Scent Super Senses: The Secret Power of Animals

Our human senses are incredible.

We have excellent vision.

Precise hearing.

And can detect the slightest fragrance drifting on the breeze.

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

Imagine a world where you could see with sound.

These images are just phenomenal.

Hear thunderstorms from hundreds of kilometres away.

That's incredible. They've all stopped.

Imagine seeing the world in slow motion.

Or through some of the sharpest eyes in nature.

HE GASPS

He's so fast!

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

We reveal the strange and wonderful world of animal senses.

Light is emitted. Look at that!

Another one!

This is brilliant!

I'm Dr Helen Czerski.

I'm a physicist and I want to find out how animals tap into an amazing

range of light, scent and sound.

I'm Patrick Aryee.

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 the invisible world of scent.

To discover the bizarre and extraordinary ways

creatures use their sense of smell to survive.

Experience the world through animal senses.

The plains of Africa.

As I walk through this landscape, my eyes see the golden sunlight.

And my ears hear exotic birdsong.

But there's another sensory world here that I can barely detect.

In the air are millions of minute scent particles,

given off by the earth...

..by plants and by animals.

Our human noses pick up only a tiny fraction of these smells.

But for any animal that lives here, this is vital information.

In this landscape, scent is king.

On the plains of Africa,

smell is crucial to the survival of most animals.

And over millions of years

life has tapped into this invisible chemical world

in some extraordinary ways.

The Great Migration,

where more than a million wildebeest journey across the savanna,

is driven by scent.

Each wildebeest leaves a trail of a musky odour that others follow.

Scent keeps these herds together on their epic journey.

Lions also produce a distinctive scent

and cubs recognize their mothers through her unique smell.

But there are some unlikely little creatures that also survive out here

thanks to their amazing noses.

Meerkats.

I'm going to spend the day with them to see just how they do it.

These meerkats have just come out of their burrow.

The gang is made up of 14 adults and four newly emerged pups,

and they're just heating up as the sun rises.

These pups are just enchanting,

and they're only about four weeks old.

You can see they are sticking very close by.

With pups around, the gang must be extra vigilant.

And whilst meerkats are known for their excellent vision,

it's their long pointed noses that can offer vital surveillance.

Meerkats are constantly alert.

And whilst this sentry is on the lookout for any predators

in his visual range,

Meerkats can smell danger long before they see it.

And that's their way of keeping their pups safe.

Meerkats have many predators...

like jackals

and cobras.

Their predators may "appear" camouflaged,

but they cannot hide their scent.

Wherever they go, they leave minute traces.

To show how well their noses are tuned into these dangerous smells

I've got a little test.

Now, this set up might seem a bit strange, but inside this bag

I've got a small rock with a piece of wildcat hair attached to it.

Now, wildcats are one of the major predators of meerkats -

so let's see what happens when I put it down here,

a few meters from where they're foraging.

It takes just a few moments for the scent to reach the meerkats' noses.

Ah, you can see they've definitely picked up the scent

and now they're coming in.

And you can see that classic mobbing behaviour -

their backs are arched, their tails held high.

And they're all clustered together as well.

It's a powerful display of how finely tuned

the meerkats' noses are to danger.

But it's not the only way these little gangsters

use their sense of smell.

They also use it to defend their patch.

Meerkats spend their lives constantly locked in a turf war

against rival gangs,

so it's absolutely crucial they can identify friend from foe.

Meerkats are highly territorial.

Scraping out a living in this arid desert is tough,

so staking claim to their patch and food source is an absolute priority.

They do this by scent marking.

It's such a powerful instinct.

If anything new is placed in their territory - like this stick -

they have an irresistible urge to mark it.

OK, here we go. It looks like one of the more dominant meerkats

is about to stake its claim.

And it's scenting all over the post -

making clear that it belongs to this group of meerkats.

Meerkats produce the scent from glands just below their tails.

There it goes.

And there it goes, again.

Anything unfamiliar, any object in their patch,

they'll want to claim as their own.

Now, I've got a rather mischievous trick.

I'm going to take this post and present it to another gang

to see if they recognize their rival's scent.

This marks the border between two rival gangs of meerkats.

And visually there's not much to see,

but if you could enter their olfactory world

you'd see this entire area awash with odours and meerkat scent.

