All language subtitles for The Life of Mammals 3of9 Plant Predators 720p HDTV x264 AAC MVGroup org_Subtitles01.ENG

The biggest predator
to walk the Earth today
faces a continuous struggle.

Its prey is heavily armoured,

often indigestible,
sometimes even poisonous.

What makes this struggle between
predator and prey surprising

is that the predators are elephants
and the prey are plants.

These herds are the task force

in a war that has been fought
for millions of years

and has produced some of
the most complex and highly evolved
relationships in the natural world.

In this tree, there is one of the
most extraordinary plant predators.

It's one animal
that I don't need to sneak up on.

Boo!

This extraordinary creature
is half-blind, half-deaf,

and this is just about as fast
as it can move.

That's what can happen to you
if you live on nothing but leaves.

It's a sloth.

It's not exactly
an enthusiastic leaf-eater.

A couple of half-hearted chews
and the leaves
go straight down to its stomach.

Leaves, however,
are not easily digested.

The sloth's technique
is to give them time.

Then eventually, this mobile
compost heap pulls itself together

and starts on a long
and dangerous journey.

This is a very unusual sight -
a sloth in a hurry.

It wants to defecate and the only
place it is happy doing that -
oddly enough - is down on the ground.

It only does it about once a week,

but why does it come down
to the ground to do it?

And why does it nearly always choose
to do so in exactly the same place?

Whatever the reason, it must be
very important, for a sloth
on the ground is almost helpless.

Any predator could attack it and
it doesn't have the speed to escape.

Why it comes down in this way
is a mystery. Nobody knows.

Now it's finished

and back it goes,
up to the safety of the canopy.

Leaves are not very nutritious.

The sloth's way of compensating
for that is not to eat more
but to do less.

Its claws hook over the branches,
so that the sloth can hang

without any effort of its muscles,
which have been reduced
to thin ribbons.

And to save energy, it spends
most of its time hanging around,

half-asleep, in the tree-tops.

So with very little muscle,
and a reaction time
only a quarter as fast as ours,

how does a sloth's day
compare with our day?

In the time it takes me
to write a few letters, the sloth
just about manages to groom itself.

While we have our lunch,
the sloth nibbles a few leaves.

And then, as we film a sequence
for the series,
it's time for another nap.

Not surprisingly, many mammals in
the world are dependent upon plants.

We live, after all,
on a green planet.

Plants capture the energy
they need to grow from the sun

and turn much of the Earth's surface
into a vast and varied salad bowl.

But the leaves' nutriment
is locked away within a mesh
of cellulose walls.

No mammal, by itself,
can digest cellulose

and those that eat leaves
rely on bacteria in their stomachs
to break through this dense lattice.

Broad-leaved trees first appeared
on Earth about 100,000,000 years ago.

Gradually they spread, eventually
forming lush rainforests,
like this one in South America.

And it was in places like this

that the early mammals first started
to eat leaves in a wholesale way.

One of those primitive plant
predators, with very little change,
still survives here today.

There's its track.

The prints are very fresh,
so it could be quite close.

The animal I'm following is said to
be as difficult to see as a jaguar.

And I must be careful because it's
also said to be quite dangerous.

There it is.

This is the largest animal
in the whole of the South American
rainforests.

It's a tapir.

There's a female on the left and a
small half-grown calf on the right.

With a calf there,
she could be a bit aggressive.

I'd better not get too close.

They're feeding on leaves.

In fact, most of their meals
are made on leaves.

You would think they've got
more than enough to choose from,

but they are extremely selective
about which leaves they choose.

And you can see why.

Many of the leaves are protected
by spines.

Branches and trunks are armoured,
too, and spikes like these
can inflict real damage.

Even plants which appear harmless
may have such defences,
if you look close enough.

Their tissues are loaded with poison,

some of which are really powerful,
such as strychnine.

But tapirs have found ways
of dealing with THAT problem.

They eat only a little of any one
kind of leaf, then move onto another,

so that they don't get a lethal dose
of any particular one.

And they have another defence
against poison.

This river bank is a special place
that has been visited by tapirs
over many generations.

It's eating earth -

kaolin, a special kind of clay,
that binds to poisons,

neutralising them
before they cause any harm.

The kaolin is a medicine.
We ourselves use it for the same
purpose when we have stomach-ache.

So, in spite of all the defences
that plants have evolved,

tapirs manage to find all the food
they need in these forests.

