0:00:04 > 0:00:06Our human senses are incredible.
0:00:09 > 0:00:10We have excellent vision.
0:00:13 > 0:00:14Precise hearing.
0:00:18 > 0:00:21And can detect the slightest fragrance drifting on the breeze.
0:00:24 > 0:00:28But we only experience a tiny fraction of what's out there.
0:00:30 > 0:00:33Imagine a world where you could see with sound.
0:00:35 > 0:00:37These images are just phenomenal.
0:00:42 > 0:00:45Hear thunderstorms from hundreds of kilometres away.
0:00:47 > 0:00:49That's incredible. They've all stopped.
0:00:51 > 0:00:55Imagine seeing the world in slow motion.
0:00:55 > 0:00:58Or through some of the sharpest eyes in nature.
0:00:58 > 0:00:59HE GASPS
0:00:59 > 0:01:00He's so fast!
0:01:02 > 0:01:04Travelling to some of the wildest places on Earth...
0:01:06 > 0:01:10We reveal the strange and wonderful world of animal senses.
0:01:10 > 0:01:13Light is emitted. Look at that!
0:01:14 > 0:01:16Another one!
0:01:16 > 0:01:18This is brilliant!
0:01:18 > 0:01:19I'm Dr Helen Czerski.
0:01:20 > 0:01:25I'm a physicist and I want to find out how animals tap into an amazing
0:01:25 > 0:01:27range of light, scent and sound.
0:01:31 > 0:01:33I'm Patrick Aryee.
0:01:33 > 0:01:36And as a biologist, I'm fascinated by what the world appears like
0:01:36 > 0:01:39through animal senses far superior to our own.
0:01:43 > 0:01:45In this episode,
0:01:45 > 0:01:48we are on a journey through the invisible world of scent.
0:01:51 > 0:01:54To discover the bizarre and extraordinary ways
0:01:54 > 0:01:58creatures use their sense of smell to survive.
0:02:01 > 0:02:03Experience the world through animal senses.
0:02:22 > 0:02:24The plains of Africa.
0:02:25 > 0:02:30As I walk through this landscape, my eyes see the golden sunlight.
0:02:31 > 0:02:34And my ears hear exotic birdsong.
0:02:35 > 0:02:39But there's another sensory world here that I can barely detect.
0:02:41 > 0:02:45In the air are millions of minute scent particles,
0:02:45 > 0:02:47given off by the earth...
0:02:49 > 0:02:52..by plants and by animals.
0:02:54 > 0:02:59Our human noses pick up only a tiny fraction of these smells.
0:02:59 > 0:03:04But for any animal that lives here, this is vital information.
0:03:08 > 0:03:11In this landscape, scent is king.
0:03:11 > 0:03:12On the plains of Africa,
0:03:12 > 0:03:16smell is crucial to the survival of most animals.
0:03:16 > 0:03:17And over millions of years
0:03:17 > 0:03:20life has tapped into this invisible chemical world
0:03:20 > 0:03:22in some extraordinary ways.
0:03:25 > 0:03:26The Great Migration,
0:03:26 > 0:03:31where more than a million wildebeest journey across the savanna,
0:03:31 > 0:03:33is driven by scent.
0:03:35 > 0:03:39Each wildebeest leaves a trail of a musky odour that others follow.
0:03:41 > 0:03:45Scent keeps these herds together on their epic journey.
0:03:47 > 0:03:49Lions also produce a distinctive scent
0:03:49 > 0:03:53and cubs recognize their mothers through her unique smell.
0:03:58 > 0:04:03But there are some unlikely little creatures that also survive out here
0:04:03 > 0:04:05thanks to their amazing noses.
0:04:07 > 0:04:08Meerkats.
0:04:10 > 0:04:13I'm going to spend the day with them to see just how they do it.
0:04:17 > 0:04:20These meerkats have just come out of their burrow.
0:04:21 > 0:04:26The gang is made up of 14 adults and four newly emerged pups,
0:04:26 > 0:04:28and they're just heating up as the sun rises.
0:04:33 > 0:04:36These pups are just enchanting,
0:04:36 > 0:04:39and they're only about four weeks old.
0:04:40 > 0:04:42You can see they are sticking very close by.
0:04:46 > 0:04:50With pups around, the gang must be extra vigilant.
0:04:50 > 0:04:53And whilst meerkats are known for their excellent vision,
0:04:53 > 0:04:58it's their long pointed noses that can offer vital surveillance.
0:05:00 > 0:05:02Meerkats are constantly alert.
0:05:02 > 0:05:06And whilst this sentry is on the lookout for any predators
0:05:06 > 0:05:08in his visual range,
0:05:08 > 0:05:13Meerkats can smell danger long before they see it.
0:05:13 > 0:05:16And that's their way of keeping their pups safe.
0:05:19 > 0:05:24Meerkats have many predators...
0:05:24 > 0:05:26like jackals
0:05:26 > 0:05:28and cobras.
0:05:28 > 0:05:31Their predators may "appear" camouflaged,
0:05:31 > 0:05:34but they cannot hide their scent.
0:05:34 > 0:05:37Wherever they go, they leave minute traces.
0:05:38 > 0:05:42To show how well their noses are tuned into these dangerous smells
0:05:42 > 0:05:44I've got a little test.
0:05:44 > 0:05:48Now, this set up might seem a bit strange, but inside this bag
0:05:48 > 0:05:53I've got a small rock with a piece of wildcat hair attached to it.
0:05:53 > 0:05:57Now, wildcats are one of the major predators of meerkats -
0:05:57 > 0:05:59so let's see what happens when I put it down here,
0:05:59 > 0:06:02a few meters from where they're foraging.
0:06:06 > 0:06:11It takes just a few moments for the scent to reach the meerkats' noses.
0:06:12 > 0:06:15Ah, you can see they've definitely picked up the scent
0:06:15 > 0:06:17and now they're coming in.
0:06:17 > 0:06:20And you can see that classic mobbing behaviour -
0:06:20 > 0:06:24their backs are arched, their tails held high.
0:06:24 > 0:06:26And they're all clustered together as well.
0:06:28 > 0:06:30It's a powerful display of how finely tuned
0:06:30 > 0:06:33the meerkats' noses are to danger.
0:06:33 > 0:06:36But it's not the only way these little gangsters
0:06:36 > 0:06:37use their sense of smell.
0:06:41 > 0:06:43They also use it to defend their patch.
0:06:45 > 0:06:48Meerkats spend their lives constantly locked in a turf war
0:06:48 > 0:06:50against rival gangs,
0:06:50 > 0:06:54so it's absolutely crucial they can identify friend from foe.
0:06:57 > 0:07:00Meerkats are highly territorial.
0:07:01 > 0:07:04Scraping out a living in this arid desert is tough,
0:07:04 > 0:07:09so staking claim to their patch and food source is an absolute priority.
0:07:11 > 0:07:13They do this by scent marking.
0:07:15 > 0:07:17It's such a powerful instinct.
0:07:17 > 0:07:21If anything new is placed in their territory - like this stick -
0:07:21 > 0:07:24they have an irresistible urge to mark it.
0:07:32 > 0:07:35OK, here we go. It looks like one of the more dominant meerkats
0:07:35 > 0:07:37is about to stake its claim.
0:07:42 > 0:07:44And it's scenting all over the post -
0:07:44 > 0:07:48making clear that it belongs to this group of meerkats.
0:07:48 > 0:07:52Meerkats produce the scent from glands just below their tails.
0:07:52 > 0:07:54There it goes.
0:07:54 > 0:07:56And there it goes, again.
0:07:57 > 0:08:00Anything unfamiliar, any object in their patch,
0:08:00 > 0:08:01they'll want to claim as their own.
0:08:03 > 0:08:06Now, I've got a rather mischievous trick.
0:08:06 > 0:08:10I'm going to take this post and present it to another gang
0:08:10 > 0:08:12to see if they recognize their rival's scent.
