Themes and Variations Life on Earth

Although all those creatures are different,

they are in fact closely related to one another. They are all mammals.

But how have they become so varied?

And what is the ancestral form, the basic theme, on which they are all variations?

You can find a close approximation to that theme in the jungles of South-East Asia.

It's properly called a tupaia,

and it's certainly a mammal, with a hairy coat and warm blood.

But what kind? It looks very like a squirrel.

A close look at its anatomy reveals resemblances to a rabbit,

but it doesn't gnaw nuts and it doesn't nibble grass, it catches insects.

Its teeth are small, numerous and spiky, like a shrew's.

Indeed, its popular name is tree shrew,

but its large brain and those grasping hands

have suggested to some that it's related to monkeys.

It seems in fact to contain hints of many different mammals.

One thing, though, is clear.

It's very like the earliest of mammals,

that were living when the dinosaurs dominated the Earth 100 million years ago.

The basic pattern on which there's been such a multitude of variations.

Some of those variations are so extreme that it's difficult to believe

there's any connection between them and the basic theme,

were it not for the evidence of fossils and the anatomy of the living animals.

The tree shrew's continuous activity and swift reactions are typical of a mammal.

A consequence of its ability to generate heat within its body

so that its chemistry works fast and provides it with abundant energy.

This talent probably developed a very long time ago indeed,

at a time when the dinosaurs dominated the Earth.

For fossils of a creature remarkably similar to the living tree shrew

have been found in rocks that are 200 million years old.

Its numerous spiky teeth suggest that it ate insects,

and the shape of its limbs that it was a swift runner.

In fact, its lifestyle was not unlike a tree shrew.

And creatures like it survived alongside the dinosaurs throughout their reign,

probably scampering about at night

when the colder-blooded dinosaurs became torpid in the cold.

Then, 65 million years ago, the dinosaurs disappeared.

The forests and swamps of the world were suddenly empty of large creatures.

Primitive birds flapped through the sky,

but on the ground there were few creatures other than insects and other invertebrates

and those small warm-blooded primitive mammals that fed on them.

And here and there, in odd corners of the world,

their descendants still survive, little changed.

The tree shrew of Malaysia is one.

Here in the streams of the Pyrenees lives another little-known and very engaging one.

It's called a desman.

Like most of these primitive mammals, the desman has a stupendous appetite.

It eats two-thirds of its own body weight every day

and seems never to stop the hunt for more.

Its nose does most of the searching.

It scents the faintest changes in the taste of the water with its nostrils,

and feels its way around with all those whiskers.

Its feet are a combination of web and claw, for both swimming and clambering.

Its eyes are tiny, minute beads hidden in its long fur.

When at last it does find something good, it doesn't give up easily.

Its snorkel nose allows it to snatch a breath

with the minimum of interruption in the struggle.

Its hard-fought-for worm will now keep it going for another hour or so.

Another of these primitive survivals lives along the streams of North America.

It's not only an energetic swimmer, but a burrower as well.

It's possible the swimming way of life and the body design to go with it

led to a similar activity not in water, but underground.

What were paddles have become spades.

This is the star-nosed mole.

The odd fleshy flower on its nose is another highly sensitive smelling device.

It may have yet another way of investigating its surroundings.

Many of these little insect-eaters, such as shrews,

make squeaks so high-pitched that we can't hear them,

and the echo they produce helps the animals to find their way around.

Moles, like desmans and shrews, have formidable appetites

and have to eat every few hours.

Their tunnels are not simply passageways, but traps.

Worms and insects burrowing through the soil drop into them,

and the mole collects whatever turns up.

If its appetite is momentarily sated,

then it paralyses a surplus worm with sharp bites and stores it away in a special larder

before setting off again on its never-ending patrols.

Variations on the theme of the small insect-eater

began to appear soon after the dinosaurs vanished.

Creatures developed that specialised in feeding on one kind of insect - ants and termites.

This is another digger, the aardvark, from Africa.

And this is its South American equivalent, the giant anteater.

The essential equipment for a diet of ants and termites, it seems,

is an elongated snout for poking inside the nests

and a long sticky tongue for collecting the insects.

