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The ancient eucalypt forests of Australia.

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A prehistoric world teeming with bizarre
and captivating sounds.

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{\an8}Isolated for over 35 million years...

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the animals here have evolved weird
and wonderful ways to communicate.

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Especially when it comes
to finding a mate.

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The superb lyrebird, one of the world's
oldest surviving songbirds.

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Each winter to attract a mate,

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males take part
in the ultimate avian talent show.

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At 15 years old, this experienced male
is something of a star.

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But with younger rivals jostling
for the airwaves,

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this season he will have to put on
the performance of a lifetime.

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First, he clears his arena.

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Everything has to be just right.

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Then his show begins.

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Lyrebirds are master mimics.

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First, a whipbird.

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It's a convincing start.

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And it's caught the attention of a female.

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She will choose a partner
who does the most accurate impressions.

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So next,
he pulls out an Australian classic.

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The Kookaburra.

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Not bad.

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The female seems interested.
So he continues with a cockatoo.

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Seventy percent of his repertoire
is mimicry,

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learned from listening to older lyrebirds
and copying them.

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To sing such complex and varied songs,

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lyrebirds are able to produce
multiple notes at the same time.

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Slowing their song down
reveals the secret.

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They have the avian equivalent
of two voice boxes.

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One produces lower tones.

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The other higher ones.

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Allowing them
to switch notes effortlessly.

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Barely stopping for breath.

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Drawn in by his song...

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the female approaches.

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And this is when he reveals
his showstopper.

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A perfectly choreographed song
and dance routine.

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But only when she gets close,
does he reveal his grand finale.

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The laser gun.

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For a female, it's irresistible.

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Lyrebirds may be the star performers
in Australia's forests...

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but across the year,
the sound of the forest builds

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as different animals take turns
to broadcast their courtship calls.

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We can hear this change on
an acoustic journey through the seasons.

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In spring, the airwaves
are filled with the love songs

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of more than 300 bird species.

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In the sweltering summers,
insects buzz to attract a female.

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And at the start of the wet season...

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the patter of rain builds.

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Ninety percent of the annual rainfall
occurs in just six months,

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turning rain forest rivers
into thundering waterfalls.

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Now, animals must shout to be heard.

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And it's some of the forest's smallest
inhabitants that have the biggest voices.

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Australia has over 200 frog species.

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Each with a unique mating call.

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This female orange-thighed tree frog
may only lay eggs once a year.

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So tonight she's listening out
for Mr. Right.

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But there's so many species calling,
it's hard to pick out who's who.

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Luckily,
frogs have a special way of hearing

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that's totally unique
in the animal kingdom.

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Their lungs are connected
to their ear drums.

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And when inflated, it's believed
they dampen out background sound.

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Rather like noise canceling headphones.

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It allows this female
to filter out the hubbub...

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singling out the perfect partner.

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Amidst the cacophony of the frog chorus.

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The wet season triggers other
strange sounds in the Australian forests.

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More subtle calls that are used not
to attract a mate...

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but to protect precious young.

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Along the rivers
of the northern territory...

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lurks Australia's most fearsome predator.

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The saltwater crocodile.

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Up to 20 feet long,
these giants can live for 70 years.

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But to survive the first day
of their lives, sound is key.

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This female is soon to be a mother.

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Three months ago,
she laid a clutch of 30 eggs

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and buried them deep inside a nest
of sticks and mud...

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to incubate and protect them.

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But as soon as the eggs hatch,
they'll face dangers.

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Pied herons scour this riverbank

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and could easily pick off
a croc hatchling.

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To keep guard,

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mum has stuck close
by her nest for the last 90 days.

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She's been listening out
for one particular sound.

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A low-pitched murmur
from inside one of the eggs.

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Soon another egg joins in.

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Then another.

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The baby crocs are talking to each other.

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And they're hatching a plan
to all break out at the same time.

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Mum is able to hear these faint chirps
from over 30 feet away

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through ear slits just behind her eyes.

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She heads to the nest to check it out.

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Crocs don't just hear through their ears.

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Placing her head on the nest,
she uses receptors along her jaw...

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to pick up vibrations from below.

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When she knows the time is right,
she starts to dig.

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Surprisingly, scientists have discovered

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that the sound of digging
encourages the eggs to hatch.

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As the first youngsters emerge
into the world...

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their mother leads the way to the river...

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and waits.

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Chirping to keep in touch,

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the young crocodiles
scramble towards the water

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and the safety of mum.

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But all this noise hasn't gone unnoticed.

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Herons start to gather.

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One last hatchling remains.

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Alone, the odds are against him.

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But female crocodiles
are fiercely protective.

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With all the youngsters in the river,
the immediate danger is over.

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Continuing to chirp to each other,

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the hatchlings will use these calls
for the next five weeks...

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to stick together in a crocodile crèche.

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Whilst many animals
in the Australian forests

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produce sounds that are audible
to the human ear,

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some creatures detect sound
in a completely different way.

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A female orb weaver spider...

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the size of a human hand.

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Spiders don't have ears.

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Instead, they pick up vibrations
through receptors in their legs.

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Feeling sound rather than hearing it.

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Now, using laser technology
that transforms vibrations into sound...

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we can tune in to the spider's web
and enter an alien world.

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Virtually blind,
her web acts as a giant acoustic sensor,

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allowing her to listen in
to the world around her.

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She's waiting for one sound
above all others.

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The impact sends ripples
up each silk string,

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allowing her to pinpoint her prey...

