1 00:01:54,200 --> 00:01:59,660 These mountains stand in the middle of the biggest desert on earth, the Sahara. 2 00:02:00,540 --> 00:02:02,910 It stretches right across the width of Africa, 3 00:02:03,160 --> 00:02:06,290 three and a half million square miles of it. 4 00:02:07,630 --> 00:02:11,460 At night, it gets so cold that it can freeze. 5 00:02:11,670 --> 00:02:15,380 During the day, the sun strikes it so ferociously 6 00:02:15,550 --> 00:02:19,510 that the highest land temperatures ever recorded were measured here: 7 00:02:19,680 --> 00:02:24,310 58 degrees centigrade, 137 degrees Fahrenheit. 8 00:02:24,690 --> 00:02:30,360 And, in turn, those oven-like temperatures rob the land of all its moisture. 9 00:02:30,650 --> 00:02:36,240 All in all, there could hardly be a more hostile environment for life on earth. 10 00:02:36,570 --> 00:02:38,450 But it wasn't always this way. 11 00:02:40,490 --> 00:02:43,200 And if you want evidence of that, here it is. 12 00:02:48,920 --> 00:02:51,550 A group of antelope, probably sable. 13 00:02:51,800 --> 00:02:54,920 Creatures that can't live anywhere in the Sahara today, 14 00:02:55,090 --> 00:02:57,680 because there's simply not enough vegetation for them. 15 00:02:57,930 --> 00:03:01,600 These aren't the only wild animals painted on these rocks. 16 00:03:05,350 --> 00:03:06,730 A giraffe. 17 00:03:09,230 --> 00:03:12,070 A kind of wild goat, probably a moufflon. 18 00:03:12,980 --> 00:03:14,150 And antelope. 19 00:03:14,400 --> 00:03:18,990 Obviously, at the time these pictures were painted, there was good grazing here. 20 00:03:19,950 --> 00:03:25,290 Indeed, there was sufficient vegetation to sustain not only wild animals, 21 00:03:25,450 --> 00:03:27,710 but whole herds of cattle. 22 00:03:29,580 --> 00:03:32,090 We don't know exactly who drew these pictures. 23 00:03:32,250 --> 00:03:34,460 The artists may have been ancestors of the nomads 24 00:03:34,630 --> 00:03:40,640 who today follow their herds of long-horned piebald cattle just south of the Sahara. 25 00:03:41,050 --> 00:03:44,560 But we know what they looked like, because they left their portraits. 26 00:03:44,970 --> 00:03:47,890 They lived here, it seems, some 5,000 years ago. 27 00:03:48,100 --> 00:03:51,860 But eventually the rains began to fail, the pastures disappeared, 28 00:03:52,060 --> 00:03:54,650 and with it the cattle and their keepers. 29 00:03:54,980 --> 00:03:58,820 But there are one or two living survivors from that time. 30 00:04:01,870 --> 00:04:06,700 This ancient cypress, judging from the number of rings in the trunks of others like it 31 00:04:07,040 --> 00:04:10,620 is probably between 2,000 and 3,000 years old. 32 00:04:10,920 --> 00:04:17,090 In fact, it was probably already growing here when the last paintings were being made. 33 00:04:17,420 --> 00:04:19,510 It still bears fertile seed, 34 00:04:19,680 --> 00:04:23,300 but there are no little seedlings growing here to replace it. 35 00:04:23,680 --> 00:04:26,810 The land now is far too dry for them. 36 00:04:27,100 --> 00:04:29,310 Indeed, the cypress itself only survives 37 00:04:29,480 --> 00:04:35,020 because it sends its huge roots deep into the ground to tap underground water. 38 00:04:36,110 --> 00:04:39,490 The drying out of the Sahara seems to be connected 39 00:04:39,650 --> 00:04:44,070 with the great changes in climate at the end of the last ice age. 40 00:04:44,410 --> 00:04:48,080 As the glaciers retreated northwards across Europe, 41 00:04:48,330 --> 00:04:51,540 so the rains that fell regularly along their southern edge 42 00:04:51,750 --> 00:04:57,960 left Africa and moved up into Europe, and the Sahara was robbed of its rains. 43 00:04:58,380 --> 00:05:04,470 Indeed, it seems that most of the world's great deserts were formed around that time, 44 00:05:04,640 --> 00:05:09,390 and most, if not all of them, are therefore comparatively recent environments. 45 00:05:10,930 --> 00:05:15,900 To see why deserts lie where they do, we can look at the Sahara from the west. 46 00:05:16,190 --> 00:05:20,440 The equator runs away from us across the width of Africa. 47 00:05:20,900 --> 00:05:25,370 It's along this line that the sun's rays strike from directly overhead, 48 00:05:25,530 --> 00:05:28,160 and therefore with the greatest strength. 49 00:05:28,660 --> 00:05:30,620 The heated air rises along the equator 50 00:05:30,790 --> 00:05:34,580 and flows away, north and south, to cooler parts of the world. 51 00:05:34,880 --> 00:05:38,420 Because it's warm, it carries a lot of moisture. 52 00:05:38,670 --> 00:05:43,680 But as it rises and cools, the moisture condenses first to form clouds 53 00:05:43,880 --> 00:05:45,590 and then to fall as rain. 54 00:05:45,970 --> 00:05:51,430 When the air comes down again over the Sahara to the left and the Kalahari to the right, 55 00:05:51,730 --> 00:05:55,850 it's lost most of its moisture and creates few clouds. 56 00:05:56,440 --> 00:05:59,440 The Sahara, with few clouds to shield it from the sun, 57 00:05:59,650 --> 00:06:02,400 becomes roastingly hot during the day. 58 00:06:02,780 --> 00:06:08,830 And at night, with no blanket of clouds to keep in its warmth, it gets desperately cold. 59 00:06:09,620 --> 00:06:13,410 Deserts are not placed symmetrically around the world, 60 00:06:13,620 --> 00:06:17,790 because the continents themselves are distributed in a very irregular way. 61 00:06:18,040 --> 00:06:23,050 They're ridged with great mountain ranges, which interfere with the even flow of air, 62 00:06:23,260 --> 00:06:27,430 and the planet's spin creates vast eddies in the atmosphere, 63 00:06:27,720 --> 00:06:29,430 which further complicates things. 