0:08:14 > 0:08:18This marks the border between two rival gangs of meerkats.
0:08:18 > 0:08:21And visually there's not much to see,
0:08:21 > 0:08:24but if you could enter their olfactory world
0:08:24 > 0:08:29you'd see this entire area awash with odours and meerkat scent.
0:08:29 > 0:08:31This heavily scented area is
0:08:31 > 0:08:35a meerkat's equivalent of a no trespassing sign.
0:08:37 > 0:08:39For a passing meerkat from a rival gang,
0:08:39 > 0:08:41you'd cross this border at your peril.
0:08:44 > 0:08:47I set up the post close to the rival's burrow.
0:08:49 > 0:08:52Within minutes they pick up the scent.
0:08:56 > 0:08:59Here we go. We've got a meerkat coming up to our post.
0:09:02 > 0:09:04And now more of them are coming in.
0:09:06 > 0:09:09So, they've detected that the scent that's been left
0:09:09 > 0:09:10is from a rival meerkat.
0:09:11 > 0:09:13And now they're on high alert.
0:09:13 > 0:09:16And one of them is standing on its hind legs -
0:09:16 > 0:09:18and its eyes are trained on the horizon.
0:09:18 > 0:09:21He's trying to see if he can spot an intruder.
0:09:23 > 0:09:26And if he were to see a rival meerkat,
0:09:26 > 0:09:29it would rally the rest of the troops into a violent attack.
0:09:30 > 0:09:32Using their noses,
0:09:32 > 0:09:36meerkats detect dangers their eyes and ears just can't pick up.
0:09:38 > 0:09:41Scent is hugely important in their world.
0:09:41 > 0:09:43And there's a good reason for this.
0:09:45 > 0:09:49Leave some water out in the open and it quickly evaporates in the heat.
0:09:51 > 0:09:56That's exactly what happens with scent as it disperses into the air.
0:09:56 > 0:09:59And that's why places with warmer climates
0:09:59 > 0:10:01have more smells in the atmosphere.
0:10:04 > 0:10:09For the meerkats, this vital source of sensory information
0:10:09 > 0:10:11is key to keeping the gang safe.
0:10:15 > 0:10:18The meerkats are back from their days foraging,
0:10:18 > 0:10:19safe and sound in their burrow
0:10:19 > 0:10:22and the young pups are accounted for as well.
0:10:22 > 0:10:26And that's in part thanks to their amazing little noses
0:10:26 > 0:10:29that act as excellent early warning systems.
0:10:32 > 0:10:37Although meerkats are brilliant at detecting lots of different smells,
0:10:37 > 0:10:41their noses aren't very sensitive to tiny traces of scent.
0:10:44 > 0:10:46But in the frozen wilds of Alaska,
0:10:46 > 0:10:51there's one mammal that can sniff out food from more than a kilometre away.
0:10:53 > 0:10:56That requires one of the most acute noses on the planet.
0:11:01 > 0:11:04You know those cold, clear days
0:11:04 > 0:11:08where the world just seems to be fresh and clean?
0:11:08 > 0:11:11One of the reasons that everything seems so clean is that
0:11:11 > 0:11:14you actually can't smell most of what's going on.
0:11:14 > 0:11:16And that's because cold environments like this one
0:11:16 > 0:11:19make it a lot harder for those scent molecules to evaporate
0:11:19 > 0:11:23and to drift up into the atmosphere.
0:11:23 > 0:11:26And just to show you how cold it is, look at this.
0:11:26 > 0:11:29I've got a beaker here full of just boiled water. Watch.
0:11:35 > 0:11:37At minus 20 degrees Celsius,
0:11:37 > 0:11:41the boiling water freezes before it hits the ground.
0:11:43 > 0:11:47If you're a carnivore and you can survive in this environment,
0:11:47 > 0:11:50you need a really impressive sense of smell.
0:11:52 > 0:11:55And there's one tenacious mammal that thrives here
0:11:55 > 0:11:59thanks to its extraordinary nose.
0:11:59 > 0:12:01It's the giant of the weasel family.
0:12:02 > 0:12:04The wolverine.
0:12:06 > 0:12:09Wolverines are scavengers, and they're really good at it.
0:12:09 > 0:12:12They can travel up to 25 miles a day
0:12:12 > 0:12:14through terrain like this looking for food,
0:12:14 > 0:12:16and what they're looking for is dead animals
0:12:16 > 0:12:19that are buried underneath the snow.
0:12:19 > 0:12:20Their Latin name is brilliant,
0:12:20 > 0:12:23it's Gulo gulo, which means the glutton,
0:12:23 > 0:12:24and they're voracious eaters,
0:12:24 > 0:12:27they chomp trough a carcass, bones and all.
0:12:28 > 0:12:32To find out how they locate their food in this frozen wilderness,
0:12:32 > 0:12:36I've come to meet one of the world's only habituated wolverines.
0:12:39 > 0:12:41A two-year-old orphan named Jasper.
0:12:42 > 0:12:47Jasper was rescued by arctic wildlife expert Steve Kroschel,
0:12:47 > 0:12:51who exercises him on a lead to contain his boundless energy.
0:12:54 > 0:12:56So, he's sniffing around all the time,
0:12:56 > 0:12:59- getting used to the environment? - Yes. Never a dull moment.
0:12:59 > 0:13:01They're always very active, their metabolism is very high.
0:13:01 > 0:13:04No such thing really as a fat wolverine.
0:13:04 > 0:13:06And they don't hibernate, like a bear.
0:13:06 > 0:13:08- So, they just keep going all winter? - That's right.
0:13:08 > 0:13:11They don't have much time, it's all about survival, you know,
0:13:11 > 0:13:14and mother nature doesn't give an animal like this
0:13:14 > 0:13:15a lot of margin for error.
0:13:18 > 0:13:22As scavengers, most of a wolverine's diet is dead animals.
0:13:26 > 0:13:28And to find their food,
0:13:28 > 0:13:33wolverines take advantage of one of nature's most destructive forces.
0:13:35 > 0:13:36Avalanches.
0:13:39 > 0:13:41As snow thunders down the mountainside,
0:13:41 > 0:13:43mammals are often buried beneath.
0:13:45 > 0:13:49And it's thought that wolverines can sniff out frozen carcasses
0:13:49 > 0:13:52even deep beneath the snow.
0:13:58 > 0:14:00I want to put this to the test,
0:14:00 > 0:14:04so I'm going to bury some meat and leave it for several hours.
0:14:04 > 0:14:07Then, I'm going to see if Jasper can sniff it out.
0:14:11 > 0:14:13I could walk along this path now
0:14:13 > 0:14:16and never know that the chicken was down there.
0:14:16 > 0:14:17But here's the amazing thing.
0:14:17 > 0:14:22Even though it's frozen solid, and it will stay frozen solid,
0:14:22 > 0:14:26molecules of that chicken are evaporating in tiny quantities.
0:14:26 > 0:14:30They're drifting out through the snow and out into the air just here.
0:14:30 > 0:14:35I can't detect them, but a wolverine could use them to find dinner.
0:14:39 > 0:14:41Before we set Jasper his challenge,
0:14:41 > 0:14:46I'm going to demonstration how our noses and the wolverine's work.
0:14:47 > 0:14:50To show you I'll need an orange.
0:14:53 > 0:14:55When I sniff the orange...
0:14:57 > 0:14:58It's really strong, this one.
0:15:00 > 0:15:04..is that the scent molecules from the orange are going up my nose
0:15:04 > 0:15:06and they're going up to the top here,
0:15:06 > 0:15:09and there's a structure up here called the epithelial membrane
0:15:09 > 0:15:12and that is where the odour detectors are.
0:15:12 > 0:15:15It's thought smell works like a lock and key.
0:15:16 > 0:15:21The scent molecule is the key and will only fit certain "locks" -
0:15:21 > 0:15:24the odour detectors.
