01x03 - Chasing Colour

[bunniefuu]

[Attenborough] Colour can be crucial

in the lives of animals.

They use it to win a mate,

to hide

and even to warn.

But to discover

how animals themselves perceive colour

is not easy.

Making this series

took ingenious experiments

innovative camera systems

[man] Look at that! That's extraordinary.

[Attenborough]

and lots of patience.

Our camera team's task

was not only to film colour as we see it,

but as animals do

including colours

that are invisible to our eyes.

And, as our climate changes,

we worked with experts

to understand the challenges

that these changes cause

and learn that colour can sometimes

be part of the fight to survive.

Our eyes enable us to see

all the colours of the rainbow,

from red to violet.

But many animals

can see colours beyond that spectrum.

In the ultraviolet.

To capture their view of the world,

we needed a highly specialised camera,

which I was able

to see in action for myself

in a flower garden near London.

Cameraman Mark Payne-Gill

showed me how it works.

[Payne-Gill] Actually, it's quite simple.

It's got one camera

that sees in ordinary vision,

our colour world,

and the other sees in ultraviolet,

how an insect would see it.

What's the basis of it?

So the principle

lies behind this filter here.

So it's an ultraviolet filter.

It allows ultraviolet light

to pass through it,

but the clever bit

is it also reflects white light,

normal light,

straight into this camera here,

which can see the world as we see it.

And the end product is then we can cut

between the two of them as we will?

Exactly, and see how we see the world

and how the insects see the world as well.

[Attenborough] Okay, show me it in action.

[Payne-Gill] Okay, here we go.

The first picture you'll see

is the white-light view,

how we see the world.

[Attenborough] Yeah.

[Payne-Gill] Then, there you go.

- [Attenborough] There it is.

- Very different.

It seems to have black marks

on each of the petals.

- We can't see those?

- That's right.

When we look at our white-light view,

it's just pure yellow,

but hidden in that is the ultraviolet.

You think there's a lot more

still out there to discover?

I think there's an awful lot,

but personally, I quite like the fact

it remains a secret.

- Leave something for the children.

- Exactly, yeah.

[Attenborough] Many birds, lizards,

insects, and some fish

can see ultraviolet,

so they're often reacting

to things that we can't see.

Scientists are only

just beginning to reveal

how animals use these ultraviolet colours.

Our Australian team joined a scientist

who has been studying

a small and unassuming butterfly,

which has a secret trick.

Cameraman Ben Cunningham

caught up with Dr. Darrell Kemp,

from Macquarie University,

at Coffs Harbour, New South Wales.

[man 1] You got one already?

- [man 2] I've caught this lovely male.

- Look at that guy.

Hypolimnas bolina. Yeah.

Also known as the blue moon butterfly.

[Attenborough] Darrell studies

mating displays in butterflies,

and the blue moon butterfly

has something special to show off about.

[Kemp] That's fantastic

in giving us a really great view

of the colour patch

that we're interested in here.

[Attenborough] The butterflies

have iridescent blue patches,

which certainly look very pretty

to our eyes.

But their display is even more startling

in the ultraviolet range

that we can't see.

[Kemp] We're unfortunate in that regard.

We miss out on a quarter of the brilliance

of the natural world.

[Attenborough] Which is where

our ultraviolet camera can help.

[Kemp] I'm excited to see

what the camera can produce.

It's almost going to allow us

to get as close as possible

to understanding

the full brilliance of these butterflies.

[Attenborough] But first,

the beam splitter camera

needed to be assembled,

and that wasn't exactly straightforward.

Fiddly, fiddly, fiddly.

Where does this one go again?

[Kemp] It's amazing how complex

a system needs to be

in order for us to reconstruct

quite simply what a butterfly can do

with its normal compound eye.

[Attenborough] Finally,

the camera was up and functioning.

And it gave Darrell

a view of his butterflies

that he had never witnessed before.

[tranquil piano music playing]

[Kemp] Here, it's almost like

you have a beam of light

being shone from the wing.

Because, really, those ultraviolet patches

are little mirrors

that essentially

reflect the intensity of ultraviolet light

that's coming from the sun.

[Attenborough] And, as a consequence,

Darrell was able to see things

crucial to his understanding

of his butterfly's behaviour.

[Kemp] I'm excited by itbecause

the ultraviolet part of the markings

of the male of this species

is really the critical thing

that females are judging

when they're deciding

who they would ideally mate with.

