Insect Hunters The Life of Mammals

100 million years ago, forests like these were just developing,

and were dominated by dinosaurs.

But as the giant reptiles slept,

tiny creatures were stirring.

They were the early mammals.

Despite this humble beginning, their descendants would ultimately take over the whole world.

Yet the rise of this great dynasty was founded on the most surprising diet.

Creatures very like those first mammals are still around today - shrews.

They hunted insects at night,

when most dinosaurs were sleeping.

They were able to generate heat in their tiny bodies

so that they could stay active in the cold night air.

But doing this burns a lot of food, so they had to eat almost continuously - as shrews still do.

There's never enough food for a shrew.

Rivals fight over hunting rights with extraordinary ferocity.

HISSING

This little insect-eater has now staked his claim to the food in this part of the woodland.

When he meets a female, he is almost as aggressive towards her as he is towards a rival male.

After testing one another's strength, the female accepts the male as a contestant and as a mate.

Two weeks later, the young are born.

Their mother has nourished them inside her womb, so they arrive comparatively well-developed.

Caring for the young is a crucial part of a mammals' winning design -

something few reptiles do.

A mother shrew will even quench her babies' thirst with her own saliva if necessary.

Most important of all,

she provides them with that uniquely mammalian food - milk.

This milk is so rich that it takes just two weeks for the young to approach their mother in size.

By this time, they are quite a handful and need to be weaned from the nipple, despite their protests.

Their mum doesn't abandon them. She leads them into the world outside.

And the young have their own particular way of ensuring that they don't get lost.

The first mammals lived alongside the dinosaurs for a very long time,

but then, about 65 million years ago, when the dinosaurs so suddenly and dramatically disappeared,

they had their chance to colonise new environments.

At first, they remained much the same - small, scurrying creatures.

But that is a versatile body pattern, and one of them, without much change, took to the water.

It hunts as frenetically as its cousins do on land, but it has a different way of catching insects.

The water shrew's fur is oily and sheds water with a slight flick.

Its splendid whiskers are long to help it feel for prey underwater.

Its ankles are hairy, so its feet serve as excellent paddles.

It shines like silver, glistening from the bubbles trapped within its fur as it searches for prey.

Clinging to a root, a dragonfly larva -

but the shrew's whiskers don't touch it, and it's missed.

But not this time!

In Africa's Namib Desert, another insect-hunter swims after prey -

but without a drop of water in sight.

It's a sand swimmer -

a golden mole.

Sand, unlike water, scratches and it isn't transparent either.

The mole's eyes are covered with skin, its head a wedge with which it forces its way through sand.

As it digs,

sand collapses behind it, making it impossible for a tunnel to form,

so it doesn't dig through the sand - it really does swim.

Sound travels well in sand. Unlike shrews, which are adapted to hear high-pitched sound,

this mole detects very low ones -

like the faint vibrations made by foraging termites.

Propelled by its flippers and guided by sound, the mole homes in

on its prey.

In North America, another mole has paws that look like flippers.

These help it to swim under ice

to collect insects - but this is not their primary purpose.

This creature is a digger - a star-nosed mole.

Its paws are spades for pushing aside soil,

while it tries to locate its prey with its astonishing nose.

This has 22 fleshy arms.

Each is so packed with nerve endings,

that the mole could touch a pinhead with its nose in 600 places at once,

allowing it to locate the tiniest of prey.

Living in soil rather than sand,

this mole can dig proper tunnels.

It constructs a labyrinth of passages

and patrols them to collect any prey that drops into them.

The star-nose, underground, is largely beyond the reach of predators.

Other insect-hunters, however,

run along trails above ground and they are not so lucky.

And one of these trail runners lives here in the scrublands of East Africa.

This tiny pathway through the withered grass

shows the insect-hunting rights for this area have been taken.

To advertise the fact, the owner has left a little pile of its dung.

But what could have made it?

Well, to find out, I can use this tiny surveillance camera.

