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These mountains stand in the middle
of the biggest desert on earth, the Sahara.
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00:02:04,440 --> 00:02:07,080
It stretches right across the width of Africa,
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three and a half million square miles of it.
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00:02:11,680 --> 00:02:15,400
At night, it gets so cold that it can freeze.
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00:02:15,560 --> 00:02:19,200
During the day, the sun strikes it so ferociously
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00:02:19,360 --> 00:02:23,280
that the highest land temperatures
ever recorded were measured here:
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58 degrees centigrade,
137 degrees Fahrenheit.
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And, in turn, those oven-like temperatures
rob the land of all its moisture.
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All in all, there could hardly be
a more hostile environment for life on earth.
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00:02:39,400 --> 00:02:42,360
But it wasn't always this way.
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00:02:42,840 --> 00:02:46,680
And if you want evidence of that, here it is.
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A group of antelope, probably sable.
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00:02:53,880 --> 00:02:56,920
Creatures that can't live
anywhere in the Sahara today,
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because there's simply
not enough vegetation for them.
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00:03:00,120 --> 00:03:03,640
These aren't the only wild animals
painted on these rocks.
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00:03:06,840 --> 00:03:08,840
A giraffe.
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00:03:10,400 --> 00:03:13,000
A kind of wild goat, probably a moufflon.
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00:03:13,680 --> 00:03:15,280
And antelope.
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00:03:15,440 --> 00:03:20,360
Obviously, at the time these pictures
were painted, there was good grazing here.
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00:03:20,560 --> 00:03:26,120
Indeed, there was sufficient vegetation
to sustain not only wild animals,
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00:03:26,280 --> 00:03:28,880
but whole herds of cattle.
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00:03:29,760 --> 00:03:32,240
We don't know exactly who drew these pictures.
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The artists may have been
ancestors of the nomads
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00:03:35,240 --> 00:03:40,440
who today follow their herds of long-horned
piebald cattle just south of the Sahara.
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00:03:40,600 --> 00:03:44,600
But we know what they looked like,
because they left their portraits.
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00:03:44,760 --> 00:03:47,800
They lived here, it seems, some 5,000 years ago.
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But eventually the rains began to fail,
the pastures disappeared,
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and with it the cattle and their keepers.
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00:03:54,400 --> 00:03:58,440
But there are one or two living survivors
from that time.
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00:04:00,720 --> 00:04:05,840
This ancient cypress, judging from
the number of rings in the trunks of others like it,
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00:04:06,000 --> 00:04:09,120
is probably between 2,000 and 3,000 years old.
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00:04:09,920 --> 00:04:15,760
In fact, it was probably already growing here
when the last paintings were being made.
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00:04:15,920 --> 00:04:17,920
It still bears fertile seed,
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00:04:18,080 --> 00:04:21,720
but there are no little seedlings
growing here to replace it.
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00:04:21,880 --> 00:04:24,560
The land now is far too dry for them.
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00:04:25,280 --> 00:04:27,480
Indeed, the cypress itself only survives
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00:04:27,640 --> 00:04:33,000
because it sends its huge roots
deep into the ground to tap underground water.
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00:04:33,960 --> 00:04:37,360
The drying out of the Sahara
seems to be connected
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00:04:37,520 --> 00:04:41,880
with the great changes in climate
at the end of the last ice age.
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00:04:42,040 --> 00:04:45,320
As the glaciers
retreated northwards across Europe,
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00:04:45,480 --> 00:04:48,840
so the rains that fell regularly
along their southern edge
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00:04:49,000 --> 00:04:54,480
left Africa and moved up into Europe,
and the Sahara was robbed of its rains.
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00:04:55,480 --> 00:05:01,080
Indeed, it seems that most of the world's
great deserts were formed around that time,
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00:05:01,240 --> 00:05:06,240
and most, if not all of them,
are therefore comparatively recent environments.
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00:05:07,080 --> 00:05:12,280
To see why deserts lie where they do,
we can look at the Sahara from the west.
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00:05:12,440 --> 00:05:16,600
The equator runs away from us
across the width of Africa.
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00:05:16,760 --> 00:05:21,200
It's along this line
that the sun's rays strike from directly overhead,
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00:05:21,360 --> 00:05:23,720
and therefore with the greatest strength.
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00:05:23,880 --> 00:05:26,240
The heated air rises along the equator
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00:05:26,400 --> 00:05:30,080
and flows away, north and south,
to cooler parts of the world.
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00:05:30,240 --> 00:05:33,600
Because it's warm, it carries a lot of moisture.
52
00:05:33,760 --> 00:05:38,520
But as it rises and cools,
the moisture condenses first to form clouds
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00:05:38,680 --> 00:05:40,880
and then to fall as rain.
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00:05:41,040 --> 00:05:46,120
When the air comes down again over the Sahara
to the left and the Kalahari to the right,
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00:05:46,280 --> 00:05:50,000
it's lost most of its moisture
and creates few clouds.
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00:05:50,160 --> 00:05:53,680
The Sahara,
with few clouds to shield it from the sun,
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00:05:53,840 --> 00:05:56,880
becomes roastingly hot during the day.
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00:05:57,040 --> 00:06:02,760
And at night, with no blanket of clouds
to keep in its warmth, it gets desperately cold.
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00:06:04,120 --> 00:06:07,080
Deserts are not placed
symmetrically around the world,
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00:06:07,240 --> 00:06:11,440
because the continents themselves
are distributed in a very irregular way.
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00:06:11,600 --> 00:06:16,400
They're ridged with great mountain ranges,
which interfere with the even flow of air,
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00:06:16,560 --> 00:06:20,400
and the planet's spin
creates vast eddies in the atmosphere,
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00:06:20,560 --> 00:06:22,640
which further complicates things.