64 00:06:29,850 --> 00:06:35,100 But even so, deserts do lie in two broad zones on either side of the equator. 65 00:06:35,810 --> 00:06:38,900 The pattern in Africa, with the Sahara in the north 66 00:06:39,110 --> 00:06:44,400 and the Kalahari and the Namib in the south, has its equivalent in the Americas. 67 00:06:46,030 --> 00:06:49,320 South of the lush equatorial jungles of the Amazon, 68 00:06:49,570 --> 00:06:54,540 beyond the great range of the Andes, lies the Atacama desert. 69 00:06:54,910 --> 00:07:00,130 On the other side of the equator, beyond the drenched tropical rainforests of Panama, 70 00:07:00,460 --> 00:07:03,550 stretch the deserts of Mexico and Arizona. 71 00:07:05,090 --> 00:07:09,140 Across the Pacific, the greatest expanse of water on the globe, 72 00:07:09,430 --> 00:07:15,230 lies, south of the equator, Australia, most of which is covered by desert. 73 00:07:15,600 --> 00:07:20,650 Its northern tip gets close enough to the equator to collect some rain. 74 00:07:20,980 --> 00:07:22,520 Farther north still, 75 00:07:22,730 --> 00:07:27,150 beyond the jungle that blankets Indonesia and Malaysia, Thailand and Burma, 76 00:07:27,400 --> 00:07:30,570 across the great snow-covered range of the Himalayas, 77 00:07:30,910 --> 00:07:35,870 stretch the vast deserts of central Asia: Mongolia and the Gobi. 78 00:07:36,540 --> 00:07:39,540 Beyond them, as we complete the circuit of the globe, 79 00:07:39,750 --> 00:07:44,380 the huge desert of the Middle East that covers Iran, Iraq and Jordan, 80 00:07:44,630 --> 00:07:48,930 Syria and Israel, the vast sandy emptiness of Arabia, 81 00:07:49,130 --> 00:07:52,050 and runs on to join the Sahara. 82 00:07:58,980 --> 00:08:03,320 This is the biggest expanse of waterless land on earth. 83 00:08:03,820 --> 00:08:09,490 Here, as in deserts everywhere, almost nothing moves during the heat of the day. 84 00:08:11,740 --> 00:08:13,740 But animals are here. 85 00:08:25,210 --> 00:08:27,210 If you want to see what made these tracks, 86 00:08:27,510 --> 00:08:31,840 you have to wait until the sun sinks and the desert begins to cool. 87 00:08:41,020 --> 00:08:46,610 A striped hyena, one of the commonest of the bigger desert animals in this part of the world. 88 00:08:57,580 --> 00:08:58,950 A fennec fox. 89 00:09:08,050 --> 00:09:13,680 Fennecs usually live in small family groups, and clearly enjoy one another's company. 90 00:09:22,730 --> 00:09:26,400 But there's not much time for frolicking. Food must be found. 91 00:09:26,730 --> 00:09:32,070 Faint smells from the sand tell them who has moved where since they were last out. 92 00:10:02,430 --> 00:10:05,850 As the moon rises, many more creatures emerge. 93 00:10:06,690 --> 00:10:09,190 A gecko. Just what the fennec wants. 94 00:10:21,620 --> 00:10:25,710 Ajerboa. It, too, is looking for food. Seeds. 95 00:10:34,840 --> 00:10:37,890 Another little seed-eating rodent, a gerbil. 96 00:10:41,350 --> 00:10:47,400 And a caracal, a kind of cat, which loves both gerbils and jerboas, if it can get them. 97 00:11:04,200 --> 00:11:08,420 A smaller hunter, but nonetheless a deadly one: A scorpion. 98 00:11:08,920 --> 00:11:11,710 It is searching for beetles or other small insects 99 00:11:11,920 --> 00:11:14,840 But sometimes the hunter becomes the hunted. 100 00:11:15,340 --> 00:11:20,260 A black widow spider has set her snare of silk underneath a thorn bush. 101 00:12:01,800 --> 00:12:06,180 In the intense struggle, the black widow loses one of her legs. 102 00:12:17,150 --> 00:12:22,820 She manages to get more ropes of silk around the scorpion, hampering it still further. 103 00:12:28,040 --> 00:12:31,960 The scorpion hangs on to its trophy, but to no purpose. 104 00:12:32,210 --> 00:12:33,920 The battle is as good as lost. 105 00:12:46,260 --> 00:12:51,480 Methodically, the spider trusses up her victim and hangs it in her larder. 106 00:12:58,400 --> 00:13:02,990 Wolves, perhaps surprisingly, are quite common in these Middle Eastern deserts, 107 00:13:03,200 --> 00:13:05,990 but they're not like those farther north. 108 00:13:06,240 --> 00:13:10,210 They're smaller, lighter-coloured, and with only the thinnest fur, 109 00:13:10,500 --> 00:13:13,040 and they scavenge as much as they hunt. 110 00:13:17,750 --> 00:13:21,170 The cool night is coming to an end. Hunting is over. 111 00:13:21,380 --> 00:13:24,260 The animals must go back to their dens and hiding places 112 00:13:24,430 --> 00:13:27,260 to shelter from the heat that is to come. 113 00:13:50,080 --> 00:13:55,420 The sun returns, and very soon the desert will be heating up once again. 114 00:13:55,630 --> 00:13:59,590 The mammals that were active during the night have to find shelter. 115 00:13:59,800 --> 00:14:01,920 The day belongs not to them, 116 00:14:02,090 --> 00:14:07,220 but to creatures that get their heat directly from the sun: Reptiles. 117 00:14:07,470 --> 00:14:11,270 This is the desert of the American west in Arizona, 118 00:14:11,430 --> 00:14:16,940 and we've come here to look at one very special desert reptile: This one. 119 00:14:19,480 --> 00:14:26,200 This is the Gila monster, one of only two poisonous lizards in the world. 