If I put that there

and then, just in front of it, put some twigs across the path...

The creator of these runways is fastidious and with any luck,

it will stop to clear the twigs

and then give us a chance to have a good look at it.

This is the picture from the camera I've just placed,

and this is from another camera farther up the same trail.

And now all I have to do is to wait.

It's an elephant shrew!

He's not going to like that!

There you go. He's clearing his trail.

Oh!

Oh, dear!

I'm afraid I have put in too much!

The elephant shrew or sengi keeps its trails immaculate for a very good reason.

It sprints to evade its enemies.

Even the smallest twig could cause a stumble that could be disastrous.

The goshawk has such keen eyesight that spotting a sengi is no problem.

Catching one is another matter.

The sengi holds a map of its trails in its mind

so that in emergencies it can cut corners to dive for cover.

Even a brush with death doesn't put a sengi off its food.

Like all small insect-hunters, it needs to constantly fuel its internal fires.

That is especially important when there are young to feed.

Incredibly, this sengi is only a few hours old.

Few mammals are born as well-developed as a baby sengi.

This gives them a survival edge.

Daytime in the African bush is no place for the helpless. Sengis are born to run.

Its appetite for milk is unquenchable -

growing at this speed gives it constant hunger.

Its mother has nipples near her shoulders, which makes them easier to reach and helps a quick get-away.

The baby will take solid food from its mother on its very first day if it gets a chance.

SCREECHING

With continued help from its mother, the youngster will be almost fully grown within a week

and be able to run as fast as her along their racetracks.

Catching insects one by one takes a lot of time and a lot of energy,

and very few creatures that feed that way can get enough to build and sustain big bodies.

But some insect-eaters, about 40 million years ago,

solved that problem by broadening their diet.

And one of their descendants lives right here in my garden in London.

And I can tempt it out with a wide variety of food, including, for example, minced meat.

The hedgehog is a creature of the night, but it's too big to hide in the leaves.

That makes it vulnerable to attack from animals like foxes.

To make up for this, its hairs have become a cloak of prickles.

And if it thinks it is in real danger, it's got a special trick.

The hedgehog will stay an impregnable spiny ball like this

until it decides that danger is passed.

But one thing is guaranteed to make a male hedgehog drop his guard -

an amorous liaison.

If you are outside on a spring evening,

you may be lucky enough to witness an extraordinary sight.

You might think that having a coat of spines on your back

would be something of a handicap when it comes to the intimacies of courtship.

Classical naturalists thought that hedgehogs actually mated

belly to belly.

The male noses the female's spines, which seems to excite her.

Although, as far as he is concerned,

it does look rather painful.

Whether the female flattens her prickles to help the male is unclear,

but it does seem that the old joke that asks, "How do hedgehogs mate?" is true.

The answer is, of course, with great care.

The early American insect-eaters also needed to protect themselves,

but they did so, not with spines, but with armour plating.

Armadillos, like hedgehogs, grew large by broadening their diet.

Their tastes change with the seasons.

Fruit is easy to collect, but the nine-banded armadillo is not fussy

and will pick up anything that looks edible.

It still eats insects,

but ants present it with a problem.

Its armour protects it from large predators, but it isn't good defence against small prey.

One extraordinary African insect-hunter has no such trouble.

It's a pangolin. Its horny scales, like the hedgehog's prickles, are made from modified hair.

Its big front claws are useless for walking.

It trundles along on its hind legs,

balancing its torso with its tail.

Its front claws are reserved for digging up ants.

As it does so, it swallows stones.

They accumulate in its stomach and help to grind up the ants.

But these small underground ant colonies

are mere snacks to a pangolin.

This is a real meal - a full-size ants' nest.

There are a million or so of them in here.

The pangolin smashes through the nest wall with formidable power.

Only an adult has the strength to do this.

Young ones stay with their mother, feeding in her wake until they are big enough to dig for themselves.

The angry ants swarm all over their attacker,

but the pangolin's armour is a very effective defence.

