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00:00Join me on a journey through the fantastic world of the human senses.
00:08I'm Nigel Marvin. I've spent my life meeting amazing animals.
00:15And now I'm going to discover what they can reveal about the most extraordinary creature of them all.
00:22Us.
00:25We'll see why we love tasting foods that even a shark can't stomach.
00:34And how, when it comes to our sense of touch, we're just like an elephant.
00:43I'll be going through an assault course through each of my senses.
00:48Oh, that is obnoxious.
00:52To find out just how much we can take.
00:57I'll reveal how every one of us can miss what's right in front of our eyes.
01:02Sorry, what were you saying?
01:04Would you spot the changeover?
01:06Discover the secrets of the human senses and your life will never feel the same again.
01:16Woohoo!
01:33Tonight, it's smell. Across the animal kingdom, it's the most powerful sense of all.
01:41Bears can detect the sweet smell of honey from over three miles away.
01:46So let's see what they make of this.
01:56It's working. The honey's like a smell magnet.
02:17We respond as strongly as any other animal to some smells.
02:22I'm going to reveal what makes a smell good or bad.
02:26I'm going to find the smells with the most powerful effects on us.
02:30And I'm also going to discover the worst smell on the planet.
02:39I'll be going through an assault course through each of my senses.
02:44I'll put my nose to the test to reveal the secrets of our sense of smell.
02:51I'll be finding out why some smells can make us physically sick.
03:00Before finally taking in a suffocating stench that's designed to be utterly unbearable.
03:14Mmm, exquisite.
03:17Nothing can beat that, the delicate aroma of blossom on a spring morning.
03:25But you can beat it. It's not nice flowery smells that pack the most powerful punch.
03:34It's things like the foul stench of rotting manure.
03:38It's things like the foul stench of rotting manure.
03:42We react most strongly to the nasty ones.
03:45And if I really want to understand what gets our sense of smell going,
03:50I need to get out there and experience the most unpleasant stenches of them all.
03:55I'm a zoologist, so my search for stinks has to start here in New Mexico,
04:01with a showdown with the smelliest animal in the West.
04:13In terms of stinkiness, the animal I'm after is a wild boar.
04:19In terms of stinkiness, the animal I'm after today is a legend.
04:24Mephitus mephitus is its Latin name, and that means bad odour.
04:30The skunk, and I'm going to test whether its smell is as bad as it's cracked up to be,
04:36with the help of Jerry Dragoo, who rescues skunks nesting under people's houses.
04:42A lot of people don't like them living under their house, but they don't necessarily want them killed either,
04:45so that's where we come in.
04:46So we're going back to your house now, which is skunk heaven.
04:49That's right.
04:52At his home outside Albuquerque, Jerry's got all sorts of skunks waiting to be returned to the wild.
05:01But I'm not here just to get a whiff myself.
05:04I actually want to get covered in skunk spray, so I can test the effect of their smell on other people.
05:10I want to be skunked. Call me mad, but that's what we've come here for.
05:15Right.
05:16And this is the best chance. This one's got to go back to the wild.
05:18Right. We're getting ready to release this one now, so we'll probably get a lot of opportunity to get sprayed.
05:23People have described the smell like garlic, like rotting eggs.
05:27I mean, I've never really smelt it close up, so this is going to be an interesting experience.
05:33But I've got to take some precautions.
05:35They can bite, can't they?
05:36Yeah.
05:37Again, they'll be defending themselves, and if the spray doesn't work, they'll try to grab a hold of you with their teeth.
05:41And if the spray gets you in the eyes, that really is painful, isn't it?
05:44Yes.
05:45It's a burning sensation.
05:46It'll sting a little bit, so it'll probably keep your eyes watering for about 15 minutes.
05:50You were just trying to help me there. It will sting more than a little bit, won't it?
05:53Yeah.
05:54I mean, it's excruciating, so I should have these in case I get a direct hit, and that's what they aim for, the eyes of predators.
05:59I'll be looking for your eyes.
06:01So I've got these couple of precautions.
06:05Now, the moment of truth.
06:10There he is.
06:16You can see that yellow fluid.
06:18What a spray.
06:19All over it.
06:22And that is an indescribable stench.
06:25It's a terrible stench.
06:27It's a terrible stench.
06:29It's a terrible stench.
06:31It's an indescribable stench.
