Chisellers The Life of Mammals

Morning in the rainforest of Panama.

Capuchin monkeys are having a rather wasteful breakfast.

Inside these nuts, there is a kernel that is very, very good eating.

But the tree has protected them with 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 is hard to believe, but nonetheless true,

that there's an animal in these Central American forests that can open these nuts with its bare teeth.

But it's certainly NOT these monkeys.

The best they can do is to scrape off the thin, fleshy coating.

The nut itself, they simply throw away.

But another animal is listening for that telltale sound.

GENTLE THUD AND RUSTLE

It's an agouti.

WHISPERS: 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.

Agoutis belong to a large and highly successful group of mammals, the rodents.

And rodents specialise in being able to chisel their way through almost anything.

But 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 something hard wears down the dentine faster than the enamel,

leaving an edge as sharp as a carpenter's chisel.

And unlike the front teeth of many other mammals, a rodent's grow continuously,

replacing what gets worn away.

Armed with these formidable teeth,

the rodents have become the most successful and numerous of all mammals.

The largest group of them are the rats and mice.

There are 1,300 species of those and they are 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 use their teeth to open cones and collect seeds that few other mammals can reach.

Abert squirrels use their teeth, not only to cut into cones,

but to nip off the tips of the branches of ponderosa pines,

where the underlying bark is particularly nutritious.

This is a valuable food reserve if the pine cones run out during the long northern winters.

Abert squirrels are now such specialised feeders

that they can only survive where there are plenty of ponderosa pines.

Most squirrels, however, have more general tastes and high on their menus are often acorns.

Oak trees usually produce a heavy crop of acorns each autumn.

This is good news for squirrels like these greys,

because they must eat a great deal to fatten themselves up for winter.

They must also bury some to eat later on when times get hard.

But for squirrels here in the woodlands of the eastern United States, in Virginia,

things are not quite so simple.

Here there are two kinds of oaks - red oaks and white oaks -

and their acorns are very similar.

This is the acorn of a white oak.

And this, a red oak. This one is just 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 between the two and treat them differently.

When a squirrel finds a white acorn, it always eats it immediately,

because otherwise it would soon germinate.

On the other hand, red acorns, like this one, are almost always buried as a store for winter.

Squirrels are colour-blind, so they can't recognise the difference between the two acorns by sight.

It's the smell that tells them which should be eaten straightaway and which should be buried for later.

Some years, the white oaks produce their usual bonanza of acorns,

while the red oaks provide hardly any at all.

When this happens - as now - the squirrels change their tactics.

You might think this one is tucking into a white acorn as usual,

but after only a quick nibble, it takes it away and buries it.

What's it up to?

It has neatly cut away the tip, beneath which lies the embryo of the seed.

And once that has gone, the acorn will never germinate.

But its food store will remain fresh throughout winter for the squirrel.

So, with dexterous paws to hold the acorn

and sharp front teeth to cut out the embryo with the precision of a surgeon,

the squirrels get the best possible value out of these apparently well-protected nuts.

Seed-producing plants grow pretty well everywhere,

so, potentially, seed-eating rodents can live everywhere, too.

But there is one problem.

Although all seeds are full of nutriment, most of them are small, like these from these desert plants.

You'd need about 500 of these to make the food equivalent of a typical nut like an acorn.

Any rodent wanting to store the tiny seeds produced by plants like these faces a transport problem.

It would be hopelessly impractical to carry them away one by one, as squirrels carry acorns.

Here in California's Mojave Desert, there is a little rodent that has solved that problem,

and its burrows are all around me.

Like most desert animals,

the creature that owns this hole usually doesn't come out until after dark, when it's cooler.

But 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 much later, when the moon was up, that the burrow's owner eventually showed itself.

A kangaroo rat.

It can carry hundreds of seeds at one time because it has expandable cheek pouches.

The pink toes, poking out from the fur,

are those of its back legs, which are huge and support its whole body.

That's why it is called a kangaroo rat.

