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This is one of the coldest places on Earth - the high Arctic.
The temperature drops to 50 degrees below freezing.
Without this specialist clothing, the cold would kill me in minutes.
Yet there are animals that live here all the time.
One of the most remarkable is hunting over there...
an Arctic fox.
The only reason it and I don't freeze solid
is that we are mammals and have the mammal's ability
to use food to heat our bodies. We are warm-blooded.
The reason it is more at home up here than I am
is that it has more of that other mammalian characteristic, hair, than I have.
Its body is insulated with fur.
Warm-bloodedness is one of the key factors
that enabled mammals to conquer the earth
and to develop the most complex bodies in the animal kingdom.
In this series, we will travel the world
to discover how varied and astonishing mammals are.
We go to Africa, where mammals are at their most spectacular.
The plains are thronged with specialist grass-eaters.
There are other mammals here too, with different tastes.
Some hunting mammals have become the fastest creatures on Earth.
Those they hunt have had to respond or die.
Some mammals have become fearsomely strong and aggressive.
They fight for mates.
They fight for food.
Some even have to fight for a place to live.
Wherever you go, you find a bewildering variety of mammals.
Some are miniatures, a few inches long.
Others are massive.
The biggest of those on land are dwarfed by those in the sea.
I can see its tail, just under my boat here.
It's coming up!
There! The blue whale!
It's the biggest creature that exists, or has ever existed, on the planet.
Mammals are as at home in the water as they are on land.
Some lounge around on the surface.
Others prefer to do so on the beach.
We will go underground to track them.
And up into the tops of the tallest trees.
Mammals have even taken to the air and challenged the birds.
They can congregate in astronomical numbers.
They thrive almost everywhere.
Precisely how they do so depends,
as in so much in life, on what they eat.
Between them, they tackle everything that's edible.
Some are very particular about their food.
Others will take the best of what is around at the time.
Top of the menu right now is...salmon.
We will look at the lives of our closest relatives.
(HE CHORTLES)
And they will lead us to ourselves -
perhaps the most successful variation of the mammal's winning design.
To glimpse the beginnings of the mammalian dynasty,
we must travel to Australia.
(CLICK OF RECEIVER)
I'm looking for one of the most ancient of all mammals.
So ancient, it shares at least one characteristic with reptiles.
It's a very elusive creature, but here in South Australia,
there's a population that's been fitted with radio transmitters
and I can track them with this aerial.
I've got a very strong signal.
At first glance you might think this is a sort of hedgehog...
or a porcupine.
But it's weirdly different from a hedgehog, porcupine
or any other kind of mammal.
(WHISPERS) It's an echidna.
You can tell it's a mammal because it's got hair.
Only mammals have hair.
Some of its hairs have been enlarged and strengthened
and have turned into big spines, giving it an effective armour.
This hair keeps the echidna warm,
ensuring it doesn't lose valuable body heat to the cold air.
The fuel with which it and all mammals generates that heat is food.
On a cold winter's day like this, it has to spend its time
searching for its next meal to make sure it's fully stoked up.
Although echidnas have good eyesight and excellent hearing,
it's their sense of smell which guides them to food.
They sniff out insects and grubs,
then get at them by ripping open the nests and tunnels
with their immensely strong claws.
That beak-like snout pokes into holes,
then out comes a long sticky tongue,
that flicks into crevices to lick up whatever's worth eating.
Echidnas are particularly fond of ants and termites,
and will even climb trees to find them.
This particular female has an unusually healthy appetite
because she's about to breed.
And the way she does so is why the echidna is such a weird mammal.
The echidna doesn't give birth to live babies, she lays an egg.
It's hidden in her fur in a shallow depression on her underside.
It's no bigger than a marble.
Inside it, a young echidna is developing.
After hatching, she carries it around on her underside
for about 50 days, until it develops spines.
She then deposits it in a burrow, where it stays and grows for nearly seven months.
But how does she feed it during this long time?
For the answer, we must find the only other egg-laying mammal.
It too lives in Australia.
