Original subtitles

Our human senses are incredible.

We have excellent vision...

..precise hearing...

..and can detect the slightest fragrance drifting on the breeze.

But we only experience a tiny fraction of what's out there.

Imagine a world where you could see with sound.

These images are just phenomenal.

Hear storms from hundreds of kilometres away.

That's incredible. They've all stopped.

Imagine seeing the world in slow motion

or through some of the sharpest eyes in nature.

HE GASPS

So fast!

Travelling to some of the wildest places on Earth...

..we reveal the strange and wonderful world of animal senses.

Light is emitted. Look at that.

Another one!

This is brilliant.

I'm Dr Helen Czerski.

I'm a physicist, and I want to find out how animals

tap into an amazing range of light, scent and sound.

I'm Patrick Aryee.

As a biologist I'm fascinated by what the world appears like

through animal senses far superior to our own.

Experience the world through animal senses.

Wherever we are, no matter how tranquil it seems,

we are constantly surrounded by sound.

Our ears are incredibly sensitive

and hear a huge range of tones.

But still, we detect only a tiny fraction of the sounds around us.

I've come to Mexico's Sea Of Cortez, where two very different creatures

have pushed sound to extremes.

One very low pitched, the other incredibly high.

I can hear

..squeaks and whistles and occasional series of clicks.

And it's really busy. It sounds like a busy city street.

DOLPHIN 'CLICKS'

Dolphins see their world through sound.

Their high-frequency clicks reflect off objects around them,

allowing them to build up an acoustic image.

This is nature's sonar.

And the thing is, I'm only hearing a tiny bit of all the sound

that's down there, because most of the dolphins' calls are at

frequencies above my hearing range.

These extreme high frequencies are known as ultrasound,

meaning they're too high for our ears to detect.

But here in the same waters,

other mammals operate at the other extreme of the sound spectrum.

LOW RUMBLE

Blue whales.

Their songs are infrasonic - too low for our ears to detect.

It's only when they're sped up that we can hear them.

WHALE MAKES LOW RUMBLING

These deep, haunting songs allow them to keep

in touch with each other over hundreds of kilometres.

Whales and dolphins operate on the outer limits of the spectrum,

but across the planet, animals are tuned into every

frequency of sound in-between.

In this episode, we're going on a journey through the world of sound,

from the deep sounds, far lower than the ones we can hear,

up to where the dolphins are calling at frequencies far higher than

we can hear, and there are ways of perceiving sound

that are way beyond our human capabilities.

Prepare to enter a bizarre world of sound,

beyond human hearing.

Our journey starts in Southern Africa, where one of nature's

true giants makes the deepest sounds of any land animal.

LOW RUMBLE

The African elephant.

The frequency, or pitch of sound, is measured in hertz,

and their low rumbles reach around 250 hertz.

But elephants also produce and hear sounds below 20 hertz.

These are sounds our ears struggle to detect, called infrasound.

Infrasound travels a long way, so elephants use it

to keep in touch with each other over many kilometres.

RUMBLING

But it's now suspected they also use their infrasonic hearing to

listen in to a secret sound of our planet.

THUNDERCLAPS

We can hear thunderstorms from 20, occasionally 30 kilometres away

but it's now thought that elephants can hear them

from distances of up to 500 kilometres.

That's roughly the equivalent of someone in London

listening to a storm in Edinburgh.

This may seem impossible

but at the end of the dry season, elephants often make sudden

and unpredictable changes in direction.

For no obvious reason, herds turn and march for days on end.

But when checked against weather records, it seems the elephants

are heading towards rainstorms up to 500 kilometres away.

I want to find out if this is a coincidence

or whether elephants really can recognise the deep,

infrasonic sounds of a storm over vast distances.

We're going to try something that has never been done before.

We're going to play the infrasonic part of a thunderstorm to a herd

of elephants and see just how they react.

And to do that, we're going to take a camper van

and turn it into a giant speaker.