[Attenborough]

The brighter the ultraviolet,

the more attractive the male.

But this attractiveness comes at a cost.

[Kemp] And those markings,

being as bright as they are,

are likely to be equally apparent

to the main predators of this species,

which are birds.

[Attenborough]

And birds, with their sharp eyes

that can see colours that we can't,

are quick to spot prey.

[Kemp] In some ways,

that's both the blessing and the curse

of this colour patch for this species.

Yes, you need it to impress a female,

but flying around

with a really bright signal like that,

visible to birds,

is almost a handicap

that these guys have to bear.

[Attenborough] But, for the males,

it's worth being

the brightest of the bunch.

[Kemp] It's a male's sole goal

to mate with females

and perpetuate his genes

to the next generation.

So, if by being bright,

he's able to achieve two or three matings

and then die, even at a young age,

that would be evolutionarily favoured.

[Cunningham] Even if he meets

a grim death, it's success.

[Kemp] Absolutely.

[Attenborough]

The rocky hills of Northern India.

[menacing instrumental music playing]

The open forests here

are the hunting grounds

of one of the world's

most dramatic predators.

This is the home of the tiger.

Tigers have to k*ll at least once a week

if they're not to starve.

They can't run as fast as a deer,

their favorite prey,

so if they're going to catch one,

they have to get quite close, unseen,

before they charge.

Not this time.

But why are tigers orange

with black stripes?

At the University of Bristol's

Camouflage Laboratory,

scientists John Fennell and Laszlo Talas

have been finding out

how tigers are perceived by their prey.

To help them test their research,

they're joined by Max Hug Williams,

a wildlife cameraman experienced

in filming tigers in the wild.

I've always wondered,

"Why are they this bright orange colour?"

You'd think they'd stick out

from the environment.

There's no form of camouflage at all.

We tend to think of everything we see

only in terms of our own visual system.

We're pretty good at picking up colour,

but many animals don't have that.

And that's true of most mammals, in fact.

[Attenborough] Including deer.

Their eyes lack red receptors,

so are only sensitive

to blue and green light.

Deer, in common

with around 5% of human beings,

are red-green colour-blind,

and to them,

orange and green look very much the same.

John and Laszlo are working with glasses

that allow us to see

in much the same way as deer do.

They could give Max an idea

of how they view the world.

Before you look at tigers,

we thought it would be useful

to do a standard test.

And your task is to read out the number

that you see.

So we got a That's a 74.

And in the middle there, 15,

popping out from the orange.

- How about that one?

- [Hug Williams] Three?

[Attenborough] Max started well,

but putting on the glasses

made things much more difficult.

Wow.

That's just completely

completely disappeared.

Can't pick out those numbers at all now.

They're just gone.

Twenty-nine.

Without the glasses, nothing.

With the glasses Amazing.

It completely vanishes.

Those numbers have gone.

[Fennell] The glasses are effectively

filtering out the red lights.

That's amazing.

So a deer would not see

these numbers at all.

[Attenborough] Max's next task

was to look at photographs

of a tiger stalking in the wild

and try to point it out

as quickly as possible.

- Right.

- [Talas] You ready to go?

Let's go.

Right, there's one.

Okay, there's a tiger. Top right.

That's easy.

Right bottom. Right there behind the bush.

Getting my eye in now.

- So how am I doing?

- [Fennell] You're doing pretty well, Max.

Your time is probably an average

of about two seconds per frame.

- Is that good? sharpsh**ter?

- [Fennell] It's good.

[Attenborough]

It's not surprising that Max,

a wildlife cameraman,

was fast to spot an animal,

but could he do as well

with the deer vision glasses,

which render him colour-blind?

[tense ambient music playing]

[Hug Williams] Everything's green.

This is just ridiculous.

- It just completely disappeared.

- [Fennell] Yeah.

It's amazing.

So this is what a deer would be seeing?

[Fennell] Yes, that's right.

[Hug Williams]

As soon as there's vegetation,

it's nearly impossible.

Top top left.

Where is the

Wow.

I'm guessing I didn't do well.

Yes, you were

about half the speed you were

when you did this without the glasses on.

That time difference might equate

to as much as a seven-metre difference

for the tiger

when it's attacking its prey.

[Attenborough]

So tigers don't look orange to deer,

but why aren't they green?

All kinds of very different animals,

birds, reptiles, amphibians, insects,

are coloured green.

So why aren't tigers?

