[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 it because 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 kill 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? Sharpshooter? -[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 killing 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.