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00:06:22,800 --> 00:06:28,000
But even so, deserts do lie
in two broad zones on either side of the equator.
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00:06:28,160 --> 00:06:31,560
The pattern in Africa, with the Sahara in the north
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00:06:31,720 --> 00:06:37,280
and the Kalahari and the Namib in the south,
has its equivalent in the Americas.
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00:06:38,280 --> 00:06:41,680
South of the lush equatorial jungles
of the Amazon,
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00:06:41,840 --> 00:06:46,760
beyond the great range of the Andes,
lies the Atacama desert.
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00:06:46,920 --> 00:06:51,960
On the other side of the equator, beyond
the drenched tropical rainforests of Panama,
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00:06:52,120 --> 00:06:55,080
stretch the deserts of Mexico and Arizona.
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00:06:56,720 --> 00:07:00,640
Across the Pacific,
the greatest expanse of water on the globe,
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00:07:00,800 --> 00:07:06,360
lies, south of the equator, Australia,
most of which is covered by desert.
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00:07:06,520 --> 00:07:11,360
Its northern tip gets close enough to the equator
to collect some rain.
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00:07:11,520 --> 00:07:13,200
Farther north still,
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00:07:13,360 --> 00:07:17,840
beyond the jungle that blankets
Indonesia and Malaysia, Thailand and Burma,
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00:07:18,000 --> 00:07:21,240
across the great snow-covered range
of the Himalayas,
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00:07:21,400 --> 00:07:26,360
stretch the vast deserts
of central Asia: Mongolia and the Gobi.
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00:07:26,520 --> 00:07:29,600
Beyond them,
as we complete the circuit of the globe,
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00:07:29,760 --> 00:07:34,320
the huge desert of the Middle East
that covers Iran, Iraq and Jordan,
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00:07:34,480 --> 00:07:38,760
Syria and Israel,
the vast sandy emptiness of Arabia,
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00:07:38,920 --> 00:07:41,800
and runs on to join the Sahara.
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00:07:48,040 --> 00:07:52,600
This is the biggest expanse
of waterless land on earth.
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00:07:52,760 --> 00:07:58,760
Here, as in deserts everywhere,
almost nothing moves during the heat of the day.
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00:08:00,040 --> 00:08:02,320
But animals are here.
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00:08:12,800 --> 00:08:15,440
If you want to see what made these tracks,
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you have to wait until the sun sinks
and the desert begins to cool.
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00:08:28,520 --> 00:08:34,560
A striped hyena, one of the commonest of
the bigger desert animals in this part of the world.
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00:08:43,960 --> 00:08:45,960
A fennec fox.
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00:08:54,400 --> 00:09:00,240
Fennecs usually live in small family groups,
and clearly enjoy one another's company.
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00:09:08,520 --> 00:09:12,120
But there's not much time for frolicking.
Food must be found.
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00:09:12,280 --> 00:09:17,840
Faint smells from the sand tell them
who has moved where since they were last out.
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00:09:48,120 --> 00:09:50,640
As the moon rises,
many more creatures emerge.
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00:09:50,800 --> 00:09:53,920
A gecko. Just what the fennec wants.
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00:10:05,520 --> 00:10:08,920
A jerboa.
It, too, is looking for food. Seeds.
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00:10:17,600 --> 00:10:20,960
Another little seed-eating rodent, a gerbil.
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00:10:24,240 --> 00:10:30,600
And a caracal, a kind of cat, which loves
both gerbils and jerboas, if it can get them.
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00:10:45,800 --> 00:10:50,320
A smaller hunter,
but nonetheless a deadly one: A scorpion.
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00:10:50,520 --> 00:10:54,240
It is searching for beetles or other small insects.
99
00:10:54,400 --> 00:10:56,920
But sometimes the hunter becomes the hunted.
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00:10:57,080 --> 00:11:02,360
A black widow spider has set
her snare of silk underneath a thorn bush.
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00:11:40,960 --> 00:11:45,960
In the intense struggle,
the black widow loses one of her legs.
102
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She manages to get more ropes of silk
around the scorpion, hampering it still further.
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00:12:05,720 --> 00:12:10,520
The scorpion hangs on to its trophy,
but to no purpose.
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00:12:10,680 --> 00:12:12,880
The battle is as good as lost.
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00:12:23,840 --> 00:12:29,560
Methodically, the spider trusses up her victim
and hangs it in her larder.
106
00:12:35,160 --> 00:12:39,840
Wolves, perhaps surprisingly, are quite common
in these Middle Eastern deserts,
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00:12:40,000 --> 00:12:42,920
but they're not like those farther north.
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00:12:43,080 --> 00:12:47,080
They're smaller, lighter-coloured,
and with only the thinnest fur,
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00:12:47,240 --> 00:12:50,480
and they scavenge as much as they hunt.
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00:12:53,600 --> 00:12:57,600
The cool night is coming to an end.
Hunting is over.
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00:12:57,760 --> 00:13:00,960
The animals must go back to their dens
and hiding places
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00:13:01,120 --> 00:13:04,640
to shelter from the heat that is to come.
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00:13:25,200 --> 00:13:30,400
The sun returns, and very soon
the desert will be heating up once again.
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00:13:30,560 --> 00:13:34,440
The mammals that were active
during the night have to find shelter.
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00:13:34,600 --> 00:13:36,880
The day belongs not to them,
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00:13:37,040 --> 00:13:41,840
but to creatures that get their heat
directly from the sun: Reptiles.
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00:13:42,000 --> 00:13:45,760
This is the desert
of the American west in Arizona,
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and we've come here to look
at one very special desert reptile: This one.
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This is the Gila monster,
one of only two poisonous lizards in the world.
