1 00:01:03,878 --> 00:01:06,472 (CICADAS CHIRP) 2 00:01:28,238 --> 00:01:31,230 It's the end of another African day, 3 00:01:31,398 --> 00:01:34,993 and the game animals are preparing for the night. 4 00:01:35,158 --> 00:01:41,347 Baboons are climbing up into the branches of the trees and birds are coming in to roost. 5 00:01:42,238 --> 00:01:46,277 All these animals rely upon their eyes to find their way around, 6 00:01:46,438 --> 00:01:49,430 as indeed do I. 7 00:01:49,598 --> 00:01:52,431 In a short while it will be totally dark. 8 00:01:52,598 --> 00:01:55,510 Without a torch, I would be very well advised 9 00:01:55,678 --> 00:01:59,307 not to try and stumble around in the darkness. 10 00:01:59,478 --> 00:02:05,189 But not all animals rely on sight. 0thers use other senses to find their way around. 11 00:02:05,358 --> 00:02:09,067 And in a short while, they will be venturing out. 12 00:02:22,038 --> 00:02:27,237 Spotted hyena. They hunt almost entirely during the hours of darkness. 13 00:02:29,518 --> 00:02:32,510 They may travel up to 60 miles in one night, 14 00:02:32,678 --> 00:02:36,956 and they rely very much on smell to find their way around. 15 00:02:38,078 --> 00:02:42,629 Specially scented signposts tell them of other hyenas that have been this way. 16 00:02:42,798 --> 00:02:48,794 They add their own signatures by drawing grass stems across the glands beneath the tail. 17 00:02:54,078 --> 00:02:57,388 These registrations will remain detectable for up to a month, 18 00:02:57,558 --> 00:03:00,356 so each marking station is full of information 19 00:03:00,518 --> 00:03:04,511 about the comings and goings since they were last here. 20 00:03:06,958 --> 00:03:11,076 The hyenas also deposit their urine and dung in special places. 21 00:03:11,238 --> 00:03:13,832 To them, with their hypersensitive noses, 22 00:03:13,998 --> 00:03:19,948 these dunging stations must shine like beacons for miles through the blackness of the night. 23 00:03:35,078 --> 00:03:37,638 Bushbabies - galagos. 24 00:03:37,798 --> 00:03:41,996 They use regular pathways through the branches of the trees 25 00:03:42,158 --> 00:03:44,752 and mark them with great care. 26 00:03:44,918 --> 00:03:48,513 They deliberately urinate on their hands. 27 00:03:50,518 --> 00:03:55,751 Now every branch along which they run will be impregnated with smell. 28 00:03:59,758 --> 00:04:03,307 An intruder is quickly detected and chased off. 29 00:04:03,478 --> 00:04:07,266 The residents all keep a close nose on who is around 30 00:04:07,438 --> 00:04:11,556 and whether any of the females are coming into breeding condition. 31 00:04:32,838 --> 00:04:36,626 Tent caterpillars are also on the move during the night, 32 00:04:36,798 --> 00:04:40,677 marching out from their silken camp in search of food. 33 00:04:44,278 --> 00:04:50,114 When they've stripped one bush of its leaves, a single scout sets out to find a new supply. 34 00:04:51,998 --> 00:04:55,786 Scent glands on its rear end leave a trail of smell. 35 00:04:55,958 --> 00:05:01,555 That is for the scout's benefit, to enable it to find its own way back to the tent. 36 00:05:13,718 --> 00:05:20,510 A leaf, a meal. Having eaten its fill, it heads back, following its own scent trail. 37 00:05:24,038 --> 00:05:28,031 But it's still laying down scent from its rear end. 38 00:05:28,198 --> 00:05:34,194 The rest of the caterpillars can differentiate between a single trail and a double one. 39 00:05:34,358 --> 00:05:39,876 They may tell from its smell whether its creator has had a good meal before it returned. 40 00:05:40,038 --> 00:05:44,031 So they know whether the trail will lead them to food. 41 00:05:45,278 --> 00:05:49,794 Before long, a network of smelly pathways covers the branches, 42 00:05:49,958 --> 00:05:55,555 ensuring that the caterpillars lose no time in the constant rush for food. 43 00:06:13,278 --> 00:06:17,271 Smell isn't the only way of finding your way in the dark. 44 00:06:17,438 --> 00:06:22,831 This massive cave in Borneo is the home of birds that use a completely different system. 45 00:06:22,998 --> 00:06:29,392 These are swiftlets. It's evening, and they're pouring into the cave to roost in thousands. 46 00:06:29,558 --> 00:06:35,155 At the moment there's still enough light outside for them to see and they are relatively quiet, 47 00:06:35,318 --> 00:06:41,314 but just listen to them when I follow them down into the real blackness of the cave itself. 48 00:06:41,478 --> 00:06:43,867 (CLICKING) 49 00:06:49,638 --> 00:06:55,827 These ladders were built by people who collect the swiftlets' nests to make bird's-nest soup. 50 00:06:57,278 --> 00:07:03,194 They lead down to a rickety, slimy platform hanging 180 feet above the cave floor, 51 00:07:03,358 --> 00:07:06,555 alongside the nests stuck to the rock wall. 52 00:07:06,718 --> 00:07:09,437 The chorus of clicks that you can hear 53 00:07:09,598 --> 00:07:13,591 is made by the birds as they fly through the pitch blackness. 