0:01:26 > 0:01:33The end of another African day, and the game animals prepare for night.
0:01:33 > 0:01:38Baboons are climbing up into the branches of the thorn trees
0:01:38 > 0:01:42and the birds are coming in to roost.
0:01:42 > 0:01:48These animals rely upon their eyes to find their way around, as do I.
0:01:48 > 0:01:51In a short while, it will be totally dark,
0:01:51 > 0:01:57and without a torch, I would be very well advised not to stumble around in the darkness.
0:01:57 > 0:02:04But not all animals rely on sight. Others use other senses to find their way around,
0:02:04 > 0:02:07and soon they'll be venturing out.
0:02:19 > 0:02:26Spotted hyena. They hunt almost entirely during the hours of darkness.
0:02:26 > 0:02:30They may travel up to 60 miles in one night,
0:02:30 > 0:02:36and they rely very much on smell to find their way around.
0:02:36 > 0:02:41Specially scented signposts reveal other hyenas have been this way
0:02:41 > 0:02:48and they add their own signatures by drawing the grass-stems across the glands beneath the tail.
0:02:52 > 0:02:57These registrations remain detectable for up to a month,
0:02:57 > 0:03:01so each marking station is full of information for them.
0:03:06 > 0:03:11The hyenas also deposit their urine and dung in special places.
0:03:11 > 0:03:15To them, with their hypersensitive noses,
0:03:15 > 0:03:21these dung stations must shine like beacons through the black night.
0:03:34 > 0:03:41Bush babies galagos. They use regular pathways through the trees
0:03:41 > 0:03:44and mark them with great care.
0:03:44 > 0:03:54They urinate on their hands. Now every branch on which they run will be impregnated with smell.
0:03:58 > 0:04:02An intruder is quickly detected.
0:04:02 > 0:04:10The residents keep a close nose on who is around and whether any females are coming into season.
0:04:31 > 0:04:36Tent caterpillars are also on the move during the night,
0:04:36 > 0:04:41marching out from their silken camp in search of food.
0:04:42 > 0:04:51When they've stripped one bush of its leaves, a single scout sets out to find a new supply.
0:04:51 > 0:05:00Scent glands on its rear end leave a trail of smell. This enables it to find its way back to the tent.
0:05:12 > 0:05:20A leaf a meal. Having eaten its fill, it heads back, following its own scent trail.
0:05:23 > 0:05:26But it's still laying down scent.
0:05:26 > 0:05:35The rest of the caterpillars can differentiate between a single trail and a double one.
0:05:35 > 0:05:41And they can also tell whether the creator has eaten a good meal.
0:05:41 > 0:05:45So they know whether the trail will lead them to food.
0:05:45 > 0:05:50Before long, a network of smelly pathways covers the branches,
0:05:50 > 0:05:58ensuring that the caterpillars lose no time in the constant rush for food.
0:06:10 > 0:06:15Smell isn't the only way of finding your way in the dark.
0:06:15 > 0:06:20This massive cave in Borneo houses birds that use a different system.
0:06:20 > 0:06:28These are swiftlets. It's evening and they're pouring into the cave to roost in thousands.
0:06:28 > 0:06:35There's still enough light outside for them to see, and they are relatively quiet.
0:06:35 > 0:06:40But just listen to them in the blackness of the cave itself!
0:06:40 > 0:06:44SHRILL TWITTERING
0:06:48 > 0:06:55These ladders were built by locals who come here to collect swiftlets' nests to make bird's-nest soup.
0:06:55 > 0:07:01They lead down to a slimy platform 180 feet above the cave floor
0:07:01 > 0:07:05but right alongside the nests stuck to the cave wall.
0:07:05 > 0:07:12The chorus of clicks you can hear is made by the birds as they fly through the blackness.
0:07:12 > 0:07:19Each bird is guiding itself by listening to the echo of its call bouncing off the cave wall.
0:07:19 > 0:07:26Amazingly, it's able to distinguish the echo of its own call from that of all the other birds.
0:07:26 > 0:07:31So that although there are over a million swiftlets in this cave,
0:07:31 > 0:07:37each one can find its way back to its own nest in the blackness.
0:07:41 > 0:07:47Birds aren't the only animals to have evolved echolocation.
0:07:47 > 0:07:52It was developed first by mammals bats.
0:07:52 > 0:07:57This sea of pink is a mass of naked, newly-born bats.