This struggle between mammals
and the plants they feed on
is waged all over the world.

The Canadian Rocky Mountains,
and the beginning of an autumn day.

A pika,
a member of a small community

that lives among the tumbled
boulders bordering a mountain meadow
where they all feed.

SQUEAK! SQUEAK!

That's a warning call, telling other
pikas that this patch is now taken.

Pikas start their foraging
early in the morning.

They eat all parts of a plant, not
just leaves, but the flowers as well.

Grazing out in the open is dangerous.

There are eagles around,
so the pikas never stray very far
from the safety of the rocks.

There may seem to be
plenty of food now,

but soon there will be the first
flurries of snow, the flowers will
die back and winter will be upon us.

What happens then
when little is growing?

Well, watch what happens to these
if I leave them just there.

It's not eating my flowers -
at least, not yet.

It's stacking them in its larder,

creating a store that will last it
through the hard days to come, when
this valley will be covered in snow.

It will need a stack several feet
thick if it's to survive the winter.

The strange thing is that many of
these leaves are extremely poisonous.

So, why does the pika collect them?

Well, the poison acts
as a natural preservative,

and the leaves remain fresh until
midwinter, so in the end the poison
works to the pika's advantage.

But the pika's preparations are
more subtle than they might seem.

It collects a variety of plants.

Those with only a little poison
will become edible quite quickly,

whereas those with a lot will remain
fresh until almost the end of winter.

Each little pika may make several
hundred trips a day, literally
making hay while the sun shines.

Sometimes the problem is not what's
in your food, but what is not.

Dealing with dietary deficiencies
has had a dramatic outcome

here on the flanks of Mount Elgon
in East Africa.

The first Europeans
to visit these caves noticed
marks like these in the walls

and they imagined that maybe they
had been made by ancient Egyptians

who came here to mine for gold
and precious stones.

These grooves do look like the marks
made by a pick-axe,

but to discover
what actually made them,
you have to wait until nightfall.

We've set up infrared lights
that the animals can't see,
but our cameras can.

I will be able to keep watch
from the safety of a side chamber.

The bats are preparing to leave
to search for their food
in the night skies outside.

In a few minutes' time, it will be
as dark outside as it is in here.

Something is moving.

Bushbuck.

They're looking extremely nervous.

And that's why.
There's a buffalo close by.

They're only a few feet apart,
but they can't see one another.

You've got to remember
that, as far as it is concerned,
it's in pitch blackness.

It seems to be searching
for something.

It's eating.

I can see its throat as it swallows,

and it's understandably
very nervous and apprehensive.

It's licking salt.

The bushbuck has heard something.

It sounds like distant thunder.

It's an elephant.

IT TRUMPETS SOFTLY

Every foot's being placed
very carefully.

THUD

Oh!

He bumped his head,
Well, no-one's perfect.

This deep rumble, this resonating
noise that's coming from him,

that's probably a signal to others
waiting outside the cave, because
he's by himself at the moment.

RUMBLING SOUND

That's the picture
from our cave-mouth camera.

The rest of the herd have arrived
and are climbing up to the entrance.

How they are managing
this steep slope, I just don't know.

There's even a young calf among them.

Maybe the male's rumbles were
messages to say that all is safe.

They are following exactly
the same path that the male took.

Look how the female is using
her trunk to guide her calf
over the cave floor.

Has she detected one of our cameras?

Maybe not.

But they clearly know
where they are going.

The passage here is so narrow,
the big male can only just
squeeze through.

SCRAPING

And now I can hear that noise.
He's using his tusks
to gouge out the salt.

And of course it's falling
to the ground.

So what he does now is use his trunk
to sniff it up

and then blow it into his mouth.
You can hear that, too.

Elephants must have been coming here
like this for centuries,

each generation
deepening the cave a little

and passing onto the next
its knowledge of the route through
the darkness to the precious salt.

So the marks near the cave entrance
were not made by ancient Egyptians
but by elephants.

Could this great cavern
have been created by them?

It's surely an extraordinary thing
that elephants should choose
to come to a cave,

go into its depths,
then travel for hundreds of yards
through total blackness.

It's a dramatic demonstration
of how important
a mineral can be to an animal.

So the demands of diet have had
the extraordinary effect of turning
elephants into salt miners.

