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Our human senses are incredible.
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We have excellent vision...
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..precise hearing...
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..and can detect the slightest
fragrance drifting on the breeze.
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But we only experience a tiny
fraction of what's out there.
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Imagine a world where
you could see with sound.
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These images are just phenomenal.
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Hear storms from hundreds
of kilometres away.
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That's incredible.
They've all stopped.
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Imagine seeing the world
in slow motion
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or through some of the sharpest
eyes in nature.
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HE GASPS
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So fast!
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Travelling to some of the wildest
places on Earth...
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..we reveal the strange and
wonderful world of animal senses.
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Light is emitted. Look at that.
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Another one!
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This is brilliant.
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I'm Dr Helen Czerski.
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I'm a physicist,
and I want to find out how animals
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tap into an amazing range
of light, scent and sound.
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I'm Patrick Aryee.
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As a biologist I'm fascinated
by what the world appears like
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through animal senses
far superior to our own.
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Experience the world
through animal senses.
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Wherever we are, no matter how
tranquil it seems,
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we are constantly
surrounded by sound.
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Our ears are incredibly sensitive
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and hear a huge range of tones.
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But still, we detect only a tiny
fraction of the sounds around us.
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I've come to Mexico's Sea Of Cortez,
where two very different creatures
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have pushed sound to extremes.
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One very low pitched,
the other incredibly high.
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I can hear
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..squeaks and whistles
and occasional series of clicks.
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And it's really busy.
It sounds like a busy city street.
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DOLPHIN 'CLICKS'
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Dolphins see their world
through sound.
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Their high-frequency clicks
reflect off objects around them,
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allowing them to build up
an acoustic image.
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This is nature's sonar.
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And the thing is, I'm only hearing
a tiny bit of all the sound
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that's down there, because most of
the dolphins' calls are at
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frequencies above my hearing range.
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These extreme high frequencies
are known as ultrasound,
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meaning they're too high
for our ears to detect.
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But here in the same waters,
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other mammals operate at the other
extreme of the sound spectrum.
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LOW RUMBLE
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Blue whales.
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Their songs are infrasonic -
too low for our ears to detect.
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It's only when they're sped up
that we can hear them.
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WHALE MAKES LOW RUMBLING
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These deep, haunting songs
allow them to keep
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in touch with each other
over hundreds of kilometres.
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Whales and dolphins operate
on the outer limits of the spectrum,
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but across the planet,
animals are tuned into every
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frequency of sound in-between.
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In this episode, we're going on a
journey through the world of sound,
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from the deep sounds,
far lower than the ones we can hear,
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up to where the dolphins are calling
at frequencies far higher than
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we can hear, and there are ways
of perceiving sound
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that are way beyond
our human capabilities.
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Prepare to enter a bizarre world
of sound,
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beyond human hearing.
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Our journey starts in Southern
Africa, where one of nature's
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true giants makes the deepest
sounds of any land animal.
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LOW RUMBLE
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The African elephant.
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The frequency, or pitch of sound,
is measured in hertz,
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and their low rumbles
reach around 250 hertz.
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00:05:56,508 --> 00:06:01,708
But elephants also produce
and hear sounds below 20 hertz.
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These are sounds our ears struggle
to detect, called infrasound.
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00:06:11,468 --> 00:06:16,068
Infrasound travels a long way,
so elephants use it
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to keep in touch with each other
over many kilometres.
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RUMBLING
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But it's now suspected they also
use their infrasonic hearing to
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listen in to a secret sound
of our planet.
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THUNDERCLAPS
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We can hear thunderstorms from 20,
occasionally 30 kilometres away
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but it's now thought
that elephants can hear them
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from distances of up to
500 kilometres.
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That's roughly the equivalent
of someone in London
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listening to a storm in Edinburgh.
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This may seem impossible
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but at the end of the dry season,
elephants often make sudden
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and unpredictable changes
in direction.
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For no obvious reason, herds turn
and march for days on end.
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00:07:19,308 --> 00:07:22,908
But when checked against weather
records, it seems the elephants
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are heading towards rainstorms
up to 500 kilometres away.
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I want to find out
if this is a coincidence
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or whether elephants really can
recognise the deep,
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infrasonic sounds of a storm
over vast distances.
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We're going to try something
that has never been done before.
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We're going to play the infrasonic
part of a thunderstorm to a herd
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of elephants and see just
how they react.
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00:07:56,508 --> 00:07:59,948
And to do that, we're going
to take a camper van
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and turn it into a giant speaker.
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'It may seem an unusual
choice of speaker,
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'but broadcasting infrasound
requires large volumes of air
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00:08:13,748 --> 00:08:17,588
'and the inside of this camper van
offers the perfect space.
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'So with the help of infrasonic
expert Bruce Thigpen,
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'we're transforming it
into a massive subwoofer.'
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00:08:27,148 --> 00:08:30,828
So can we use this camper van,
this infrasonic speaker,
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to replicate the sound
of a thunderstorm?
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Yes, we have an actual thunderstorm
recording of thunderclaps,
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the rumble of the sound
after the lightning strike.
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We've recorded that,
we've took an actual recording
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and we filtered it, so it just
plays the lowest frequencies.
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So, even though we're going to be
quite close to the elephants, our
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00:08:51,948 --> 00:08:55,388
infrasonic speaker is going to play
the sound of a distant thunderstorm.
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Exactly.
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'Thunderstorms are full of different
frequencies of sound
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00:09:01,668 --> 00:09:04,708
'and these travel
different distances.'
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00:09:04,708 --> 00:09:07,068
THUNDERCLAPS
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The higher sounds,
like the thunderclap,
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are quickly absorbed into the
atmosphere, so don't travel far.
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00:09:28,708 --> 00:09:33,428
Storms also produce low rumbles
that carry much further.
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00:09:34,828 --> 00:09:39,108
But the very deepest sounds
are below our hearing range
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00:09:39,108 --> 00:09:42,948
and these infrasonic parts
of the storm are known to travel
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much greater distances.
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Infrasound can travel through
its environment without getting
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absorbed, and that's why
the infrasound from rainstorms
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can travel for hundreds
of kilometres.
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00:10:12,348 --> 00:10:16,028
But could elephants really
be hearing the infrasound
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from these distant storms?
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00:10:22,308 --> 00:10:24,908
Andre, I believe Tembo
is the perfect elephant.
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'Andre Kotze has worked with
elephants for 25 years
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'and can recognise the behaviours
that will show
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'if the elephants are hearing
our infrasonic storm.'
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Andre, even though these are rescued
elephants, do you still see
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a change in behaviour
when a thunderstorm is approaching,
like they would in the wild?
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When they hear a thunderstorm,
they will more than likely
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turn their backsides together,
facing to the thunderstorm,
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ears out with a spontaneous freeze,
like it's a secret message or
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something that happens
and they just stand still for it.
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After the spontaneous freeze
you are more than likely
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to find that they start chatting
amongst each other. Low rumbles.
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If they respond in that way
to our thunderstorms
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then that's proof, in a way,
that they can hear
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a part of the sound spectrum
that we can't even attempt to.
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Absolutely, absolutely,
without a question of doubt.
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Although the speaker is positioned
close to the elephants,
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the infrasound it produces
will have the intensity
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of a distant thunderstorm.
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The herd is busy feeding,
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so we're looking for a definite
change in behaviour.
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Bruce, I think we're ready
to play the speaker.
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OK, Patrick, audio in two seconds.
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It may seem strange, but because
the camper van is generating sounds
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below our hearing threshold
we can't hear it,
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but we can certainly see it, as air
inside vibrates with sound energy.
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The elephants react immediately...
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..turning to the speaker.
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They're clearly reacting to the
sound, but I can't hear a thing.
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That's incredible,
they've all stopped
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and they've changed their behaviour,
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as soon as Bruce started playing
that sound from the camper van.