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00:14:05,000 --> 00:14:08,640
Actually, he very seldom
uses his poison in defence,
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00:14:08,800 --> 00:14:11,400
and it's still quite early in the morning
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and he is so cold that he isn't very active.
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00:14:15,920 --> 00:14:21,280
But in only about an hour, the desert will
get so hot that he won't be able to stand it,
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and he, too, will have to seek shade.
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00:14:25,200 --> 00:14:29,480
So in this short period of the early morning
and in the cool of the evening
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00:14:29,640 --> 00:14:32,720
is the time when he hunts.
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A tortoise, but he's far too big
and well-armoured for a Gila monster to tackle.
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This great nest of sticks,
however, looks much more promising.
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(HISS)
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(SQUEAK)
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The victim: A desert mouse.
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The tortoise is on the lookout for food, too,
but it is a vegetarian.
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00:15:59,720 --> 00:16:02,240
The day is now several hours old.
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00:16:02,400 --> 00:16:05,600
Cool dawn is changing to baking noon.
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00:16:05,760 --> 00:16:10,000
It's time for even a reptile to get out of the sun.
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Movement generates heat,
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so now nothing moves unless it absolutely has to.
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00:16:22,400 --> 00:16:25,480
And there are some creatures
that remain motionless
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even when you get within a few inches of them.
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One of them is on the ground
right in front of me now,
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though you may find it difficult to see
because it's so well camouflaged.
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00:16:38,080 --> 00:16:41,040
It's a poorwill, a kind of nightjar.
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Fluttering the throat evaporates moisture
from the mouth and so cools the bird.
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00:16:50,040 --> 00:16:53,880
It consumes much less energy
than heaving the chest and panting,
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as many mammals would do in this situation.
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00:16:57,680 --> 00:17:01,200
The sand grouse
of Africa uses the same trick.
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The sand grouse chicks start doing it
almost as soon as they emerge from the shell.
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They also immediately peck for seeds,
but there's little moisture in a seed,
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and unless they drink, they will die.
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00:17:24,520 --> 00:17:29,080
The responsibility for providing that
rests entirely with the male.
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00:17:30,200 --> 00:17:34,520
Every day he flies to water,
maybe as much as 25 miles from the nest.
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First he fills his own stomach with water.
153
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But then, very deliberately,
he soaks his belly feathers.
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These feathers have a special spongy structure
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so that they can absorb lots of water.
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Once he has a full load, he flies back to his family.
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At last the chicks get their drink.
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00:18:15,360 --> 00:18:19,240
No other bird
has such an ingenious water-carrying device.
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00:18:28,760 --> 00:18:30,920
The roadrunner of the American deserts
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provides water for its chicks quite differently.
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This parent bird
has collected a cicada for its family.
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00:18:39,200 --> 00:18:41,520
Its nest is in a cholla cactus.
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The parent doesn't give its chicks
their food immediately.
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The chick is gulping.
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00:18:53,880 --> 00:19:00,160
The parent bird is producing liquid from
its stomach and letting it trickle down its beak.
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Each youngster gets its share.
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Another ration of solid food. This time, a lizard.
168
00:19:35,760 --> 00:19:40,120
Each time, before the meal is handed over,
the chicks get a drink,
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whether they like it or not.
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00:19:53,360 --> 00:19:58,040
During the day, the parents sit on the nest,
not to keep the chicks warm,
171
00:19:58,200 --> 00:20:01,520
but, on the contrary,
to keep them cool by shading them.
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The bird not only flutters its throat,
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but protects itself from the sun
by using its tail as a parasol.
174
00:20:12,400 --> 00:20:16,520
The ground squirrel
of the Namib desert does the same thing,
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and very effectively, too,
carefully angling itself as far as possible
176
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to keep its body in the shade.
177
00:20:33,800 --> 00:20:37,680
Many animals
keep their blood cool with radiators.
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00:20:37,840 --> 00:20:41,960
The hedgehog that lives in the desert
of the Middle East has unusually large ears.
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00:20:42,120 --> 00:20:45,640
Blood circulates through capillaries
close to the surface of the skin
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00:20:45,800 --> 00:20:48,520
and is cooled by the breeze.
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The fennec fox's huge ears
serve the same purpose.
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So do those of the American jackrabbit,
183
00:21:04,440 --> 00:21:08,800
which perhaps has the biggest ears of all
in proportion to its body.
184
00:21:13,840 --> 00:21:16,480
The dorcas gazelle also has radiator ears
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00:21:16,640 --> 00:21:19,960
and is one of the best-adapted
desert mammals.
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00:21:20,120 --> 00:21:23,440
It's one of few
that can survive without drinking at all.
187
00:21:23,600 --> 00:21:26,920
It gets all the liquid it needs from vegetation.
188
00:21:29,480 --> 00:21:36,080
It doesn't waste liquid as urine,
but gets rid of its uric acid as small dry pellets.
189
00:21:52,840 --> 00:21:57,320
It's now approaching noon,
the hottest time of the day.
190
00:21:57,480 --> 00:22:00,440
It's summer, the hottest time of the year,
191
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and I'm in one of the hottest places on earth:
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Death Valley in the western United States.
193
00:22:07,680 --> 00:22:14,000
A thermometer on the ground here
has risen to 201 degrees Fahrenheit.
194
00:22:14,160 --> 00:22:17,840
That's about 94 degrees centigrade.
195
00:22:18,000 --> 00:22:22,600
It's so hot that no creature
can survive permanently out here.
196
00:22:22,760 --> 00:22:27,640
Even at the edge of these sand flats,
where the ground is more broken,
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00:22:27,800 --> 00:22:31,160
there is no sign of animal life whatever.
