The Living Planet

These mountains stand in the middle of the biggest desert on earth, the Sahara.

It stretches right across the width of Africa,

three and a half million square miles of it.

At night, it gets so cold that it can freeze.

During the day, the sun strikes it so ferociously

that the highest land temperatures ever recorded were measured here:

58 degrees centigrade, 137 degrees Fahrenheit.

And, in turn, those oven-like temperatures rob the land of all its moisture.

All in all, there could hardly be a more hostile environment for life on earth.

But it wasn't always this way.

And if you want evidence of that, here it is.

A group of antelope, probably sable.

Creatures that can't live anywhere in the Sahara today,

because there's simply not enough vegetation for them.

These aren't the only wild animals painted on these rocks.

A giraffe.

A kind of wild goat, probably a moufflon.

And antelope.

Obviously, at the time these pictures were painted, there was good grazing here.

Indeed, there was sufficient vegetation to sustain not only wild animals,

but whole herds of cattle.

We don't know exactly who drew these pictures.

The artists may have been ancestors of the nomads

who today follow their herds of long-horned piebald cattle just south of the Sahara.

But we know what they looked like, because they left their portraits.

They lived here, it seems, some 5,000 years ago.

But eventually the rains began to fail, the pastures disappeared,

and with it the cattle and their keepers.

But there are one or two living survivors from that time.

This ancient cypress, judging from the number of rings in the trunks of others like it,

is probably between 2,000 and 3,000 years old.

In fact, it was probably already growing here when the last paintings were being made.

It still bears fertile seed,

but there are no little seedlings growing here to replace it.

The land now is far too dry for them.

Indeed, the cypress itself only survives

because it sends its huge roots deep into the ground to tap underground water.

The drying out of the Sahara seems to be connected

with the great changes in climate at the end of the last ice age.

As the glaciers retreated northwards across Europe,

so the rains that fell regularly along their southern edge

left Africa and moved up into Europe, and the Sahara was robbed of its rains.

Indeed, it seems that most of the world's great deserts were formed around that time,

and most, if not all of them, are therefore comparatively recent environments.

To see why deserts lie where they do, we can look at the Sahara from the west.

The equator runs away from us across the width of Africa.

It's along this line that the sun's rays strike from directly overhead,

and therefore with the greatest strength.

The heated air rises along the equator

and flows away, north and south, to cooler parts of the world.

Because it's warm, it carries a lot of moisture.

But as it rises and cools, the moisture condenses first to form clouds

and then to fall as rain.

When the air comes down again over the Sahara to the left and the Kalahari to the right,

it's lost most of its moisture and creates few clouds.

The Sahara, with few clouds to shield it from the sun,

becomes roastingly hot during the day.

And at night, with no blanket of clouds to keep in its warmth, it gets desperately cold.

Deserts are not placed symmetrically around the world,

because the continents themselves are distributed in a very irregular way.

They're ridged with great mountain ranges, which interfere with the even flow of air,

and the planet's spin creates vast eddies in the atmosphere,

which further complicates things.

But even so, deserts do lie in two broad zones on either side of the equator.

The pattern in Africa, with the Sahara in the north

and the Kalahari and the Namib in the south, has its equivalent in the Americas.

South of the lush equatorial jungles of the Amazon,

beyond the great range of the Andes, lies the Atacama desert.

On the other side of the equator, beyond the drenched tropical rainforests of Panama,

stretch the deserts of Mexico and Arizona.

Across the Pacific, the greatest expanse of water on the globe,

lies, south of the equator, Australia, most of which is covered by desert.

Its northern tip gets close enough to the equator to collect some rain.

Farther north still,

beyond the jungle that blankets Indonesia and Malaysia, Thailand and Burma,

across the great snow-covered range of the Himalayas,

stretch the vast deserts of central Asia: Mongolia and the Gobi.

Beyond them, as we complete the circuit of the globe,

the huge desert of the Middle East that covers Iran, Iraq and Jordan,

Syria and Israel, the vast sandy emptiness of Arabia,

and runs on to join the Sahara.

This is the biggest expanse of waterless land on earth.

Here, as in deserts everywhere, almost nothing moves during the heat of the day.

But animals are here.

If you want to see what made these tracks,

you have to wait until the sun sinks and the desert begins to cool.

A striped hyena, one of the commonest of the bigger desert animals in this part of the world.

A fennec fox.

Fennecs usually live in small family groups, and clearly enjoy one another's company.

But there's not much time for frolicking. Food must be found.

Faint smells from the sand tell them who has moved where since they were last out.

As the moon rises, many more creatures emerge.

A gecko. Just what the fennec wants.

A jerboa. It, too, is looking for food. Seeds.

Another little seed-eating rodent, a gerbil.

And a caracal, a kind of cat, which loves both gerbils and jerboas, if it can get them.

