1 00:01:49,476 --> 00:01:54,709 These mountains stand in the middle of the biggest desert on earth, the Sahara. 2 00:01:55,582 --> 00:01:57,846 It stretches right across the width of Africa, 3 00:01:58,084 --> 00:02:01,076 three and a half million square miles of it. 4 00:02:02,355 --> 00:02:06,052 At night, it gets so cold that it can freeze. 5 00:02:06,259 --> 00:02:09,786 During the day, the sun strikes it so ferociously 6 00:02:09,963 --> 00:02:13,763 that the highest land temperatures ever recorded were measured here: 7 00:02:13,933 --> 00:02:18,393 58 degrees centigrade, 137 degrees Fahrenheit. 8 00:02:18,738 --> 00:02:24,199 And, in turn, those oven-like temperatures rob the land of all its moisture. 9 00:02:24,477 --> 00:02:29,813 All in all, there could hardly be a more hostile environment for life on earth. 10 00:02:30,150 --> 00:02:31,947 But it wasn't always this way. 11 00:02:33,920 --> 00:02:36,514 And if you want evidence of that, here it is. 12 00:02:41,995 --> 00:02:44,555 A group of antelope, probably sable. 13 00:02:44,764 --> 00:02:47,756 Creatures that can't live anywhere in the Sahara today, 14 00:02:47,934 --> 00:02:50,425 because there's simply not enough vegetation for them. 15 00:02:50,670 --> 00:02:54,162 These aren't the only wild animals painted on these rocks. 16 00:02:57,744 --> 00:02:59,075 A giraffe. 17 00:03:01,514 --> 00:03:04,210 A kind of wild goat, probably a moufflon. 18 00:03:05,084 --> 00:03:06,244 And antelope. 19 00:03:06,486 --> 00:03:10,855 Obviously, at the time these pictures were painted, there was good grazing here. 20 00:03:11,758 --> 00:03:16,889 Indeed, there was sufficient vegetation to sustain not only wild animals, 21 00:03:17,063 --> 00:03:19,258 but whole herds of cattle. 22 00:03:21,034 --> 00:03:23,434 We don't know exactly who drew these pictures. 23 00:03:23,603 --> 00:03:25,696 The artists may have been ancestors of the nomads 24 00:03:25,872 --> 00:03:31,674 who today follow their herds of long-horned piebald cattle just south of the Sahara. 25 00:03:32,045 --> 00:03:35,412 But we know what they looked like, because they left their portraits. 26 00:03:35,782 --> 00:03:38,615 They lived here, it seems, some 5,000 years ago. 27 00:03:38,818 --> 00:03:42,447 But eventually the rains began to fail, the pastures disappeared, 28 00:03:42,622 --> 00:03:45,090 and with it the cattle and their keepers. 29 00:03:45,425 --> 00:03:49,088 But there are one or two living survivors from that time. 30 00:03:52,031 --> 00:03:56,695 This ancient cypress, judging from the number of rings in the trunks of others like i 31 00:03:56,970 --> 00:04:00,428 is probably between 2,000 and 3,000 years old. 32 00:04:00,740 --> 00:04:06,679 In fact, it was probably already growing here when the last paintings were being made. 33 00:04:06,946 --> 00:04:08,937 It still bears fertile seed, 34 00:04:09,115 --> 00:04:12,642 but there are no little seedlings growing here to replace it. 35 00:04:12,952 --> 00:04:15,944 The land now is far too dry for them. 36 00:04:16,256 --> 00:04:18,383 Indeed, the cypress itself only survives 37 00:04:18,558 --> 00:04:23,860 because it sends its huge roots deep into the ground to tap underground water. 38 00:04:24,897 --> 00:04:28,128 The drying out of the Sahara seems to be connected 39 00:04:28,301 --> 00:04:32,567 with the great changes in climate at the end of the last ice age. 40 00:04:32,872 --> 00:04:36,399 As the glaciers retreated northwards across Europe, 41 00:04:36,643 --> 00:04:39,737 so the rains that fell regularly along their southern edge 42 00:04:39,912 --> 00:04:45,873 left Africa and moved up into Europe, and the Sahara was robbed of its rains. 43 00:04:46,286 --> 00:04:52,122 Indeed, it seems that most of the world's great deserts were formed around that time, 44 00:04:52,292 --> 00:04:56,820 and most, if not all of them, are therefore comparatively recent environments. 45 00:04:58,331 --> 00:05:03,098 To see why deserts lie where they do, we can look at the Sahara from the west. 46 00:05:03,403 --> 00:05:07,464 The equator runs away from us across the width of Africa. 47 00:05:07,907 --> 00:05:12,173 It's along this line that the sun's rays strike from directly overhead, 48 00:05:12,345 --> 00:05:14,870 and therefore with the greatest strength. 49 00:05:15,348 --> 00:05:17,248 The heated air rises along the equator 50 00:05:17,417 --> 00:05:21,046 and flows away, north and south, to cooler parts of the world. 51 00:05:21,321 --> 00:05:24,757 Because it's warm, it carries a lot of moisture. 52 00:05:24,957 --> 00:05:29,792 But as it rises and cools, the moisture condenses first to form clouds 53 00:05:29,962 --> 00:05:31,623 and then to fall as rain. 54 00:05:31,964 --> 00:05:37,231 When the air comes down again over the Sahara to the left and the Kalahari to the right, 55 00:05:37,503 --> 00:05:41,462 it's lost most of its moisture and creates few clouds. 56 00:05:42,008 --> 00:05:44,875 The Sahara, with few clouds to shield it from the sun, 57 00:05:45,078 --> 00:05:47,774 becomes roastingly hot during the day. 58 00:05:48,114 --> 00:05:53,916 And at night, with no blanket of clouds to keep in its warmth, it gets desperately cold. 59 00:05:54,687 --> 00:05:58,316 Deserts are not placed symmetrically around the world, 60 00:05:58,524 --> 00:06:02,517 because the continents themselves are distributed in a very irregular way. 61 00:06:02,795 --> 00:06:07,562 They're ridged with great mountain ranges, which interfere with the even flow of air, 62 00:06:07,800 --> 00:06:11,793 and the planet's spin creates vast eddies in the atmosphere, 63 00:06:12,038 --> 00:06:13,699 which further complicates things. 