120 00:14:31,080 --> 00:14:35,120 Actually, he very seldom uses his poison in defence, 121 00:14:35,290 --> 00:14:38,340 and it's still quite early in the morning 122 00:14:38,500 --> 00:14:42,300 and he is so cold that he isn't very active. 123 00:14:42,590 --> 00:14:48,720 But in only about an hour, the desert will get so hot that he won't be able to stand it, 124 00:14:48,970 --> 00:14:51,720 and he, too, will have to seek shade. 125 00:14:51,930 --> 00:14:56,310 So in this short period of the early morning and in the cool of the evening 126 00:14:56,520 --> 00:14:59,060 is the time when he hunts. 127 00:15:14,040 --> 00:15:19,460 A tortoise, but he's far too big and well-armoured for a Gila monster to tackle. 128 00:15:31,180 --> 00:15:35,230 This great nest of sticks, however, looks much more promising. 129 00:15:50,370 --> 00:15:52,950 The victim: A desert mouse. 130 00:16:08,090 --> 00:16:12,760 The tortoise is on the lookout for food, too, but it is a vegetarian. 131 00:16:31,070 --> 00:16:33,740 The day is now several hours old. 132 00:16:33,950 --> 00:16:37,080 Cool dawn is changing to baking noon. 133 00:16:37,330 --> 00:16:40,710 It's time for even a reptile to get out of the sun 134 00:16:48,090 --> 00:16:49,880 Movement generates heat, 135 00:16:50,130 --> 00:16:54,350 so now nothing moves unless it absolutely has to. 136 00:16:54,640 --> 00:16:57,560 And there are some creatures that remain motionless 137 00:16:57,720 --> 00:17:00,730 even when you get within a few inches of them. 138 00:17:00,980 --> 00:17:05,440 One of them is on the ground right in front of me now, 139 00:17:05,730 --> 00:17:09,530 though you may find it difficult to see because it's so well camouflaged. 140 00:17:10,610 --> 00:17:13,490 It's a poorwill, a kind of nightjar. 141 00:17:18,620 --> 00:17:23,040 Fluttering the throat evaporates moisture from the mouth and so cools the bird. 142 00:17:23,460 --> 00:17:26,920 It consumes much less energy than heaving the chest and panting, 143 00:17:27,090 --> 00:17:29,840 as many mammals would do in this situation. 144 00:17:31,010 --> 00:17:33,680 The sand grouse of Africa uses the same trick. 145 00:17:42,310 --> 00:17:46,690 The sand grouse chicks start doing it almost as soon as they emerge from the shell. 146 00:17:52,400 --> 00:17:56,410 They also immediately peck for seeds, but there's little moisture in a seed, 147 00:17:56,620 --> 00:17:58,790 and unless they drink, they will die. 148 00:17:59,290 --> 00:18:03,500 The responsibility for providing that rests entirely with the male. 149 00:18:04,540 --> 00:18:09,210 Every day he flies to water, maybe as much as 25 miles from the nest. 150 00:18:10,050 --> 00:18:13,180 First he fills his own stomach with water. 151 00:18:18,600 --> 00:18:22,730 But then, very deliberately, he soaks his belly feathers. 152 00:18:27,770 --> 00:18:30,480 These feathers have a special spongy structure 153 00:18:30,650 --> 00:18:33,360 so that they can absorb lots of water. 154 00:18:33,530 --> 00:18:37,780 Once he has a full load, he flies back to his family. 155 00:18:48,750 --> 00:18:51,210 At last the chicks get their drink. 156 00:18:51,460 --> 00:18:55,550 No other bird has such an ingenious water-carrying device. 157 00:19:06,400 --> 00:19:08,360 The roadrunner of the American deserts 158 00:19:08,520 --> 00:19:11,570 provides water for its chicks quite differently. 159 00:19:11,780 --> 00:19:15,400 This parent bird has collected a cicada for its family. 160 00:19:17,110 --> 00:19:19,320 Its nest is in a cholla cactus. 161 00:19:19,570 --> 00:19:23,000 The parent doesn't give its chicks their food immediately. 162 00:19:30,130 --> 00:19:32,210 The chick is gulping. 163 00:19:32,420 --> 00:19:37,090 The parent bird is producing liquid from its stomach and letting it trickle down its beak. 164 00:19:43,850 --> 00:19:46,310 Each youngster gets its share. 165 00:20:02,700 --> 00:20:06,250 Another ration of solid food. This time, a lizard. 166 00:20:17,590 --> 00:20:21,470 Each time, before the meal is handed over, the chicks get a drink, 167 00:20:21,640 --> 00:20:23,260 whether they like it or not. 168 00:20:35,860 --> 00:20:39,950 During the day, the parents sit on the nest, not to keep the chicks warm, 169 00:20:40,110 --> 00:20:43,660 but, on the contrary, to keep them cool by shading them. 170 00:20:43,990 --> 00:20:46,120 The bird not only flutters its throat, 171 00:20:46,290 --> 00:20:50,670 but protects itself from the sun by using its tail as a parasol. 172 00:20:55,340 --> 00:20:59,090 The ground squirrel of the Namib desert does the same thing, 173 00:20:59,300 --> 00:21:03,340 and very effectively, too, carefully angling itself as far as possible 174 00:21:03,550 --> 00:21:05,220 to keep its body in the shade. 175 00:21:17,820 --> 00:21:20,860 Many animals keep their blood cool with radiators. 176 00:21:21,110 --> 00:21:25,870 The hedgehog that lives in the desert of the Middle East has unusually large ears. 177 00:21:26,160 --> 00:21:29,330 Blood circulates through capillaries close to the surface of the skin 178 00:21:29,540 --> 00:21:31,620 and is cooled by the breeze. 179 00:21:39,510 --> 00:21:42,840 The fennec fox's huge ears serve the same purpose. 