0:07:57 > 0:08:06And the mother has to be able to steer her way through the cave and land beside her baby to suckle it.
0:08:06 > 0:08:14The bats' echolocating equipment is much more highly developed than that of the swiftlets.
0:08:14 > 0:08:19They have huge ears that constantly twitch to pick up faint echoes.
0:08:21 > 0:08:29They also have complex flaps on the nose that concentrate the sounds into a narrow beam.
0:08:30 > 0:08:35Most important of all, the frequency of sound they use is very much higher.
0:08:35 > 0:08:41As a result, the bats' echolocation is very much more efficient.
0:08:41 > 0:08:46So as the swiftlets come back because they cannot see to hunt,
0:08:46 > 0:08:49the bats are just setting out.
0:08:49 > 0:08:57They can find and catch insect prey less than half a millimetre across in the pitch blackness.
0:08:59 > 0:09:05Caves are not the only places that are permanently dark. So are some rivers.
0:09:05 > 0:09:13Much of the Amazon is thick with suspended mud, and here, another mammal has developed echolocation.
0:09:18 > 0:09:26The river dolphin has a bulge on its forehead through which it transmits beams of ultrasound.
0:09:26 > 0:09:33Although it's virtually blind, echolocation enables it to avoid obstacles in its path,
0:09:33 > 0:09:36and catch even the smallest fish.
0:09:51 > 0:09:59This same river is home to other animals that literally feel their way through the gloom.
0:10:09 > 0:10:14There are several hundred different species of catfish in the Amazon
0:10:14 > 0:10:21and they're all equipped with long feelers. Some on the throat search for prey in the sand.
0:10:25 > 0:10:31Others on the snout reach ahead to detect obstacles to be avoided.
0:10:42 > 0:10:47Here on the Amazon, there are also other fish that use the most extraordinary method of all.
0:10:49 > 0:10:54Living among this tangle of trunks and branches of the flooded forest,
0:10:54 > 0:11:02they find their way about not by touch, or by smell, or by sight or even by echolocation,
0:11:02 > 0:11:05but by electricity.
0:11:05 > 0:11:09I can fish for them using a device like this.
0:11:09 > 0:11:15When I turn it on, it emits a stream of electronic pulses at either end
0:11:15 > 0:11:22which these fish, with their extreme sensitivity to electricity, should find irresistible. Let's see.
0:11:31 > 0:11:36And here they are within seconds electric eels, six feet long.
0:11:37 > 0:11:45They can discharge massive electric shocks which can stun and even kill their prey.
0:11:45 > 0:11:48But they also generate continuous low-voltage signals
0:11:48 > 0:11:53that enable them to visualise their surroundings and maybe even to recognise one another.
0:11:53 > 0:11:56So they're very interested in my version.
0:12:14 > 0:12:22The electric field they create around themselves is distorted by any object in the water,
0:12:22 > 0:12:29and they detect these changes with a line of sensory cells along their flanks.
0:12:29 > 0:12:37So for dark places as well as dark hours, animals have developed many techniques to find their way about.
0:12:37 > 0:12:42But for many, the time of activity is during the light.
0:12:49 > 0:12:56The rufous elephant shrew of Africa guides itself with its eyes as it careers along.
0:12:56 > 0:13:02It spends three quarters of its day keeping its tracks clear.
0:13:02 > 0:13:05A single twig could trip it up and bring disaster.
0:13:05 > 0:13:13Its safety depends on knowing every twist and curve so that it can outrun its enemies
0:13:13 > 0:13:17like the black-shouldered kite.
0:13:35 > 0:13:40If it's really threatened by a bird, it can take shortcuts,
0:13:40 > 0:13:47leaving one trail and going onto another to outsmart its enemy.
0:13:47 > 0:13:55Clearly elephant shrews have a good mental picture of the layout of their trails.
0:13:55 > 0:14:01It is useful for escaping predators and also for finding food.
0:14:04 > 0:14:09Each autumn in English oakwoods, jays find and bury acorns,
0:14:09 > 0:14:14giving each one its own hiding place and covering it with a leaf.
0:14:21 > 0:14:29In one season, a jay will bury several thousand acorns throughout its territory.
0:14:29 > 0:14:34It relies on these for food during the winter months.
0:14:37 > 0:14:43All through the spring and early summer it keeps on recovering them.