Surfacing beside me here
is one of the most extraordinary animals in the world.
It's so bizarre that when specimens of it
were first sent from here to Europe,
people thought it must be a fake.
But it's not. It's real, it's alive, it's a platypus.
That bill looks as though it should belong to a duck.
But it's not hard like a bird's beak, it's rubbery.
Like the echidna, the platypus feeds on small invertebrates,
but it looks for them under water.
Once it has collected a mouthful,
it takes them to the surface and grinds them to a pulp.
It has no teeth. Horny plates in the bill do the job.
How does it find that food?
Underwater, it closes its eyelids tight so it can't see anything.
It has a remote sensing device, its bill.
As it sweeps it from side to side like a metal detector,
sensors in it pick up infinitesimally tiny currents
given off by all living things.
There were few mammals on Earth 100 million years ago
when the first platypus appeared.
But there was another kind of animal hunting in the rivers...
..birds.
As the platypus grubs around on the river bed,
it attracts fish, which the cormorant snaps up.
Water birds are among the most ancient bird families,
so this could be a scene from just after the death of the dinosaurs,
when a new kind of animal appeared on Earth -
one with warm blood and fur.
The platypus has had enough.
She returns to her breeding burrow,
where, at the end of a tunnel that may be 20 yards long,
safe in a leaf-lined nesting chamber, she's laid an egg.
Exactly what goes on inside a nest, no one really knew.
No one had succeeded in breeding platypus in captivity
until very recently, and certainly no one at all
had seen inside an occupied platypus's nest, until now.
We have bored, very carefully, a hole into the nest
that lies below here and inserted this tube.
This is an optical probe with a little light on the end,
and I can manipulate it like this so that I can scan it.
If I then insert that inside this tube,
I'll be able to see something no one has ever seen before.
That's her in close-up. There's her eye, her ear.
It looks as though she's seen us.
(HE CHUCKLES)
She's...she's nibbling it.
No, not worth eating.
She doesn't seem particularly disturbed by it.
But has her egg hatched?
I think that quivering may have something to do with feeding.
I'll move the camera to see if I can see what's going on.
Yes, there it is...it's milk.
Milk is the perfect food.
It provides the growing youngster with everything it wants.
Only mammals produce milk.
In most mammals, it comes from the nipple,
but in this very primitive mammal, it simply oozes through the skin.
She's leaving. Off she goes.
The end of her furry tail.
What's that among the leaves?
There it is, that's her baby!
I'll try and zoom in on it.
Now you can see it - a tiny grub-like creature, naked and blind.
On the end of its bill you can see a tiny spike.
It's an egg tooth, used to cut through its shell,
in the same way as reptiles and birds do.
Only a few days old.
The platypus and the echidna
are the only mammals alive today that lay eggs,
links with the egg-laying reptiles from which mammals are descended.
Both are so well-adapted to their particular ways of life
that they are still successful and widespread in Australia.
Quite an achievement,
for they've been around for 100 million years,
as the fossil evidence makes clear.
Most of that evidence is just tiny fragments,
but here in Riversleigh in northern Australia, it's a very different story.
Fifty million years ago,
Australia was a much wetter country than it is today,
and just here was then a swampy area.
The bones of animals that died in or around those swamps
became buried in limey mud at the bottom of the pools
and are now preserved in limestone.
This rock is full of bone.
Here's the rectangular bony plate from the back of a crocodile.
The rest looks like bird bone.
The limestone in which they're embedded is so hard
that the only way to get them out
is to put the whole block in a bath of acid for a few weeks.
The limestone then dissolves away,
and what is left is sometimes extraordinary bones, beautifully preserved.
This is the skull of an extinct platypus,
about 15 million years old.
It's been called Obdurodon, which means ''enduring tooth'',
because, unlike today's platypus, which has no teeth,
this one still has them.
There are the empty molar sockets. There, two little pre-molars.
What was this place like 15 million years ago,
when Obdurodon was alive?
The night sky would have been full of the calls of animals
in the surrounding lush tropical forests.
