Morning in the rainforest of Panama.
Capuchin monkeys are having a rather wasteful breakfast.
Inside these nuts, there's a kernel that is very, very good eating,
but it has such a hard shell,
it's almost impossible to get at the kernel inside...even...
..if I hit it with a stone.
Unbroken. So it's hard to believe, but nonetheless true,
that one animal in these Central American forests
can open these nuts with its bare teeth.
But it's certainly not these monkeys.
The best they can do is scrape off the thin, fleshy coating.
The nut itself, they simply throw away.
But another animal is listening for that telltale sound.
(NUT DROPS)
It's an agouti.
(QUIETLY) The agouti is certainly no bigger or stronger than a monkey,
but it has the right tools to deal with any nut...
a pair of extremely strong and sharp front teeth
that can cut through even the toughest casing.
They belong to a large, successful group of mammals - the rodents.
Rodents specialise in being able to chisel their way
through almost anything.
What is so special about an agouti's front teeth that enables them
to cut a hole in the toughest of nuts?
They have a layer of strong enamel, but only on the front surface.
Behind it, the body of the tooth is made of a softer material: dentine.
Gnawing wears down the dentine faster than the enamel,
leaving an edge as sharp as a chisel.
Unlike the front teeth of other mammals, a rodent's grow continuously,
replacing what gets worn away.
Armed with these formidable teeth, rodents have become
the most successful and numerous of all mammals.
The largest group are rats and mice.
There are 1,300 species of those,
and they're all accomplished stealers of seeds.
Other rodents use their teeth to cut up vegetation.
Some even tackle wood.
The rodent body is very versatile and has evolved
into an astonishing variety of shapes and sizes to suit many different lifestyles.
So you might find a rodent almost anywhere.
Some are amazingly nimble tree-climbers - the squirrels.
They open cones and collect seeds that few other mammals can reach.
Abert's squirrels use their teeth to nip off the tips of the branches
of ponderosa pines, where the bark is particularly nutritious.
This is a valuable food reserve
if the pine cones run out during the long northern winters.
Abert's squirrels are such specialised feeders,
they can only survive where there are ponderosa pines.
Most squirrels have more general tastes.
High on their menus are often acorns.
Oak trees produce a heavy crop of acorns each autumn.
Good news for squirrels like these greys.
They need to eat a great deal to fatten themselves up for winter.
They must also bury some to eat later on, when times get hard.
For squirrels here in the woodlands of the eastern US, in Virginia,
things are not quite so simple.
Here, there are two kind of oaks:
red oaks and white oaks. Their acorns are very similar.
This is the acorn of a white oak, and this a red oak.
This one is slightly darker.
But the acorns of the white oak germinate almost immediately,
using up their food supply.
The red oaks, on the other hand, don't germinate until next spring.
The squirrels recognise the difference
and treat them differently.
When a squirrel finds a white acorn, it eats it immediately,
because otherwise it would soon germinate.
On the other hand, red acorns are almost always buried
as a store for winter.
Squirrels are colour blind, so can't recognise the difference by sight.
The smell tells them which should be eaten
and which should be buried for later.
In some years, the white oaks produce their usual bonanza,
while the red oaks provide hardly any at all.
When this happens, the squirrels change tactics.
You might think it's tucking in to a white acorn as usual,
but, after only a quick nibble, it takes it away and buries it.
What's it up to?
It's neatly cut away the tip, beneath which lies the embryo of the seed.
Once that has gone, the acorn will never germinate,
but its food store will remain fresh all winter for the squirrel.
So with dexterous paws to hold the acorn
and sharp front teeth to cut out the embryo,
the squirrels get the best value
out of these apparently well-protected nuts.
Seed-producing plants grow everywhere,
so, potentially, seed-eating rodents can live everywhere, too.
But there is one problem.
All seeds are full of nutriment, but most are very small,
like these from these desert plants.
You need about 500 of these to make the food equivalent
of a typical nut like an acorn.
Rodents wanting to store the seeds from these face a transport problem.
It'd be impractical to carry them one by one
as squirrels carry acorns.
Here in California's Mojave Desert, a rodent has solved that problem.
Its burrows are all around me.
Like most desert animals, the creature in this hole
usually doesn't come out until dark, when it's cooler.
If I scatter a few seeds around its entrance,
maybe that will tempt him to come out a little earlier.
Well, as it happens, it didn't.
It wasn't until later, when the moon was up,
that the burrow's owner showed itself.
A kangaroo rat.
It can carry hundreds of seeds in expandable cheek pouches.
The pink toes poking out are those of its back legs,
which support its whole body.
That's why it's called a kangaroo rat.