0:14:43 > 0:14:52It must remember where many are for its recovery rate is much greater than if it were by chance.
0:14:52 > 0:15:00It seems that just as we may remember the position of a shop by relating it to a big building,
0:15:00 > 0:15:08so the jays use prominent trees as landmarks, and tend to bury their acorns around them.
0:15:10 > 0:15:15Jays live in places that are full of distinctive features,
0:15:15 > 0:15:19but all animals are not so lucky.
0:15:22 > 0:15:26This must be the easiest place in the world to get lost.
0:15:26 > 0:15:31I'm in the great sea of sand in the eastern Sahara.
0:15:31 > 0:15:38Behind me, to the south, wave upon wave of dunes stretch for hundreds of miles.
0:15:38 > 0:15:44It would be hard to imagine a landscape with fewer features.
0:15:44 > 0:15:49With temperatures up to 50 degrees, getting lost here could be lethal.
0:15:49 > 0:15:57And yet this is the home of one of the most remarkable animal travellers
0:15:57 > 0:16:00an ant that leaves its sandy home
0:16:00 > 0:16:08and sets out on the longest overland journey made by any insect. It's called cataglyphis.
0:16:08 > 0:16:13It emerges in the middle of the day when others die from the heat.
0:16:13 > 0:16:22It searches for these casualties when it's so hot even it must seek relief from the burning surface.
0:16:26 > 0:16:32At first it forages randomly over the sand.
0:16:32 > 0:16:36But when it finds its exhausted prey,
0:16:36 > 0:16:41astonishingly it returns in a dead straight line to its nest.
0:16:43 > 0:16:52It's so hot that even cataglyphis has to get back as quickly as possible so as not to risk death.
0:16:54 > 0:17:02These journeys are equivalent in human terms to a trek of 40 miles over featureless territory.
0:17:02 > 0:17:10And yet the ants, even if they wander about in searching for food, can return directly to their nest.
0:17:10 > 0:17:13How do they achieve that?
0:17:13 > 0:17:18Have a closer look at one leaving on a journey.
0:17:18 > 0:17:22It keeps stopping and making a turn.
0:17:23 > 0:17:26Stop and turn.
0:17:26 > 0:17:29Stop and turn.
0:17:37 > 0:17:43As it turns, it looks up at the sun, checking its position.
0:17:43 > 0:17:49It moves on...and checks the sun and the pattern of polarised light.
0:17:49 > 0:17:57It can measure the distance between stops, and it always takes a bearing at every one.
0:17:57 > 0:18:06When it eventually finds food a quick calculation and it knows exactly the shortest way home.
0:18:15 > 0:18:23If you can use a beacon like the sun, then you're no longer restricted to your home ground.
0:18:23 > 0:18:29You can venture into unknown territory, go long distances to find new feeding grounds.
0:18:29 > 0:18:32Great journeys are now possible.
0:18:32 > 0:18:39The death's-head hawkmoth lives in Africa, but every year, some, seeking new territory,
0:18:39 > 0:18:44fly across the Mediterranean, keeping the setting sun to the left.
0:18:46 > 0:18:53They fly right through the night, using the moon to hold their northward course.
0:18:56 > 0:19:03They continue into Europe, climbing higher to cross the Alps, and then on into France.
0:19:03 > 0:19:08Their speed is only about 15 mph, but they continue doggedly on.
0:19:08 > 0:19:11A few may cross the Channel.
0:19:20 > 0:19:28Now they're exhausted, and they find one sight irresistible hives of honeybees.
0:19:42 > 0:19:48And the hungry traveller restores its energy with stolen honey,
0:19:48 > 0:19:52before it looks for potato plants on which it will lay its eggs.
0:19:58 > 0:20:04Honeybees not only steer by the sun, they use it to pass on instructions to one another.
0:20:05 > 0:20:09When a forager finds nectar in newly-opened flowers,
0:20:09 > 0:20:17it fills its crop and flies back to the hive, guiding itself by the position of the sun.
0:20:17 > 0:20:20And inside it dances.
0:20:22 > 0:20:25It waggles across the comb
0:20:25 > 0:20:29so that the angle of its waggled path to the vertical
0:20:29 > 0:20:35tells the other bees they must fly out at the same angle to the sun.
0:20:39 > 0:20:45So other workers are able to fly off directly to the same flower.
0:20:52 > 0:20:57As the day goes on, the sun, of course, moves.