0:15:24 > 0:15:27We have 400 different types of locks.
0:15:27 > 0:15:32But scent molecules can hit many combinations of detectors
0:15:32 > 0:15:36so we can identify thousands of different smells.
0:15:38 > 0:15:41And that's how its works for everything you've ever smelt,
0:15:41 > 0:15:44from new shoes to perfume to oranges,
0:15:44 > 0:15:48it's all about the combination of receptors that those molecules hit.
0:15:49 > 0:15:52All mammals detect scent in this way.
0:15:54 > 0:15:58But for wolverines it's all about sensitivity.
0:15:58 > 0:16:02Detecting the tiniest traces of scent, even deep under the snow.
0:16:04 > 0:16:07So, let's see how Jasper gets on with finding the buried meat.
0:16:10 > 0:16:13We start several hundred meters away
0:16:13 > 0:16:15and let the wolverine choose his course.
0:16:16 > 0:16:19He's smelling things right now, Helen.
0:16:19 > 0:16:21And it is not only what is on the snow,
0:16:21 > 0:16:23it's the air drifting past that he's smelling.
0:16:35 > 0:16:37Oh, having a bit of a sniff here.
0:16:38 > 0:16:41He quickly picks up a scent, but is it the right one?
0:16:46 > 0:16:48He definitely knows there is something here.
0:16:54 > 0:16:56He's onto some scent there, for sure.
0:16:59 > 0:17:01Those big paws are pretty efficient, aren't they?
0:17:01 > 0:17:04- Yes.- He looks like nothing will get in his way.
0:17:04 > 0:17:05- He's shifting a lot of snow.- Yeah.
0:17:07 > 0:17:09Oh, they're we go - he's got it!
0:17:09 > 0:17:12He got it. He found it.
0:17:12 > 0:17:15He might even recache it if he's not all that hungry.
0:17:15 > 0:17:20In the wild, wolverines cache, or bury, their food
0:17:20 > 0:17:21so they can eat it later.
0:17:22 > 0:17:24He's heading for a hole over here.
0:17:24 > 0:17:27Oh, he's going to put it in there, isn't he?
0:17:27 > 0:17:30Straight in there. So, that's his larder now in there.
0:17:30 > 0:17:34So, Jasper succeeded in sniffing out the buried food.
0:17:34 > 0:17:36I couldn't smell a thing.
0:17:37 > 0:17:40But if their noses work in the same way as ours,
0:17:40 > 0:17:43why is their sense of smell so much better?
0:17:45 > 0:17:49It all comes down to an extraordinary adaptation inside their noses.
0:17:51 > 0:17:54This is the skull of a wolverine.
0:17:54 > 0:17:57And the really interesting bit is his nasal cavity, which is here.
0:17:57 > 0:18:00If you look straight down it
0:18:00 > 0:18:03what you see are these amazing honeycomb structures.
0:18:06 > 0:18:08This is where the scent detectors are found.
0:18:09 > 0:18:13The honeycomb pattern greatly enlarges the surface area
0:18:13 > 0:18:16so there's room for many more detectors.
0:18:16 > 0:18:19Unravelled, it would be the size of a plate.
0:18:21 > 0:18:23We have a similar structure,
0:18:23 > 0:18:26but ours is only the size of a ten pence piece.
0:18:29 > 0:18:32It means that up the nose of a wolverine
0:18:32 > 0:18:35is an enormous detector for scent.
0:18:36 > 0:18:40In fact, the wolverine's nose has twice the surface area
0:18:40 > 0:18:42of a bloodhound's -
0:18:42 > 0:18:46a dog purposefully bred for its amazing sense of smell.
0:18:46 > 0:18:50That's how they're able to track down frozen carcasses
0:18:50 > 0:18:53even deep beneath the snow.
0:18:53 > 0:18:57And it's their secret to survival in this icy world.
0:18:59 > 0:19:04But one creature has evolved an even more precise way of detecting scent.
0:19:06 > 0:19:09They can smell the world in stereo,
0:19:09 > 0:19:12allowing them to pinpoint prey with deadly accuracy.
0:19:14 > 0:19:16Rattlesnakes.
0:19:17 > 0:19:21For decades, a mystery has surrounded exactly how these snakes
0:19:21 > 0:19:24are able to hone in on their prey.
0:19:25 > 0:19:29It's only now that we can unlock their sensory secret.
0:19:30 > 0:19:34The answer lies in the unique way it smells the world
0:19:34 > 0:19:37and the brilliant way it uses its venom.
0:19:39 > 0:19:42I've come to the great prairie lands of Colorado
0:19:42 > 0:19:43to see these snakes in action.
0:19:49 > 0:19:52And I've teamed up with Professor Steve Mackessy
0:19:52 > 0:19:56who's advised me to wear snake-proof boots.
0:19:56 > 0:19:59Steve, where's the best place to find some snakes?
0:19:59 > 0:20:03Well, we have a rattlesnake den site about 30 meters up ahead here.
0:20:03 > 0:20:06And we're going to see if the snakes are out and about.
0:20:06 > 0:20:07So, watch your feet.
0:20:07 > 0:20:10Right. So, how many snakes are we likely to find today?
0:20:10 > 0:20:11Well, we could find as many as...
0:20:11 > 0:20:13HE GASPS
0:20:13 > 0:20:15There's one! There's one there! Look! Look!
0:20:15 > 0:20:16- Oh, my goodness.- There it is.
0:20:21 > 0:20:22They move so quickly.
0:20:27 > 0:20:30That rattlesnake was perfectly camouflaged,
0:20:30 > 0:20:32we had no idea it was there.
0:20:32 > 0:20:33Oh, lordy, lord.
0:20:35 > 0:20:38- I think it's pretty clear they're on the move today.- Yeah.
0:20:40 > 0:20:43I'm going to be a bit more careful because after seeing that snake
0:20:43 > 0:20:46I can tell you I've got goosebumps all over my skin.
0:20:49 > 0:20:52But to show how rattlesnakes use scent to hunt down prey
0:20:52 > 0:20:55I need to get closer.
0:20:55 > 0:20:57A lot closer.
0:20:57 > 0:21:01Luckily, the next snake we find is a little calmer.
0:21:02 > 0:21:05And reveals its extraordinary sensory trick.
0:21:06 > 0:21:10Now, you can see that tongue flickering in and out of it's mouth.
0:21:10 > 0:21:14And what it's doing is using that tongue to pick out
0:21:14 > 0:21:16the scent molecules in the air.
0:21:16 > 0:21:19Some of which I'll be giving off right now.
0:21:19 > 0:21:21So, he can taste my presence.
0:21:23 > 0:21:28For the rattlesnake, its sense of taste and smell are the same thing.
0:21:30 > 0:21:34When it detects a scent, it brings its tongue back into its mouth
0:21:34 > 0:21:37and presses it on a patch of sensory cells.
0:21:39 > 0:21:42This highly sensitive area identifies the smell.
0:21:44 > 0:21:47It's called the Jacobson's organ, and this is what gives
0:21:47 > 0:21:50the rattlesnake its serpentine supersense.
0:21:52 > 0:21:54But this is just part of the story.
0:21:56 > 0:21:59Many rattlesnakes are strike and release predators.
0:22:00 > 0:22:02They lie in wait,
0:22:02 > 0:22:06strike their prey just once then let them go.
0:22:07 > 0:22:10But the venom doesn't kill immediately,
0:22:10 > 0:22:14so the prey can move a distance away before the toxins take hold.
0:22:16 > 0:22:19This presents a huge sensory challenge for the rattlesnake
0:22:19 > 0:22:22because now they must track down their dinner
0:22:22 > 0:22:24in these wide-open prairies.
0:22:25 > 0:22:29And that can be like finding a needle in a haystack.
0:22:31 > 0:22:35Within these vast grasslands are a complex and tangled network
0:22:35 > 0:22:39of scent trails left by countless mice and other small mammals.