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You can even hear them vocalising.
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Their ears fanning out.
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It's absolutely amazing how it
completely changes their behaviour.
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Bruce, it worked!
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There's absolutely
no question about it
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and their ability to determine
the direction the sound
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was coming from, I was really
impressed with that.
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The elephants are back feeding now
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but virtually the entire herd
turned and faced our infrasonic
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speaker, listening in to that
secret sound of the storm.
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This hidden channel of infrasound
could explain a great mystery
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of the natural world.
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How elephants know where to go
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when they migrate vast distances
in search of water.
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But storms aren't the only elemental
forces to produce infrasound.
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Even things we think of as silent
are in fact making very
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low-frequency sounds.
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The spectacular aurora borealis
produces infrasonic rumbles
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of a hundredth of a hertz.
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Volcanoes produce
even lower frequencies.
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These are some of the deepest
sounds on the planet.
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And amazingly, there's evidence
elephants may be detecting
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other natural sources of infrasound.
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The most extraordinary example
is the tsunami that swept across
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the Indian Ocean in 2004.
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When the tsunami hit
the shores of Sri Lanka,
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there were numerous reports
of elephants acting erratically
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and moving inland well before
the tsunami struck.
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Now, this apparent sixth sense
could be down to the large
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amounts of infrasound
being produced by the tsunami.
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As it built up, the sound it was
producing was moving faster
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than the approaching wave.
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So the theory is that elephants
could hear this low-pitched sound,
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like it was an alarm, and were able
to move off into safety.
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ELEPHANT MAKES LOW RUMBLE
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Elephants are one of the few animals
on Earth that hear
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and produce infrasound.
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But in the vast wetlands
of the Florida Everglades,
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an ancient predator has also
harnessed the power of sounds
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too deep for us to hear.
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They use it to put on one
of the most extraordinary
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displays in the animal kingdom.
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The American alligator.
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Every spring, the male alligators
put on a spectacular mating display.
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They sink down in the water
so their backs are just below
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the surface, and then make
really low-frequency sounds.
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And the consequence of that
is that water droplets on their back
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look like they're dancing.
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00:16:10,708 --> 00:16:14,748
And soon it becomes a water
dance-off,
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as rival males compete
by displaying to females.
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00:16:20,788 --> 00:16:24,228
I've never really had any desire
to be close to a bellowing
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alligator but I do want to see this,
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and to do it, I've got to trigger
a chorus of amorous alligators.
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To see this spectacle, I need
to encourage some alligators
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to start dancing.
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00:16:45,068 --> 00:16:49,268
And to do that, I need to replicate
their infrasonic calls
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so they think that
there's a larger male close by.
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00:16:53,148 --> 00:16:56,788
That requires speakers even bigger
than a camper van.
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The alligators are producing
infrasound in water
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but we want to do it in air, to send
sound waves out across the lake
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00:17:09,548 --> 00:17:12,868
and the physics works a little bit
differently in air, so we've built
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00:17:12,868 --> 00:17:16,708
special speakers that do one job
and they do it really well.
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But to make it work,
they need to look like this.
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These speakers produce sounds
at 19 hertz, the same deep
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frequency that the alligators
bellow at.
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00:17:29,068 --> 00:17:33,948
So let's see if they can entice a
grumpy alligator to start flirting.
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So that's it.
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00:17:50,868 --> 00:17:54,468
Those are the big infrasound
speakers sending sound out
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over the lake here, and now
we just have to wait and see
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if any of the alligators react.
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Oh, straight over there, tail
up in the air, getting ready to call.
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ALLIGATOR BELLOWS
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There are two parts to this display.
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00:18:16,508 --> 00:18:19,948
One is a deep but audible bellow
from their mouths.
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ALLIGATOR BELLOWS
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It's like hearing dinosaurs.
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The other part is the water dance.
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This is produced by sound that is
too low for us to hear.
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It's a really deep hum coming
straight from
238
00:18:41,508 --> 00:18:45,468
the alligator's body, that makes
the water dance at the surface.
239
00:18:52,988 --> 00:18:57,268
There's two things going on here.
There's two indicators of size.
240
00:18:57,268 --> 00:19:00,268
And one of them
is the infrasound itself,
241
00:19:00,268 --> 00:19:03,748
a noise that's really deep. You need
to be big, like in the same way
242
00:19:03,748 --> 00:19:06,348
that a big bell makes
a deeper noise.
243
00:19:06,348 --> 00:19:09,828
You need to have scale, size,
to make that kind of deep noise.
244
00:19:09,828 --> 00:19:12,668
But the other thing is what
the alligators are doing
245
00:19:12,668 --> 00:19:13,988
just before they call.
246
00:19:13,988 --> 00:19:15,828
They lift up their tail
and their head
247
00:19:15,828 --> 00:19:19,188
and you can see the full length
of the alligator, and they're big.
248
00:19:19,188 --> 00:19:20,908
These are enormous creatures.
249
00:19:24,868 --> 00:19:26,308
I'm feeling very small.
250
00:19:28,748 --> 00:19:32,588
Putting on a water dance requires
huge amounts of energy.
251
00:19:32,588 --> 00:19:34,188
So why go to all that effort?
252
00:19:37,268 --> 00:19:41,308
To understand, I need to venture
deeper into the gator's natural home.
253
00:19:44,908 --> 00:19:47,788
I'm on the north edge of
the Florida Everglades
254
00:19:47,788 --> 00:19:49,268
and these wetlands stretch
255
00:19:49,268 --> 00:19:52,788
south for hundreds of kilometres
from here.
256
00:19:52,788 --> 00:19:57,588
This place, where muddy brown water
touches blue sky,
257
00:19:57,588 --> 00:19:59,828
is prime alligator territory.
258
00:20:06,548 --> 00:20:10,588
Alligators live on the boundary
between air and water,
259
00:20:10,588 --> 00:20:15,028
in a low world where vision is
obscured by tangled vegetation.
260
00:20:19,908 --> 00:20:22,988
So, to stand any chance
of attracting a mate,
261
00:20:22,988 --> 00:20:25,908
males have to make sure
they stand out.
262
00:20:28,868 --> 00:20:32,188
Imagine there's a female 300 metres
away over there
263
00:20:32,188 --> 00:20:34,548
and an alligator here is calling.
264
00:20:34,548 --> 00:20:37,388
Sight isn't much good
because she's too far away
265
00:20:37,388 --> 00:20:40,908
and there's too much in the way,
but sound can travel through the
266
00:20:40,908 --> 00:20:44,908
water, and that is what the audible
part of the alligator's bellow does.
267
00:20:44,908 --> 00:20:48,788
And when she's come in closer, the
sound isn't as much use any more.
268
00:20:50,868 --> 00:20:54,468
But the water dance is splashing up
above the surface of the water,
269
00:20:54,468 --> 00:20:58,468
so she can see that and go right
to the male that produced it.
270
00:20:58,468 --> 00:21:02,348
For these ancient predators,
the water dance is essential
271
00:21:02,348 --> 00:21:03,428
for survival.
272
00:21:06,628 --> 00:21:10,508
But the most extraordinary thing
is how they use infrasound
273
00:21:10,508 --> 00:21:11,908
to put on the display.
274
00:21:14,228 --> 00:21:17,908
To show you, I'm going to create
my own water dance.
275
00:21:28,028 --> 00:21:30,068
This is a Chinese singing bowl.
276
00:21:30,068 --> 00:21:32,988
They've been around for well
over 2,000 years
277
00:21:32,988 --> 00:21:35,308
and the reason that they
are special is that
278
00:21:35,308 --> 00:21:39,108
when you rub on the handles, you get
this splashing from the bowl.
279
00:21:42,068 --> 00:21:46,628
That's because vibrations of the bowl
send low-frequency sounds
280
00:21:46,628 --> 00:21:47,988
through the water.