0:21:02 > 0:21:10In the hive, the original forager often continues dancing for hours, unable to see the sun's movement.
0:21:14 > 0:21:22But remarkably, to match the sun's movement, the dancer steadily shifts the direction of its dance.
0:21:42 > 0:21:50So the continuous stream of departing workers are always given the correct angle of flight.
0:21:50 > 0:21:58All animals that steer by the sun must be able to compensate for its movements in this way.
0:21:58 > 0:22:05But the sun isn't visible to all. What do you do, for instance, if you live underwater?
0:22:07 > 0:22:12In the calm shallow seas of the Bahamas live spiny lobsters.
0:22:12 > 0:22:19Lobsters like calm, clear water, but in autumn the Bahamas are swept by serious storms.
0:22:27 > 0:22:33Suddenly, as the waters become cloudy, the lobsters decide to move
0:22:33 > 0:22:36and seek refuge at greater depths.
0:22:55 > 0:22:59They usually start in the evening, travelling in pairs.
0:22:59 > 0:23:05By morning, the pairs have joined into long columns.
0:23:22 > 0:23:29In queues 30 or 40 strong, they head for the drop-off on the ocean side of the lagoon.
0:23:38 > 0:23:46It seems that they know the way from the overall current and swell which stays constant.
0:23:53 > 0:23:58Lines join together into longer lines.
0:23:58 > 0:24:02Sometimes 60 lobsters will be one behind the other.
0:24:02 > 0:24:06The migration takes place within a few days each year
0:24:06 > 0:24:11and then the whole lagoon floor is covered with parallel marching columns.
0:24:11 > 0:24:19Travelling in line reduces the drag of the water on an individual by as much as half.
0:24:19 > 0:24:22But there's another reason for marching this way.
0:24:22 > 0:24:28If they are threatened, they can form defensive circles.
0:24:28 > 0:24:32A trigger fish one of their main enemies.
0:24:32 > 0:24:37It wants to attack the legs, but can't get past the sharp antennae.
0:24:46 > 0:24:50But a solitary traveller is in trouble.
0:24:57 > 0:25:00First, it's disarmed.
0:25:02 > 0:25:05Then the rest is easy.
0:25:26 > 0:25:32There are others ready to pick flesh from the broken limbs.
0:25:35 > 0:25:40Within a few minutes, all that is left is an empty shell.
0:25:51 > 0:25:59When the survivors reach the reefs that run along the edge of the ocean drop-off,
0:25:59 > 0:26:03they abandon the caravans and each makes its own way.
0:26:05 > 0:26:09They clamber down the slope to even greater depths
0:26:09 > 0:26:16where they will be safe from the storms that churn the waters hundreds of feet above.
0:26:18 > 0:26:23Lobsters travel 30 miles but aren't the greatest marine migrants.
0:26:23 > 0:26:32These same reefs are the feeding ground of green turtles. They, like the lobsters, do not breed here.
0:26:32 > 0:26:39To do that, they must leave the reef and head out into the open ocean.
0:26:39 > 0:26:44Those on the eastern coast of South America swim for 1,000 miles
0:26:44 > 0:26:49to the tiny island of Ascension in the middle of the Atlantic.
0:26:49 > 0:26:54Others in the Pacific head for the small cluster of the Galapagos.
0:26:57 > 0:27:05They have to come to the surface regularly to breathe, and they may therefore use the sun as a guide.
0:27:05 > 0:27:12The direction of the waves and the ocean swell may also provide clues.
0:27:15 > 0:27:22But they also swim at greater depths and take advantage of the powerful currents.
0:27:22 > 0:27:27In this deep blue water, they may be guided by the Earth's magnetic field.
0:27:27 > 0:27:31They have particles of iron-oxide in their heads
0:27:31 > 0:27:39and these must be sensitive to the earth's magnetism, just as our own magnetic compasses are.
0:27:44 > 0:27:53As they near the islands, they may also detect the fresh water that flows from them, however faintly.
0:27:53 > 0:27:59By swimming so that the taste grows stronger, they reach the Galapagos.
0:28:01 > 0:28:05Here they meet others and here they mate.
0:28:13 > 0:28:18The sheltered beaches provide the females with nesting sites.
0:28:31 > 0:28:38Weeks later, after the adults have resumed their wanderings, the young dig their way to the surface.