180 00:21:45,930 --> 00:21:47,810 So do those of the American jackrabbit, 181 00:21:47,970 --> 00:21:51,980 which perhaps has the biggest ears of all in proportion to its body. 182 00:21:59,280 --> 00:22:02,150 The dorcas gazelle also has radiator ears 183 00:22:02,320 --> 00:22:05,280 and is one of the best-adapted desert mammals. 184 00:22:05,490 --> 00:22:09,240 It's one of few that can survive without drinking at all. 185 00:22:09,540 --> 00:22:12,750 It gets all the liquid it needs from vegetation. 186 00:22:15,620 --> 00:22:21,800 It doesn't waste liquid as urine, but gets rid of its uric acid as small dry pellets 187 00:22:39,610 --> 00:22:44,200 It's now approaching noon, the hottest time of the day. 188 00:22:44,400 --> 00:22:47,320 It's summer, the hottest time of the year, 189 00:22:47,530 --> 00:22:51,200 and I'm in one of the hottest places on earth: 190 00:22:51,450 --> 00:22:54,830 Death Valley in the western United States. 191 00:22:55,120 --> 00:23:01,800 A thermometer on the ground here has risen to 201 degrees Fahrenheit. 192 00:23:02,000 --> 00:23:05,550 That's about 94 degrees centigrade. 193 00:23:05,800 --> 00:23:11,140 It's so hot that no creature can survive permanently out here. 194 00:23:11,350 --> 00:23:15,730 Even at the edge of these sand flats, where the ground is more broken, 195 00:23:15,940 --> 00:23:19,610 there is no sign of animal life whatever. 196 00:23:20,020 --> 00:23:26,400 All animals now have sought the shade and shelter from this ferocious sun. 197 00:23:26,650 --> 00:23:30,570 But some organisms can't get out of the sun. 198 00:23:31,580 --> 00:23:34,040 Plants, being fixed to the ground, 199 00:23:34,200 --> 00:23:39,630 have to stay out in the heat of the day and simply endure. 200 00:23:39,960 --> 00:23:43,750 But all of them have special devices to help them to do so. 201 00:23:44,050 --> 00:23:49,220 The desert holly. Its leaves grow at about 70 degrees to the vertical, 202 00:23:49,590 --> 00:23:55,140 so that in the morning when it's less hot and in the evening when the plant needs light, 203 00:23:55,310 --> 00:23:58,520 the face of its leaves face the light. 204 00:23:58,730 --> 00:24:04,570 During the middle of the day, it shows only the edges and doesn't heat up so much. 205 00:24:05,110 --> 00:24:10,570 Not only that, but the plant extracts salt from the salt-laden ground 206 00:24:10,780 --> 00:24:14,540 and excretes it as a white coating on the leaf, 207 00:24:14,700 --> 00:24:19,120 which, like the white costume of an athlete, reflects the heat 208 00:24:19,290 --> 00:24:22,000 and so keeps the plant that much cooler. 209 00:24:22,750 --> 00:24:25,170 And this, the creosote bush. 210 00:24:26,590 --> 00:24:30,720 This is one of the most widespread of plants in American deserts, 211 00:24:31,010 --> 00:24:37,060 and its roots are better at extracting the last molecule of water from parched sands 212 00:24:37,270 --> 00:24:40,020 than those of any other American plant. 213 00:24:40,310 --> 00:24:45,150 This has led to an extraordinary state of affairs that's only just been discovered. 214 00:24:46,190 --> 00:24:50,030 It seems that the creosote bush was the first plant to establish itself 215 00:24:50,240 --> 00:24:54,330 in the arid Mojave desert when the desert first appeared. 216 00:24:54,660 --> 00:24:57,830 Once it had established its extensive root system, 217 00:24:58,040 --> 00:25:01,620 it extracted moisture from the sand so efficiently 218 00:25:01,790 --> 00:25:06,170 that it was extremely difficult for any other plant to grow alongside it. 219 00:25:06,500 --> 00:25:12,510 And that applied not only to any other kind of plant but also to its own seedlings. 220 00:25:14,100 --> 00:25:20,100 So an individual creosote bush tended to spread not by setting seeds 221 00:25:20,310 --> 00:25:22,020 and producing a new generation, 222 00:25:22,230 --> 00:25:26,900 but by sending out new stems around its base. 223 00:25:27,270 --> 00:25:32,320 And as these spread outwards, so the stems in the middle tended to die away, 224 00:25:32,660 --> 00:25:37,030 and the bush grew into a ring shape like this. 225 00:25:37,240 --> 00:25:43,290 So these are not separate individual creosote bushes, as it might appear, 226 00:25:43,460 --> 00:25:49,130 but this is just one big ring-shaped individual plant. 227 00:25:50,670 --> 00:25:55,430 Over the centuries, the rings widened and changed their shape 228 00:25:55,590 --> 00:26:01,230 until now some are over 25 yards across, like this one. 229 00:26:02,350 --> 00:26:08,730 Of course, the individual stems and leaves of this plant are not very ancient. 230 00:26:09,030 --> 00:26:14,950 The first ones to grow, which appeared in the middle, decayed and disappeared long ago. 231 00:26:15,160 --> 00:26:19,200 Now it's estimated that this plant began growing 232 00:26:19,410 --> 00:26:22,710 between 10,000 and 12,000 years ago, 233 00:26:22,960 --> 00:26:26,670 in fact, when the Mojave desert first appeared, 234 00:26:26,880 --> 00:26:32,010 and that makes it the oldest known living organism in the world. 235 00:26:37,600 --> 00:26:42,980 In the Mojave, the plants may have to survive for as long as ten years without rain, 236 00:26:43,230 --> 00:26:47,900 but if rain falls a little more frequently, as it does nearby in Arizona, 237 00:26:48,060 --> 00:26:51,150 plants can have different survival strategies. 