0:00:04 > 0:00:07Our human senses are incredible.
0:00:09 > 0:00:10We have excellent vision...
0:00:12 > 0:00:13..precise hearing...
0:00:18 > 0:00:22..and can detect the slightest fragrance drifting on the breeze.
0:00:25 > 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.
0:00:43 > 0:00:45Hear storms from hundreds of kilometres away.
0:00:47 > 0:00:50That'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 > 0:01:00So fast!
0:01:02 > 0:01:05Travelling to some of the wildest places on Earth...
0:01:06 > 0:01:11..we reveal the strange and wonderful world of animal senses.
0:01:11 > 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 > 0:01:20I'm Dr Helen Czerski.
0:01:20 > 0:01:24I'm a physicist, and I want to find out how animals
0:01:24 > 0:01:27tap into an amazing range of light, scent and sound.
0:01:31 > 0:01:33I'm Patrick Aryee.
0:01:33 > 0:01:37As a biologist I'm fascinated by what the world appears like
0:01:37 > 0:01:39through animal senses far superior to our own.
0:01:46 > 0:01:49Experience the world through animal senses.
0:02:10 > 0:02:15Wherever we are, no matter how tranquil it seems,
0:02:15 > 0:02:18we are constantly surrounded by sound.
0:02:21 > 0:02:24Our ears are incredibly sensitive
0:02:24 > 0:02:27and hear a huge range of tones.
0:02:32 > 0:02:38But still, we detect only a tiny fraction of the sounds around us.
0:02:41 > 0:02:47I've come to Mexico's Sea Of Cortez, where two very different creatures
0:02:47 > 0:02:50have pushed sound to extremes.
0:02:51 > 0:02:56One very low pitched, the other incredibly high.
0:02:56 > 0:02:57I can hear
0:02:59 > 0:03:03..squeaks and whistles and occasional series of clicks.
0:03:04 > 0:03:08And it's really busy. It sounds like a busy city street.
0:03:08 > 0:03:11DOLPHIN 'CLICKS'
0:03:18 > 0:03:22Dolphins see their world through sound.
0:03:22 > 0:03:26Their high-frequency clicks reflect off objects around them,
0:03:26 > 0:03:29allowing them to build up an acoustic image.
0:03:29 > 0:03:31This is nature's sonar.
0:03:33 > 0:03:37And the thing is, I'm only hearing a tiny bit of all the sound
0:03:37 > 0:03:40that's down there, because most of the dolphins' calls are at
0:03:40 > 0:03:42frequencies above my hearing range.
0:03:45 > 0:03:49These extreme high frequencies are known as ultrasound,
0:03:49 > 0:03:52meaning they're too high for our ears to detect.
0:03:55 > 0:03:57But here in the same waters,
0:03:57 > 0:04:02other mammals operate at the other extreme of the sound spectrum.
0:04:02 > 0:04:04LOW RUMBLE
0:04:04 > 0:04:06Blue whales.
0:04:14 > 0:04:19Their songs are infrasonic - too low for our ears to detect.
0:04:19 > 0:04:23It's only when they're sped up that we can hear them.
0:04:23 > 0:04:29WHALE MAKES LOW RUMBLING
0:04:32 > 0:04:35These deep, haunting songs allow them to keep
0:04:35 > 0:04:39in touch with each other over hundreds of kilometres.
0:04:41 > 0:04:46Whales and dolphins operate on the outer limits of the spectrum,
0:04:46 > 0:04:49but across the planet, animals are tuned into every
0:04:49 > 0:04:52frequency of sound in-between.
0:04:56 > 0:05:00In this episode, we're going on a journey through the world of sound,
0:05:00 > 0:05:03from the deep sounds, far lower than the ones we can hear,
0:05:03 > 0:05:07up to where the dolphins are calling at frequencies far higher than
0:05:07 > 0:05:10we can hear, and there are ways of perceiving sound
0:05:10 > 0:05:13that are way beyond our human capabilities.
0:05:17 > 0:05:21Prepare to enter a bizarre world of sound,
0:05:21 > 0:05:23beyond human hearing.
0:05:27 > 0:05:32Our journey starts in Southern Africa, where one of nature's
0:05:32 > 0:05:38true giants makes the deepest sounds of any land animal.
0:05:38 > 0:05:41LOW RUMBLE
0:05:41 > 0:05:42The African elephant.
0:05:45 > 0:05:50The frequency, or pitch of sound, is measured in hertz,
0:05:50 > 0:05:54and their low rumbles reach around 250 hertz.
0:05:57 > 0:06:02But elephants also produce and hear sounds below 20 hertz.
0:06:02 > 0:06:07These are sounds our ears struggle to detect, called infrasound.
0:06:11 > 0:06:16Infrasound travels a long way, so elephants use it
0:06:16 > 0:06:20to keep in touch with each other over many kilometres.
0:06:20 > 0:06:21RUMBLING
0:06:23 > 0:06:28But it's now suspected they also use their infrasonic hearing to
0:06:28 > 0:06:32listen in to a secret sound of our planet.
0:06:32 > 0:06:34THUNDERCLAPS
0:06:41 > 0:06:46We can hear thunderstorms from 20, occasionally 30 kilometres away
0:06:46 > 0:06:49but it's now thought that elephants can hear them
0:06:49 > 0:06:52from distances of up to 500 kilometres.
0:06:52 > 0:06:55That's roughly the equivalent of someone in London
0:06:55 > 0:06:57listening to a storm in Edinburgh.
0:07:01 > 0:07:03This may seem impossible
0:07:03 > 0:07:07but at the end of the dry season, elephants often make sudden
0:07:07 > 0:07:10and unpredictable changes in direction.
0:07:12 > 0:07:18For no obvious reason, herds turn and march for days on end.
0:07:19 > 0:07:23But when checked against weather records, it seems the elephants
0:07:23 > 0:07:27are heading towards rainstorms up to 500 kilometres away.
0:07:35 > 0:07:39I want to find out if this is a coincidence
0:07:39 > 0:07:42or whether elephants really can recognise the deep,
0:07:42 > 0:07:46infrasonic sounds of a storm over vast distances.
0:07:47 > 0:07:51We're going to try something that has never been done before.
0:07:51 > 0:07:54We're going to play the infrasonic part of a thunderstorm to a herd
0:07:54 > 0:07:57of elephants and see just how they react.
0:07:57 > 0:08:00And to do that, we're going to take a camper van
0:08:00 > 0:08:02and turn it into a giant speaker.
0:08:05 > 0:08:09'It may seem an unusual choice of speaker,
0:08:09 > 0:08:14'but broadcasting infrasound requires large volumes of air
0:08:14 > 0:08:18'and the inside of this camper van offers the perfect space.
0:08:19 > 0:08:23'So with the help of infrasonic expert Bruce Thigpen,
0:08:23 > 0:08:27'we're transforming it into a massive subwoofer.'
0:08:27 > 0:08:31So can we use this camper van, this infrasonic speaker,
0:08:31 > 0:08:34to replicate the sound of a thunderstorm?