Plants make themselves indigestible,
defend themselves with spines and
poisons and are so poor in nutriment

that their predators
have to go to great lengths
to get dietary supplements.

Yet despite all this, plant-eating
mammals are a great success story,

and nowhere more spectacularly so
than out here
on the open plains of Africa.

Here plant predators
gather in unparalleled numbers,

the greatest concentration
of mammals to be found on Earth.

The leaves they seek are those of
one particular kind of plant - grass.

The relationship between them
and their prey is very complex.

Grass is not as passive
as it might appear.

The edges of its leaves
are armoured with tiny spines.

And inside its tissues
there are needles of silica.

Grazers, in response,
have developed countermeasures.

They have teeth
that grow continuously up
just as fast as they are worn down.

And they digest everything twice.

Each mouthful, after being chewed,
goes down into a multi-chambered
stomach for a first processing

and is then brought up again
for further mastication.

This second chewing can be done
at leisure and in relative safety,

for instead of having your head down
to graze, you can now keep it up,
watching out for danger.

The leaves go back
for a final treatment in a different
chamber of the stomach.

What nutriment is left is returned
to re-fertilise the plants
from which it came.

But there is a season each year
when the rains stop
and the grass shrivels.

The grazers have to find food
elsewhere.

The annual migration has started.

Such yearly compulsions
grip grazers all over the world.

In Alaska, caribou also have to move
to escape the worst privations
of the Arctic winter.

But wherever the migrating
plant predators travel,
they are beset by animal predators.

Only from the air can you get a real
impression of the vast scale
of these annual upheavals.

Every year, millions of animals
travel hundreds of miles

across burning hot plains
and freezing cold tundra.

But what is the real reason for
these extraordinary, risky journeys?

Speed up the movements of the herds
and a pattern appears.

The wildebeest are following
special trails in the grass.

Grass may all look the same,
but in fact it varies
in one particular component

that we now know is essential
for the survival of the wildebeest -
phosphorus.

Wildebeest can tell which grass is
rich in phosphorus and which is not,

so they graze some parts
and ignore others.

They time their migration
to arrive on the short-grass plains
of the Serengeti

just as phosphorus-rich grasses
are beginning to sprout.

But before long this grass
will also dry out and then the herds
will be forced to move again.

Although the wildebeest
rob the grass of its leaves,

they don't damage the stems,
so the grass continues to sprout.

A greater threat to its survival
comes not from an animal, but
another plant - small acacia bushes.

In due course,
it may grow into a big tree.

If it does, it will compete
so effectively with grass
for natural resources

that grass and therefore grazers
are driven away and the trees
will extend their territory.

But every plant has its predator.

The dik-dik is the smallest antelope
on the plains and it browses
on the acacia's lowest leaves.

Its delicate pointed muzzle enables
it to avoid the hooks and spines

that protect the acacia's branches
from clumsier,
more wholesale browsers.

The dik-dik is so small,
it can't reach leaves that are more
than a couple of feet above ground.

Others attack the higher branches.

The impala, with its larger muzzle
and longer neck, can reach three
times higher than the dik-dik.

Having taken what they need,
the impala herd moves on.

But the acacia has to withstand
the assault of yet another attacker.

The gerenuk is able to crop leaves
that are far beyond the reach
of even an impala.

Its head is very small
for its height, so it can get
in between the thorny branches.

And its lips and tongue
are particularly mobile.

Standing erect
demands special adaptations.

The gerenuk's hip joints
swivel so far

that its backbone can swing up and
continue the line of its hind legs.

A feeding group may have
all the grace of a corps de ballet
standing on their points.

But even gerenuks have to step aside
when the world's tallest
plant predator appears.

The giraffe.

They travel in groups of up to 30

and launch their attacks
from necks that are seven-feet long.

The acacia's defences on its upper
branches would deter most browsers.

But the giraffe's weaponry
is formidable indeed.

Its tongue is 18 inches long
and so muscular that it has a grasp.

Its neck joint is so mobile that
its head can tip vertically upwards.

And its lips are so leathery,
they are impervious to thorns.

The acacia is under attack
from bottom to top.

With such a diversity of predators,
you might think that the march of
the acacia would be held in check.

But the acacia has other plans,
and they're revealed
during the dry season.

Throughout the year,
the acacia has tantalised animals

with the chance of eating some
but not all of its leaves.

Now that the time has come
to shed its seeds,

that has ensured that there's
a wide range of animals around
to pick them up and disperse them.