Obdurodon, like the platypus, would swim in the pools.
In the trees around, there were mammals of a different kind -
marsupials.
There were different kinds of possums, similar to those alive today.
On the ground, there were less familiar creatures,
like this large marsupial leaf-eater.
Nothing like it is alive today.
There were great numbers of small mouse-sized animals
which, judging from their teeth, ate insects...
and others with a taste for flesh.
Preying on all these small animals, a marsupial lion.
(GROWLS)
It was big enough to make a meal of an unwary Obdurodon.
As the millions of years passed, Australia began to dry out.
The rainforests retreated and were replaced by grassy plains.
As the landscape changed, so did the marsupial mammals.
They thrived and diversified into many different species
and are still abundant today.
They differ from the platypus and echidna in reproduction.
Instead of laying eggs,
they produce young without protective shells,
and this grey kangaroo is about to do so.
Out comes, not a shelled egg,
but a tiny, under-developed little worm.
It weighs less than a lump of sugar.
It has no back legs but it has forelegs,
just strong enough to pull it through its mother's fur.
It has started on an extraordinary journey.
To survive, it must get to a pouch higher on its mother's belly.
Instinctively, this tiny living particle climbs upwards,
against the pull of gravity, towards the smell of the pouch.
After about three minutes,
it reaches the lip of the pouch and clambers to safety inside.
There it clamps its tiny mouth on its mother's nipple
and takes its first meal, of milk.
As it grows, the ingredients of the milk from the nipple change
to ensure the infant gets exactly the nutrients it needs
for each stage of its development.
By the time it's nine months old, it's getting cramped.
It's time to enter the outside world.
It's almost like a second birth.
He's a little unsteady at first, but Mum offers a helping hand.
Now he's known as a ''joey''.
It's all a bit much for one day
and he heads back to the security of Mother's pouch.
It will be another year before he's fully independent.
(BIRDS CALLING)
Other marsupials have taken to the trees - koalas.
They too have pouches.
It's the Latin word ''marsupium'' - meaning ''pouch or purse'' -
that gives the whole group its name.
When a koala joey emerges,
it clings tight to mother for several days before going solo.
Koalas feed exclusively on the leaves of gum trees, eucalyptus.
They are hardly an ideal food.
They're tough, indigestible and full of unpleasant chemicals.
By sticking close to Mother, they learn how to pick the best trees
with the most palatable leaves.
Even these contain little nourishment,
so they have to eat a lot of them,
and spend almost all their waking hours doing so.
When not feeding, they conserve their energies and sleep.
Only koalas can live on a diet of these particular gum leaves.
Australia seems full of difficult diets in awkward places,
but there are marsupials that can deal with almost every one.
This vast continent stretches from the temperate and sometimes chilly south
right up into the tropics.
In the centre, there are dry sun-baked deserts,
where it's only too easy to die from thirst.
There are great mountain ranges, which in winter are crested with snow.
But the mammalian characteristics of warm blood and insulating fur
enables the marsupials to cope with almost anything.
The wombat's fur is so thick it can remain active
throughout the winter, even in the coldest parts of Australia.
It feeds on grass and other plants,
and the strong front limbs with which it digs itself burrows
are equally good at clearing snow to find food.
Its pouch opens backwards,
so the youngster doesn't get a faceful of snow as Mum digs for food.
Numbats live in woodland, but it can get cold at night
and this family are warming themselves in the early morning sun.
Fur must be in prime condition if it's to function as an insulator,
so grooming is essential.
These dry eucalyptus forests look unpromising as a source of food,
but there are plenty of termites.
Numbats have just the right equipment to collect them.
That spectacular tongue
has to be kept well-anointed with sticky saliva,
and numbats spend some time making quite sure that it is.
With gear like that, a numbat can collect 20,000 termites a day.
This creature's ancestors
might also have used their tongues to collect insects,
but the mammal tongue is an adaptable instrument
and the honey possum uses it to gather pollen and nectar.