0:28:56 > 0:28:59As they enter the sea,
0:28:59 > 0:29:05they get a taste of the coastal water they'll remember for 30 years.
0:29:05 > 0:29:10For it's only after 30 years that they breed.
0:29:10 > 0:29:17Then they will use that memory to guide them back to mate on these very same beaches.
0:29:53 > 0:29:58This is the high Arctic Spitzbergen.
0:29:58 > 0:30:06It's midnight, although the sun is high in the sky, because we're only 600 miles from the North Pole.
0:30:06 > 0:30:12The sea is usually covered with ice but in the brief summer, it melts
0:30:12 > 0:30:19and it's the time that the Arctic tern comes here to nest. It's at the edge of its range.
0:30:19 > 0:30:25No bird nests farther north than this.
0:30:25 > 0:30:31And there's a very good reason for the birds to come here.
0:30:31 > 0:30:3724 hours of daylight means 24 hours in which to get food for the chicks.
0:30:37 > 0:30:40Fishing need never stop.
0:30:40 > 0:30:46The sea is so rich that the chicks grow faster here
0:30:46 > 0:30:50than anywhere else in the Arctic tern's range.
0:30:53 > 0:30:58This tiny little chick, only a few days old,
0:30:58 > 0:31:06in a few weeks' time before the ice returns, will have to set out to fly south to reach a place
0:31:06 > 0:31:13which is as far away from here as it is possible to be, without leaving the planet.
0:31:13 > 0:31:21By the beginning of August, darkness is returning and the temperature is falling.
0:31:21 > 0:31:24The sea will be covered with ice and fishing will be impossible.
0:31:24 > 0:31:29The terns must leave and start their 12,000 mile journey south.
0:31:31 > 0:31:36The juveniles, who fed so continuously and grew so fast,
0:31:36 > 0:31:41are now strong enough to follow their parents.
0:31:41 > 0:31:45From Spitzbergen they head for Norway.
0:31:45 > 0:31:52Then south, down the coast of Scandinavia, past Britain, on to southern Europe and North Africa.
0:32:01 > 0:32:08It's a continuous two-month flight, and the birds feed, drink and sleep at sea.
0:32:13 > 0:32:16They continue, following the coast down to the Cape of Good Hope
0:32:16 > 0:32:18and then across the Southern Ocean.
0:32:28 > 0:32:33Eventually they reach the ice again Antarctic ice.
0:32:33 > 0:32:38They have followed the sun to the edge of the southern continent.
0:32:38 > 0:32:45And here, of course, the summer is just beginning and there is round-the-clock fishing.
0:32:45 > 0:32:50For eight months of the year, they never see the sun set.
0:32:50 > 0:32:59And then, once more, the adults head off on the 12,000 mile journey back to Spitzbergen to breed again.
0:33:03 > 0:33:09- AGITATED ALARM-CALLS - These parent birds,
0:33:09 > 0:33:17so vigorously defending their nest, lay their eggs within a few inches of the previous year's nest site.
0:33:17 > 0:33:22When they were in the Antarctic, the pair separated,
0:33:22 > 0:33:29but they reunite once they come back here onto their own patch of shingle.
0:33:29 > 0:33:33What's more, they do that year after year.
0:33:33 > 0:33:39One pair, here in Spitzbergen, have been known to do it for 18 years.
0:33:39 > 0:33:46Such accurate route-finding can't be achieved simply by following a compass direction.
0:33:46 > 0:33:53So in addition to a compass, you have to have a map. You have to navigate.
0:33:53 > 0:34:02This rufous hummingbird has a route-map of the Rocky Mountain Chain in its brain.
0:34:02 > 0:34:06It's used it to fly from Mexico to Alaska.
0:34:06 > 0:34:11No other tropical bird ventures as far north as this,
0:34:11 > 0:34:15and here it will spend the summer.
0:34:20 > 0:34:23CHEEPING
0:34:25 > 0:34:32During these short weeks, there's a rich supply of nectar and insects for its young.
0:34:36 > 0:34:44Only the female rears the chicks, so in June, the male starts the 4,000 mile journey to Mexico.
0:34:49 > 0:34:53The female stays a week longer to feed the chicks.
0:34:53 > 0:34:58Then she will leave them and they will follow independently.
0:35:05 > 0:35:13If you consider body-size, the hummingbird's migration is even more impressive than the terns.