54 00:07:13,758 --> 00:07:18,274 Each bird is guiding itself by listening to the echo that its call makes 55 00:07:18,438 --> 00:07:20,952 as it bounces from the rock wall of the cave. 56 00:07:21,118 --> 00:07:25,316 Amazingly, it's able to distinguish the echo of its own call 57 00:07:25,478 --> 00:07:27,946 from that of all the rest of the birds. 58 00:07:28,118 --> 00:07:32,111 So that although there are a million swiftlets in this cave, 59 00:07:32,278 --> 00:07:38,274 each one is able to find its way back to its own particular nest in the pitch blackness. 60 00:07:40,678 --> 00:07:43,988 (DIN 0F CLICKING) 61 00:07:44,878 --> 00:07:48,871 Birds aren't the only animals to have evolved echolocation. 62 00:07:49,038 --> 00:07:52,428 It was developed first by mammals - bats. 63 00:07:54,278 --> 00:07:58,669 This sea of pink is a mass of naked, newly-born bats. 64 00:07:58,838 --> 00:08:03,832 A mother has to be able to steer her way through the winding passages of the cave 65 00:08:03,998 --> 00:08:07,991 and land alongside her baby in order to give it milk. 66 00:08:09,958 --> 00:08:15,351 The bats' equipment for echolocating is more highly developed than that of the swiftlets. 67 00:08:15,518 --> 00:08:20,387 First, they have huge ears that are constantly twitching to pick up faint echoes. 68 00:08:22,878 --> 00:08:26,666 Second, they have complex flaps around the nose 69 00:08:26,838 --> 00:08:30,831 that concentrate the sound signals into a narrow beam. 70 00:08:33,398 --> 00:08:37,107 Most important of all, the frequency of sound they use is very much higher. 71 00:08:37,278 --> 00:08:42,671 As a result, bat echolocation is very much more efficient than that used by the swiftlets. 72 00:08:42,838 --> 00:08:48,549 In the evenings, when the swiftlets come back into the cave because they cannot see to hunt, 73 00:08:48,718 --> 00:08:51,312 the bats themselves are just setting out. 74 00:08:51,478 --> 00:08:53,912 With their echolocation, 75 00:08:54,078 --> 00:08:59,391 they can find and catch insect prey less than half a millimetre across in the pitch blackness. 76 00:09:00,758 --> 00:09:04,751 Caves are not the only places that are permanently dark. 77 00:09:04,918 --> 00:09:06,556 So are some of the world's great rivers. 78 00:09:08,958 --> 00:09:12,667 Much of the Amazon is thick with suspended mud, 79 00:09:12,838 --> 00:09:16,831 and here, another mammal has developed echolocation. 80 00:09:21,198 --> 00:09:27,194 The river dolphin has a bulge on its forehead through which it transmits beams of ultrasound. 81 00:09:27,358 --> 00:09:29,747 (CLICKING) 82 00:09:29,918 --> 00:09:35,311 Although it's virtually blind, echolocation enables it to avoid obstacles in its path 83 00:09:35,478 --> 00:09:38,072 and catch even the smallest fish. 84 00:09:51,518 --> 00:09:57,309 This same river is home to other animals that literally feel their way through the gloom. 85 00:10:10,598 --> 00:10:14,591 There are several hundred species of catfish in the Amazon, 86 00:10:14,758 --> 00:10:17,636 and they're all equipped with long feelers. 87 00:10:17,798 --> 00:10:21,586 Some, on the throat, search for prey in the sand. 88 00:10:25,598 --> 00:10:30,797 0thers, on the snout, reach ahead to detect obstacles that must be avoided. 89 00:10:42,918 --> 00:10:45,910 Here on the Amazon, there are other fish 90 00:10:46,078 --> 00:10:50,594 that use perhaps the most extraordinary method of navigation of all. 91 00:10:50,758 --> 00:10:56,151 Living amongst this tangle of fallen trunks and branches of the flooded forest, 92 00:10:56,318 --> 00:11:01,711 they find their ways about not by touch or by smell or by sight, 93 00:11:01,878 --> 00:11:05,871 or even by echolocation, but by electricity. 94 00:11:06,038 --> 00:11:10,031 And I can fish for them using a device like this. 95 00:11:10,198 --> 00:11:16,876 When I turn it on, it emits a stream of electronic pulses from either end 96 00:11:17,038 --> 00:11:22,908 which these fish, with their extreme sensitivity to electricity, should find irresistible. 97 00:11:23,078 --> 00:11:25,069 Let's see. 98 00:11:27,038 --> 00:11:29,427 (CLICKING) 99 00:11:32,198 --> 00:11:36,988 And here they are, within seconds. Electric eels, six feet long. 100 00:11:40,358 --> 00:11:45,557 They can discharge massive electric shocks which can stun and even kill their prey. 101 00:11:45,718 --> 00:11:51,588 They generate continuous low-voltage signals that enable them to visualise their surroundings 102 00:11:51,758 --> 00:11:54,591 and even, maybe, to recognise one another. 103 00:11:54,758 --> 00:11:57,795 So they're very interested in my version. 104 00:12:16,438 --> 00:12:21,751 The electric field they create around themselves is distorted by any object in the water, 105 00:12:21,918 --> 00:12:26,912 and they detect these changes with a line of sensory cells along their flanks. 106 00:12:29,798 --> 00:12:32,995 For the dark places as well as the dark hours, 107 00:12:33,158 --> 00:12:37,754 animals have developed a variety of techniques for finding their way around. 108 00:12:37,918 --> 00:12:42,708 But for many, the time of activity is not during darkness, but during the light. 