198
00:22:31,320 --> 00:22:37,960
All animals now have sought the shade
and shelter from this ferocious sun.
199
00:22:38,120 --> 00:22:41,560
But some organisms can't get out of the sun.
200
00:22:42,680 --> 00:22:45,120
Plants, being fixed to the ground,
201
00:22:45,280 --> 00:22:50,520
have to stay out in the heat of the day
and simply endure.
202
00:22:50,680 --> 00:22:54,800
But all of them
have special devices to help them to do so.
203
00:22:54,960 --> 00:22:59,760
The desert holly. Its leaves grow
at about 70 degrees to the vertical,
204
00:22:59,920 --> 00:23:05,320
so that in the morning when it's less hot
and in the evening when the plant needs light,
205
00:23:05,480 --> 00:23:09,120
the face of its leaves face the light.
206
00:23:09,280 --> 00:23:14,240
During the middle of the day, it shows
only the edges and doesn't heat up so much.
207
00:23:15,360 --> 00:23:20,280
Not only that, but the plant
extracts salt from the salt-laden ground
208
00:23:20,440 --> 00:23:23,960
and excretes it as a white coating on the leaf,
209
00:23:24,120 --> 00:23:28,440
which, like the white costume of an athlete,
reflects the heat
210
00:23:28,600 --> 00:23:31,640
and so keeps the plant that much cooler.
211
00:23:31,800 --> 00:23:34,880
And this, the creosote bush.
212
00:23:35,040 --> 00:23:39,720
This is one of the most widespread of plants
in American deserts,
213
00:23:39,880 --> 00:23:45,320
and its roots are better at extracting
the last molecule of water from parched sands
214
00:23:45,480 --> 00:23:48,280
than those of any other American plant.
215
00:23:48,440 --> 00:23:54,040
This has led to an extraordinary state of affairs
that's only just been discovered.
216
00:23:54,200 --> 00:23:58,200
It seems that the creosote bush
was the first plant to establish itself
217
00:23:58,360 --> 00:24:02,240
in the arid Mojave desert
when the desert first appeared.
218
00:24:02,400 --> 00:24:05,720
Once it had established its extensive root system,
219
00:24:05,880 --> 00:24:09,080
it extracted moisture
from the sand so efficiently
220
00:24:09,240 --> 00:24:13,680
that it was extremely difficult
for any other plant to grow alongside it.
221
00:24:13,840 --> 00:24:20,560
And that applied not only to any other
kind of plant but also to its own seedlings.
222
00:24:21,680 --> 00:24:26,440
So an individual creosote bush
tended to spread not by setting seeds
223
00:24:26,600 --> 00:24:28,760
and producing a new generation,
224
00:24:28,920 --> 00:24:33,440
but by sending out new stems around its base.
225
00:24:33,600 --> 00:24:38,720
And as these spread outwards,
so the stems in the middle tended to die away,
226
00:24:38,880 --> 00:24:43,040
and the bush grew into a ring shape like this.
227
00:24:43,200 --> 00:24:49,040
So these are not separate
individual creosote bushes, as it might appear,
228
00:24:49,200 --> 00:24:54,920
but this is just
one big ring-shaped individual plant.
229
00:24:55,640 --> 00:25:01,040
Over the centuries,
the rings widened and changed their shape
230
00:25:01,200 --> 00:25:06,760
until now some are over 25 yards across,
like this one.
231
00:25:06,920 --> 00:25:13,520
Of course, the individual stems and leaves
of this plant are not very ancient.
232
00:25:13,680 --> 00:25:19,640
The first ones to grow, which appeared
in the middle, decayed and disappeared long ago.
233
00:25:19,800 --> 00:25:23,360
Now it's estimated
that this plant began growing
234
00:25:23,520 --> 00:25:26,960
between 10,000 and 12,000 years ago,
235
00:25:27,120 --> 00:25:30,720
in fact, when the Mojave desert first appeared,
236
00:25:30,880 --> 00:25:36,240
and that makes it the oldest known
living organism in the world.
237
00:25:41,000 --> 00:25:46,600
In the Mojave, the plants may have to survive
for as long as ten years without rain,
238
00:25:46,760 --> 00:25:51,200
but if rain falls a little more frequently,
as it does nearby in Arizona,
239
00:25:51,360 --> 00:25:53,920
plants can have different survival strategies.
240
00:25:55,560 --> 00:26:00,000
To many of us,
the very symbol of the desert is the cactus.
241
00:26:00,160 --> 00:26:06,040
But in fact, this family of fleshy-stemmed
plants lives only in the Americas.
242
00:26:06,200 --> 00:26:11,760
There are several hundred species of them,
but among the biggest is the saguaro.
243
00:26:13,560 --> 00:26:15,840
The saguaro has solved the problems
244
00:26:16,000 --> 00:26:20,880
of surviving in great heat and drought
very successfully indeed.
245
00:26:22,000 --> 00:26:27,200
Its stem is pleated like an accordion,
so when rain does fall, the cactus can expand
246
00:26:27,360 --> 00:26:31,200
and quickly absorb as much water as possible
before it disappears.
247
00:26:31,360 --> 00:26:37,640
After a single storm, a saguaro
can take up as much as a ton in a few days.
248
00:26:38,760 --> 00:26:40,080
Its leaves have become thorns,
249
00:26:40,240 --> 00:26:45,240
so reducing the surface area from which
the plant might lose water by evaporation.
250
00:26:45,400 --> 00:26:50,000
The stem itself is green
and has taken over the job of photosynthesis.
251
00:26:50,160 --> 00:26:53,560
The thorns protect the young plant from browsers,
252
00:26:53,720 --> 00:26:58,560
but they also break up the wind currents,
so that the cactus is wrapped in still air,
253
00:26:58,720 --> 00:27:03,600
and evaporation of moisture
from the stem is kept very low.