Impala and other browsers
crunch the pods,

but the seeds are indigestible
and they will emerge unharmed
with the eater's droppings.

Dik-dik might take them
just a few hundred yards,
impala - for a mile or so.

Giraffe can transport seeds
for ten miles or even more.

But there is one predator against
which the acacia has no defence.

Even the stoutest, sharpest spines
don't deter an elephant

and it has a simple but devastating
way of getting the branches
that even a giraffe can't reach.

Its reward is a relatively
spine-free meal,

for the acacia neglects to grow
spines on its topmost branches

since they are beyond the reach
of most browsers.

Elephants have
a range of power tools
with which to collect their meals.

Tusk and trunk together can cut up
anything their owner fancies.

The woodier a branch,
the more difficult it is to digest,

but elephants have such vast
stomachs that they can allow their
meals to stew for about three days.

The need for a big stomach
may be one of the reasons
why elephants have grown so large.

But being jumbo-sized
brings other advantages as well.

Wherever there are plant-eaters,
there are meat-eaters.

But even the biggest of them is
not big enough to tackle an elephant.

Smaller plant-eaters
are more vulnerable.

How can they defend themselves?

One way is to gather together
in large numbers.

And that's what grazers do,
all over the world.

If you live in a herd,
there are many others around
to help you in detecting danger.

Ears can be rotated to detect sound
from all directions.

Sensitive noses can pick up
the first faint whiff of an enemy.

Eyes with elongated pupils can
keep watch across the whole horizon.

And when heads go down,

eyes swivel
in their sockets to ensure
that the pupil stays horizontal.

So, even when you are grazing,
you can still keep an eye
on what is watching YOU.

With eyes on the side of your head,

you can see both in front and behind
at the same time.

This really is wrap-around vision.

Hunters' eyes point directly ahead,
giving them the ability
to assess range.

The targets, on the other hand, have
to hold their heads sideways if they
are to keep an eye on the hunter.

Sometimes the prey appears
to be stalking the predator.

Grazers even taunt a hunter to make
quite sure that there is no way
it can launch a surprise attack.

And at this point,
many hunters would give up...

but not always!

An attack is now imminent.

Sound the alarm!

VARIOUS ANIMAL CRIES

Now the time has come to run.

Herbivores have powerfully muscled
hind legs that give them
superlative acceleration,

invaluable
if you are caught unawares.

Once again, numbers bring safety.

The North American pronghorn
is the second fastest sprinter
on the planet,

but over long distances
it's the world champion.

But all large herbivores
have to be able to run fast.

They run on tip-toe, so that they
cover more ground with each stride.

Muscles are bunched
at the top of the legs,
so that the limbs are streamlined.

Some grazers flaunt
their athleticism,

as if to say, "I'm fit, so save your
energy and pick on someone weaker."

Cheetahs may be the fastest
sprinters, but gazelles are better
at dodging and jinking.

Slimline legs, however,
trip only too easily.

And having eyes on the side of your
head, so that you can't see directly
forward, can be catastrophic.

Even so, herbivores manage
to outmanoeuvre their enemies
more often than you might suppose.

Kicking hooves and thrusting horns
are formidable weapons.

This mother is going to defend
her fawn, come what may.

A buffalo has incautiously
strayed away from its herd.

It surely can have no defence
against a group of lions.

But the rest of the herd
have noticed.

Faced with the threat
of hundreds of tons of massed anger,
the lioness turns tail.

The male lion, however,
seems unwilling to give up.

The buffalo,
with their heavy armament,
have won this particular battle.

But the war on the plains
is a never-ending one.

You might think that these weapons
are just a defence
against carnivores, but not so.

Their primary use
is to fight one another.

And that's the drawback
of living in herds.

The Badlands of North America.

Bull bison are preparing
for the annual rut.

There are only a few females on heat
at any one time,

so each male tries to sniff them out
before rivals approach.

The males walk in parallel,
assessing one another.

Pumped up with testosterone,
they paw the ground
to show off their strength.

They spray the earth with their
urine and then roll in it, so that
they reek of their own hormones.

This combination of rolling
and roaring is a clear sign
that there will be a fight.

Most contests are resolved
in seconds.

A few, however, escalate
into full-scale battle.

The attack is usually head-on. At
full gallop, the impact is titanic.

One ton moving at 30mph
meeting another
coming in the opposite direction.