109 00:12:49,958 --> 00:12:52,950 The rufous elephant shrew of Africa 110 00:12:53,118 --> 00:12:57,111 guides itself with its eyes as it careers along its runways. 111 00:12:58,398 --> 00:13:03,677 It spends three quarters of its waking hours keeping its network of tracks clear. 112 00:13:03,838 --> 00:13:07,353 A single twig could trip it up and bring disaster. 113 00:13:07,518 --> 00:13:12,592 Its safety depends on knowing every curve and twist in its pathways 114 00:13:12,758 --> 00:13:17,548 so that it can outrun most of its enemies, like a black-shouldered kite. 115 00:13:35,398 --> 00:13:41,189 Most remarkably, if it's really threatened by a stooping bird, it can take shortcuts, 116 00:13:41,358 --> 00:13:46,557 leaving one trail and going straight on to another to outsmart its enemy. 117 00:13:46,718 --> 00:13:51,269 Elephant shrews have a good mental picture of the layout of their trails, 118 00:13:51,438 --> 00:13:54,236 just as we have of our own neighbourhood. 119 00:13:54,398 --> 00:13:58,391 Such local knowledge is not only useful for escaping predators, 120 00:13:58,558 --> 00:14:01,152 it's also valuable in finding food. 121 00:14:06,918 --> 00:14:11,912 Each autumn, in English oak woods, jays find and bury acorns, 122 00:14:12,078 --> 00:14:16,868 giving each one its own hiding place and covering it with a leaf. 123 00:14:22,278 --> 00:14:26,351 In one season, a jay will bury several thousand acorns 124 00:14:26,518 --> 00:14:29,237 in different places throughout its territory. 125 00:14:29,398 --> 00:14:33,107 It relies on these for food during the winter months. 126 00:14:41,078 --> 00:14:45,276 All through the spring and early summer, it continues recovering them. 127 00:14:45,438 --> 00:14:48,714 It must remember where many are, 128 00:14:48,878 --> 00:14:53,668 for its recovery rate is much greater than can be accounted for by chance. 129 00:14:54,678 --> 00:15:00,674 Just as we may remember the position of a shop by relating it to a big building like a church, 130 00:15:00,838 --> 00:15:06,834 so the jays use prominent trees as landmarks, and tend to bury their acorns around them. 131 00:15:12,838 --> 00:15:18,834 Jays live in places that are full of distinctive features, but all animals are not so lucky. 132 00:15:24,238 --> 00:15:28,436 This must be the easiest place in the world to get lost. 133 00:15:28,598 --> 00:15:32,989 I'm in the great sea of sand in the eastern Sahara. 134 00:15:33,158 --> 00:15:35,547 Behind me, to the south, 135 00:15:35,718 --> 00:15:39,711 wave upon wave of dunes stretch for hundreds of miles. 136 00:15:39,878 --> 00:15:44,076 It would be hard to imagine a landscape with fewer features to it. 137 00:15:44,238 --> 00:15:48,629 And with temperatures rising to 50 degrees centigrade during the day, 138 00:15:48,798 --> 00:15:51,107 getting lost here could be lethal. 139 00:15:51,278 --> 00:15:56,068 And yet this is the home of one of the most remarkable animal travellers, 140 00:15:56,238 --> 00:16:00,436 an ant that regularly leaves its home in these sands 141 00:16:00,598 --> 00:16:05,388 and sets out on the longest overland journey made by any insect. 142 00:16:05,558 --> 00:16:07,947 It's called cataglyphis, 143 00:16:08,118 --> 00:16:13,511 and it comes out during the middle of the day when other insects die from heat exhaustion. 144 00:16:13,678 --> 00:16:16,476 Cataglyphis searches for these casualties 145 00:16:16,638 --> 00:16:21,632 when it's so hot that even it seeks relief from the burning surface when it can. 146 00:16:27,838 --> 00:16:31,547 At first, it forages randomly over the sand. 147 00:16:32,438 --> 00:16:34,906 But when it finds its exhausted prey, 148 00:16:35,078 --> 00:16:39,594 astonishingly, it returns in a dead straight line to its nest. 149 00:16:45,318 --> 00:16:47,957 It's so hot in the desert that even cataglyphis 150 00:16:48,118 --> 00:16:53,351 has to get back as quickly as possible to its nest if it's not to risk death. 151 00:16:55,958 --> 00:16:59,951 These foraging journeys are equivalent in human terms 152 00:17:00,118 --> 00:17:04,111 to a trek of 40 miles over completely featureless territory. 153 00:17:04,278 --> 00:17:08,749 And yet the ants, even if they wander about in searching for their food, 154 00:17:08,918 --> 00:17:11,716 are able to return directly to their nest. 155 00:17:11,878 --> 00:17:14,346 How do they achieve that? 156 00:17:14,518 --> 00:17:18,830 Well, have a closer look at one leaving on one of these journeys. 157 00:17:20,598 --> 00:17:23,396 It keeps stopping and making a turn. 158 00:17:25,158 --> 00:17:27,626 Stop, and turn. 159 00:17:27,798 --> 00:17:30,107 Stop, and turn. 160 00:17:38,798 --> 00:17:43,110 As it turns, it looks up at the sun, checking its position. 161 00:17:43,278 --> 00:17:45,872 It moves on again, 162 00:17:46,038 --> 00:17:50,031 and checks the sun and the pattern of polarised light. 