It's one of the most specialised feeders of all mammals.
Its tongue has a brush on its tip
which soaks up nectar from even the deepest flowers.
These fields of boulders are home to a less fussy marsupial,
which will collect whatever food happens to be around.
At the moment, there's an unusual delicacy -
these moths, sheltering from the summer sun.
The mountain pygmy possum may be small, but it has a huge appetite.
Moths provide a fast-food snack, high in energy-rich fat,
and it will eat as much as it can while it can get it,
and put on a little fat to see it through leaner times.
Only the indigestible wings are discarded.
At other times of the year, it lives on berries and seeds,
picking them off with its nimble fingers.
The striped possum has a particular taste for grubs.
It lives in the few fragments of rainforest
that survive in north-eastern Australia.
It's got what's necessary to collect them -
an excellent sense of smell, strong teeth to chew the bark
and a long, sticky tongue.
Perhaps the most challenging of all Australian environments
is the arid hot desert at the continent's heart.
Here there is little to eat or drink and few places to hide.
But marsupials have colonised this country too.
Everybody would recognise those as kangaroos.
But the kangaroos belong to a very big family.
There are kangaroos, wallaroos and wallabies,
big ones and small ones.
These are red kangaroos, the biggest of the family,
and they're particularly at home in this dry country.
This can be one of the hottest places on Earth,
so red kangaroos don't have to worry about keeping warm.
Their problem is over-heating.
All mammals sweat to lose heat, but water is in short supply here,
and red kangaroos only do so when they're on the move.
Instead, during the hottest part of the day,
they make use of whatever shade they can find.
Wiping saliva on their forearms helps to lose unwanted heat.
There's a rich supply of blood vessels near the skin surface,
and as the saliva evaporates, the blood cools.
They only feed in the morning and evening when it's cooler.
When they do, it's hard not to notice
the extraordinary way by which they get about.
The tail acts rather like a fifth leg,
propping it up as it swings forwards its huge hind limbs.
It looks ungainly when they're moving slowly,
but when a kangaroo senses danger,
the advantage of these unusual proportions becomes obvious.
Hopping at full speed, a kangaroo can outpace a racehorse.
They're the only large mammals in the world
that have developed this way of getting about,
but it's a very efficient way of doing so.
Tendons in the back legs act like giant springs,
storing energy as it lands,
then releasing it to propel the animal forward again.
By recycling energy like this,
kangaroos can quickly cover vast distances
to escape predators or to search for food and water.
It's not just out on the flat that hopping works well.
Some marsupials even hop around on cliffs.
The rock wallaby's key to success lies in its feet.
The soles have thick corrugated skin,
pads which give them a grip on every kind of surface,
and a wallaby can bounce about this difficult terrain with extraordinary confidence.
There's little to drink here,
and though adults get the fluid they need from their diet,
growing youngsters may find that difficult.
This youngster is after an extra drink from its mother.
They are able to bring up fluid from the stomach
to ensure their young don't go thirsty.
It's a special adaptation to this arid environment.
Grey kangaroos live out on the relatively well-watered grassy plains.
They're among the most sociable of all Australian marsupials.
But living in groups can lead to problems in getting on together.
Last season's joeys are fast approaching independence,
so the mothers will soon be ready to mate again.
Males use their sense of smell
to find out if a female is sexually available,
and will court her for several days.
Having found a promising one,
the male stays close to her side
to try and ensure that he, and no other male, mates with her.
The most dominant male is likely to be the one
to father most of the next generation,
and that is something worth fighting for.
(SNORTS AND BRAYS)
Joeys also fight, but it's just play-boxing,
a way of learning skills that will be important later.
It's not always a fair fight.
Fortunately, this little one still has Mother to see off the neighbourhood bully.
Marsupials first appeared about 100 million years ago,
towards the end of the age of the dinosaurs.
Then Australia was part of a great super-continent.
As the millions of years rolled by,
that continent began to split apart.
One fragment drifted south. That was Antarctica.