0:08:34 > 0:08:39Yes, we have an actual thunderstorm recording of thunderclaps,
0:08:39 > 0:08:42the rumble of the sound after the lightning strike.
0:08:42 > 0:08:45We've recorded that, we've took an actual recording
0:08:45 > 0:08:49and we filtered it, so it just plays the lowest frequencies.
0:08:49 > 0:08:52So, even though we're going to be quite close to the elephants, our
0:08:52 > 0:08:55infrasonic speaker is going to play the sound of a distant thunderstorm.
0:08:55 > 0:08:57Exactly.
0:08:57 > 0:09:02'Thunderstorms are full of different frequencies of sound
0:09:02 > 0:09:05'and these travel different distances.'
0:09:05 > 0:09:07THUNDERCLAPS
0:09:18 > 0:09:21The higher sounds, like the thunderclap,
0:09:21 > 0:09:25are quickly absorbed into the atmosphere, so don't travel far.
0:09:29 > 0:09:33Storms also produce low rumbles that carry much further.
0:09:35 > 0:09:39But the very deepest sounds are below our hearing range
0:09:39 > 0:09:43and these infrasonic parts of the storm are known to travel
0:09:43 > 0:09:44much greater distances.
0:09:52 > 0:09:56Infrasound can travel through its environment without getting
0:09:56 > 0:10:00absorbed, and that's why the infrasound from rainstorms
0:10:00 > 0:10:02can travel for hundreds of kilometres.
0:10:12 > 0:10:16But could elephants really be hearing the infrasound
0:10:16 > 0:10:18from these distant storms?
0:10:22 > 0:10:25Andre, I believe Tembo is the perfect elephant.
0:10:25 > 0:10:29'Andre Kotze has worked with elephants for 25 years
0:10:29 > 0:10:32'and can recognise the behaviours that will show
0:10:32 > 0:10:35'if the elephants are hearing our infrasonic storm.'
0:10:36 > 0:10:40Andre, even though these are rescued elephants, do you still see
0:10:40 > 0:10:43a change in behaviour when a thunderstorm is approaching, like they would in the wild?
0:10:43 > 0:10:46When they hear a thunderstorm, they will more than likely
0:10:46 > 0:10:49turn their backsides together, facing to the thunderstorm,
0:10:49 > 0:10:53ears out with a spontaneous freeze, like it's a secret message or
0:10:53 > 0:10:56something that happens and they just stand still for it.
0:10:56 > 0:10:58After the spontaneous freeze you are more than likely
0:10:58 > 0:11:01to find that they start chatting amongst each other. Low rumbles.
0:11:01 > 0:11:04If they respond in that way to our thunderstorms
0:11:04 > 0:11:07then that's proof, in a way, that they can hear
0:11:07 > 0:11:10a part of the sound spectrum that we can't even attempt to.
0:11:10 > 0:11:13Absolutely, absolutely, without a question of doubt.
0:11:21 > 0:11:24Although the speaker is positioned close to the elephants,
0:11:24 > 0:11:27the infrasound it produces will have the intensity
0:11:27 > 0:11:29of a distant thunderstorm.
0:11:34 > 0:11:36The herd is busy feeding,
0:11:36 > 0:11:39so we're looking for a definite change in behaviour.
0:11:41 > 0:11:43Bruce, I think we're ready to play the speaker.
0:11:43 > 0:11:46OK, Patrick, audio in two seconds.
0:11:51 > 0:11:56It may seem strange, but because the camper van is generating sounds
0:11:56 > 0:12:00below our hearing threshold we can't hear it,
0:12:00 > 0:12:05but we can certainly see it, as air inside vibrates with sound energy.
0:12:12 > 0:12:14The elephants react immediately...
0:12:18 > 0:12:20..turning to the speaker.
0:12:21 > 0:12:26They're clearly reacting to the sound, but I can't hear a thing.
0:12:26 > 0:12:28That's incredible, they've all stopped
0:12:28 > 0:12:30and they've changed their behaviour,
0:12:30 > 0:12:34as soon as Bruce started playing that sound from the camper van.
0:12:34 > 0:12:36You can even hear them vocalising.
0:12:39 > 0:12:40Their ears fanning out.
0:12:42 > 0:12:46It's absolutely amazing how it completely changes their behaviour.
0:12:51 > 0:12:52Bruce, it worked!
0:12:52 > 0:12:55There's absolutely no question about it
0:12:55 > 0:12:58and their ability to determine the direction the sound
0:12:58 > 0:13:00was coming from, I was really impressed with that.
0:13:02 > 0:13:04The elephants are back feeding now
0:13:04 > 0:13:08but virtually the entire herd turned and faced our infrasonic
0:13:08 > 0:13:11speaker, listening in to that secret sound of the storm.
0:13:18 > 0:13:23This hidden channel of infrasound could explain a great mystery
0:13:23 > 0:13:24of the natural world.
0:13:26 > 0:13:28How elephants know where to go
0:13:28 > 0:13:32when they migrate vast distances in search of water.
0:13:36 > 0:13:40But storms aren't the only elemental forces to produce infrasound.
0:13:42 > 0:13:47Even things we think of as silent are in fact making very
0:13:47 > 0:13:48low-frequency sounds.
0:13:49 > 0:13:54The spectacular aurora borealis produces infrasonic rumbles
0:13:54 > 0:13:56of a hundredth of a hertz.
0:14:07 > 0:14:11Volcanoes produce even lower frequencies.
0:14:11 > 0:14:15These are some of the deepest sounds on the planet.
0:14:17 > 0:14:21And amazingly, there's evidence elephants may be detecting
0:14:21 > 0:14:23other natural sources of infrasound.
0:14:25 > 0:14:29The most extraordinary example is the tsunami that swept across
0:14:29 > 0:14:31the Indian Ocean in 2004.
0:14:33 > 0:14:36When the tsunami hit the shores of Sri Lanka,
0:14:36 > 0:14:39there were numerous reports of elephants acting erratically
0:14:39 > 0:14:43and moving inland well before the tsunami struck.
0:14:43 > 0:14:47Now, this apparent sixth sense could be down to the large
0:14:47 > 0:14:51amounts of infrasound being produced by the tsunami.
0:14:51 > 0:14:56As it built up, the sound it was producing was moving faster
0:14:56 > 0:14:58than the approaching wave.
0:14:58 > 0:15:02So the theory is that elephants could hear this low-pitched sound,
0:15:02 > 0:15:05like it was an alarm, and were able to move off into safety.
0:15:10 > 0:15:14ELEPHANT MAKES LOW RUMBLE
0:15:14 > 0:15:17Elephants are one of the few animals on Earth that hear
0:15:17 > 0:15:19and produce infrasound.
0:15:25 > 0:15:29But in the vast wetlands of the Florida Everglades,
0:15:29 > 0:15:33an ancient predator has also harnessed the power of sounds
0:15:33 > 0:15:35too deep for us to hear.