This heavily scented area is

a meerkat's equivalent of a no trespassing sign.

For a passing meerkat from a rival gang,

you'd cross this border at your peril.

I set up the post close to the rival's burrow.

Within minutes they pick up the scent.

Here we go. We've got a meerkat coming up to our post.

And now more of them are coming in.

So, they've detected that the scent that's been left

is from a rival meerkat.

And now they're on high alert.

And one of them is standing on its hind legs -

and its eyes are trained on the horizon.

He's trying to see if he can spot an intruder.

And if he were to see a rival meerkat,

it would rally the rest of the troops into a violent attack.

Using their noses,

meerkats detect dangers their eyes and ears just can't pick up.

Scent is hugely important in their world.

And there's a good reason for this.

Leave some water out in the open and it quickly evaporates in the heat.

That's exactly what happens with scent as it disperses into the air.

And that's why places with warmer climates

have more smells in the atmosphere.

For the meerkats, this vital source of sensory information

is key to keeping the gang safe.

The meerkats are back from their days foraging,

safe and sound in their burrow

and the young pups are accounted for as well.

And that's in part thanks to their amazing little noses

that act as excellent early warning systems.

Although meerkats are brilliant at detecting lots of different smells,

their noses aren't very sensitive to tiny traces of scent.

But in the frozen wilds of Alaska,

there's one mammal that can sniff out food from more than a kilometre away.

That requires one of the most acute noses on the planet.

You know those cold, clear days

where the world just seems to be fresh and clean?

One of the reasons that everything seems so clean is that

you actually can't smell most of what's going on.

And that's because cold environments like this one

make it a lot harder for those scent molecules to evaporate

and to drift up into the atmosphere.

And just to show you how cold it is, look at this.

I've got a beaker here full of just boiled water. Watch.

At minus 20 degrees Celsius,

the boiling water freezes before it hits the ground.

If you're a carnivore and you can survive in this environment,

you need a really impressive sense of smell.

And there's one tenacious mammal that thrives here

thanks to its extraordinary nose.

It's the giant of the weasel family.

The wolverine.

Wolverines are scavengers, and they're really good at it.

They can travel up to 25 miles a day

through terrain like this looking for food,

and what they're looking for is dead animals

that are buried underneath the snow.

Their Latin name is brilliant,

it's Gulo gulo, which means the glutton,

and they're voracious eaters,

they chomp trough a carcass, bones and all.

To find out how they locate their food in this frozen wilderness,

I've come to meet one of the world's only habituated wolverines.

A two-year-old orphan named Jasper.

Jasper was rescued by arctic wildlife expert Steve Kroschel,

who exercises him on a lead to contain his boundless energy.

So, he's sniffing around all the time,

-getting used to the environment?

-Yes. Never a dull moment.

They're always very active, their metabolism is very high.

No such thing really as a fat wolverine.

And they don't hibernate, like a bear.

-So, they just keep going all winter?

-That's right.

They don't have much time, it's all about survival, you know,

and mother nature doesn't give an animal like this

a lot of margin for error.

As scavengers, most of a wolverine's diet is dead animals.

And to find their food,

wolverines take advantage of one of nature's most destructive forces.

Avalanches.

As snow thunders down the mountainside,

mammals are often buried beneath.

And it's thought that wolverines can sniff out frozen carcasses

even deep beneath the snow.

I want to put this to the test,

so I'm going to bury some meat and leave it for several hours.

Then, I'm going to see if Jasper can sniff it out.

I could walk along this path now

and never know that the chicken was down there.

But here's the amazing thing.

Even though it's frozen solid, and it will stay frozen solid,

molecules of that chicken are evaporating in tiny quantities.

They're drifting out through the snow and out into the air just here.

I can't detect them, but a wolverine could use them to find dinner.

Before we set Jasper his challenge,

I'm going to demonstration how our noses and the wolverine's work.

To show you I'll need an orange.

When I sniff the orange...

It's really strong, this one.

..is that the scent molecules from the orange are going up my nose

and they're going up to the top here,

and there's a structure up here called the epithelial membrane

and that is where the odour detectors are.

It's thought smell works like a lock and key.

The scent molecule is the key and will only fit certain "locks" -

the odour detectors.

We have 400 different types of locks.

