"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 i 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 nort 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 worl" "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." "Ajerboa." "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 fami" "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." "The desert holly." "Its leaves grow at about 70 degrees to the vertical, so that in the morning when it's less hot and in the evening when the plant needs light, the face of its leaves face the light." "During the middle of the day, it shows only the edges and doesn't heat up so much." "Not only that, but the plant extracts salt from the salt-laden ground and excretes it as a white coating on the leaf, which, like the white costume of an athlete, reflects the heat and so keeps the plant that much cooler." "And this, the creosote bush." "This is one of the most widespread of plants in American deserts, and its roots are better at extracting the last molecule of water from parched sands than those of any other American plant." "This has led to an extraordinary state of affairs that's only just been discovered." "It seems that the creosote bush was the first plant to establish itself in the arid Mojave desert when the desert first appeared." "Once it had established its extensive root system, it extracted moisture from the sand so efficiently that it was extremely difficult for any other plant to grow alongside it." "And that applied not only to any other kind of plant but also to its own seedlings." "So an individual creosote bush tended to spread not by setting seeds and producing a new generation, but by sending out new stems around its base." "And as these spread outwards, so the stems in the middle tended to die away, and the bush grew into a ring shape like this." "So these are not separate individual creosote bushes, as it might appear, but this is just one big ring-shaped individual plant." "Over the centuries, the rings widened and changed their shape until now some are over 25 yards across, like this one." "Of course, the individual stems and leaves of this plant are not very ancient." "The first ones to grow, which appeared in the middle, decayed and disappeared long ago." "Now it's estimated that this plant began growing between 10,000 and 12,000 years ago, in fact, when the Mojave desert first appeared, and that makes it the oldest known living organism in the world." "In the Mojave, the plants may have to survive for as long as ten years without rain, but if rain falls a little more frequently, as it does nearby in Arizona, plants can have different survival strategies." "To many of us, the very symbol of the desert is the cactus." "But in fact, this family of fleshy-stemmed plants lives only in the Americas." "There are several hundred species of them, but among the biggest is the saguaro." "The saguaro has solved the problems of surviving in great heat and drought very successfully indeed." "Its stem is pleated like an accordion, so when rain does fall, the cactus can expand and quickly absorb as much water as possible before it disappears." "After a single storm, a saguaro can take up as much as a ton in a few days." "Its leaves have become thorns, so reducing the surface area from which the plant might lose water by evaporation." "The stem itself is green and has taken over the job of photosynthesis." "The thorns protect the young plant from browsers, but they also break up the wind currents, so that the cactus is wrapped in still air, and evaporation of moisture from the stem is kept very low." "These huge saguaro cacti can live for over 200 years and stand nearly 50 feet high." "If I was dying of thirst in this desert," "I'd be tempted to cut inside that saguaro and raid its reservoir of water." "But that would probably be a mistake, because the water in the saguaro contains a poison." "But there are lots of desert-living plants which do have drinkable water within them, and desert-living people all over the world have become expert botanists, able to recognise from just the tiniest little leaflet or straggling where they can get a good drink." "None are more skilled than the Bushman people of the Kalahari." "By the end of the dry season, all their water holes have usually dried up." "For liquid, they must now rely almost entirely on plants and their ability to recognise the right ones." "This tuber is a kind that provides good drinking water." "This much larger one is also full of liquid, but, unfortunately, it's so bitter, it's undrinkable." "But it's worth having nonetheless." "To extract the water, the root must be grated and pulped." "The bigger root is grated as well." "Drier though it is, it still contains valuable flu" "Since it cannot be drunk, people use it to moisten their skin, and as it evaporates, it brings a delicious refreshing coolness." "200 miles west of the Kalahari lies an even hotter, drier desert:" "The Namib." "Very few plants indeed can survive in these parched sands." "Patches of grass sprouted after a rare shower and lived for a few weeks, but that was over four years ago and now only the dusty withered stems are left." "There is one plant that grows here, though, and nowhere else, and one that is very odd indeed." "The scientist who first described this extraordinary plant was an Austrian called Dr Welwitsch, who came here in the last century." "He discovered many plants in Africa, but this perhaps is his most famous and the one that bears his name:" "Welwitschia." "There are male plants and female plants." "This one is a