Its eyes are protected by thick lids,

and its nostrils and ears have special valves to keep the biting insects out.

For its size, the pangolin has the longest tongue of any mammal -

and the stickiest saliva.

But mammals didn't always have ant colonies to feed on.

The rise of social insects, 60 million years after mammals arrived, was a landmark in evolution.

It was then that termites and ants

started to build huge nests, each containing millions of individuals.

Here was so much food that insect-eaters could grow big.

There are termites in the Americas

just as there are in Africa,

so of course, there are termite-eaters, too.

And here in Brazil is the biggest of them all - the giant anteater.

Its eyesight is very poor

and it relies mostly on its sense of smell, which is very acute.

But if I keep downwind of it,

I may not disturb it too much.

The truth is that ants and termites aren't very nutritious,

so the giant anteater has to do all it can to conserve energy,

and one way of doing that is to sleep for 15 out of 24 hours.

It covers itself, too, with that big bushy tail

to reduce heat loss to a minimum.

And it also keeps its body at as low a temperature as any mammal - 32 degrees.

That means, of course, that its brain does not work very fast.

It's not an animal with lightning reactions or dazzling intelligence,

but then you don't really need that if you are an anteater. And now I think I will get out of its way.

Termite mounds are more numerous here than anywhere,

but the challenges facing a termite-eater are considerable.

Anteaters and pangolins have different ancestors

but the demands of their diet has shaped them in similar ways.

Both have big claws - the giant's are the largest of any mammal -

and both have an immensely long tongue that slips through the tube formed by the toothless jaws -

so that both can virtually drink termites.

He may lack teeth, but I am going to treat him with caution,

because, in fact, those huge claws and those powerful front legs can be very dangerous.

He can rip apart this termite hill,

and if he wants to defend himself,

he will use those big bowed legs and their claws and grip you.

It has even been said that the carcass of a jaguar was found in the embrace of one of these.

It only collected a few hundred termites on that brief visit.

As soon as it breaks into a mound, the inhabitants attack it so ferociously that they drive it away.

But quick sampling like this does have an advantage.

The termites will soon replace the ones they have lost, so, in effect, the anteater is harvesting

the termite hills in its territory

to ensure a continuous supply.

It may not have a dazzling intelligence,

but nothing exploits termites more effectively than the giant anteater.

If you want to explore the origins of this extraordinary animal,

you would have to go to a very surprising place.

I'm near Messel in Germany.

Behind me is a quarry rich in the fossilised remains of animals that died 50 million years ago,

and that was a pivotal time in the history of the mammals.

Even though these animals lived a very long time ago, some of them look remarkably familiar.

This is a tree anteater, very like the tamandua anteater that lives in South America today.

All the insect-collecting equipment is there -

huge claws on the front legs,

no teeth, and jaws fused into a tube, through which a long tongue would have flicked.

And alongside the anteater, the first known pangolin.

Once more it has huge claws - no teeth.

And again it looks identical to its living equivalent,

the African pangolin of today.

Why have these animals remained unchanged for 50 million years?

The rocks of Messel provide an answer to that.

From them has come a termite - more importantly, the queen of a termite colony.

It's the same in every important respect as its living relatives,

and this is the key.

If termites haven't changed for 50 million years, why change the design of the termite-eater?

Even back then, the majority of insects were airborne and out of reach of ground-dwelling mammals.

But one mammal followed the insects into the air,

and fossils of it have also been found in the Messel deposits.

It's a bat.

The ability to catch insects on the wing is an extraordinary achievement. How do the bats do it?

This is a great place for bats. There are many insects flying around.

Just now, birds are feeding on them and bats are asleep in their roosts.

But soon, it will get dark

and then the birds will go to roost and the bats will come out to claim their share.

At night, there are even more flying insects than there were during the day

and by the mill stream is a colony of Daubenton's bats that are already stirring.