163 00:17:51,158 --> 00:17:53,956 It can measure the distance between stops, 164 00:17:54,118 --> 00:17:58,987 and it always takes a bearing on the sun at every one of them. 165 00:17:59,158 --> 00:18:02,434 When it finds food, a quick calculation 166 00:18:02,598 --> 00:18:05,590 and it knows exactly the shortest way home. 167 00:18:15,118 --> 00:18:19,908 If you can use a beacon that's with you wherever you go, like the sun, 168 00:18:20,078 --> 00:18:24,788 then, of course, you're no longer restricted to your familiar home ground. 169 00:18:24,958 --> 00:18:31,352 You can venture into unknown territory. You can go long distances to find new feeding grounds. 170 00:18:31,518 --> 00:18:34,032 Great journeys are now possible. 171 00:18:35,878 --> 00:18:38,472 The death's-head hawk moth lives in Africa, 172 00:18:38,638 --> 00:18:43,234 but every year, some, seeking new territory, fly across the Mediterranean, 173 00:18:43,398 --> 00:18:45,992 keeping the setting sun to the left. 174 00:18:47,918 --> 00:18:53,311 They fly right through the night, using the moon to hold their northward course. 175 00:18:58,158 --> 00:19:03,869 They continue into Europe, climbing higher to cross the Alps, and then on into France. 176 00:19:04,038 --> 00:19:08,748 Their speed is only about 15 miles an hour, but they continue doggedly on. 177 00:19:08,918 --> 00:19:12,911 After several weeks, a few may cross the Channel. 178 00:19:21,438 --> 00:19:28,037 Now they're exhausted, and they find one sight - or, more accurately, one smell - irresistible. 179 00:19:28,198 --> 00:19:30,507 Hives of honey bees. 180 00:19:30,678 --> 00:19:33,067 (BUZZING) 181 00:19:44,398 --> 00:19:48,391 The hungry traveller restores its energy with stolen honey 182 00:19:48,558 --> 00:19:53,074 before it starts looking for potato plants, on which it will lay its eggs. 183 00:19:58,718 --> 00:20:04,907 Honey bees not only steer by the sun. They use it to pass on instructions to one another. 184 00:20:07,118 --> 00:20:12,112 When a forager finds a fresh source of nectar in newly-opened flowers, 185 00:20:12,278 --> 00:20:17,671 it fills its crop and flies back to the hive, guiding itself by the position of the sun. 186 00:20:20,438 --> 00:20:22,827 And inside, it dances. 187 00:20:25,118 --> 00:20:30,112 It waggles across the comb so that the angle of its waggled path to the vertical 188 00:20:30,278 --> 00:20:35,796 tells the other bees that to find new food, they must fly out at the same angle 189 00:20:35,958 --> 00:20:38,347 with respect to the sun. 190 00:20:40,518 --> 00:20:46,707 So other workers who've witnessed the dance are able to fly off directly to the same flower. 191 00:20:55,318 --> 00:20:58,708 As the day goes on, the sun, of course, moves. 192 00:21:02,998 --> 00:21:08,595 In the dark of the hive, the original forager often continues dancing for several hours, 193 00:21:08,758 --> 00:21:11,226 unable to see the moving sun. 194 00:21:16,478 --> 00:21:20,676 But, remarkably, to match exactly the sun's movements, 195 00:21:20,838 --> 00:21:24,626 the dancer steadily shifts the direction of its dance. 196 00:21:43,558 --> 00:21:49,906 So the continuous stream of departing workers are always given the correct angle of flight. 197 00:21:53,358 --> 00:21:58,955 All animals that steer by the sun must be able to compensate for its movements in this way. 198 00:21:59,118 --> 00:22:01,916 0f course, the sun is not visible to everyone. 199 00:22:02,078 --> 00:22:05,593 What do you do, for instance, if you live underwater? 200 00:22:10,198 --> 00:22:14,988 In the calm shallow seas of the Bahamas live spiny lobsters. 201 00:22:15,158 --> 00:22:18,070 Lobsters like calm, clear water, 202 00:22:18,238 --> 00:22:22,550 but in autumn the Bahamas are swept by serious storms. 203 00:22:29,318 --> 00:22:32,310 Suddenly, as the waters become more and more cloudy, 204 00:22:32,478 --> 00:22:37,074 the lobsters decide to move and seek refuge at greater depths. 205 00:22:58,478 --> 00:23:02,790 They usually start in the evening, travelling in pairs. 206 00:23:02,958 --> 00:23:07,156 By morning, the pairs have joined into long columns. 207 00:23:23,318 --> 00:23:25,912 In queues 30 or 40 strong, 208 00:23:26,078 --> 00:23:30,469 they head for the drop-off on the ocean side of the lagoon. 209 00:23:41,478 --> 00:23:46,074 It seems that they know the way from the overall direction of current and swell, 210 00:23:46,238 --> 00:23:48,957 which remains constant at this depth. 211 00:23:55,198 --> 00:23:57,792 Lines join together into longer lines. 212 00:23:57,958 --> 00:24:02,349 Sometimes 60 lobsters will be marching one behind the other. 213 00:24:02,518 --> 00:24:06,306 The migration takes place within a few days each year, 214 00:24:06,478 --> 00:24:11,472 and then the whole lagoon floor is covered with parallel marching columns. 215 00:24:13,318 --> 00:24:18,312 Travelling in line reduces the drag of the water on an individual by as much as half. 216 00:24:18,478 --> 00:24:22,869 But there's another reason why it's better to march in this way. 217 00:24:24,278 --> 00:24:28,669 If they are threatened, they can form defensive circles. 