281
00:21:47,988 --> 00:21:51,588
When it's loud enough, this causes
the water surface to break
282
00:21:51,588 --> 00:21:54,748
into special waves
called Faraday waves.
283
00:22:01,708 --> 00:22:05,148
Faraday waves are almost like a way
of concentrating energy.
284
00:22:05,148 --> 00:22:06,668
Once they start to grow,
285
00:22:06,668 --> 00:22:10,068
they keep growing, and so if you
rub on the bowl hard enough
286
00:22:10,068 --> 00:22:13,468
you make the amplitude of the waves
loud enough, those Faraday waves
287
00:22:13,468 --> 00:22:17,348
get so high they start to spit
little droplets of water upwards.
288
00:22:21,988 --> 00:22:25,908
And, incredibly, footage from
our high-speed camera shows
289
00:22:25,908 --> 00:22:28,788
the alligators are also creating
Faraday waves.
290
00:22:32,788 --> 00:22:35,628
So this makes it easier to see
what's going on.
291
00:22:35,628 --> 00:22:38,428
The alligator's back is just below
the surface of the water,
292
00:22:38,428 --> 00:22:40,868
its lungs are full
so its body is really big.
293
00:22:42,788 --> 00:22:47,548
As the alligator starts to vibrate
its lungs, the top of its back
294
00:22:47,548 --> 00:22:51,988
is acting like a piston, it's pushing
up on the water above it
295
00:22:51,988 --> 00:22:56,028
and that's driving the surface
into this splashing pattern.
296
00:22:56,028 --> 00:22:57,308
It's really dramatic.
297
00:22:59,988 --> 00:23:01,868
And you can see it takes
a lot of energy
298
00:23:01,868 --> 00:23:05,988
because after they have called maybe
seven or eight times they stop
299
00:23:05,988 --> 00:23:08,028
and they rest, they're exhausted.
300
00:23:18,788 --> 00:23:24,828
It's thought alligators have been
calling like this for at least
70 million years,
301
00:23:24,828 --> 00:23:28,428
so they were doing it when
the dinosaurs were around.
302
00:23:33,428 --> 00:23:37,388
And what stimulates them to call is
hearing other alligators calling
303
00:23:37,388 --> 00:23:41,628
or other sources of infrasound, and
that leads to something really cool
304
00:23:41,628 --> 00:23:45,388
because Cape Canaveral is just 70
miles that way. And when the shuttle
305
00:23:45,388 --> 00:23:48,748
was landing there, when there were
shuttle flights, the infrasound
306
00:23:48,748 --> 00:23:53,228
from the sonic boom would set off
the bellowing of the alligators,
307
00:23:53,228 --> 00:23:56,028
so it's like the space age
touching the dinosaurs.
308
00:24:13,108 --> 00:24:17,108
As we move into the lower part of
the sound spectrum that human ears
309
00:24:17,108 --> 00:24:22,268
can hear, sounds above 20 hertz
still travel long distances.
310
00:24:25,028 --> 00:24:28,748
But these deeper tones
don't just move through air,
311
00:24:28,748 --> 00:24:31,348
they also travel through the ground.
312
00:24:35,468 --> 00:24:38,468
And in Southern Africa's
Namib Desert,
313
00:24:38,468 --> 00:24:41,548
one bizarre little predator
can hear so brilliantly
314
00:24:41,548 --> 00:24:47,108
underground that they can find tiny
prey in this vast expanse of sand.
315
00:24:50,388 --> 00:24:51,508
The golden mole.
316
00:24:52,708 --> 00:24:55,468
They're such weird looking animals.
317
00:24:55,468 --> 00:24:58,868
They've got no eyes
and no external ears
318
00:24:58,868 --> 00:25:02,228
and they spend most of their time
beneath the ground.
319
00:25:02,228 --> 00:25:05,508
And yet they can do something
truly remarkable.
320
00:25:05,508 --> 00:25:08,948
They can hear the faintest of sounds
through the sand.
321
00:25:10,428 --> 00:25:14,268
In fact, their hearing is
so sensitive they can find
322
00:25:14,268 --> 00:25:16,908
a tiny termite from 20 metres away.
323
00:25:20,988 --> 00:25:25,108
Golden moles feed on termites
and other small insects.
324
00:25:26,748 --> 00:25:31,508
To show how they find them, I first
need to track down a golden mole.
325
00:25:32,948 --> 00:25:37,188
That's not easy,
because they're very shy
326
00:25:37,188 --> 00:25:38,828
and only active at night.
327
00:25:40,868 --> 00:25:43,228
But they do leave
distinctive tracks.
328
00:25:44,468 --> 00:25:49,388
As they travel across the sand, they
leave these strange indentations
329
00:25:49,388 --> 00:25:51,988
every few metres, and that gives us
330
00:25:51,988 --> 00:25:55,828
a clue as to how this blind creature
is finding the termites.
331
00:25:55,828 --> 00:25:59,148
What they're doing is dipping
their head into the sand
332
00:25:59,148 --> 00:26:03,228
and listening in for vibrations that
are travelling through the ground.
333
00:26:08,908 --> 00:26:12,108
The mole's trail ends
at a grass mound,
334
00:26:12,108 --> 00:26:15,068
so I've cordoned off the area
and left it overnight.
335
00:26:19,948 --> 00:26:23,308
And look...at that.
336
00:26:24,668 --> 00:26:28,308
It's the cutest animal
I've ever seen, look.
337
00:26:30,188 --> 00:26:34,468
Perfectly shaped for swimming
through sand, wedge-shaped head.
338
00:26:35,948 --> 00:26:40,588
Even though it's so tiny, you can
feel the power in those front legs,
339
00:26:40,588 --> 00:26:43,188
perfectly adapted for swimming
through sand.
340
00:26:46,348 --> 00:26:50,468
And incredibly, you can't see
any eyes.
341
00:26:50,468 --> 00:26:54,588
And that's because from a young age,
their eyelids fuse over
342
00:26:54,588 --> 00:26:57,868
and where their eyes would be,
just covered in fur.
343
00:27:00,708 --> 00:27:04,668
And because it can't see, it relies
entirely on its sense of hearing.
344
00:27:06,188 --> 00:27:07,988
Such a beautiful animal.
345
00:27:12,508 --> 00:27:16,748
Golden moles may be able to hear me
coming, but there's no way
346
00:27:16,748 --> 00:27:20,788
they could detect a termite's
footsteps from 20 metres.
347
00:27:20,788 --> 00:27:23,188
So how do they do it?
348
00:27:23,188 --> 00:27:27,308
Well, one bizarre theory suggests
the moles are actually listening out
349
00:27:27,308 --> 00:27:29,868
for the sound of grass
blowing in the breeze.
350
00:27:32,588 --> 00:27:36,868
The desert landscape is constantly
shifting and changing.
351
00:27:36,868 --> 00:27:40,188
The only fixed points are these
tussock grasses
352
00:27:40,188 --> 00:27:42,508
and it's in these mounds
beneath the grass
353
00:27:42,508 --> 00:27:44,548
that the termites make their home.
354
00:27:46,148 --> 00:27:50,748
So could the golden moles really be
detecting the sound of tussock grass
355
00:27:50,748 --> 00:27:53,188
and using it to track down termites?
356
00:28:00,588 --> 00:28:03,188
There's only one way to find out.
357
00:28:03,188 --> 00:28:06,108
I'm going to play the sound
of blowing grass through
358
00:28:06,108 --> 00:28:09,948
the ground and see if
the golden mole approaches.
359
00:28:11,468 --> 00:28:14,588
But first, I've got to record it.
360
00:28:25,588 --> 00:28:27,028
DISTANT-SEEMING RUMBLE
361
00:28:27,028 --> 00:28:30,068
Wow, that's such an alien sound.