And the giant anteater has the most extreme version of both that exists.

Termites are easily crushed,

so the anteater has no need of teeth and has lost them all.

Termites' nests, however, can be as hard as cement,

and strong claws and muscular legs are needed to tear them open.

The anteater is very fussy about its food. In spite of its name it seldom eats ants.

Termites, like these, are a much more usual meal,

and even then, it prefers some termites to others.

There are a dozen or so species of mammal round the world

that have specialised in living on ants and termites.

As a lifestyle it doesn't seem to require

a particularly quick intelligence or vivacious disposition.

And all these anteaters are relatively slow-moving creatures.

Because of that and their total lack of teeth,

they might seem to be easy meat for a hunter.

But the giant anteater's front legs are so strong that its hug is lethal,

and few creatures interfere with it.

The termite-eating specialist of Africa, the pangolin,

is much smaller and not nearly such a powerful digger.

It's developed a flexible armour of scales

and can curl itself up into a ball so that it's virtually impregnable.

Its muscular tail also acts as a counterbalance

so that the creature can trundle along with most of its weight on its back legs,

and its front legs at the ready for digging into termite mounds, like this one.

It's so confident of its defences that it takes little notice of any other creature around,

unless they molest it.

Smallest of all, the pygmy silky-furred anteater of South America.

It does seem to be defenceless and can't move fast enough

to escape even the clumsiest hunter.

But it keeps out of the way up in the branches, living almost entirely on tree ants.

This one has a baby on its back, and it may be either male or female,

for both parents take a share in carrying the load.

There's yet another kind of specialist anteater in South America,

intermediate between the giant and the pygmy - the tamandua.

It feeds mostly at night.

Its thick, bristly fur is supposed to protect the tamandua

from the bites of infuriated ants, swarming from their shattered nest.

But when you watch the animal feeding,

you can't help wondering just how effective that protection really is.

Ants and termites are among the most numerous insects, particularly in the tropics.

And the tamandua and its relatives around the world

have little difficulty in finding more than enough to eat.

And there are insects not only in water and in the soil and all over plants,

but in the air, and particularly at night.

It's difficult to realise just how many there are

until you put up a mercury vapour lamp in the tropics.

Here, within a few minutes, you've got all sorts of creatures.

Small moths, crickets, huge beetles, mantises,

big moths, insects of all kinds.

The insects first took to the air about 300 million years ago.

They had it to themselves for about 100 million years,

at least until the arrival of the reptiles.

Whether there were any night-flying, insect-hunting reptiles, we don't know,

but it seems unlikely because reptiles, being cold-blooded,

are usually active during the day.

And then about 150 million years ago, the birds developed.

But there's no reason to suppose

there were any more night-flying birds in the past

than there are today, and that's precious few.

So this great feast of insects

awaited any creature that could master the tricky technique of flying at night.

And one group of the mammals did.

The bats.

The majority of them are hunters of flying insects,

such as moths, mosquitoes or even beetles.

Caught on the wing and eaten at the roost as the bats hang upside down.

Bats began to fly a very long time ago.

These fossil bones of what is undoubtedly a bat are about 50 million years old.

The bat skeleton is very similar to the tree shrew's.

Seen here from above, and now side on.

But how did this flying variation arise?

It may be that the early insect-eaters sought their food up in the branches of trees,

as indeed some kinds of tree shrews do today.

And as they leapt about trying to snatch flying insects from the air,

some may have developed flaps of skin between their arms and the sides of their body

so that they could glide, as the living flying squirrels can today.

They then supported those flaps with their fingers and strengthened the arm muscles

until, eventually, they were able to flap their newly developed wings

and fly in search of their insect prey, and so became bats.

But living on insects has one great disadvantage.

In many parts of the world insects disappear almost totally during the winter.

What does an insect-eater do then?

It hibernates in any sheltered place it can find

where the temperature might remain a few degrees higher than elsewhere,

as it is inside this old Canadian mine.

These tiny lumps, as cold as stone, are living bats.

