Sight Super Senses: The Secret Power of Animals

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.

ELEPHANT RUMBLES

..hear thunderstorms from hundreds of miles 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!

-SHE LAUGHS

-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,

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

through animal senses far superior to our own.

In this episode, we are on a journey through a world of sight.

We'll discover the bizarre adaptations

and ingenious ways animals use sight to survive.

Experience the world through animal senses.

The African grassland.

Home to a hunter with some of the most incredible eyes on Earth...

the cheetah.

This female has been reared by humans,

so I can get an unusually close-up view.

Looking into the eyes of this cheetah,

it's easy to imagine that they view the world like we do -

that it sees me like I see myself in the mirror.

But it's actually a completely different story.

Its eyes look similar to ours...

..but appearances can be deceptive.

As we walk through this landscape,

it's seeing the world very differently to me.

Surprisingly, our eyes are sharper than the cheetah's.

But whereas our most precise vision

is in the centre of our field of view...

..the cheetah sees most clearly across a long narrow band.

This bizarre view of the world might seem odd to us,

but it's critical for the cheetah's survival.

It allows them to spot prey anywhere on the wide open savanna...

..making them a formidable ambush predator.

The way any creature sees is finely adapted to where and how it lives.

The cheetah's view is just the tip of the iceberg.

Some animals have a far stranger view of the world.

Despite the fact that we have much better visual acuity than cheetahs,

our eyes aren't better than theirs.

And even though they are brilliant

and can spot the slightest of movements on the horizon,

their eyes aren't better than ours.

We've just got different views of the world

that are both ideally suited to our needs.

Every tiny detail of every creature on Earth

is adapted to give them an edge in the challenge of staying alive.

CHEETAH PURRS

This is the beauty of evolution -

and through the eye, we can tell this most remarkable story.

You may think you know what the world looks like,

but prepare to think again.

Animals have many weird and wonderful ways

of seeing the world around them...

..and it's all to do with how they detect light.

It's easy to think we see the whole picture...

..but there's much more to light than meets the eye.

We get a huge amount of information about our world

in the form of visible light -

but that doesn't mean that's all the light there is.

In fact, we can only see a tiny fraction of what's out there.

I can reveal what our eyes can and, more importantly, can't see

using a prism.

So, the sun's up there

and it's shining white light down onto the prism,

and if I get the angle right...

you can see there's a rainbow on this rock here.

Sunlight appears white,

but it's actually made up of a spectrum of many colours of light,

and the prism splits them up.

And what's happening is that as the white light comes in

and it passes through the glass, it gets bent,

but the different colours get bent by different amounts -

and we can see all the colours of the rainbow lined up here.

This is the entire spectrum of light that's visible to our eyes,

and animals have evolved amazing ways of using every bit of it.

But some creatures can see more,

because there are parts of the spectrum we can't see.

Beyond blue is the hidden world of ultraviolet,

and beyond red is the invisible world of infrared.

And so what we can see is a tiny, tiny part

of this enormous spectrum of light.

And just because we can't see it doesn't mean it isn't there.

On our journey through sight, we're going to reveal the amazing ways

animals use every colour of the rainbow

right across this colourful spectrum...

and beyond.

We start with ultraviolet

and work our way across the spectrum to infrared

to show nature's most bizarre and extraordinary ways

of seeing the world.

We begin in one of the most remote places on Earth.

A wilderness where one unlikely animal

has tapped into ultraviolet to get an edge over its archrival.

Alaska.

This is an icy world, where one creature rules supreme.

The wolf.

This hunter patrols much of North America's snowy landscape.

Their camouflaged fur allows them to blend into the snowy background -

for a hunter, disguise is essential.

The wolf is a formidable predator,

and in Alaska, it pursues one animal relentlessly.

Caribou - or reindeer, as they're known in Europe.

These gentle giants seem vulnerable to the wolf pack...