0:22:39 > 0:22:42So, even with this supersense,
0:22:42 > 0:22:44a rattlesnake somehow needs to distinguish between all
0:22:44 > 0:22:49the other mice and the one that has been injected with its venom.
0:22:49 > 0:22:51How do they do this?
0:22:52 > 0:22:56I'm hoping this test will offer an intriguing clue.
0:22:57 > 0:23:00Using sponges, I lay two trails in the sand.
0:23:02 > 0:23:05The one on the right contains the scent of rattlesnake's venom.
0:23:07 > 0:23:08The one on the left doesn't.
0:23:10 > 0:23:13Which one will the snake follow?
0:23:26 > 0:23:29So, you can see our snake is already on the trail.
0:23:29 > 0:23:32The forked tongue gives them their directional sense of smell.
0:23:33 > 0:23:37This snake is detecting more scent molecules on the right fork,
0:23:37 > 0:23:40so is moving in that direction.
0:23:44 > 0:23:49He's using that forked tongue to pick out the scented sponge.
0:23:49 > 0:23:51Look, he's going the right way now.
0:23:52 > 0:23:55It's a great demonstration of how a rattlesnake
0:23:55 > 0:23:57will find its prey in the wild.
0:24:02 > 0:24:04There you go, it's right by the sponge.
0:24:06 > 0:24:09I really wasn't sure if our experiment was going to work.
0:24:09 > 0:24:13But it goes to show that the rattlesnake's sense of smell
0:24:13 > 0:24:15is absolutely remarkable.
0:24:16 > 0:24:19The snake followed the trail that containing the venom.
0:24:20 > 0:24:24So, it seems there's something in the deadly toxin that allows
0:24:24 > 0:24:27the snake to track down its prey.
0:24:27 > 0:24:28But what is it?
0:24:30 > 0:24:34Steve has been carefully extracting venom to unravel this mystery.
0:24:37 > 0:24:39So, when you have them up close here you can see fangs.
0:24:39 > 0:24:41Yeah, I can see them.
0:24:41 > 0:24:43So, using these capillary tubes
0:24:43 > 0:24:46- we should be able to get some of that venom out.- Right.
0:24:46 > 0:24:50Prairie rattlesnakes have long, delicate fangs.
0:24:50 > 0:24:54That's why the snakes release their prey as soon as they've struck.
0:24:54 > 0:24:57It means the fangs don't get damaged.
0:24:57 > 0:24:59Wow.
0:24:59 > 0:25:02- So, I'm going to put this in our tube here.- Yes.
0:25:04 > 0:25:06That's incredible.
0:25:06 > 0:25:09HE EXHALES
0:25:10 > 0:25:13Rattlesnake venom contains more than 100 different molecules.
0:25:15 > 0:25:20By freeze-drying it and isolating all the components, Steve discovered
0:25:20 > 0:25:24a group of molecules that appear to help the snake track its prey.
0:25:25 > 0:25:28They're called disintegrins.
0:25:28 > 0:25:31Well, the disintegrins seem to act as a chemical tag,
0:25:31 > 0:25:35so it's marking of prey that's been bitten by the rattlesnake as
0:25:35 > 0:25:39the envenomated prey, and that's what lays down the chemical trail
0:25:39 > 0:25:41- that the snake is following.- Yeah.
0:25:41 > 0:25:45And so by using that very exquisite sense of taste and smell,
0:25:45 > 0:25:49the snake is able to follow the prey very, very carefully and very,
0:25:49 > 0:25:53very exactly to where it's run off and expired.
0:25:53 > 0:25:57Disintegrins are the mystery molecules in the snake's venom
0:25:57 > 0:26:01that allows them to track down their prey with such precision.
0:26:02 > 0:26:06In the rattlesnake's world, where the ground is awash with scent,
0:26:06 > 0:26:09this is how they find their dinner in
0:26:09 > 0:26:12the vastness of the great American prairies.
0:26:17 > 0:26:21So far we've seen how animals are able to detect
0:26:21 > 0:26:24tiny traces of scent in the air around them.
0:26:30 > 0:26:34But it's an amazing thought that our oceans are also awash
0:26:34 > 0:26:35with different smells.
0:26:37 > 0:26:39And creatures beneath have developed
0:26:39 > 0:26:42even more sophisticated ways of sensing them.
0:26:46 > 0:26:50I'm just off the Bahamas and I'm here to show you one of
0:26:50 > 0:26:53the most powerful uses of scent in the natural world.
0:26:56 > 0:27:02In our oceans, there's a complex mix of thousands of chemical compounds
0:27:02 > 0:27:04carried on swirling currents.
0:27:07 > 0:27:10And I'm in search of a master of this sensory world.
0:27:12 > 0:27:13The shark.
0:27:22 > 0:27:27These sharks are swimming through a world that I just can't sense.
0:27:27 > 0:27:29But they're masters at it.
0:27:31 > 0:27:35The water all around me here is full of tiny amounts of chemicals
0:27:35 > 0:27:37that I can't detect.
0:27:37 > 0:27:39But there's a huge amount of information
0:27:39 > 0:27:41just flowing around my head.
0:27:43 > 0:27:44And these animals,
0:27:44 > 0:27:48they're fantastically well equipped to swim through this world
0:27:48 > 0:27:53and to be able to decode that information and use it to survive.
0:27:56 > 0:28:00I want to discover the extraordinary way sharks use scent
0:28:00 > 0:28:06and debunk an age-old myth about their thirst for blood.
0:28:10 > 0:28:12But to start with,
0:28:12 > 0:28:16I want to find out how they are able to smell underwater.
0:28:16 > 0:28:21For that I need to get hands on with a baby lemon shark.
0:28:21 > 0:28:24Look at this, I'm holding a shark. I've never done this before.
0:28:24 > 0:28:27He's quite calm down here,
0:28:27 > 0:28:31but there is a trick where we can calm him down a little bit more.
0:28:31 > 0:28:34What I'm going to do is turn him over.
0:28:34 > 0:28:39If I turn the shark upside down he becomes very passive.
0:28:39 > 0:28:41It's called tonic immobility,
0:28:41 > 0:28:44and it gives me a chance to look at his nostrils.
0:28:45 > 0:28:49As water flows through the nostrils, or naries, it enters
0:28:49 > 0:28:54the olfactory chamber, lined with hundreds of thin membranes.
0:28:54 > 0:28:57These are covered in odour detectors.
0:28:57 > 0:29:00And the water is always flowing in the same direction
0:29:00 > 0:29:03so the sharks are smelling continuously.
0:29:07 > 0:29:11It's a brilliant system, but just how sensitive is it?
0:29:14 > 0:29:19To find out I've teamed up with shark biologist Dr Tristan Guttridge
0:29:19 > 0:29:24to test a scent that sharks are renowned for detecting.
0:29:26 > 0:29:28The smell of blood.
0:29:28 > 0:29:32So, we're standing in a pen of lemon sharks here.
0:29:32 > 0:29:34Yup, these are semi-captive pens.
0:29:34 > 0:29:36As you can see, they are exposed to the ambient conditions,
0:29:36 > 0:29:39and we have about 12 juvenile lemon sharks.
0:29:39 > 0:29:40And as they are swimming along,
0:29:40 > 0:29:43they are continually sensing the water around them.
0:29:43 > 0:29:45Exactly, so they will be swimming around
0:29:45 > 0:29:48and they're determining whether there is something interesting.
0:29:48 > 0:29:50And if there is anything interesting that smells good,
0:29:50 > 0:29:52they'll then direct themselves towards that source.
0:29:52 > 0:29:54And what sort of thing interests them?
0:29:54 > 0:29:57These little lemon sharks, they feed primarily on fishes.
0:29:57 > 0:30:00So, they will be sniffing around hunting for fish.