But scent molecules can hit many combinations of detectors

so we can identify thousands of different smells.

And that's how its works for everything you've ever smelt,

from new shoes to perfume to oranges,

it's all about the combination of receptors that those molecules hit.

All mammals detect scent in this way.

But for wolverines it's all about sensitivity.

Detecting the tiniest traces of scent, even deep under the snow.

So, let's see how Jasper gets on with finding the buried meat.

We start several hundred meters away

and let the wolverine choose his course.

He's smelling things right now, Helen.

And it is not only what is on the snow,

it's the air drifting past that he's smelling.

Oh, having a bit of a sniff here.

He quickly picks up a scent, but is it the right one?

He definitely knows there is something here.

He's onto some scent there, for sure.

Those big paws are pretty efficient, aren't they?

-Yes.

-He looks like nothing will get in his way.

-He's shifting a lot of snow.

-Yeah.

Oh, they're we go - he's got it!

He got it. He found it.

He might even recache it if he's not all that hungry.

In the wild, wolverines cache, or bury, their food

so they can eat it later.

He's heading for a hole over here.

Oh, he's going to put it in there, isn't he?

Straight in there. So, that's his larder now in there.

So, Jasper succeeded in sniffing out the buried food.

I couldn't smell a thing.

But if their noses work in the same way as ours,

why is their sense of smell so much better?

It all comes down to an extraordinary adaptation inside their noses.

This is the skull of a wolverine.

And the really interesting bit is his nasal cavity, which is here.

If you look straight down it

what you see are these amazing honeycomb structures.

This is where the scent detectors are found.

The honeycomb pattern greatly enlarges the surface area

so there's room for many more detectors.

Unravelled, it would be the size of a plate.

We have a similar structure,

but ours is only the size of a ten pence piece.

It means that up the nose of a wolverine

is an enormous detector for scent.

In fact, the wolverine's nose has twice the surface area

of a bloodhound's -

a dog purposefully bred for its amazing sense of smell.

That's how they're able to track down frozen carcasses

even deep beneath the snow.

And it's their secret to survival in this icy world.

But one creature has evolved an even more precise way of detecting scent.

They can smell the world in stereo,

allowing them to pinpoint prey with deadly accuracy.

Rattlesnakes.

For decades, a mystery has surrounded exactly how these snakes

are able to hone in on their prey.

It's only now that we can unlock their sensory secret.

The answer lies in the unique way it smells the world

and the brilliant way it uses its venom.

I've come to the great prairie lands of Colorado

to see these snakes in action.

And I've teamed up with Professor Steve Mackessy

who's advised me to wear snake-proof boots.

Steve, where's the best place to find some snakes?

Well, we have a rattlesnake den site about 30 meters up ahead here.

And we're going to see if the snakes are out and about.

So, watch your feet.

Right. So, how many snakes are we likely to find today?

Well, we could find as many as...

HE GASPS

There's one! There's one there! Look! Look!

-Oh, my goodness.

-There it is.

They move so quickly.

That rattlesnake was perfectly camouflaged,

we had no idea it was there.

Oh, lordy, lord.

-I think it's pretty clear they're on the move today.

-Yeah.

I'm going to be a bit more careful because after seeing that snake

I can tell you I've got goosebumps all over my skin.

But to show how rattlesnakes use scent to hunt down prey

I need to get closer.

A lot closer.

Luckily, the next snake we find is a little calmer.

And reveals its extraordinary sensory trick.

Now, you can see that tongue flickering in and out of it's mouth.

And what it's doing is using that tongue to pick out

the scent molecules in the air.

Some of which I'll be giving off right now.

So, he can taste my presence.

For the rattlesnake, its sense of taste and smell are the same thing.

When it detects a scent, it brings its tongue back into its mouth

and presses it on a patch of sensory cells.

This highly sensitive area identifies the smell.

It's called the Jacobson's organ, and this is what gives

the rattlesnake its serpentine supersense.

But this is just part of the story.

Many rattlesnakes are strike and release predators.

They lie in wait,

strike their prey just once then let them go.

But the venom doesn't kill immediately,

so the prey can move a distance away before the toxins take hold.

This presents a huge sensory challenge for the rattlesnake

because now they must track down their dinner

in these wide-open prairies.

And that can be like finding a needle in a haystack.