'It may seem an unusual choice of speaker,

'but broadcasting infrasound requires large volumes of air

'and the inside of this camper van offers the perfect space.

'So with the help of infrasonic expert Bruce Thigpen,

'we're transforming it into a massive subwoofer.'

So can we use this camper van, this infrasonic speaker,

to replicate the sound of a thunderstorm?

Yes, we have an actual thunderstorm recording of thunderclaps,

the rumble of the sound after the lightning strike.

We've recorded that, we've took an actual recording

and we filtered it, so it just plays the lowest frequencies.

So, even though we're going to be quite close to the elephants, our

infrasonic speaker is going to play the sound of a distant thunderstorm.

Exactly.

'Thunderstorms are full of different frequencies of sound

'and these travel different distances.'

THUNDERCLAPS

The higher sounds, like the thunderclap,

are quickly absorbed into the atmosphere, so don't travel far.

Storms also produce low rumbles that carry much further.

But the very deepest sounds are below our hearing range

and these infrasonic parts of the storm are known to travel

much greater distances.

Infrasound can travel through its environment without getting

absorbed, and that's why the infrasound from rainstorms

can travel for hundreds of kilometres.

But could elephants really be hearing the infrasound

from these distant storms?

Andre, I believe Tembo is the perfect elephant.

'Andre Kotze has worked with elephants for 25 years

'and can recognise the behaviours that will show

'if the elephants are hearing our infrasonic storm.'

Andre, even though these are rescued elephants, do you still see

a change in behaviour when a thunderstorm is approaching, like they would in the wild?

When they hear a thunderstorm, they will more than likely

turn their backsides together, facing to the thunderstorm,

ears out with a spontaneous freeze, like it's a secret message or

something that happens and they just stand still for it.

After the spontaneous freeze you are more than likely

to find that they start chatting amongst each other. Low rumbles.

If they respond in that way to our thunderstorms

then that's proof, in a way, that they can hear

a part of the sound spectrum that we can't even attempt to.

Absolutely, absolutely, without a question of doubt.

Although the speaker is positioned close to the elephants,

the infrasound it produces will have the intensity

of a distant thunderstorm.

The herd is busy feeding,

so we're looking for a definite change in behaviour.

Bruce, I think we're ready to play the speaker.

OK, Patrick, audio in two seconds.

It may seem strange, but because the camper van is generating sounds

below our hearing threshold we can't hear it,

but we can certainly see it, as air inside vibrates with sound energy.

The elephants react immediately...

..turning to the speaker.

They're clearly reacting to the sound, but I can't hear a thing.

That's incredible, they've all stopped

and they've changed their behaviour,

as soon as Bruce started playing that sound from the camper van.

You can even hear them vocalising.

Their ears fanning out.

It's absolutely amazing how it completely changes their behaviour.

Bruce, it worked!

There's absolutely no question about it

and their ability to determine the direction the sound

was coming from, I was really impressed with that.

The elephants are back feeding now

but virtually the entire herd turned and faced our infrasonic

speaker, listening in to that secret sound of the storm.

This hidden channel of infrasound could explain a great mystery

of the natural world.

How elephants know where to go

when they migrate vast distances in search of water.

But storms aren't the only elemental forces to produce infrasound.

Even things we think of as silent are in fact making very

low-frequency sounds.

The spectacular aurora borealis produces infrasonic rumbles

of a hundredth of a hertz.

Volcanoes produce even lower frequencies.

These are some of the deepest sounds on the planet.

And amazingly, there's evidence elephants may be detecting

other natural sources of infrasound.

The most extraordinary example is the tsunami that swept across

the Indian Ocean in 2004.

When the tsunami hit the shores of Sri Lanka,

there were numerous reports of elephants acting erratically

and moving inland well before the tsunami struck.

Now, this apparent sixth sense could be down to the large

amounts of infrasound being produced by the tsunami.

As it built up, the sound it was producing was moving faster

than the approaching wave.