Well, tigers are mammals,

and the pigments in mammalian hair and fur

come from just two substances,

eumelanin and pheomelanin.

It is actually biochemically impossible

for a

to manufacture green colour

using these two pigments.

So we suspect that evolution

came up with the next best thing,

to make the tiger's fur orange-brown,

which actually looks the same colour

to its prey.

Amazing.

So, while mammals can't be green,

they've evolved alongside the prey

to become the perfect,

camouflaged k*lling machine.

[Attenborough]

Tigers are just as colour-blind as deer.

So a tiger doesn't even know

that it's orange.

Luckily, Max does.

I don't think I'll take these glasses

with me on my next filming trip.

- You can't see anything.

- [Fennell] That's wise.

- I think you'd be dinner.

- [Hug Williams chuckling]

[Attenborough] Whilst tigers use colour

to become inconspicuous,

another very different animal

uses colour in a very different way.

The poison dart frog.

It's no bigger than a fingernail,

and the skin of some species

contains a poison so powerful

that local people used it

to tip their blow-pipe darts.

The strawberry poison dart frog

is not quite so lethal,

but toxic nonetheless.

They live on a remote archipelago

called Bocas del Toro in Panama.

Dr. Yusan Yang

has been researching the function

that colour plays in their lives.

[Yang]

So Bocas del Toro is very interesting

because

the strawberry poison dart frogs here

On different islands,

you have all kinds of different colour.

They're not slight differences in colour.

They're dramatic differences.

We have red frogs, we have yellow frogs,

we have green, we have blue.

They look different,

but they're the same species.

[Attenborough] These are the most varied

and brightly coloured frogs in the world,

and Yusan has been running experiments

to find out the significance

of their wide colour range.

She's made robot frogs

to test how the territory holders react

to different-coloured individuals.

These are 3D-printed model frogs,

and I hand-painted them,

so that they look like the different

colour types we found on these islands.

[Attenborough] Her equipment enables her

to simulate a territory invasion.

She glues the 3D-printed model

onto a motor arm

so that she can control its movements.

[whirring]

Then she plays the frogs' call,

which is the same on all the islands.

[recording of frog call playing]

A frog will attack any other frog

that enters its territory.

They are famous for their wrestling skill,

but it's a very civilised sport.

[Yang] I always like to describe them

as two gummy bears going at each other,

because they don't have claws or teeth

and can't hurt each other.

They're just trying

to pin each other down.

[Attenborough]

First, she tested an orange frog

on an island

where orange frogs predominate.

[recording of frog call playing]

The males of this species

vigorously defend their territories.

So if one of them hears the calls

of another male,

he will think his territory

is being invaded,

and he won't allow that.

He becomes physically aggressive,

trying to wrestle with the model

and pin it down.

Next,

Yusan tried the orange territory holder

with a blue model.

[recording of frog call playing]

This time, the male reacted to the sound,

but didn't seem to recognise

the blue male as a threat.

[Yang] In the red populations,

the frog will be more aggressive

toward a red model.

And in the blue population,

the frog will be more aggressive

toward the blue model.

But if it's of a different colour,

then a lot of times, the frogs

wouldn't recognise it as a competitor.

[Attenborough]

So if the frogs react only to the colours

with which they are familiar,

what is the function of different colours?

The answer seems to be

that the frogs are not signalling

to each other,

but to predators.

[Yang] So these poison dart frogs,

as their name suggested,

they are poisonous.

So the bright colour is actually

a warning signal to the predators,

telling them,

"I am poisonous. Don't eat me."

[Attenborough] But the frogs

are not all equally poisonous.

Their poison comes from toxic alkaloids

that occur in their food,

which is largely ants and mites.

The particular diet on some islands

makes some frogs

more poisonous than others,

and this affects their colour.

So researchers have found that the colour

is related to their toxicity,

and the ones that are duller,

that are a more camouflaged colour,

they are actually less toxic.

[Attenborough] Red and orange ones,

on the other hand,

are extremely poisonous

and make that quite clear

before they are attacked.

Most birds and lizards

have excellent colour vision

and are well able

to see these red warning signals.

So whilst the tiger's orange colour

conceals it from colour-blind prey,

the poison dart frog's similar colour

advertises that it's poisonous.

There is yet another equally important way

in which colour is used

in the natural world.

To attract a mate.

The peacock does that on a grand scale.

[jaunty instrumental music playing]

And so does this tiny spider,

in a remarkably similar way.

This one belongs to a group

called the jumping spiders.