Within these vast grasslands are a complex and tangled network

of scent trails left by countless mice and other small mammals.

So, even with this supersense,

a rattlesnake somehow needs to distinguish between all

the other mice and the one that has been injected with its venom.

How do they do this?

I'm hoping this test will offer an intriguing clue.

Using sponges, I lay two trails in the sand.

The one on the right contains the scent of rattlesnake's venom.

The one on the left doesn't.

Which one will the snake follow?

So, you can see our snake is already on the trail.

The forked tongue gives them their directional sense of smell.

This snake is detecting more scent molecules on the right fork,

so is moving in that direction.

He's using that forked tongue to pick out the scented sponge.

Look, he's going the right way now.

It's a great demonstration of how a rattlesnake

will find its prey in the wild.

There you go, it's right by the sponge.

I really wasn't sure if our experiment was going to work.

But it goes to show that the rattlesnake's sense of smell

is absolutely remarkable.

The snake followed the trail that containing the venom.

So, it seems there's something in the deadly toxin that allows

the snake to track down its prey.

But what is it?

Steve has been carefully extracting venom to unravel this mystery.

So, when you have them up close here you can see fangs.

Yeah, I can see them.

So, using these capillary tubes

-we should be able to get some of that venom out.

-Right.

Prairie rattlesnakes have long, delicate fangs.

That's why the snakes release their prey as soon as they've struck.

It means the fangs don't get damaged.

Wow.

-So, I'm going to put this in our tube here.

-Yes.

That's incredible.

HE EXHALES

Rattlesnake venom contains more than 100 different molecules.

By freeze-drying it and isolating all the components, Steve discovered

a group of molecules that appear to help the snake track its prey.

They're called disintegrins.

Well, the disintegrins seem to act as a chemical tag,

so it's marking of prey that's been bitten by the rattlesnake as

the envenomated prey, and that's what lays down the chemical trail

-that the snake is following.

-Yeah.

And so by using that very exquisite sense of taste and smell,

the snake is able to follow the prey very, very carefully and very,

very exactly to where it's run off and expired.

Disintegrins are the mystery molecules in the snake's venom

that allows them to track down their prey with such precision.

In the rattlesnake's world, where the ground is awash with scent,

this is how they find their dinner in

the vastness of the great American prairies.

So far we've seen how animals are able to detect

tiny traces of scent in the air around them.

But it's an amazing thought that our oceans are also awash

with different smells.

And creatures beneath have developed

even more sophisticated ways of sensing them.

I'm just off the Bahamas and I'm here to show you one of

the most powerful uses of scent in the natural world.

In our oceans, there's a complex mix of thousands of chemical compounds

carried on swirling currents.

And I'm in search of a master of this sensory world.

The shark.

These sharks are swimming through a world that I just can't sense.

But they're masters at it.

The water all around me here is full of tiny amounts of chemicals

that I can't detect.

But there's a huge amount of information

just flowing around my head.

And these animals,

they're fantastically well equipped to swim through this world

and to be able to decode that information and use it to survive.

I want to discover the extraordinary way sharks use scent

and debunk an age-old myth about their thirst for blood.

But to start with,

I want to find out how they are able to smell underwater.

For that I need to get hands on with a baby lemon shark.

Look at this, I'm holding a shark. I've never done this before.

He's quite calm down here,

but there is a trick where we can calm him down a little bit more.

What I'm going to do is turn him over.

If I turn the shark upside down he becomes very passive.

It's called tonic immobility,

and it gives me a chance to look at his nostrils.

As water flows through the nostrils, or naries, it enters

the olfactory chamber, lined with hundreds of thin membranes.

These are covered in odour detectors.

And the water is always flowing in the same direction

so the sharks are smelling continuously.

It's a brilliant system, but just how sensitive is it?

To find out I've teamed up with shark biologist Dr Tristan Guttridge

to test a scent that sharks are renowned for detecting.

The smell of blood.

So, we're standing in a pen of lemon sharks here.

Yup, these are semi-captive pens.

As you can see, they are exposed to the ambient conditions,

and we have about 12 juvenile lemon sharks.

And as they are swimming along,

they are continually sensing the water around them.

Exactly, so they will be swimming around

and they're determining whether there is something interesting.