So the theory is that elephants could hear this low-pitched sound,

like it was an alarm, and were able to move off into safety.

ELEPHANT MAKES LOW RUMBLE

Elephants are one of the few animals on Earth that hear

and produce infrasound.

But in the vast wetlands of the Florida Everglades,

an ancient predator has also harnessed the power of sounds

too deep for us to hear.

They use it to put on one of the most extraordinary

displays in the animal kingdom.

The American alligator.

Every spring, the male alligators put on a spectacular mating display.

They sink down in the water so their backs are just below

the surface, and then make really low-frequency sounds.

And the consequence of that is that water droplets on their back

look like they're dancing.

And soon it becomes a water dance-off,

as rival males compete by displaying to females.

I've never really had any desire to be close to a bellowing

alligator but I do want to see this,

and to do it, I've got to trigger a chorus of amorous alligators.

To see this spectacle, I need to encourage some alligators

to start dancing.

And to do that, I need to replicate their infrasonic calls

so they think that there's a larger male close by.

That requires speakers even bigger than a camper van.

The alligators are producing infrasound in water

but we want to do it in air, to send sound waves out across the lake

and the physics works a little bit differently in air, so we've built

special speakers that do one job and they do it really well.

But to make it work, they need to look like this.

These speakers produce sounds at 19 hertz, the same deep

frequency that the alligators bellow at.

So let's see if they can entice a grumpy alligator to start flirting.

So that's it.

Those are the big infrasound speakers sending sound out

over the lake here, and now we just have to wait and see

if any of the alligators react.

Oh, straight over there, tail up in the air, getting ready to call.

ALLIGATOR BELLOWS

There are two parts to this display.

One is a deep but audible bellow from their mouths.

ALLIGATOR BELLOWS

It's like hearing dinosaurs.

The other part is the water dance.

This is produced by sound that is too low for us to hear.

It's a really deep hum coming straight from

the alligator's body, that makes the water dance at the surface.

There's two things going on here. There's two indicators of size.

And one of them is the infrasound itself,

a noise that's really deep. You need to be big, like in the same way

that a big bell makes a deeper noise.

You need to have scale, size, to make that kind of deep noise.

But the other thing is what the alligators are doing

just before they call.

They lift up their tail and their head

and you can see the full length of the alligator, and they're big.

These are enormous creatures.

I'm feeling very small.

Putting on a water dance requires huge amounts of energy.

So why go to all that effort?

To understand, I need to venture deeper into the gator's natural home.

I'm on the north edge of the Florida Everglades

and these wetlands stretch

south for hundreds of kilometres from here.

This place, where muddy brown water touches blue sky,

is prime alligator territory.

Alligators live on the boundary between air and water,

in a low world where vision is obscured by tangled vegetation.

So, to stand any chance of attracting a mate,

males have to make sure they stand out.

Imagine there's a female 300 metres away over there

and an alligator here is calling.

Sight isn't much good because she's too far away

and there's too much in the way, but sound can travel through the

water, and that is what the audible part of the alligator's bellow does.

And when she's come in closer, the sound isn't as much use any more.

But the water dance is splashing up above the surface of the water,

so she can see that and go right to the male that produced it.

For these ancient predators, the water dance is essential

for survival.

But the most extraordinary thing is how they use infrasound

to put on the display.

To show you, I'm going to create my own water dance.

This is a Chinese singing bowl.

They've been around for well over 2,000 years

and the reason that they are special is that

when you rub on the handles, you get this splashing from the bowl.

That's because vibrations of the bowl send low-frequency sounds

through the water.

When it's loud enough, this causes the water surface to break

into special waves called Faraday waves.

Faraday waves are almost like a way of concentrating energy.

Once they start to grow,

they keep growing, and so if you rub on the bowl hard enough

you make the amplitude of the waves loud enough, those Faraday waves

get so high they start to spit little droplets of water upwards.

And, incredibly, footage from our high-speed camera shows

the alligators are also creating Faraday waves.

So this makes it easier to see what's going on.