Dr. Lisa Taylor,

from the University of Florida,

goes to extreme lengths

to study how they view the world.

There's more than 6,000 species

of jumping spiders,

and we're just now learning a lot

about their colour vision.

[Attenborough] Lisa also studies a group

called the Habronattus spiders,

which can see a range of colours,

from ultraviolet all the way into the red.

The males have bright red faces,

and Lisa is investigating why.

As a starting point for understanding

what those colours are communicating,

one way to do that

is to block out the colours completely

and then ask the females what she thinks.

To block out the male's colours,

we gently apply liquid eyeliner.

We've tested a lot of products

to make sure they're safe for the spiders

and that the spiders behave normally

after the eyeliner is applied.

[Attenborough] By giving some males

a makeover in this way,

Lisa discovered

that a female regards a red face

as a very important quality in a male.

[Taylor]

When the males are courting females,

under

really good quality lighting conditions,

the females were very attentive

to the colour.

[Attenborough]

The red-faced male on the left

is clearly holding the attention

of the female.

But a male with a pale face

is largely ignored.

There have been a lot of studies

trying to understand

what the brightness of a colour might tell

a female about a potential mate.

Usually, it's the brightest males

that are advertising their good quality.

That's what we've actually found

in the jumping spiders too.

So males with brighter colours

seem to be better quality

than males with darker, duller colours.

[Attenborough] But these male spiders

have an extra need to impress.

So females are really voracious predators.

They go after almost anything that moves,

and they take down prey

that's a lot bigger than themselves.

So when a male is courting a female,

he has to take that into account.

[Attenborough]

And if she doesn't accept him as a mate,

she will eat him.

The redness of his face

makes all the difference.

Red is a strong, bright colour

that stands out against most backgrounds,

so it's used as a warning of danger

by many animals,

including ourselves.

And in Habronattus,

it could be the difference

between life and death.

When a male encounters a female,

he has a very limited amount of time

in order to impress that female.

So the female could quickly attack him.

So we think that maybe the males

incorporate this red into their display

to give them an extra second,

so they have a little bit more time

before the female pounces on him

and cannibalises him.

[gentle instrumental music playing]

[Attenborough] By adopting the red colour

used by their toxic prey,

a male spider gains

an extra second or two,

during which he can make his case

and win over a mate.

So colour can affect

the way animals hide and display,

but some animals can see light

in a completely different way

to ourselves.

They can detect

and respond to polarised light,

light that vibrates in only one plane,

as it does when it's reflected

from a shiny surface.

Polarised light plays a crucial part

in the lives of some animals,

including these small fiddler crabs

in Darwin, Australia.

Our team worked with Prof. Viktor Gruev,

from the University of Illinois,

to develop a unique

and pioneering camera system

to view this hidden world.

Put it in front.

[Attenborough]

The camera detects areas of polarisation,

such as the light

that passes through polarising sunglasses.

100% here.

[Attenborough] It then combines vertical

and horizontal polarisation

to show the contrast

between polarised and unpolarised light.

With this new camera,

the team hoped to find out

how fiddler crabs use polarised light

to signal to each other.

But this camera had been developed

in sterile, controlled conditions,

and these fiddler crabs live in

one of the least sterile environments

on earth,

Australia's tropical mudflats.

When we designed this technology,

we usually test it in the lab.

And it performs well there.

Taking it out in nature, out in the open,

it's a very different challenge.

[Attenborough] And also a challenge

for the cameraman, Mark Lamble.

That mudflat, it's just

a really extreme environment to work.

Blazing sun overhead,

really high humidity,

and almost no airflow.

[Attenborough]

To make matters even more difficult,

the camera needed to be half buried in mud

to get a fiddler-crab's-eye view.

And there's another problem.

One of the things that's really tricky

about being on the mudflats

is the fact that the water that comes

in there is salt water. It's seawater.

But as the day goes on,

the water evaporates,

leaving it more and more and more salty.

So by the time it's starting to dry out,

it's really, really severe brine,

and if you get your hands in it,

it just literally peels the skin off.

[Attenborough]

Whether the camera would work here,

no one could be sure.

[Gruev] I'm slightly worried.

Hopefully, we're not going to miss

that special moment

as the camera is not going to work,

but I think we'll be okay.

- Good luck.

- Thank you.

[Attenborough]

Once in position, Mark settled down

for an uncomfortable wait.

[Lamble] It's incredibly hot.

The air temperature is somewhere

around about 36 to 37 degrees Celsius.