They fed voraciously during the summer, building up reserves of fat,

but now a profound change has taken place in their bodies.

Their heat has seeped away, and their body processes have slowed down

to almost, but not quite, a complete halt.

They must keep their body chemistry ticking over

just enough to generate sufficient heat to prevent them from freezing solid,

for that they can't survive.

Not all of them are successful.

Sometimes an individual cannot stave off the chill,

falls and is entombed in the ice.

You might think they huddle together to keep warm.

But careful measurements have shown those in groups get just as cold

as those hanging by themselves.

It may be that grouping protects them from another hazard,

the loss of moisture during breathing.

That does seem to be less for those in clusters.

Other creatures also take refuge in the mine,

the very ones which in summer are food for the bats: moths.

Both hunters and hunted shelter together from that overwhelming killer, cold.

In other parts of the world, as here in New Mexico,

bats solve the problem of lack of insect food by migrating.

From this cave they will fly south some 1,000km for the winter.

They have to, to find enough food,

for their populations are measured in millions,

and tonnes of insects are needed.

Caves like these contain the densest populations of mammals

to be found anywhere on Earth.

How is it that all those bats flying at such a speed

can find their way around in the dark?

The answer is echolocation.

Although I can only hear just the faintest twitter,

in fact, each bat is emitting a more or less continuous stream

of high-frequency sound beyond the range of my ears.

But I can translate those into sounds that I can hear using a machine like this.

A bat detector. Listen.

TWITTERING

The system is based on those high-pitched ultrasounds,

like those produced today by shrews

and which the early insect-eaters may have used as well.

The bats have developed that ability

into a highly sophisticated technique called sonar.

Every bat sends out a stream of short squeaks,

which can be as many as 20 or 30 a second, or even more.

From the echoes it can gauge its distance from an object,

whether it's a cave wall or an insect in the air.

The horseshoe bat produces such ultrasounds from its nostrils,

and that construction around the nose serves as a megaphone,

focusing the sound into a beam.

The easiest way to study these signals is to pick them up with a special microphone

and relay them to an oscilloscope so that they can be analysed in a visual form.

The oscilloscope tells us that the bat is producing sounds,

though they are beyond the range of our hearing.

But with the right equipment we can translate those ultrasounds

into sounds that we can hear.

HIGH-PITCHED PULSE

With the oscilloscope as well, we can both see and hear

the variations that the bat can make.

FASTER HIGH-PITCHED PULSE

SLOWER PULSE

FASTER PULSE

Another way to analyse the bat's signals

is to slow them down using a special tape recorder.

These are the horseshoe bat's ultrasounds slowed down 32 times.

LONG WHISTLING NOTES

This is a different species also slowed down.

And it's emitting the ultrasounds through its mouth.

With these echolocating signals bouncing back off the prey,

bats can home in very accurately, raising the rate of output as they approach.

BAT CHIRRUPS

Both sound and action are slowed down 16 times.

The bait, a mealworm, is located precisely by sonar,

and the bat, a pipistrelle, catches it first with its wing membrane,

then flicks it across to its tail membrane,

which is then brought up to its head so the mealworm is passed to the mouth.

The tail membrane is still over the head.

Now it's pulled back, and the bat continues to eat its prey in flight.

Since bats evolved to take advantage of the rich insect larder,

the insects themselves have developed their own countermeasures. Watch.

The lacewing's escape technique

is to close its wings and fall out of the path of the bat.

Lacewings have tiny ears on their wings.

So in this conflict between predator and prey, the insect has tuned in

to be able to hear the bat coming, and therefore take avoiding action.

Some moths, including tiger moths, have an even more elaborate defence.

Not only can they hear bats coming and then dive or spiral away,

but, as a last resort, they can emit their own sounds.

First we will see and hear the sound of the threatening bat,

and then the reaction of the moth.

CRACKLING

The moth has either jammed the bat's signals

or sent some kind of warning which puts the bat off.

Anyway, the moth nearly always escapes.

We may assume the battle of techniques will continue to evolve

as bats further develop their sonar equipment.

The apparatus often dominates the faces of bats.

