All language subtitles for The Trials of Life s01e05 Finding the Way.eng

(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,

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 of...patch 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.

Repair and Synchronization by
Easy Subtitles Synchronizer 1.0.0.0