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 the birds are coming into 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.
And 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.
Others 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 the 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 hyper-sensitive 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 also 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 in the encampment
can differentiate between a single trail and a double one.
They may also tell from its smell
whether its creator has had 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
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.
There's still enough light outside for them to see and they're relatively quiet,
but listen to them when I follow them down into the real blackness of the cave.
These ladders were built by local people
who come here to collect the swiftlets' nests to make bird's-nest soup.
They lead to a rickety, slimy platform hanging 180 feet above the cave floor,
but right alongside the nests stuck to the rock wall.
The chorus of clicks that you can hear around me
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 wall of the cave.
And, amazingly, it's able to distinguish the echo of its own call
from that of all the rest of the birds.
So although there are a million swiftlets in this cave,
each one is able to find its way back to its own nest in the pitch blackness.
(TREMENDOUS DIN OF 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.
The mother has to be able to steer through the cave's winding passages
and land alongside her baby in order to give it milk.
The bats' equipment for echolocating
is much more highly developed than the swiftlets'.
First, they have huge ears that constantly twitch to pick up faint echoes.
Second, they have complex flaps around the nose
that concentrate these 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 much more efficient than that of the swiftlets.
In the evenings, like now, when the swiftlets are forced back into the cave
because they can no longer 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.
But many animals, of course, are active not in the dark, but in the daylight.
The rufous elephant shrew of Africa
guides itself with its eyes as it careers along its runways.
It spends 3/4 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.
Clearly, elephant shrews have a mental picture of the layout of their trails,
just as we have of our own neighbourhood.
But you can only get that in places that have distinctive landmarks.
Not all animals are 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.
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.
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 eventually finds food, a quick calculation
and it knows exactly the shortest way home.
By using a beacon that you can see wherever you go,
you're no longer restricted to familiar home ground
and great journeys into unknown territory become possible.
But what if the sun is no longer visible?
What if, for example, you live underwater?
In the 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.
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 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 trigger fish, one of their main enemies.
It wants to attack the vulnerable legs,
but 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.
Within a few minutes, all that is left is an empty shell.
When the survivors reach the shelter of the rich 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'll be safe from the storms 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.
Others, in the Pacific, head for the little cluster of the Galapagos.
They have to 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 our own 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 the taste grows ever stronger,
they at last reach the rich waters of the Galapagos.
Here they meet others and here they mate.
The sheltered beaches provide the nesting sites the females 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 help 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
because we're only 600 miles from the North Pole.
Most of the year, the sea is ice-covered,
but now, during the brief 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 very good reason the birds 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 Ocean.
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 the 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 breed again on exactly the same spot in Spitzbergen.
But perhaps the most impressive navigator of all is the royal albatross.
It spends years on the wing entirely over the ocean.
And one individual is probably the greatest animal traveller alive.
Here in Taiaroa Head in South Island, New Zealand, back in 1937,
a young female albatross was given an identification ring.
She'd spent the previous eight years
flying round and round the Antarctic continent 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'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 unerringly to their nest
on a tiny island isolated in a vast empty tract of the Southern Ocean.
Maybe they recognise wave patterns.
Perhaps the clouds that build up over oceanic islands serve as guides
for they are visible many miles away.
There's some evidence that they use the sun and the stars.
But, in truth, we're still unsure of exactly how they do it.
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 humans 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.
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.
Only 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 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,
maybe because they swim so deep they're beyond the reach of any normal net,
and that they can't be caught by hook with bait
because they don't feed ever again in their lives.
But how do they guide themselves on these astonishing journeys?
As young elvers, they can't be guided by their parents
because they make their journey across the Atlantic by themselves.
And, as adults, they 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.
The fact of the matter is, we've still got a lot to learn
about the ways in which animals find their way around.
When things are scarce in this harsh world,
you may have to fight to get your fair share of them.
In this case, it's food, meat.
But you may have to fight, too, to get the best available mate that's around
or territory which will support you and your family.
So the ability to fight is needed by animals of all kinds.
Even sea anemones fight on occasion.
This group, catching floating particles of food with their tentacles,
is harmonious enough,
for these individuals are all sisters, budded off one from the other
and, therefore, genetically identical.
This, however, is a stranger.
The residents can detect chemically that these tentacles are no relations of theirs.
Someone else is clambering onto their stone, seeking their feeding space.
This is war.
The battle is difficult for us to recognise,
for anemones work on a rather slower time-scale than ourselves,
but speed things up and the fight becomes obvious.
The lashing tentacles are armed with stings.
The resident anemones have had enough and off they go.
Male strawberry frogs in the jungles of Costa Rica,
when breeding time comes, fight over territory.
They wrestle.
(CONSTANT CHIRPING CROAKS)
They call throughout the contest.
If one stops, it's a sign that he's given in.
If he continues to call, even if he's retreating,
his opponent will continue to pursue him and take him on for another round.
These bouts may go on for as long as half an hour.
In fact, the result is pretty well a foregone conclusion.
Unless the intruder is very much bigger,
the resident, the one who was here first, nearly always wins.
The loser will hop off to find an unoccupied square yard of forest floor,
and no great damage has been done to either contestant.
But many animals can injure one another only too easily.
The hummingbird, like many other animals,
feeds with a device that can also be used as a weapon.