The alligator's back is just below the surface of the water,

its lungs are full so its body is really big.

As the alligator starts to vibrate its lungs, the top of its back

is acting like a piston, it's pushing up on the water above it

and that's driving the surface into this splashing pattern.

It's really dramatic.

And you can see it takes a lot of energy

because after they have called maybe seven or eight times they stop

and they rest, they're exhausted.

It's thought alligators have been calling like this for at least 70 million years,

so they were doing it when the dinosaurs were around.

And what stimulates them to call is hearing other alligators calling

or other sources of infrasound, and that leads to something really cool

because Cape Canaveral is just 70 miles that way. And when the shuttle

was landing there, when there were shuttle flights, the infrasound

from the sonic boom would set off the bellowing of the alligators,

so it's like the space age touching the dinosaurs.

As we move into the lower part of the sound spectrum that human ears

can hear, sounds above 20 hertz still travel long distances.

But these deeper tones don't just move through air,

they also travel through the ground.

And in Southern Africa's Namib Desert,

one bizarre little predator can hear so brilliantly

underground that they can find tiny prey in this vast expanse of sand.

The golden mole.

They're such weird looking animals.

They've got no eyes and no external ears

and they spend most of their time beneath the ground.

And yet they can do something truly remarkable.

They can hear the faintest of sounds through the sand.

In fact, their hearing is so sensitive they can find

a tiny termite from 20 metres away.

Golden moles feed on termites and other small insects.

To show how they find them, I first need to track down a golden mole.

That's not easy, because they're very shy

and only active at night.

But they do leave distinctive tracks.

As they travel across the sand, they leave these strange indentations

every few metres, and that gives us

a clue as to how this blind creature is finding the termites.

What they're doing is dipping their head into the sand

and listening in for vibrations that are travelling through the ground.

The mole's trail ends at a grass mound,

so I've cordoned off the area and left it overnight.

And look...at that.

It's the cutest animal I've ever seen, look.

Perfectly shaped for swimming through sand, wedge-shaped head.

Even though it's so tiny, you can feel the power in those front legs,

perfectly adapted for swimming through sand.

And incredibly, you can't see any eyes.

And that's because from a young age, their eyelids fuse over

and where their eyes would be, just covered in fur.

And because it can't see, it relies entirely on its sense of hearing.

Such a beautiful animal.

Golden moles may be able to hear me coming, but there's no way

they could detect a termite's footsteps from 20 metres.

So how do they do it?

Well, one bizarre theory suggests the moles are actually listening out

for the sound of grass blowing in the breeze.

The desert landscape is constantly shifting and changing.

The only fixed points are these tussock grasses

and it's in these mounds beneath the grass

that the termites make their home.

So could the golden moles really be detecting the sound of tussock grass

and using it to track down termites?

There's only one way to find out.

I'm going to play the sound of blowing grass through

the ground and see if the golden mole approaches.

But first, I've got to record it.

DISTANT-SEEMING RUMBLE

Wow, that's such an alien sound.

CREAKING

It's kind of a low knocking sound and that's perfect for

the golden mole, because low frequencies travel

really well through the ground.

And inside the golden mole's skull,

there's a clue that suggests they may be tuned into these

low-frequency sounds.

This is a 3-D model of the inner ear of a golden mole,

it's been enlarged by about 15 times.

Now, this section of the ear is responsible for converting

vibrational energy into nerve impulses that the brain

can interpret.

And the section that we are most interested in is right here,

this coiled area, known as a cochlea.

Now, in the golden mole this area is

twice as long as it is in European moles, and it's thought that it

helps extend the hearing range into lower frequencies.

Think of it as a piano.

If you've got an extended number of keys, you can play lower

and lower octaves.

It's time to put our golden mole's low frequency hearing to the test.

I find an area of sand free from tussock grass and set up

a rig of night-time cameras that can be monitored remotely.

And this is our key piece of kit. It's a transducer.