64 00:06:14,073 --> 00:06:19,136 But even so, deserts do lie in two broad zones on either side of the equator. 65 00:06:19,846 --> 00:06:22,781 The pattern in Africa, with the Sahara in the nort 66 00:06:22,949 --> 00:06:28,046 and the Kalahari and the Namib in the south, has its equivalent in the Americas. 67 00:06:29,655 --> 00:06:32,818 South of the lush equatorial jungles of the Amazon, 68 00:06:33,025 --> 00:06:37,826 beyond the great range of the Andes, lies the Atacama desert. 69 00:06:38,131 --> 00:06:43,159 On the other side of the equator, beyond the drenched tropical rainforests of Panama, 70 00:06:43,469 --> 00:06:46,461 stretch the deserts of Mexico and Arizona. 71 00:06:47,907 --> 00:06:51,843 Across the Pacific, the greatest expanse of water on the globe, 72 00:06:52,078 --> 00:06:57,675 lies, south of the equator, Australia, most of which is covered by desert. 73 00:06:57,984 --> 00:07:02,887 Its northern tip gets close enough to the equator to collect some rain. 74 00:07:03,189 --> 00:07:04,656 Farther north still, 75 00:07:04,891 --> 00:07:09,089 beyond the jungle that blankets Indonesia and Malaysia, Thailand and Burma, 76 00:07:09,362 --> 00:07:12,388 across the great snow-covered range of the Himalayas, 77 00:07:12,732 --> 00:07:17,499 stretch the vast deserts of central Asia: Mongolia and the Gobi. 78 00:07:18,104 --> 00:07:20,971 Beyond them, as we complete the circuit of the globe, 79 00:07:21,207 --> 00:07:25,667 the huge desert of the Middle East that covers Iran, Iraq and Jordan, 80 00:07:25,912 --> 00:07:29,973 Syria and Israel, the vast sandy emptiness of Arabia, 81 00:07:30,216 --> 00:07:32,980 and runs on to join the Sahara. 82 00:07:39,659 --> 00:07:43,857 This is the biggest expanse of waterless land on earth. 83 00:07:44,297 --> 00:07:49,758 Here, as in deserts everywhere, almost nothing moves during the heat of the day. 84 00:07:51,938 --> 00:07:53,838 But animals are here. 85 00:08:04,851 --> 00:08:06,785 If you want to see what made these tracks, 86 00:08:07,019 --> 00:08:11,217 you have to wait until the sun sinks and the desert begins to cool. 87 00:08:19,999 --> 00:08:25,369 A striped hyena, one of the commonest of the bigger desert animals in this part of the worl 88 00:08:35,948 --> 00:08:37,245 A fennec fox. 89 00:08:45,992 --> 00:08:51,362 Fennecs usually live in small family groups, and clearly enjoy one another's company. 90 00:09:00,039 --> 00:09:03,600 But there's not much time for frolicking. Food must be found. 91 00:09:03,910 --> 00:09:09,041 Faint smells from the sand tell them who has moved where since they were last out. 92 00:09:38,144 --> 00:09:41,443 As the moon rises, many more creatures emerge. 93 00:09:42,248 --> 00:09:44,682 A gecko. Just what the fennec wants. 94 00:09:56,596 --> 00:10:00,532 Ajerboa. It, too, is looking for food. Seeds. 95 00:10:09,275 --> 00:10:12,210 Another little seed-eating rodent, a gerbil. 96 00:10:15,514 --> 00:10:21,316 And a caracal, a kind of cat, which loves both gerbils and jerboas, if it can get them. 97 00:10:37,470 --> 00:10:41,497 A smaller hunter, but nonetheless a deadly one: A scorpion. 98 00:10:42,008 --> 00:10:44,670 It is searching for beetles or other small insects 99 00:10:44,877 --> 00:10:47,710 But sometimes the hunter becomes the hunted. 100 00:10:48,147 --> 00:10:52,914 A black widow spider has set her snare of silk underneath a thorn bush. 101 00:11:32,792 --> 00:11:36,990 In the intense struggle, the black widow loses one of her legs. 102 00:11:47,506 --> 00:11:52,944 She manages to get more ropes of silk around the scorpion, hampering it still further. 103 00:11:57,983 --> 00:12:01,714 The scorpion hangs on to its trophy, but to no purpose. 104 00:12:01,954 --> 00:12:03,615 The battle is as good as lost. 105 00:12:15,434 --> 00:12:20,428 Methodically, the spider trusses up her victim and hangs it in her larder. 106 00:12:27,113 --> 00:12:31,516 Wolves, perhaps surprisingly, are quite common in these Middle Eastern deserts, 107 00:12:31,717 --> 00:12:34,379 but they're not like those farther north. 108 00:12:34,620 --> 00:12:38,420 They're smaller, lighter-coloured, and with only the thinnest fur, 109 00:12:38,724 --> 00:12:41,158 and they scavenge as much as they hunt. 110 00:12:45,664 --> 00:12:48,997 The cool night is coming to an end. Hunting is over. 111 00:12:49,168 --> 00:12:51,932 The animals must go back to their dens and hiding places 112 00:12:52,104 --> 00:12:54,834 to shelter from the heat that is to come. 113 00:13:16,695 --> 00:13:21,860 The sun returns, and very soon the desert will be heating up once again. 114 00:13:22,067 --> 00:13:25,867 The mammals that were active during the night have to find shelter. 115 00:13:26,038 --> 00:13:28,097 The day belongs not to them, 116 00:13:28,274 --> 00:13:33,177 but to creatures that get their heat directly from the sun: Reptiles. 117 00:13:33,412 --> 00:13:37,075 This is the desert of the American west in Arizona, 118 00:13:37,249 --> 00:13:42,482 and we've come here to look at one very special desert reptile: This one. 119 00:13:44,957 --> 00:13:51,385 This is the Gila monster, one of only two poisonous lizards in the world. 