Finding the Way The Trials of Life

The end of another African day, and the game animals prepare for night.

Baboons are climbing up into the branches of the thorn trees

and the birds are coming in to roost.

These animals rely upon their eyes to find their way around, as do I.

In a short while, it will be totally dark,

and without a torch, I would be very well advised not to stumble around in the darkness.

But not all animals rely on sight. Others use other senses to find their way around,

and soon they'll 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 reveal other hyenas have been this way

and they add their own signatures by drawing the grass-stems across the glands beneath the tail.

These registrations remain detectable for up to a month,

so each marking station is full of information for them.

The hyenas also deposit their urine and dung in special places.

To them, with their hypersensitive noses,

these dung stations must shine like beacons through the black night.

Bush babies galagos. They use regular pathways through the trees

and mark them with great care.

They urinate on their hands. Now every branch on which they run will be impregnated with smell.

An intruder is quickly detected.

The residents keep a close nose on who is around and whether any females are coming into season.

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. This enables it to find its 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.

The rest of the caterpillars can differentiate between a single trail and a double one.

And they can also tell whether the creator has eaten a good meal.

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 houses birds that use a different system.

These are swiftlets. It's evening and they're pouring into the cave to roost in thousands.

There's still enough light outside for them to see, and they are relatively quiet.

But just listen to them in the blackness of the cave itself!

SHRILL TWITTERING

These ladders were built by locals who come here to collect swiftlets' nests to make bird's-nest soup.

They lead down to a slimy platform 180 feet above the cave floor

but right alongside the nests stuck to the cave wall.

The chorus of clicks you can hear is made by the birds as they fly through the blackness.

Each bird is guiding itself by listening to the echo of its call bouncing off the cave wall.

Amazingly, it's able to distinguish the echo of its own call from that of all the other birds.

So that although there are over a million swiftlets in this cave,

each one can find its way back to its own nest in the blackness.

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.

And the mother has to be able to steer her way through the cave and land beside her baby to suckle it.

The bats' echolocating equipment is much more highly developed than that of the swiftlets.

They have huge ears that constantly twitch to pick up faint echoes.

They also have complex flaps on the nose that concentrate the sounds into a narrow beam.

Most important of all, the frequency of sound they use is very much higher.

As a result, the bats' echolocation is very much more efficient.

So as the swiftlets come back because they cannot see to hunt,

the bats are just setting out.

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 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.

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 different species of catfish in the Amazon

and they're all equipped with long feelers. Some on the throat search for prey in the sand.

Others on the snout reach ahead to detect obstacles to be avoided.

Here on the Amazon, there are also other fish that use the most extraordinary method of all.

Living among this tangle of trunks and branches of the flooded forest,

they find their way about not by touch, or by smell, or by sight or even by echolocation,

but by electricity.

I can fish for them using a device like this.

When I turn it on, it emits a stream of electronic pulses at either end

which these fish, with their extreme sensitivity to electricity, should find irresistible. Let's see.

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.

But they also generate continuous low-voltage signals

that enable them to visualise their surroundings and maybe even 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.

So for dark places as well as dark hours, animals have developed many techniques to find their way about.

But for many, the time of activity is during the light.

The rufous elephant shrew of Africa guides itself with its eyes as it careers along.

It spends three quarters of its day keeping its tracks clear.

A single twig could trip it up and bring disaster.

Its safety depends on knowing every twist and curve so that it can outrun its enemies

like the black-shouldered kite.

If it's really threatened by a bird, it can take shortcuts,

leaving one trail and going onto another to outsmart its enemy.

Clearly elephant shrews have a good mental picture of the layout of their trails.

It is useful for escaping predators and also for finding food.

Each autumn in English oakwoods, 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 throughout its territory.

It relies on these for food during the winter months.

All through the spring and early summer it keeps on recovering them.

It must remember where many are for its recovery rate is much greater than if it were by chance.