Its front legs, hidden beneath its chin, are shovelling the seeds into its mouth.

Normally a kangaroo rat would have to travel quite a long way to find enough seeds to fill its pouches.

But because of what I have supplied,

this one's shopping bags are soon full enough to take to its burrow.

There we are!

And it covers them over with sand, just to keep them safe.

Its deep burrow is a relatively safe place

and the kangaroo rat sleeps here throughout the day and much of the night.

Even so, the slightest suspicious sound from outside will wake it.

WHISHING

A gopher snake is crawling 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, of course, are not the only part of a plant that is edible.

Some rodents, like these marmots that live in the European Alps, eat the entire thing.

And they fight over grazing rights.

When Alpine marmots come out of their burrows in late spring, after their winter sleep,

they immediately set about re-establishing their territorial boundaries.

Calling and tail-flagging are only the preliminaries. WHISTLING CALL

If that doesn't work, there is nothing for it but to go to battle.

The senior males do most of the fighting,

while their female partners guard the family burrow and watch. CALLING

When a male wins a territorial scrap,

he marks the boundaries just established 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 together with a few of last year's offspring,

all of them females.

Last year's males have already been chased away.

Although daughters are allowed to stay, their mothers constantly beat them up, particularly in spring.

They persecute them so severely that, if any of the youngsters have become pregnant, they abort.

The reason for this strange and merciless behaviour doesn't become apparent until later in the year.

By midsummer, relative peace has come to the mountainside.

This season's babies are about to emerge from the maternity burrow for the first time.

There are usually between four and six babies in a litter

and they immediately get down to the important business of grazing.

Alpine summers are short,

so marmots have to feed as fast as they can, while they can.

Their incisors slice with ease through the tough stalks of flowers and grasses.

The babies put on weight rapidly, but even so,

by the end of the summer, they still do not have fat reserves to match those of their parents.

In the autumn, the family start on their final harvest -

hay to line their deep burrow, in which they will sleep through the winter.

By October, the weather has turned really cold.

Down in the burrow, the marmots' body temperature has dropped to 2 degrees centigrade

and their hearts are beating only two or three times a minute. They are hibernating.

They are sustained only by their fat reserves which will have to last them until the following April.

They snuggle together to minimise the loss of heat.

The youngsters are always in the middle of the pile.

With less body weight, they can't afford to let their temperatures drop as low as the adults.

The thermal-imaging camera

shows that a baby, glowing orange, is several degrees 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 of youngsters warm enough throughout the winter.

That is why the dominant female made quite sure that none of her youngsters produced any babies.

But hibernating isn't the only way to survive the winter in the mountains.

Here in North America, the winters can be just as severe as they are in the high Alps.

Yet here there is a rodent that manages to find food throughout the winter months,

and 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 a family that lives here in this lake

at the foot of the Teton Mountains in Wyoming.

While beavers can get around perfectly well on land, they are most at home in the water,

where their webbed hind feet and paddle-like tail make them powerful swimmers above and below the surface.

Like marmots, beavers feed on all kinds of vegetation,

and eat wood as well as leaves.

And they are accomplished engineers - this great pond is entirely their own creation.

Only a few years ago, this shallow, pebbly stream flowed straight 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 has been lined with logs, some of them big and quite heavy.

And on this side, it has been packed with mud and vegetation.

It has 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.

And the lake it has created stretches upstream for almost a mile.

So important is their dam to them,

that if they detect the slightest leak - usually by hearing the sound of 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 the beavers several advantages,

one of which is that it floods the surrounding woodlands

and so enables them to swim in safety 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 incisor teeth to strip off the bark from a fallen tree trunk,

while still being close enough to water to slip away should a bear or a mountain lion turn up.

Their network of channels also enables them to ferry whole branches back to their pond.

And there, 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 the vegetation will keep fresh through the long winter when the pond is covered with ice.

Stocking the fridge takes a lot of work

and the beavers are at their busiest in autumn.