120 00:13:56,101 --> 00:13:59,969 Actually, he very seldom uses his poison in defence, 121 00:14:00,139 --> 00:14:03,040 and it's still quite early in the morning 122 00:14:03,209 --> 00:14:06,838 and he is so cold that he isn't very active. 123 00:14:07,146 --> 00:14:13,016 But in only about an hour, the desert will get so hot that he won't be able to stand it, 124 00:14:13,252 --> 00:14:15,914 and he, too, will have to seek shade. 125 00:14:16,088 --> 00:14:20,320 So in this short period of the early morning and in the cool of the evening 126 00:14:20,492 --> 00:14:22,926 is the time when he hunts. 127 00:14:37,343 --> 00:14:42,508 A tortoise, but he's far too big and well-armoured for a Gila monster to tackle. 128 00:14:53,759 --> 00:14:57,661 This great nest of sticks, however, looks much more promising. 129 00:15:12,211 --> 00:15:14,645 The victim: A desert mouse. 130 00:15:29,228 --> 00:15:33,688 The tortoise is on the lookout for food, too, but it is a vegetarian. 131 00:15:51,283 --> 00:15:53,808 The day is now several hours old. 132 00:15:54,053 --> 00:15:57,022 Cool dawn is changing to baking noon. 133 00:15:57,289 --> 00:16:00,520 It's time for even a reptile to get out of the sun 134 00:16:07,599 --> 00:16:09,362 Movement generates heat, 135 00:16:09,535 --> 00:16:13,596 so now nothing moves unless it absolutely has to. 136 00:16:13,906 --> 00:16:16,670 And there are some creatures that remain motionless 137 00:16:16,842 --> 00:16:19,709 even when you get within a few inches of them. 138 00:16:19,979 --> 00:16:24,279 One of them is on the ground right in front of me now, 139 00:16:24,516 --> 00:16:28,213 though you may find it difficult to see because it's so well camouflaged. 140 00:16:29,221 --> 00:16:31,985 It's a poorwill, a kind of nightjar. 141 00:16:36,929 --> 00:16:41,161 Fluttering the throat evaporates moisture from the mouth and so cools the bird. 142 00:16:41,533 --> 00:16:44,866 It consumes much less energy than heaving the chest and panting, 143 00:16:45,037 --> 00:16:47,665 as many mammals would do in this situation. 144 00:16:48,807 --> 00:16:51,401 The sand grouse of Africa uses the same trick. 145 00:16:59,651 --> 00:17:03,849 The sand grouse chicks start doing it almost as soon as they emerge from the shell. 146 00:17:09,361 --> 00:17:13,195 They also immediately peck for seeds, but there's little moisture in a seed, 147 00:17:13,399 --> 00:17:15,492 and unless they drink, they will die. 148 00:17:15,934 --> 00:17:19,995 The responsibility for providing that rests entirely with the male. 149 00:17:21,006 --> 00:17:25,500 Every day he flies to water, maybe as much as 25 miles from the nest. 150 00:17:26,278 --> 00:17:29,304 First he fills his own stomach with water. 151 00:17:34,520 --> 00:17:38,456 But then, very deliberately, he soaks his belly feathers. 152 00:17:43,328 --> 00:17:45,888 These feathers have a special spongy structure 153 00:17:46,065 --> 00:17:48,659 so that they can absorb lots of water. 154 00:17:48,834 --> 00:17:52,895 Once he has a full load, he flies back to his fami 155 00:18:03,449 --> 00:18:05,781 At last the chicks get their drink. 156 00:18:06,051 --> 00:18:09,953 No other bird has such an ingenious water-carrying device. 157 00:18:20,399 --> 00:18:22,264 The roadrunner of the American deserts 158 00:18:22,434 --> 00:18:25,335 provides water for its chicks quite differently. 159 00:18:25,571 --> 00:18:29,029 This parent bird has collected a cicada for its family. 160 00:18:30,642 --> 00:18:32,769 Its nest is in a cholla cactus. 161 00:18:33,011 --> 00:18:36,344 The parent doesn't give its chicks their food immediately. 162 00:18:43,155 --> 00:18:45,146 The chick is gulping. 163 00:18:45,390 --> 00:18:49,827 The parent bird is producing liquid from its stomach and letting it trickle down its beak. 164 00:18:56,335 --> 00:18:58,701 Each youngster gets its share. 165 00:19:14,453 --> 00:19:17,820 Another ration of solid food. This time, a lizard. 166 00:19:27,766 --> 00:19:31,497 Each time, before the meal is handed over, the chicks get a drink, 167 00:19:31,670 --> 00:19:33,228 whether they like it or not. 168 00:19:45,317 --> 00:19:49,219 During the day, the parents sit on the nest, not to keep the chicks warm, 169 00:19:49,388 --> 00:19:52,789 but, on the contrary, to keep them cool by shading them. 170 00:19:53,125 --> 00:19:55,150 The bird not only flutters its throat, 171 00:19:55,327 --> 00:19:59,525 but protects itself from the sun by using its tail as a parasol. 172 00:20:04,002 --> 00:20:07,631 The ground squirrel of the Namib desert does the same thing, 173 00:20:07,806 --> 00:20:11,708 and very effectively, too, carefully angling itself as far as possible 174 00:20:11,877 --> 00:20:13,504 to keep its body in the shade. 175 00:20:25,591 --> 00:20:28,526 Many animals keep their blood cool with radiators. 176 00:20:28,727 --> 00:20:33,323 The hedgehog that lives in the desert of the Middle East has unusually large ears. 177 00:20:33,599 --> 00:20:36,659 Blood circulates through capillaries close to the surface of the skin 178 00:20:36,835 --> 00:20:38,803 and is cooled by the breeze. 179 00:20:46,411 --> 00:20:49,608 The fennec fox's huge ears serve the same purpose. 