This male is lucky to escape
a fatal stabbing.

The largest horns in proportion
to body size are carried
by American bighorn sheep.

When armaments reach this size,

their indiscriminate deployment
could be catastrophic.

Smaller males can be warned off
with a simple kick.

But closely-matched males
will have to fight.

The rules are strict.
Contestants must meet head-on.

If contact is unbalanced, both
fighters could break their necks.

An impact like that would crush
a human skull like an eggshell.

So, how does the bighorn survive?

Well, its skull is heavily
reinforced internally with bone,

but it also has a number
of hairline cracks in it

and these flex,
so acting like shock absorbers.

The bighorn's weapon is a battering
ram, but there are also swords,
scimitars and daggers.

All are ridged and pointed at the
tips and both those characteristics
have important functions.

Before any physical contact is made,
the males, no matter what their
species, size one another up.

If neither retreats,
horns will clash.

The V-shaped gap
between the horns is always narrower
than the width of a single horn,

so that it is not possible
for a fighter to strike his opponent
directly on the skull.

Having made contact,

the contestants wrestle and now the
function of the ridges becomes clear.

They prevent the horns from slipping
and enable the contestants
to test each other's strength.

Now if there is a chance,
the pointed tips will be used to
stab a rival in the flank or belly.

A competitor will not waste
his energy in starting a fight
if he is obviously outgunned,

so horns are continually flaunted.

These male topi
are even putting on war paint.

By plastering their horns with mud,

they make themselves
more intimidating to other males,
and more attractive to females.

Each in this gathering of several
hundred must establish a small patch
of territory for himself.

At first, the females
wander through the pasture,
perhaps sizing up the males.

And the males are torn
between pursuing particular females

and battling with one another
to establish their individual
stamping grounds.

Again and again, a male has to fight.

Eventually
each male has his own patch.

The females decide
which they like best
and present him with his reward.

The mating rituals
go on for many days.

Males dare not leave their territory
in case rivals claim it

and they have to fight repeatedly
to maintain their ownership.

As the days pass, they become
more exhausted and eventually
they can barely stand.

They are so tired that
their normal defences are down.

The most powerful males
have claimed territories
in the centre of the breeding ground.

The less strong have to accept
those on the fringes.

And that is not a good place to be.

In spite of the circling hyenas, the
males won't leave their territories.

If they did,
they would have no chance of mating.

But they no longer have the will
or the strength to confront
the hyenas, unless they are attacked.

For most of the year,
when the topi grazed in the herd,
they kept watch for one another,

but the competition
to breed has changed all that.

The dangers of eating grass
out on the open plain led the topi
to live in herds.

Now the price of doing so
is being paid...

by the weaker males.

From the topi's battle to breed
to the great migrations of the world,

the underground mines of Mount Elgon

and the extraordinary shape and size
of the wonderful creatures
that made them,

all these stem from the apparently
simple act of eating leaves.

So, as always
in the Life of Mammals,

what you eat determines what you are.

Elephants are surely the most
impressive, the most formidable
of all plant predators.

They are, after all,
the biggest of all land animals.

And yet they can suddenly
appear or disappear
absolutely silently in the bush.

They're so powerful,
they can flatten your Land Rover,
if they have a mind to do so.

And they're so intelligent,
they have such long memories,

and they communicate within
their families in ways which we are
only beginning to understand.

Sitting in a canoe, watching
elephants coming down to the river
to drink is a marvellous experience.

But to be in a cave
in the pitch blackness
where you can't see them,

and yet you can hear the creak of
their bodies, and that low, rumbling
call, that is something else.

And to tell you about that,

here is Justine Evans,

who has spent night after night
in that cave in Mount Elgon
with her cameras.

It was not so much scary... I don't
know even if it was dangerous,

but it felt intimidating

and quite overwhelming.
It sounded worse than it was
because of the cave walls.

They made the sound of the
elephants resonate, so I felt that
they were roaring right next to me.

There's...one, two, three, four,
five...at least five in here
at once, which is amazing.

They came all the way to the back
and I got all sorts of shots.

The baby was standing in the dark,
obviously really bored because
it doesn't know how to tusk yet.

He was just doing this with his
trunk and going round in circles!

The cave elephant families follow
ancient traditional pathways
used by many previous generations.

There's some kind of inherited
culture.