0:35:13 > 0:35:20They follow the mountain chains half flying down the Rockies, the others down the Sierra Nevada.
0:35:24 > 0:35:31For tiny birds weighing only three grams, the flight demands great expenditure of energy
0:35:31 > 0:35:35and they have to find flowers to refuel.
0:35:35 > 0:35:42Up in the mountains, the shrinking snows have exposed meadows where flowers are in bloom.
0:35:42 > 0:35:51The young birds find these meadows on their first journey south, and return to the same ones each year.
0:35:57 > 0:36:02They continue south along the canyons of Utah and Colorado.
0:36:02 > 0:36:10They must be unforgettable landmarks on the route-map they use to find their way.
0:36:10 > 0:36:15After two months, they reach the mountains of southern Mexico,
0:36:15 > 0:36:18where they will spend the winter.
0:36:21 > 0:36:27This is a rich tropical area full of flowering plants providing nectar for the winter.
0:36:41 > 0:36:47These birds do not return just to the same general area.
0:36:47 > 0:36:52Each winter, many are found back on the same flowering bush.
0:36:56 > 0:37:04They're highly territorial, and use traditional perches to defend their patch,
0:37:04 > 0:37:09calling to warn off intruders.
0:37:09 > 0:37:14A large-scale mental map gets them back to the right area
0:37:14 > 0:37:22and the same territorial knowledge that helps the jay find acorns takes them to the same bush.
0:37:25 > 0:37:33But not all birds use geographical features as guides. The royal albatross migrates over the sea.
0:37:33 > 0:37:38One can claim to be the greatest animal traveller of all.
0:37:38 > 0:37:43Here in Taiaroa Head on South Island, New Zealand, back in 1937,
0:37:43 > 0:37:48a young female albatross was given an identification ring.
0:37:48 > 0:37:55She had spent the previous eight years flying round the Antarctic until she was ready to breed.
0:37:55 > 0:38:00And in that year, she bred here for the very first time.
0:38:00 > 0:38:05In the half century since then, she has come back here alternate years,
0:38:05 > 0:38:09in between times making more circuits of Antarctica.
0:38:09 > 0:38:13She's affectionately known as Grandma.
0:38:13 > 0:38:20She hasn't reappeared this season so presumably she's still out at sea.
0:38:20 > 0:38:24She's certainly the best-travelled animal we know about.
0:38:24 > 0:38:28But all albatross are superb aeronauts.
0:39:06 > 0:39:10By using tags that can be traced by satellite,
0:39:10 > 0:39:15we know an albatross may fly 800 miles to collect food for a chick
0:39:15 > 0:39:22and still find their way back to their nest on a tiny island in the Southern Ocean.
0:39:32 > 0:39:39Maybe they recognise the patterns made by the waves on the surface of the sea.
0:39:39 > 0:39:47Perhaps the clouds that build up over oceanic islands may help them. They're visible many miles away.
0:39:47 > 0:39:51It could be that the sun gives them navigational information.
0:39:51 > 0:39:56The nearer you are to the Pole, the lower it will be at midday.
0:39:56 > 0:40:03So if you have an accurate sense of time, the sun's altitude will tell you your latitude.
0:40:03 > 0:40:08So far there is no evidence that birds can navigate in this way.
0:40:08 > 0:40:15But they do have remarkable abilities to use celestial clues.
0:40:15 > 0:40:18SQUAWKING
0:40:20 > 0:40:25Research shows many birds with a view of the sky from their nest
0:40:25 > 0:40:29learn to orientate themselves by the stars.
0:40:29 > 0:40:36This is far harder than using the sun. There are thousands of stars in the sky.
0:40:36 > 0:40:38CHEEPING
0:40:40 > 0:40:46Individual chicks, however, learn to recognise star patterns.
0:40:50 > 0:40:58Different chicks may select different constellations and watch them as they circle around the sky.
0:40:58 > 0:41:06By relating the position of their particular group of stars to the North Star,
0:41:06 > 0:41:13which stays in a constant position, the chicks can always find north without an internal clock.
0:41:13 > 0:41:21In the southern hemisphere, they use the patch of the night sky around which the stars rotate...
0:41:21 > 0:41:24until it's blacked out by a parent!
0:41:29 > 0:41:36Whether they use the sun or the stars, an internal compass, or a very detailed memory,
0:41:36 > 0:41:40animals achieve immense journeys with great accuracy.