218 00:24:29,638 --> 00:24:32,630 A triggerfish, one of their main enemies. 219 00:24:32,798 --> 00:24:35,517 It wants to attack the vulnerable legs, 220 00:24:35,678 --> 00:24:40,388 but it has little chance of getting past the ring of spear-like antennae. 221 00:24:49,278 --> 00:24:52,350 But a solitary traveller is in trouble. 222 00:24:59,518 --> 00:25:01,986 First, it's disarmed. 223 00:25:04,838 --> 00:25:07,306 Then the rest is easy. 224 00:25:28,878 --> 00:25:33,349 There are others ready to pick flesh from the broken limbs. 225 00:25:36,678 --> 00:25:40,796 Within a few minutes, all that is left is an empty shell. 226 00:25:52,878 --> 00:25:58,475 When the survivors reach the shelter of reefs that run along the edge of the ocean drop-off, 227 00:25:58,638 --> 00:26:02,631 they abandon the caravans and each makes its own way. 228 00:26:05,478 --> 00:26:09,869 0ne by one, they clamber down the slope to even greater depths, 229 00:26:10,038 --> 00:26:15,635 where they will be safe from the storms that churn the waters hundreds of feet above. 230 00:26:19,478 --> 00:26:21,946 Lobsters travel about 30 miles, 231 00:26:22,118 --> 00:26:26,111 but they're not by any means the greatest marine migrants. 232 00:26:26,278 --> 00:26:30,669 These same reefs are the feeding ground of green turtles. 233 00:26:30,838 --> 00:26:33,830 They, like the lobsters, do not breed down here. 234 00:26:33,998 --> 00:26:38,788 To do that, they must leave the reef and head out into the open ocean. 235 00:26:41,318 --> 00:26:44,037 Those on the eastern coast of South America 236 00:26:44,198 --> 00:26:49,591 swim for 1,000 miles to the tiny island of Ascension in the middle of the Atlantic. 237 00:26:50,918 --> 00:26:55,708 0thers, in the Pacific, head for the little cluster of the Galapagos. 238 00:26:59,838 --> 00:27:02,830 They come to the surface regularly to breathe, 239 00:27:02,998 --> 00:27:06,991 and they may use these glimpses of the sun as a guide. 240 00:27:07,158 --> 00:27:12,152 The direction of the waves and the ocean swell may also provide clues. 241 00:27:17,678 --> 00:27:20,476 But they also swim at greater depths, 242 00:27:20,638 --> 00:27:25,234 and take advantage of the powerful currents that help them on their way. 243 00:27:25,398 --> 00:27:29,994 In this deep blue water, they may be guided by the earth's magnetic field. 244 00:27:30,158 --> 00:27:32,831 They have iron oxide particles in their heads, 245 00:27:32,998 --> 00:27:38,595 and these must be sensitive to the earth's magnetism, just as magnetic compasses are. 246 00:27:45,358 --> 00:27:50,148 As they near the islands, they may also detect the fresh water that flows from them, 247 00:27:50,318 --> 00:27:52,627 faint though it must be. 248 00:27:52,798 --> 00:27:58,794 By swimming so that the taste grows stronger, they at last reach the rich waters of the Galapagos. 249 00:28:03,918 --> 00:28:07,035 Here they meet others, and here they mate. 250 00:28:14,918 --> 00:28:20,117 The sheltered beaches provide the females with the nesting sites they need. 251 00:28:32,998 --> 00:28:37,514 Weeks later, after the adults have resumed their ocean-wide wanderings, 252 00:28:37,678 --> 00:28:40,317 the young dig their way to the surface. 253 00:28:57,958 --> 00:29:00,267 As they enter the sea, 254 00:29:00,438 --> 00:29:05,432 they get a taste of the coastal water that will remain with them for at least 30 years. 255 00:29:05,598 --> 00:29:09,386 For it's only after 30 years that they're ready to breed. 256 00:29:09,558 --> 00:29:13,551 Then they will use that memory to guide them back to mate and nest 257 00:29:13,718 --> 00:29:17,427 on these very beaches where they were hatched. 258 00:29:56,678 --> 00:29:59,909 This is the high Arctic, Spitzbergen. 259 00:30:00,078 --> 00:30:04,071 It's the middle of the night, although the sun is high in the sky. 260 00:30:04,238 --> 00:30:07,230 We're only 600 miles from the North Pole. 261 00:30:07,398 --> 00:30:10,196 Most of the year, the sea is covered with ice. 262 00:30:10,358 --> 00:30:13,156 Now, during the summer, the ice has melted. 263 00:30:13,318 --> 00:30:17,516 Now is the time that the Arctic tern comes up here to nest. 264 00:30:17,678 --> 00:30:20,556 It's at the extreme edge of its range. 265 00:30:20,718 --> 00:30:23,835 No bird nests farther north than this. 266 00:30:26,998 --> 00:30:29,910 There's a good reason for birds to come here. 267 00:30:30,078 --> 00:30:35,277 24 hours of daylight means 24 hours in which to collect food for the chicks. 268 00:30:38,278 --> 00:30:40,667 Fishing need never stop. 269 00:30:44,558 --> 00:30:47,026 The Barents Sea is so rich 270 00:30:47,198 --> 00:30:52,591 that the chicks here grow faster than anywhere else in the Arctic tern's range. 271 00:30:55,718 --> 00:30:59,427 This tiny little chick, only a few days old, 272 00:30:59,598 --> 00:31:02,590 in a few weeks' time, before the ice returns, 273 00:31:02,758 --> 00:31:07,388 will have to set out to fly south in an attempt to reach a place 274 00:31:07,558 --> 00:31:13,554 which is as far away from here as it's possible to be without actually leaving the planet. 