0:15:37 > 0:15:40They use it to put on one of the most extraordinary
0:15:40 > 0:15:42displays in the animal kingdom.
0:15:46 > 0:15:48The American alligator.
0:15:52 > 0:15:57Every spring, the male alligators put on a spectacular mating display.
0:15:57 > 0:16:01They sink down in the water so their backs are just below
0:16:01 > 0:16:06the surface, and then make really low-frequency sounds.
0:16:06 > 0:16:09And the consequence of that is that water droplets on their back
0:16:09 > 0:16:11look like they're dancing.
0:16:11 > 0:16:15And soon it becomes a water dance-off,
0:16:15 > 0:16:18as rival males compete by displaying to females.
0:16:21 > 0:16:24I've never really had any desire to be close to a bellowing
0:16:24 > 0:16:27alligator but I do want to see this,
0:16:27 > 0:16:31and to do it, I've got to trigger a chorus of amorous alligators.
0:16:37 > 0:16:41To see this spectacle, I need to encourage some alligators
0:16:41 > 0:16:42to start dancing.
0:16:45 > 0:16:49And to do that, I need to replicate their infrasonic calls
0:16:49 > 0:16:52so they think that there's a larger male close by.
0:16:53 > 0:16:57That requires speakers even bigger than a camper van.
0:17:03 > 0:17:05The alligators are producing infrasound in water
0:17:05 > 0:17:10but we want to do it in air, to send sound waves out across the lake
0:17:10 > 0:17:13and the physics works a little bit differently in air, so we've built
0:17:13 > 0:17:17special speakers that do one job and they do it really well.
0:17:17 > 0:17:19But to make it work, they need to look like this.
0:17:22 > 0:17:26These speakers produce sounds at 19 hertz, the same deep
0:17:26 > 0:17:29frequency that the alligators bellow at.
0:17:29 > 0:17:34So let's see if they can entice a grumpy alligator to start flirting.
0:17:49 > 0:17:51So that's it.
0:17:51 > 0:17:54Those are the big infrasound speakers sending sound out
0:17:54 > 0:17:58over the lake here, and now we just have to wait and see
0:17:58 > 0:17:59if any of the alligators react.
0:18:09 > 0:18:12Oh, straight over there, tail up in the air, getting ready to call.
0:18:12 > 0:18:14ALLIGATOR BELLOWS
0:18:14 > 0:18:17There are two parts to this display.
0:18:17 > 0:18:20One is a deep but audible bellow from their mouths.
0:18:20 > 0:18:23ALLIGATOR BELLOWS
0:18:23 > 0:18:25It's like hearing dinosaurs.
0:18:27 > 0:18:30The other part is the water dance.
0:18:34 > 0:18:38This is produced by sound that is too low for us to hear.
0:18:38 > 0:18:42It's a really deep hum coming straight from
0:18:42 > 0:18:46the alligator's body, that makes the water dance at the surface.
0:18:53 > 0:18:57There's two things going on here. There's two indicators of size.
0:18:57 > 0:19:00And one of them is the infrasound itself,
0:19:00 > 0:19:04a noise that's really deep. You need to be big, like in the same way
0:19:04 > 0:19:06that a big bell makes a deeper noise.
0:19:06 > 0:19:10You need to have scale, size, to make that kind of deep noise.
0:19:10 > 0:19:13But the other thing is what the alligators are doing
0:19:13 > 0:19:14just before they call.
0:19:14 > 0:19:16They lift up their tail and their head
0:19:16 > 0:19:19and you can see the full length of the alligator, and they're big.
0:19:19 > 0:19:21These are enormous creatures.
0:19:25 > 0:19:26I'm feeling very small.
0:19:29 > 0:19:33Putting on a water dance requires huge amounts of energy.
0:19:33 > 0:19:34So why go to all that effort?
0:19:37 > 0:19:41To understand, I need to venture deeper into the gator's natural home.
0:19:45 > 0:19:48I'm on the north edge of the Florida Everglades
0:19:48 > 0:19:49and these wetlands stretch
0:19:49 > 0:19:53south for hundreds of kilometres from here.
0:19:53 > 0:19:58This place, where muddy brown water touches blue sky,
0:19:58 > 0:20:00is prime alligator territory.
0:20:07 > 0:20:11Alligators live on the boundary between air and water,
0:20:11 > 0:20:15in a low world where vision is obscured by tangled vegetation.
0:20:20 > 0:20:23So, to stand any chance of attracting a mate,
0:20:23 > 0:20:26males have to make sure they stand out.
0:20:29 > 0:20:32Imagine there's a female 300 metres away over there
0:20:32 > 0:20:35and an alligator here is calling.
0:20:35 > 0:20:37Sight isn't much good because she's too far away
0:20:37 > 0:20:41and there's too much in the way, but sound can travel through the
0:20:41 > 0:20:45water, and that is what the audible part of the alligator's bellow does.
0:20:45 > 0:20:49And when she's come in closer, the sound isn't as much use any more.
0:20:51 > 0:20:55But the water dance is splashing up above the surface of the water,
0:20:55 > 0:20:58so she can see that and go right to the male that produced it.
0:20:58 > 0:21:02For these ancient predators, the water dance is essential
0:21:02 > 0:21:04for survival.
0:21:07 > 0:21:11But the most extraordinary thing is how they use infrasound
0:21:11 > 0:21:12to put on the display.
0:21:14 > 0:21:18To show you, I'm going to create my own water dance.
0:21:28 > 0:21:30This is a Chinese singing bowl.
0:21:30 > 0:21:33They've been around for well over 2,000 years
0:21:33 > 0:21:35and the reason that they are special is that
0:21:35 > 0:21:39when you rub on the handles, you get this splashing from the bowl.
0:21:42 > 0:21:47That's because vibrations of the bowl send low-frequency sounds
0:21:47 > 0:21:48through the water.
0:21:48 > 0:21:52When it's loud enough, this causes the water surface to break
0:21:52 > 0:21:55into special waves called Faraday waves.
0:22:02 > 0:22:05Faraday waves are almost like a way of concentrating energy.
0:22:05 > 0:22:07Once they start to grow,
0:22:07 > 0:22:10they keep growing, and so if you rub on the bowl hard enough
0:22:10 > 0:22:14you make the amplitude of the waves loud enough, those Faraday waves
0:22:14 > 0:22:17get so high they start to spit little droplets of water upwards.
0:22:22 > 0:22:26And, incredibly, footage from our high-speed camera shows
0:22:26 > 0:22:29the alligators are also creating Faraday waves.
0:22:33 > 0:22:36So this makes it easier to see what's going on.
0:22:36 > 0:22:38The alligator's back is just below the surface of the water,
0:22:38 > 0:22:41its lungs are full so its body is really big.
0:22:43 > 0:22:48As the alligator starts to vibrate its lungs, the top of its back
0:22:48 > 0:22:52is acting like a piston, it's pushing up on the water above it
0:22:52 > 0:22:56and that's driving the surface into this splashing pattern.