Huge ears for detecting the echoes,

and on the nose, leaves, flanges and spears for directing the sound,

so they look as grotesque as any gargoyle produced by the medieval imagination.

Some bats tackle insects much bigger than mosquitoes or lacewings.

This one is quite prepared to alight on the forest floor

and grapple with a giant cockroach.

The insect-eating teeth, inherited from the shrew-like ancestor,

are essential here to break up the tough chitin of the insect's body.

When it hangs up, the wings form a kind of tent,

preventing bits of the prey from dropping out. This is a top view.

And an even tougher adversary for the pallid bat, a scorpion.

The poisonous sting is to be carefully avoided.

And some bats are real carnivores.

This huge silk cotton tree contains a small colony of them.

They are hanging at the very top of the hollow interior,

sharing the tree with other species of bats.

This is strange, because this carnivorous species feeds on other bats,

but here it leaves its neighbours in peace.

The also feed on birds, which they catch on their roosts at night.

This is Vampyrum spectrum, but it doesn't actually suck blood.

This is not the true vampire.

This is. Its teeth and mouth

are very specialised for feeding on blood.

Vampires may have originally fed on insects that cluster around grazing animals

and chased them on or near the ground.

By shaving away the skin with razor teeth

and having a saliva that prevents oozing blood clotting,

the vampire shows how extremely specialised a mammal can become.

And it probably all started with insects.

And the originally insect-eating bat

evolved in yet another direction in Arizona and Mexico.

This is the land of big plants, like cactus, yuccas and agaves.

The agave flowers, branching from a mast some six metres high,

attract hummingbirds that feed on the nectar.

And insects, too.

It was probably these that attracted bats in the first place.

Nectar feeding came later.

The bats, in small parties, move from plant to plant,

dipping and sipping at the energy-rich nectar.

Often they get covered in pollen.

In this way they ferry it from plant to plant, so bringing about cross-fertilisation.

So both bat and plant have evolved together to become unlikely partners.

As in other bats, this feeding specialisation involves adaptation.

Long noses and long tongues enable them to reach deep into the flowers.

When flying, seen here in slow motion,

they emit a weak sonar, so they've been called whispering bats.

FAST IRREGULAR PULSE

Carrying pollen and dripping nectar,

this bat will fly on to another agave, where cross-fertilisation will occur.

When the bat has helped that to happen, the fruit will appear.

And fruit, too, has become a food for bats.

This one, lapping at a banana with its tongue,

was the same kind as the one biting into a cockroach with its teeth.

For some bats have developed broad tastes.

Some, however, are exclusively fruit-eaters,

and they include the biggest of all.

These hardly ever live in caves,

but instead hang themselves up in great roosts in trees, called camps.

Their wings are immense, up to two metres across.

Just as birds have to groom their feathers with great care to keep themselves airworthy,

so bats spend a lot of time meticulously cleaning the elastic membrane of skin

on which they depend.

Fruit bats are often called flying foxes, and indeed their faces do look rather foxy.

The fact that they have large eyes

and no immense ears or grotesque ornaments on their noses is significant.

They have no sonar and rely instead on vision to find their way around.

In fact they are so different from insect-eating bats

that they may well be descended from a different branch of the primitive mammals.

They're powerful flyers and regularly go off on journeys of 50km

just to find a tree in fruit.

These are in slow motion.

The structure of a bat's wing is very different from that of a bird's.

The bird's, in effect, is formed from just one finger fringed with feathers.

All the other fingers have been effectively lost.

But the bat's ancestors didn't have feathers with long stiff quills.

They created a broad wing by a different method.

By retaining all their fingers and greatly elongating them

to support the wing membrane.

Their feet also help, for the membrane goes right down to the ankle.

Only the thumb remains free,

and that the bat needs for its toilet

and to hook onto branches as it clambers about.

When, over 50 million years ago, the first mammals flew,

they opened up great possibilities for their descendants.

They had the night sky virtually to themselves,

and they developed into a multitude of different forms to take full advantage of it.

Today there are nearly 1,000 different species of them,

flying through the skies of the world.