275 00:31:15,998 --> 00:31:20,708 By the beginning of August, darkness is returning and the temperature falling. 276 00:31:20,878 --> 00:31:25,269 The sea will soon be covered with ice and fishing will be impossible. 277 00:31:25,438 --> 00:31:30,148 The terns must leave and start on the 12,000-mile journey south. 278 00:31:32,958 --> 00:31:37,270 The juveniles, who've fed so continuously and grown so fast, 279 00:31:37,438 --> 00:31:40,430 are now strong enough to follow their parents. 280 00:31:43,278 --> 00:31:45,997 From Spitzbergen, they head for Norway, 281 00:31:46,158 --> 00:31:49,275 then south down the coasts of Scandinavia, 282 00:31:49,438 --> 00:31:53,750 past Britain, and on to southern Europe and North Africa. 283 00:32:03,238 --> 00:32:08,949 It's a continuous two-month flight, and the birds feed, drink and sleep at sea. 284 00:32:14,478 --> 00:32:18,676 They continue, following the coast down to the Cape of Good Hope 285 00:32:18,838 --> 00:32:21,830 and then out across the Southern 0cean. 286 00:32:29,478 --> 00:32:33,471 Eventually, they reach the ice again. Antarctic ice. 287 00:32:33,638 --> 00:32:38,632 They've followed the sun to the very edge of the great southern continent. 288 00:32:38,798 --> 00:32:41,790 Here, of course, the summer is just beginning. 289 00:32:41,958 --> 00:32:45,746 And once again, there is round-the-clock fishing. 290 00:32:45,918 --> 00:32:48,386 So, for eight months of their year, 291 00:32:48,558 --> 00:32:52,551 these indefatigable fishermen never see the sun set. 292 00:32:52,718 --> 00:32:57,508 And then, once more, the adults head off on their 12,000-mile journey 293 00:32:57,678 --> 00:33:00,431 back to Spitzbergen to breed again. 294 00:33:08,238 --> 00:33:13,028 These parent birds so vigorously defending their nest 295 00:33:13,198 --> 00:33:18,909 lay their eggs within a few inches of the previous year's nest site. 296 00:33:19,078 --> 00:33:23,868 When they were down in the Antarctic, the pair separated. 297 00:33:24,038 --> 00:33:27,030 But they reunite once they come back here 298 00:33:27,198 --> 00:33:30,270 onto their own patch of...patch of shingle. 299 00:33:31,358 --> 00:33:34,953 What's more, they do that year after year. 300 00:33:35,118 --> 00:33:40,795 0ne pair here in Spitzbergen have been known to do it for 18 years in succession. 301 00:33:40,958 --> 00:33:45,952 Such accurate route-finding can't be achieved simply by following a compass direction. 302 00:33:46,118 --> 00:33:48,632 You have to know where you are. 303 00:33:48,798 --> 00:33:54,794 So in addition to a compass, you have to have a map. In short, you have to navigate. 304 00:33:56,038 --> 00:34:01,510 This rufous hummingbird has a route map of the Rocky Mountain chain in its brain. 305 00:34:01,678 --> 00:34:05,876 It's used it to fly from Mexico all the way up here to Alaska, 306 00:34:06,038 --> 00:34:09,428 which is almost as far north as Spitzbergen. 307 00:34:09,598 --> 00:34:13,591 No other tropical bird ventures as far north as this, 308 00:34:13,758 --> 00:34:16,352 and here it will spend the summer. 309 00:34:27,918 --> 00:34:33,675 During these short weeks, there's a rich supply of nectar and insects with which to feed its young. 310 00:34:38,198 --> 00:34:40,666 0nly the female rears the chicks, 311 00:34:40,838 --> 00:34:45,992 so in June the male can start the 4,000-mile journey back south to Mexico. 312 00:34:51,158 --> 00:34:55,151 The female stays a week longer to feed the chicks. 313 00:34:55,318 --> 00:34:59,709 Then she will leave them, and they will follow independently. 314 00:35:06,718 --> 00:35:09,107 If you consider body size, 315 00:35:09,278 --> 00:35:14,636 the hummingbirds' migration is even more impressive than the terns'. 316 00:35:14,798 --> 00:35:18,996 They follow the mountain chains, half of them flying down the Rockies, 317 00:35:19,158 --> 00:35:23,754 the others travelling nearer the coast, down the Sierra Nevada range. 318 00:35:23,918 --> 00:35:29,914 For tiny birds weighing only three grams, the flight demands great expenditure of energy, 319 00:35:30,078 --> 00:35:32,911 and they have to find flowers to refuel. 320 00:35:34,798 --> 00:35:40,395 Up in the mountains, the shrinking snows have exposed meadows where flowers are in bloom. 321 00:35:40,558 --> 00:35:45,154 0nly here, at this time of the year, can they get the nectar they need. 322 00:35:45,318 --> 00:35:49,709 The young birds discover these meadows on their first journey south. 323 00:35:49,878 --> 00:35:54,394 0ften, the same birds will return to the same meadows each year. 324 00:35:57,198 --> 00:36:01,589 They continue south along the canyons of Utah and Colorado. 325 00:36:01,758 --> 00:36:06,149 These great geographical features must be unforgettable landmarks 326 00:36:06,318 --> 00:36:10,709 on the route map they use to find their way with such accuracy. 327 00:36:14,278 --> 00:36:18,271 After two months, they reach the mountains of southern Mexico, 328 00:36:18,438 --> 00:36:20,906 where they will spend the winter. 