0:30:04 > 0:30:07To start our experiment, I take a sample of fish blood.
0:30:09 > 0:30:13I'm going to release a tiny amount into the water to see if they react.
0:30:20 > 0:30:22We're looking for a corkscrewing behaviour,
0:30:22 > 0:30:25where the sharks turn repeatedly.
0:30:26 > 0:30:28It's a sign they've detected the smell
0:30:28 > 0:30:30and are trying to find the source.
0:30:30 > 0:30:32There you go.
0:30:32 > 0:30:33Oh, he turned.
0:30:37 > 0:30:39- So they're all turning now, looking. - Yeah.
0:30:41 > 0:30:42That guy in particular
0:30:42 > 0:30:44just turned around at least three or four times -
0:30:44 > 0:30:46there you go, he's trying to find it on the floor.
0:30:46 > 0:30:48See this one here turning as well?
0:30:48 > 0:30:49He knows what's going on.
0:30:49 > 0:30:51And now all of them are interested.
0:30:51 > 0:30:54- Oh, they've all come in, look at that.- Yeah.
0:30:54 > 0:30:58So just a few drops of fish blood triggered this response.
0:31:00 > 0:31:01But what about human blood?
0:31:03 > 0:31:07It's been said that it can attract sharks from many kilometres away.
0:31:12 > 0:31:14So it's spreading out just like the fish blood did.
0:31:14 > 0:31:16Let's see what these guys do.
0:31:16 > 0:31:18No change whatsoever -
0:31:18 > 0:31:21still in their nice, big circle formation.
0:31:21 > 0:31:24Let's see if they come through it again - these two here.
0:31:24 > 0:31:27Yeah, they would have picked it up by now.
0:31:27 > 0:31:30So they're not interested in humans or human blood at all.
0:31:30 > 0:31:34No. We're intact and they didn't react to the human blood.
0:31:34 > 0:31:38Our simple test shows these sharks are not attracted to my blood.
0:31:39 > 0:31:41And despite many myths,
0:31:41 > 0:31:46there's no evidence to suggest that larger sharks - like great whites -
0:31:46 > 0:31:48are interested in human blood either.
0:31:53 > 0:31:59But there is one mysterious scent that all sharks detest.
0:32:02 > 0:32:06In fact, it's so reviled by these predators
0:32:06 > 0:32:09that it's currently being trialled as a shark repellent.
0:32:12 > 0:32:18So we've come to a reef where sharks gather in large numbers...
0:32:18 > 0:32:21and I've been volunteered as a guinea pig.
0:32:25 > 0:32:27There's a lot of sharks down there.
0:32:28 > 0:32:32The plan is to go down to let the sharks get used to my presence,
0:32:32 > 0:32:35and then to release this and watch how they react.
0:32:43 > 0:32:46Three, two, one - jump.
0:33:00 > 0:33:04The sharks come right up to investigate.
0:33:04 > 0:33:08Their noses are detecting the minute chemical cues
0:33:08 > 0:33:11I give off as I move through the water.
0:33:14 > 0:33:15Look at this!
0:33:17 > 0:33:21They look bigger down here than they looked up on the surface!
0:33:22 > 0:33:23Oh...
0:33:29 > 0:33:31I just love it being down here,
0:33:31 > 0:33:33sitting in the middle of all this.
0:33:38 > 0:33:42You can see their nostrils right on the front of their snout.
0:33:42 > 0:33:45Imagine water coming towards them -
0:33:45 > 0:33:48so they're taking it into this chemical laboratory
0:33:48 > 0:33:50that's inside their nose.
0:33:55 > 0:33:57They're amazing animals.
0:34:02 > 0:34:05When the sharks are really close, I release the scent.
0:34:20 > 0:34:22They've gone.
0:34:22 > 0:34:24All the other species are still here.
0:34:26 > 0:34:27The sharks have vanished.
0:34:30 > 0:34:34So why are sharks so repulsed by this mystery smell?
0:34:36 > 0:34:41Dr Patrick Rice is a marine biologist who studies shark olfaction,
0:34:41 > 0:34:44and he's helping to develop this shark-repelling scent
0:34:44 > 0:34:46called a necromone.
0:34:46 > 0:34:48A necromone is a chemical signal
0:34:48 > 0:34:51that tells sharks that there's other predators in the area.
0:34:51 > 0:34:54There's evidence from nature that when a predator comes in
0:34:54 > 0:34:55and eats another shark,
0:34:55 > 0:34:57the little bits left behind may start to decay
0:34:57 > 0:34:58and emit this signal,
0:34:58 > 0:34:59and that chemical signal
0:34:59 > 0:35:02tells a shark that there's a predator in the area.
0:35:02 > 0:35:03So it's a protective thing -
0:35:03 > 0:35:06so even though we think of sharks as being invincible,
0:35:06 > 0:35:08they're actually - even they have to look out for themselves.
0:35:08 > 0:35:09That's correct.
0:35:11 > 0:35:13Necromones - or the scent of dead shark -
0:35:13 > 0:35:16are a sure sign there's danger around.
0:35:16 > 0:35:20So sharks are hardwired to flee when they detect it.
0:35:21 > 0:35:26It's believed this is a response they've had since prehistoric times.
0:35:26 > 0:35:30So this evolved at a time when sharks weren't top predators -
0:35:30 > 0:35:31they were just little things in the sea,
0:35:31 > 0:35:34- and they were basically food for even bigger animals.- That's right.
0:35:34 > 0:35:37There was a monster shark called the megaladon -
0:35:37 > 0:35:40a prehistoric great white shark that was about 60 feet long,
0:35:40 > 0:35:41and it preyed on everything,
0:35:41 > 0:35:43and it was the king of the ocean at the time.
0:35:44 > 0:35:47And today they're still vulnerable.
0:35:47 > 0:35:51Almost half of baby sharks don't survive their first year -
0:35:51 > 0:35:54many because they're eaten by bigger species.
0:35:55 > 0:36:00Even the most feared ocean predator - the great white shark -
0:36:00 > 0:36:02can be attacked by a pod of killer whales.
0:36:08 > 0:36:12So the ability to smell danger is still key to their survival.
0:36:18 > 0:36:21Although sharks detect scent underwater,
0:36:21 > 0:36:25their noses work in a similar way to ours - and other mammals.
0:36:27 > 0:36:31It's all about taking scent inside the body
0:36:31 > 0:36:32and analysing it.
0:36:37 > 0:36:41But some creatures smell the world through the outside of their bodies,
0:36:41 > 0:36:46and that allows them to use scent in even more complex ways.
0:36:46 > 0:36:48BUZZING
0:36:49 > 0:36:51Bees.
0:36:52 > 0:36:56Their world is controlled by smell.
0:36:56 > 0:37:00And if you can tap into this, you can harness the power of the swarm.
0:37:02 > 0:37:04I've come to Arizona's Sonoran Desert
0:37:04 > 0:37:09to show the overwhelming power of scent in the bee's world.
0:37:09 > 0:37:12This is a swarm of European honeybees,
0:37:12 > 0:37:15and there must about 5,000 of them.
0:37:15 > 0:37:19And I'm going to move them from here...into here.
0:37:19 > 0:37:23And I'm going to do it simply by tapping into their world of scent.
0:37:30 > 0:37:32And this is my secret weapon.
0:37:51 > 0:37:52This is amazing.
0:37:52 > 0:37:54You can see all the bees are reacting,
0:37:54 > 0:37:57and they're instantly swarming into the case.
0:38:04 > 0:38:05And it's that sense of smell
0:38:05 > 0:38:07that's driving that behaviour.
0:38:12 > 0:38:17The bees are reacting to minute traces of scent in the air.
0:38:17 > 0:38:19A scent I can't pick up at all.
0:38:21 > 0:38:24In humans, we smell by breathing,
0:38:24 > 0:38:28but bees detect scent in a completely different way.
0:38:28 > 0:38:32They use their antennae and that's what they are doing here.