..but the caribou have a secret that helps them to see wolves coming.

And it's all to do with ultraviolet light that's invisible to us.

When we think of ultraviolet light, or UV,

we normally think about the potential it has

to damage ourselves.

It's what causes our skin to tan or to burn,

and it's what sunscreen protects us against.

We can't see UV, because our eyes have a filter

that blocks these harmful rays, so we're blind to them.

But caribou don't have this filter,

so they are one of the only mammals on Earth

that can see this ultraviolet light.

And in this frozen landscape,

UV vision is a particularly powerful weapon.

Snow reflects ultraviolet light,

so, although we can't see it, this is a world awash with UV.

More importantly for the caribou,

anything that doesn't reflect UV - like wolf fur -

stands out against the bright UV background.

Even if it looks perfectly camouflaged to our eyes.

To show you how effective

this hidden world of UV is for the caribou,

I need to get close to a wolf.

This is a habituated wolf.

You can see that she's pretty well camouflaged against her background.

If you were looking from a long way away against a big landscape

you'd find this wolf pretty hard to spot.

But that's not how the caribou see the world.

And to get an idea of what they see,

I'm going to take a picture using this UV camera.

This specially modified camera can see the UV light

that's invisible to us.

Objects that reflect UV appear white,

and things that absorb UV appear black.

The pictures are really clear -

the snow is bright, bright white because the UV is reflecting off,

there's loads of UV around in the snowy background.

And then when the wolf comes into shot

it stands out, because it's so dark.

And that's because the wolf's fur absorbs UV light.

The image is particularly striking

because of the huge amounts of UV

being reflected off the snowy landscape behind.

And so, for a caribou,

being able to see in the ultraviolet is really useful.

You'd see white snow and a very, very obvious wolf.

By tapping into the ultraviolet light

at this extreme end of the spectrum,

caribou have stripped their archpredator of its camouflage.

What's fascinating is that we have adaptations to protect us

from UV light - but the caribou have evolved to use it.

Out here in this snowy white world,

it could be the difference between life and death.

To survive out here,

the caribou have pushed vision far further than ours,

seeing light beyond the edge of the visible light spectrum.

But as we move across the spectrum and away from ultraviolet,

we enter the world of light our eyes can see.

FLUTTERING

I've come to Cuba to find one amazing little predator

that specialises in hunting on the blue edge of the colour spectrum.

Their bizarre, alien-like eyes

have become highly tuned to blue light

and can perform one of nature's most astonishing tricks...

..warping time.

Dragonflies -

master predators.

They have an incredible hunting success rate,

capturing prey 95% of the time.

When you compare that with apex predators like Great White sharks -

who are successful 50% of the time -

or lions - only 40% - then dragonflies really are outstanding.

They hunt tiny insects in midair

by spotting their silhouettes against the bright sky.

To help them,

their eyes have become especially sensitive to blue light,

which makes the sky appear dazzlingly bright.

So much so, even the tiniest little insect...

..casts a dark silhouette.

Their unusual colour vision

gives them a huge advantage spotting their prey -

but to actually catch it,

they need another, even more extraordinary, visual trick.

Just by looking at them,

you can see how vision dominates their world.

They have the largest and possibly the best eyes of all insects,

which takes up almost their entire head.

And their bizarre-looking eyes

give dragonflies almost unbelievable visual powers.

Because these nippy little predators can see the world in slow motion.

To show you how spectacular their time-warping vision is,

I'm going to try to recreate one of their hunts using a high-speed lure.

Dragonflies live in a world where every millisecond counts.

Most of the airborne insects they eat move incredibly fast.

If they're to stand any chance of catching one,

they have to see it,

anticipate what it's going to do,

and react.

All in a fraction of a second.

So I'm going to test their eyes

using a dragonfly that hunts by ambushing its prey.

Just in front of me is a perching dragonfly.

When they see a fly, they take off, catch it in midair,

and return back to their perch.