Here in Trinidad, hummingbirds defend patches of flowers
full of nectar on which they feed.
And an owner threatens an intruder by displaying brilliant colours.
But make no mistake, these ritualised threats are backed up with real force.
Watch what happens if an intruder appears in this defended patch
and stays put, as this little stuffed hummingbird presumably will do.
First, the threats.
Spreading the tail and angling the brilliantly-coloured reflective feathers
on the throat and the head, so that they catch the light.
Usually, this is quite enough to frighten off a rival.
But it's not working in this case.
So now he decides to use force.
And still his rival won't go.
Maybe it's not a rival. Maybe it's a particularly stoical female eager to mate.
So he tries that.
But with such an unresponsive partner, it's difficult to find the right place to do so.
Another hummingbird sees what's going on and interrupts him.
So another intruder has to be seen off.
Then he returns to his first problem.
The needle-sharp beak, which he normally uses to thrust into a flower for nectar,
he now wields like a murderous stiletto.
It's an impressive demonstration of what can happen
if an opponent doesn't give up.
This tiny pool in Panama
is the nursery of another powerfully-armed animal,
a damselfly larva.
It's weapons are jaws that shoot out from beneath its head.
But it holds these in reserve and, before launching an attack,
it makes threatening signals
with leaf-shaped gills on the end of its abdomen.
It's opponent - any other damselfly larva that comes near.
There's not enough food in this part of the pool for two.
If they come to blows, the loser will be killed and eaten.
But there's enough room here for them to separate, and both survive.
These animals are even more powerfully armed.
They are mantis shrimps.
The mantis shrimp lives in burrows.
This female is guarding eggs
and won't abandon them or her hole without a fight.
Outside, a male looking for a home.
Both animals can deliver a blow with their forelegs
that can crack the shell of their prey or an opponent.
If they're going to argue, they must be very careful.
She threatens him by displaying the bold pattern on her forelegs.
But he's not intimidated. He badly needs a hole to shelter from predators.
He shields himself with his tail, which has specially thick armour.
That foreleg, enlarged at its end into a fearsome club,
is the weapon that does the damage.
She's lost her eggs and her home.
Considering the force of the blows they've exchanged,
she's lucky not to have lost her life as well.
But male and female must meet sometime,
even among mantis shrimps, if they're to breed.
Once again, it's the female who's in possession of a burrow.
And she is not going to surrender it willingly to anyone else.
He has to convince her that he has something else in mind.
She blocks the entrance with her armoured tail.
He tickles it, reassuring her about his intentions.
Wafting his scent into the hole also helps to convince her.
A quick coupling, and it's all over.
That, in mantis shrimp terms, was comparatively easy.
This male, on the same mission, is rather more apprehensive.
She is, after all, bigger than him,
and she could crack him open with a single punch.
Understandably, he shields himself from such a disaster.
But faint hearts never did win fair ladies.
She's not receiving visitors today.
Rattlesnakes have one of the most lethal of all venoms.
If they fought, they would be quite capable of killing one another.
Yet they, too, have their rivalries.
Males meeting in the autumn, at the beginning of the breeding season,
have arguments to settle.
But their fights have rules.
The aim, it seems, is not to bite your opponent, but to slam him to the ground.
Wolves, too, could kill one another just as they can kill a deer.
But most of their disputes are settled by a complex series of gestures.
Initial threats are made by lifting the lips to expose the weapons they could use:
their teeth.
Even when tempers flare, there is still restraint.
Jaws may grip throats and heads, but they don't bite.
Disputes among wolves, as among most dogs and cats,
are largely a matter of threatening snarls.
It's hardly surprising that meat eaters,
which have, by their very nature, to be ferocious and aggressive,
should also be a bit quarrelsome on occasion.
But you might think that, by contrast, grass eaters, like these zebras,
would be peaceable, gentle creatures.
But actually, the male stallion zebra, on occasions, has to fight his own kind.
Look at this lot.
These are young males.
They're just sparring,
practising for the crucial duels they'll have to fight as adults.
Then their fights will determine their success as breeders.
Now they're largely a matter of high spirits.
Since zebras don't kill other animals for food,
they don't have death-dealing stings and fangs.
Their only weapons are their hooves and their relatively short, blunt teeth.
So, paradoxically, their fights can be very vigorous
and involve a great deal of barging and nipping.
Giraffes have no teeth at all in the front of the upper jaw,
so they could hardly give a devastating bite even if they wanted to.
Instead, they fight with their necks.
Even here there is restraint.
A giraffe's kick can disembowel a lion.
But rival giraffes never kick one another.
They just use their heads like sledgehammers.
Hippos use their leathery lips to gather plants,
which they grind to pulp with huge molars at the back of the mouth.
The tusk-like teeth at the front of the mouth,
which this baby has yet to develop and which its mother already possesses,
are used almost entirely in arguments between one another.
The bulls have even more spectacular tusks.
They yawn not because they're tired, but to display these weapons.
And if rivals don't take proper notice, they'll use them.
There's little point in risking injury if the result of a fight is predictable,
so assessing the strength of your rival brings benefit all round.
These Malaysian flies, roosting on hanging rootlets,
have developed a very precise way of doing that
to within a fraction of a millimetre.