0:22:56 > 0:22:57It's really dramatic.
0:23:00 > 0:23:02And you can see it takes a lot of energy
0:23:02 > 0:23:06because after they have called maybe seven or eight times they stop
0:23:06 > 0:23:08and they rest, they're exhausted.
0:23:19 > 0:23:25It's thought alligators have been calling like this for at least 70 million years,
0:23:25 > 0:23:29so they were doing it when the dinosaurs were around.
0:23:33 > 0:23:37And what stimulates them to call is hearing other alligators calling
0:23:37 > 0:23:42or other sources of infrasound, and that leads to something really cool
0:23:42 > 0:23:45because Cape Canaveral is just 70 miles that way. And when the shuttle
0:23:45 > 0:23:49was landing there, when there were shuttle flights, the infrasound
0:23:49 > 0:23:53from the sonic boom would set off the bellowing of the alligators,
0:23:53 > 0:23:56so it's like the space age touching the dinosaurs.
0:24:13 > 0:24:17As we move into the lower part of the sound spectrum that human ears
0:24:17 > 0:24:22can hear, sounds above 20 hertz still travel long distances.
0:24:25 > 0:24:29But these deeper tones don't just move through air,
0:24:29 > 0:24:31they also travel through the ground.
0:24:35 > 0:24:38And in Southern Africa's Namib Desert,
0:24:38 > 0:24:42one bizarre little predator can hear so brilliantly
0:24:42 > 0:24:47underground that they can find tiny prey in this vast expanse of sand.
0:24:50 > 0:24:52The golden mole.
0:24:53 > 0:24:55They're such weird looking animals.
0:24:55 > 0:24:59They've got no eyes and no external ears
0:24:59 > 0:25:02and they spend most of their time beneath the ground.
0:25:02 > 0:25:05And yet they can do something truly remarkable.
0:25:05 > 0:25:09They can hear the faintest of sounds through the sand.
0:25:10 > 0:25:14In fact, their hearing is so sensitive they can find
0:25:14 > 0:25:17a tiny termite from 20 metres away.
0:25:21 > 0:25:25Golden moles feed on termites and other small insects.
0:25:27 > 0:25:31To show how they find them, I first need to track down a golden mole.
0:25:33 > 0:25:37That's not easy, because they're very shy
0:25:37 > 0:25:39and only active at night.
0:25:41 > 0:25:43But they do leave distinctive tracks.
0:25:44 > 0:25:49As they travel across the sand, they leave these strange indentations
0:25:49 > 0:25:52every few metres, and that gives us
0:25:52 > 0:25:56a clue as to how this blind creature is finding the termites.
0:25:56 > 0:25:59What they're doing is dipping their head into the sand
0:25:59 > 0:26:03and listening in for vibrations that are travelling through the ground.
0:26:09 > 0:26:12The mole's trail ends at a grass mound,
0:26:12 > 0:26:15so I've cordoned off the area and left it overnight.
0:26:20 > 0:26:23And look...at that.
0:26:25 > 0:26:28It's the cutest animal I've ever seen, look.
0:26:30 > 0:26:35Perfectly shaped for swimming through sand, wedge-shaped head.
0:26:36 > 0:26:41Even though it's so tiny, you can feel the power in those front legs,
0:26:41 > 0:26:43perfectly adapted for swimming through sand.
0:26:46 > 0:26:50And incredibly, you can't see any eyes.
0:26:50 > 0:26:55And that's because from a young age, their eyelids fuse over
0:26:55 > 0:26:58and where their eyes would be, just covered in fur.
0:27:01 > 0:27:05And because it can't see, it relies entirely on its sense of hearing.
0:27:06 > 0:27:08Such a beautiful animal.
0:27:12 > 0:27:17Golden moles may be able to hear me coming, but there's no way
0:27:17 > 0:27:21they could detect a termite's footsteps from 20 metres.
0:27:21 > 0:27:23So how do they do it?
0:27:23 > 0:27:27Well, one bizarre theory suggests the moles are actually listening out
0:27:27 > 0:27:30for the sound of grass blowing in the breeze.
0:27:33 > 0:27:37The desert landscape is constantly shifting and changing.
0:27:37 > 0:27:40The only fixed points are these tussock grasses
0:27:40 > 0:27:43and it's in these mounds beneath the grass
0:27:43 > 0:27:45that the termites make their home.
0:27:46 > 0:27:51So could the golden moles really be detecting the sound of tussock grass
0:27:51 > 0:27:53and using it to track down termites?
0:28:01 > 0:28:03There's only one way to find out.
0:28:03 > 0:28:06I'm going to play the sound of blowing grass through
0:28:06 > 0:28:10the ground and see if the golden mole approaches.
0:28:12 > 0:28:15But first, I've got to record it.
0:28:26 > 0:28:27DISTANT-SEEMING RUMBLE
0:28:27 > 0:28:30Wow, that's such an alien sound.
0:28:30 > 0:28:32CREAKING
0:28:35 > 0:28:38It's kind of a low knocking sound and that's perfect for
0:28:38 > 0:28:41the golden mole, because low frequencies travel
0:28:41 > 0:28:43really well through the ground.
0:28:48 > 0:28:51And inside the golden mole's skull,
0:28:51 > 0:28:54there's a clue that suggests they may be tuned into these
0:28:54 > 0:28:57low-frequency sounds.
0:28:57 > 0:29:00This is a 3-D model of the inner ear of a golden mole,
0:29:00 > 0:29:03it's been enlarged by about 15 times.
0:29:03 > 0:29:06Now, this section of the ear is responsible for converting
0:29:06 > 0:29:10vibrational energy into nerve impulses that the brain
0:29:10 > 0:29:11can interpret.
0:29:11 > 0:29:14And the section that we are most interested in is right here,
0:29:14 > 0:29:18this coiled area, known as a cochlea.
0:29:18 > 0:29:20Now, in the golden mole this area is
0:29:20 > 0:29:24twice as long as it is in European moles, and it's thought that it
0:29:24 > 0:29:29helps extend the hearing range into lower frequencies.
0:29:29 > 0:29:31Think of it as a piano.
0:29:31 > 0:29:35If you've got an extended number of keys, you can play lower
0:29:35 > 0:29:36and lower octaves.
0:29:38 > 0:29:42It's time to put our golden mole's low frequency hearing to the test.
0:29:46 > 0:29:51I find an area of sand free from tussock grass and set up
0:29:51 > 0:29:54a rig of night-time cameras that can be monitored remotely.
0:29:57 > 0:30:00And this is our key piece of kit. It's a transducer.
0:30:00 > 0:30:05I'm going to use this to play back the sound of the tussock grass
0:30:05 > 0:30:07I recorded earlier.
0:30:07 > 0:30:11If I put my hand on that speaker, I can feel the gentle vibrations
0:30:11 > 0:30:13that are being played out through the sand.
0:30:16 > 0:30:19So, if the tussock grass theory is correct,
0:30:19 > 0:30:23our mole should associate this sound with termites
0:30:23 > 0:30:25and move towards it.