I'm going to use this to play back the sound of the tussock grass

I recorded earlier.

If I put my hand on that speaker, I can feel the gentle vibrations

that are being played out through the sand.

So, if the tussock grass theory is correct,

our mole should associate this sound with termites

and move towards it.

Let's see if it works.

Oh, look.

You can see he's moving.

And there.

That's the behaviour we are looking for,

that classic head-dipping movement.

The head just couples with the sand perfectly and the vibrations

of the sound waves travel really well

and that is what it's picking up. That's what it's detecting.

Here we go. He's running around, really fast.

Oh, look, he's just darted out of frame!

It seems we've lost our mole to the open desert.

But minutes later, he's back.

Oh, look, there he is!

He's just run in, dipped his head in the sand.

He's just run off again.

I mean, he hasn't gone directly to the speaker,

but he's, kind of, gone in that general direction.

By head dipping so close to the transducer,

it seems the mole was attracted to the sound of the tussock grass.

Perhaps, tonight, he just wasn't hungry.

But over a few nights in this remote desert,

I gain a unique insight into the secret lives

of these rare and shy little mammals...

..including a mole struggling to find a termite on the surface,

until he burrows down to listen to where the sound is coming from.

By tapping into this hidden world of underground sound,

the Golden Mole has become master of these sand dunes.

Who'd have thought the sound of grass blowing in the wind

would be the secret of desert survival?

BIRDSONG

As sound gets higher in pitch,

our ears become much better at detecting it.

Our hearing is most sensitive around 1,000 Hertz,

the frequency range around human speech.

But many animals also tap into these frequencies...

..nowhere more so than the tropical rainforest.

BIRDSONG AND MONKEY CALLS

In this dense, tangled world, animals can be heard, but rarely seen.

So, there's an acoustic battle for the airwaves,

as creatures fight to make themselves heard.

In the jungles of Puerto Rico, the calls of one surprising creature

drown out all others.

They're the giants of this acoustic world.

They're almost as loud as a pneumatic drill

and if it wasn't for a really clever evolutionary adaptation,

they'd deafen themselves with their own call.

Meet the Coqui Frog, thought to be the loudest amphibian on the planet.

A fully-grown Coqui Frog is around the size of a 2p piece,

but what they lack in size, they definitely make up for in volume.

So, what's driven these little frogs to become so loud?

An extraordinary piece of recording technology,

that lets me SEE sound, will help me find out.

This is an acoustic camera.

It's got 48 microphones arranged around a normal camera in the middle

and what it lets us do is to take the normal images

and overlay on top of them where the sound is coming from.

So, this is going to help me find Coqui Frogs,

when everything around me is pitch black.

The acoustic camera also records the intensity, or loudness, of sound,

measured in decibels.

At the volume I'm talking at the moment, the computer is registering

about 70 decibels, but if I clap, it will register 90.

So, by pointing this in the darkness, we'll get a direct measure

of how loud these little frogs really are.

BIRDSONG AND MONKEY CALLS

The Coqui chorus starts around sunset.

And the noise they make is overwhelming

and comes from all directions.

LOUD CHIRRUPING

They're all around me.

There's one.

So loud.

I feel like I must be being stared at by millions of frogs,

because there's clearly so many of them.

It's the male frog's call that gives them their name.

There are two parts - the "co" and the "kee".

The "co" warns off rival males,

while the "kee" lets any females nearby know he's available.

CO-KEE SOUND

First, I want to record just how loudly a frog can call.

So, there he is, our calling frog.

We've moved the acoustic camera in quite close

and we're, pretty much, a metre away from him now.

And the fact that we're so close

means that we can actually measure how loud he is.

CO-KEE SOUND

This little frog is calling at nearly 80 decibels,

but they have been recorded up to 95.

For their size, they're one of the noisiest creatures on Earth,

the equivalent of two and a half times louder than a lion's roar

and three times louder than an elephant.

So, how does such a tiny creature make such a massive noise?

The secret lies in the balloon-like vocal sac.