The whole hunt takes place in the blink of a human eye.

THUNK

This peashooter might be low-tech,

but it's the perfect tool to recreate a high-speed target

for our dragonfly.

It fires a seed so quickly

I can't possibly see it.

But is the dragonfly's vision quick enough to spot it?

We're going to have a look back at our slow-motion clip.

Will the dragonfly detect the tiny pea?

Our dragonfly is completely still.

And the head definitely turns before we see the seed come into frame.

And then the dragonfly almost takes off -

but it has enough time to assess that it's not a fly,

and it changes its mind and stays on its perch.

That's incredible.

The head definitely moves first.

The dragonfly's vision is so quick it can track the flying object -

and work out it's not prey -

all in less than five hundredths of a second.

It's partly due to the speed at which they process information.

Dragonflies experience time in a completely different way to us.

They have a reaction time of about 30 milliseconds.

The route from their eyes to their brain

and then back to their muscles is much shorter.

The whole process of seeing and catching a fly

can happen in just over 100 milliseconds.

This is about the same time

it takes us to simply react.

But there's something even more astonishing

about the dragonfly's vision.

To demonstrate,

I've got a flicker book.

If I flick this book fast enough...

the images begin to animate.

And that's because the pages are moving so fast,

we hardly notice them turning.

It's essentially an optical illusion.

Although our vision appears seamless,

our eyes actually work

by capturing up to 60 images a second,

which our brain then combines

to create the illusion of a continuous moving image.

And because the pages are turning faster than that,

it brings the animation to life.

To a dragonfly, however, this would look completely different.

The images would appear slowed down,

and it would see each individual page turning.

And that's because dragonflies see faster than we do.

Whereas we see 60 images per second,

they see around 200.

And so they can observe things

that are just too fast for us to even process.

In real time, it's impossible for us

to see exactly what this dragonfly's doing.

But using our high-speed camera that slows down the action 80 times,

we're able to reveal the astonishing accuracy

of a dragonfly's vision...

..as it catches a tiny midge in midair.

Dragonflies have been around for 300 million years -

since before the dinosaurs.

In this time, they've finely tuned their eyes

to see their world in slow motion.

Dragonflies clearly have the edge when it comes to high-speed vision,

which explains why they're so difficult for us to catch.

With their time-warping eyesight, they can definitely see us coming.

And it's this visual adaptation

that has enabled these spellbinding insects

to become masters of their world.

In the battle to spot their prey,

these little predators have evolved astonishing eyes.

But the dragonfly's super slow-mo vision comes at a cost.

For peak performance, they need bright, blue skies.

On our journey across the light spectrum,

we show the many unique ways that animals see.

Some see colours beyond our vision -

others see in slow motion.

But they all rely on one thing -

light from the sun.

Darkness poses the single biggest challenge for sight,

so nocturnal animals have evolved special adaptations

to allow them to see.

Many have huge eyes - because, often,

the bigger the eye, the better it can detect tiny traces of light.

But some animals have come up with an even more unusual way

of dealing with darkness -

by illuminating the world around them.

I'm in Puerto Rico,

because it's the perfect place

to show you one of nature's most dazzling displays.

All around me now there's something incredible.

I can see it,

but for you to see it too, we'll have to switch to a special camera.

Rather like big eyes,

our camera is very sensitive in low light conditions.

The water around me might appear pitch black -

but looks can be deceptive.

THUNDER ROLLS

This is bioluminescence.

It's found out in the deep ocean and also in coastal waters,

and this is one of the best places in the world to see it.

And it's magical! Look at this!

I feel like I'm a witch casting spells.

This is all my childhood dreams come true.

And what's happening is,

down here there are thousands of little organisms

that each give out a little bit of light when they're disturbed.

And all together, they make these amazing plumes.

This is living light.

These tiny organisms are one of nature's wonders -

but what is the secret to making light?