And if there is anything interesting that smells good,

they'll then direct themselves towards that source.

And what sort of thing interests them?

These little lemon sharks, they feed primarily on fishes.

So, they will be sniffing around hunting for fish.

To start our experiment, I take a sample of fish blood.

I'm going to release a tiny amount into the water to see if they react.

We're looking for a corkscrewing behaviour,

where the sharks turn repeatedly.

It's a sign they've detected the smell

and are trying to find the source.

There you go.

Oh, he turned.

-So they're all turning now, looking.

-Yeah.

That guy in particular

just turned around at least three or four times -

there you go, he's trying to find it on the floor.

See this one here turning as well?

He knows what's going on.

And now all of them are interested.

-Oh, they've all come in, look at that.

-Yeah.

So just a few drops of fish blood triggered this response.

But what about human blood?

It's been said that it can attract sharks from many kilometres away.

So it's spreading out just like the fish blood did.

Let's see what these guys do.

No change whatsoever -

still in their nice, big circle formation.

Let's see if they come through it again - these two here.

Yeah, they would have picked it up by now.

So they're not interested in humans or human blood at all.

No. We're intact and they didn't react to the human blood.

Our simple test shows these sharks are not attracted to my blood.

And despite many myths,

there's no evidence to suggest that larger sharks - like great whites -

are interested in human blood either.

But there is one mysterious scent that all sharks detest.

In fact, it's so reviled by these predators

that it's currently being trialled as a shark repellent.

So we've come to a reef where sharks gather in large numbers...

and I've been volunteered as a guinea pig.

There's a lot of sharks down there.

The plan is to go down to let the sharks get used to my presence,

and then to release this and watch how they react.

Three, two, one - jump.

The sharks come right up to investigate.

Their noses are detecting the minute chemical cues

I give off as I move through the water.

Look at this!

They look bigger down here than they looked up on the surface!

Oh...

I just love it being down here,

sitting in the middle of all this.

You can see their nostrils right on the front of their snout.

Imagine water coming towards them -

so they're taking it into this chemical laboratory

that's inside their nose.

They're amazing animals.

When the sharks are really close, I release the scent.

They've gone.

All the other species are still here.

The sharks have vanished.

So why are sharks so repulsed by this mystery smell?

Dr Patrick Rice is a marine biologist who studies shark olfaction,

and he's helping to develop this shark-repelling scent

called a necromone.

A necromone is a chemical signal

that tells sharks that there's other predators in the area.

There's evidence from nature that when a predator comes in

and eats another shark,

the little bits left behind may start to decay

and emit this signal,

and that chemical signal

tells a shark that there's a predator in the area.

So it's a protective thing -

so even though we think of sharks as being invincible,

they're actually - even they have to look out for themselves.

That's correct.

Necromones - or the scent of dead shark -

are a sure sign there's danger around.

So sharks are hardwired to flee when they detect it.

It's believed this is a response they've had since prehistoric times.

So this evolved at a time when sharks weren't top predators -

they were just little things in the sea,

-and they were basically food for even bigger animals.

-That's right.

There was a monster shark called the megaladon -

a prehistoric great white shark that was about 60 feet long,

and it preyed on everything,

and it was the king of the ocean at the time.

And today they're still vulnerable.

Almost half of baby sharks don't survive their first year -

many because they're eaten by bigger species.

Even the most feared ocean predator - the great white shark -

can be attacked by a pod of killer whales.

So the ability to smell danger is still key to their survival.

Although sharks detect scent underwater,

their noses work in a similar way to ours - and other mammals.

It's all about taking scent inside the body

and analysing it.

But some creatures smell the world through the outside of their bodies,

and that allows them to use scent in even more complex ways.

BUZZING

Bees.

Their world is controlled by smell.

And if you can tap into this, you can harness the power of the swarm.

I've come to Arizona's Sonoran Desert

to show the overwhelming power of scent in the bee's world.

This is a swarm of European honeybees,

and there must about 5,000 of them.

And I'm going to move them from here...into here.

And I'm going to do it simply by tapping into their world of scent.

And this is my secret weapon.

This is amazing.

You can see all the bees are reacting,

and they're instantly swarming into the case.

And it's that sense of smell

that's driving that behaviour.

The bees are reacting to minute traces of scent in the air.

A scent I can't pick up at all.