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and devour it.

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For the miniscule male...

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just a tenth of her size,
this makes courtship dangerous.

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He must tread carefully,

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or she may mistake him for pray
and eat him.

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So he turns the web
into a musical instrument...

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and serenades her.

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Plucking the silk strings,

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he reassures the female
he's friend, not foe.

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Next, he shakes his abdomen to produce
a pulsing vibration across the web.

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Known as tremulation.

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It puts the female into a docile trance.

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These sound signals allow the male
orb weaver to edge closer to the female.

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And if he's lucky, to mate with her.

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This time,
music really is the food of love.

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But there's one love song
in the Australian forests

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that is so deep and resonant,
it defies the rules of physics.

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And it's produced by
the most unlikely of animals.

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The koala.

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Famous for not doing very much at all,

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these eucalyptus-munching marsupials
are quiet for much of the year.

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But each spring, for just a few weeks,
all that changes.

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This big male is searching for a mate.

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High in the canopy,
it's hard to see potential partners,

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so the best way to make yourself known
is to do this.

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A call this low should only be possible
in an animal 20 times the size of a koala.

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But scientists have discovered
their secret.

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Males have an extra set of vocal cords
that are 15 times wider than normal...

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allowing the koala to make a sound
as deep as an elephant's rumble.

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His deep bellows can be heard
from half a mile away...

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and catch the attention of a female.

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But there are other male suitors here
eager to impress...

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and soon they join the battle
for the airwaves.

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Smaller males can't call as low.

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And unfortunately for them,

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females prefer males
with the lowest voices.

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Because deep bellows are a sign of size
and strength.

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Eventually,
the big male silences the competition...

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and can now approach the female
without being challenged.

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But first, he must track her down.

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Koalas only come to the ground to move
between trees, but are surprisingly agile.

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He spots the female and approaches.

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Has she been impressed?

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This isn't the response he was hoping for.

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A rejection call.

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Today, even the deepest bellow
isn't enough to win her over.

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For a moment, things are quiet.

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But it won't be long

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before another captivating song
fills the airwaves

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of Australia's ancient forests.

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To capture the enchanting chorus
of Australian tree frogs,

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the <i>Earthsounds</i> team traveled
into the rain forest

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of northern Queensland
to record a particular species

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that only calls one or two nights
of the year.

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To help, they're joined
by renowned frog scientist, Jodi Rowley.

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{\an8}I'm really hoping that we hear a species
that we haven't heard in quite some time,

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{\an8}which is the orange-thighed tree frog.

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These rarely seen
little frogs live high up in the canopy,

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only coming down to breed after rain
forms puddles on the forest floor.

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And tonight, conditions are perfect.

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It's rained really, really heavily today.

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So I'm hoping that we'll find
some orange-thighed tree frogs

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because they love it when it pours
with rain.

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In the thick bush,

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these tiny frogs
are incredibly hard to spot.

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So while I'm actually looking
for frogs here with these torches,

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the best way to actually find frogs
is just to listen.

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The tree frogs aren't
the only ones out in this wet weather.

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Up to 40 species of frog are calling,

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bringing the forest to life
with their ribbits...

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barks,

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and honks.

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00:28:59,656 --> 00:29:04,161
Okay. I can hear a barred frog
somewhere in this leaf litter.

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I have no idea where it is now.

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There it is.

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00:29:13,128 --> 00:29:17,007
So, look at how camouflaged
this beautiful frog is.

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If he hadn't called out,
I would never have found him.

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I love these guys.

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00:29:24,097 --> 00:29:25,891
But time is of the essence.

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00:29:26,934 --> 00:29:31,021
If Jodi is going to hear
the elusive tree frogs in this cacophony,

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she will need to keep moving.

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I can hear them.

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This is really, really exciting.
I can hear the orange-thighed tree frogs.

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00:29:42,115 --> 00:29:46,662
You know, I haven't actually seen them
or heard them for 20 years.

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00:29:47,246 --> 00:29:48,622
Think we're getting closer.

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00:29:49,998 --> 00:29:50,999
Very close.

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00:29:52,042 --> 00:29:54,336
There. Look at that.

216
00:29:55,337 --> 00:29:56,922
Look at the beautiful frogs.

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00:29:58,382 --> 00:30:02,010
You can see one over there.
One there, one there.

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00:30:02,010 --> 00:30:04,555
Oh, my gosh.
There's a lot more of them than I thought.

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00:30:06,557 --> 00:30:09,893
There is just so many male frogs calling
right now

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that I can barely hear myself talk.

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Listen to them all.

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This cacophony is a sign
of a balanced ecosystem.

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Frogs are incredibly sensitive
to environmental change

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and their calls can be used
to monitor the health of the rain forest.

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But for Jodi, it's also a magical moment.

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I feel like I've hit the jackpot.

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This is something that you don't get
to see very often.

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And for me being in a frog chorus
and a chorus of this species,

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which is pretty hard to get in chorus,
is heaven.

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I'm in heaven right now.
This is really, really awesome.

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Having so little time,
the team moves quickly...

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recording the sounds of the chorus
and the unique calls of individual frogs.

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We could come a 100 times to this stream,

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to this spot, and not see them.

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00:31:21,840 --> 00:31:23,884
But we've hit the night.

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With Jodi's expertise and a bit of luck,

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the team manages to record
the rarely heard chorus

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of one of the smallest but loudest
inhabitants of Australia's forests.