whales off California.
0:50:19 > 0:50:27How they could have gone unnoticed for so long I don't know, but it was an extraordinary discovery.
0:50:27 > 0:50:31But the key to understanding is to identify and follow individuals.
0:50:31 > 0:50:37Since the early 1970s, scientists have identified killer whales
0:50:37 > 0:50:40using the shape of the dorsal fin.
0:50:40 > 0:50:46Similarly, humpback whales have individual patterns on their tail fins.
0:50:46 > 0:50:51Blue whales proved more of a challenge, but in the early '80s,
0:50:51 > 0:50:58biologist Richard Sears showed that individual blue whales could be identified by their mottled flanks.
0:51:00 > 0:51:06Now we can recognise over 1,300 of the whales in the California group.
0:51:07 > 0:51:14It's like a human mugshot. Imagine the CIA files for criminals. This is the equivalent for blue whales.
0:51:14 > 0:51:17You build up catalogues of an entire population.
0:51:17 > 0:51:22This is the bricks and mortar of blue whale research now.
0:51:22 > 0:51:26When the whales dive out of camera range,
0:51:26 > 0:51:31it's still possible to follow them by listening to their calls.
0:51:31 > 0:51:36This, too, is a way to identify individuals and families.
0:51:36 > 0:51:41Surprisingly, the end of the Cold War brought the next advance.
0:51:41 > 0:51:46The US navy used very sensitive hydrophones to track submarines.
0:51:46 > 0:51:50Its operatives had listened in to whales for decades.
0:51:50 > 0:51:56The crux point was in 1993 when Chris Clark of Cornell University
0:51:56 > 0:52:01was given permission to use this system to eavesdrop on whales.
0:52:01 > 0:52:08For whale biologists all over the world, this was like Christmas a thousand times over. It was great.
0:52:08 > 0:52:13It meant he could listen to one whale as it crossed an ocean basin.
0:52:13 > 0:52:18The first afternoon Chris and his colleagues listened in
0:52:18 > 0:52:21with the help of navy analysts,
0:52:21 > 0:52:28they actually heard more blue whales than had ever been written about by scientists ever before.
0:52:28 > 0:52:31It was mind-boggling stuff.
0:52:31 > 0:52:35The latest technology gives even more detail.
0:52:35 > 0:52:42A harmless tag sends biological data up to a satellite each time the whale surfaces to breathe.
0:52:42 > 0:52:47It provides vital information for whale biologist Bruce Mate.
0:52:48 > 0:52:54There's a low population of blue whales worldwide now -
0:52:54 > 0:52:56we've lost 92% of them to whaling.
0:52:56 > 0:53:02With 8% left, and the Californian coast having 25% of those,
0:53:02 > 0:53:08we really need to know where they go to breed and calve in the winter to protect them.
0:53:08 > 0:53:12We only know where they are in the summer,
0:53:12 > 0:53:18so tagging them here and tracking them in the winter is important.
0:53:18 > 0:53:23Our understanding of whales, dolphins and other marine mammals
0:53:23 > 0:53:30has increased enormously over recent years thanks to the researches of marine biologists.
0:53:30 > 0:53:37It's THEIR expertise that has enabled us to get our camera teams to the right place at the right time.
0:53:37 > 0:53:42Nonetheless, filming whales still poses enormous problems.
0:53:47 > 0:53:51Sometimes, it's as simple as get on a boat, point the camera.
0:53:51 > 0:53:56In that case, you're only seeing them for a small part of their life.
0:53:59 > 0:54:05We think that to see the subject properly, you have to go underwater.
0:54:05 > 0:54:12Entering the underwater world obviously brings up a whole set of fresh challenges.
0:54:12 > 0:54:15For a start, you can't breathe.
0:54:15 > 0:54:20A lot of times, marine mammals live in water which is quite cold -
0:54:20 > 0:54:26temperate or polar zones - so you have to wear a dry suit rather than a wet suit.
0:54:26 > 0:54:34Your extremities, like your hands, tend to get quite cold, so we put hot water into our gloves,
0:54:34 > 0:54:39which means our fingers are slower to get cold.
0:54:39 > 0:54:47You either use a scuba or a rebreather - which doesn't give out so many bubbles.
0:54:47 > 0:54:54But sometimes the best technique is to go completely simple - simply use your snorkel and breath hold.
0:54:54 > 0:55:00As a testament, this is one of the most memorable whale images.
0:55:00 > 0:55:06It's the sei whale, with a mouth the size of a dustcart, passing right by the lens.
0:55:06 > 0:55:12Cameraman Doug Anderson managed to film it holding his breath
0:55:12 > 0:55:16and with an amazingly steady camera from just two metres.
0:55:21 > 0:55:25Being close to a blue whale is something I'll never forget.
0:55:25 > 0:55:32But conveying its sheer size visually requires a whole range of skills
0:55:32 > 0:55:35and poses a considerable challenge.
0:55:45 > 0:55:50The idea of this sequence was to get over how huge the blue whale is.
0:55:50 > 0:55:57Although there's a great shot of David in the boat, there's no sense of scale.
0:55:57 > 0:56:04We got the skeleton through the internet - there were images of one that's outside a Californian museum.
0:56:04 > 0:56:11We thought it would be a great location, cos there was plenty of access, plenty of space.
0:56:11 > 0:56:17To prepare for shooting, we had to put up a drape to hide the car park behind it
0:56:17 > 0:56:23and then we lit it with arc lamps on cherry pickers from way above,
0:56:23 > 0:56:28and then got David to walk around and through it as we filmed him.
0:56:30 > 0:56:37When we arrived on location, we noticed one problem - the inside was supported by a black frame
0:56:37 > 0:56:40that would appear in every shot.
0:56:40 > 0:56:44We had to paint it electronically.
0:56:44 > 0:56:49Back in Bristol, for each shot we created a scene in the 3-D computer,
0:56:49 > 0:56:57built models of the organs, lit them, animated them and composited them with the shots of David
0:56:57 > 0:57:03to give the final sequence. It's fun AND informative with the organs appearing next to David.
0:57:03 > 0:57:07We brought something new to the screen.
0:57:07 > 0:57:13Finally, we can show you the essence of the greatest mammal on the planet.
0:57:13 > 0:57:19But even now, we've only touched the surface of our understanding.
0:57:19 > 0:57:25The more we care for and study these magnificent creatures, the closer our relationship will become.
0:57:56 > 0:57:59Subtitles by Red Bee Media Ltd