06:34Garlic sewage.
06:36It is unbelievable.
06:37You can read all about it, but actually to get it first-hand like that.
06:41Skunk spray contains chemicals like the ones that give garlic its eye-watering burn
06:46and rotting eggs their sulfurous stench.
06:49The combination is a powerful repellent.
06:52Got him.
06:53All right, bring him over here into the cage.
06:54Yeah.
06:55I've got him right here.
06:56Look at that.
06:57He's going to the water.
06:58There we go.
07:00Great.
07:01Thanks for that demonstration.
07:03That was great.
07:04And now, we'll see what happens when I walk around with this smell on me.
07:13If you see a skunk, you're expecting a stink.
07:17But if I go up to people wearing a shirt drenched in skunk spray,
07:21I can test people's genuine reaction to the smell on its own.
07:30Skunk spray.
07:32Skunk spray.
07:58So skunk definitely smells bad.
08:00But it's not totally unbearable.
08:02It's not a bad smell from a distance, actually.
08:06Skunk is okay downwind. It doesn't bother me.
08:09I mean, I wouldn't buy it as a cologne.
08:13As long as I keep moving, I'm fine.
08:15Yeah, yeah.
08:16I don't want to sit in it.
08:17I think people exaggerate more when they smell it.
08:19They make a big thing out of it. It's not that bad.
08:22It's not the worst smell in the world.
08:23It's not. It's not.
08:25So if I'm after the bad smells with the most gut-wrenching effects on us,
08:30there must be something worse than a skunk.
08:36At the Monell Chemical Senses Centre,
08:38scientists are working on something far, far nastier.
08:46They've spent the last five years deliberately trying to create the worst smell on earth.
08:52A smell so awful it could be used for crowd control.
09:00Pam Dalton is in charge of the project.
09:03Lots of people had looked for agreement about good smells,
09:06but we were one of the first people that started looking specifically at bad odours.
09:11Pam and the team wanted to see whether they could create something totally harmless
09:16that smelled so bad that everyone on the planet would find it utterly revolting.
09:22The first step was to find out what people reckoned were the nastiest smells.
09:28Certain types of vomit, saliva, spit.
09:33It's probably going into the toilet after he's been in there, would be my guess.
09:36Dead body smell.
09:39Yeah, that'd never go away. You know, that smell would never go away.
09:42I've been counting that several times.
09:45He's a cop.
09:47Just so you know.
09:48Hey!
09:53Oh, what about this one?
10:01I don't even want to know what that is, but it's absolutely disgusting.
10:07Everyone tends to find the smell of things like vomit, excrement and decomposing bodies deeply unpleasant.
10:13Oh, not good, whatever it is.
10:15But it's only a few bad stinks that really get up our noses.
10:23Smell was the first sense our animal ancestors developed.
10:27But as we evolved into humans, we lost much of that ability to smell.
10:32Come on.
10:34Around three million years ago, our ape-man ancestors stood upright for the first time.
10:40And when they did that, they sacrificed our sense of smell.
10:45The best sniffers keep their noses low,
10:48because smells sink down through the air, forming an invisible fog just above the ground.
10:57And compared to some animals, our noses just aren't very sensitive.
11:09Bloodhounds can track people days after they've walked by.
11:13They can smell the minute traces of body scent which have seeped through the soles of the shoes and lodged on the ground.
11:25Yet our noses work in the same way as a dog's.
11:29The main difference? Well, it's the size of the snout.
11:33Dogs have around 220 million receptors for smell.
11:37We have a mere 10 million.
11:39For us to smell as well as a dog can, we'd need to have a muzzle as large as theirs.
11:45But a big nose, it isn't really good for pulling faces.
11:49And for our monkey ancestors, communicating with facial expressions, that was crucial.
11:55So they sacrificed their acute sense of smell.
12:10But even with our limited sense of smell, bad odours are capable of giving us a real jolt.
12:20To find out why, I've come to Africa to sample a stench which, more than any other,
12:26has the ability to evoke the most stomach-churning reactions in us.
12:32This is flesh that has been left rotting in the African sun.
12:42I've got a bit of a stomach ache, but I've got to go.
12:45I've got to go.
12:47I've got to go.
12:49I've got to go.
12:51I've got to go.
12:53I've got to go.
12:55I've got to go.
12:57I've got to go.
12:59I've got to go.
