The ancient eucalypt forests of Australia.
A prehistoric world teeming with bizarre and captivating sounds.
Isolated for over 35 million years...
the animals here have evolved weird and wonderful ways to communicate.
Especially when it comes to finding a mate.
The superb lyrebird, one of the world's oldest surviving songbirds.
Each winter to attract a mate,
males take part in the ultimate avian talent show.
At 15 years old, this experienced male is something of a star.
But with younger rivals jostling for the airwaves,
this season he will have to put on the performance of a lifetime.
First, he clears his arena.
Everything has to be just right.
Then his show begins.
Lyrebirds are master mimics.
First, a whipbird.
It's a convincing start.
And it's caught the attention of a female.
She will choose a partner who does the most accurate impressions.
So next, he pulls out an Australian classic.
The Kookaburra.
Not bad.
The female seems interested. So he continues with a cockatoo.
Seventy percent of his repertoire is mimicry,
learned from listening to older lyrebirds and copying them.
To sing such complex and varied songs,
lyrebirds are able to produce multiple notes at the same time.
Slowing their song down reveals the secret.
They have the avian equivalent of two voice boxes.
One produces lower tones.
The other higher ones.
Allowing them to switch notes effortlessly.
Barely stopping for breath.
Drawn in by his song...
the female approaches.
And this is when he reveals his showstopper.
A perfectly choreographed song and dance routine.
But only when she gets close, does he reveal his grand finale.
The laser gun.
For a female, it's irresistible.
Lyrebirds may be the star performers in Australia's forests...
but across the year, the sound of the forest builds
as different animals take turns to broadcast their courtship calls.
We can hear this change on an acoustic journey through the seasons.
In spring, the airwaves are filled with the love songs
of more than 300 bird species.
In the sweltering summers, insects buzz to attract a female.
And at the start of the wet season...
the patter of rain builds.
Ninety percent of the annual rainfall occurs in just six months,
turning rain forest rivers into thundering waterfalls.
Now, animals must shout to be heard.
And it's some of the forest's smallest inhabitants that have the biggest voices.
Australia has over 200 frog species.
Each with a unique mating call.
This female orange-thighed tree frog may only lay eggs once a year.
So tonight she's listening out for Mr. Right.
But there's so many species calling, it's hard to pick out who's who.
Luckily, frogs have a special way of hearing
that's totally unique in the animal kingdom.
Their lungs are connected to their ear drums.
And when inflated, it's believed they dampen out background sound.
Rather like noise canceling headphones.
It allows this female to filter out the hubbub...
singling out the perfect partner.
Amidst the cacophony of the frog chorus.
The wet season triggers other strange sounds in the Australian forests.
More subtle calls that are used not to attract a mate...
but to protect precious young.
Along the rivers of the northern territory...
lurks Australia's most fearsome predator.
The saltwater crocodile.
Up to 20 feet long, these giants can live for 70 years.
But to survive the first day of their lives, sound is key.
This female is soon to be a mother.
Three months ago, she laid a clutch of 30 eggs
and buried them deep inside a nest of sticks and mud...
to incubate and protect them.
But as soon as the eggs hatch, they'll face dangers.
Pied herons scour this riverbank
and could easily pick off a croc hatchling.
To keep guard,
mum has stuck close by her nest for the last 90 days.
She's been listening out for one particular sound.
A low-pitched murmur from inside one of the eggs.
Soon another egg joins in.
Then another.
The baby crocs are talking to each other.
And they're hatching a plan to all break out at the same time.
Mum is able to hear these faint chirps from over 30 feet away
through ear slits just behind her eyes.
She heads to the nest to check it out.
Crocs don't just hear through their ears.
Placing her head on the nest, she uses receptors along her jaw...
to pick up vibrations from below.
When she knows the time is right, she starts to dig.
Surprisingly, scientists have discovered
that the sound of digging encourages the eggs to hatch.
As the first youngsters emerge into the world...
their mother leads the way to the river...
and waits.
Chirping to keep in touch,
the young crocodiles scramble towards the water
and the safety of mum.
But all this noise hasn't gone unnoticed.
Herons start to gather.
One last hatchling remains.
Alone, the odds are against him.
But female crocodiles are fiercely protective.
With all the youngsters in the river, the immediate danger is over.
Continuing to chirp to each other,
the hatchlings will use these calls for the next five weeks...
to stick together in a crocodile crèche.
Whilst many animals in the Australian forests
produce sounds that are audible to the human ear,
some creatures detect sound in a completely different way.
A female orb weaver spider...
the size of a human hand.
Spiders don't have ears.
Instead, they pick up vibrations through receptors in their legs.
Feeling sound rather than hearing it.