At one side of the lake, stands their lodge -

a fortress built of branches and boulders that is so strong that not even a bear could break into it.

The only entrance is through tunnels that open underwater

and the beavers take refuge here whenever they are alarmed.

That was a warning signal to say that danger was around - that is me.

And now I may not see the beavers for some time.

They can stay underwater for five minutes at a time - up to 15, if they need to.

They can get back to the safety of their lodge without putting their head above water for a single second.

Most lodges have at least two different entrances.

By October, winter is well under way, but whereas marmots would now be hibernating,

the beavers are still active and will remain that way throughout the winter.

Even when the pond ices over completely,

they are still able to swim under the ice to get back and forth to their lodge.

No-one knew exactly what went on inside the lodge during winter,

so when the beavers were away, we installed a couple of infrared cameras in order to find out.

A branch from the fridge is being brought back to the lodge for the whole family to feed on.

And another.

No wonder they don't need to hibernate, with this ingenious set-up.

The lodge is warm and safe, even in midwinter,

and 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 actually know, in the pitch blackness, that there are strangers among them?

We noticed that the muskrats regularly left the lodge to forage under the ice.

And on several occasions, they returned a few minutes later with a load of fresh reeds.

Perhaps the muskrats are 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 very safe home for a beaver.

Other rodents take more active steps to defend themselves, and one is positively dangerous.

Darkness provides one of the best forms of defence

and it is very dark right now.

The only reason you can see me in the middle of the African night is that we are using a starlight camera.

So, many rodents don't come out except under cover of darkness -

and that applies even to one of the most ferocious and well-armed of all rodents.

I think there could be one around here.

WHISPERS: 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 that they are half erect is a sign that it's not too happy to have me so close.

Those long quills are VERY sharp.

And I'm quite sure it knows how to use them.

It is turning its back on me - but don't be fooled. That's not because it is 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.

Porcupines feed mainly on roots and tubers, but they are big animals

and to find enough to eat, they must travel long distances every night across open savanna.

That means that they have to be prepared to take on all comers.

This young leopard has probably never been close to a porcupine before,

so it doesn't realise how dangerous it would be to interfere with it.

The porcupine, on the other hand,

seems to have total confidence in its armoury.

If it's not very careful,

that naive young 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 the hills of California and make their burrows close to one another

so there are plenty of watchers to give warning of the slightest danger.

This is particularly important in 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 are nearly all related - mothers, sisters and daughters -

and each looks after her nephews and nieces as carefully as she does her own offspring.

When one of the guards spots a potential predator, such as an eagle, she sounds a preliminary warning.

Those trills mean, "Danger seen but evasive action not needed."

A bobcat - and once again a warning.

HIGH-PITCHED SQUEAK

Each female is trying to keep the enemy in view

to ensure it doesn't sneak up and catch them by surprise.

But the bobcat is getting closer. Now the warning will change.

LOUD CHIRRUP That means, "Run for it!"

Another small rodent that would make a tasty meal for a predator

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 burrowing equipment, they dig a great 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 founder of the colony.

All the rest of them - and there may be as many as a hundred - are the founder's children,

and none of them will breed here as long as their mother is alive.

Their job is to tunnel away, searching for tubers and roots,

creating a vast network of tunnels that stretches for hundreds of yards under the African plains.

Above ground, you have to look very hard

to find any sign of a plant that might provide mole rats with an underground meal.

Mostly, there is just dry grass and the odd acacia seedling.

When mole rats meet, they assess one another's status with a sniff,

and the junior one has to pass underneath the senior one.

They are blind, and in the darkness they are just as happy travelling backwards as forwards.

I've now come about a quarter of a mile from where I saw those mole rats digging,

and this is the plant I have 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, there is treasure.

And here it is.

Tubers like this are few and far between here

and it is a matter of pure chance as to whether the mole rats, burrowing underground blindly, bump into one,

which is why mole rat colonies have to 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 also drag back lumps to the central chamber to be shared by the rest of the colony.

This colonial arrangement, where all the 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 of any kind that have adopted it.