0:30:25 > 0:30:27Let's see if it works.
0:30:39 > 0:30:41Oh, look.
0:30:42 > 0:30:43You can see he's moving.
0:30:47 > 0:30:48And there.
0:30:48 > 0:30:50That's the behaviour we are looking for,
0:30:50 > 0:30:52that classic head-dipping movement.
0:30:53 > 0:30:57The head just couples with the sand perfectly and the vibrations
0:30:57 > 0:31:01of the sound waves travel really well
0:31:01 > 0:31:04and that is what it's picking up. That's what it's detecting.
0:31:07 > 0:31:10Here we go. He's running around, really fast.
0:31:16 > 0:31:18Oh, look, he's just darted out of frame!
0:31:19 > 0:31:23It seems we've lost our mole to the open desert.
0:31:27 > 0:31:29But minutes later, he's back.
0:31:29 > 0:31:32Oh, look, there he is!
0:31:32 > 0:31:35He's just run in, dipped his head in the sand.
0:31:36 > 0:31:39He's just run off again.
0:31:39 > 0:31:41I mean, he hasn't gone directly to the speaker,
0:31:41 > 0:31:44but he's, kind of, gone in that general direction.
0:31:44 > 0:31:48By head dipping so close to the transducer,
0:31:48 > 0:31:52it seems the mole was attracted to the sound of the tussock grass.
0:31:52 > 0:31:56Perhaps, tonight, he just wasn't hungry.
0:31:59 > 0:32:03But over a few nights in this remote desert,
0:32:03 > 0:32:06I gain a unique insight into the secret lives
0:32:06 > 0:32:08of these rare and shy little mammals...
0:32:17 > 0:32:23..including a mole struggling to find a termite on the surface,
0:32:23 > 0:32:27until he burrows down to listen to where the sound is coming from.
0:32:37 > 0:32:40By tapping into this hidden world of underground sound,
0:32:40 > 0:32:44the Golden Mole has become master of these sand dunes.
0:32:44 > 0:32:48Who'd have thought the sound of grass blowing in the wind
0:32:48 > 0:32:51would be the secret of desert survival?
0:32:54 > 0:33:00BIRDSONG
0:33:01 > 0:33:03As sound gets higher in pitch,
0:33:03 > 0:33:05our ears become much better at detecting it.
0:33:06 > 0:33:11Our hearing is most sensitive around 1,000 Hertz,
0:33:11 > 0:33:13the frequency range around human speech.
0:33:14 > 0:33:18But many animals also tap into these frequencies...
0:33:21 > 0:33:23..nowhere more so than the tropical rainforest.
0:33:23 > 0:33:27BIRDSONG AND MONKEY CALLS
0:33:35 > 0:33:41In this dense, tangled world, animals can be heard, but rarely seen.
0:33:43 > 0:33:47So, there's an acoustic battle for the airwaves,
0:33:47 > 0:33:50as creatures fight to make themselves heard.
0:33:56 > 0:34:00In the jungles of Puerto Rico, the calls of one surprising creature
0:34:00 > 0:34:02drown out all others.
0:34:03 > 0:34:06They're the giants of this acoustic world.
0:34:06 > 0:34:09They're almost as loud as a pneumatic drill
0:34:09 > 0:34:13and if it wasn't for a really clever evolutionary adaptation,
0:34:13 > 0:34:16they'd deafen themselves with their own call.
0:34:16 > 0:34:22Meet the Coqui Frog, thought to be the loudest amphibian on the planet.
0:34:24 > 0:34:29A fully-grown Coqui Frog is around the size of a 2p piece,
0:34:29 > 0:34:34but what they lack in size, they definitely make up for in volume.
0:34:38 > 0:34:41So, what's driven these little frogs to become so loud?
0:34:48 > 0:34:51An extraordinary piece of recording technology,
0:34:51 > 0:34:55that lets me SEE sound, will help me find out.
0:34:59 > 0:35:01This is an acoustic camera.
0:35:01 > 0:35:05It's got 48 microphones arranged around a normal camera in the middle
0:35:05 > 0:35:08and what it lets us do is to take the normal images
0:35:08 > 0:35:11and overlay on top of them where the sound is coming from.
0:35:11 > 0:35:14So, this is going to help me find Coqui Frogs,
0:35:14 > 0:35:16when everything around me is pitch black.
0:35:19 > 0:35:25The acoustic camera also records the intensity, or loudness, of sound,
0:35:25 > 0:35:26measured in decibels.
0:35:28 > 0:35:32At the volume I'm talking at the moment, the computer is registering
0:35:32 > 0:35:37about 70 decibels, but if I clap, it will register 90.
0:35:37 > 0:35:40So, by pointing this in the darkness, we'll get a direct measure
0:35:40 > 0:35:42of how loud these little frogs really are.
0:35:42 > 0:35:46BIRDSONG AND MONKEY CALLS
0:35:48 > 0:35:51The Coqui chorus starts around sunset.
0:35:55 > 0:35:58And the noise they make is overwhelming
0:35:58 > 0:36:00and comes from all directions.
0:36:00 > 0:36:03LOUD CHIRRUPING
0:36:03 > 0:36:05They're all around me.
0:36:05 > 0:36:06There's one.
0:36:07 > 0:36:09So loud.
0:36:11 > 0:36:15I feel like I must be being stared at by millions of frogs,
0:36:15 > 0:36:17because there's clearly so many of them.
0:36:17 > 0:36:21It's the male frog's call that gives them their name.
0:36:22 > 0:36:25There are two parts - the "co" and the "kee".
0:36:28 > 0:36:31The "co" warns off rival males,
0:36:31 > 0:36:36while the "kee" lets any females nearby know he's available.
0:36:36 > 0:36:38CO-KEE SOUND
0:36:38 > 0:36:42First, I want to record just how loudly a frog can call.
0:36:44 > 0:36:46So, there he is, our calling frog.
0:36:46 > 0:36:51We've moved the acoustic camera in quite close
0:36:51 > 0:36:55and we're, pretty much, a metre away from him now.
0:36:56 > 0:36:58And the fact that we're so close
0:36:58 > 0:37:01means that we can actually measure how loud he is.
0:37:04 > 0:37:09CO-KEE SOUND
0:37:09 > 0:37:14This little frog is calling at nearly 80 decibels,
0:37:14 > 0:37:17but they have been recorded up to 95.
0:37:21 > 0:37:25For their size, they're one of the noisiest creatures on Earth,
0:37:25 > 0:37:30the equivalent of two and a half times louder than a lion's roar
0:37:30 > 0:37:33and three times louder than an elephant.
0:37:35 > 0:37:39So, how does such a tiny creature make such a massive noise?
0:37:41 > 0:37:46The secret lies in the balloon-like vocal sac.
0:37:48 > 0:37:52And you can see that, as he pushes air out of his lungs,
0:37:52 > 0:37:55it goes into that big vocal sac and back again.