254
00:27:03,760 --> 00:27:10,600
These huge saguaro cacti can live
for over 200 years and stand nearly 50 feet high.
255
00:27:10,760 --> 00:27:16,840
A big one like this may weigh
as much as eight tons, and 90% of that is water.
256
00:27:17,960 --> 00:27:20,920
If I was dying of thirst in this desert,
257
00:27:21,080 --> 00:27:26,520
I'd be tempted to cut inside that saguaro
and raid its reservoir of water.
258
00:27:26,680 --> 00:27:32,040
But that would probably be a mistake, because
the water in the saguaro contains a poison.
259
00:27:32,200 --> 00:27:38,440
But there are lots of desert-living plants
which do have drinkable water within them,
260
00:27:38,600 --> 00:27:43,000
and desert-living people all over the world
have become expert botanists,
261
00:27:43,160 --> 00:27:47,360
able to recognise
from just the tiniest little leaflet or straggling stem
262
00:27:47,520 --> 00:27:50,280
where they can get a good drink.
263
00:27:51,920 --> 00:27:55,800
None are more skilled
than the Bushman people of the Kalahari.
264
00:28:08,760 --> 00:28:13,600
By the end of the dry season,
all their water holes have usually dried up.
265
00:28:13,760 --> 00:28:17,560
For liquid, they must now
rely almost entirely on plants
266
00:28:17,720 --> 00:28:21,760
and their ability to recognise the right ones.
267
00:28:28,760 --> 00:28:32,640
This tuber is a kind
that provides good drinking water.
268
00:28:43,160 --> 00:28:45,880
This much larger one is also full of liquid,
269
00:28:46,040 --> 00:28:49,160
but, unfortunately,
it's so bitter, it's undrinkable.
270
00:28:49,320 --> 00:28:52,240
But it's worth having nonetheless.
271
00:28:55,040 --> 00:28:58,960
To extract the water,
the root must be grated and pulped.
272
00:29:29,760 --> 00:29:32,880
The bigger root is grated as well.
273
00:29:37,320 --> 00:29:41,320
Drier though it is, it still contains valuable fluid.
274
00:29:47,680 --> 00:29:51,000
Since it cannot be drunk,
people use it to moisten their skin,
275
00:29:51,160 --> 00:29:54,680
and as it evaporates,
it brings a delicious refreshing coolness.
276
00:30:15,280 --> 00:30:17,840
200 miles west of the Kalahari
277
00:30:18,000 --> 00:30:21,560
lies an even hotter, drier desert: The Namib.
278
00:30:21,720 --> 00:30:25,640
Very few plants indeed
can survive in these parched sands.
279
00:30:25,800 --> 00:30:30,800
Patches of grass sprouted
after a rare shower and lived for a few weeks,
280
00:30:30,960 --> 00:30:36,160
but that was over four years ago
and now only the dusty withered stems are left.
281
00:30:36,320 --> 00:30:39,920
There is one plant that grows here, though,
and nowhere else,
282
00:30:40,080 --> 00:30:42,480
and one that is very odd indeed.
283
00:30:45,880 --> 00:30:50,520
The scientist who first described
this extraordinary plant
284
00:30:50,680 --> 00:30:55,880
was an Austrian called Dr Welwitsch,
who came here in the last century.
285
00:30:56,040 --> 00:31:00,760
He discovered many plants in Africa,
but this perhaps is his most famous
286
00:31:00,920 --> 00:31:04,240
and the one that bears his name: Welwitschia.
287
00:31:04,400 --> 00:31:07,800
There are male plants and female plants.
288
00:31:07,960 --> 00:31:11,640
This one is a female,
and these are the female's structures.
289
00:31:11,800 --> 00:31:14,600
These are young ones, which sprouted this year,
290
00:31:14,760 --> 00:31:17,960
and these are
fully developed ones from last year.
291
00:31:18,120 --> 00:31:22,280
In structure, they are very
like the cones of a fir tree.
292
00:31:25,040 --> 00:31:28,920
The male plant has growths
rather like stamens, which produce pollen,
293
00:31:29,080 --> 00:31:33,200
so welwitschia seems to be
a kind of link between coniferous trees
294
00:31:33,360 --> 00:31:36,040
and true flowering plants.
295
00:31:38,360 --> 00:31:41,600
But the oddest thing about it are its leaves.
296
00:31:41,760 --> 00:31:46,800
They grow from the top of its central trunk,
and do so extremely slowly,
297
00:31:46,960 --> 00:31:51,800
so that this leaf would have taken
about 70 years to be produced.
298
00:31:52,000 --> 00:31:57,400
But if it hadn't frayed at the edges,
it would be about 400 yards long,
299
00:31:57,560 --> 00:32:02,760
because this individual plant
is thought to be about 1,500 years old.
300
00:32:05,560 --> 00:32:10,960
It's these amazing leaves that enable the plant
to collect water in this rainless country.
301
00:32:11,120 --> 00:32:14,480
The Namib lies close
to the western coast of Africa.
302
00:32:14,640 --> 00:32:18,600
At dawn,
fogs regularly roll in from the Atlantic.
303
00:32:18,960 --> 00:32:24,440
As they swirl around the welwitschia,
their moisture condenses on the huge leaves.
304
00:32:24,600 --> 00:32:30,040
Some droplets are absorbed
through cracks in the leaves' skin.
305
00:32:31,160 --> 00:32:33,840
The rest is channelled down to the ground,
306
00:32:34,000 --> 00:32:39,520
where it's sucked up by roots
just below the surface of the sand.
307
00:32:40,640 --> 00:32:44,760
The fog also provides life-saving drinks
for some of the desert animals.