180 00:20:52,584 --> 00:20:54,381 So do those of the American jackrabbit, 181 00:20:54,553 --> 00:20:58,387 which perhaps has the biggest ears of all in proportion to its body. 182 00:21:05,397 --> 00:21:08,127 The dorcas gazelle also has radiator ears 183 00:21:08,300 --> 00:21:11,133 and is one of the best-adapted desert mammals. 184 00:21:11,370 --> 00:21:14,931 It's one of few that can survive without drinking at all. 185 00:21:15,240 --> 00:21:18,334 It gets all the liquid it needs from vegetation. 186 00:21:21,079 --> 00:21:26,984 It doesn't waste liquid as urine, but gets rid of its uric acid as small dry pellets 187 00:21:44,102 --> 00:21:48,505 It's now approaching noon, the hottest time of the day. 188 00:21:48,707 --> 00:21:51,540 It's summer, the hottest time of the year, 189 00:21:51,743 --> 00:21:55,235 and I'm in one of the hottest places on earth: 190 00:21:55,480 --> 00:21:58,745 Death Valley in the western United States. 191 00:21:58,984 --> 00:22:05,412 A thermometer on the ground here has risen to 201 degrees Fahrenheit. 192 00:22:05,624 --> 00:22:08,991 That's about 94 degrees centigrade. 193 00:22:09,261 --> 00:22:14,358 It's so hot that no creature can survive permanently out here. 194 00:22:14,599 --> 00:22:18,797 Even at the edge of these sand flats, where the ground is more broken, 195 00:22:18,970 --> 00:22:22,497 there is no sign of animal life whatever. 196 00:22:22,874 --> 00:22:29,006 All animals now have sought the shade and shelter from this ferocious sun. 197 00:22:29,281 --> 00:22:33,012 But some organisms can't get out of the sun. 198 00:22:33,985 --> 00:22:36,351 Plants, being fixed to the ground, 199 00:22:36,521 --> 00:22:41,754 have to stay out in the heat of the day and simply endure. 200 00:22:42,027 --> 00:22:45,690 But all of them have special devices to help them to do so. 201 00:22:45,964 --> 00:22:50,924 The desert holly. Its leaves grow at about 70 degrees to the vertical, 202 00:22:51,269 --> 00:22:56,639 so that in the morning when it's less hot and in the evening when the plant needs light, 203 00:22:56,808 --> 00:22:59,868 the face of its leaves face the light. 204 00:23:00,045 --> 00:23:05,677 During the middle of the day, it shows only the edges and doesn't heat up so much. 205 00:23:06,184 --> 00:23:11,451 Not only that, but the plant extracts salt from the salt-laden ground 206 00:23:11,623 --> 00:23:15,218 and excretes it as a white coating on the leaf, 207 00:23:15,394 --> 00:23:19,660 which, like the white costume of an athlete, reflects the heat 208 00:23:19,831 --> 00:23:22,391 and so keeps the plant that much cooler. 209 00:23:23,101 --> 00:23:25,433 And this, the creosote bush. 210 00:23:26,838 --> 00:23:30,797 This is one of the most widespread of plants in American deserts, 211 00:23:31,042 --> 00:23:36,878 and its roots are better at extracting the last molecule of water from parched sands 212 00:23:37,048 --> 00:23:39,710 than those of any other American plant. 213 00:23:39,951 --> 00:23:44,650 This has led to an extraordinary state of affairs that's only just been discovered. 214 00:23:45,624 --> 00:23:49,321 It seems that the creosote bush was the first plant to establish itself 215 00:23:49,494 --> 00:23:53,453 in the arid Mojave desert when the desert first appeared. 216 00:23:53,765 --> 00:23:56,791 Once it had established its extensive root system, 217 00:23:56,968 --> 00:24:00,426 it extracted moisture from the sand so efficiently 218 00:24:00,605 --> 00:24:04,837 that it was extremely difficult for any other plant to grow alongside it. 219 00:24:05,110 --> 00:24:10,912 And that applied not only to any other kind of plant but also to its own seedlings. 220 00:24:12,417 --> 00:24:18,185 So an individual creosote bush tended to spread not by setting seeds 221 00:24:18,356 --> 00:24:19,983 and producing a new generation, 222 00:24:20,225 --> 00:24:24,719 but by sending out new stems around its base. 223 00:24:25,063 --> 00:24:29,932 And as these spread outwards, so the stems in the middle tended to die away, 224 00:24:30,202 --> 00:24:34,434 and the bush grew into a ring shape like this. 225 00:24:34,639 --> 00:24:40,441 So these are not separate individual creosote bushes, as it might appear, 226 00:24:40,612 --> 00:24:46,016 but this is just one big ring-shaped individual plant. 227 00:24:47,519 --> 00:24:52,081 Over the centuries, the rings widened and changed their shape 228 00:24:52,257 --> 00:24:57,661 until now some are over 25 yards across, like this one. 229 00:24:58,730 --> 00:25:04,896 Of course, the individual stems and leaves of this plant are not very ancient. 230 00:25:05,136 --> 00:25:10,836 The first ones to grow, which appeared in the middle, decayed and disappeared long ago. 231 00:25:11,009 --> 00:25:14,911 Now it's estimated that this plant began growing 232 00:25:15,080 --> 00:25:18,277 between 10,000 and 12,000 years ago, 233 00:25:18,517 --> 00:25:22,078 in fact, when the Mojave desert first appeared, 234 00:25:22,254 --> 00:25:27,191 and that makes it the oldest known living organism in the world. 235 00:25:32,564 --> 00:25:37,729 In the Mojave, the plants may have to survive for as long as ten years without rain, 236 00:25:38,003 --> 00:25:42,440 but if rain falls a little more frequently, as it does nearby in Arizona, 237 00:25:42,641 --> 00:25:45,576 plants can have different survival strategies. 238 00:25:46,678 --> 00:25:51,274 To many of us, the very symbol of the desert is the cactus. 