Many of them have remained insect-feeders, like their earth-bound ancestors,

but fruit, nectar, blood, birds and even other bats

is by no means the complete list of the diets they've discovered for themselves.

And one of them has actually become a fisherman.

It lives in Central America.

Its closest relatives are all insect-feeders, and it too will take a beetle, like this one,

though in a unique way.

It caught that beetle by using its hind legs as grapnels,

and it goes after fish in the same way. Watch.

It hooked the fish, but not well enough.

So back it comes.

Like other bats, it immediately transfers its capture into its mouth.

And only eats it when it gets back to its roost, stuffing some of it into cheek pouches.

The membrane doesn't go down to the ankle, like most bats', so it's kept clear of the water.

And the claws are as sharp as needles.

But how does it know where to trawl? The answer's ultrasounds again.

BAT CHIRRUPS

It's able to detect the ripple of a fish at the surface

and home in on it with deadly accuracy.

But fishing, for the bats, is a rare and very recently acquired talent.

The first really accomplished fisherman amongst the mammals

appeared early on in the history of the group.

When the great ocean-going reptiles, the ichthyosaurs and plesiosaurs,

disappeared at the end of the age of the dinosaurs,

the mammals were very quick indeed

to fill the space that was left in the economy of the sea.

At first, doubtless, the creature lived partly in the water, partly on land,

rather as the hippopotamus does today,

but very soon, within a few million years,

truly specialised mammalian swimmers appeared.

Some of them grew to be bigger even than the biggest of the dinosaurs - the whales.

And here in the blue waters of the Pacific, off the Hawaiian islands,

every year humpback whales assemble to give birth and to court.

And if you have a lot of patience and even more luck, you may be able to swim among them.

I was lucky enough to dive with a group of whale experts

who knew just how to get close to these magnificent creatures.

And there, in the distance, a 40-tonne mother and her baby.

The changes that have taken place during the descent of these vast creatures

from their little furry ancestors are obviously immense.

But they're all adaptations to a sea-going life.

The forelegs have become flippers and the back legs lost.

But what about their huge increase in size?

The larger you are, the lower the ratio between your volume and your surface area,

and the easier it is to retain heat.

Dinosaurs also had problems about getting chilled

and solved it in a similar way by getting big.

Their size, however, was limited by the strength of bone.

Above a certain weight, leg bones would simply break.

But whales are less hampered.

Their bodies are not supported by legs, but by the water.

So they have grown into the biggest animals the world has seen,

some of them four times bigger than the largest known dinosaur.

As far as we can tell, the whales, when in Hawaiian waters, don't feed at all.

They come here only to court and to give birth to their young.

Around April they begin to swim north, up to the Arctic.

Many of them assemble off the coast of Alaska,

and here they begin to feed.

As they gather on their feeding grounds in ever-increasing numbers,

they begin to behave in a most dramatic way.

The breaching, 40 tonnes of animal right out of the water,

may be something to do with the establishment of territories.

The need to breathe air, bequeathed to them by their mammalian ancestors,

might seem to be a major handicap.

But the whales have minimised the problem by breathing particularly efficiently.

Human beings only clear about 15% of the air in their lungs with a normal breath.

The whale, in great exhalations, gets rid of about 90% of its spent air.

It also has a well-developed system for storing oxygen in the muscles,

and some can swim for up to 40 minutes without drawing breath if they want to.

Humpbacks are one of the group of whales

that feed on shoals of shrimp-like creatures, krill.

Sometimes they concentrate the krill with a ring of bubbles from the blowhole,

and the mouthful is filtered through plates of whalebone hanging from the upper jaw.

In a way they parallel the anteaters.

Both creatures have modified their jaws and lost their teeth

in order to collect swarms of tiny invertebrates.

Another group of whales tackle much bigger prey.

These whales have kept their teeth

and become among the fiercest creatures in the sea.

These are killer whales, and they're hunting seals.

That dot is the head of a seal, desperately searching for safety,

but it has no chance.

That most dramatic and elusive creature, the narwhal, is another of the toothed whales.

And one of its teeth has grown enormous.

Only the males have this impressive tusk,

but no-one yet has discovered just what it's for.