It seems that just as we may remember the position of a shop by relating it to a big building,

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.

With temperatures up to 50 degrees, getting lost here could be lethal.

And yet this is the home of one of the most remarkable animal travellers

an ant that leaves its sandy home

and sets out on the longest overland journey made by any insect. It's called cataglyphis.

It emerges in the middle of the day when others die from the heat.

It searches for these casualties when it's so hot even it must seek relief from the burning surface.

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 that even cataglyphis has to get back as quickly as possible so as not to risk death.

These journeys are equivalent in human terms to a trek of 40 miles over featureless territory.

And yet the ants, even if they wander about in searching for food, can return directly to their nest.

How do they achieve that?

Have a closer look at one leaving on a journey.

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...and checks the sun and the pattern of polarised light.

It can measure the distance between stops, and it always takes a bearing at every one.

When it eventually finds food a quick calculation and it knows exactly the shortest way home.

If you can use a beacon like the sun, then you're no longer restricted to your home ground.

You can venture into unknown territory, go long distances to find new feeding grounds.

Great journeys are now possible.

The death's-head hawkmoth 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 mph, but they continue doggedly on.

A few may cross the Channel.

Now they're exhausted, and they find one sight irresistible hives of honeybees.

And the hungry traveller restores its energy with stolen honey,

before it looks for potato plants on which it will lay its eggs.

Honeybees not only steer by the sun, they use it to pass on instructions to one another.

When a forager finds 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 they must fly out at the same angle to the sun.

So other workers are able to fly off directly to the same flower.

As the day goes on, the sun, of course, moves.

In the hive, the original forager often continues dancing for hours, unable to see the sun's movement.

But remarkably, to match the sun's movement, 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.

But the sun isn't visible to all. 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 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 current and swell which stays constant.

Lines join together into longer lines.

Sometimes 60 lobsters will be 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 for marching this way.

If they are threatened, they can form defensive circles.

A trigger fish one of their main enemies.

It wants to attack the legs, but can't get past the sharp 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 reefs that run along the edge of the ocean drop-off,

they abandon the caravans and each makes its own way.

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 30 miles but aren't the greatest marine migrants.

These same reefs are the feeding ground of green turtles. They, like the lobsters, do not breed 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.

Others in the Pacific head for the small cluster of the Galapagos.

They have to come to the surface regularly to breathe, and they may therefore use 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.

In this deep blue water, they may be guided by the Earth's magnetic field.

They have particles of iron-oxide in their heads

and these must be sensitive to the earth's magnetism, just as our own magnetic compasses are.

As they near the islands, they may also detect the fresh water that flows from them, however faintly.

By swimming so that the taste grows stronger, they reach the Galapagos.

Here they meet others and here they mate.

The sheltered beaches provide the females with nesting sites.

Weeks later, after the adults have resumed their wanderings, the young dig their way to the surface.

As they enter the sea,

they get a taste of the coastal water they'll remember for 30 years.

For it's only after 30 years that they breed.

Then they will use that memory to guide them back to mate on these very same beaches.

This is the high Arctic Spitzbergen.

It's midnight, although the sun is high in the sky, because we're only 600 miles from the North Pole.

The sea is usually covered with ice but in the brief summer, it melts

and it's the time that the Arctic tern comes here to nest. It's at the edge of its range.

No bird nests farther north than this.

And there's a very good reason for the birds to come here.

24 hours of daylight means 24 hours in which to get food for the chicks.

Fishing need never stop.

The sea is so rich that the chicks grow faster here

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 to reach a place

which is as far away from here as it is possible to be, without leaving the planet.

By the beginning of August, darkness is returning and the temperature is falling.

The sea will be covered with ice and fishing will be impossible.

The terns must leave and start their 12,000 mile journey south.

The juveniles, who fed so continuously and grew so fast,

are now strong enough to follow their parents.

From Spitzbergen they head for Norway.

Then south, down the coast of Scandinavia, past Britain, 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 across the Southern Ocean.