239 00:25:51,483 --> 00:25:57,388 But in fact, this family of fleshy-stemmed plants lives only in the Americas. 240 00:25:57,622 --> 00:26:02,992 There are several hundred species of them, but among the biggest is the saguaro. 241 00:26:05,030 --> 00:26:07,191 The saguaro has solved the problems 242 00:26:07,365 --> 00:26:12,234 of surviving in great heat and drought very successfully indeed. 243 00:26:12,804 --> 00:26:18,470 Its stem is pleated like an accordion, so when rain does fall, the cactus can expand 244 00:26:18,643 --> 00:26:22,511 and quickly absorb as much water as possible before it disappears. 245 00:26:22,881 --> 00:26:27,682 After a single storm, a saguaro can take up as much as a ton in a few days. 246 00:26:29,487 --> 00:26:31,387 Its leaves have become thorns, 247 00:26:31,590 --> 00:26:36,584 so reducing the surface area from which the plant might lose water by evaporation. 248 00:26:36,895 --> 00:26:41,229 The stem itself is green and has taken over the job of photosynthesis. 249 00:26:41,533 --> 00:26:44,093 The thorns protect the young plant from browsers, 250 00:26:44,269 --> 00:26:50,071 but they also break up the wind currents, so that the cactus is wrapped in still air, 251 00:26:50,308 --> 00:26:53,903 and evaporation of moisture from the stem is kept very low. 252 00:26:55,347 --> 00:27:01,980 These huge saguaro cacti can live for over 200 years and stand nearly 50 feet high. 253 00:27:09,160 --> 00:27:12,095 If I was dying of thirst in this desert, 254 00:27:12,263 --> 00:27:17,826 I'd be tempted to cut inside that saguaro and raid its reservoir of water. 255 00:27:18,003 --> 00:27:23,305 But that would probably be a mistake, because the water in the saguaro contains a poison. 256 00:27:23,608 --> 00:27:28,978 But there are lots of desert-living plants which do have drinkable water within them, 257 00:27:29,147 --> 00:27:33,607 and desert-living people all over the world have become expert botanists, 258 00:27:33,785 --> 00:27:38,449 able to recognise from just the tiniest little leaflet or straggling 259 00:27:38,623 --> 00:27:40,523 where they can get a good drink. 260 00:27:43,495 --> 00:27:47,454 None are more skilled than the Bushman people of the Kalahari. 261 00:28:01,012 --> 00:28:04,812 By the end of the dry season, all their water holes have usually dried up. 262 00:28:05,083 --> 00:28:08,814 For liquid, they must now rely almost entirely on plants 263 00:28:08,987 --> 00:28:11,615 and their ability to recognise the right ones. 264 00:28:21,099 --> 00:28:24,466 This tuber is a kind that provides good drinking water. 265 00:28:34,779 --> 00:28:37,077 This much larger one is also full of liquid, 266 00:28:37,248 --> 00:28:40,445 but, unfortunately, it's so bitter, it's undrinkable. 267 00:28:40,885 --> 00:28:42,819 But it's worth having nonetheless. 268 00:28:46,658 --> 00:28:49,627 To extract the water, the root must be grated and pulped. 269 00:29:21,092 --> 00:29:23,526 The bigger root is grated as well. 270 00:29:28,800 --> 00:29:32,361 Drier though it is, it still contains valuable flu 271 00:29:38,576 --> 00:29:42,410 Since it cannot be drunk, people use it to moisten their skin, 272 00:29:42,614 --> 00:29:46,516 and as it evaporates, it brings a delicious refreshing coolness. 273 00:30:07,038 --> 00:30:09,029 200 miles west of the Kalahari 274 00:30:09,207 --> 00:30:12,665 lies an even hotter, drier desert: The Namib. 275 00:30:13,211 --> 00:30:16,840 Very few plants indeed can survive in these parched sands. 276 00:30:17,415 --> 00:30:22,045 Patches of grass sprouted after a rare shower and lived for a few weeks, 277 00:30:22,320 --> 00:30:27,155 but that was over four years ago and now only the dusty withered stems are left. 278 00:30:27,559 --> 00:30:31,086 There is one plant that grows here, though, and nowhere else, 279 00:30:31,329 --> 00:30:33,729 and one that is very odd indeed. 280 00:30:37,135 --> 00:30:41,799 The scientist who first described this extraordinary plant 281 00:30:41,973 --> 00:30:47,172 was an Austrian called Dr Welwitsch, who came here in the last century. 282 00:30:47,478 --> 00:30:52,006 He discovered many plants in Africa, but this perhaps is his most famous 283 00:30:52,183 --> 00:30:55,516 and the one that bears his name: Welwitschia. 284 00:30:55,854 --> 00:30:58,288 There are male plants and female plants. 285 00:30:58,656 --> 00:31:02,786 This one is a female, and these are the female's structures. 286 00:31:03,094 --> 00:31:05,392 These are young ones, which sprouted this year, 287 00:31:05,563 --> 00:31:08,327 and these are fully developed ones from last year. 288 00:31:08,566 --> 00:31:13,560 In structure, they are very like the cones of a fir tree. 289 00:31:16,307 --> 00:31:20,107 The male plant has growths rather like stamens, which produce pollen, 290 00:31:20,311 --> 00:31:24,213 so welwitschia seems to be a kind of link between coniferous trees 291 00:31:24,382 --> 00:31:26,612 and true flowering plants. 292 00:31:29,921 --> 00:31:32,913 But the oddest thing about it are its leaves. 293 00:31:33,157 --> 00:31:38,493 They grow from the top of its central trunk, and do so extremely slowly, 294 00:31:38,663 --> 00:31:43,430 so that this leaf would have taken about 70 years to be produced. 