female, and these are the female's structures." "These are young ones, which sprouted this year, and these are fully developed ones from last year." "In structure, they are very like the cones of a fir tree." "The male plant has growths rather like stamens, which produce pollen, so welwitschia seems to be a kind of link between coniferous trees and true flowering plants." "But the oddest thing about it are its leaves." "They grow from the top of its central trunk, and do so extremely slowly, so that this leaf would have taken about 70 years to be produced." "But if it hadn't frayed at the edges, it would be about 400 yards long, because this individual plant is thought to be about 1,500 years old." "It's these amazing leaves that enable the plant to collect water in this rainless country." "The Namib lies close to the western coast of Africa." "At dawn, fogs regularly roll in from the Atlantic." "As they swirl around the welwitschia, their moisture condenses on the huge leaves." "Some droplets are absorbed through cracks in the leaves' skin." "The rest is channelled down to the ground, where it's sucked up by roots just below the surface of the sand." "The fog also provides life-saving drinks for some of the desert animals." "These are darkling beetles." "On foggy mornings, they climb to the top of the dunes and stand in lines, head down, abdomen up, slowly marking time." "Droplets of water from the fog collect on legs and antennae, and then, as the beetle lifts its feet, trickle down towards its mouth." "The Namib's fogs never penetrate very far inland." "Deserts that lie far from the sea, therefore, can never receive moisture in such a way." "Their water must come from the clouds." "Often, the clouds that do build up above a desert sail off elsewhere without bursting, and the land remains parched." "But when eventually rain does come, it's the trigger for immediate and urgent action." "One or two drops are all that's necessary to activate these dead stems." "Within half a minute, they're upright." "Other plants begin to open their seed-heads." "None of these plants is alive." "All their movements are simply the result of the dead tissues absorbing water." "The dead seed-heads have held the seeds securely during the drought." "Now, since it's rained and there's a chance of them germinating, they can be distributed." "For some plants, the heavy raindrops are enough to dislodge the seeds." "Others utilise the physical effects of absorbing water to shoot the seeds away." "Now the seeds themselves, lying on the ground, begin to move." "As the hairs absorb water, they swell and stiffen, so raising the seed into the right position for its first rootlets to strike straight downwards into the ground." "But sometimes in the Arizona desert, maybe once in several years, there are real cloudbursts, and the desert is transformed." "In the aftermath of the flood, new faces appear." "A spadefoot toad." "The males are the first to emerge from the soil where they've been buried for the past year or more." "Hastily, they make their way to one of the pools that have appeared in the desert, and there they begin calling, summoning the females." "There is great urgency." "If they don't mate on this night, they may have lost their chance." "Within 24 hours, the eggs have been laid and fertilised and are beginning to hatch." "A day later, the pool is full of tadpoles." "Other creatures have appeared as if from nowhere." "Fairy shrimp have hatched from tiny eggs blown with the dust all over the desert." "The tadpoles are growing fast." "These with small mouths feed on algae and bacteria, a diet usually abundant in these desert pools." "But other individuals from the same batch of eggs develop bigger heads and more powerfully muscled jaws." "They have become meat-eaters." "Not all pools will provide enough food for them, but here they are fortunate." "They even eat their vegetarian brothers." "With such a protein-rich diet, they grow even faster than the algal feeders." "Here they are the favoured few, more likely to survive if the pool evaporates quickly." "They're an insurance for the continuation of the species, for which the payments are their vegetarian brothers." "But now the pool is shrinking fast." "Another couple of days and it's almost gone." "Unless there is another shower of rain, all the tadpoles will die." "If they do die, their bodies will not be wasted." "They will decompose and fertilise the sand, so that when the next rains come and another pool collects in this hollow, the algae will grow fast and well." "Ants are quick to attack the stricken tadpoles." "But at the last minute, there is a reprieve." "A shower of rain." "Some tadpoles, though they still have a tail, now have legs, and they're able to leave the puddle just two weeks after hatching." "Even among this tiny proportion of survivors, the mortality will be huge." "But with luck, a few will join the adults as the desert dries and bury themselves to wait for the next shower of rain many months from now." "For several weeks after the rains, the desert blooms." "The seeds shed by the shrivelled plants have sprouted and burst into flower." "And deserts after rain all over the world, in Arizona and Australia, the Namib and the Sahara, put on one of the most dazzling displays of colour that you can see anywhere." "Deserts are shaped by the sun and the wind." "The roasting of rocks during the day, their chilling during cold nights, eventually makes their surface crumble." "Some of their minerals splinter and fray into dust" "But quartz, the commonest, is very hard, and that remains as