0:38:32 > 0:38:35Capturing that scent and moving towards it.
0:38:37 > 0:38:40Their antennae are packed full of odour receptors,
0:38:40 > 0:38:44which pick up scent molecules as they fly though the air.
0:38:47 > 0:38:48Just look at that -
0:38:48 > 0:38:52virtually the entire swarm is now in this case.
0:38:52 > 0:38:54But what's the secret to my bee trickery?
0:38:56 > 0:38:59It's all to do with a group of odours called pheromones
0:38:59 > 0:39:02that bees use to communicate.
0:39:02 > 0:39:05It's as if they can talk through scent.
0:39:06 > 0:39:09My secret weapon was a synthetic pheromone
0:39:09 > 0:39:12that exactly replicates the one produced by the queen bee.
0:39:12 > 0:39:15It encourages the workers to surround her,
0:39:15 > 0:39:17and that's what's happened here.
0:39:17 > 0:39:20The bees have detected the scent and moved into the case.
0:39:26 > 0:39:28So there we have it -
0:39:28 > 0:39:305,000 bees in a briefcase,
0:39:30 > 0:39:32simply by the power of scent.
0:39:33 > 0:39:37This is just one example of how bees use odour to talk to each other.
0:39:39 > 0:39:41They have a language of scent -
0:39:41 > 0:39:43with more than 15 pheromones
0:39:43 > 0:39:46triggering wildly different behaviours.
0:39:47 > 0:39:51They use them to navigate, find food and to reproduce.
0:39:53 > 0:39:57But there's one pheromone that triggers a response so dangerous
0:39:57 > 0:40:01that it can only be studied well away from human habitation.
0:40:02 > 0:40:04To see this in action,
0:40:04 > 0:40:08I've come to a remote research base deep in the desert.
0:40:09 > 0:40:13This is home to some of the most dangerous insects on the planet.
0:40:15 > 0:40:18Behind me are hives full of Africanised bees,
0:40:18 > 0:40:21also known as killer bees.
0:40:21 > 0:40:22And to protect their home,
0:40:22 > 0:40:25they'll wage war on anything or anyone that approaches.
0:40:25 > 0:40:29And they do this entirely through their sense of smell.
0:40:31 > 0:40:35I've teamed up with entomologist Dr Steve Thoenes
0:40:35 > 0:40:39who is studying what makes this subspecies of bee so aggressive.
0:40:40 > 0:40:43Once a month, he checks on the health of the hives.
0:40:43 > 0:40:47To do this, he has to get up close.
0:40:47 > 0:40:50Here we have the research apiary for Africanised bees.
0:40:50 > 0:40:52Africanised bees have a lot of good qualities.
0:40:52 > 0:40:53They are very hard working,
0:40:53 > 0:40:55they produce a lot of honey
0:40:55 > 0:40:58and they don't get any of the diseases that European bees do.
0:40:58 > 0:40:59Their problem is
0:40:59 > 0:41:01that they are really, really defensive of their nest.
0:41:01 > 0:41:04If you disturb one bee, she will release alarm pheromone.
0:41:04 > 0:41:06The alarm pheromone spreads,
0:41:06 > 0:41:09and you have this whole cascade where up to 50% of the workers
0:41:09 > 0:41:11- will come out to defend their colony.- Wow.
0:41:11 > 0:41:15The main area that Africanised bees are going to come is at your face
0:41:15 > 0:41:18because they have been attacked for millions of years by mammals.
0:41:18 > 0:41:21- And we all breathe in oxygen and we breathe out CO2...- CO2, yep.
0:41:21 > 0:41:24..so they are going to come right at the CO2 source,
0:41:24 > 0:41:25which is your mouth and nose.
0:41:25 > 0:41:27Just don't panic.
0:41:27 > 0:41:29Let's get you all suited up, and we'll go look.
0:41:31 > 0:41:36In defence mode, the bees will find any chink in our bee armour.
0:41:36 > 0:41:38So nothing's left to chance.
0:41:41 > 0:41:44Steve uses smoke to keep the bees calm.
0:41:45 > 0:41:48This blocks the scent receptors on their antennae,
0:41:48 > 0:41:51making them less sensitive to alarm pheromone.
0:41:53 > 0:41:55But it doesn't last long.
0:41:57 > 0:41:58So, Steve, we've got our smoke,
0:41:58 > 0:42:00which is going to pacify the bees, yeah?
0:42:00 > 0:42:03Yes, that's the best tool a bee keeper has.
0:42:04 > 0:42:06Gosh, I can feel my heart beat.
0:42:08 > 0:42:11I've been nervous around bees since I was a child,
0:42:11 > 0:42:14so I'm going to have to overcome one of my greatest fears.
0:42:17 > 0:42:19How long do you think we'll have here?
0:42:19 > 0:42:21Couple of minutes.
0:42:21 > 0:42:23Let's see what's inside of here.
0:42:25 > 0:42:27OK, they're coming at me.
0:42:27 > 0:42:30They're definitely interested in what we're doing now.
0:42:30 > 0:42:33They're getting very, very defensive.
0:42:33 > 0:42:35This is serious. This is serious stuff.
0:42:36 > 0:42:38This is just starting.
0:42:38 > 0:42:39Oh, you're joking.
0:42:41 > 0:42:43Steve needs to check through the hive
0:42:43 > 0:42:47to ensure the honey stocks are good, and there's no sign of disease.
0:42:48 > 0:42:53But his work triggers the bees to start releasing alarm pheromone.
0:42:53 > 0:42:57And when one starts - others quickly join in.
0:42:57 > 0:42:58Oh, whoa.
0:42:58 > 0:43:00Oh, wow.
0:43:00 > 0:43:01Wow!
0:43:01 > 0:43:03Oh, Lord.
0:43:03 > 0:43:04Oh...
0:43:04 > 0:43:07Within minutes, thousands of bees are in the air
0:43:07 > 0:43:09and in full attack mode.
0:43:11 > 0:43:14So this is the defence of Africanised bees.
0:43:14 > 0:43:16Where they've coordinated all this by odour,
0:43:16 > 0:43:18and there's an odour in the air.
0:43:18 > 0:43:21- You can smell a little of it. - That smells...
0:43:21 > 0:43:24- That smells just like...- Bananas. - ..off bananas, yeah.- Right.
0:43:24 > 0:43:27And that's the alarm pheromone that the humans can smell.
0:43:27 > 0:43:30And they're coordinating their attack by releasing alarm pheromone
0:43:30 > 0:43:32and responding to that.
0:43:32 > 0:43:34How does it feel to be a predator of bees?
0:43:34 > 0:43:36I don't want to be a predator of bees!
0:43:36 > 0:43:38- I want to leave them alone.- OK.
0:43:38 > 0:43:43Despite our hostile reception, it seems all is well with the hive.
0:43:44 > 0:43:47So the brood's good, the honey stores are good
0:43:47 > 0:43:49and the whole hive is thriving.
0:43:49 > 0:43:51Great. It's time for us to, er, get out of Dodge!
0:43:51 > 0:43:54- It's getting too much. - OK, I'll put it back together again
0:43:54 > 0:43:56- and we can head off down towards the truck.- Let's do that.
0:43:56 > 0:43:58It's been a stark demonstration
0:43:58 > 0:44:01of the power of scent in the bees' world,
0:44:01 > 0:44:03and I'm just happy to be heading back.
0:44:05 > 0:44:07That was overwhelming.
0:44:07 > 0:44:09There must have been 5,000 Africanised bees
0:44:09 > 0:44:11swarming all around us.
0:44:11 > 0:44:14We've left the hives, but they're still following.
0:44:14 > 0:44:16This has got to be one of the ultimate examples
0:44:16 > 0:44:19of how an animal uses scent to defend their home.
0:44:21 > 0:44:24The intricate ways bees use scent to communicate
0:44:24 > 0:44:28have made them one of the most successful insects on Earth.