238 00:26:52,280 --> 00:26:57,070 To many of us, the very symbol of the desert is the cactus. 239 00:26:57,280 --> 00:27:03,450 But in fact, this family of fleshy-stemmed plants lives only in the Americas. 240 00:27:03,700 --> 00:27:09,250 There are several hundred species of them, but among the biggest is the saguaro. 241 00:27:11,380 --> 00:27:13,670 The saguaro has solved the problems 242 00:27:13,840 --> 00:27:18,930 of surviving in great heat and drought very successfully indeed. 243 00:27:19,510 --> 00:27:25,390 Its stem is pleated like an accordion, so when rain does fall, the cactus can expand 244 00:27:25,600 --> 00:27:29,610 and quickly absorb as much water as possible before it disappears. 245 00:27:29,980 --> 00:27:34,990 After a single storm, a saguaro can take up as much as a ton in a few days. 246 00:27:36,900 --> 00:27:38,860 Its leaves have become thorns, 247 00:27:39,070 --> 00:27:44,290 so reducing the surface area from which the plant might lose water by evaporation. 248 00:27:44,580 --> 00:27:49,130 The stem itself is green and has taken over the job of photosynthesis. 249 00:27:49,420 --> 00:27:52,090 The thorns protect the young plant from browsers, 250 00:27:52,290 --> 00:27:58,300 but they also break up the wind currents, so that the cactus is wrapped in still air, 251 00:27:58,590 --> 00:28:02,300 and evaporation of moisture from the stem is kept very low. 252 00:28:03,850 --> 00:28:10,730 These huge saguaro cacti can live for over 200 years and stand nearly 50 feet high. 253 00:28:18,240 --> 00:28:21,240 If I was dying of thirst in this desert, 254 00:28:21,450 --> 00:28:27,250 I'd be tempted to cut inside that saguaro and raid its reservoir of water. 255 00:28:27,410 --> 00:28:32,960 But that would probably be a mistake, because the water in the saguaro contains a poison. 256 00:28:33,250 --> 00:28:38,840 But there are lots of desert-living plants which do have drinkable water within them, 257 00:28:39,050 --> 00:28:43,680 and desert-living people all over the world have become expert botanists, 258 00:28:43,850 --> 00:28:48,730 able to recognise from just the tiniest little leaflet or straggling 259 00:28:48,890 --> 00:28:50,900 where they can get a good drink. 260 00:28:53,980 --> 00:28:58,110 None are more skilled than the Bushman people of the Kalahari. 261 00:29:12,210 --> 00:29:16,170 By the end of the dry season, all their water holes have usually dried up. 262 00:29:16,460 --> 00:29:20,340 For liquid, they must now rely almost entirely on plants 263 00:29:20,510 --> 00:29:23,260 and their ability to recognise the right ones. 264 00:29:33,150 --> 00:29:36,650 This tuber is a kind that provides good drinking water. 265 00:29:47,410 --> 00:29:49,790 This much larger one is also full of liquid, 266 00:29:50,000 --> 00:29:53,290 but, unfortunately, it's so bitter, it's undrinkable. 267 00:29:53,750 --> 00:29:55,750 But it's worth having nonetheless. 268 00:29:59,760 --> 00:30:02,880 To extract the water, the root must be grated and pulped. 269 00:30:35,620 --> 00:30:38,170 The bigger root is grated as well. 270 00:30:43,670 --> 00:30:47,390 Drier though it is, it still contains valuable flu 271 00:30:53,850 --> 00:30:57,860 Since it cannot be drunk, people use it to moisten their skin, 272 00:30:58,060 --> 00:31:02,150 and as it evaporates, it brings a delicious refreshing coolness. 273 00:31:23,510 --> 00:31:25,550 200 miles west of the Kalahari 274 00:31:25,760 --> 00:31:29,350 lies an even hotter, drier desert: The Namib. 275 00:31:29,930 --> 00:31:33,720 Very few plants indeed can survive in these parched sands. 276 00:31:34,310 --> 00:31:39,110 Patches of grass sprouted after a rare shower and lived for a few weeks, 277 00:31:39,400 --> 00:31:44,440 but that was over four years ago and now only the dusty withered stems are left. 278 00:31:44,900 --> 00:31:48,530 There is one plant that grows here, though, and nowhere else, 279 00:31:48,820 --> 00:31:51,280 and one that is very odd indeed. 280 00:31:54,830 --> 00:31:59,710 The scientist who first described this extraordinary plant 281 00:31:59,880 --> 00:32:05,300 was an Austrian called Dr Welwitsch, who came here in the last century. 282 00:32:05,630 --> 00:32:10,340 He discovered many plants in Africa, but this perhaps is his most famous 283 00:32:10,510 --> 00:32:14,020 and the one that bears his name: Welwitschia. 284 00:32:14,350 --> 00:32:16,850 There are male plants and female plants. 285 00:32:17,270 --> 00:32:21,560 This one is a female, and these are the female's structures. 286 00:32:21,900 --> 00:32:24,320 These are young ones, which sprouted this year, 287 00:32:24,480 --> 00:32:27,320 and these are fully developed ones from last year. 288 00:32:27,610 --> 00:32:32,780 In structure, they are very like the cones of a fir tree. 289 00:32:35,620 --> 00:32:39,580 The male plant has growths rather like stamens, which produce pollen, 290 00:32:39,790 --> 00:32:43,880 so welwitschia seems to be a kind of link between coniferous trees 291 00:32:44,050 --> 00:32:46,380 and true flowering plants. 292 00:32:49,840 --> 00:32:52,930 But the oddest thing about it are its leaves. 293 00:32:53,180 --> 00:32:58,770 They grow from the top of its central trunk, and do so extremely slowly, 294 00:32:58,930 --> 00:33:03,940 so that this leaf would have taken about 70 years to be produced. 