13:05It's been left...
13:09I've never had a smell like this.
13:11It is really making my eyes water.
13:15We find the reek of rotting flesh utterly disgusting.
13:19But some animals have a very different attitude to a decomposing carcass.
13:30Kevin spotted hyenas, the ultimate carrion eaters.
13:34They're magnificent, aren't they?
13:36That's right, Nigel.
13:38In fact, I've seen these guys follow a smell up to 20, 30 kilometres.
13:43So that smell that's so disgusting to us is crucial to them in their survival?
13:47No, it's absolutely attractive.
13:49This is like a fillet steak.
13:52It's a real treat.
13:54And they're competing for the meat.
13:56They're competing for it.
13:57You can see it's not like they're just eating it because they have to.
14:00They're eating it because they want to.
14:04The reek of rotting flesh is caused by sulphur-containing chemicals
14:08released as bacteria break down protein and fat in the meat.
14:13By the time it smells off,
14:15rotting meat contains such high levels of sulphur
14:18that it can't be digested.
14:20By the time it smells off,
14:22rotting meat contains such high levels of bacteria
14:25that eating it could be fatal.
14:29So we find the smell disgusting
14:31to stop us putting rotting flesh anywhere near our mouths.
14:36Our sense of smell is warning us of danger.
14:43Help!
14:45Just take the big piece.
14:51Hey!
14:56The smell doesn't bother hyenas
14:58because rotting flesh isn't dangerous for them.
15:01They've evolved to be scavengers,
15:03so they have an immune system that can deal with deadly microbes
15:07in rotting meat that would kill a human.
15:09I mean, there's even been cases of wild hyena eating anthrax.
15:12So they can actually cope with meat that's covered in anthrax spores?
15:16Pretty much cast iron stomachs, so to speak.
15:21So we find smells like rotting flesh unpleasant to warn us off,
15:25and our noses are actually more sensitive to these bad smells
15:29than they are to most pleasant ones.
15:32Oh!
15:35Oh, idiots.
15:39All smells, nice or nasty,
15:41are made of tiny molecules floating in the air.
15:45The sulphur compounds, which are the main ingredient of most bad smells,
15:49are lighter, faster-moving molecules.
15:52So they have no trouble reaching our smell sensors,
15:55which are tucked away in a layer of mucus
15:57right at the back of the nasal cavity.
16:00Even if there's just a faint trace of a bad smell,
16:03some of the molecules will reach the smell-sensing nerve cells,
16:07which send their signals to the brain.
16:10So because of the design of our noses,
16:12we can detect most bad smells
16:14at much lower concentrations than pleasant ones.
16:22Despite having a puny sense of smell compared to most animals,
16:26we're particularly sensitive to bad odours.
16:32So you might think that creating a smell that everyone knows about
16:37So you might think that creating a smell that everyone in the world
16:40finds totally unbearable wouldn't be that difficult,
16:44but it turns out that it is.
16:47Pam Dalton has found that people react to bad smells
16:51in surprisingly different ways.
16:54Even the stench of human faeces,
16:56or as they politely call it, bathroom mal-odour.
17:02Brace yourself. Here you go.
17:07Oh, dear, that is revolting.
17:10It must be a component in the world's worst smell.
17:13Why wouldn't it work just on its own?
17:16Well, it does work, but it wasn't quite there yet.
17:24Ooh! Maybe a toe.
17:29Don't smell it!
17:31Don't smell it.
17:33Amazingly, some people just aren't too bothered by a smell
17:37others find totally unbearable.
17:40Why do we react so differently to the same bad smell?
17:44It's not the worst.
17:48It's because as we grow up, our responses to smells develop gradually.
17:53When we're tiny, bad smells don't seem to give us too much of a problem.
17:59Babies hardly notice the stench of their dirty nappy.
18:05But slowly we're influenced by our parents telling us that poo is nasty stuff,
18:11and our feelings about the smell gradually get stronger.
18:17We all tend to find some smells unpleasant because they warn us of danger,
18:22but how strongly we react is all a matter of personal experience.
18:29COUGHING
18:37At the University of California,
18:39they're studying how we each develop powerful reactions to certain smells.
18:54OK, Angel, actually, we're looking at your brain now as we speak,
18:57and your brain looks just fine.
19:03Noam Sobel and his team are using an MRI scanner to see what happens in my brain.