Eventually they reach the ice again Antarctic ice.

They have followed the sun to the edge of the southern continent.

And here, of course, the summer is just beginning and there is round-the-clock fishing.

For eight months of the year, they never see the sun set.

And then, once more, the adults head off on the 12,000 mile journey back to Spitzbergen to breed again.

-AGITATED ALARM-CALLS

-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 in the Antarctic, the pair separated,

but they reunite once they come back here onto their own patch of shingle.

What's more, they do that year after year.

One pair, here in Spitzbergen, have been known to do it for 18 years.

Such accurate route-finding can't be achieved simply by following a compass direction.

So in addition to a compass, you have to have a map. 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 to Alaska.

No other tropical bird ventures as far north as this,

and here it will spend the summer.

CHEEPING

During these short weeks, there's a rich supply of nectar and insects for its young.

Only the female rears the chicks, so in June, the male starts the 4,000 mile journey 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 hummingbird's migration is even more impressive than the terns.

They follow the mountain chains half flying down the Rockies, the others down the Sierra Nevada.

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.

The young birds find these meadows on their first journey south, and return to the same ones each year.

They continue south along the canyons of Utah and Colorado.

They must be unforgettable landmarks on the route-map they use to find their way.

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 providing 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 area

and the same territorial knowledge that helps the jay find acorns takes them to the same bush.

But not all birds use geographical features as guides. The royal albatross migrates over the sea.

One can claim to be the greatest animal traveller of all.

Here in Taiaroa Head on 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 the Antarctic until she was ready to breed.

And in that year, she bred here for the very first time.

In the half century since then, she has come back here alternate years,

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.

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 an albatross may fly 800 miles to collect food for a chick

and still find their way back to their nest on a tiny island in the Southern Ocean.

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. They're visible many miles away.

It could be that the sun gives them navigational information.

The nearer you are to the Pole, the lower it will be at midday.

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.

But they do have remarkable abilities to use celestial clues.

SQUAWKING

Research shows many birds with a view of the sky from 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.

CHEEPING

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 stays in a constant position, the chicks can always find north without an internal clock.

In the southern hemisphere, they use the patch of the night sky around which the stars rotate...

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.

One of the most extraordinary journeys comes to its climax 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 across the Atlantic.

You might suppose that fish capable of such an immense journey

and then forcing their way up a waterfall, would be big, powerful creatures.

Well, these are they!

Elvers baby eels. And 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 struggle.

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.

Hatchlings don't look like eels.

They have no fins, barring a fringe around their 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 Europe, they have become slimmer,

developed fins, and are beginning to look more like eels.

In these coastal seas they can detect fresh water.

They seem drawn to it and they swim into the estuaries.

But now they have no oceanic current to aid them.

Now indeed they have to swim against the current to fresh water.

Moving from salt water to fresh, their body chemistry must change.

Thousands upon thousands of them will die from one cause or another.

Only a tiny percentage of them get as far as this. As the rivers narrow, the going gets harder.

They continue to travel by day and by night.

Millions of them pass through our riverside towns unnoticed.

They assemble below waterfalls, preparing 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 leave a pond

and cross dew-drenched meadows, if that's necessary to reach a waterway to the sea.

Down the rivers they go, into the estuaries and out into the sea.

When the adult eels swim across the Continental Shelf, they disappear.

No-one has ever caught one more than 50 miles from the coast.

That may be because they swim at such a depth they evade nets,

and they can't be caught by a baited hook, because they don't feed ever again in their lives.

How do they guide themselves on these astonishing journeys?

Young elvers aren't guided by their parents. They cross alone.

And as adults, they can't guide themselves by the sun and the stars because they swim at great depths.

Maybe they have some kind of inbuilt compass. Perhaps they use a sense we haven't yet identified.

The fact is, we've still got a lot to learn about the ways in which animals find their way around.

Subtitles by Alison Loudon BBC Scotland 1990