grains of sand." "The wind catches them, sweeps them away, and collects them together as sand dunes." "Dunes may be hundreds of feet high." "If the wind is more or less constant, it blows the grains up the gently sloping side and over the steep front so that the dune marches slowly across the desert." "Trudging up the face of a dune like this is extremely hard work." "The sand is so dry and the grains are so polished by the wind rubbing them together that the surface is continuously on the move, and it's quite impossible to get any firm foothold." "And, of course, that problem faces notjust me, but all the animals that live among these dunes." "Some of them have developed some extremely ingenious solutions to the difficulty." "These extraordinary tracks have been made by one of the swiftest movers across the dunes." "The sidewinder, a kind of rattlesnake." "It skims across the surface by throwing its body into loops, which only touch the sand at two points." "This not only enables it to move fast, but keeps most of its body off the hot surface." "At midday, the sand is so hot that it's painful to touch." "The Namib fringe-toed lizard prevents its feet from scorching by gymnastics." "But eventually it gets so hot, the only thing to do is to shelter beneath the surface where the sand is very cool." "Burrowing through this kind of sand also has problems." "An animal can't construct a tunnel like a mouse hole or a rabbit burrow because the sand simply falls in behind it." "So instead it has to wriggle through the sand almost as though it's swimming." "And that's precisely what this little creature does." "It may look like a worm, but in fact it's a lizard that has lost its legs." "You can see that it's a lizard when you look closely at its face." "Its mouth and eyes are covered by transparent scales that protect them in the sand." "It's a blind skink." "It lives by hunting for insects below the sand surface, and when I put it down, it'll wriggle away, just like an eel." "The most extremely specialised of these hunters in the dunes is not a reptile but a mammal." "It's very rarely seen, and your best chance of finding it is at night." "These are its tracks, and that depression a place where it caught something." "This is where it has burrowed again, and where, with luck, and if I dig very fast, I might catch it." "Here it is, a golden mole." "This one is a baby, but like its parents, it's totally blind." "Eyes are of no use beneath the sand." "Nor are ears, and it hasn't got those either." "Its head ends in a leathery wedge with which it pushes through the sand, or alternatively, through my fingers." "Golden moles will eat quite large creatures:" "A blind skink, if it encounters one, or other creatures that might be wandering unsuspectingly across the surface." "A cricket would do nicely." "The great sandy deserts of the world in Arabia, central Australia and the Sahara have repelled even the hardiest of human travellers." "Few people have managed to survive in them for long totally unaided." "But some manage to make regularjourneys through these wildernesses." "These are the Tuareg." "They travel from one side of the Sahara to the other, carrying great cakes of salt, which they trade for cloth and grain and dates." "But even the Tuareg can only make these journeys with the help of an animal desert specialist:" "The camel." "They have to take all the food that they and their camels will need with them." "Water is carried in skins slung beneath the camels' bellies to minimise evaporation and keep it as cool as possible." "The camel is marvellously adapted to life in the desert." "Its toes are reduced to two, but connected by skin, so that they splay out on the sand and don't sink deeply into it." "Their nostrils are closable, so they can shut out sand grains during a sandstorm." "The hair on their body is restricted to the top, where it shields against the sun." "Elsewhere, for coolness, their skin is virtually naked." "Their hump is full of fat, which in emergencies can be converted to water." "But the process wastes the fat's calories and is only used when the camel hasn't drunk for a long time." "It can live without liquid water for four times as long as a donkey and ten times as long as a man." "But eventually even a camel has to drink." "At one or two places in the Sahara, water can be reached by digging deep into the ground." "Here, camels can at last refill their stomachs, and they take a lot of filling." "If the Tuareg can't cross the Sahara without the camel, the camel can't do so without the Tuareg, for only men can dig for the essential water." "Spring water is the key which unlocks abundant fertility." "At Saharan oases like this one, all kinds of crops can be produced from the sand if it's watered:" "Dates and vegetables and fruit." "Insects whizz and buzz over the gurgling irrigation channels and birds sing in the palm trees." "But these small islands of life are under constant threat." "If the wind veers and blows steadily from another direction, nothing can stop the sand." "Eventually the advancing dunes may well overwhelm this oasis, and then this small world that's been brought into existence in the desert by the presence of water will be extinguished." "The force that drives the dune, of course, is the wind, and the wind, too, has its own world of living organisms." "Many of the spiders and beetles and other insects that live in the oasis arrived by air." "And many of the plants, too, coming as windblown seeds or carried by birds." "And that world, the world of the wind and the sky, we'll be exploring next time."