308
00:32:44,920 --> 00:32:47,120
These are darkling beetles.
309
00:32:47,280 --> 00:32:51,760
On foggy mornings, they climb
to the top of the dunes and stand in lines,
310
00:32:51,920 --> 00:32:56,040
head down, abdomen up, slowly marking time.
311
00:33:16,800 --> 00:33:21,680
Droplets of water
from the fog collect on legs and antennae,
312
00:33:21,840 --> 00:33:26,280
and then, as the beetle lifts its feet,
trickle down towards its mouth.
313
00:33:31,320 --> 00:33:35,520
The Namib's fogs never penetrate very far inland.
314
00:33:35,680 --> 00:33:40,880
Deserts that lie far from the sea, therefore,
can never receive moisture in such a way.
315
00:33:41,040 --> 00:33:44,200
Their water must come from the clouds.
316
00:33:46,280 --> 00:33:49,960
Often, the clouds that do build up above a desert
317
00:33:50,120 --> 00:33:55,160
sail off elsewhere without bursting,
and the land remains parched.
318
00:34:04,640 --> 00:34:10,720
But when eventually rain does come,
it's the trigger for immediate and urgent action.
319
00:34:13,520 --> 00:34:18,600
One or two drops are all that's necessary
to activate these dead stems.
320
00:34:29,000 --> 00:34:31,760
Within half a minute, they're upright.
321
00:34:35,080 --> 00:34:38,680
Other plants begin to open their seed-heads.
322
00:34:46,040 --> 00:34:48,040
None of these plants is alive.
323
00:34:48,240 --> 00:34:54,040
All their movements are simply
the result of the dead tissues absorbing water.
324
00:35:04,960 --> 00:35:10,520
The dead seed-heads have held the seeds
securely during the drought.
325
00:35:11,640 --> 00:35:17,120
Now, since it's rained and there's a chance
of them germinating, they can be distributed.
326
00:35:18,680 --> 00:35:23,480
For some plants, the heavy raindrops
are enough to dislodge the seeds.
327
00:35:41,560 --> 00:35:47,520
Others utilise the physical effects
of absorbing water to shoot the seeds away.
328
00:35:56,800 --> 00:36:01,600
Now the seeds themselves,
lying on the ground, begin to move.
329
00:36:07,240 --> 00:36:10,240
As the hairs absorb water,
they swell and stiffen,
330
00:36:10,400 --> 00:36:12,920
so raising the seed into the right position
331
00:36:13,080 --> 00:36:17,360
for its first rootlets
to strike straight downwards into the ground.
332
00:36:21,720 --> 00:36:24,560
But sometimes in the Arizona desert,
maybe once in several years,
333
00:36:24,720 --> 00:36:29,160
there are real cloudbursts,
and the desert is transformed.
334
00:37:19,120 --> 00:37:22,360
In the aftermath of the flood, new faces appear.
335
00:37:33,560 --> 00:37:37,680
A spadefoot toad.
The males are the first to emerge from the soil
336
00:37:37,840 --> 00:37:41,240
where they've been buried
for the past year or more.
337
00:37:46,680 --> 00:37:51,680
Hastily, they make their way to one of the pools
that have appeared in the desert,
338
00:37:51,840 --> 00:37:55,800
and there they begin calling,
summoning the females.
339
00:37:55,960 --> 00:38:01,200
There is great urgency. If they don't mate
on this night, they may have lost their chance.
340
00:38:19,200 --> 00:38:24,720
Within 24 hours, the eggs have been laid
and fertilised and are beginning to hatch.
341
00:38:35,160 --> 00:38:38,800
A day later, the pool is full of tadpoles.
342
00:38:42,120 --> 00:38:45,800
Other creatures
have appeared as if from nowhere.
343
00:38:45,960 --> 00:38:51,080
Fairy shrimp have hatched from tiny eggs
blown with the dust all over the desert.
344
00:38:55,400 --> 00:38:57,960
The tadpoles are growing fast.
345
00:38:58,120 --> 00:39:00,920
These with small mouths
feed on algae and bacteria,
346
00:39:01,080 --> 00:39:05,240
a diet usually abundant in these desert pools.
347
00:39:12,560 --> 00:39:17,040
But other individuals from the same batch of eggs
develop bigger heads
348
00:39:17,200 --> 00:39:22,040
and more powerfully muscled jaws.
They have become meat-eaters.
349
00:39:25,000 --> 00:39:30,840
Not all pools will provide enough food for them,
but here they are fortunate.
350
00:39:40,200 --> 00:39:43,720
They even eat their vegetarian brothers.
351
00:39:48,840 --> 00:39:53,960
With such a protein-rich diet,
they grow even faster than the algal feeders.
352
00:39:54,120 --> 00:40:00,440
Here they are the favoured few, more likely
to survive if the pool evaporates quickly.
353
00:40:00,600 --> 00:40:03,800
They're an insurance
for the continuation of the species,
354
00:40:03,960 --> 00:40:06,480
for which the payments
are their vegetarian brothers.
355
00:40:07,960 --> 00:40:13,080
But now the pool is shrinking fast.
Another couple of days and it's almost gone.
356
00:40:13,240 --> 00:40:17,840
Unless there is another shower of rain,
all the tadpoles will die.
357
00:40:23,160 --> 00:40:26,160
If they do die, their bodies will not be wasted.
358
00:40:26,320 --> 00:40:31,000
They will decompose and fertilise the sand,
so that when the next rains come
359
00:40:31,160 --> 00:40:36,120
and another pool collects in this hollow,
the algae will grow fast and well.
360
00:40:39,520 --> 00:40:43,040
Ants are quick to attack the stricken tadpoles.