0:37:55 > 0:38:00And what the vocal sac is doing is acting like
0:38:00 > 0:38:03the sound board on a guitar, the front face for a guitar,
0:38:03 > 0:38:07and it's helping to transmit that sound really efficiently
0:38:07 > 0:38:09into the outside world.
0:38:11 > 0:38:15I love watching his body work like that.
0:38:17 > 0:38:22In fact, Coqui Frogs are so loud they should deafen themselves,
0:38:22 > 0:38:25But they don't, thanks to a bizarre adaptation.
0:38:28 > 0:38:33Inside the frog, the lungs and the vocal sac are connected to the ears.
0:38:35 > 0:38:40That means that, when he calls, the sound travels out into the air,
0:38:40 > 0:38:42but also through the frog's body.
0:38:44 > 0:38:48If the call was just hitting the eardrum from one side,
0:38:48 > 0:38:50it would rupture it.
0:38:53 > 0:38:57But because the sound hits the eardrum from inside and outside
0:38:57 > 0:39:01the frog's body at the same time, the effect is cancelled out.
0:39:08 > 0:39:11But why have these frogs in the jungles of Puerto Rico
0:39:11 > 0:39:14pushed sound to such extremes?
0:39:16 > 0:39:18The acoustic camera reveals a possible answer.
0:39:18 > 0:39:22This is brilliant, because it's a completely different way
0:39:22 > 0:39:25of understanding what's going on.
0:39:25 > 0:39:30When I look out there, you know, I see blackness and leaves,
0:39:30 > 0:39:34but when I look here, there's these really bright splotches of light
0:39:34 > 0:39:36and those are these little frogs.
0:39:36 > 0:39:40The camera reveals the sheer density of frogs.
0:39:40 > 0:39:45There can be 80 in an area the size of a tennis court.
0:39:45 > 0:39:51With so many frogs calling, they've had to become louder
0:39:51 > 0:39:53and louder to make themselves heard.
0:39:55 > 0:39:59It's like being at a crowded party, when you raise your voice
0:39:59 > 0:40:02to be heard, but so does everyone else, so you end up shouting.
0:40:05 > 0:40:09This acoustic arms race may explain why the Coqui Frog
0:40:09 > 0:40:11is so exceptionally loud.
0:40:14 > 0:40:17I'm completely bathed in sound.
0:40:19 > 0:40:21For the Puerto Ricans, this is the sound of home,
0:40:21 > 0:40:24but for the frogs, it's different.
0:40:24 > 0:40:27From their point of view, what's surrounding you
0:40:27 > 0:40:32is an organised, precise, flow of information.
0:40:33 > 0:40:38And if you want to survive out here, understanding the information
0:40:38 > 0:40:41that all this sound is giving you is essential.
0:40:45 > 0:40:49COQUI FROGS CHIRP
0:40:50 > 0:40:54As we journey further up the sound spectrum,
0:40:54 > 0:40:56our ears become less sensitive.
0:40:56 > 0:40:59We don't hear high-pitched sounds very well.
0:41:02 > 0:41:04Our countryside is full of sounds,
0:41:04 > 0:41:07like birdsong, that we can appreciate.
0:41:07 > 0:41:12But it's also awash with the squeaks of small mammals, like voles.
0:41:13 > 0:41:18It's just that our ears can't detect them, unless we're really close.
0:41:20 > 0:41:24But we have one amazing creature that can hear these tiny sounds
0:41:24 > 0:41:26from great distances away.
0:41:29 > 0:41:30The Barn Owl.
0:41:32 > 0:41:36For their young to survive, a pair of adults must catch
0:41:36 > 0:41:403,000 voles a year. That's eight every single night.
0:41:43 > 0:41:46And the only information they've got to go on
0:41:46 > 0:41:50are the little squeaks of the voles and the rustling, as they
0:41:50 > 0:41:53move around in the undergrowth. It's not much. So, when the owls
0:41:53 > 0:41:57are out hunting, they're floating over a landscape like this
0:41:57 > 0:42:00and it's not enough for them to know that dinner is out there somewhere.
0:42:00 > 0:42:03They need to be able to pinpoint it accurately.
0:42:03 > 0:42:06They want to pounce and get the vole first time.
0:42:11 > 0:42:16But how do they pinpoint prey to the millimetre in this open landscape,
0:42:16 > 0:42:19just using their ears?
0:42:23 > 0:42:25Usually, when we think of good hearing,
0:42:25 > 0:42:27we think about things with big ears.
0:42:27 > 0:42:31We associate having big ears with being able to hear better.
0:42:31 > 0:42:36Now, this owl has fabulous hearing, but it doesn't have external ears.
0:42:36 > 0:42:41If you look at these feathers here, this thick ring around,
0:42:41 > 0:42:45that defines the facial disc and they're basically forming
0:42:45 > 0:42:48a cup, just like when you put your hand behind ear, and they're
0:42:48 > 0:42:51doing the job that our ears do, but they are built into his face.
0:42:51 > 0:42:55And this dish here, this dish of feathers,
0:42:55 > 0:42:59is directing sound into his ears and it's directional.
0:42:59 > 0:43:01If an owl looks at you, it's listening to you.
0:43:03 > 0:43:06I want to put the owl's hearing to the test,
0:43:06 > 0:43:08so I've got a phone with an unusual ring tone.
0:43:08 > 0:43:11SOFT SQUEAKING
0:43:14 > 0:43:17These sounds are the high-pitched squeaks that voles make and
0:43:17 > 0:43:21the rustling sound that you get as they move around in the undergrowth.
0:43:21 > 0:43:24So, that's what an owl's got to listen out for if it wants dinner.
0:43:26 > 0:43:28I'm going to hide the phone in the long grass in just the sort
0:43:28 > 0:43:31of a place where a vole might be and then I'm going to hide and call
0:43:31 > 0:43:35the phone and, when it rings, that squeaking noise will start
0:43:35 > 0:43:39and we will see whether the owl can locate it just using that sound.
0:43:41 > 0:43:44But before I let the owl loose on the phone, I'm going to see
0:43:44 > 0:43:49how I get on with locating this faint sound.
0:43:49 > 0:43:52To help me out, I've got a piece of owl-like technology.
0:43:55 > 0:43:57This is a parabolic microphone
0:43:57 > 0:44:00and the reason I've got it is that the shape of the inside
0:44:00 > 0:44:04of it is similar to the shape of the owl's feathers, that facial disc.
0:44:07 > 0:44:09So, just let me call the phone here.
0:44:12 > 0:44:16S, the phone's about 60 or 70 metres over there and it's ringing,
0:44:16 > 0:44:19but I can't hear anything.
0:44:19 > 0:44:20Let's see if this'll help.
0:44:29 > 0:44:33WIND WHISTLING AND BIRDSONG
0:44:33 > 0:44:38So, there's a little bit of birdsong in there, as well.