And you can see that, as he pushes air out of his lungs,

it goes into that big vocal sac and back again.

And what the vocal sac is doing is acting like

the sound board on a guitar, the front face for a guitar,

and it's helping to transmit that sound really efficiently

into the outside world.

I love watching his body work like that.

In fact, Coqui Frogs are so loud they should deafen themselves,

But they don't, thanks to a bizarre adaptation.

Inside the frog, the lungs and the vocal sac are connected to the ears.

That means that, when he calls, the sound travels out into the air,

but also through the frog's body.

If the call was just hitting the eardrum from one side,

it would rupture it.

But because the sound hits the eardrum from inside and outside

the frog's body at the same time, the effect is cancelled out.

But why have these frogs in the jungles of Puerto Rico

pushed sound to such extremes?

The acoustic camera reveals a possible answer.

This is brilliant, because it's a completely different way

of understanding what's going on.

When I look out there, you know, I see blackness and leaves,

but when I look here, there's these really bright splotches of light

and those are these little frogs.

The camera reveals the sheer density of frogs.

There can be 80 in an area the size of a tennis court.

With so many frogs calling, they've had to become louder

and louder to make themselves heard.

It's like being at a crowded party, when you raise your voice

to be heard, but so does everyone else, so you end up shouting.

This acoustic arms race may explain why the Coqui Frog

is so exceptionally loud.

I'm completely bathed in sound.

For the Puerto Ricans, this is the sound of home,

but for the frogs, it's different.

From their point of view, what's surrounding you

is an organised, precise, flow of information.

And if you want to survive out here, understanding the information

that all this sound is giving you is essential.

COQUI FROGS CHIRP

As we journey further up the sound spectrum,

our ears become less sensitive.

We don't hear high-pitched sounds very well.

Our countryside is full of sounds,

like birdsong, that we can appreciate.

But it's also awash with the squeaks of small mammals, like voles.

It's just that our ears can't detect them, unless we're really close.

But we have one amazing creature that can hear these tiny sounds

from great distances away.

The Barn Owl.

For their young to survive, a pair of adults must catch

3,000 voles a year. That's eight every single night.

And the only information they've got to go on

are the little squeaks of the voles and the rustling, as they

move around in the undergrowth. It's not much. So, when the owls

are out hunting, they're floating over a landscape like this

and it's not enough for them to know that dinner is out there somewhere.

They need to be able to pinpoint it accurately.

They want to pounce and get the vole first time.

But how do they pinpoint prey to the millimetre in this open landscape,

just using their ears?

Usually, when we think of good hearing,

we think about things with big ears.

We associate having big ears with being able to hear better.

Now, this owl has fabulous hearing, but it doesn't have external ears.

If you look at these feathers here, this thick ring around,

that defines the facial disc and they're basically forming

a cup, just like when you put your hand behind ear, and they're

doing the job that our ears do, but they are built into his face.

And this dish here, this dish of feathers,

is directing sound into his ears and it's directional.

If an owl looks at you, it's listening to you.

I want to put the owl's hearing to the test,

so I've got a phone with an unusual ring tone.

SOFT SQUEAKING

These sounds are the high-pitched squeaks that voles make and

the rustling sound that you get as they move around in the undergrowth.

So, that's what an owl's got to listen out for if it wants dinner.

I'm going to hide the phone in the long grass in just the sort

of a place where a vole might be and then I'm going to hide and call

the phone and, when it rings, that squeaking noise will start

and we will see whether the owl can locate it just using that sound.

But before I let the owl loose on the phone, I'm going to see

how I get on with locating this faint sound.

To help me out, I've got a piece of owl-like technology.

This is a parabolic microphone

and the reason I've got it is that the shape of the inside

of it is similar to the shape of the owl's feathers, that facial disc.

So, just let me call the phone here.

S, the phone's about 60 or 70 metres over there and it's ringing,

but I can't hear anything.

Let's see if this'll help.