295 00:33:04,150 --> 00:33:09,150 But if it hadn't frayed at the edges, it would be about 400 yards long, 296 00:33:09,360 --> 00:33:15,280 because this individual plant is thought to be about 1,500 years old. 297 00:33:18,120 --> 00:33:23,670 It's these amazing leaves that enable the plant to collect water in this rainless country. 298 00:33:24,040 --> 00:33:26,960 The Namib lies close to the western coast of Africa. 299 00:33:27,130 --> 00:33:31,380 At dawn, fogs regularly roll in from the Atlantic. 300 00:33:31,720 --> 00:33:37,260 As they swirl around the welwitschia, their moisture condenses on the huge leaves. 301 00:33:37,890 --> 00:33:41,730 Some droplets are absorbed through cracks in the leaves' skin. 302 00:33:44,310 --> 00:33:47,480 The rest is channelled down to the ground, 303 00:33:47,690 --> 00:33:51,450 where it's sucked up by roots just below the surface of the sand. 304 00:33:53,990 --> 00:33:58,620 The fog also provides life-saving drinks for some of the desert animals. 305 00:33:58,870 --> 00:34:00,830 These are darkling beetles. 306 00:34:01,120 --> 00:34:05,710 On foggy mornings, they climb to the top of the dunes and stand in lines, 307 00:34:06,000 --> 00:34:09,760 head down, abdomen up, slowly marking time. 308 00:34:32,490 --> 00:34:36,160 Droplets of water from the fog collect on legs and antennae, 309 00:34:36,410 --> 00:34:41,580 and then, as the beetle lifts its feet, trickle down towards its mouth. 310 00:34:47,380 --> 00:34:50,920 The Namib's fogs never penetrate very far inland. 311 00:34:51,510 --> 00:34:57,090 Deserts that lie far from the sea, therefore, can never receive moisture in such a way. 312 00:34:57,600 --> 00:35:00,350 Their water must come from the clouds. 313 00:35:03,520 --> 00:35:06,600 Often, the clouds that do build up above a desert 314 00:35:06,770 --> 00:35:12,110 sail off elsewhere without bursting, and the land remains parched. 315 00:35:23,200 --> 00:35:29,170 But when eventually rain does come, it's the trigger for immediate and urgent action. 316 00:35:32,420 --> 00:35:36,970 One or two drops are all that's necessary to activate these dead stems. 317 00:35:48,230 --> 00:35:50,520 Within half a minute, they're upright. 318 00:35:54,730 --> 00:35:58,030 Other plants begin to open their seed-heads. 319 00:36:06,580 --> 00:36:08,750 None of these plants is alive. 320 00:36:09,040 --> 00:36:14,000 All their movements are simply the result of the dead tissues absorbing water. 321 00:36:25,890 --> 00:36:30,020 The dead seed-heads have held the seeds securely during the drought. 322 00:36:32,690 --> 00:36:37,690 Now, since it's rained and there's a chance of them germinating, they can be distributed. 323 00:36:40,490 --> 00:36:44,530 For some plants, the heavy raindrops are enough to dislodge the seeds. 324 00:37:04,350 --> 00:37:10,140 Others utilise the physical effects of absorbing water to shoot the seeds away. 325 00:37:20,450 --> 00:37:24,450 Now the seeds themselves, lying on the ground, begin to move. 326 00:37:30,870 --> 00:37:33,830 As the hairs absorb water, they swell and stiffen, 327 00:37:34,040 --> 00:37:36,500 so raising the seed into the right position 328 00:37:36,670 --> 00:37:41,170 for its first rootlets to strike straight downwards into the ground. 329 00:37:44,720 --> 00:37:48,680 But sometimes in the Arizona desert, maybe once in several years, 330 00:37:48,850 --> 00:37:53,020 there are real cloudbursts, and the desert is transformed. 331 00:38:45,740 --> 00:38:49,620 In the aftermath of the flood, new faces appear. 332 00:39:00,170 --> 00:39:04,420 A spadefoot toad. The males are the first to emerge from the soil 333 00:39:04,590 --> 00:39:07,890 where they've been buried for the past year or more. 334 00:39:14,930 --> 00:39:19,440 Hastily, they make their way to one of the pools that have appeared in the desert, 335 00:39:19,650 --> 00:39:22,780 and there they begin calling, summoning the females. 336 00:39:24,280 --> 00:39:29,370 There is great urgency. If they don't mate on this night, they may have lost their chance. 337 00:39:48,550 --> 00:39:53,930 Within 24 hours, the eggs have been laid and fertilised and are beginning to hatch. 338 00:40:04,990 --> 00:40:08,740 A day later, the pool is full of tadpoles. 339 00:40:12,450 --> 00:40:15,000 Other creatures have appeared as if from nowhere. 340 00:40:15,200 --> 00:40:20,170 Fairy shrimp have hatched from tiny eggs blown with the dust all over the desert. 341 00:40:26,050 --> 00:40:27,800 The tadpoles are growing fast. 342 00:40:27,970 --> 00:40:31,720 These with small mouths feed on algae and bacteria, 343 00:40:31,890 --> 00:40:35,390 a diet usually abundant in these desert pools. 344 00:40:43,940 --> 00:40:48,320 But other individuals from the same batch of eggs develop bigger heads 345 00:40:48,490 --> 00:40:52,910 and more powerfully muscled jaws. They have become meat-eaters. 346 00:40:57,200 --> 00:41:01,920 Not all pools will provide enough food for them, but here they are fortunate. 347 00:41:13,090 --> 00:41:16,060 They even eat their vegetarian brothers. 348 00:41:22,350 --> 00:41:26,980 With such a protein-rich diet, they grow even faster than the algal feeders. 349 00:41:27,150 --> 00:41:32,950 Here they are the favoured few, more likely to survive if the pool evaporates quickly. 350 00:41:33,110 --> 00:41:35,990 They're an insurance for the continuation of the species, 351 00:41:36,160 --> 00:41:39,540 for which the payments are their vegetarian brothers. 