362
00:28:30,068 --> 00:28:31,668
CREAKING
363
00:28:35,028 --> 00:28:38,388
It's kind of a low knocking sound
and that's perfect for
364
00:28:38,388 --> 00:28:40,628
the golden mole,
because low frequencies travel
365
00:28:40,628 --> 00:28:42,668
really well through the ground.
366
00:28:47,948 --> 00:28:50,588
And inside the golden mole's skull,
367
00:28:50,588 --> 00:28:54,428
there's a clue that suggests
they may be tuned into these
368
00:28:54,428 --> 00:28:56,468
low-frequency sounds.
369
00:28:56,468 --> 00:28:59,948
This is a 3-D model of the inner
ear of a golden mole,
370
00:28:59,948 --> 00:29:02,948
it's been enlarged by about
15 times.
371
00:29:02,948 --> 00:29:06,308
Now, this section of the ear
is responsible for converting
372
00:29:06,308 --> 00:29:09,668
vibrational energy into nerve
impulses that the brain
373
00:29:09,668 --> 00:29:10,908
can interpret.
374
00:29:10,908 --> 00:29:14,028
And the section that we are most
interested in is right here,
375
00:29:14,028 --> 00:29:17,788
this coiled area,
known as a cochlea.
376
00:29:17,788 --> 00:29:20,268
Now, in the golden mole this area is
377
00:29:20,268 --> 00:29:24,388
twice as long as it is in European
moles, and it's thought that it
378
00:29:24,388 --> 00:29:28,548
helps extend the hearing range
into lower frequencies.
379
00:29:28,548 --> 00:29:30,548
Think of it as a piano.
380
00:29:30,548 --> 00:29:34,628
If you've got an extended number
of keys, you can play lower
381
00:29:34,628 --> 00:29:35,868
and lower octaves.
382
00:29:37,708 --> 00:29:42,188
It's time to put our golden mole's
low frequency hearing to the test.
383
00:29:46,348 --> 00:29:50,548
I find an area of sand free
from tussock grass and set up
384
00:29:50,548 --> 00:29:54,228
a rig of night-time cameras
that can be monitored remotely.
385
00:29:56,748 --> 00:29:59,908
And this is our key piece of kit.
It's a transducer.
386
00:29:59,908 --> 00:30:04,868
I'm going to use this to play back
the sound of the tussock grass
387
00:30:04,868 --> 00:30:06,788
I recorded earlier.
388
00:30:06,788 --> 00:30:10,868
If I put my hand on that speaker,
I can feel the gentle vibrations
389
00:30:10,868 --> 00:30:13,268
that are being played out
through the sand.
390
00:30:16,308 --> 00:30:19,068
So, if the tussock grass theory
is correct,
391
00:30:19,068 --> 00:30:22,508
our mole should associate
this sound with termites
392
00:30:22,508 --> 00:30:25,028
and move towards it.
393
00:30:25,028 --> 00:30:26,668
Let's see if it works.
394
00:30:39,028 --> 00:30:40,588
Oh, look.
395
00:30:41,868 --> 00:30:43,308
You can see he's moving.
396
00:30:46,548 --> 00:30:47,948
And there.
397
00:30:47,948 --> 00:30:49,788
That's the behaviour
we are looking for,
398
00:30:49,788 --> 00:30:51,748
that classic head-dipping movement.
399
00:30:53,508 --> 00:30:56,908
The head just couples with the sand
perfectly and the vibrations
400
00:30:56,908 --> 00:31:00,548
of the sound waves
travel really well
401
00:31:00,548 --> 00:31:03,948
and that is what it's picking up.
That's what it's detecting.
402
00:31:07,268 --> 00:31:09,508
Here we go.
He's running around, really fast.
403
00:31:15,828 --> 00:31:17,868
Oh, look,
he's just darted out of frame!
404
00:31:19,148 --> 00:31:22,628
It seems we've lost our mole
to the open desert.
405
00:31:27,308 --> 00:31:29,188
But minutes later, he's back.
406
00:31:29,188 --> 00:31:32,068
Oh, look, there he is!
407
00:31:32,068 --> 00:31:34,388
He's just run in,
dipped his head in the sand.
408
00:31:36,308 --> 00:31:38,588
He's just run off again.
409
00:31:38,588 --> 00:31:41,268
I mean, he hasn't gone directly
to the speaker,
410
00:31:41,268 --> 00:31:44,068
but he's, kind of,
gone in that general direction.
411
00:31:44,068 --> 00:31:47,668
By head dipping
so close to the transducer,
412
00:31:47,668 --> 00:31:52,388
it seems the mole was attracted
to the sound of the tussock grass.
413
00:31:52,388 --> 00:31:55,828
Perhaps, tonight,
he just wasn't hungry.
414
00:31:58,468 --> 00:32:02,828
But over a few nights
in this remote desert,
415
00:32:02,828 --> 00:32:05,588
I gain a unique insight
into the secret lives
416
00:32:05,588 --> 00:32:08,308
of these rare and shy
little mammals...
417
00:32:16,988 --> 00:32:23,068
..including a mole struggling
to find a termite on the surface,
418
00:32:23,068 --> 00:32:27,308
until he burrows down to listen
to where the sound is coming from.
419
00:32:37,108 --> 00:32:40,348
By tapping into this hidden world
of underground sound,
420
00:32:40,348 --> 00:32:44,348
the Golden Mole has become master
of these sand dunes.
421
00:32:44,348 --> 00:32:48,068
Who'd have thought the sound
of grass blowing in the wind
422
00:32:48,068 --> 00:32:50,468
would be the secret
of desert survival?
423
00:32:53,748 --> 00:32:59,428
BIRDSONG
424
00:33:00,428 --> 00:33:02,708
As sound gets higher in pitch,
425
00:33:02,708 --> 00:33:04,868
our ears become much better
at detecting it.
426
00:33:06,508 --> 00:33:10,828
Our hearing is most sensitive
around 1,000 Hertz,
427
00:33:10,828 --> 00:33:12,828
the frequency range
around human speech.
428
00:33:13,828 --> 00:33:17,788
But many animals also tap into
these frequencies...
429
00:33:20,548 --> 00:33:23,188
..nowhere more so
than the tropical rainforest.
430
00:33:23,188 --> 00:33:27,388
BIRDSONG AND MONKEY CALLS
431
00:33:34,748 --> 00:33:41,188
In this dense, tangled world, animals
can be heard, but rarely seen.
432
00:33:43,308 --> 00:33:46,548
So, there's an acoustic battle
for the airwaves,
433
00:33:46,548 --> 00:33:49,588
as creatures fight
to make themselves heard.
434
00:33:55,628 --> 00:33:59,948
In the jungles of Puerto Rico, the
calls of one surprising creature
435
00:33:59,948 --> 00:34:02,188
drown out all others.
436
00:34:03,348 --> 00:34:06,188
They're the giants
of this acoustic world.
437
00:34:06,188 --> 00:34:09,388
They're almost as loud
as a pneumatic drill
438
00:34:09,388 --> 00:34:12,708
and if it wasn't for a really
clever evolutionary adaptation,
439
00:34:12,708 --> 00:34:15,668
they'd deafen themselves
with their own call.
440
00:34:15,668 --> 00:34:21,868
Meet the Coqui Frog, thought to be
the loudest amphibian on the planet.
441
00:34:24,348 --> 00:34:29,188
A fully-grown Coqui Frog is around
the size of a 2p piece,
442
00:34:29,188 --> 00:34:33,468
but what they lack in size, they
definitely make up for in volume.
443
00:34:38,068 --> 00:34:40,948
So, what's driven these
little frogs to become so loud?
444
00:34:48,308 --> 00:34:51,268
An extraordinary piece
of recording technology,
445
00:34:51,268 --> 00:34:54,548
that lets me SEE sound,
will help me find out.