Although those long, chisel 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 their 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,

that is up to nearly 30 young, in a single season,

provided that the adults can find enough seeds to feed on.

The young leave the nest when they are only two weeks old and start searching for food by themselves.

To begin with, they are not nearly as agile as their parents.

In fact, sometimes they seem quite unsure about just where they can safely tread.

These little creatures have been living in our cornfields,

taking a small proportion of our crops ever since farming began.

Usually they only became visible when the crops are gathered, which is why they were called harvest mice.

Even after harvesting, there is still the spilt grain to feed on.

But in the vast wheat belt of Australia,

a different mouse takes advantage of this plentiful supply of food,

digging their burrows along the edge of the sandy fields.

These are common house mice, the most prolific breeders of them all.

Each female is capable of becoming pregnant at only five weeks old,

and if there is a good supply of grain, she will breed every month or so.

In some years, if all the grain out in the fields gets eaten,

the mice head for the nearest farm buildings to look for food.

As they leave the stripped fields, it becomes clear just how many have been feasting out there.

Once they do find their way into a grain store, they become a plague.

But eventually, 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 measured system of breeding - the Patagonian cavies or maras.

Although this litter may appear to be very large, 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, in fact, is a communal creche.

It is based around a hole that was often dug initially by burrowing owls.

The maras have simply enlarged it.

Although the adults guard each others' offspring, each female only gives her milk to her own babies.

The others will have to wait until their own mother arrives to take her turn as guard.

Here comes the next pair.

The adults are not necessarily 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 out on the plains,

the maras have evolved long legs, so that they look more like antelopes than rodents.

That may be the reason that they are not particularly good at digging

and prefer to steal holes from the burrowing owls.

A plover warns of approaching danger.

The babies, thanks to the owls, have a hole to go to.

But it's not big enough for the adults. All THEY can do is run.

There is yet another kind of rodent

that gather together in vast numbers to graze.

They really do remind one

of the huge herds that you see on the savannahs of East Africa.

But we are in South America,

and these are not wildebeeste or antelope.

These are the biggest rodents in the world - capybara.

Capybara are, like the mara, related to guinea pigs,

but these are real giants.

And they use their rodent teeth to crop grass - lots of it.

BARKS

When they want to, they can gallop as fast as a small horse.

Although they are 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 vigorously chase off any rival males

that venture too close to his family, and 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

so that they can remain almost completely submerged

but able to hear, see and smell what is happening on dry land nearby.

Even the male's scent gland, which most other rodents

carry on their cheeks, is on the top of his nose.

During the rainy seasons,

the plains flood and the capybara are truly in their element.

If the maras are the South American equivalent of antelope,

then these surely are the rodents' version of hippopotamus.

Rodents truly are the most adaptable of mammals.

Rodents really do have the most extraordinary spatial memory.

I've often wondered, watching grey squirrels pinching all those peanuts that we provide in such quantities

and burying them in the lawn, how many of those nuts they will, in fact, retrieve.

A lot of experimental work has been done on that, and the results are astonishing.

This is a true master of food storage -

one that even prepares its food to ensure it doesn't germinate.

But squirrels also need complex survival strategies

to see them through the winter.

It's a fine balance between hoarding enough food

and remembering where it all is. Get it wrong and you face starvation.

The kangaroo rat has a simpler strategy.

It crams as much as it can

into its food pouches then stores the seeds in an underground larder.

They'll need to be checked from time to time to prevent them going mouldy,

but the kangaroo rat doesn't have to memorise a set of different locations.

Likewise, the chipmunks that hibernate through the Siberian winter have a larder next to the bed,

so there's no need to venture out into the cold.

If larder hoarding is so successful, why do squirrels scatter their caches around?

Scatter-caching is related to a risk-averse strategy,

so that you're not going to suffer any major losses.

It also seems to be associated with an inability to defend.

So, if you have lots of species of seed-eater around and you can't defend that larder against them,

then you could die over the winter if you lose your larder.