Its front legs are hidden beneath its chin,
shovelling seeds into its mouth.
Normally, it has to travel quite a long way
to find enough seeds to fill its pouches.
Tonight, because of this supply, its shopping bags
are soon full enough to take back to its burrow.
There we are. It covers them over with sand just to keep them safe.
Its deep burrow is a relatively safe place.
The rat sleeps here all day and much of the night.
Even so, the slightest suspicious sound from outside will wake it.
(WHOOSHING)
A gopher snake crawls towards the burrow. It might find its way inside.
It had better be seen off.
Foot stamping is the first warning.
That seems to have no effect.
Perhaps kicking sand in its face will do the trick.
The snake doesn't like that at all.
Victory to the kangaroo rat.
Seeds are not the only part of a plant that is edible.
Some rodents, like marmots in the European Alps,
eat the entire thing.
And they fight over grazing rights.
When they come out of their burrows in late spring after their winter sleep,
they set about re-establishing their territorial boundaries.
Calling and tail-flagging are only the preliminaries.
(SQUEAKS)
If that doesn't work, there's nothing for it but to go to battle.
Senior males do most of the fighting,
while their female partners guard the family burrow and watch.
When a male wins a scrap, he marks the boundaries
with scent from a gland on his cheek.
Marmots form strong pair bonds,
and the females usually only mate with their one permanent partner.
By late spring, a marmot family consists of the adult pair
with a few of last year's offspring, all of them females.
Last year's males have already been chased away.
Although daughters can stay,
their mothers constantly beat them up, particularly in spring.
They're persecuted so severely that, if any become pregnant, they abort.
The reason for this merciless behaviour
doesn't become apparent until later in the year.
By mid-summer, relative peace has come to the mountainside.
This season's babies emerge from the burrow for the first time.
There are usually between four and six babies,
and they immediately get down to the important business of grazing.
Alpine summers are short, so marmots feed as fast as they can
while they can.
Incisors slice with ease through the tough stalks of flowers and grasses.
The babies put on weight rapidly but, by the end of the summer,
they still do not have fat reserves to match those of their parents.
In autumn, the family start on their final harvest -
hay to line the deep burrow where they will sleep through the winter.
By October, the weather has turned really cold.
In the burrow, the marmots' body temperature
has dropped to two degrees centigrade.
Their hearts are beating only two or three times a minute. They're hibernating.
Their fat reserves will have to last them
until the following April.
They snuggle together to minimise heat loss.
Youngsters are in the middle of the pile.
With less bodyweight, they can't let their temperatures drop as low as the adults'.
Thermal imaging shows that a baby, glowing orange,
is warmer than the adults, which show green.
The burrow, which registers blue, is well below freezing point.
The adults couldn't keep more than one litter warm.
That is why the dominant female made sure that none of her youngsters had babies.
But hibernating isn't the only way to survive the winter in the mountains.
In North America, winters can be just as severe as in the high Alps.
Yet here, there's a rodent that manages to find food
throughout the winter months.
It does so with an extremely ingenious device:
a refrigerator - a pool of deep, cold water like this.
And over there is its builder and owner.
A beaver, one of the family that lives here in this lake
at the foot of the Teton Mountains in Wyoming.
Beavers can get around on land, but they're most at home in water,
where their webbed hind feet and large paddle-like tail
make them powerful swimmers above and below the surface.
Beavers feed on all kinds of vegetation and eat wood as well as leaves.
They're accomplished engineers.
This great pond is entirely their own creation.
Only a few years ago, this shallow stream flowed down the valley,
then a family of beavers moved in and built a dam.
The main body of it is built of boulders.
On the downstream side, it's been lined with logs,
some big and quite heavy.
On this side, it's been packed with mud and vegetation.
It's been built so accurately that it is, to within a few inches,
horizontal across its entire length of about 150 yards
from one side to the other.
The lake it's created stretches upstream for almost a mile.
So important is their dam to them that, if they detect any leak,
usually by hearing trickling water, they start repair work immediately.
Mud is needed as well as logs.
The repair team will labour away until the leak is fully repaired.
Maintaining the water at a high level brings beavers several advantages.
It floods the surrounding woodlands and so enables them to swim safely
to their main source of food.
They increase the distance they can swim by digging channels
that lead into the very heart of the woodland.
Here they can use their sharp incisors
to strip off the bark from a fallen tree and nibble at it,
while being close enough to water to slip away
should a bear or a mountain lion turn up.
Their network of channels enables them to ferry whole branches
back to their pond.
Where the water is deepest,
they dive down and push each branch firmly into the mud
at the bottom.
This is the beavers' fridge, where vegetation will keep fresh
through the long winter when the pond is covered with ice.