Their faces are so like a shrew's

that it's easy to imagine shrew-like ancestors in the trees, jumping from branch to branch,

chasing insects.

Ever larger flaps of skin between their fingers extended those jumps

until, eventually, they could fly.

And how they can fly!

The change from a scurrying animal like a shrew to a fluttering bat

is surely the most magical in the whole history of the mammals.

The bats' mastery of flight is so complete

that few insects can outmanoeuvre them in the air.

The bat scoops up the moth with the membrane around its tail

and passes it forward to the mouth.

Their ground-living ancestors probably used sound

to find their way in the night, as shrews still do,

but bats perfected that technique, using sound frequencies beyond our hearing.

A bat detector makes those calls audible to us.

Bats emit high-intensity pulses of sound and then listen to the echoes that bounce back.

Their brains process these reflections

to give them a 3-D image of their surroundings

and their prey.

Moths, with their laborious flight, are relatively easy to catch.

But then some evolved a defence -

a simple ear, so that when they hear the sonar of a bat approaching, they can swerve out of the way.

So, one bat changed tactics.

The long-eared doesn't hunt with sonar.

It uses its enormous ears to listen for prey.

It can filter the sound of a moth's wing beats

through the noise of the rushing water.

Its sonar guides it through the branches,

but as it nears the moth, it turns that off and enters "stealth mode".

Now it's guided solely by the noise of the moth's wing beats. But the system isn't perfect.

The bat can hear the moth through the leaf

and is approaching it from the wrong side.

A lucky escape for the moth.

But now the bat has come round to the other side. If the moth stays still,

it makes no noise, so the bat can't locate it.

But, sooner or later, the moth will have to move.

And that is its undoing.

But how could a bat catch prey that is silent in a place like this, so cluttered with vegetation

that echolocation shouldn't work?

These places are tricky to navigate, but full of food.

Spiders are more nutritious than moths, but they're silent, venomous,

and construct webs so strong that a bat could easily become entangled in the sticky silk.

Here comes Natterer's bat.

It seems well aware of the almost microscopically thin silken threads

and, with surgical precision, removes the spider from its web.

It even reverses away from the web to avoid getting entangled.

To detect the threads and recognise on which side the spider is sitting

must be the ultimate refinement of sonar.

Mexican free-tailed bats.

They form some of the biggest and the densest assemblages of mammals to be found anywhere on the planet.

There are 12 million in this cave.

But where do such a vast number of individuals find food within flying distance of where they roost?

That puzzle baffled people for a long time.

But now we're beginning to discover what they feed on and where they find it - and it's very surprising.

A few years ago, pilots flying above Texas reported seeing bats at high altitude.

Scientists investigated and made an extraordinary discovery.

As I climb into the evening sky, the weather conditions seem good.

But the local weather radar shows a storm nearby, growing with alarming speed.

However, I needn't worry.

This is not a storm. It's the bats we just left, leaving their roost.

They start from a number of points, each the mouth of a different cave.

The swarms are vast, with up to 20 million bats leaving one entrance.

Some fly low over Texas,

but, curiously, most start to climb.

At 10,000 feet up, bats are so widely dispersed

that it is very difficult to see them,

but I've got my bat detector.

-HIGH-PITCHED CLICKING

-And there's one. And, what is more, that is a feeding buzz.

So, they're eating something. The question is - what?

They're a kilometre above the ground and most are still climbing.

But now the radar picks up another front blowing in from New Mexico

and the bats are flying towards it.

What could attract them to these great heights?

Scientists find out what's flying high in the sky at night with a device like this.

And in it...

moths.

Vast numbers of these insects use the prevailing winds at altitude

to travel from the tropics to feed.

Bats climb up to three kilometres into the night sky to catch them.

Bats are so numerous and so voracious that the individuals in this one cave below me

eat 120 tons of insects every night.

So, if bats have such ravenous appetites,

how do they survive in the winter, when there are no flying insects?

In Texas, they migrate.

Here, in Canada, they have a truly radical solution.