A smaller hunter, but nonetheless a deadly one: A scorpion.

It is searching for beetles or other small insects.

But sometimes the hunter becomes the hunted.

A black widow spider has set her snare of silk underneath a thorn bush.

In the intense struggle, the black widow loses one of her legs.

She manages to get more ropes of silk around the scorpion, hampering it still further.

The scorpion hangs on to its trophy, but to no purpose.

The battle is as good as lost.

Methodically, the spider trusses up her victim and hangs it in her larder.

Wolves, perhaps surprisingly, are quite common in these Middle Eastern deserts,

but they're not like those farther north.

They're smaller, lighter-coloured, and with only the thinnest fur,

and they scavenge as much as they hunt.

The cool night is coming to an end. Hunting is over.

The animals must go back to their dens and hiding places

to shelter from the heat that is to come.

The sun returns, and very soon the desert will be heating up once again.

The mammals that were active during the night have to find shelter.

The day belongs not to them,

but to creatures that get their heat directly from the sun: Reptiles.

This is the desert of the American west in Arizona,

and we've come here to look at one very special desert reptile: This one.

This is the Gila monster, one of only two poisonous lizards in the world.

Actually, he very seldom uses his poison in defence,

and it's still quite early in the morning

and he is so cold that he isn't very active.

But in only about an hour, the desert will get so hot that he won't be able to stand it,

and he, too, will have to seek shade.

So in this short period of the early morning and in the cool of the evening

is the time when he hunts.

A tortoise, but he's far too big and well-armoured for a Gila monster to tackle.

This great nest of sticks, however, looks much more promising.

The victim: A desert mouse.

The tortoise is on the lookout for food, too, but it is a vegetarian.

The day is now several hours old.

Cool dawn is changing to baking noon.

It's time for even a reptile to get out of the sun.

Movement generates heat,

so now nothing moves unless it absolutely has to.

And there are some creatures that remain motionless

even when you get within a few inches of them.

One of them is on the ground right in front of me now,

though you may find it difficult to see because it's so well camouflaged.

It's a poorwill, a kind of nightjar.

Fluttering the throat evaporates moisture from the mouth and so cools the bird.

It consumes much less energy than heaving the chest and panting,

as many mammals would do in this situation.

The sand grouse of Africa uses the same trick.

The sand grouse chicks start doing it almost as soon as they emerge from the shell.

They also immediately peck for seeds, but there's little moisture in a seed,

and unless they drink, they will die.

The responsibility for providing that rests entirely with the male.

Every day he flies to water, maybe as much as 25 miles from the nest.

First he fills his own stomach with water.

But then, very deliberately, he soaks his belly feathers.

These feathers have a special spongy structure

so that they can absorb lots of water.

Once he has a full load, he flies back to his family.

At last the chicks get their drink.

No other bird has such an ingenious water-carrying device.

The roadrunner of the American deserts

provides water for its chicks quite differently.

This parent bird has collected a cicada for its family.

Its nest is in a cholla cactus.

The parent doesn't give its chicks their food immediately.

The chick is gulping.

The parent bird is producing liquid from its stomach and letting it trickle down its beak.

Each youngster gets its share.

Another ration of solid food. This time, a lizard.

Each time, before the meal is handed over, the chicks get a drink,

whether they like it or not.

During the day, the parents sit on the nest, not to keep the chicks warm,

but, on the contrary, to keep them cool by shading them.

The bird not only flutters its throat,

but protects itself from the sun by using its tail as a parasol.

The ground squirrel of the Namib desert does the same thing,

and very effectively, too, carefully angling itself as far as possible

to keep its body in the shade.

Many animals keep their blood cool with radiators.

The hedgehog that lives in the desert of the Middle East has unusually large ears.

Blood circulates through capillaries close to the surface of the skin

and is cooled by the breeze.

The fennec fox's huge ears serve the same purpose.

So do those of the American jackrabbit,

which perhaps has the biggest ears of all in proportion to its body.

The dorcas gazelle also has radiator ears

and is one of the best-adapted desert mammals.

It's one of few that can survive without drinking at all.

It gets all the liquid it needs from vegetation.

It doesn't waste liquid as urine, but gets rid of its uric acid as small dry pellets.

It's now approaching noon, the hottest time of the day.

It's summer, the hottest time of the year,

and I'm in one of the hottest places on earth:

Death Valley in the western United States.

A thermometer on the ground here has risen to 201 degrees Fahrenheit.

That's about 94 degrees centigrade.

It's so hot that no creature can survive permanently out here.

Even at the edge of these sand flats, where the ground is more broken,

there is no sign of animal life whatever.

All animals now have sought the shade and shelter from this ferocious sun.

But some organisms can't get out of the sun.

Plants, being fixed to the ground,

have to stay out in the heat of the day and simply endure.

But all of them have special devices to help them to do so.