In humans, we smell by breathing,

but bees detect scent in a completely different way.

They use their antennae and that's what they are doing here.

Capturing that scent and moving towards it.

Their antennae are packed full of odour receptors,

which pick up scent molecules as they fly though the air.

Just look at that -

virtually the entire swarm is now in this case.

But what's the secret to my bee trickery?

It's all to do with a group of odours called pheromones

that bees use to communicate.

It's as if they can talk through scent.

My secret weapon was a synthetic pheromone

that exactly replicates the one produced by the queen bee.

It encourages the workers to surround her,

and that's what's happened here.

The bees have detected the scent and moved into the case.

So there we have it -

5,000 bees in a briefcase,

simply by the power of scent.

This is just one example of how bees use odour to talk to each other.

They have a language of scent -

with more than 15 pheromones

triggering wildly different behaviours.

They use them to navigate, find food and to reproduce.

But there's one pheromone that triggers a response so dangerous

that it can only be studied well away from human habitation.

To see this in action,

I've come to a remote research base deep in the desert.

This is home to some of the most dangerous insects on the planet.

Behind me are hives full of Africanised bees,

also known as killer bees.

And to protect their home,

they'll wage war on anything or anyone that approaches.

And they do this entirely through their sense of smell.

I've teamed up with entomologist Dr Steve Thoenes

who is studying what makes this subspecies of bee so aggressive.

Once a month, he checks on the health of the hives.

To do this, he has to get up close.

Here we have the research apiary for Africanised bees.

Africanised bees have a lot of good qualities.

They are very hard working,

they produce a lot of honey

and they don't get any of the diseases that European bees do.

Their problem is

that they are really, really defensive of their nest.

If you disturb one bee, she will release alarm pheromone.

The alarm pheromone spreads,

and you have this whole cascade where up to 50% of the workers

-will come out to defend their colony.

-Wow.

The main area that Africanised bees are going to come is at your face

because they have been attacked for millions of years by mammals.

-And we all breathe in oxygen and we breathe out CO2...

-CO2, yep.

..so they are going to come right at the CO2 source,

which is your mouth and nose.

Just don't panic.

Let's get you all suited up, and we'll go look.

In defence mode, the bees will find any chink in our bee armour.

So nothing's left to chance.

Steve uses smoke to keep the bees calm.

This blocks the scent receptors on their antennae,

making them less sensitive to alarm pheromone.

But it doesn't last long.

So, Steve, we've got our smoke,

which is going to pacify the bees, yeah?

Yes, that's the best tool a bee keeper has.

Gosh, I can feel my heart beat.

I've been nervous around bees since I was a child,

so I'm going to have to overcome one of my greatest fears.

How long do you think we'll have here?

Couple of minutes.

Let's see what's inside of here.

OK, they're coming at me.

They're definitely interested in what we're doing now.

They're getting very, very defensive.

This is serious. This is serious stuff.

This is just starting.

Oh, you're joking.

Steve needs to check through the hive

to ensure the honey stocks are good, and there's no sign of disease.

But his work triggers the bees to start releasing alarm pheromone.

And when one starts - others quickly join in.

Oh, whoa.

Oh, wow.

Wow!

Oh, Lord.

Oh...

Within minutes, thousands of bees are in the air

and in full attack mode.

So this is the defence of Africanised bees.

Where they've coordinated all this by odour,

and there's an odour in the air.

-You can smell a little of it.

-That smells...

-That smells just like...

-Bananas.

-..off bananas, yeah.

-Right.

And that's the alarm pheromone that the humans can smell.

And they're coordinating their attack by releasing alarm pheromone

and responding to that.

How does it feel to be a predator of bees?

I don't want to be a predator of bees!

-I want to leave them alone.

-OK.

Despite our hostile reception, it seems all is well with the hive.

So the brood's good, the honey stores are good

and the whole hive is thriving.

Great. It's time for us to, er, get out of Dodge!

-It's getting too much.

-OK, I'll put it back together again

-and we can head off down towards the truck.

-Let's do that.

It's been a stark demonstration

of the power of scent in the bees' world,

and I'm just happy to be heading back.

That was overwhelming.

There must have been 5,000 Africanised bees

swarming all around us.

We've left the hives, but they're still following.