295 00:31:43,635 --> 00:31:48,436 But if it hadn't frayed at the edges, it would be about 400 yards long, 296 00:31:48,673 --> 00:31:54,373 because this individual plant is thought to be about 1,500 years old. 297 00:31:57,081 --> 00:32:02,417 It's these amazing leaves that enable the plant to collect water in this rainless country. 298 00:32:02,754 --> 00:32:05,552 The Namib lies close to the western coast of Africa. 299 00:32:05,723 --> 00:32:09,819 At dawn, fogs regularly roll in from the Atlantic. 300 00:32:10,128 --> 00:32:15,430 As they swirl around the welwitschia, their moisture condenses on the huge leaves. 301 00:32:16,067 --> 00:32:19,730 Some droplets are absorbed through cracks in the leaves' skin. 302 00:32:22,240 --> 00:32:25,266 The rest is channelled down to the ground, 303 00:32:25,443 --> 00:32:29,072 where it's sucked up by roots just below the surface of the sand. 304 00:32:31,482 --> 00:32:35,942 The fog also provides life-saving drinks for some of the desert animals. 305 00:32:36,220 --> 00:32:38,085 These are darkling beetles. 306 00:32:38,356 --> 00:32:42,759 On foggy mornings, they climb to the top of the dunes and stand in lines, 307 00:32:43,027 --> 00:32:46,622 head down, abdomen up, slowly marking time. 308 00:33:08,486 --> 00:33:11,978 Droplets of water from the fog collect on legs and antennae, 309 00:33:12,256 --> 00:33:17,216 and then, as the beetle lifts its feet, trickle down towards its mouth. 310 00:33:22,734 --> 00:33:26,170 The Namib's fogs never penetrate very far inland. 311 00:33:26,704 --> 00:33:32,108 Deserts that lie far from the sea, therefore, can never receive moisture in such a way. 312 00:33:32,543 --> 00:33:35,239 Their water must come from the clouds. 313 00:33:38,249 --> 00:33:41,218 Often, the clouds that do build up above a desert 314 00:33:41,386 --> 00:33:46,517 sail off elsewhere without bursting, and the land remains parched. 315 00:33:56,200 --> 00:34:01,900 But when eventually rain does come, it's the trigger for immediate and urgent action. 316 00:34:05,043 --> 00:34:09,412 One or two drops are all that's necessary to activate these dead stems. 317 00:34:20,224 --> 00:34:22,419 Within half a minute, they're upright. 318 00:34:26,497 --> 00:34:29,591 Other plants begin to open their seed-heads. 319 00:34:37,809 --> 00:34:39,902 None of these plants is alive. 320 00:34:40,211 --> 00:34:44,978 All their movements are simply the result of the dead tissues absorbing water. 321 00:34:56,394 --> 00:35:00,353 The dead seed-heads have held the seeds securely during the drought. 322 00:35:02,900 --> 00:35:07,701 Now, since it's rained and there's a chance of them germinating, they can be distributed. 323 00:35:10,408 --> 00:35:14,276 For some plants, the heavy raindrops are enough to dislodge the seeds. 324 00:35:33,297 --> 00:35:38,860 Others utilise the physical effects of absorbing water to shoot the seeds away. 325 00:35:48,713 --> 00:35:52,615 Now the seeds themselves, lying on the ground, begin to move. 326 00:35:58,756 --> 00:36:01,623 As the hairs absorb water, they swell and stiffen, 327 00:36:01,792 --> 00:36:04,158 so raising the seed into the right position 328 00:36:04,328 --> 00:36:08,662 for its first rootlets to strike straight downwards into the ground. 329 00:36:12,036 --> 00:36:15,836 But sometimes in the Arizona desert, maybe once in several years, 330 00:36:16,007 --> 00:36:20,034 there are real cloudbursts, and the desert is transformed. 331 00:37:10,661 --> 00:37:14,358 In the aftermath of the flood, new faces appear. 332 00:37:24,475 --> 00:37:28,571 A spadefoot toad. The males are the first to emerge from the soil 333 00:37:28,746 --> 00:37:31,874 where they've been buried for the past year or more. 334 00:37:38,656 --> 00:37:42,956 Hastily, they make their way to one of the pools that have appeared in the desert, 335 00:37:43,160 --> 00:37:46,186 and there they begin calling, summoning the females. 336 00:37:47,632 --> 00:37:52,535 There is great urgency. If they don't mate on this night, they may have lost their chance. 337 00:38:10,921 --> 00:38:16,086 Within 24 hours, the eggs have been laid and fertilised and are beginning to hatch. 338 00:38:26,737 --> 00:38:30,298 A day later, the pool is full of tadpoles. 339 00:38:33,844 --> 00:38:36,335 Other creatures have appeared as if from nowhere. 340 00:38:36,514 --> 00:38:41,281 Fairy shrimp have hatched from tiny eggs blown with the dust all over the desert. 341 00:38:46,924 --> 00:38:48,619 The tadpoles are growing fast. 342 00:38:48,793 --> 00:38:52,354 These with small mouths feed on algae and bacteria, 343 00:38:52,530 --> 00:38:55,897 a diet usually abundant in these desert pools. 344 00:39:04,075 --> 00:39:08,307 But other individuals from the same batch of eggs develop bigger heads 345 00:39:08,479 --> 00:39:12,745 and more powerfully muscled jaws. They have become meat-eaters. 346 00:39:16,854 --> 00:39:21,348 Not all pools will provide enough food for them, but here they are fortunate. 347 00:39:32,069 --> 00:39:34,936 They even eat their vegetarian brothers. 348 00:39:40,978 --> 00:39:45,415 With such a protein-rich diet, they grow even faster than the algal feeders. 349 00:39:45,583 --> 00:39:51,146 Here they are the favoured few, more likely to survive if the pool evaporates quickly. 350 00:39:51,322 --> 00:39:54,052 They're an insurance for the continuation of the species, 351 00:39:54,225 --> 00:39:57,490 for which the payments are their vegetarian brothers. 