Now, using laser technology that transforms vibrations into sound...
we can tune in to the spider's web and enter an alien world.
Virtually blind, her web acts as a giant acoustic sensor,
allowing her to listen in to the world around her.
She's waiting for one sound above all others.
The impact sends ripples up each silk string,
allowing her to pinpoint her prey...
and devour it.
For the miniscule male...
just a tenth of her size, this makes courtship dangerous.
He must tread carefully,
or she may mistake him for pray and eat him.
So he turns the web into a musical instrument...
and serenades her.
Plucking the silk strings,
he reassures the female he's friend, not foe.
Next, he shakes his abdomen to produce a pulsing vibration across the web.
Known as tremulation.
It puts the female into a docile trance.
These sound signals allow the male orb weaver to edge closer to the female.
And if he's lucky, to mate with her.
This time, music really is the food of love.
But there's one love song in the Australian forests
that is so deep and resonant, it defies the rules of physics.
And it's produced by the most unlikely of animals.
The koala.
Famous for not doing very much at all,
these eucalyptus-munching marsupials are quiet for much of the year.
But each spring, for just a few weeks, all that changes.
This big male is searching for a mate.
High in the canopy, it's hard to see potential partners,
so the best way to make yourself known is to do this.
A call this low should only be possible in an animal 20 times the size of a koala.
But scientists have discovered their secret.
Males have an extra set of vocal cords that are 15 times wider than normal...
allowing the koala to make a sound as deep as an elephant's rumble.
His deep bellows can be heard from half a mile away...
and catch the attention of a female.
But there are other male suitors here eager to impress...
and soon they join the battle for the airwaves.
Smaller males can't call as low.
And unfortunately for them,
females prefer males with the lowest voices.
Because deep bellows are a sign of size and strength.
Eventually, the big male silences the competition...
and can now approach the female without being challenged.
But first, he must track her down.
Koalas only come to the ground to move between trees, but are surprisingly agile.
He spots the female and approaches.
Has she been impressed?
This isn't the response he was hoping for.
A rejection call.
Today, even the deepest bellow isn't enough to win her over.
For a moment, things are quiet.
But it won't be long
before another captivating song fills the airwaves
of Australia's ancient forests.
To capture the enchanting chorus of Australian tree frogs,
the Earthsounds team traveled into the rain forest
of northern Queensland to record a particular species
that only calls one or two nights of the year.
To help, they're joined by renowned frog scientist, Jodi Rowley.
I'm really hoping that we hear a species that we haven't heard in quite some time,
which is the orange-thighed tree frog.
These rarely seen little frogs live high up in the canopy,
only coming down to breed after rain forms puddles on the forest floor.
And tonight, conditions are perfect.
It's rained really, really heavily today.
So I'm hoping that we'll find some orange-thighed tree frogs
because they love it when it pours with rain.
In the thick bush,
these tiny frogs are incredibly hard to spot.
So while I'm actually looking for frogs here with these torches,
the best way to actually find frogs is just to listen.
The tree frogs aren't the only ones out in this wet weather.
Up to 40 species of frog are calling,
bringing the forest to life with their ribbits...
barks,
and honks.
Okay. I can hear a barred frog somewhere in this leaf litter.
I have no idea where it is now.
There it is.
So, look at how camouflaged this beautiful frog is.
If he hadn't called out, I would never have found him.
I love these guys.
But time is of the essence.
If Jodi is going to hear the elusive tree frogs in this cacophony,
she will need to keep moving.
I can hear them.
This is really, really exciting. I can hear the orange-thighed tree frogs.
You know, I haven't actually seen them or heard them for 20 years.
Think we're getting closer.
Very close.
There. Look at that.
Look at the beautiful frogs.
You can see one over there. One there, one there.
Oh, my gosh. There's a lot more of them than I thought.
There is just so many male frogs calling right now
that I can barely hear myself talk.
Listen to them all.
This cacophony is a sign of a balanced ecosystem.
Frogs are incredibly sensitive to environmental change
and their calls can be used to monitor the health of the rain forest.
But for Jodi, it's also a magical moment.
I feel like I've hit the jackpot.
This is something that you don't get to see very often.
And for me being in a frog chorus and a chorus of this species,
which is pretty hard to get in chorus, is heaven.
I'm in heaven right now. This is really, really awesome.
Having so little time, the team moves quickly...
recording the sounds of the chorus and the unique calls of individual frogs.
We could come a 100 times to this stream,
to this spot, and not see them.
But we've hit the night.
With Jodi's expertise and a bit of luck,
the team manages to record the rarely heard chorus
of one of the smallest but loudest inhabitants of Australia's forests.