446
00:34:59,268 --> 00:35:01,308
This is an acoustic camera.
447
00:35:01,308 --> 00:35:05,348
It's got 48 microphones arranged
around a normal camera in the middle
448
00:35:05,348 --> 00:35:08,188
and what it lets us do is to take
the normal images
449
00:35:08,188 --> 00:35:10,948
and overlay on top of them
where the sound is coming from.
450
00:35:10,948 --> 00:35:14,068
So, this is going
to help me find Coqui Frogs,
451
00:35:14,068 --> 00:35:15,948
when everything around me
is pitch black.
452
00:35:18,988 --> 00:35:24,628
The acoustic camera also records
the intensity, or loudness, of sound,
453
00:35:24,628 --> 00:35:26,268
measured in decibels.
454
00:35:28,148 --> 00:35:31,588
At the volume I'm talking at the
moment, the computer is registering
455
00:35:31,588 --> 00:35:36,868
about 70 decibels, but if I clap,
it will register 90.
456
00:35:36,868 --> 00:35:39,988
So, by pointing this in the darkness,
we'll get a direct measure
457
00:35:39,988 --> 00:35:42,108
of how loud
these little frogs really are.
458
00:35:42,108 --> 00:35:45,588
BIRDSONG AND MONKEY CALLS
459
00:35:48,628 --> 00:35:50,868
The Coqui chorus
starts around sunset.
460
00:35:55,348 --> 00:35:57,948
And the noise they make
is overwhelming
461
00:35:57,948 --> 00:36:00,388
and comes from all directions.
462
00:36:00,388 --> 00:36:03,228
LOUD CHIRRUPING
463
00:36:03,228 --> 00:36:04,548
They're all around me.
464
00:36:04,548 --> 00:36:06,228
There's one.
465
00:36:07,308 --> 00:36:08,948
So loud.
466
00:36:10,548 --> 00:36:15,028
I feel like I must be being stared at
by millions of frogs,
467
00:36:15,028 --> 00:36:17,308
because there's clearly
so many of them.
468
00:36:17,308 --> 00:36:20,668
It's the male frog's call
that gives them their name.
469
00:36:21,948 --> 00:36:25,268
There are two parts -
the "co" and the "kee".
470
00:36:27,628 --> 00:36:31,108
The "co" warns off rival males,
471
00:36:31,108 --> 00:36:35,748
while the "kee" lets any females
nearby know he's available.
472
00:36:35,748 --> 00:36:37,908
CO-KEE SOUND
473
00:36:37,908 --> 00:36:42,108
First, I want to record
just how loudly a frog can call.
474
00:36:43,748 --> 00:36:46,268
So, there he is, our calling frog.
475
00:36:46,268 --> 00:36:50,948
We've moved the acoustic camera
in quite close
476
00:36:50,948 --> 00:36:54,428
and we're, pretty much,
a metre away from him now.
477
00:36:55,908 --> 00:36:57,908
And the fact that we're so close
478
00:36:57,908 --> 00:37:00,828
means that we can actually measure
how loud he is.
479
00:37:03,948 --> 00:37:08,788
CO-KEE SOUND
480
00:37:08,788 --> 00:37:13,508
This little frog is calling
at nearly 80 decibels,
481
00:37:13,508 --> 00:37:16,348
but they have been recorded up to 95.
482
00:37:21,028 --> 00:37:25,068
For their size, they're one
of the noisiest creatures on Earth,
483
00:37:25,068 --> 00:37:29,508
the equivalent of two and a half
times louder than a lion's roar
484
00:37:29,508 --> 00:37:32,708
and three times louder
than an elephant.
485
00:37:34,668 --> 00:37:39,348
So, how does such a tiny creature
make such a massive noise?
486
00:37:41,468 --> 00:37:45,708
The secret lies
in the balloon-like vocal sac.
487
00:37:47,708 --> 00:37:51,948
And you can see that,
as he pushes air out of his lungs,
488
00:37:51,948 --> 00:37:55,068
it goes into that big vocal sac
and back again.
489
00:37:55,068 --> 00:37:59,628
And what the vocal sac is doing
is acting like
490
00:37:59,628 --> 00:38:02,708
the sound board on a guitar,
the front face for a guitar,
491
00:38:02,708 --> 00:38:07,028
and it's helping to transmit
that sound really efficiently
492
00:38:07,028 --> 00:38:08,868
into the outside world.
493
00:38:11,268 --> 00:38:14,428
I love watching his body
work like that.
494
00:38:17,108 --> 00:38:21,588
In fact, Coqui Frogs are so loud
they should deafen themselves,
495
00:38:21,588 --> 00:38:24,788
But they don't,
thanks to a bizarre adaptation.
496
00:38:28,268 --> 00:38:32,748
Inside the frog, the lungs and the
vocal sac are connected to the ears.
497
00:38:34,948 --> 00:38:39,548
That means that, when he calls,
the sound travels out into the air,
498
00:38:39,548 --> 00:38:42,068
but also through the frog's body.
499
00:38:44,348 --> 00:38:48,388
If the call was just hitting
the eardrum from one side,
500
00:38:48,388 --> 00:38:49,788
it would rupture it.
501
00:38:52,668 --> 00:38:57,148
But because the sound hits
the eardrum from inside and outside
502
00:38:57,148 --> 00:39:00,388
the frog's body at the same time,
the effect is cancelled out.
503
00:39:08,108 --> 00:39:11,148
But why have these frogs
in the jungles of Puerto Rico
504
00:39:11,148 --> 00:39:14,348
pushed sound to such extremes?
505
00:39:15,548 --> 00:39:18,468
The acoustic camera
reveals a possible answer.
506
00:39:18,468 --> 00:39:22,108
This is brilliant, because
it's a completely different way
507
00:39:22,108 --> 00:39:24,868
of understanding what's going on.
508
00:39:24,868 --> 00:39:29,868
When I look out there, you know,
I see blackness and leaves,
509
00:39:29,868 --> 00:39:33,708
but when I look here, there's these
really bright splotches of light
510
00:39:33,708 --> 00:39:36,228
and those are these little frogs.
511
00:39:36,228 --> 00:39:40,388
The camera reveals
the sheer density of frogs.
512
00:39:40,388 --> 00:39:44,828
There can be 80 in an area
the size of a tennis court.
513
00:39:44,828 --> 00:39:50,588
With so many frogs calling,
they've had to become louder
514
00:39:50,588 --> 00:39:53,388
and louder
to make themselves heard.
515
00:39:55,308 --> 00:39:58,708
It's like being at a crowded party,
when you raise your voice
516
00:39:58,708 --> 00:40:01,508
to be heard, but so does everyone
else, so you end up shouting.
517
00:40:04,988 --> 00:40:08,628
This acoustic arms race
may explain why the Coqui Frog
518
00:40:08,628 --> 00:40:11,268
is so exceptionally loud.
519
00:40:14,268 --> 00:40:16,948
I'm completely bathed in sound.
520
00:40:18,988 --> 00:40:21,428
For the Puerto Ricans,
this is the sound of home,
521
00:40:21,428 --> 00:40:23,788
but for the frogs, it's different.
522
00:40:23,788 --> 00:40:27,108
From their point of view,
what's surrounding you
523
00:40:27,108 --> 00:40:31,988
is an organised,
precise, flow of information.
524
00:40:33,388 --> 00:40:37,588
And if you want to survive out here,
understanding the information
525
00:40:37,588 --> 00:40:40,428
that all this sound
is giving you is essential.
526
00:40:44,948 --> 00:40:48,428
COQUI FROGS CHIRP
527
00:40:50,468 --> 00:40:54,308
As we journey further up
the sound spectrum,
528
00:40:54,308 --> 00:40:56,148
our ears become less sensitive.