The most familiar toothed whales of all are dolphins and porpoises.

They're the friendliest and also the smallest.

They were among the first whales to be kept in tanks.

As a result, we've been able to watch the moment

that must be among the trickiest of a sea mammal's life, the moment of birth.

This is the mother-to-be. Her belly is swollen, and birth is imminent.

The baby's tail is just showing.

Now it's half out.

And there is the puff of red blood,

as the umbilical cord breaks and the youngster swims free.

Here is that remarkable moment again.

The baby can swim immediately,

but the mother helps it to the surface for its first breath of air.

Now it swims alongside her, gliding just as fast as she does,

seemingly without any difficulty.

Soon, as it swims, it will suckle at that other mammalian device,

the nipple on its mother's underside, to take its first meal of milk.

The dolphins' gymnastic skills, their ability to copy from one another

and their apparent eagerness to learn new tricks from their trainers

have made them the most accomplished and popular performers in oceanaria.

But how intelligent are they?

SQUEAKING

Speculations about dolphin intelligence have been stimulated in particular

by these calls.

Some people have even suggested that dolphins have a true language,

and that if only we were clever enough, we would not only be able to understand,

but might be able to speak it and convey quite complex messages to dolphins.

It's true dolphins not only make sounds when they have their heads above water,

but do so almost continuously below water.

And we can listen to them do it with an underwater microphone.

CLICKING

Over 20 different kinds of calls have been identified.

Some serve to keep a school together when they're travelling at top speed,

and they can swim at 20mph, and they go on long migrations.

Some sounds are warning cries, some call signs

that enable one animal to be recognised at a distance by another.

Complex though these calls are,

no-one has yet demonstrated that dolphins ever put calls together

to form the equivalent of a two-word sentence.

That can be regarded as the beginning of a true language.

But those aren't the only sounds they make.

They also use sound for echolocation in rather the same way that bats do.

That's to say they emit a series of very high-pitched clicks and squeaks,

and by sensing the echoes,

they can detect the presence of objects in the water around them.

The frequencies they use are around 200,000 vibrations per second,

which is about the same as that used by bats

and way, way above the range of the human ear.

But by once again using the bat detector,

this time connected to the underwater microphone,

we can translate those clicks into sounds that we can hear.

Normally, of course, the dolphins use their eyesight in conjunction with echolocation,

but just to show how accurate that echolocation can be,

we're going to blindfold the dolphins.

The dolphin has been trained to retrieve this hoop.

What's more, it can find it in the water

and distinguish it from these two shapes blindfold. Watch.

CLICKING

And just to show that that's no fluke, let's try it again.

The waters around Hawaii are also filled with strange sounds,

but these we know far less about.

DEEP GROANS

A moment ago we made this recording with an underwater microphone

here in the Pacific near Hawaii. Just listen to this.

DEEP GROANS

LOW WAIL

This is the sound of a humpback whale

that's lying in the water about 100 feet below us in the sea.

There are many extraordinary things about its song.

To start with, they're so long.

They may last anything from a quarter of an hour to half an hour.

Although the various themes within the song may be repeated a varying number of times,

the actual themes themselves and the order in which they appear in the song is unvarying.

And even more remarkable, all the singing whales within this area

sing the same song.

WHALES SING

After the breeding season they disperse. Next year they'll be back,

but next year they'll have a slightly different song

which contains themes that have never been heard before.

And all of them will be singing the same song.

And that song can be heard echoing throughout these waters

for miles and miles and miles.

WHALES SINGING

It seems extraordinary that a creature like this

could have given rise to whales, as well as to moles and bats and anteaters.

Those swimming, burrowing, flying specialists

appeared a few million years after the dinosaurs disappeared.

The only mammals around from which they could have sprung

were these small furry insect-eaters.

So this tiny theme has proved to be one of the most fruitful in the animal kingdom.

There are still some variations we haven't looked at yet -

the vegetarians, the leaf-and-grass-eaters,

and the carnivores that developed to prey on them.

They will have to have a programme of their own.

Subtitles by Red Bee Media Ltd