352 00:39:59,630 --> 00:40:04,431 But now the pool is shrinking fast. Another couple of days and it's almost gone. 353 00:40:04,602 --> 00:40:08,436 Unless there is another shower of rain, all the tadpoles will die. 354 00:40:14,979 --> 00:40:17,539 If they do die, their bodies will not be wasted. 355 00:40:17,715 --> 00:40:22,311 They will decompose and fertilise the sand, so that when the next rains come 356 00:40:22,486 --> 00:40:27,617 and another pool collects in this hollow, the algae will grow fast and well. 357 00:40:31,028 --> 00:40:34,225 Ants are quick to attack the stricken tadpoles. 358 00:40:37,468 --> 00:40:40,801 But at the last minute, there is a reprieve. A shower of rain. 359 00:40:40,971 --> 00:40:44,998 Some tadpoles, though they still have a tail, now have legs, 360 00:40:45,176 --> 00:40:49,203 and they're able to leave the puddle just two weeks after hatching. 361 00:40:56,754 --> 00:41:01,054 Even among this tiny proportion of survivors, the mortality will be huge. 362 00:41:01,292 --> 00:41:05,388 But with luck, a few will join the adults as the desert dries 363 00:41:05,563 --> 00:41:11,126 and bury themselves to wait for the next shower of rain many months from now. 364 00:41:33,224 --> 00:41:36,853 For several weeks after the rains, the desert blooms. 365 00:41:37,027 --> 00:41:41,623 The seeds shed by the shrivelled plants have sprouted and burst into flower. 366 00:41:41,799 --> 00:41:44,563 And deserts after rain all over the world, 367 00:41:44,735 --> 00:41:48,068 in Arizona and Australia, the Namib and the Sahara, 368 00:41:48,239 --> 00:41:53,199 put on one of the most dazzling displays of colour that you can see anywhere. 369 00:42:38,422 --> 00:42:40,890 Deserts are shaped by the sun and the wind. 370 00:42:41,058 --> 00:42:44,824 The roasting of rocks during the day, their chilling during cold nights, 371 00:42:44,995 --> 00:42:47,156 eventually makes their surface crumble. 372 00:42:47,398 --> 00:42:50,128 Some of their minerals splinter and fray into dust 373 00:42:50,301 --> 00:42:55,534 But quartz, the commonest, is very hard, and that remains as grains of sand. 374 00:42:55,773 --> 00:43:01,678 The wind catches them, sweeps them away, and collects them together as sand dunes. 375 00:43:29,173 --> 00:43:31,437 Dunes may be hundreds of feet high. 376 00:43:31,709 --> 00:43:33,506 If the wind is more or less constant, 377 00:43:33,677 --> 00:43:38,307 it blows the grains up the gently sloping side and over the steep front 378 00:43:38,515 --> 00:43:41,882 so that the dune marches slowly across the desert. 379 00:43:55,899 --> 00:44:00,529 Trudging up the face of a dune like this is extremely hard work. 380 00:44:00,771 --> 00:44:03,262 The sand is so dry 381 00:44:03,507 --> 00:44:08,001 and the grains are so polished by the wind rubbing them together 382 00:44:08,212 --> 00:44:11,739 that the surface is continuously on the move, 383 00:44:11,915 --> 00:44:15,146 and it's quite impossible to get any firm foothold. 384 00:44:15,386 --> 00:44:18,878 And, of course, that problem faces notjust me, 385 00:44:19,056 --> 00:44:22,685 but all the animals that live among these dunes. 386 00:44:23,060 --> 00:44:28,521 Some of them have developed some extremely ingenious solutions to the difficulty. 387 00:44:30,768 --> 00:44:35,899 These extraordinary tracks have been made by one of the swiftest movers across the dunes. 388 00:44:40,077 --> 00:44:42,068 The sidewinder, a kind of rattlesnake. 389 00:44:42,312 --> 00:44:46,146 It skims across the surface by throwing its body into loops, 390 00:44:46,316 --> 00:44:49,149 which only touch the sand at two points. 391 00:44:49,787 --> 00:44:51,880 This not only enables it to move fast, 392 00:44:52,056 --> 00:44:55,082 but keeps most of its body off the hot surface. 393 00:45:01,298 --> 00:45:05,667 At midday, the sand is so hot that it's painful to touch. 394 00:45:05,869 --> 00:45:11,239 The Namib fringe-toed lizard prevents its feet from scorching by gymnastics. 395 00:45:28,592 --> 00:45:30,116 But eventually it gets so hot, 396 00:45:30,294 --> 00:45:35,027 the only thing to do is to shelter beneath the surface where the sand is very cool. 397 00:45:37,067 --> 00:45:40,400 Burrowing through this kind of sand also has problems. 398 00:45:40,571 --> 00:45:45,270 An animal can't construct a tunnel like a mouse hole or a rabbit burrow 399 00:45:45,442 --> 00:45:48,502 because the sand simply falls in behind it. 400 00:45:48,879 --> 00:45:53,179 So instead it has to wriggle through the sand almost as though it's swimming. 401 00:45:53,417 --> 00:45:56,181 And that's precisely what this little creature does. 402 00:45:57,020 --> 00:46:02,014 It may look like a worm, but in fact it's a lizard that has lost its legs. 403 00:46:02,259 --> 00:46:05,922 You can see that it's a lizard when you look closely at its face. 404 00:46:07,731 --> 00:46:12,794 Its mouth and eyes are covered by transparent scales that protect them in the sand. 405 00:46:13,070 --> 00:46:18,133 It's a blind skink. It lives by hunting for insects below the sand surface, 406 00:46:18,308 --> 00:46:22,369 and when I put it down, it'll wriggle away, just like an eel. 407 00:46:29,186 --> 00:46:33,088 The most extremely specialised of these hunters in the dunes 408 00:46:33,257 --> 00:46:36,715 is not a reptile but a mammal. 409 00:46:37,094 --> 00:46:41,394 It's very rarely seen, and your best chance of finding it is at night. 