529
00:40:56,148 --> 00:40:59,188
We don't hear high-pitched sounds
very well.
530
00:41:01,668 --> 00:41:03,668
Our countryside is full of sounds,
531
00:41:03,668 --> 00:41:06,748
like birdsong,
that we can appreciate.
532
00:41:06,748 --> 00:41:11,588
But it's also awash with the squeaks
of small mammals, like voles.
533
00:41:13,228 --> 00:41:17,708
It's just that our ears can't detect
them, unless we're really close.
534
00:41:19,548 --> 00:41:23,868
But we have one amazing creature
that can hear these tiny sounds
535
00:41:23,868 --> 00:41:26,148
from great distances away.
536
00:41:28,868 --> 00:41:29,988
The Barn Owl.
537
00:41:31,948 --> 00:41:35,788
For their young to survive,
a pair of adults must catch
538
00:41:35,788 --> 00:41:40,228
3,000 voles a year.
That's eight every single night.
539
00:41:43,428 --> 00:41:46,348
And the only information
they've got to go on
540
00:41:46,348 --> 00:41:49,588
are the little squeaks of the voles
and the rustling, as they
541
00:41:49,588 --> 00:41:53,228
move around in the undergrowth.
It's not much. So, when the owls
542
00:41:53,228 --> 00:41:56,588
are out hunting, they're floating
over a landscape like this
543
00:41:56,588 --> 00:42:00,548
and it's not enough for them to know
that dinner is out there somewhere.
544
00:42:00,548 --> 00:42:03,188
They need to be able
to pinpoint it accurately.
545
00:42:03,188 --> 00:42:06,228
They want to pounce
and get the vole first time.
546
00:42:11,308 --> 00:42:16,268
But how do they pinpoint prey to the
millimetre in this open landscape,
547
00:42:16,268 --> 00:42:18,428
just using their ears?
548
00:42:22,908 --> 00:42:25,268
Usually,
when we think of good hearing,
549
00:42:25,268 --> 00:42:27,028
we think about things with big ears.
550
00:42:27,028 --> 00:42:30,828
We associate having big ears
with being able to hear better.
551
00:42:30,828 --> 00:42:36,108
Now, this owl has fabulous hearing,
but it doesn't have external ears.
552
00:42:36,108 --> 00:42:41,148
If you look at these feathers here,
this thick ring around,
553
00:42:41,148 --> 00:42:44,668
that defines the facial disc
and they're basically forming
554
00:42:44,668 --> 00:42:48,108
a cup, just like when you put
your hand behind ear, and they're
555
00:42:48,108 --> 00:42:51,348
doing the job that our ears do,
but they are built into his face.
556
00:42:51,348 --> 00:42:54,748
And this dish here,
this dish of feathers,
557
00:42:54,748 --> 00:42:58,468
is directing sound into his ears
and it's directional.
558
00:42:58,468 --> 00:43:01,068
If an owl looks at you,
it's listening to you.
559
00:43:02,908 --> 00:43:05,828
I want to put the owl's
hearing to the test,
560
00:43:05,828 --> 00:43:07,988
so I've got a phone
with an unusual ring tone.
561
00:43:07,988 --> 00:43:11,228
SOFT SQUEAKING
562
00:43:14,308 --> 00:43:17,508
These sounds are the high-pitched
squeaks that voles make and
563
00:43:17,508 --> 00:43:20,788
the rustling sound that you get as
they move around in the undergrowth.
564
00:43:20,788 --> 00:43:24,388
So, that's what an owl's got to
listen out for if it wants dinner.
565
00:43:25,708 --> 00:43:28,308
I'm going to hide the phone
in the long grass in just the sort
566
00:43:28,308 --> 00:43:31,348
of a place where a vole might be
and then I'm going to hide and call
567
00:43:31,348 --> 00:43:35,428
the phone and, when it rings,
that squeaking noise will start
568
00:43:35,428 --> 00:43:39,068
and we will see whether the owl
can locate it just using that sound.
569
00:43:41,468 --> 00:43:44,508
But before I let the owl loose
on the phone, I'm going to see
570
00:43:44,508 --> 00:43:48,708
how I get on with locating
this faint sound.
571
00:43:48,708 --> 00:43:52,148
To help me out, I've got
a piece of owl-like technology.
572
00:43:54,508 --> 00:43:57,148
This is a parabolic microphone
573
00:43:57,148 --> 00:43:59,588
and the reason I've got it
is that the shape of the inside
574
00:43:59,588 --> 00:44:03,428
of it is similar to the shape of the
owl's feathers, that facial disc.
575
00:44:06,668 --> 00:44:08,508
So, just let me call the phone here.
576
00:44:12,148 --> 00:44:16,228
S, the phone's about 60 or 70
metres over there and it's ringing,
577
00:44:16,228 --> 00:44:18,868
but I can't hear anything.
578
00:44:18,868 --> 00:44:20,388
Let's see if this'll help.
579
00:44:28,628 --> 00:44:32,508
WIND WHISTLING AND BIRDSONG
580
00:44:32,508 --> 00:44:37,508
So, there's a little bit
of birdsong in there, as well.
581
00:44:37,508 --> 00:44:39,068
SOFT SQUEAKING
582
00:44:39,068 --> 00:44:40,708
That's it there,
583
00:44:40,708 --> 00:44:45,268
the squeaking, and if I move
the dish even a little bit
584
00:44:45,268 --> 00:44:46,908
to either side, it's gone.
585
00:44:51,308 --> 00:44:54,428
There's a surprising reason this
parabolic microphone and the owl's
586
00:44:54,428 --> 00:44:59,028
facial disc are both so effective
at picking up these squeaks.
587
00:45:00,828 --> 00:45:03,228
It's all to do with
the pitch of the sound.
588
00:45:05,588 --> 00:45:08,308
This parabolic shape
has a cut-off frequency,
589
00:45:08,308 --> 00:45:11,708
so it doesn't work for very
low frequencies and for the owls
590
00:45:11,708 --> 00:45:16,668
that cut-off is at about 3,000 hertz,
so if you tap a very thin
591
00:45:16,668 --> 00:45:21,268
wine glass with a spoon, that's
about that sort of note, 3,000 hertz.
592
00:45:21,268 --> 00:45:24,828
So, above that, the owl's got
really good directional hearing.
593
00:45:24,828 --> 00:45:27,988
Below that, it doesn't hear as well
and that's actually really useful,
594
00:45:27,988 --> 00:45:29,828
because the rustling
and squeaking
595
00:45:29,828 --> 00:45:33,028
is at those high frequencies
and all the background noise,
596
00:45:33,028 --> 00:45:36,468
the low frequencies that might be
distracting, they're all cut out.
597
00:45:37,908 --> 00:45:42,148
So, now let's see how our Barn Owl
gets on with locating the vole phone.
598
00:45:42,148 --> 00:45:45,548
It's hidden in the grass
about 60 metres away,
599
00:45:45,548 --> 00:45:47,548
with a small camera close by.
600
00:45:49,508 --> 00:45:52,388
This is just the time of day
when owls would hunt.
601
00:45:52,388 --> 00:45:54,748
The voles are starting to come out.
602
00:46:00,028 --> 00:46:02,148
The owl quickly responds.
603
00:46:05,988 --> 00:46:09,948
Its facial disc helps filter out
background noise,
604
00:46:09,948 --> 00:46:13,228
so it can focus on the high-pitched
squeak from our phone.
605
00:46:13,228 --> 00:46:16,228
SOFT SQUEAKING
606
00:46:19,308 --> 00:46:21,948
Then it strikes.
607
00:46:21,948 --> 00:46:25,388
SQUEAKING
608
00:46:28,708 --> 00:46:31,508
So, our owl got it. It did the job.