It comes down to some simple chemistry with beautiful results.

What they need is a way of storing energy

that they can convert into light just at the moment they need it.

So they produce a chemical called luciferin,

and also an enzyme called luciferase,

And when you mix those two things together...

..light is emitted. Look at that!

Wow.

This is a fascinating adaptation,

because it allows animals to generate light in complete darkness.

What's even cleverer is that most ocean bioluminescence is blue,

because blue light travels further through water

than any other colour in the visible light spectrum.

So if you want to light up your underwater world -

blue is the colour to go for.

But what's in it for these little organisms?

Why go to all the effort of creating this strange light?

I'm hoping to demonstrate using these cardinal fish

and some tiny bioluminescent creatures called ostracods.

These fish feed on plankton,

so ostracods are just the sort of thing that they might normally eat.

I'm going to put some ostracods in the tank,

and that might sound a little bit mean,

but have a look at what happens.

SHE LAUGHS

It's like a little fish firework!

Oh, look at that!

Oh there we go, look!

So what's happening here

is that the cardinal fish ate an ostracod,

and the ostracod immediately put out an intense burst of light.

And that made the cardinal fish spit it out.

And the reason for that is that cardinal fish also has predators,

and if they can see it, they'll come and eat it.

So the cardinal fish don't want to be seen.

So the ostracod lets out a burst of light, the fish spits it out,

and then both go their separate ways.

Oh, there we go!

SHE LAUGHS

Oh, that time I saw the ostracod swim away!

Oh, another one!

-SHE LAUGHS

-This is brilliant.

I never thought that watching fish vomit could be this much fun!

SHE LAUGHS

You can see it coming out through its gills!

SHE LAUGHS

These bioluminescent creatures illuminate when threatened

because their dazzling glare acts like a security light

deterring predators.

Most ostracods live near the surface of the ocean,

but the vast majority of bioluminescent animals

live far deeper down.

As you descend further into the dark depths of the abyss,

the world of bioluminescence gets far weirder.

Down here, this is the only light there is.

And in the deep ocean abyss,

one of the planet's most mysterious giants

uses this alien light to survive.

To do this it has supersized its eye to a grotesque dimension,

far beyond any other animal on Earth.

Just think about all the creatures on planet Earth

that need to be able to see.

They go right the way from tiny insects,

all the way up to gigantic blue whales.

And if you look at the eyes themselves,

they start with tiny, microscopic, light-sensitive patches,

and get bigger and bigger and bigger,

until you get to about nine or ten centimetres -

and that's about the size of an orange.

And there they stop.

So all the eyes on planet Earth are this size or smaller.

But there is one massive exception -

an animal whose eye has grown to the size of a human head.

The mysterious giant squid.

These images, filmed over 600 metres below the ocean surface,

show it in its natural habitat for the first time ever.

They are true giants,

measuring over 14 metres in length -

and even relative to their colossal body size,

their eyes are still enormous.

But what's in it for the squid?

Why is seeing this bioluminescent light so vital to its survival?

As I row across this bay, the water offers a clue.

As my oars move through the water,

they're disturbing these organisms,

and they feel threatened so they light up,

producing this wonderful glow.

And that brings us back to the giant squid eye.

The biggest eye in the world.

Because what those eyes are really good at

is detecting the faint glow of bioluminescence in the deep ocean.

The squid needs gigantic eyes

because a fearsome predator is searching for it in the darkness.

A giant that descends into the pitch-black abyss

to hunt it down -

the sperm whale.

Now, sperm whales also don't produce bioluminescence,

but as such a big animal moves through the water,

it'll disturb little organisms like this,

and so it'll be surrounded by a faint ghostly glow -

and that is what the giant squid eyes are looking out for.

The giant squid's huge eyes

allows it to peer further through the gloom

and detect these traces of blue bioluminescent light

as the sperm whale approaches.

It's this spectacular eye

that has allowed them to survive in this inky black world.