WIND WHISTLING AND BIRDSONG

So, there's a little bit of birdsong in there, as well.

SOFT SQUEAKING

That's it there,

the squeaking, and if I move the dish even a little bit

to either side, it's gone.

There's a surprising reason this parabolic microphone and the owl's

facial disc are both so effective at picking up these squeaks.

It's all to do with the pitch of the sound.

This parabolic shape has a cut-off frequency,

so it doesn't work for very low frequencies and for the owls

that cut-off is at about 3,000 hertz, so if you tap a very thin

wine glass with a spoon, that's about that sort of note, 3,000 hertz.

So, above that, the owl's got really good directional hearing.

Below that, it doesn't hear as well and that's actually really useful,

because the rustling and squeaking

is at those high frequencies and all the background noise,

the low frequencies that might be distracting, they're all cut out.

So, now let's see how our Barn Owl gets on with locating the vole phone.

It's hidden in the grass about 60 metres away,

with a small camera close by.

This is just the time of day when owls would hunt.

The voles are starting to come out.

The owl quickly responds.

Its facial disc helps filter out background noise,

so it can focus on the high-pitched squeak from our phone.

SOFT SQUEAKING

Then it strikes.

SQUEAKING

So, our owl got it. It did the job.

And the fact that it was a phone ring tone it found

showed that it couldn't have done it by smell and it couldn't have

done it by sight, it must have been using its hearing.

And it pinpointed it so accurately, swooped right down in on it.

How did it do this with such precision?

By comparing minuscule time differences between the sound

hitting the left and right ear,

they work out which direction that sound is coming from.

But whilst our ears are symmetrical, the barn owls ears are skewed.

He's got one ear on each side of his face,

but they're not in the same place.

The one on this side, on the right, is just below his eye,

and that, combined with the shape of the facial disc,

is mostly listening to sound that is coming from above,

and the other side, the ear is just above his eye,

and the facial disc is funnelling mostly sound from below.

So, by listening and comparing the sound coming in both ears,

he can tell how high or low something is coming from

and that, combined with his ability to tell where

things are horizontally, is what lets him pinpoint his prey.

The Barn Owl's amazing hearing has allowed it to become the most

widespread and successful owl species on Earth.

We're all really familiar with owls and the image of an owl,

but now look at an owl and see it for what it is.

It's got this face, a dish which is collecting sound.

And isn't that just a fantastic idea that instead of having ears

that stick out which would get in the way if you flew,

it's all built into his face?

But even owls, with their extreme auditory adaptations,

are unable to hear the sounds at the highest end of the spectrum.

This is where our ears stop working completely.

Our ability to hear high-pitched sounds changes

throughout our lives.

We start out being able to hear really high frequencies

and then this decreases with age.

So someone in their sixties will be able to hear up

HIGH-PITCHED RINGING

to around 10,000 hertz, around there.

But someone in their 20s can probably hear up to

around 16,000 hertz and actually my hearing starts to go around 15.

But it's only young children that can hear even higher frequencies,

up to 20,000 hertz.

I can't hear anything there.

Now any sound above this is referred to being ultrasonic,

which means it is above the human hearing range.

But for some animals, there are great advantages to hearing

and calling at this extreme end of the spectrum.

If an animal can call at a frequency that its predators can't hear,

but members of its own species can, then this opens up a secret

channel of sound that they can use to communicate.

And around the world there are a few animals

that have tapped into this strategy perfectly.

In the pine forests of Canada,

Flying Squirrels produce ultrasonic alarm calls.

At 50,000 hertz, this is way above our hearing range

and that of their predators.

In South East Asia, Tarsiers push their calls to even greater

extremes, up to 70,000 hertz.

But one group of alien-like creatures

reaches even higher pitches.

Katydids, or Bush Crickets.

Their secretive love songs have been

recorded at a staggering 150,000 hertz.

We can only hear the sound by slowing it down 30 times.

They produce these extreme pitches by rubbing the wing cases together.