329 00:36:22,758 --> 00:36:25,067 This is a rich, tropical area 330 00:36:25,238 --> 00:36:30,232 full of flowering plants that will provide them with nectar for the winter. 331 00:36:43,398 --> 00:36:47,277 These birds do not return just to the same general area. 332 00:36:47,438 --> 00:36:51,829 Each winter, many are found back on the same flowering bush. 333 00:36:57,598 --> 00:37:02,308 They're highly territorial, and use traditional perches to defend their patch, 334 00:37:02,478 --> 00:37:05,072 calling to warn off intruders. 335 00:37:11,078 --> 00:37:15,549 A large-scale mental map gets them back to the right part of Mexico, 336 00:37:15,718 --> 00:37:20,314 and then the sort of territorial knowledge that enables the jay to find acorns 337 00:37:20,478 --> 00:37:23,390 takes them to the same flowering bush. 338 00:37:27,118 --> 00:37:31,157 But not all birds have geographical features to serve as guides during migration. 339 00:37:31,318 --> 00:37:34,549 The royal albatross migrates over the sea. 340 00:37:34,718 --> 00:37:39,746 And one of them has claims to be the greatest animal traveller of all. 341 00:37:39,918 --> 00:37:45,709 Here in Taiaroa Head in South Island, New Zealand, back in 1937, 342 00:37:45,878 --> 00:37:50,190 a young female albatross was given an identification ring. 343 00:37:50,358 --> 00:37:55,352 She had spent the previous eight years flying round and round the Antarctic continent 344 00:37:55,518 --> 00:37:57,907 until she was ready to breed. 345 00:37:58,078 --> 00:38:01,468 In that year, she bred here for the first time. 346 00:38:01,638 --> 00:38:06,234 In the half-century since then, she's come back here every other year, 347 00:38:06,398 --> 00:38:10,676 in between times making more circuits of Antarctica. 348 00:38:10,838 --> 00:38:13,750 She's affectionately known as Grandma. 349 00:38:13,918 --> 00:38:18,628 She hasn't reappeared this season, so presumably she's still out at sea. 350 00:38:18,798 --> 00:38:23,394 But she's certainly the best-travelled animal we know about. 351 00:38:23,558 --> 00:38:27,551 But all albatross are superb aeronauts. 352 00:39:07,718 --> 00:39:10,915 By using tags that can be traced by satellite, 353 00:39:11,078 --> 00:39:15,993 we know that an albatross may fly 800 miles to collect food for their chick, 354 00:39:16,158 --> 00:39:18,956 and still find their way back to their nest 355 00:39:19,118 --> 00:39:23,873 on a tiny island isolated in a vast, empty tract of the Southern 0cean. 356 00:39:34,638 --> 00:39:39,632 Maybe they recognise the patterns made by the waves on the surface of the sea. 357 00:39:39,798 --> 00:39:44,189 Perhaps the clouds that build up over oceanic islands may help them, 358 00:39:44,358 --> 00:39:46,952 for they are visible many miles away. 359 00:39:47,918 --> 00:39:51,911 It could be that the sun gives them navigational information. 360 00:39:52,078 --> 00:39:56,469 The nearer you are to the pole, the lower its altitude at midday will be. 361 00:39:56,638 --> 00:40:02,429 So if you have an accurate sense of time, the sun's altitude will tell you your latitude. 362 00:40:05,118 --> 00:40:09,316 So far, there is no evidence that birds can navigate in this way. 363 00:40:09,478 --> 00:40:14,108 However, they certainly do have remarkable abilities to use celestial clues 364 00:40:14,278 --> 00:40:16,348 both during the day and the night. 365 00:40:21,798 --> 00:40:27,191 Evidence is growing that many young birds with a view of the sky as they sit in their nest 366 00:40:27,358 --> 00:40:30,555 learn to orientate themselves by the stars. 367 00:40:32,998 --> 00:40:37,992 This is far harder than using the sun. There are thousands of stars in the sky. 368 00:40:43,158 --> 00:40:47,436 Individual chicks, however, learn to recognise star patterns. 369 00:40:52,358 --> 00:40:56,351 Different chicks may select different constellations, 370 00:40:56,518 --> 00:40:59,510 and watch them as they circle around the sky. 371 00:41:03,158 --> 00:41:07,788 By relating the position of their particular group of stars to the North Star, 372 00:41:07,958 --> 00:41:10,472 which remains in a constant position, 373 00:41:10,638 --> 00:41:14,916 the chicks can always find north without requiring an internal clock. 374 00:41:15,078 --> 00:41:21,267 In the southern hemisphere, they use the patch of the night sky around which the stars rotate. 375 00:41:22,118 --> 00:41:27,112 It's a remarkable feat of observation, until it's blacked out by a parent. 376 00:41:31,278 --> 00:41:38,070 Whether they use the sun or the stars, an internal compass or a very detailed memory, 377 00:41:38,238 --> 00:41:42,231 animals achieve immense journeys with great accuracy. 378 00:41:42,398 --> 00:41:45,549 Even relatively simple creatures can navigate 379 00:41:45,718 --> 00:41:51,429 with a skill which human beings have only managed to rival within the past few centuries. 