19:20When we smell something, the nerve signals from the nose
19:23travel to two different areas of the brain.
19:26The signals travel up to the frontal lobe, which works out what the smell is.
19:31But they also get sent to a set of structures called the limbic system,
19:35the part of the brain where we store memories of any intense emotions
19:39we've had smelling that smell.
19:41So the smell triggers a strong feeling based on our previous experiences.
19:46Fear is one type of response that we see here.
19:49Disgust is, of course, another type of response that we commonly see here to odours,
19:53and we can all be quite familiar with that.
19:55Oh, that is obnoxious.
20:05Smells make us feel good or bad according to our own personal experiences of that smell,
20:11whether nice or nasty.
20:14Oh, that's a nice one.
20:16It reminds me of chasing butterflies in flower-filled meadows in the summer.
20:26Particular smells can trigger amazingly specific memories,
20:30which affect how we feel about that smell.
20:37I was invited to a garden party at Buckingham Palace
20:40and took my mother along as my guest.
20:43And as we walked into the gardens,
20:46the smell of that cut grass took me back to when I was about six years old.
20:54A smell most of us would rate as mildly pleasant
20:57has become the loveliest thing in the world.
21:03A perfume intended to bring pleasure can be tinged with feelings of sadness.
21:08The last time I smelt this out of nowhere,
21:11I had this scene of me as a little kid in a big fairground,
21:15flashing lights everywhere.
21:17I remember everything being really high up.
21:19My nan, who used to wear this,
21:22was towering above me and there were lots of lights
21:25and, you know, coloured light bulbs everywhere.
21:28The strange thing is I didn't even know I had that memory
21:31until the scent came along
21:33and then I was kind of dropped into this little world all of a sudden.
21:43Personal experience can dramatically alter how we respond to smells.
21:50So the team at Monell found that different people
21:54often had slightly different reactions to bad smells.
21:58But there was one stench which produced an amazing range of responses.
22:10What was that stench?
22:13That is, in fact, vomit odour.
22:16To most of us, it's pretty unpleasant.
22:19Oh, it's not a very pleasant smell.
22:23Vomit.
22:25It's vomit.
22:27The chemical that gives vomit its acrid punch is called butyric acid.
22:31Oh, my goodness gracious me, that's absolutely horrible.
22:34But butyric acid isn't just found in vomit.
22:41Yeah, it's like some sort of cheese.
22:43Yeah, it's like some sort of cheese.
22:45Stale cheese.
22:47So the smell isn't that bad
22:49if the first thing that comes into your head is something you like.
22:53Oh, Parmesan.
22:58We're all born with mild aversions to some smells to guard us from danger.
23:03But as we grow up, our reactions are shaped by what actually happens to us.
23:09For each of us, some bad smells really kick through
23:13and others we end up almost liking.
23:21So for Pam Dalton's team,
23:23creating a smell no-one can bear has been quite a challenge.
23:27They had to find a way to trigger everyone's most awful memories.
23:32Their brew contains every disgusting smell.
23:36Their brew contains every disgusting stink I've ever encountered.
23:44Cadaverine, the smell of rotting flesh, dead bodies,
23:48the smell of a bathroom malodor.
23:51Their exact recipe is a secret,
23:53but they had to get precisely the right amounts of each ingredient
23:57to make sure they all come through in the mix.
24:00The smell of vomit, butyric acid, which would be the smell of vomit.
24:06They call their secret formula stench soup.
24:10So is this really the most potent combination of bad smells ever created?
24:19I've agreed to get a noseful of it with some other brave volunteers.
24:29Chris is going to be hooking you up to all of the test equipment
24:33so that we can monitor a lot of your physiological responses,
24:36your breathing, your heart rate, what your stomach is doing.
24:39I feel like a guinea pig. What's going to happen now?
24:42Well, first I'm just going to apply a few electrodes to your skin.
24:46OK.
25:01They aren't taking any chances.
25:03Even though I'm in an airtight room, the smell is so powerful
25:07they'll only let me sniff it from inside a suction hood
25:10to make absolutely sure none of the stench escapes.
25:16Don't move.
25:20OK. Everything seems to be good here.
25:23Now it looks as though you are ready to be exposed to the world's worst odor.
25:27And I'm ready to leave the room.
25:29That's for you also, Paul.
25:31All right. Good luck.