Stocking the fridge takes a lot of work.
The beavers are at their busiest in autumn.
At one side of the lake stands their lodge, a fortress of branches and boulders
that's so strong that not even a bear could break into it.
The only entrance is through underwater tunnels.
The beavers take refuge here whenever they are alarmed.
That was a warning signal
to say that danger was around, that's to say - me.
Now I may not see the beavers for some time.
They can stay underwater for five minutes, up to 15 if they need to.
They can get back to their lodge
without putting their head above the surface for a second.
Most lodges have at least two different entrances.
By October, winter is underway.
Marmots would now be hibernating, but the beavers are still active,
and will remain that way throughout the winter.
Even when the pond ices over,
they can still swim under the ice to get back and forth to their lodge.
No one knew what went on inside the lodge in winter,
so when the beavers were away, we installed infrared cameras
in order to find out.
A branch from the fridge is brought back for the whole family to feed on.
And another.
No wonder they don't need to hibernate.
The lodge is warm and safe even in mid-winter.
The only sign of activity in the snug home beneath the snow
is hot air rising from the vent at the top.
Inside, our cameras catch a glimpse of what, at first sight,
looks like a very small beaver.
It's a muskrat. There are a pair of them in here.
This is a new observation.
Do the beavers know, in the dark,
that there are strangers among them?
The muskrats regularly left the lodge to forage under the ice.
On several occasions, they returned a few minutes later with fresh reeds.
Perhaps they're paying rent
by regularly providing fresh bedding for the lodge.
Maybe that is why the beavers accept them
and even allow them to share their food.
Our infrared lights, however, are no longer welcome, it seems.
A lodge makes a safe home for a beaver.
Other rodents defend themselves more actively, and one is positively dangerous.
Darkness provides one of the best forms of defence,
and it's very dark right now.
The only reason you can see me
is that we're using a starlight camera.
Many rodents only come out under cover of darkness.
That applies even to one of the most ferocious, well-armed of all rodents.
I think there could be one around here.
(QUIETLY) And there it is, and what a formidable sight -
an African crested porcupine.
It seems to have found something to eat underneath that spiny bush.
Most of the time, those quills would be lying flat.
The fact they're half erect is a sign that it's not happy that I'm so close.
Those long quills are very sharp,
and I'm quite sure it knows how to use them.
It's turning its back on me, but that's not because it's about to run away.
If it gets really angry, it will attack by suddenly sprinting backwards.
And it's off.
That's not the sort of thing you want to stumble into
in the middle of the African night.
They feed mainly on roots and tubers, but they're big animals
and to find enough to eat, they travel long distances across open savannah.
And that means they have to be prepared to take on all comers.
This young leopard has probably never been close to a porcupine.
He doesn't realise it would be dangerous to interfere with it.
The porcupine seems to have total confidence in its armoury.
If it's not careful, that naive leopard will end up with a face full of quills.
And the porcupine goes back to digging for food.
There's confidence for you.
Belding's ground squirrels don't have that kind of nerve.
They live in California and burrow close to one another,
so there's always plenty of watchers to give warning of any danger.
This is important during the summer when there are babies about.
There are always at least one or two adults on watch nearby,
so that the young can feed in relative safety.
Only the females care for the young. They're nearly all related.
Mothers, sisters and daughters look after nephews and nieces
as carefully as they do their own offspring.
When a guard spots a potential predator such as an eagle,
she sounds a warning.
Those trills mean ''Danger seen, but evasive action not needed''.
(SQUEAKS)
A bobcat. Another warning.
(SQUEAKS)
Each female is trying to keep the enemy in view
so it doesn't catch them by surprise.
The bobcat is getting closer. Now the warning will change.
That means ''Run for it!''
Another small rodent that would make a tasty meal
lives here in the arid north of Kenya.
There are dozens within a few yards of me.
But this one has adopted the safest strategy of all -
to spend nearly all of its time below ground.
And this is all that most people will ever see of it.
A naked mole rat.
They use their teeth for digging.
Uniquely, their lips close behind the teeth.
That stops earth getting into their mouth.
With this superb equipment, they dig a network of tunnels
radiating from a central chamber
where the colony gathers and the babies are kept.
All the babies are produced by one big female, the colony's founder.
The rest, and there may be as many as 100,
are her children and none will breed here
as long as their mother is alive.
Their job is to tunnel away, searching for tubers and roots.
By doing that, they create a vast network of tunnels
stretching for hundreds of yards under the African plains.
Above ground, it's hard to find any plants
that might provide mole rats with an underground meal.
Mostly there's just dry grass and the odd acacia seedling.
When mole rats meet, they assess one another's status with a sniff.