410 00:46:45,769 --> 00:46:51,571 These are its tracks, and that depression a place where it caught something. 411 00:46:59,650 --> 00:47:01,117 This is where it has burrowed again, 412 00:47:01,285 --> 00:47:05,051 and where, with luck, and if I dig very fast, I might catch it. 413 00:47:14,798 --> 00:47:17,323 Here it is, a golden mole. 414 00:47:20,003 --> 00:47:23,632 This one is a baby, but like its parents, it's totally blind. 415 00:47:23,841 --> 00:47:28,540 Eyes are of no use beneath the sand. Nor are ears, and it hasn't got those either. 416 00:47:28,745 --> 00:47:31,509 Its head ends in a leathery wedge 417 00:47:31,682 --> 00:47:36,142 with which it pushes through the sand, or alternatively, through my fingers. 418 00:47:50,200 --> 00:47:52,725 Golden moles will eat quite large creatures: 419 00:47:52,903 --> 00:47:55,565 A blind skink, if it encounters one, 420 00:47:55,739 --> 00:47:59,835 or other creatures that might be wandering unsuspectingly across the surface. 421 00:48:13,957 --> 00:48:16,084 A cricket would do nicely. 422 00:49:05,709 --> 00:49:10,373 The great sandy deserts of the world in Arabia, central Australia and the Sahara 423 00:49:10,547 --> 00:49:13,641 have repelled even the hardiest of human travellers. 424 00:49:13,817 --> 00:49:18,481 Few people have managed to survive in them for long totally unaided. 425 00:49:18,789 --> 00:49:22,816 But some manage to make regularjourneys through these wildernesses. 426 00:49:27,864 --> 00:49:32,164 These are the Tuareg. They travel from one side of the Sahara to the other, 427 00:49:32,336 --> 00:49:37,638 carrying great cakes of salt, which they trade for cloth and grain and dates. 428 00:49:52,022 --> 00:49:54,115 But even the Tuareg can only make these journeys 429 00:49:54,291 --> 00:49:58,091 with the help of an animal desert specialist: The camel. 430 00:49:59,229 --> 00:50:03,359 They have to take all the food that they and their camels will need with them. 431 00:50:08,071 --> 00:50:11,472 Water is carried in skins slung beneath the camels' bellies 432 00:50:11,641 --> 00:50:15,133 to minimise evaporation and keep it as cool as possible. 433 00:50:17,381 --> 00:50:20,407 The camel is marvellously adapted to life in the desert. 434 00:50:20,650 --> 00:50:24,177 Its toes are reduced to two, but connected by skin, 435 00:50:24,354 --> 00:50:27,721 so that they splay out on the sand and don't sink deeply into it. 436 00:50:35,198 --> 00:50:40,158 Their nostrils are closable, so they can shut out sand grains during a sandstorm. 437 00:50:49,679 --> 00:50:54,446 The hair on their body is restricted to the top, where it shields against the sun. 438 00:50:54,618 --> 00:50:57,917 Elsewhere, for coolness, their skin is virtually naked. 439 00:50:58,355 --> 00:51:02,314 Their hump is full of fat, which in emergencies can be converted to water. 440 00:51:02,559 --> 00:51:05,221 But the process wastes the fat's calories 441 00:51:05,395 --> 00:51:09,092 and is only used when the camel hasn't drunk for a long time. 442 00:51:09,299 --> 00:51:13,531 It can live without liquid water for four times as long as a donkey 443 00:51:13,703 --> 00:51:16,137 and ten times as long as a man. 444 00:51:17,074 --> 00:51:20,134 But eventually even a camel has to drink. 445 00:51:28,218 --> 00:51:33,451 At one or two places in the Sahara, water can be reached by digging deep into the ground. 446 00:51:33,690 --> 00:51:36,887 Here, camels can at last refill their stomachs, 447 00:51:37,060 --> 00:51:39,153 and they take a lot of filling. 448 00:52:10,627 --> 00:52:13,357 If the Tuareg can't cross the Sahara without the camel, 449 00:52:13,530 --> 00:52:15,760 the camel can't do so without the Tuareg, 450 00:52:15,932 --> 00:52:19,800 for only men can dig for the essential water. 451 00:52:22,706 --> 00:52:26,608 Spring water is the key which unlocks abundant fertility. 452 00:52:26,943 --> 00:52:28,808 At Saharan oases like this one, 453 00:52:28,979 --> 00:52:32,779 all kinds of crops can be produced from the sand if it's watered: 454 00:52:33,049 --> 00:52:35,609 Dates and vegetables and fruit. 455 00:52:35,852 --> 00:52:39,379 Insects whizz and buzz over the gurgling irrigation channels 456 00:52:39,556 --> 00:52:42,252 and birds sing in the palm trees. 457 00:52:44,561 --> 00:52:48,190 But these small islands of life are under constant threat. 458 00:52:48,632 --> 00:52:54,036 If the wind veers and blows steadily from another direction, nothing can stop the sand. 459 00:53:04,614 --> 00:53:09,278 Eventually the advancing dunes may well overwhelm this oasis, 460 00:53:09,452 --> 00:53:11,386 and then this small world 461 00:53:11,555 --> 00:53:17,323 that's been brought into existence in the desert by the presence of water will be extinguished. 462 00:53:17,661 --> 00:53:20,926 The force that drives the dune, of course, is the wind, 463 00:53:21,164 --> 00:53:24,395 and the wind, too, has its own world of living organisms. 464 00:53:24,634 --> 00:53:27,432 Many of the spiders and beetles 465 00:53:27,604 --> 00:53:31,802 and other insects that live in the oasis arrived by air. 466 00:53:32,042 --> 00:53:37,241 And many of the plants, too, coming as windblown seeds or carried by birds. 467 00:53:37,480 --> 00:53:42,747 And that world, the world of the wind and the sky, we'll be exploring next time.