[whimsical instrumental music playing]

[Attenborough] If the crabs detect

the slightest movement,

they disappear into their burrows.

Again

and again.

I have to be really still, or

they will not come out at all.

I'd love to be able to have

an umbrella over me.

So, yes, anything over the top of me,

anything higher than me or the camera

is just not tolerated

by the fiddler crabs.

They just won't come up.

[Attenborough] But amazingly,

the camera survived the heat,

the humidity, and the caustic brine,

and eventually, Mark was able to capture,

for the first time,

a fiddler crab's world in polarised light.

Light reflected

from the crabs' bodies is unpolarised,

so they look dark.

This makes them stand out

against the mudflats,

from which the reflected light

is polarised.

They can see things

that we can only imagine.

When you look up

and you see a bird fly over,

it's a white bird against a white sky.

Whereas when they look up,

it's just this total silhouette

with the polarisation,

and they can see birds coming

from miles away.

And often, I'm filming,

and they'll all bolt down their holes,

and I'll wonder why.

And it's just because

they've spotted a bird

way earlier

than I would've been able to see it.

So polarised light helps the crabs

pick out distant potential mates,

rivals, and predators

more quickly

against their bright polarised background.

And for Viktor,

it was the first time he had seen

the camera he had developed in the lab

revealing the world

in the way these tiny creatures see it.

It's an amazing footage

you've captured, Mark.

It's really amazing.

You really put the system

to its limits today.

[Attenborough] But there was one

even bigger challenge for the camera.

One that lay farther out to sea.

Underwater, only crustaceans,

cephalopods, and a few fish

are known to be able to see

and react to polarised light.

But there is one animal here

that exploits this ability

in a really complex way.

The peacock mantis shrimp.

It's not only able to detect polarisation

but has patches on its body

that reflect light in a polarised form,

and it uses them to signal

to others of their own kind

in ways that we cannot normally see.

Prof. Justin Marshall

of Queensland University

has adapted the polarising camera

to work underwater.

[Marshall] Here we go.

This is the camera

that's gonna show us polarisation.

[Attenborough] Rory McGuinness,

the team's underwater cameraman,

arrived to see

the latest version of the camera.

You've obviously done a lot of work

to get this into an underwater housing.

Yeah, that's right.

You can see in here there's a computer

that runs the camera.

There's quite a lot of engineering

going on in there.

[suspenseful piano music playing]

[Attenborough] Taking the camera

for its first test underwater

was a tense moment.

Computers and salt water

don't usually mix well.

Having found a suitable spot,

it was time

for the camera's first critical test.

A leak could be disastrous.

But all is well.

Now they need a mantis shrimp.

[McGuinness] Looks like

a promising area, Justin.

[Marshall] It looks perfect, Rory.

So we're looking for a hole

with coral around it.

Hey, look.

Is that a mantis shrimp hole?

[Attenborough]

The hole's resident soon appeared.

It was time

for the camera to show what it could do.

As the shrimp turns,

the polarised camera shows

that its tail has a shimmering fringe

invisible in normal light.

[Marshall]

Look at that! That's extraordinary.

Life in polarised light.

Now, this is the first time

we've been able to do this

with this very special camera.

[Attenborough] The light

on the ocean floor is unpolarised.

So, in complete reverse

to the fiddler crabs,

the mantis shrimps use polarisation

to stand out

against the unpolarised background.

Special pigments polarise the light

reflected from parts of their body,

allowing them to signal to deter intruders

and attract mates.

This camera has revealed to us

a first glimpse into a world of light

that we are only beginning to be aware of,

let alone understand.

Science has shown us

that colour is crucial

for survival for many animals.

So what happens when their world

suddenly changes colour?

That happens, of course,

every year in some parts of the world.

Sometimes, even overnight.

[soaring electronic music playing]

During the making of this series,

we went to the Cairngorm Mountains

in Scotland,

in the middle of winter,

to look for a very special bird.

Here, I met Jim Cornfoot,

a land manager

and an expert

on the natural history of these mountains.

How long have you been here now?

Over 30 years since I started up here, so

I've seen a lot of different changes.

In what way?

[Cornfoot] On the Cairngorm Plateau,

there's areas where we have

snow patches lasting all year round,

but if you look at the last 20 years,

there's five, six times

where the snow's completely gone.

And, you know,

over sort of 200, 250 years,

there's only been seven times

that that's happened.

Has that had a great effect

on the wildlife?