361
00:40:45,320 --> 00:40:49,640
But at the last minute, there is a reprieve.
A shower of rain.
362
00:40:49,800 --> 00:40:53,800
Some tadpoles, though they still have a tail,
now have legs,
363
00:40:53,960 --> 00:40:58,520
and they're able to leave the puddle
just two weeks after hatching.
364
00:41:05,040 --> 00:41:09,840
Even among this tiny proportion of survivors,
the mortality will be huge.
365
00:41:10,000 --> 00:41:14,280
But with luck,
a few will join the adults as the desert dries
366
00:41:14,440 --> 00:41:20,160
and bury themselves to wait for
the next shower of rain many months from now.
367
00:41:41,640 --> 00:41:45,480
For several weeks after the rains,
the desert blooms.
368
00:41:45,640 --> 00:41:50,520
The seeds shed by the shrivelled plants
have sprouted and burst into flower.
369
00:41:50,680 --> 00:41:53,320
And deserts after rain all over the world,
370
00:41:53,480 --> 00:41:56,920
in Arizona and Australia,
the Namib and the Sahara,
371
00:41:57,080 --> 00:42:02,360
put on one of the most dazzling displays
of colour that you can see anywhere.
372
00:42:46,480 --> 00:42:49,200
Deserts are shaped by the sun and the wind.
373
00:42:49,360 --> 00:42:53,480
The roasting of rocks during the day,
their chilling during cold nights,
374
00:42:53,640 --> 00:42:55,920
eventually makes their surface crumble.
375
00:42:56,080 --> 00:42:58,920
Some of their minerals splinter and fray into dust.
376
00:42:59,080 --> 00:43:04,320
But quartz, the commonest, is very hard,
and that remains as grains of sand.
377
00:43:04,480 --> 00:43:10,280
The wind catches them, sweeps them away,
and collects them together as sand dunes.
378
00:43:37,600 --> 00:43:40,360
Dunes may be hundreds of feet high.
379
00:43:40,520 --> 00:43:42,920
If the wind is more or less constant,
380
00:43:43,080 --> 00:43:47,600
it blows the grains up the gently sloping side
and over the steep front
381
00:43:47,760 --> 00:43:50,720
so that the dune marches slowly
across the desert.
382
00:44:04,520 --> 00:44:09,600
Trudging up the face of a dune like this
is extremely hard work.
383
00:44:09,760 --> 00:44:12,320
The sand is so dry
384
00:44:12,480 --> 00:44:16,880
and the grains are so polished
by the wind rubbing them together
385
00:44:17,040 --> 00:44:20,080
that the surface is continuously on the move,
386
00:44:20,240 --> 00:44:24,000
and it's quite impossible
to get any firm foothold.
387
00:44:24,160 --> 00:44:27,480
And, of course, that problem faces not just me,
388
00:44:27,640 --> 00:44:31,520
but all the animals that live among these dunes.
389
00:44:31,680 --> 00:44:37,200
Some of them have developed some extremely
ingenious solutions to the difficulty.
390
00:44:39,080 --> 00:44:45,600
These extraordinary tracks have been made
by one of the swiftest movers across the dunes.
391
00:44:48,480 --> 00:44:50,840
The sidewinder, a kind of rattlesnake.
392
00:44:51,000 --> 00:44:55,240
It skims across the surface
by throwing its body into loops,
393
00:44:55,400 --> 00:44:58,120
which only touch the sand at two points.
394
00:44:58,280 --> 00:45:00,760
This not only enables it to move fast,
395
00:45:00,920 --> 00:45:04,440
but keeps most of its body off the hot surface.
396
00:45:09,160 --> 00:45:14,920
At midday,
the sand is so hot that it's painful to touch.
397
00:45:15,080 --> 00:45:20,800
The Namib fringe-toed lizard
prevents its feet from scorching by gymnastics.
398
00:45:36,600 --> 00:45:39,200
But eventually it gets so hot,
399
00:45:39,360 --> 00:45:45,000
the only thing to do is to shelter
beneath the surface where the sand is very cool.
400
00:45:46,120 --> 00:45:49,200
Burrowing through this kind of sand
also has problems.
401
00:45:49,360 --> 00:45:54,040
An animal can't construct a tunnel
like a mouse hole or a rabbit burrow
402
00:45:54,200 --> 00:45:56,960
because the sand simply falls in behind it.
403
00:45:57,120 --> 00:46:01,760
So instead it has to wriggle through the sand
almost as though it's swimming.
404
00:46:01,920 --> 00:46:05,000
And that's precisely
what this little creature does.
405
00:46:05,160 --> 00:46:10,520
It may look like a worm,
but in fact it's a lizard that has lost its legs.
406
00:46:10,680 --> 00:46:14,920
You can see that it's a lizard
when you look closely at its face.
407
00:46:16,240 --> 00:46:21,520
Its mouth and eyes are covered by
transparent scales that protect them in the sand.
408
00:46:21,680 --> 00:46:26,680
It's a blind skink. It lives by hunting
for insects below the sand surface,
409
00:46:26,840 --> 00:46:31,640
and when I put it down,
it'll wriggle away, just like an eel.
410
00:46:37,360 --> 00:46:41,800
The most extremely specialised
of these hunters in the dunes
411
00:46:41,960 --> 00:46:45,600
is not a reptile but a mammal.
412
00:46:45,760 --> 00:46:51,960
It's very rarely seen,
and your best chance of finding it is at night.
413
00:46:54,240 --> 00:47:00,200
These are its tracks, and that depression
a place where it caught something.
414
00:47:07,600 --> 00:47:09,800
This is where it has burrowed again,
415
00:47:09,960 --> 00:47:14,680
and where, with luck,
and if I dig very fast, I might catch it.