The junior one has to pass underneath the senior one.
Being blind, they're just as happy travelling backwards as forwards.
I'm about a quarter of a mile from where I saw the mole rats digging.
This is the plant I've been looking for.
Now, in the dry season, it's nothing more than a curly stem
and a few withered leaves,
but beneath the ground here, there's treasure.
And here it is.
Tubers like this are few and far between around here.
It's a matter of pure chance as to whether the mole rats bump into one.
So mole rat colonies must have several dozen workers,
but if they do find one like this,
it could sustain the colony for two or three weeks.
Their sharp incisors cut into the tough tuber.
Some of it, the lucky finders will eat on the spot.
They drag back lumps to the central chamber to be shared by the colony.
This arrangement, where workers are sterile and labour away
to support their mother, is also the basis for the colonies of bees and ants.
But naked mole rats are the only mammals
that have adopted it.
Although those long chiselled front teeth are rodent hallmarks,
rodents are also famous for reproducing with extraordinary rapidity.
Mole rats certainly do that,
but the most prolific members of the whole family are the mice.
Tiny harvest mice produce babies in a carefully woven nest
among the stems of reeds or tall grasses.
Litters of up to eight babies
fed on their mother's rich milk mature in only a few weeks.
Each pair can produce three litters, nearly 30 young, in a single season,
provided that the adults can find enough seeds to feed on.
Youngsters leave the nest at only two weeks old
and start searching for food.
To begin with, they're not nearly as agile as their parents.
In fact, sometimes they seem quite unsure about where they can safely tread.
These little creatures have lived in our cornfields,
taking a small proportion of our crops, ever since farming began.
Usually they were only visible when crops were gathered,
which is why they were called harvest mice.
Even after harvesting, there's still the spilled grain to feed on.
In the vast wheat belt of Australia, a different mouse takes advantage
of this plentiful supply of food,
digging burrows along the edge of the sandy fields.
These are common house mice, the most prolific breeders of all.
Each new female can become pregnant at only five weeks old.
If there's a good supply of grain, she will breed every month or so.
In some years, if all the grain gets eaten,
the mice head for the nearest farm buildings to look for food.
As they leave the stripped fields, it's clear how many have been feasting.
Once they do find their way into a grain store, they become a plague.
When they finish the grain, their numbers will fall as quickly as they rose.
On the flat, windswept plains of Patagonia lives a much larger rodent,
with a more leisured system of breeding.
These are Patagonian cavies or maras.
Although this litter appears very large,
you can see all the young are of slightly different ages.
That is because they belong to different parents.
Only one pair is on guard. The others are away grazing.
This is a communal creche.
It's based around a hole often dug initially by burrowing owls.
The maras have simply enlarged it.
The adults guard each other's offspring,
but each female will only give her milk to her own babies.
The others must wait until their own mother arrives to take her turn as guard.
Here comes the next pair.
Adults are not always related, so the changing of the guard
can be a somewhat tense business.
The mara couples behave as though they were rivals rather than friends.
Galloping around, grazing, the maras have evolved long legs.
They look more like antelopes.
That may be why they're not good at digging
and prefer to steal holes from the burrowing owls.
A plover warns of approaching danger.
(PLOVER CHIRRUPS)
The babies, thanks to the owls, have a hole to go to.
It's not big enough for the adults. All they can do is run.
There is yet another kind of rodent that gather in vast numbers to graze.
They remind one of the huge herds you see
on the savannahs of East Africa.
But we're in South America,
and these are not wildebeest or antelope.
These are the biggest rodents in the world: capybara.
Capybara are, like mara, related to guinea pigs,
but these are real giants that use their rodent teeth to crop grass,
lots of it.
(SQUEAKING BARK)
When they want to, they can gallop as fast as a small horse.
Although they're very agile on land, they are equally at home in water.
Within these herds, each family sticks together in groups of 20 or so.
There are mothers and daughters, children and grandchildren.
And just one large, dominant male. That's him at the back.
He marks his territory with scent from a gland on the top of his nose.
And he will chase off any rival males that venture too close to his family,
particularly to his females.
Capybara will graze out on the dry savannahs,
but they're most at home in the water.
They have oily, water-resistant fur, partially webbed feet,
and their eyes and nostrils are on the top of their heads.
Almost completely submerged, they can hear, see, and smell
what is happening on dry land nearby.
Even the male's scent gland, which other rodents carry on their cheeks,
has migrated up to the top of his nose.
During the rainy seasons, the plains flood.
The capybara are in their element.
If maras are the South American equivalent of antelope,
these surely are the rodent's version of hippopotamus.
Rodents truly are the most adaptable of mammals.