0:44:38 > 0:44:39SOFT SQUEAKING
0:44:39 > 0:44:41That's it there,
0:44:41 > 0:44:45the squeaking, and if I move the dish even a little bit
0:44:45 > 0:44:47to either side, it's gone.
0:44:51 > 0:44:55There's a surprising reason this parabolic microphone and the owl's
0:44:55 > 0:44:59facial disc are both so effective at picking up these squeaks.
0:45:01 > 0:45:03It's all to do with the pitch of the sound.
0:45:06 > 0:45:08This parabolic shape has a cut-off frequency,
0:45:08 > 0:45:12so it doesn't work for very low frequencies and for the owls
0:45:12 > 0:45:17that cut-off is at about 3,000 hertz, so if you tap a very thin
0:45:17 > 0:45:21wine glass with a spoon, that's about that sort of note, 3,000 hertz.
0:45:21 > 0:45:25So, above that, the owl's got really good directional hearing.
0:45:25 > 0:45:28Below that, it doesn't hear as well and that's actually really useful,
0:45:28 > 0:45:30because the rustling and squeaking
0:45:30 > 0:45:33is at those high frequencies and all the background noise,
0:45:33 > 0:45:37the low frequencies that might be distracting, they're all cut out.
0:45:38 > 0:45:42So, now let's see how our Barn Owl gets on with locating the vole phone.
0:45:42 > 0:45:46It's hidden in the grass about 60 metres away,
0:45:46 > 0:45:48with a small camera close by.
0:45:49 > 0:45:52This is just the time of day when owls would hunt.
0:45:52 > 0:45:55The voles are starting to come out.
0:46:00 > 0:46:02The owl quickly responds.
0:46:06 > 0:46:10Its facial disc helps filter out background noise,
0:46:10 > 0:46:13so it can focus on the high-pitched squeak from our phone.
0:46:13 > 0:46:16SOFT SQUEAKING
0:46:19 > 0:46:22Then it strikes.
0:46:22 > 0:46:25SQUEAKING
0:46:29 > 0:46:32So, our owl got it. It did the job.
0:46:32 > 0:46:34And the fact that it was a phone ring tone it found
0:46:34 > 0:46:37showed that it couldn't have done it by smell and it couldn't have
0:46:37 > 0:46:39done it by sight, it must have been using its hearing.
0:46:39 > 0:46:43And it pinpointed it so accurately, swooped right down in on it.
0:46:47 > 0:46:50How did it do this with such precision?
0:46:53 > 0:46:57By comparing minuscule time differences between the sound
0:46:57 > 0:46:59hitting the left and right ear,
0:46:59 > 0:47:03they work out which direction that sound is coming from.
0:47:04 > 0:47:09But whilst our ears are symmetrical, the barn owls ears are skewed.
0:47:13 > 0:47:16He's got one ear on each side of his face,
0:47:16 > 0:47:17but they're not in the same place.
0:47:17 > 0:47:21The one on this side, on the right, is just below his eye,
0:47:21 > 0:47:24and that, combined with the shape of the facial disc,
0:47:24 > 0:47:27is mostly listening to sound that is coming from above,
0:47:27 > 0:47:31and the other side, the ear is just above his eye,
0:47:31 > 0:47:34and the facial disc is funnelling mostly sound from below.
0:47:36 > 0:47:39So, by listening and comparing the sound coming in both ears,
0:47:39 > 0:47:43he can tell how high or low something is coming from
0:47:43 > 0:47:46and that, combined with his ability to tell where
0:47:46 > 0:47:50things are horizontally, is what lets him pinpoint his prey.
0:47:55 > 0:47:58The Barn Owl's amazing hearing has allowed it to become the most
0:47:58 > 0:48:02widespread and successful owl species on Earth.
0:48:05 > 0:48:09We're all really familiar with owls and the image of an owl,
0:48:09 > 0:48:13but now look at an owl and see it for what it is.
0:48:13 > 0:48:18It's got this face, a dish which is collecting sound.
0:48:21 > 0:48:25And isn't that just a fantastic idea that instead of having ears
0:48:25 > 0:48:27that stick out which would get in the way if you flew,
0:48:27 > 0:48:29it's all built into his face?
0:48:38 > 0:48:43But even owls, with their extreme auditory adaptations,
0:48:43 > 0:48:48are unable to hear the sounds at the highest end of the spectrum.
0:48:48 > 0:48:51This is where our ears stop working completely.
0:48:52 > 0:48:56Our ability to hear high-pitched sounds changes
0:48:56 > 0:48:57throughout our lives.
0:48:57 > 0:49:00We start out being able to hear really high frequencies
0:49:00 > 0:49:03and then this decreases with age.
0:49:03 > 0:49:06So someone in their sixties will be able to hear up
0:49:06 > 0:49:07HIGH-PITCHED RINGING
0:49:07 > 0:49:11to around 10,000 hertz, around there.
0:49:11 > 0:49:14But someone in their 20s can probably hear up to
0:49:14 > 0:49:22around 16,000 hertz and actually my hearing starts to go around 15.
0:49:22 > 0:49:25But it's only young children that can hear even higher frequencies,
0:49:25 > 0:49:27up to 20,000 hertz.
0:49:29 > 0:49:30I can't hear anything there.
0:49:30 > 0:49:35Now any sound above this is referred to being ultrasonic,
0:49:35 > 0:49:37which means it is above the human hearing range.
0:49:38 > 0:49:42But for some animals, there are great advantages to hearing
0:49:42 > 0:49:46and calling at this extreme end of the spectrum.
0:49:47 > 0:49:52If an animal can call at a frequency that its predators can't hear,
0:49:52 > 0:49:56but members of its own species can, then this opens up a secret
0:49:56 > 0:50:00channel of sound that they can use to communicate.
0:50:00 > 0:50:02And around the world there are a few animals
0:50:02 > 0:50:04that have tapped into this strategy perfectly.
0:50:07 > 0:50:10In the pine forests of Canada,
0:50:10 > 0:50:13Flying Squirrels produce ultrasonic alarm calls.
0:50:15 > 0:50:19At 50,000 hertz, this is way above our hearing range
0:50:19 > 0:50:21and that of their predators.
0:50:24 > 0:50:28In South East Asia, Tarsiers push their calls to even greater
0:50:28 > 0:50:31extremes, up to 70,000 hertz.
0:50:35 > 0:50:38But one group of alien-like creatures
0:50:38 > 0:50:40reaches even higher pitches.
0:50:42 > 0:50:44Katydids, or Bush Crickets.
0:50:45 > 0:50:48Their secretive love songs have been
0:50:48 > 0:50:52recorded at a staggering 150,000 hertz.
0:50:57 > 0:51:02We can only hear the sound by slowing it down 30 times.
0:51:04 > 0:51:09They produce these extreme pitches by rubbing the wing cases together.
0:51:13 > 0:51:16But there's one group of animals that have pushed sound
0:51:16 > 0:51:18higher than any other.
0:51:20 > 0:51:21Bats.
0:51:22 > 0:51:26Their ultrasonic pulses have been recorded at over
0:51:26 > 0:51:28200,000 hertz.