416
00:47:22,680 --> 00:47:25,760
Here it is, a golden mole.
417
00:47:28,360 --> 00:47:32,280
This one is a baby,
but like its parents, it's totally blind.
418
00:47:32,440 --> 00:47:37,560
Eyes are of no use beneath the sand.
Nor are ears, and it hasn't got those either.
419
00:47:37,720 --> 00:47:40,120
Its head ends in a leathery wedge
420
00:47:40,280 --> 00:47:46,880
with which it pushes through the sand,
or alternatively, through my fingers.
421
00:47:58,560 --> 00:48:01,360
Golden moles will eat quite large creatures:
422
00:48:01,520 --> 00:48:04,120
A blind skink, if it encounters one,
423
00:48:04,280 --> 00:48:08,960
or other creatures that might be wandering
unsuspectingly across the surface.
424
00:48:22,800 --> 00:48:25,280
A cricket would do nicely.
425
00:49:14,120 --> 00:49:19,120
The great sandy deserts of the world
in Arabia, central Australia and the Sahara
426
00:49:19,280 --> 00:49:22,560
have repelled
even the hardiest of human travellers.
427
00:49:22,720 --> 00:49:27,120
Few people have managed
to survive in them for long totally unaided.
428
00:49:27,280 --> 00:49:32,360
But some manage to make regular journeys
through these wildernesses.
429
00:49:36,040 --> 00:49:41,040
These are the Tuareg. They travel
from one side of the Sahara to the other,
430
00:49:41,200 --> 00:49:46,800
carrying great cakes of salt,
which they trade for cloth and grain and dates.
431
00:49:59,640 --> 00:50:02,720
But even the Tuareg
can only make these journeys
432
00:50:02,880 --> 00:50:07,480
with the help of an animal desert specialist:
The camel.
433
00:50:07,640 --> 00:50:13,960
They have to take all the food
that they and their camels will need with them.
434
00:50:16,480 --> 00:50:20,280
Water is carried in skins
slung beneath the camels' bellies
435
00:50:20,440 --> 00:50:24,280
to minimise evaporation
and keep it as cool as possible.
436
00:50:25,520 --> 00:50:29,280
The camel is marvellously adapted
to life in the desert.
437
00:50:29,440 --> 00:50:32,720
Its toes are reduced to two,
but connected by skin,
438
00:50:32,880 --> 00:50:38,240
so that they splay out on the sand
and don't sink deeply into it.
439
00:50:44,200 --> 00:50:49,520
Their nostrils are closable, so they can
shut out sand grains during a sandstorm.
440
00:50:57,680 --> 00:51:03,160
The hair on their body is restricted
to the top, where it shields against the sun.
441
00:51:03,320 --> 00:51:06,240
Elsewhere, for coolness,
their skin is virtually naked.
442
00:51:06,400 --> 00:51:11,240
Their hump is full of fat,
which in emergencies can be converted to water.
443
00:51:11,400 --> 00:51:13,920
But the process wastes the fat's calories
444
00:51:14,080 --> 00:51:17,880
and is only used
when the camel hasn't drunk for a long time.
445
00:51:18,040 --> 00:51:22,480
It can live without liquid water
for four times as long as a donkey
446
00:51:22,640 --> 00:51:25,280
and ten times as long as a man.
447
00:51:25,440 --> 00:51:28,920
But eventually even a camel has to drink.
448
00:51:36,400 --> 00:51:41,920
At one or two places in the Sahara, water
can be reached by digging deep into the ground.
449
00:51:42,080 --> 00:51:45,800
Here, camels can at last refill their stomachs,
450
00:51:45,960 --> 00:51:48,880
and they take a lot of filling.
451
00:52:18,720 --> 00:52:22,200
If the Tuareg
can't cross the Sahara without the camel,
452
00:52:22,360 --> 00:52:25,000
the camel can't do so without the Tuareg,
453
00:52:25,160 --> 00:52:29,200
for only men can dig for the essential water.
454
00:52:31,520 --> 00:52:34,800
Spring water is the key
which unlocks abundant fertility.
455
00:52:34,960 --> 00:52:37,160
At Saharan oases like this one,
456
00:52:37,320 --> 00:52:41,440
all kinds of crops can be produced
from the sand if it's watered:
457
00:52:41,600 --> 00:52:44,760
Dates and vegetables and fruit.
458
00:52:44,920 --> 00:52:49,240
Insects whizz and buzz
over the gurgling irrigation channels
459
00:52:49,400 --> 00:52:51,960
and birds sing in the palm trees.
460
00:52:53,200 --> 00:52:57,000
But these small islands of life
are under constant threat.
461
00:52:57,160 --> 00:53:02,760
If the wind veers and blows steadily
from another direction, nothing can stop the sand.
462
00:53:13,040 --> 00:53:18,760
Eventually the advancing dunes
may well overwhelm this oasis,
463
00:53:18,920 --> 00:53:20,840
and then this small world
464
00:53:21,000 --> 00:53:26,320
that's been brought into existence in the desert
by the presence of water will be extinguished.
465
00:53:26,480 --> 00:53:29,560
The force that drives the dune,
of course, is the wind,
466
00:53:29,720 --> 00:53:33,200
and the wind, too,
has its own world of living organisms.
467
00:53:33,360 --> 00:53:36,120
Many of the spiders and beetles
468
00:53:36,280 --> 00:53:41,280
and other insects
that live in the oasis arrived by air.
469
00:53:41,440 --> 00:53:46,160
And many of the plants, too, coming
as windblown seeds or carried by birds.
470
00:53:46,320 --> 00:53:52,120
And that world, the world of the wind
and the sky, we'll be exploring next time.
471
00:53:52,170 --> 00:53:56,720
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