0:41:40 > 0:41:45Even relatively simple creatures can navigate with a skill
0:41:45 > 0:41:50which human beings have only managed to rival within the past few centuries.
0:41:50 > 0:41:56One of the most extraordinary journeys comes to its climax here.
0:41:57 > 0:42:01This waterfall on the west coast of Ireland
0:42:01 > 0:42:06is the last major obstacle on a journey that began three years ago
0:42:06 > 0:42:10and 6,000 miles away across the Atlantic.
0:42:15 > 0:42:20You might suppose that fish capable of such an immense journey
0:42:20 > 0:42:27and then forcing their way up a waterfall, would be big, powerful creatures.
0:42:27 > 0:42:29Well, these are they!
0:42:29 > 0:42:35Elvers baby eels. And at this time of the year,
0:42:35 > 0:42:43this Irish river, like most rivers in Western Europe, is filled with countless millions of them.
0:42:43 > 0:42:48And these rocks form a jam-packed motorway, up which they struggle.
0:42:50 > 0:42:55The elvers began their journey in the warm, near-stagnant waters
0:42:55 > 0:42:59between Bermuda and the West Indies the Sargasso Sea.
0:42:59 > 0:43:05Here, at a depth of around 2,000 feet, eels lay their eggs.
0:43:05 > 0:43:09Hatchlings don't look like eels.
0:43:09 > 0:43:14They have no fins, barring a fringe around their leaf-shaped body.
0:43:14 > 0:43:22For two years they move east across the Atlantic, aided by the flow of the Gulf Stream.
0:43:22 > 0:43:26By the time they reach Europe, they have become slimmer,
0:43:26 > 0:43:30developed fins, and are beginning to look more like eels.
0:43:34 > 0:43:39In these coastal seas they can detect fresh water.
0:43:39 > 0:43:45They seem drawn to it and they swim into the estuaries.
0:43:45 > 0:43:49But now they have no oceanic current to aid them.
0:43:49 > 0:43:56Now indeed they have to swim against the current to fresh water.
0:43:56 > 0:44:01Moving from salt water to fresh, their body chemistry must change.
0:44:05 > 0:44:11Thousands upon thousands of them will die from one cause or another.
0:44:32 > 0:44:40Only a tiny percentage of them get as far as this. As the rivers narrow, the going gets harder.
0:44:44 > 0:44:49They continue to travel by day and by night.
0:44:49 > 0:44:55Millions of them pass through our riverside towns unnoticed.
0:44:58 > 0:45:05They assemble below waterfalls, preparing to wriggle upwards
0:45:05 > 0:45:08through the sodden vegetation of the banks.
0:45:11 > 0:45:16When they clear this final obstacle,
0:45:16 > 0:45:20they reach the sheltered, rich waters upstream,
0:45:20 > 0:45:25where they can rest and feed and grow into adult eels.
0:45:30 > 0:45:33They stay here for up to seven years.
0:45:36 > 0:45:41Eventually, one autumn, the urge comes upon them to spawn,
0:45:41 > 0:45:47and they start on the long journey back to the Sargasso.
0:45:47 > 0:45:52The need to return to the sea is so strong that they will leave a pond
0:45:52 > 0:46:00and cross dew-drenched meadows, if that's necessary to reach a waterway to the sea.
0:46:00 > 0:46:06Down the rivers they go, into the estuaries and out into the sea.
0:46:11 > 0:46:16When the adult eels swim across the Continental Shelf, they disappear.
0:46:16 > 0:46:21No-one has ever caught one more than 50 miles from the coast.
0:46:21 > 0:46:26That may be because they swim at such a depth they evade nets,
0:46:26 > 0:46:33and they can't be caught by a baited hook, because they don't feed ever again in their lives.
0:46:33 > 0:46:38How do they guide themselves on these astonishing journeys?
0:46:38 > 0:46:43Young elvers aren't guided by their parents. They cross alone.
0:46:43 > 0:46:52And as adults, they can't guide themselves by the sun and the stars because they swim at great depths.
0:46:52 > 0:46:59Maybe they have some kind of inbuilt compass. Perhaps they use a sense we haven't yet identified.
0:46:59 > 0:47:08The fact is, we've still got a lot to learn about the ways in which animals find their way around.
0:47:31 > 0:47:36Subtitles by Alison Loudon BBC Scotland 1990