This has got to be one of the ultimate examples

of how an animal uses scent to defend their home.

The intricate ways bees use scent to communicate

have made them one of the most successful insects on Earth.

But when it comes to detecting tiny quantities of scent

from a long way away,

the ultimate prize goes to another extraordinary insect.

Moths -

one of the planet's best smellers.

Their supersized antennae are like feather dusters,

sieving the air for minute molecules of scent.

Some species can pick up scents from nearly five kilometres away.

I've come to meet one particular moth

renowned for an amazing sense of smell.

This is the tobacco hawkmoth -

they're really large moths that live in America,

and that thing that's really distinctive about them

is that they're able to hover,

so they look a bit like the hummingbirds of the insect world.

And the thing about the hovering is that it's really energy expensive,

and so these moths need a constant supply of food.

When they're active,

they need to feed every 15 minutes.

And they're capable of flying 80 miles in a single night

just to find food.

Which is a tall order

if you're only this big.

These hawkmoths feed on nectar from flowers.

Like most moths, they're only active at night.

So all they have to guide them through the darkness

are minute wisps of scent blowing from the flowers.

To reveal the fascinating way they do this,

I've set up a hi-tech smell test.

Using a wind tunnel,

and their favourite food source -

a petunia flower -

I can recreate their natural feeding conditions.

In pitch darkness, with a night-vision camera,

I'm going to see how quickly these moths can track down their food.

When I switch the wind tunnel on,

the breeze will carry odour molecules downwind

and down there a moth might detect them,

and then it's got some detective work to do,

because it's got to solve the puzzle of where the scent came from.

And that is not as simple as it sounds.

The fans create swirling eddies

of turbulent air -

like a windy day.

If we could see the flower's fragrance,

it would appear as delicate wisps or patches of scent

constantly on the move.

So, for the moth, working out where that smell is coming from

is a real challenge.

Once inside the wind tunnel he zigzags back and forth,

trying to find the tiny traces of scent.

The patches of odour that the moths have to detect

could be just a few millimetres across,

so at the speed these moths are flying,

it would be really easy to miss the signal.

So, for the moths,

it's not enough to be able to smell well -

they've also got to be able to smell at high speed.

We can identify less than one smell a second.

That's because it takes time for our noses to refresh

and detect something new.

But these moths smell more than 30 times quicker.

And because their antennae are constantly being refreshed

they can detect and follow the tiniest traces scent.

And that's how they are able to find the source of the smell

and feed on the sweet nectar

that's so valuable to them.

It's taking these moths a lot of energy to stay in the air.

They're probably beating their wings

30 to 40 times a second.

For all organisms there has to be a balance

between the energy they take in, they eat,

and the energy they need to expend to live -

and if you look at way these moths are flying,

they're using up a huge amount of energy just to stay in the air,

so they must need a lot of food.

And if you're in that situation, the best thing you can do

is be as efficient as possible about how you find it.

And that is what their sense of smell is all about.

In the wild, these amazing hovering moths

use their supersensitive antennae

to find flowers from several kilometres away -

something they could never do through sight or hearing.

For me, this is the most incredible example

of how an animal uses scent to survive.

Our journey through the world of smell is almost complete.

But there's one creature that's harnessed the power of scent

in a highly unusual and unwelcome way.

They produce an odour so potent

it should only be handled with extreme caution.

And the fumes it gives off are so volatile

they light up with a bang.

So what animal produces this unholy odour?

Meet the striped skunk -

one of the world's smelliest creatures.

I've come to northern Arizona

to discover the amazing way these little mammals use scent

to protect themselves.

To experience it first-hand, I'm joining Kevin Overfield,

who rescues skunks

that have caused a bit of a stink.

Basically we go to homes,

people that are having nuisance, animal problems -

like the skunks are living under their house or under a deck.

We go over there, we set a live trap so that the animal's not harmed,

and we trap 'em and we bring 'em back to my home.

Once we know that they're healthy and not sick

we take them out and release them in the woods,

back in their natural habitat.

Kevin has a skunk that's ready for release.

And he's kindly offered me the chance to help him out.

So what can I expect when we try and put this skunk in a cage?

You can expect to be sprayed.

OK. And what's that musk smell like?

It's really hard to explain - there's no other smell like it.