So it might be a better strategy to spread it around. At least you won't lose everything.

The disadvantage of scatter-caching

is that the squirrels need to remember where they have buried up to 3,000 acorns.

Of course, oak trees rely on the squirrels' less-than-perfect recall,

as some of the acorns they plant will grow into new saplings.

So, how do squirrels manage to locate at least some of those buried nuts?

At Exeter University, they tested the squirrels' spatial memory.

The ring of pegs has some hazelnuts cached inside,

but only when the red marker is present.

Within days, the squirrels learn

that the red marker

is a landmark that means food

and the time it takes each one

to find the nut was carefully logged.

The experiment was removed for two months,

then the nuts and the red marker were replaced

and the squirrels timed again.

They were just as fast as before.

They had retained a detailed memory of the set-up,

including the colour of the marker, for over eight weeks.

So far, we've seen how well they can remember things over a period of a few weeks.

But if caching is to help them survive the winter, they'll have to remember things for longer than that.

How good is their long-term memory?

Caching animals do need to remember where their caches are over a long period of time.

It is quite likely that they come back to them and move their caches around,

so they're just refreshing their memory every so often.

So, the mechanisms for food caching are quite complex, but they also seem to be quite widespread,

in birds as well as mammals.

This is a family of acorn woodpeckers in North America.

There may be 60,000 acorns in THEIR larder, but it's not underground.

The woodpeckers manage the larder carefully,

since the acorns shrink as they dry out, and frequently have to be moved to smaller holes.

So the acorn woodpecker is the bird equivalent of the kangaroo rat or the chipmunk,

using a single larder and keeping a close eye on it.

But there's also a bird which uses the squirrel's strategy, a member of the crow family.

In terms of current knowledge,

I think the nutcracker is the champion

of memory for stored food and retrieval.

It's an animal that can store thousands of items and retrieve them accurately later on.

Squirrels, we see them doing this,

but we don't have a comparable piece of evidence to show that they are indeed capable of doing it.

We have also seen them rehearsing their memory in a way that is not seen in their avian counterparts.

So, this memory-refreshing may be essential for the squirrels to match the birds.

They have been studied for over 30 years,

and one extraordinary discovery

was that an area of the brain, the hippocampus,

gets larger in birds that store food over winter,

and it's also well developed in some special mammals.

Just like food-caching animals, London cabbies have to remember hundreds of landmarks and routes.

And they're constantly refreshing that memory by revisiting locations.

I'm gonna do the right into Princes Street, go left into Lothbury.

The traffic might move a bit easier.

Brain scans of cab drivers show a larger-than-average hippocampus,

and the longer you've been a cabbie, the bigger it is.

It's likely that squirrels also use their hippocampus to remember routes,

even complicated ones devised by humans.

It's easy to watch them do it, so why is it so difficult to study?

Well, if you think about the problems that you would encounter

if you had to follow one squirrel...

First, they're quite secretive.

They will pretend to cache a nut and they'll scamper away somewhere else and bury the nut there.

You'd also need to make a map of the caches,

because if you put markers by them so YOU know where they are,

that's a signal to all the other squirrels.

You'd need to check the caches on a regular basis,

to see whether they had been pilfered by other squirrels or other animals.

And you'd also need to follow your squirrel to find out which caches it was recovering,

whether it was eating them or re-caching them.

And you couldn't do it on one squirrel,

you'd need to do it on 10 or 20 squirrels

to get any reassurance that, in fact, the squirrel you're studying

is a normal example of a squirrel.

So, nobody has been able to look at precisely how many caches they're making,

where they're putting them, how long they last and when they recover them.

It would be fascinating if we could.

So, although the sight of a squirrel burying nuts

is an everyday occurrence,

it presents a challenge to science.

That they survive winter is proof of the squirrels' superb memory.

But understanding how they do what they do

will remain a test of our intelligence.

In the next programme of The Life Of Mammals, we meet the hunters,

the lords of the land, the ultimate in lethal grace and beauty.