Outside, it is 20 degrees below freezing.

Inside, icicles hang from the ceiling.

And yet these little brown bats can survive

throughout the winter without a single meal.

How do they do it?

The thermal-imaging camera is showing my face

as red and orange.

That's because it's warm. I'm a mammal.

Putting it more precisely, I am losing energy as heat.

But these little bats are blue, because they are cold -

as cold as the rock to which they are clinging.

As the bats are no longer losing any heat to their surroundings,

they are using hardly any energy

and their metabolism has slowed down almost to a stop.

Although in the deepest hibernation,

they have to wake up now and then to have a drink.

As they fire up their body chemistry,

so their image on the thermal camera glows like a furnace.

Once awake, a male seeks out the slumbering females.

He won't get a warm reception to his advances,

but he won't meet with much resistance either.

He will mate with several more

and then, after a drink, he will return to sleep until the spring.

Flight not only enabled bats to catch insects in the air -

it also allowed them to extend their range

far beyond that of any other mammal.

Bats were the first mammals to find their way to some fragments of land isolated in the South Pacific -

New Zealand.

Here, there were no cats, no rats, but lots of insects -

paradise for any insect-hunter.

So, the bats flourished and their descendants are still here...somewhere.

To see them, I must wait for darkness.

And this is the species I have been waiting for.

These bats look normal enough,

but bats are aerial predators and much of the uneaten prey in New Zealand is on the ground.

They can fly all right,

but our infrared camera reveals

that they also have a very un-bat-like way of hunting.

They land on the ground and forage through the leaf litter,

just like shrews.

They are walking on their wrists,

with the bones of their fingers pointing up and slotted into a groove along the upper arm.

Now they seem to be hunting as a pack.

Insects, or other small creatures fleeing from the jaws of one,

run straight into those of another.

Worms are a great favourite -

so much more satisfying than several hundred mosquitoes -

and they don't want to share them with one another either.

They finish off with nectar from the Hades plant, that blooms on the ground.

They are this plant's pollinators.

Relationships between a plant and its pollinator are slow to evolve,

so these bats must have been scuttling over the New Zealand forest floor for millions of years.

Worms and nectar are easy prey, but what about this?

It's a weta,

a giant flightless cricket with spiny legs and ferocious jaws.

How could bats whose ancestors ate mosquitoes tackle this?

The weta can flick its back legs forward with surprising force.

Even if you dodge that, you have to contend with its powerful jaws.

The insect gains the upper hand...

..but it's soon overwhelmed by numbers

and the bats fight one another over its remains with equal ferocity.

Evolution doesn't often go into reverse, but it seems to have here.

After several million years of aerial combat,

these bats are reverting to the techniques of their ancestors.

Mammals have pursued insects to the far corners of the earth.

They've chased them into the skies and back down to the ground.

The insect-eaters were there right at the beginning of the rise of mammals, and they are still here.

They are one of the great success stories in the life of mammals.

Bats are surely one of the most magical of mammals,

but they're also one of the most mysterious,

as they see the world in a way that is utterly different from our way.

It's SO different that we didn't even know what it was until about 60 years ago.

The action of Natterer's bats plucking spiders from their webs has never been filmed before.

The shots show the precision of their flight and their echolocation.

How did we come to understand the bat's extraordinary use of sound? That's what we'll discover now.

The story starts in 1941,

when an American biologist called Don Griffin was working with physicists at Harvard,

using a revolutionary ultrasound detector.

When a bat flew at him, he became the first human to detect the sounds it was making.

They were very loud and very high-pitched.

This discovery launched a series of experiments to prove that the bats have a sort of airborne radar,

an invention then being used in aeroplanes.

Yet bats could detect tiny targets,

processing their echoes in a brain weighing half a gram.

When echolocation was discovered, in the 1940s,

Don Griffin and Robert Galambos

presented the findings at a meeting.

There was such disbelief with the suggestion that bats use echolocation

that one of the scientists came up to Galambos,

took him by the lapels forcefully, and said, "This cannot be correct."