You can taste it, you can smell it,

you know, you can almost feel it sometimes, it's so powerful.

PATRICK CHUCKLES

-So you know when you've been sprayed by a skunk.

-Definitely.

You know when you've been sprayed.

OK. Well, here goes.

-We're going to move up slow.

-Yep.

Be aware where the cage is -

-the cage is going to be right here, OK?

-OK.

-Now, just kind of move up on him real slow.

-Mm-hm.

OK.

OK, there's your chance.

There you go.

OK, now hold him there.

Hold him like I told you. Yep.

-OK.

-SKUNK SQUEALS

-KEVIN LAUGHS

-No, put him in the cage!

There you go, got him.

Oh... Oh, God, that's disgusting.

Good job.

PATRICK COUGHS

It smells of burning rubber, stale urine and rotten eggs.

HE COUGHS HEARTILY

If you've got it on your face, just keep spitting.

Ugh, right up my nose. Ugh.

Augh!

Straight in the face.

If you look at your glasses, all the yellow on your glasses,

that's all skunk spray and on your forehead.

They can aim. They can aim very well.

-I think you did a good job.

-Oh, yeah...

-I'll hire you.

-PATRICK LAUGHS

But why do we find this smell so unpleasant?

So, this is it.

This is the skunk's secret weapon.

It's made up of seven volatile compounds called thiols.

But it's one element within these that causes the stink.

The reason we find this substance so repulsive

is because it contains sulphur -

now, from an evolutionary point of view,

sulphur is associated with areas of low concentrations of oxygen

and rotting food, so we do our best to stay away from it.

Skunks use their sulphurous scent to defend themselves

against predators - like mountain lions.

They can spray it 15 feet,

and aim for the eyes, causing temporary blindness...

..giving the skunk time to escape.

But their foul smell is precious.

They only carry enough to spray about five times,

and then it takes ten days to replenish.

So skunks only pull the trigger as a last resort.

That's why they've come up with an ingenious way

to advertise their potency.

Perhaps one of the most obvious things you notice

when you first see a skunk is that magnificent white stripe.

It's used as a warning to other animals.

It says, "Stay away - I am a skunk and I am toxic."

It's believed the skunk's odour is so notorious

that their colouring alone is enough to deter predators.

So what happens when an animal with one of nature's most sensitive noses

spots those bold warning stripes?

I've come to a bear park in Arizona to find out.

So here's the plan.

We've rustled up two decoy skunks -

one with that classic black and white stripe

and one which we've dyed completely black.

Now, in theory, any foraging bears should have no problem

approaching our black skunk -

but they should avoid this black and white one,

because they associate these markings

with that foul-smelling musk.

The stunt skunks are placed inside the bear enclosure

with on-board cameras.

And to lure the bears up close, I'm leaving some tasty snacks.

We've got our decoy skunks in position.

Now, let's see if those black and white stripes

really are a deterrent.

After some time, a big bear called Jack approaches.

OK, we've finally got a bear coming in to investigate.

He's a really big one.

This big bear is definitely approaching our black skunk.

You can see he's not bothered in the slightest.

He's confident,

he's eating up all those berries -

that skunk is in big trouble.

Now he's just taking the skunk apart.

OK, now he's turned around

and he's headed for our black and white skunk

Bears use their sensitive noses to sniff out food.

But such an acute sense of smell would be overwhelmed by the skunk.

So, in theory, Jack should spot that white stripe and steer clear.

I don't know if he's interested or not -

but is he going to go for him?

Now he's turning around.

Looks like he's not interested in the striped skunk at all.

It worked.

It may not be the most rigorous of scientific tests,

but it illustrates the point

that the skunk's success is not only down to their foul smell,

but also the brilliant way they advertise their defensive weapon.

You take him right down there by that creek.

And it's this that should keep our rescued skunk safe

as he's released back into the wild.

-Somewhere about here?

-Yeah, that'll be good.

Skunks really do get a bad press,

but having the chance to get this close to them,

you really can appreciate what beautiful creatures they are.

And just like any other animal out here,

all they're trying to do is survive.

In this episode we've seen the most extraordinary ways

animals tap into the invisible world of scent.

Smell is our most unfamiliar sense.

But in the wild it offers animals vital information

that they just can't get from sight or sound.

In nature, a keen nose is essential for survival.