0:44:29 > 0:44:33But when it comes to detecting tiny quantities of scent
0:44:33 > 0:44:34from a long way away,
0:44:34 > 0:44:38the ultimate prize goes to another extraordinary insect.
0:44:40 > 0:44:41Moths -
0:44:41 > 0:44:43one of the planet's best smellers.
0:44:45 > 0:44:49Their supersized antennae are like feather dusters,
0:44:49 > 0:44:54sieving the air for minute molecules of scent.
0:44:54 > 0:44:57Some species can pick up scents from nearly five kilometres away.
0:45:02 > 0:45:04I've come to meet one particular moth
0:45:04 > 0:45:07renowned for an amazing sense of smell.
0:45:09 > 0:45:12This is the tobacco hawkmoth -
0:45:12 > 0:45:15they're really large moths that live in America,
0:45:15 > 0:45:17and that thing that's really distinctive about them
0:45:17 > 0:45:19is that they're able to hover,
0:45:19 > 0:45:23so they look a bit like the hummingbirds of the insect world.
0:45:23 > 0:45:27And the thing about the hovering is that it's really energy expensive,
0:45:27 > 0:45:30and so these moths need a constant supply of food.
0:45:30 > 0:45:32When they're active,
0:45:32 > 0:45:34they need to feed every 15 minutes.
0:45:34 > 0:45:38And they're capable of flying 80 miles in a single night
0:45:38 > 0:45:39just to find food.
0:45:39 > 0:45:41Which is a tall order
0:45:41 > 0:45:42if you're only this big.
0:45:45 > 0:45:47These hawkmoths feed on nectar from flowers.
0:45:49 > 0:45:52Like most moths, they're only active at night.
0:45:52 > 0:45:56So all they have to guide them through the darkness
0:45:56 > 0:45:59are minute wisps of scent blowing from the flowers.
0:46:02 > 0:46:05To reveal the fascinating way they do this,
0:46:05 > 0:46:07I've set up a hi-tech smell test.
0:46:08 > 0:46:10Using a wind tunnel,
0:46:10 > 0:46:12and their favourite food source -
0:46:12 > 0:46:14a petunia flower -
0:46:14 > 0:46:17I can recreate their natural feeding conditions.
0:46:19 > 0:46:22In pitch darkness, with a night-vision camera,
0:46:22 > 0:46:27I'm going to see how quickly these moths can track down their food.
0:46:29 > 0:46:31When I switch the wind tunnel on,
0:46:31 > 0:46:34the breeze will carry odour molecules downwind
0:46:34 > 0:46:36and down there a moth might detect them,
0:46:36 > 0:46:39and then it's got some detective work to do,
0:46:39 > 0:46:42because it's got to solve the puzzle of where the scent came from.
0:46:42 > 0:46:44And that is not as simple as it sounds.
0:46:51 > 0:46:53The fans create swirling eddies
0:46:53 > 0:46:55of turbulent air -
0:46:55 > 0:46:56like a windy day.
0:46:58 > 0:47:01If we could see the flower's fragrance,
0:47:01 > 0:47:05it would appear as delicate wisps or patches of scent
0:47:05 > 0:47:06constantly on the move.
0:47:08 > 0:47:12So, for the moth, working out where that smell is coming from
0:47:12 > 0:47:13is a real challenge.
0:47:16 > 0:47:19Once inside the wind tunnel he zigzags back and forth,
0:47:19 > 0:47:23trying to find the tiny traces of scent.
0:47:24 > 0:47:27The patches of odour that the moths have to detect
0:47:27 > 0:47:29could be just a few millimetres across,
0:47:29 > 0:47:31so at the speed these moths are flying,
0:47:31 > 0:47:33it would be really easy to miss the signal.
0:47:35 > 0:47:36So, for the moths,
0:47:36 > 0:47:39it's not enough to be able to smell well -
0:47:39 > 0:47:42they've also got to be able to smell at high speed.
0:47:44 > 0:47:48We can identify less than one smell a second.
0:47:48 > 0:47:50That's because it takes time for our noses to refresh
0:47:50 > 0:47:52and detect something new.
0:47:53 > 0:47:58But these moths smell more than 30 times quicker.
0:47:58 > 0:48:01And because their antennae are constantly being refreshed
0:48:01 > 0:48:05they can detect and follow the tiniest traces scent.
0:48:12 > 0:48:16And that's how they are able to find the source of the smell
0:48:16 > 0:48:18and feed on the sweet nectar
0:48:18 > 0:48:20that's so valuable to them.
0:48:34 > 0:48:37It's taking these moths a lot of energy to stay in the air.
0:48:37 > 0:48:39They're probably beating their wings
0:48:39 > 0:48:4130 to 40 times a second.
0:48:41 > 0:48:43For all organisms there has to be a balance
0:48:43 > 0:48:46between the energy they take in, they eat,
0:48:46 > 0:48:48and the energy they need to expend to live -
0:48:48 > 0:48:50and if you look at way these moths are flying,
0:48:50 > 0:48:54they're using up a huge amount of energy just to stay in the air,
0:48:54 > 0:48:56so they must need a lot of food.
0:48:56 > 0:48:59And if you're in that situation, the best thing you can do
0:48:59 > 0:49:02is be as efficient as possible about how you find it.
0:49:02 > 0:49:05And that is what their sense of smell is all about.
0:49:09 > 0:49:12In the wild, these amazing hovering moths
0:49:12 > 0:49:14use their supersensitive antennae
0:49:14 > 0:49:18to find flowers from several kilometres away -
0:49:18 > 0:49:22something they could never do through sight or hearing.
0:49:23 > 0:49:26For me, this is the most incredible example
0:49:26 > 0:49:29of how an animal uses scent to survive.
0:49:36 > 0:49:39Our journey through the world of smell is almost complete.
0:49:42 > 0:49:46But there's one creature that's harnessed the power of scent
0:49:46 > 0:49:49in a highly unusual and unwelcome way.
0:49:52 > 0:49:54They produce an odour so potent
0:49:54 > 0:49:57it should only be handled with extreme caution.
0:49:59 > 0:50:02And the fumes it gives off are so volatile
0:50:02 > 0:50:04they light up with a bang.
0:50:11 > 0:50:14So what animal produces this unholy odour?
0:50:16 > 0:50:19Meet the striped skunk -
0:50:19 > 0:50:21one of the world's smelliest creatures.
0:50:28 > 0:50:29I've come to northern Arizona
0:50:29 > 0:50:33to discover the amazing way these little mammals use scent
0:50:33 > 0:50:35to protect themselves.
0:50:42 > 0:50:46To experience it first-hand, I'm joining Kevin Overfield,
0:50:46 > 0:50:48who rescues skunks
0:50:48 > 0:50:50that have caused a bit of a stink.
0:50:50 > 0:50:52Basically we go to homes,
0:50:52 > 0:50:54people that are having nuisance, animal problems -
0:50:54 > 0:50:56like the skunks are living under their house or under a deck.
0:50:56 > 0:51:00We go over there, we set a live trap so that the animal's not harmed,
0:51:00 > 0:51:02and we trap 'em and we bring 'em back to my home.
0:51:02 > 0:51:04Once we know that they're healthy and not sick
0:51:04 > 0:51:06we take them out and release them in the woods,
0:51:06 > 0:51:08back in their natural habitat.
0:51:08 > 0:51:12Kevin has a skunk that's ready for release.
0:51:12 > 0:51:16And he's kindly offered me the chance to help him out.
0:51:16 > 0:51:19So what can I expect when we try and put this skunk in a cage?
0:51:19 > 0:51:21You can expect to be sprayed.
0:51:21 > 0:51:24OK. And what's that musk smell like?
0:51:24 > 0:51:26It's really hard to explain - there's no other smell like it.
