The Trials of Life (1990) s01e05 Episode Script

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