And so that is why the largest eye on the planet

has evolved in the darkest place on Earth.

But as we leave the dark blue depths

and return into bright, white sunlight...

..we enter a world where most eyes perform at their peak.

It's in these bright conditions that our eyes work at their best.

And I've come to the Welsh highlands

to pit mine against an animal with some of the sharpest eyes on Earth.

Our eyes are pretty impressive.

In fact, sight is our dominant sense.

We can see things in incredible detail.

But there's a group of predators

with an even sharper view of the world.

Meet Moses!

He's a Peale's peregrine falcon, and he's pretty spectacular.

Hey-up, hup!

His owner, Lloyd, is using a lure

to help demonstrate exactly what he's capable of.

He's flying so close,

and so fast...

HE GASPS

But he's using those amazing eyes

to help avoid a collision.

Good lad.

We're going to put a peregrine to the test,

and try and find out what it's like

to see through some of the sharpest eyes on Earth!

But first I want to show you why they need such precise vision.

These birds are the fastest animals on Earth.

In a dive, they can hit 180mph.

Flying at these speeds demands exceptional visual coordination.

Any misjudgement could be fatal.

To show me how exceptional, Lloyd has an unusual test.

And it requires nerves of steel.

That was fantastic!

'Even travelling at 50mph,

'his incredible vision allows him

'to fly through my legs effortlessly.'

To fly at such speed,

peregrines have fantastic depth perception.

But where their eyes really come into their own

is in spotting prey over incredibly long distances.

There's no doubt that excellent vision is vital

to the peregrines' existence.

But exactly how precise is their eyesight?

To find out, we've set up the ultimate long-distance sight test.

The peregrine falcon is on a ridge,

just under a mile away, over there.

We're going to see if he can spot his lure

from the other side of the valley.

To make it even tougher, the visibility is poor...

..and he has no idea where we are.

But we've fitted him with a tracking device,

so we can monitor where he is.

Using this radio tracker,

we'll be able to tell when Moses is getting closer.

Fingers crossed, when his hood comes off,

he'll spot the lure and come swooping in.

It's time to put some of the sharpest eyes on Earth to the test.

Will he spot our tiny lure from so far away?

SHE SHOUTS COMMAND

I can't see anything at all.

It's quite nerve-racking, actually!

I don't know where he's coming from.

TRACKER BEEPS

I'm getting him from over here.

Here he is, here he is!

Hup, hup, hup!

-Oh!

-Hey-up!

Good boy.

That was phenomenal.

There.

He spotted us from nearly a mile away,

and flew straight to us.

-Wow!

-There's a good boy, you clever boy!

-Look how he picked us up almost instantly.

-Yeah.

It's worth remembering, he's never flown here before,

he's never even been here before,

he has no idea, when his hood comes off,

where I'm going to be in this landscape.

So you can't get any more definitive proof

about how good their eyesight is than that, I don't think.

That was really quite astonishing.

I could barely make out that there's anyone over there on that ridge,

but Moses was able to spot us and this small lure.

So what's the secret to their amazing vision?

It's all down to a brilliant adaptation they share with us

and other sharp-sighted animals called a fovea.

This is an area in the retina where light-sensitive cells -

called photoreceptors - are particularly tightly concentrated.

Like pixels on a screen, the more receptors you have,

the higher the resolution or visual acuity.

Because the fovea's so packed with receptors,

it creates a super-sharp image.

Our fovea contains 200,000 receptors per square millimetre.

But birds of prey can have nearly twice as many...

..so their vision is much sharper than ours.

This is why these birds can spot prey over such incredible distances.

Our vision is pretty impressive,

but when it comes to acuity,

the peregrine is in a class of its own.

In daylight conditions,

birds of prey have an unrivalled view of the world.

They have evolved the sharpest eyes of any animal on Earth.

We've seen how colour vision is critical for many creatures.