They're out of kilter, basically,

with what's going on around them.

Things like mountain hare, ptarmigan,

they're standing out with the browns

and the heather behind them.

- And they're still white?

- They're still white, yes.

They're not set up for that,

so if it's a very poor winter,

you know, they're suffering.

[Attenborough]

Ptarmigan, a kind of grouse,

live year-round

in this exposed environment,

where there are few places to hide.

But now, as the world warms,

things are changing dangerously.

The recent decrease in snow cover

has made them only too conspicuous.

Animals like this mountain hare,

also in its winter coat,

can be seen from far away.

And that makes life very hazardous.

These changes are affecting animals

all around the northern hemisphere.

In North America,

the reduced snow cover

has caused snowshoe hares

to be mismatched, on average,

for a week a year.

During this time,

the hare is 10% more likely

to end up as someone else's dinner.

By the end of the century,

the loss of snow cover

is predicted to expose the hares

for up to eight weeks a year,

so increasing their annual mortality

by almost a quarter.

Unless they can adapt rapidly,

they could be

in serious danger of extinction.

While a warming climate

is causing problems in northern habitats,

it's also driving colour changes

in other parts of the world

including some of the most beautiful,

colour-rich habitats on our planet.

Coral reefs.

Our Australian team spent months filming

on the Great Barrier Reef.

In these sunlit waters,

colour is everywhere.

But this habitat is being subjected

to the most drastic colour change

imaginable.

And our crew witnessed it firsthand.

[melancholy instrumental music playing]

The corals have suddenly

turned into white skeletons.

It's called coral bleaching,

and it's now happening

only too frequently.

On the Great Barrier Reef,

such events have increased

from once in every 25 years

to three events in the last five.

Prof. Jörg Wiedenmann,

from the Coral Reef Laboratory

at the University of Southampton,

has been working to discover

what is behind these changes.

The key is the relationship

between coral and the microscopic algae

that live in their tissues.

These algal partners are called symbionts.

It's they that give the coral its colour.

The algae, when they photosynthesise

during the daylight hours,

use sunlight to grow,

excreting sugars as a byproduct,

which are then absorbed by the corals.

[tranquil piano music playing]

This partnership was established

during the time of the dinosaurs

and has been such a success

that it has created structures

that are visible from space.

But warming seas

are disrupting this system.

So when the seawater temperatures rise

above a critical threshold,

the photosynthetic machinery

of the algal symbionts

starts to malfunction.

[Attenborough]

They begin to produce toxic compounds,

which cause the corals to expel them

from their tissues

so the coral loses its colour.

It bleaches.

Sometimes, the bleach corals die,

and then the entire ecosystem,

together with everything it supports,

is lost.

Almost half of the corals

in the Great Barrier Reef

have died this way over the last 15 years.

But, in the last decade,

there have been reports

from various parts of the world

of coral developing

startling neon colours.

We are just beginning to realise

that corals are using colour

to fight back.

Jörg is studying how this works.

[Wiedenmann]

This coral has lost its algal symbionts,

but instead of turning white,

it's producing

these bright neon green pigments.

The coral produces these pigments

to protect the remaining algae

inside of the tissue

from excess light stress,

so they act as a sort of sunscreen

for the symbiont algae.

[Attenborough]

This coral sunscreen makes it more likely

that the bleached coral will be able

to take back its algal partners,

restoring its food supply, its colour,

and helping it to recover.

But even this extraordinary adaptation

is not enough to protect coral

against all the changes

it is now facing.

If corals have been exposed only

to mild stress,

then they can recover from bleaching.

[Attenborough] But if corals

are subjected to prolonged

or extreme levels of heat stress,

they lose their ability

to create these sunscreen pigments

and are likely to die.

And, unfortunately,

global warming is making this more likely.

There's a severe danger that corals

will be exposed to episodes of stress

where they can't recover,

and they can't use these pigments

to bounce back from bleaching.

[Attenborough] So, although colour

might be helping coral reefs

to tolerate some of the change,

only action to halt global warming

will ensure their survival.

If warming continues, then they,

together with the beautiful array

of colour they provide,

will disappear

from the reefs of the world.

[enchanting orchestral music playing]

Science and technology

are continually unravelling

more and more details of the way

animals perceive colour and use it.

We may marvel at its beauty,

but for many animals,

it's the key to their existence.

The more we understand about its function,

the better we will be able

to protect the natural world,

in all its beauty,

for future generations.