But Griffin continued testing the bats' ability to avoid fine wires

in the flight cages at Harvard.

By 1960, it was clear that echolocation provided the bats with a detailed sense of their world.

New technology helped to explore that world.

Using high-speed tape recorders to slow down calls

showed different species used sound differently, to suit their habitat.

The intensity of bat echolocation calls really varies tremendously.

At one extreme are "whispering bats",

that we can barely detect on some of our equipment.

At the other extreme, there are bats that can make calls as loud as 130 decibels.

To put this into context, the threshold of pain in human hearing is round about 126 decibels,

so these calls would be painful to us, if we could hear them.

The calls are loud in order to produce clear echoes for the bat

and, to cope, the bat goes temporarily deaf on each call,

by synchronising the nerve impulse of the call with a muscle in the ear, which disconnects the eardrum.

This may happen 120 times a second,

one of the highest rates of muscle contraction of any mammal.

And, of course, it's what the bats hear that creates their view of the world.

Understanding that was a more complex problem.

In the '80s, Trachops, the fringe-lipped bat,

was studied by American scientists Merlin Tuttle and Mike Ryan.

Using ultrasound, the bats navigate effortlessly through the Panamanian rainforest,

but their hunting strategy was extraordinary.

These bats go for a large prey - the mud puddle frog.

Collecting them in the forest and testing them in a jungle laboratory

showed that they could distinguish different kinds of frog calls played through a loudspeaker.

As soon as we turned on the set, the bat just took off from its perch,

made a beeline to the speaker... It was obvious that this bat thought there was a frog inside that box.

By playing different tapes,

they established just what the bats were able to hear in the frog calls.

Then they tested their findings back in the forest.

Studying bats in the wild is notoriously difficult,

but it's a vital step to understanding any animal.

Whispering bats, like Trachops and the British long-eared bats,

rely only on hearing to catch prey.

They can hear the rustling of moth wings

and pluck them from bushes.

But most bats intercept insects by patrolling the air,

listening to the echoes of their own calls and attacking the ones returned by their prey.

We know that the wavelength of the call is designed to produce the best echo from moth-sized objects,

and that the bat increases the rate of calls as it homes in on the target, for greater accuracy.

This is the "feeding buzz".

It's extraordinary how fast the action happens.

We use bat detectors to hear the calls and high-speed filming to slow the flight,

simply to appreciate the behaviour.

These bats can fly six metres in a second and may catch 25 moths in a night.

The studies in the wild have also identified a wide range of different call signatures.

Each is suited to a different habitat.

The calls of woodland-edge bats are distinct from those of forest bats,

and from ones that hunt over water.

By using different frequencies and duration of call,

different species of bats have adapted to different habitats and hunting techniques.

Their ecology is based on sound.

Although research during the last 60 years has revealed extraordinary details

about the way in which bats find their way around and hunt,

there's still a great deal to learn.

How does the bat's brain process the information?

We know that the strongest echoes will be right in front of the bat, so they have a narrow field of view,

and by using a wide spectrum of call frequencies,

the bat's brain may convert the echoes into an equivalent of colour.

But can their sound perception system be as accurate as our colour vision?

One of the challenges is trying to work out

how the nervous systems of these animals cope.

In terms of time, we know from laboratory studies

that bats can make discriminations in the order of ten nanoseconds.

A nanosecond is a billionth of a second.

In terms of distance, this corresponds to a difference in range in the order of two micrometres.

Some scientists argue that animals just cannot make these sorts of discriminations,

given what we know about their nervous systems, so the challenge is trying to work out how bats do it.

And our sequence of the Natterer's bats may help to solve that mystery.

To detect a thread of spider's web from its echo is truly extraordinary.

Next week, in The Life Of Mammals,

we meet predators that prey on plants.

They are forced to fight battles with their highly-defended prey,

with one another,

and with the meat-eaters that would eat THEM.