But there's one group of animals that have pushed sound

higher than any other.

Bats.

Their ultrasonic pulses have been recorded at over

200,000 hertz.

Bats don't just use these extreme frequencies to communicate.

They use them to see their world.

I'm stood in complete darkness and I can't see a single thing

and the only reason you can see me is

because we are filming with a special infrared camera.

But I know that I'm not alone here, because I can hear and feel

the wing beats of these Egyptian fruit bats as they fly past my head.

The bats can navigate through this flight enclosure in complete

darkness using echolocation.

As the bats fly past, they click their tongues really loudly

and produce a high-frequency pulse.

High-frequency sound echoes off objects really effectively

and precisely.

So, the bat's pulses are reflecting off the sides of the enclosure

and my body.

And by detecting these echoes, they're able to build up

an acoustic image of the world around them.

That's how they're avoiding me.

But what does it actually mean to see the world through sound?

Well, it's only now that we are getting our first

glimpse into this alien world.

In an ancient British woodland,

a futuristic experiment is under way.

This is the inaugural test flight of the batcopter,

a machine that will eventually allow us to see like a bat.

This strange-looking machine is part bat,

part drone, and as it flies through the forest, it's blasting

ultrasonic pulses, just like a real bat.

This is just one of the techniques

Dr Marc Holderied is using to visualise how bats see their world

through sound.

So, Marc, you've got this really impressive machine here.

Can you tell us a bit about it and what it does?

So, this is our Octocopter here, which is a drone platform, and we

use it to carry around this grey box here, which is our artificial bat.

It has a little mouth here, loudspeaker, that sends out very

high-intensity ultrasound into the habitat that we want to survey.

Above here, we have an area of 31 microphones,

which capture the echoes coming back from the environment,

so this is why we call it our 31-ear bat, really.

I want to be able to produce a visualisation that tells me

what a bat has seen.

I want to be a bat.

Marc is still fine-tuning the batcopter's acoustic image.

But using a different technique he believes he's got a good

idea of what it will look like.

By 3D mapping a stretch of woodland with a laser,

and tracking the flight paths of bats flying through,

he's created these astonishing visualisations.

Wow, these images are just phenomenal.

It's like something out of a sci-fi movie.

What we looking at here?

So, this is a cockpit view, flythrough,

of a real bat flying through a real forest.

We have slowed this down by a factor of five.

A bat would experience and fly through this at five times

the speed that we are looking at, at the moment.

These images are a visual

representation of what the bat's world looks like.

But Marc wants to know how the same scene would appear

through echolocation.

So, he works out which of the objects in the flight path would

reflect the bat's high-frequency pulses.

This gives him the bat's acoustic image.

So what we've done is taken away all the surfaces, all the reflection

that wouldn't really scatter back sound and you see it really

dissolves into individual reflectors but it still works really well.

And this is just trying to navigate.

That's what we have to remember, this is just navigation

let alone trying to find your prey in the dark.

Yes, so imagine, one tiny insect, less than a centimetre,

and this is what you are after.

You have to find dozens if not hundred of these every night.

I wouldn't know how to do it and I'm still puzzled

and amazed by the fact that they can.

It's astonishing to think what bats achieve using a simple

acoustic image like this.

They can fly through dense woodland in pitch black,

grab motionless spiders from their webs,

and even pluck fish from beneath the surface of the water.

It's the fact that ultrasound reflects really well off small

objects that allows bats to use their echolocation with such

deadly precision.

It's captivating to get this first glimpse of what it

means to see through sound.

This is as close as we've got to entering the bat's acoustic world.

By tapping into the power of high frequency sound,

bats have become masters of the night.

In this episode, we've journeyed through the natural world of sound.

From the deepest bellows...

..the loudest calls...

..to ears tuned only to the highest pitches.

Across the planet,

animals have found extraordinary ways of using sound to survive.

Next time, we explore the invisible world of scent...

..and discover the bizarre ways

animals use their sense of smell to get an edge in the wild.