352 00:41:41,790 --> 00:41:46,800 But now the pool is shrinking fast. Another couple of days and it's almost gone. 353 00:41:46,960 --> 00:41:50,970 Unless there is another shower of rain, all the tadpoles will die. 354 00:41:57,810 --> 00:42:00,430 If they do die, their bodies will not be wasted. 355 00:42:00,600 --> 00:42:05,400 They will decompose and fertilise the sand, so that when the next rains come 356 00:42:05,610 --> 00:42:10,940 and another pool collects in this hollow, the algae will grow fast and well. 357 00:42:14,490 --> 00:42:17,830 Ants are quick to attack the stricken tadpoles. 358 00:42:21,200 --> 00:42:24,670 But at the last minute, there is a reprieve. A shower of rain. 359 00:42:24,870 --> 00:42:29,050 Some tadpoles, though they still have a tail, now have legs, 360 00:42:29,250 --> 00:42:33,430 and they're able to leave the puddle just two weeks after hatching. 361 00:42:41,270 --> 00:42:45,770 Even among this tiny proportion of survivors, the mortality will be huge. 362 00:42:46,020 --> 00:42:50,280 But with luck, a few will join the adults as the desert dries 363 00:42:50,440 --> 00:42:56,280 and bury themselves to wait for the next shower of rain many months from now. 364 00:43:19,300 --> 00:43:23,060 For several weeks after the rains, the desert blooms. 365 00:43:23,270 --> 00:43:28,020 The seeds shed by the shrivelled plants have sprouted and burst into flower. 366 00:43:28,190 --> 00:43:31,070 And deserts after rain all over the world, 367 00:43:31,270 --> 00:43:34,740 in Arizona and Australia, the Namib and the Sahara, 368 00:43:34,940 --> 00:43:40,080 put on one of the most dazzling displays of colour that you can see anywhere. 369 00:44:27,210 --> 00:44:29,750 Deserts are shaped by the sun and the wind. 370 00:44:29,920 --> 00:44:33,840 The roasting of rocks during the day, their chilling during cold nights, 371 00:44:34,000 --> 00:44:36,300 eventually makes their surface crumble. 372 00:44:36,550 --> 00:44:39,380 Some of their minerals splinter and fray into dust 373 00:44:39,590 --> 00:44:45,020 But quartz, the commonest, is very hard, and that remains as grains of sand. 374 00:44:45,270 --> 00:44:51,400 The wind catches them, sweeps them away, and collects them together as sand dunes. 375 00:45:20,090 --> 00:45:22,430 Dunes may be hundreds of feet high. 376 00:45:22,680 --> 00:45:24,550 If the wind is more or less constant, 377 00:45:24,760 --> 00:45:29,600 it blows the grains up the gently sloping side and over the steep front 378 00:45:29,810 --> 00:45:33,270 so that the dune marches slowly across the desert. 379 00:45:47,870 --> 00:45:52,710 Trudging up the face of a dune like this is extremely hard work. 380 00:45:52,960 --> 00:45:55,590 The sand is so dry 381 00:45:55,840 --> 00:46:00,470 and the grains are so polished by the wind rubbing them together 382 00:46:00,760 --> 00:46:04,390 that the surface is continuously on the move, 383 00:46:04,550 --> 00:46:07,930 and it's quite impossible to get any firm foothold. 384 00:46:08,220 --> 00:46:11,810 And, of course, that problem faces not just me, 385 00:46:12,020 --> 00:46:15,810 but all the animals that live among these dunes. 386 00:46:16,190 --> 00:46:21,860 Some of them have developed some extremely ingenious solutions to the difficulty. 387 00:46:24,200 --> 00:46:29,540 These extraordinary tracks have been made by one of the swiftest movers across the dunes. 388 00:46:33,920 --> 00:46:35,960 The sidewinder, a kind of rattlesnake. 389 00:46:36,250 --> 00:46:40,210 It skims across the surface by throwing its body into loops, 390 00:46:40,420 --> 00:46:43,340 which only touch the sand at two points. 391 00:46:44,010 --> 00:46:46,220 This not only enables it to move fast, 392 00:46:46,390 --> 00:46:49,510 but keeps most of its body off the hot surface. 393 00:46:56,020 --> 00:47:00,570 At midday, the sand is so hot that it's painful to touch. 394 00:47:00,780 --> 00:47:06,410 The Namib fringe-toed lizard prevents its feet from scorching by gymnastics. 395 00:47:24,470 --> 00:47:26,010 But eventually it gets so hot, 396 00:47:26,260 --> 00:47:31,140 the only thing to do is to shelter beneath the surface where the sand is very cool. 397 00:47:33,270 --> 00:47:36,770 Burrowing through this kind of sand also has problems. 398 00:47:36,940 --> 00:47:41,860 An animal can't construct a tunnel like a mouse hole or a rabbit burrow 399 00:47:42,030 --> 00:47:45,200 because the sand simply falls in behind it. 400 00:47:45,570 --> 00:47:50,030 So instead it has to wriggle through the sand almost as though it's swimming. 401 00:47:50,330 --> 00:47:53,160 And that's precisely what this little creature does. 402 00:47:54,040 --> 00:47:59,250 It may look like a worm, but in fact it's a lizard that has lost its legs. 403 00:47:59,540 --> 00:48:03,340 You can see that it's a lizard when you look closely at its face. 404 00:48:05,220 --> 00:48:10,510 Its mouth and eyes are covered by transparent scales that protect them in the sand. 405 00:48:10,760 --> 00:48:16,060 It's a blind skink. It lives by hunting for insects below the sand surface, 406 00:48:16,270 --> 00:48:20,480 and when I put it down, it'll wriggle away, just like an eel. 407 00:48:28,570 --> 00:48:32,620 The most extremely specialised of these hunters in the dunes 408 00:48:32,780 --> 00:48:36,410 is not a reptile but a mammal. 