0:51:26 > 0:51:28You can taste it, you can smell it,
0:51:28 > 0:51:31you know, you can almost feel it sometimes, it's so powerful.
0:51:31 > 0:51:32PATRICK CHUCKLES
0:51:32 > 0:51:35- So you know when you've been sprayed by a skunk.- Definitely.
0:51:35 > 0:51:36You know when you've been sprayed.
0:51:36 > 0:51:38OK. Well, here goes.
0:51:40 > 0:51:42- We're going to move up slow.- Yep.
0:51:44 > 0:51:45Be aware where the cage is -
0:51:45 > 0:51:47- the cage is going to be right here, OK?- OK.
0:51:51 > 0:51:53- Now, just kind of move up on him real slow.- Mm-hm.
0:51:53 > 0:51:54OK.
0:51:56 > 0:51:58OK, there's your chance.
0:52:00 > 0:52:01There you go.
0:52:01 > 0:52:03OK, now hold him there.
0:52:03 > 0:52:05Hold him like I told you. Yep.
0:52:05 > 0:52:07- OK. - SKUNK SQUEALS
0:52:07 > 0:52:09- KEVIN LAUGHS - No, put him in the cage!
0:52:10 > 0:52:12There you go, got him.
0:52:12 > 0:52:14Oh... Oh, God, that's disgusting.
0:52:14 > 0:52:15Good job.
0:52:15 > 0:52:17PATRICK COUGHS
0:52:17 > 0:52:21It smells of burning rubber, stale urine and rotten eggs.
0:52:21 > 0:52:23HE COUGHS HEARTILY
0:52:23 > 0:52:25If you've got it on your face, just keep spitting.
0:52:25 > 0:52:27Ugh, right up my nose. Ugh.
0:52:27 > 0:52:29Augh!
0:52:29 > 0:52:31Straight in the face.
0:52:31 > 0:52:34If you look at your glasses, all the yellow on your glasses,
0:52:34 > 0:52:36that's all skunk spray and on your forehead.
0:52:36 > 0:52:39They can aim. They can aim very well.
0:52:39 > 0:52:41- I think you did a good job. - Oh, yeah...
0:52:41 > 0:52:44- I'll hire you. - PATRICK LAUGHS
0:52:46 > 0:52:49But why do we find this smell so unpleasant?
0:52:50 > 0:52:52So, this is it.
0:52:52 > 0:52:56This is the skunk's secret weapon.
0:52:56 > 0:53:00It's made up of seven volatile compounds called thiols.
0:53:00 > 0:53:04But it's one element within these that causes the stink.
0:53:04 > 0:53:07The reason we find this substance so repulsive
0:53:07 > 0:53:08is because it contains sulphur -
0:53:08 > 0:53:10now, from an evolutionary point of view,
0:53:10 > 0:53:14sulphur is associated with areas of low concentrations of oxygen
0:53:14 > 0:53:19and rotting food, so we do our best to stay away from it.
0:53:19 > 0:53:22Skunks use their sulphurous scent to defend themselves
0:53:22 > 0:53:25against predators - like mountain lions.
0:53:25 > 0:53:28They can spray it 15 feet,
0:53:28 > 0:53:31and aim for the eyes, causing temporary blindness...
0:53:33 > 0:53:35..giving the skunk time to escape.
0:53:37 > 0:53:40But their foul smell is precious.
0:53:40 > 0:53:43They only carry enough to spray about five times,
0:53:43 > 0:53:46and then it takes ten days to replenish.
0:53:48 > 0:53:51So skunks only pull the trigger as a last resort.
0:53:53 > 0:53:56That's why they've come up with an ingenious way
0:53:56 > 0:53:58to advertise their potency.
0:53:59 > 0:54:01Perhaps one of the most obvious things you notice
0:54:01 > 0:54:06when you first see a skunk is that magnificent white stripe.
0:54:06 > 0:54:08It's used as a warning to other animals.
0:54:08 > 0:54:13It says, "Stay away - I am a skunk and I am toxic."
0:54:13 > 0:54:16It's believed the skunk's odour is so notorious
0:54:16 > 0:54:20that their colouring alone is enough to deter predators.
0:54:21 > 0:54:26So what happens when an animal with one of nature's most sensitive noses
0:54:26 > 0:54:28spots those bold warning stripes?
0:54:30 > 0:54:33I've come to a bear park in Arizona to find out.
0:54:36 > 0:54:37So here's the plan.
0:54:37 > 0:54:41We've rustled up two decoy skunks -
0:54:41 > 0:54:43one with that classic black and white stripe
0:54:43 > 0:54:45and one which we've dyed completely black.
0:54:45 > 0:54:49Now, in theory, any foraging bears should have no problem
0:54:49 > 0:54:51approaching our black skunk -
0:54:51 > 0:54:54but they should avoid this black and white one,
0:54:54 > 0:54:56because they associate these markings
0:54:56 > 0:54:57with that foul-smelling musk.
0:54:59 > 0:55:03The stunt skunks are placed inside the bear enclosure
0:55:03 > 0:55:05with on-board cameras.
0:55:06 > 0:55:11And to lure the bears up close, I'm leaving some tasty snacks.
0:55:16 > 0:55:18We've got our decoy skunks in position.
0:55:18 > 0:55:21Now, let's see if those black and white stripes
0:55:21 > 0:55:23really are a deterrent.
0:55:26 > 0:55:30After some time, a big bear called Jack approaches.
0:55:31 > 0:55:35OK, we've finally got a bear coming in to investigate.
0:55:35 > 0:55:37He's a really big one.
0:55:40 > 0:55:43This big bear is definitely approaching our black skunk.
0:55:45 > 0:55:47You can see he's not bothered in the slightest.
0:55:47 > 0:55:49He's confident,
0:55:49 > 0:55:51he's eating up all those berries -
0:55:51 > 0:55:53that skunk is in big trouble.
0:55:58 > 0:56:00Now he's just taking the skunk apart.
0:56:11 > 0:56:12OK, now he's turned around
0:56:12 > 0:56:15and he's headed for our black and white skunk
0:56:17 > 0:56:21Bears use their sensitive noses to sniff out food.
0:56:21 > 0:56:25But such an acute sense of smell would be overwhelmed by the skunk.
0:56:26 > 0:56:31So, in theory, Jack should spot that white stripe and steer clear.
0:56:35 > 0:56:36I don't know if he's interested or not -
0:56:36 > 0:56:39but is he going to go for him?
0:56:43 > 0:56:45Now he's turning around.
0:56:46 > 0:56:49Looks like he's not interested in the striped skunk at all.
0:56:49 > 0:56:51It worked.
0:56:54 > 0:56:57It may not be the most rigorous of scientific tests,
0:56:57 > 0:56:58but it illustrates the point
0:56:58 > 0:57:02that the skunk's success is not only down to their foul smell,
0:57:02 > 0:57:06but also the brilliant way they advertise their defensive weapon.
0:57:22 > 0:57:24You take him right down there by that creek.
0:57:24 > 0:57:28And it's this that should keep our rescued skunk safe
0:57:28 > 0:57:31as he's released back into the wild.
0:57:32 > 0:57:34- Somewhere about here? - Yeah, that'll be good.
0:57:45 > 0:57:48Skunks really do get a bad press,
0:57:48 > 0:57:50but having the chance to get this close to them,
0:57:50 > 0:57:54you really can appreciate what beautiful creatures they are.
0:57:54 > 0:57:56And just like any other animal out here,
0:57:56 > 0:57:59all they're trying to do is survive.
0:58:03 > 0:58:06In this episode we've seen the most extraordinary ways
0:58:06 > 0:58:09animals tap into the invisible world of scent.
0:58:11 > 0:58:15Smell is our most unfamiliar sense.
0:58:15 > 0:58:18But in the wild it offers animals vital information
0:58:18 > 0:58:21that they just can't get from sight or sound.
0:58:22 > 0:58:27In nature, a keen nose is essential for survival.