As we continue our journey through sight,

we enter the world of red light.

And it's here our human eyes

have evolved a surprising power of their own.

It's easy to take our vision for granted -

but compared to almost every other mammal on Earth,

our eyes can see far more colour.

Even within our close primate family,

we see things very differently.

You might think the more colours you see, the better.

But this pygmy marmoset is about to demonstrate

that sometimes seeing less is more.

He's the smallest monkey on Earth - just the size of my hand -

and he's a supreme visual predator.

And that's partly because he sees fewer colours than us.

So how does this marmoset's limited colour vision

make him such a keen little predator?

I can demonstrate this with the help of these pictures.

On this first one,

there's a cross hidden in this pattern,

but it's hard to make out at first glance

because of the two colours.

But when you take away one of those colours...

all of a sudden,

the cross becomes more prominent.

And that's because colour is a very powerful visual cue,

so when it comes to looking for patterns,

it can be quite distracting.

If you're a small monkey that eats insects

camouflaged amongst leaves and branches,

seeing fewer colours

dramatically helps you to recognise the shape of your insect prey.

Surprisingly, this little marmoset sees colour

the same way virtually every other mammal on Earth does.

Every cat, dog, bear -

they all share the same limited colour vision.

But there's one exception.

A small group of primates - including us -

have pushed colour vision even further

and evolved the ability to see a colour that no other mammal can.

BABOONS GIBBER

These chacma baboons should help me demonstrate...

..if they cooperate, that is.

I've got a little test.

Baboons love tomatoes,

and I've got two of them here - one red, and one green.

So let's see which one they decide to go for.

It seems odd,

but most mammals wouldn't be able to see the difference

between these two tomatoes.

So he's taken the red one straightaway.

Chomping away!

He knows that red fruit is the sweetest and most ripe.

Let's try again.

Every time, they're taking the red one first.

It's clear that they see the difference,

and they definitely prefer the ripe ones.

That's because, like us,

baboons can see the colour red.

We take it for granted,

but this ability we share is remarkable -

and something we can only do

thanks to a bizarre twist

in our distant past.

Deep in our eyes, we can unravel what happened.

We see the colours we do

because of light receptors in the back of the eye

called cones.

We have three main types of cones

that detect red, green and blue light.

So we can see any combination of those colours.

But our eyes haven't always been this advanced.

Going back in evolutionary time,

primates only had two types of colour receptor -

one for blue light and one for green.

But it's thought that about 40 million years ago,

a tiny genetic mutation caused a shift

from the green colour receptor to the red.

As a result, some primates - including us and the baboons -

developed the ability to see a new colour...

red...

..and this turned out to be very important.

This small change gave our primate ancestors a huge advantage,

because it allowed them to pick out ripe red fruits

from the dense, green foliage.

The way each and every animal sees colour

is slightly different,

and as a result of millions of years of evolution.

It's all about what works best in that creature's world.

We have seen the diverse ways

animals tap into every parts of the light spectrum

to survive.

But as we reach the very edge of the visible spectrum,

one predator has evolved an almost supernatural ability

to see light that's invisible to us.

And it can do so without using its eyes at all.

I'm in Cuba,

in a beautiful landscape that's teeming with wildlife,

and tonight I'm hoping to witness a special hunting spectacle.

This predator's view of the world is different to mine,

and it's evolved a way of hunting in complete darkness.

To find this remarkable animal,

I'm on my way to a cave deep in the jungle.

And to show you how well it can spot its prey in this pitch-black world,

I'm going to use a camera that works in the dark.

Down here, my eyes can't see a thing...

..yet these are the perfect hunting conditions for our predator.

It's not long before they make an appearance.

I'm in the mouth of a cave,

and over there is a Cuban boa.

It's been curled up in the crevice in the rock all day,

and now it's night-time and it's come out to hunt.

It's after an animal that seems virtually impossible to catch.