409 00:48:36,790 --> 00:48:41,290 It's very rarely seen, and your best chance of finding it is at night. 410 00:48:45,840 --> 00:48:51,890 These are its tracks, and that depression a place where it caught something. 411 00:49:00,310 --> 00:49:01,810 This is where it has burrowed again, 412 00:49:01,980 --> 00:49:05,900 and where, with luck, and if I dig very fast, I might catch it. 413 00:49:16,080 --> 00:49:18,700 Here it is, a golden mole. 414 00:49:21,500 --> 00:49:25,290 This one is a baby, but like its parents, it's totally blind. 415 00:49:25,500 --> 00:49:30,420 Eyes are of no use beneath the sand. Nor are ears, and it hasn't got those either. 416 00:49:30,630 --> 00:49:33,510 Its head ends in a leathery wedge 417 00:49:33,680 --> 00:49:38,310 with which it pushes through the sand, or alternatively, through my fingers. 418 00:49:52,950 --> 00:49:55,570 Golden moles will eat quite large creatures: 419 00:49:55,780 --> 00:49:58,580 A blind skink, if it encounters one, 420 00:49:58,740 --> 00:50:03,000 or other creatures that might be wandering unsuspectingly across the surface. 421 00:50:17,720 --> 00:50:19,930 A cricket would do nicely. 422 00:51:11,610 --> 00:51:16,450 The great sandy deserts of the world in Arabia, central Australia and the Sahara 423 00:51:16,660 --> 00:51:19,870 have repelled even the hardiest of human travellers. 424 00:51:20,080 --> 00:51:24,960 Few people have managed to survive in them for long totally unaided. 425 00:51:25,250 --> 00:51:29,420 But some manage to make regular journeys through these wildernesses. 426 00:51:34,720 --> 00:51:39,180 These are the Tuareg. They travel from one side of the Sahara to the other, 427 00:51:39,350 --> 00:51:44,890 carrying great cakes of salt, which they trade for cloth and grain and dates. 428 00:51:59,870 --> 00:52:02,030 But even the Tuareg can only make these journeys 429 00:52:02,200 --> 00:52:06,160 with the help of an animal desert specialist: The camel. 430 00:52:07,370 --> 00:52:11,670 They have to take all the food that they and their camels will need with them. 431 00:52:16,590 --> 00:52:20,090 Water is carried in skins slung beneath the camels' bellies 432 00:52:20,300 --> 00:52:23,930 to minimise evaporation and keep it as cool as possible. 433 00:52:26,270 --> 00:52:29,400 The camel is marvellously adapted to life in the desert. 434 00:52:29,690 --> 00:52:33,360 Its toes are reduced to two, but connected by skin, 435 00:52:33,520 --> 00:52:37,070 so that they splay out on the sand and don't sink deeply into it. 436 00:52:44,830 --> 00:52:50,000 Their nostrils are closable, so they can shut out sand grains during a sandstorm. 437 00:52:59,930 --> 00:53:04,850 The hair on their body is restricted to the top, where it shields against the sun. 438 00:53:05,100 --> 00:53:08,520 Elsewhere, for coolness, their skin is virtually naked. 439 00:53:08,930 --> 00:53:13,060 Their hump is full of fat, which in emergencies can be converted to water. 440 00:53:13,360 --> 00:53:16,110 But the process wastes the fat's calories 441 00:53:16,280 --> 00:53:20,150 and is only used when the camel hasn't drunk for a long time. 442 00:53:20,360 --> 00:53:24,740 It can live without liquid water for four times as long as a donkey 443 00:53:24,950 --> 00:53:27,450 and ten times as long as a man. 444 00:53:28,450 --> 00:53:31,620 But eventually even a camel has to drink. 445 00:53:40,050 --> 00:53:45,510 At one or two places in the Sahara, water can be reached by digging deep into the ground. 446 00:53:45,760 --> 00:53:49,100 Here, camels can at last refill their stomachs, 447 00:53:49,270 --> 00:53:51,440 and they take a lot of filling. 448 00:54:24,260 --> 00:54:27,050 If the Tuareg can't cross the Sahara without the camel, 449 00:54:27,260 --> 00:54:29,600 the camel can't do so without the Tuareg, 450 00:54:29,770 --> 00:54:33,810 for only men can dig for the essential water. 451 00:54:36,860 --> 00:54:40,860 Spring water is the key which unlocks abundant fertility. 452 00:54:41,240 --> 00:54:43,200 At Saharan oases like this one, 453 00:54:43,360 --> 00:54:47,320 all kinds of crops can be produced from the sand if it's watered: 454 00:54:47,620 --> 00:54:50,240 Dates and vegetables and fruit. 455 00:54:50,540 --> 00:54:54,160 Insects whizz and buzz over the gurgling irrigation channels 456 00:54:54,370 --> 00:54:57,170 and birds sing in the palm trees. 457 00:54:59,590 --> 00:55:03,380 But these small islands of life are under constant threat. 458 00:55:03,800 --> 00:55:09,470 If the wind veers and blows steadily from another direction, nothing can stop the sand. 459 00:55:20,440 --> 00:55:25,320 Eventually the advancing dunes may well overwhelm this oasis, 460 00:55:25,490 --> 00:55:27,530 and then this small world 461 00:55:27,700 --> 00:55:33,700 that's been brought into existence in the desert by the presence of water will be extinguished. 462 00:55:34,040 --> 00:55:37,460 The force that drives the dune, of course, is the wind, 463 00:55:37,710 --> 00:55:41,090 and the wind, too, has its own world of living organisms. 464 00:55:41,300 --> 00:55:44,210 Many of the spiders and beetles 465 00:55:44,420 --> 00:55:48,800 and other insects that live in the oasis arrived by air. 466 00:55:49,050 --> 00:55:54,480 And many of the plants, too, coming as windblown seeds or carried by birds.