As it got closer to the South Pole, so it got colder,
became covered in snow and ice, and its animal inhabitants died out.
A second part was Australia.
It drifted north and got warmer and marsupials flourished.
There was a third part. It too drifted north.
It too had marsupials, and they're still there.
That was South America.
It may have been in this region of the super-continent
that the marsupial mammals first appeared.
Many died out, but there are still a lot of survivors.
This is one of the most elusive of them.
Living in the Amazon forest streams, it operates only at night,
moving in the pitch blackness by feeling its way
with its front paws and luxuriant whiskers.
It's the yapok, or water opossum.
These pictures, taken with infra-red cameras,
may be the first time it's been filmed in its natural environment.
It's hunting for fish and crustaceans.
Its fur is so thick that its skin doesn't get wet.
It has webbed feet to propel it through the water.
It's too dark for even the sharpest eyes to see very much.
The yapok relies on its acute sense of smell and hearing to locate its food.
It swims with its arms apart,
groping for its prey with its highly sensitive fingers.
It usually takes its catch
to the shelter of nearby vegetation to devour it.
It doesn't only feed in the shallows.
The yapok has a large territory,
and there are many deeper pools in which to swim.
Under water, it swims with its eyes shut, like the platypus,
and hunts entirely by feel.
The female yapok can also shut her pouch,
and does so with such muscular strength
that water doesn't get in and drown her babies,
though they must be close to suffocation after a few minutes of fishing.
After a good night's hunting, the yapok retreats as day breaks.
The yapok is the only aquatic marsupial in the world.
Most marsupials in Central and South America
live high in the canopy of the rainforest.
Just how many there are up there, no one suspected
until scientists started using cranes, like this one.
Such apparatus gives easy access to this high canopy.
We can now get an accurate idea of how rich wildlife is up here.
We might think of Australia as the land of the marsupials,
but this canopy of the rainforest
may have more of them than any other kind of mammal.
Most are strictly nocturnal and, though they are abundant,
they, like everything else in the forest, can be difficult to spot.
Many are similar to this woolly opossum -
tree dwellers with few specialisations
and a broad diet which can include flowers, fruit and insects.
These marsupial mammals, of course,
reproduce in the same way as their Australian relatives.
They give birth to tiny babies
at a very early stage in their development.
The pouch is seldom as well-formed as that of a kangaroo or koala,
but their young survive very well,
clinging unprotected to their mother's underside.
Marsupial mammals dominate Australia
and flourish in the forests of Central and South America.
Alongside them lives a radically different kind of mammal,
a kind to which we ourselves belong.
And it's only that kind that you find everywhere else in the world.
The plains of Africa have an abundance of mammals,
but not one is a marsupial.
They all reproduce in a fundamentally different way.
This wildebeest has nourished her baby within her
by means of a remarkable organ growing on the wall of her womb, a placenta -
a circular pad, rich in blood vessels,
connected to her baby by a cable, the umbilical cord,
through which she has fed her growing youngster.
The baby's blood vessels run through the cords of the placenta,
passing so close to its mother's, they absorb nutrients from her blood
and carry it back to the unborn infant.
But all this is about to change.
Giving birth to such a large, highly-developed baby
places great strains on the mother.
It's pretty traumatic for the baby too.
But there is a great advantage in being born this way.
There are plenty of animals around for whom a new-born calf
would make a welcome meal.
But this mammal baby, reared with the help of a placenta,
is able to get to its feet within minutes of its birth.
While it's finding its balance, its mother is there to defend it.
(MOTHER BRAYS AND BELLOWS)
Now the baby can be fed in the same way as all mammal babies,
with its mother's milk.
Placental babies may still have months, even years to go
before they are fully independent,
but the early months, protected in their mother's body,
have given these babies an invaluable start in life.
Whether mammals lay eggs or give birth to live young,
whether their babies develop in a womb or in a pouch,
they've managed to live almost everywhere.
(BIRDS SQUAWK)
The warm-blooded, furry, milk-producing mammalian body,
in all its multitudinous variations,
really is a winning design.
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