All language subtitles for Super Senses 1of3 Sight 720p HDTV_Subtitles01.ENG

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.