609
00:46:31,508 --> 00:46:33,988
And the fact that it was
a phone ring tone it found
610
00:46:33,988 --> 00:46:37,068
showed that it couldn't have done
it by smell and it couldn't have
611
00:46:37,068 --> 00:46:39,388
done it by sight,
it must have been using its hearing.
612
00:46:39,388 --> 00:46:42,748
And it pinpointed it so accurately,
swooped right down in on it.
613
00:46:46,948 --> 00:46:49,508
How did it do this
with such precision?
614
00:46:53,228 --> 00:46:56,708
By comparing minuscule time
differences between the sound
615
00:46:56,708 --> 00:46:59,348
hitting the left and right ear,
616
00:46:59,348 --> 00:47:02,468
they work out which direction
that sound is coming from.
617
00:47:03,628 --> 00:47:08,828
But whilst our ears are symmetrical,
the barn owls ears are skewed.
618
00:47:13,388 --> 00:47:15,428
He's got one ear
on each side of his face,
619
00:47:15,428 --> 00:47:17,308
but they're not in the same place.
620
00:47:17,308 --> 00:47:20,668
The one on this side, on the right,
is just below his eye,
621
00:47:20,668 --> 00:47:24,108
and that, combined with
the shape of the facial disc,
622
00:47:24,108 --> 00:47:27,188
is mostly listening to sound
that is coming from above,
623
00:47:27,188 --> 00:47:30,628
and the other side,
the ear is just above his eye,
624
00:47:30,628 --> 00:47:34,188
and the facial disc is funnelling
mostly sound from below.
625
00:47:35,868 --> 00:47:39,388
So, by listening and comparing
the sound coming in both ears,
626
00:47:39,388 --> 00:47:43,068
he can tell how high or low
something is coming from
627
00:47:43,068 --> 00:47:46,348
and that, combined
with his ability to tell where
628
00:47:46,348 --> 00:47:49,708
things are horizontally,
is what lets him pinpoint his prey.
629
00:47:54,588 --> 00:47:58,148
The Barn Owl's amazing hearing
has allowed it to become the most
630
00:47:58,148 --> 00:48:02,228
widespread and successful
owl species on Earth.
631
00:48:05,068 --> 00:48:09,268
We're all really familiar
with owls and the image of an owl,
632
00:48:09,268 --> 00:48:12,868
but now look at an owl
and see it for what it is.
633
00:48:12,868 --> 00:48:17,748
It's got this face,
a dish which is collecting sound.
634
00:48:21,268 --> 00:48:24,668
And isn't that just a fantastic idea
that instead of having ears
635
00:48:24,668 --> 00:48:27,308
that stick out which would
get in the way if you flew,
636
00:48:27,308 --> 00:48:29,308
it's all built into his face?
637
00:48:37,868 --> 00:48:42,548
But even owls, with their extreme
auditory adaptations,
638
00:48:42,548 --> 00:48:47,628
are unable to hear the sounds
at the highest end of the spectrum.
639
00:48:47,628 --> 00:48:50,468
This is where our ears stop
working completely.
640
00:48:51,828 --> 00:48:55,748
Our ability to hear high-pitched
sounds changes
641
00:48:55,748 --> 00:48:57,148
throughout our lives.
642
00:48:57,148 --> 00:49:00,428
We start out being able
to hear really high frequencies
643
00:49:00,428 --> 00:49:02,628
and then this decreases with age.
644
00:49:02,628 --> 00:49:05,508
So someone in their sixties
will be able to hear up
645
00:49:05,508 --> 00:49:07,068
HIGH-PITCHED RINGING
646
00:49:07,068 --> 00:49:10,828
to around 10,000 hertz,
around there.
647
00:49:10,828 --> 00:49:13,428
But someone in their 20s
can probably hear up to
648
00:49:13,988 --> 00:49:21,748
around 16,000 hertz and actually
my hearing starts to go around 15.
649
00:49:21,748 --> 00:49:24,988
But it's only young children that
can hear even higher frequencies,
650
00:49:24,988 --> 00:49:27,388
up to 20,000 hertz.
651
00:49:28,988 --> 00:49:30,428
I can't hear anything there.
652
00:49:30,428 --> 00:49:34,468
Now any sound above this
is referred to being ultrasonic,
653
00:49:34,468 --> 00:49:37,188
which means it is above
the human hearing range.
654
00:49:38,268 --> 00:49:42,068
But for some animals, there are
great advantages to hearing
655
00:49:42,068 --> 00:49:45,868
and calling at this extreme end
of the spectrum.
656
00:49:47,468 --> 00:49:51,828
If an animal can call at a frequency
that its predators can't hear,
657
00:49:51,828 --> 00:49:55,868
but members of its own species can,
then this opens up a secret
658
00:49:55,868 --> 00:49:59,668
channel of sound that
they can use to communicate.
659
00:49:59,668 --> 00:50:01,748
And around the world
there are a few animals
660
00:50:01,748 --> 00:50:04,348
that have tapped into this
strategy perfectly.
661
00:50:07,268 --> 00:50:09,468
In the pine forests of Canada,
662
00:50:09,468 --> 00:50:12,708
Flying Squirrels
produce ultrasonic alarm calls.
663
00:50:14,788 --> 00:50:18,508
At 50,000 hertz, this is
way above our hearing range
664
00:50:18,508 --> 00:50:20,428
and that of their predators.
665
00:50:23,588 --> 00:50:27,588
In South East Asia, Tarsiers
push their calls to even greater
666
00:50:27,588 --> 00:50:30,988
extremes, up to 70,000 hertz.
667
00:50:35,428 --> 00:50:37,588
But one group
of alien-like creatures
668
00:50:37,588 --> 00:50:40,348
reaches even higher pitches.
669
00:50:41,548 --> 00:50:43,828
Katydids, or Bush Crickets.
670
00:50:44,828 --> 00:50:48,468
Their secretive love songs have been
671
00:50:48,468 --> 00:50:51,428
recorded at a staggering
150,000 hertz.
672
00:50:57,428 --> 00:51:01,788
We can only hear the sound
by slowing it down 30 times.
673
00:51:04,308 --> 00:51:08,868
They produce these extreme pitches
by rubbing the wing cases together.
674
00:51:13,068 --> 00:51:16,108
But there's one group of animals
that have pushed sound
675
00:51:16,108 --> 00:51:18,148
higher than any other.
676
00:51:19,948 --> 00:51:20,988
Bats.
677
00:51:22,468 --> 00:51:26,028
Their ultrasonic pulses
have been recorded at over
678
00:51:26,028 --> 00:51:27,668
200,000 hertz.
679
00:51:31,188 --> 00:51:34,988
Bats don't just use these extreme
frequencies to communicate.
680
00:51:34,988 --> 00:51:37,228
They use them to see their world.
681
00:51:41,308 --> 00:51:45,988
I'm stood in complete darkness
and I can't see a single thing
682
00:51:45,988 --> 00:51:47,828
and the only reason
you can see me is
683
00:51:47,828 --> 00:51:50,308
because we are filming
with a special infrared camera.
684
00:51:50,308 --> 00:51:54,708
But I know that I'm not alone here,
because I can hear and feel
685
00:51:54,708 --> 00:51:59,148
the wing beats of these Egyptian
fruit bats as they fly past my head.
686
00:52:02,348 --> 00:52:06,468
The bats can navigate through this
flight enclosure in complete
687
00:52:06,468 --> 00:52:08,828
darkness using echolocation.
688
00:52:17,068 --> 00:52:20,308
As the bats fly past, they click
their tongues really loudly
689
00:52:20,508 --> 00:52:23,148
and produce a high-frequency pulse.
690
00:52:23,148 --> 00:52:27,188
High-frequency sound echoes
off objects really effectively
691
00:52:27,188 --> 00:52:29,148
and precisely.