380 00:41:51,598 --> 00:41:58,071 And one of the most extraordinary of all animal journeys comes to its climax right here. 381 00:42:00,198 --> 00:42:02,996 This waterfall on the west coast of Ireland 382 00:42:03,158 --> 00:42:09,233 is the last major obstacle on a journey that began three years ago and 6,000 miles away 383 00:42:09,398 --> 00:42:12,231 on the other side of the Atlantic. 384 00:42:16,638 --> 00:42:21,189 You might suppose that fish capable of making such an immense journey 385 00:42:21,358 --> 00:42:25,033 and then forcing their way up a waterfall like this 386 00:42:25,198 --> 00:42:27,792 would be big, powerful creatures. 387 00:42:27,958 --> 00:42:30,347 Well, these are they. 388 00:42:31,358 --> 00:42:34,350 Elvers. Baby eels. 389 00:42:34,518 --> 00:42:39,512 At this time of the year, this Irish river, like most rivers in western Europe, 390 00:42:39,678 --> 00:42:42,476 is filled with countless millions of them. 391 00:42:42,638 --> 00:42:48,031 And these rocks form a jam-packed motorway, up which they're struggling. 392 00:42:52,718 --> 00:42:56,711 The elvers began their journey in the warm, near-stagnant waters 393 00:42:56,878 --> 00:43:00,871 between Bermuda and the West Indies, the Sargasso Sea. 394 00:43:01,718 --> 00:43:07,156 Here, at a depth of around 2,000 feet, eels lay their eggs. 395 00:43:08,438 --> 00:43:11,430 The hatchlings bear little resemblance to eels. 396 00:43:11,598 --> 00:43:16,592 They have no fins except for a fringe around their transparent, leaf-shaped body. 397 00:43:16,758 --> 00:43:20,751 For two years, they move east across the Atlantic, 398 00:43:20,918 --> 00:43:23,751 aided by the flow of the Gulf Stream. 399 00:43:23,918 --> 00:43:28,309 By the time they reach the continental shelf of Europe, they have become slimmer, 400 00:43:28,478 --> 00:43:32,471 developed fins, and are beginning to look more like eels. 401 00:43:36,398 --> 00:43:41,074 In these coastal seas, they're able to detect the taint of fresh water. 402 00:43:41,238 --> 00:43:45,231 They seem drawn to it, and they swim into the estuaries. 403 00:43:46,118 --> 00:43:51,112 But now the going is hard. Now they have no great oceanic current to aid them. 404 00:43:51,278 --> 00:43:56,671 Now they have to swim against the current to fresh water as it flows down the rivers. 405 00:43:56,838 --> 00:44:03,073 And as they move out of salt water into fresh, the chemistry of their bodies has to change. 406 00:44:08,518 --> 00:44:13,228 Thousands upon thousands of them will die from one cause or another. 407 00:44:33,478 --> 00:44:37,266 0nly a tiny percentage of them get as far as this. 408 00:44:37,438 --> 00:44:42,148 As the rivers narrow, so the battle against the current gets harder. 409 00:44:47,598 --> 00:44:50,749 They continue to travel by day and by night. 410 00:44:50,918 --> 00:44:55,708 Millions of them pass through our riverside towns largely unnoticed. 411 00:44:59,558 --> 00:45:02,994 At the foot of a waterfall, they assemble in swarms, 412 00:45:03,158 --> 00:45:08,949 preparing themselves to wriggle upwards through the sodden vegetation of the banks. 413 00:45:14,358 --> 00:45:17,509 When they clear this final obstacle, 414 00:45:17,678 --> 00:45:21,068 they reach the sheltered, rich waters upstream 415 00:45:21,238 --> 00:45:26,835 where they can rest and feed and grow into adult eels. 416 00:45:29,958 --> 00:45:32,950 They stay here for up to seven years. 417 00:45:37,758 --> 00:45:42,354 Eventually, one autumn, the urge comes upon them to spawn, 418 00:45:42,518 --> 00:45:46,830 and they start on the long journey back to the Sargasso. 419 00:45:48,798 --> 00:45:52,108 The need to return to the sea is so strong 420 00:45:52,278 --> 00:45:56,874 that they will wriggle out of a pond and cross dew-drenched meadows, 421 00:45:57,038 --> 00:46:01,589 if that's necessary to reach a waterway that's running down to the sea. 422 00:46:01,758 --> 00:46:07,947 Down the rivers they go, into the estuaries and out into the deep, open sea. 423 00:46:12,918 --> 00:46:18,515 When the adult eels swim across the continental shelf, they disappear into mystery. 424 00:46:18,678 --> 00:46:22,876 No one has ever caught one more than 50 miles from the coast. 425 00:46:23,038 --> 00:46:28,237 That may be because they swim at a depth that is far beyond the reach of any normal net, 426 00:46:28,398 --> 00:46:34,394 and they can't be caught by a hook with bait on it because they don't feed ever again. 427 00:46:34,558 --> 00:46:39,154 But how do they guide themselves on these astonishing journeys? 428 00:46:39,318 --> 00:46:45,507 Young elvers can't be guided by their parents because they cross the Atlantic by themselves. 429 00:46:45,678 --> 00:46:49,671 Adults can't guide themselves by the sun and the stars 430 00:46:49,838 --> 00:46:54,036 because they swim at such a depth that they can't see the sky. 431 00:46:54,198 --> 00:46:57,190 Maybe they have some kind of in-built compass. 432 00:46:57,358 --> 00:47:01,146 Perhaps they use a sense we haven't yet identified. 433 00:47:01,318 --> 00:47:07,632 We've still got a lot to learn about the ways in which animals find their way around.