Young calves, following
in their mother's footsteps,
may be too young to dig for salt,

but they're here
to learn the traditions
of Mount Elgon's elephants.

The notion that elephants might have
traditions would have been
unthinkable only a few decades ago.

But since then, scientists have
started to study elephants by living
alongside them in the field

and recognising each individual one.

No-one knows them better than
Cynthia Moss who's lived in Amboseli
for the past 30 years

and recognises every member
of 50 families, the most famous
of whom, perhaps, is Echo.

We first filmed Echo in 1993.

Since then, we've followed her reign
as head of the herd.

Echo is now a grand old matriarch.

Her crossed tusks
make her unmistakable.

This is her latest grandson.

Ella, with ragged ears,
is her second in command.

Initially,
ears identified individuals.

Their ears are never
absolutely smooth along the edge.

There's usually little nicks
or holes or whatever.

But after a while,
you recognise the whole elephant.

Wildlife cameraman Martyn Colbeck
has worked alongside Cynthia.

Together,
they gained a deeper insight.

Cynthia Moss's knowledge
of individual elephants

has been very significant for us.

It would have been difficult
for me to do it on my own

as I did not know the individuals
and how they related to each other.

Cynthia and Martyn were accepted
as part of the family.

I think one of the most
exciting things I've ever
seen and filmed with the elephants

was the birth of a calf,
the matriarch's calf, in fact.

I just never thought
that we'd ever be able to film it.

For the matriarch to trust us
enough to give birth right next
to us in the middle of the night

was quite a privilege.

And it took us several years to get
ourselves into this situation.

When the delivery finally happened,
the whole family went crazy.

They made sounds that I'd never
heard elephants make before.

They all crowded around Echo

and it was extraordinary.

It was a magical moment.

When there's such a bond
of mutual trust and understanding,

the detail of the elephants'
behaviour emerges

and the drama
of their lives is revealed.

Usually, Cynthia Moss was there
interpreting behaviour.

There's a very good example of that
when one of our family's calves
was kidnapped by another family.

That's a very rare bit of behaviour
and I would have had no idea
that that was about to happen.

Echo gets a vicious poke in the
backside when she tries to rescue
Ebony from this larger family.

The other matriarch, called Vee,
is using Ebony

to emphasise her dominance.

But help is on the way.

Reacting to Ebony's distress calls
and Echo's alarms, the rest of the
family arrive in tight formation

and plunge into the kidnappers.

FRANTIC TRUMPETING

Ebony was rescued by her family.

For some elephants, the strength
of the family is even more important

and inherited traditions are the
difference between life and death.

This is most clearly the case
for the elephants of Namibia.

They live in desert and so they're
having to move over enormous
distances to find food and water.

The matriarch is like a repository
of knowledge for the whole family.

They know exactly where they need
to go in order to be able to feed.

They must have complex mental maps
of an enormous area

because the water holes are
extremely isolated and they're
only there at certain times of year.

So their intelligence and mental
mapping ability must be phenomenal.

And that knowledge is passed down
through generation after generation.

So, now we know
that tradition is an essential
for the survival of the elephants.

But long migratory journeys
are part of the annual cycle
of many plant predators.

And it's group memory that enables
some of them to make the longest
journey of any land animal.

This is perhaps the most impressive
migration of all.

Huge herds of caribou
on their annual movements
across Alaska and Canada.

And using the same principle
of identifying and studying
known individuals,

we can understand and film
an animal tradition on a vast scale.

Very little is known about them,

although there's millions of them
in Northern America and Canada.

There's a scientist working with
one herd of about 900,000 animals.

25 of them are satellite collared,
so that you can locate an animal
in the most difficult situation.

Once a week, he got a read-out
of where those 25 animals were.

From that, we'd look at this map
about an area the size of France

and we'd be able to tell
exactly where the animals were,
so we could fly ahead and land

and film them migrating them
through an area.

Technology's opening up a whole new
field of behaviour that we can film.

One caribou tracked by satellite
moved over 3,000 miles in one year,

the record for any land mammal.

Like some migratory birds,

caribou may have a built-in compass
to help them cross unfamiliar land
on the way to their calving grounds.

But, as with the elephants,
herd traditions shape the movement.

What they gain from living
in the herd seems to be
a key factor in their survival.

In our next programme, we meet
the most numerous mammals of all,

the rodents.

They use their chisel-like teeth
in the most extraordinary ways
and manage to live almost everywhere.