692
00:52:29,148 --> 00:52:33,308
So, the bat's pulses are reflecting
off the sides of the enclosure
693
00:52:33,308 --> 00:52:34,628
and my body.
694
00:52:36,588 --> 00:52:39,228
And by detecting these echoes,
they're able to build up
695
00:52:39,228 --> 00:52:41,628
an acoustic image
of the world around them.
696
00:52:41,628 --> 00:52:43,468
That's how they're avoiding me.
697
00:52:46,148 --> 00:52:50,348
But what does it actually mean
to see the world through sound?
698
00:52:50,348 --> 00:52:52,788
Well, it's only now
that we are getting our first
699
00:52:52,788 --> 00:52:54,508
glimpse into this alien world.
700
00:53:08,028 --> 00:53:10,268
In an ancient British woodland,
701
00:53:10,268 --> 00:53:12,708
a futuristic experiment
is under way.
702
00:53:16,108 --> 00:53:20,468
This is the inaugural test flight
of the batcopter,
703
00:53:20,468 --> 00:53:24,348
a machine that will eventually
allow us to see like a bat.
704
00:53:28,148 --> 00:53:31,828
This strange-looking machine
is part bat,
705
00:53:31,828 --> 00:53:35,428
part drone, and as it flies
through the forest, it's blasting
706
00:53:35,428 --> 00:53:39,508
ultrasonic pulses,
just like a real bat.
707
00:53:42,948 --> 00:53:45,228
This is just one of the techniques
708
00:53:45,228 --> 00:53:49,268
Dr Marc Holderied is using to
visualise how bats see their world
709
00:53:49,268 --> 00:53:51,068
through sound.
710
00:53:51,068 --> 00:53:53,588
So, Marc, you've got this really
impressive machine here.
711
00:53:53,588 --> 00:53:55,988
Can you tell us a bit about it
and what it does?
712
00:53:55,988 --> 00:53:59,428
So, this is our Octocopter here,
which is a drone platform, and we
713
00:53:59,428 --> 00:54:03,668
use it to carry around this grey box
here, which is our artificial bat.
714
00:54:03,668 --> 00:54:06,188
It has a little mouth here,
loudspeaker, that sends out very
715
00:54:06,188 --> 00:54:10,828
high-intensity ultrasound into the
habitat that we want to survey.
716
00:54:10,828 --> 00:54:13,348
Above here,
we have an area of 31 microphones,
717
00:54:14,428 --> 00:54:17,868
which capture the echoes
coming back from the environment,
718
00:54:17,868 --> 00:54:20,748
so this is why we call it
our 31-ear bat, really.
719
00:54:23,828 --> 00:54:27,268
I want to be able to produce
a visualisation that tells me
720
00:54:27,268 --> 00:54:29,188
what a bat has seen.
721
00:54:29,188 --> 00:54:30,788
I want to be a bat.
722
00:54:35,188 --> 00:54:39,428
Marc is still fine-tuning
the batcopter's acoustic image.
723
00:54:39,428 --> 00:54:42,468
But using a different technique
he believes he's got a good
724
00:54:42,468 --> 00:54:44,388
idea of what it will look like.
725
00:54:46,348 --> 00:54:49,628
By 3D mapping a stretch
of woodland with a laser,
726
00:54:49,628 --> 00:54:52,788
and tracking the flight paths
of bats flying through,
727
00:54:52,788 --> 00:54:56,468
he's created
these astonishing visualisations.
728
00:55:01,988 --> 00:55:05,268
Wow,
these images are just phenomenal.
729
00:55:05,268 --> 00:55:07,628
It's like something out of
a sci-fi movie.
730
00:55:07,628 --> 00:55:09,308
What we looking at here?
731
00:55:09,308 --> 00:55:11,548
So, this is a cockpit view,
flythrough,
732
00:55:11,548 --> 00:55:14,828
of a real bat flying through
a real forest.
733
00:55:14,828 --> 00:55:17,228
We have slowed this down
by a factor of five.
734
00:55:18,588 --> 00:55:21,468
A bat would experience
and fly through this at five times
735
00:55:21,468 --> 00:55:24,948
the speed that we are looking at,
at the moment.
736
00:55:24,948 --> 00:55:26,788
These images are a visual
737
00:55:26,788 --> 00:55:29,788
representation of what
the bat's world looks like.
738
00:55:32,228 --> 00:55:35,268
But Marc wants to know how
the same scene would appear
739
00:55:35,268 --> 00:55:37,308
through echolocation.
740
00:55:38,388 --> 00:55:41,548
So, he works out which of the
objects in the flight path would
741
00:55:41,548 --> 00:55:44,708
reflect the bat's
high-frequency pulses.
742
00:55:44,708 --> 00:55:47,708
This gives him
the bat's acoustic image.
743
00:55:50,268 --> 00:55:53,188
So what we've done is taken away all
the surfaces, all the reflection
744
00:55:53,188 --> 00:55:56,668
that wouldn't really scatter back
sound and you see it really
745
00:55:56,668 --> 00:56:01,668
dissolves into individual reflectors
but it still works really well.
746
00:56:01,668 --> 00:56:03,628
And this is just trying to navigate.
747
00:56:03,628 --> 00:56:06,628
That's what we have to remember,
this is just navigation
748
00:56:06,628 --> 00:56:09,788
let alone trying to find
your prey in the dark.
749
00:56:09,788 --> 00:56:13,908
Yes, so imagine, one tiny insect,
less than a centimetre,
750
00:56:13,908 --> 00:56:16,268
and this is what you are after.
751
00:56:16,268 --> 00:56:18,988
You have to find dozens
if not hundred of these every night.
752
00:56:18,988 --> 00:56:22,028
I wouldn't know how to do it
and I'm still puzzled
753
00:56:22,028 --> 00:56:25,308
and amazed
by the fact that they can.
754
00:56:25,308 --> 00:56:29,268
It's astonishing to think
what bats achieve using a simple
755
00:56:29,268 --> 00:56:32,028
acoustic image like this.
756
00:56:33,988 --> 00:56:40,508
They can fly through dense woodland
in pitch black,
757
00:56:40,508 --> 00:56:42,828
grab motionless spiders
from their webs,
758
00:56:50,588 --> 00:56:53,788
and even pluck fish from beneath
the surface of the water.
759
00:57:09,948 --> 00:57:13,268
It's the fact that ultrasound
reflects really well off small
760
00:57:13,268 --> 00:57:17,268
objects that allows bats to
use their echolocation with such
761
00:57:17,268 --> 00:57:19,148
deadly precision.
762
00:57:26,908 --> 00:57:30,548
It's captivating to get
this first glimpse of what it
763
00:57:30,548 --> 00:57:32,988
means to see through sound.
764
00:57:34,948 --> 00:57:40,068
This is as close as we've got to
entering the bat's acoustic world.
765
00:57:40,068 --> 00:57:43,428
By tapping into the power
of high frequency sound,
766
00:57:43,428 --> 00:57:46,908
bats have become masters
of the night.
767
00:57:53,028 --> 00:57:56,868
In this episode, we've journeyed
through the natural world of sound.
768
00:57:59,508 --> 00:58:01,148
From the deepest bellows...
769
00:58:03,668 --> 00:58:05,828
..the loudest calls...
770
00:58:07,908 --> 00:58:10,868
..to ears tuned only
to the highest pitches.
771
00:58:13,508 --> 00:58:14,628
Across the planet,
772
00:58:14,628 --> 00:58:19,868
animals have found extraordinary
ways of using sound to survive.
773
00:58:22,748 --> 00:58:27,868
Next time, we explore
the invisible world of scent...
774
00:58:29,868 --> 00:58:32,228
..and discover the bizarre ways
775
00:58:32,228 --> 00:58:36,268
animals use their sense of smell
to get an edge in the wild.
67648
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