The snake's prey are just starting to come out of the cave,

and I can hear them zooming past my ears and out into the night.

They're bats,

and the snake is hoping to catch one.

To survive, these snakes have to feed.

They must somehow pinpoint...

..strike...

..and catch a bat as it speeds past -

all in complete darkness.

But these Cuban boas are specialists.

This one's got one...

..and he's coiling around it to suffocate it.

And the snake's just tucked up underneath the rock there.

To survive in this underground world,

these snakes have evolved a bizarre and brilliant visual trick.

Because even though it's pitch black,

the snakes have found an ingenious way to use light to find their prey.

But how are they doing it?

Their secret is that they can see heat.

Off the edge of the visible spectrum beyond red is infrared.

It's invisible to our eyes...

..but this is a bizarre part of the spectrum

where heat becomes light.

I've brought a special camera to the jungle

which can see this heat.

This is a thermal imaging camera,

and, just like the snakes, it can detect infrared radiation.

So the world looks almost the same, but a little bit different -

because this is an image in heat.

So, you can see that, for example,

my forehead and my neck are giving off lots of heat,

so they're bright white.

But my clothes have a thin layer of insulating air,

so the clothes themselves are cooler, so they look darker,

and my nose is also quite dark.

The secret to the snake's ability to hunt

is that it can find warm-blooded mammals like bats

by sensing their heat.

And to show you how expert they are at detecting this infrared,

I've got a test.

I've put a Cuban boa in a box that's blacked out.

I've got a balloon here, a black balloon,

which is filled with warm water,

so it's a reasonable mock-up of a small warm-blooded mammal

like a bat.

And what we're going to do is turn off all the lights...

put this in the box with the snake

and watch with the thermal camera to see how the snake reacts.

Despite the darkness,

he immediately zeroes in on our fake bat.

So, here comes the snake.

And it's completely dark in the box.

And he's just turned towards the balloon.

Gone right up to have a good look.

-Oh!

-SHE LAUGHS

So, the snake struck, burst the balloon,

and now there's hot water all over the bottom.

This snake clearly found that balloon

and struck at it accurately in complete darkness.

The snake's ability to see infrared light is incredible.

But even more remarkable is that it doesn't use its eyes to see it.

The snake's eyes are on top here.

They're really obvious.

But that's not what the snake is using

to detect this infrared radiation.

If you look along the top and bottom of its jaw,

in between the scales,

there are pits like little dimples.

And the back of each pit is sensitive to heat,

and that's what the snake's using.

Each pit generates a very crude infrared image.

Because there are so many pits,

by combining the information from all of them,

the snake can build a deadly accurate picture

of where its prey is.

So, actually, the infrared-sensing organ

is all along the front of the jaw.

It's actually much bigger than the eye.

It's a really ingenious physical solution.

But what do they actually see?

What does their world look like to them?

To show you, I'm taking my thermal camera into the cave

to try and see these bats.

I can hear the bats whooshing past me, but I can't see them.

But we can get an idea of how this scene looks to the snake.

The bats are like flying beacons in the blackness.

Like all mammals, their warm bodies radiate infrared light...

so there's no way for them to hide.

The snakes here have a huge advantage

by tapping into light that's all around us,

but that most animals just can't see.

And it lets them hunt here, even in complete darkness.

These Cuban boas have mastered

one of the most challenging places on Earth...

..because they've pushed sight to the edge of possibility.

On our journey we've shown that there's far more to sight

than meets the eye.

We live in a world that's bathed in light.

Every animal has evolved to use a part of that light,

that gives them the best chance to survive.

They can each see a part of the puzzle,

but no one animal can see it all.

Next time...

..we travel through the sound spectrum.

From the deepest bellows...

ELEPHANT RUMBLES

..to the highest pitches.

FROG CHIRPS

When I look out there, I see blackness,

but when I look here,

there's these really bright splotches of light.

We reveal the most extraordinary ways animals use sound to survive.