0:51:31 > 0:51:35Bats don't just use these extreme frequencies to communicate.
0:51:35 > 0:51:37They use them to see their world.
0:51:41 > 0:51:46I'm stood in complete darkness and I can't see a single thing
0:51:46 > 0:51:48and the only reason you can see me is
0:51:48 > 0:51:50because we are filming with a special infrared camera.
0:51:50 > 0:51:55But I know that I'm not alone here, because I can hear and feel
0:51:55 > 0:51:59the wing beats of these Egyptian fruit bats as they fly past my head.
0:52:02 > 0:52:07The bats can navigate through this flight enclosure in complete
0:52:07 > 0:52:09darkness using echolocation.
0:52:17 > 0:52:20As the bats fly past, they click their tongues really loudly
0:52:21 > 0:52:23and produce a high-frequency pulse.
0:52:23 > 0:52:27High-frequency sound echoes off objects really effectively
0:52:27 > 0:52:29and precisely.
0:52:29 > 0:52:33So, the bat's pulses are reflecting off the sides of the enclosure
0:52:33 > 0:52:35and my body.
0:52:36 > 0:52:39And by detecting these echoes, they're able to build up
0:52:39 > 0:52:42an acoustic image of the world around them.
0:52:42 > 0:52:44That's how they're avoiding me.
0:52:46 > 0:52:50But what does it actually mean to see the world through sound?
0:52:50 > 0:52:53Well, it's only now that we are getting our first
0:52:53 > 0:52:55glimpse into this alien world.
0:53:08 > 0:53:10In an ancient British woodland,
0:53:10 > 0:53:13a futuristic experiment is under way.
0:53:16 > 0:53:21This is the inaugural test flight of the batcopter,
0:53:21 > 0:53:24a machine that will eventually allow us to see like a bat.
0:53:28 > 0:53:32This strange-looking machine is part bat,
0:53:32 > 0:53:36part drone, and as it flies through the forest, it's blasting
0:53:36 > 0:53:40ultrasonic pulses, just like a real bat.
0:53:43 > 0:53:45This is just one of the techniques
0:53:45 > 0:53:49Dr Marc Holderied is using to visualise how bats see their world
0:53:49 > 0:53:51through sound.
0:53:51 > 0:53:54So, Marc, you've got this really impressive machine here.
0:53:54 > 0:53:56Can you tell us a bit about it and what it does?
0:53:56 > 0:53:59So, this is our Octocopter here, which is a drone platform, and we
0:53:59 > 0:54:04use it to carry around this grey box here, which is our artificial bat.
0:54:04 > 0:54:06It has a little mouth here, loudspeaker, that sends out very
0:54:06 > 0:54:11high-intensity ultrasound into the habitat that we want to survey.
0:54:11 > 0:54:13Above here, we have an area of 31 microphones,
0:54:14 > 0:54:18which capture the echoes coming back from the environment,
0:54:18 > 0:54:21so this is why we call it our 31-ear bat, really.
0:54:24 > 0:54:27I want to be able to produce a visualisation that tells me
0:54:27 > 0:54:29what a bat has seen.
0:54:29 > 0:54:31I want to be a bat.
0:54:35 > 0:54:39Marc is still fine-tuning the batcopter's acoustic image.
0:54:39 > 0:54:42But using a different technique he believes he's got a good
0:54:42 > 0:54:44idea of what it will look like.
0:54:46 > 0:54:50By 3D mapping a stretch of woodland with a laser,
0:54:50 > 0:54:53and tracking the flight paths of bats flying through,
0:54:53 > 0:54:57he's created these astonishing visualisations.
0:55:02 > 0:55:05Wow, these images are just phenomenal.
0:55:05 > 0:55:08It's like something out of a sci-fi movie.
0:55:08 > 0:55:09What we looking at here?
0:55:09 > 0:55:12So, this is a cockpit view, flythrough,
0:55:12 > 0:55:15of a real bat flying through a real forest.
0:55:15 > 0:55:17We have slowed this down by a factor of five.
0:55:18 > 0:55:21A bat would experience and fly through this at five times
0:55:21 > 0:55:25the speed that we are looking at, at the moment.
0:55:25 > 0:55:27These images are a visual
0:55:27 > 0:55:30representation of what the bat's world looks like.
0:55:32 > 0:55:35But Marc wants to know how the same scene would appear
0:55:35 > 0:55:37through echolocation.
0:55:38 > 0:55:41So, he works out which of the objects in the flight path would
0:55:41 > 0:55:45reflect the bat's high-frequency pulses.
0:55:45 > 0:55:48This gives him the bat's acoustic image.
0:55:50 > 0:55:53So what we've done is taken away all the surfaces, all the reflection
0:55:53 > 0:55:57that wouldn't really scatter back sound and you see it really
0:55:57 > 0:56:02dissolves into individual reflectors but it still works really well.
0:56:02 > 0:56:04And this is just trying to navigate.
0:56:04 > 0:56:07That's what we have to remember, this is just navigation
0:56:07 > 0:56:10let alone trying to find your prey in the dark.
0:56:10 > 0:56:14Yes, so imagine, one tiny insect, less than a centimetre,
0:56:14 > 0:56:16and this is what you are after.
0:56:16 > 0:56:19You have to find dozens if not hundred of these every night.
0:56:19 > 0:56:22I wouldn't know how to do it and I'm still puzzled
0:56:22 > 0:56:25and amazed by the fact that they can.
0:56:25 > 0:56:29It's astonishing to think what bats achieve using a simple
0:56:29 > 0:56:32acoustic image like this.
0:56:34 > 0:56:41They can fly through dense woodland in pitch black,
0:56:41 > 0:56:43grab motionless spiders from their webs,
0:56:51 > 0:56:54and even pluck fish from beneath the surface of the water.
0:57:10 > 0:57:13It's the fact that ultrasound reflects really well off small
0:57:13 > 0:57:17objects that allows bats to use their echolocation with such
0:57:17 > 0:57:19deadly precision.
0:57:27 > 0:57:31It's captivating to get this first glimpse of what it
0:57:31 > 0:57:33means to see through sound.
0:57:35 > 0:57:40This is as close as we've got to entering the bat's acoustic world.
0:57:40 > 0:57:43By tapping into the power of high frequency sound,
0:57:43 > 0:57:47bats have become masters of the night.
0:57:53 > 0:57:57In this episode, we've journeyed through the natural world of sound.
0:58:00 > 0:58:01From the deepest bellows...
0:58:04 > 0:58:06..the loudest calls...
0:58:08 > 0:58:11..to ears tuned only to the highest pitches.
0:58:14 > 0:58:15Across the planet,
0:58:15 > 0:58:20animals have found extraordinary ways of using sound to survive.
0:58:23 > 0:58:28Next time, we explore the invisible world of scent...
0:58:30 > 0:58:32..and discover the bizarre ways
0:58:32 > 0:58:36animals use their sense of smell to get an edge in the wild.