Original subtitles

- [birds calling] - [insects chirping]

[Tom Hiddleston] The Amazon rainforest.

There's more life here than anywhere else on Earth.

- [birds continue calling] - [insects continue chirping]

The calls of tens of thousands of different species

make this the richest soundscape on the planet.

[creature calling]

With the airwaves so packed,

the biggest challenge here

is how to make yourself heard above the competition.

And one way of doing this is to pump up the volume.

[howling]

Meet the loudest of all land animals.

The howler monkey.

Howlers live in small troops

and they spend their days feeding in the canopy.

Their favorite trees have the tastiest fruit.

Life here is pretty relaxed.

Apart from once a day

when Dad claims his family's feeding spot by howling.

[howling]

At 140 decibels,

his call is as loud as a jet engine.

[howling]

It can be heard from three miles away.

[howling in distance]

[howling continues]

The secret to this ear-splitting howl is his huge throat sac.

It contains a hollow bone known as the hyoid.

This acts like an echo chamber, deepening and amplifying his voice.

Usually, this long-range warning is enough to keep other groups away...

but not today.

Another troop is closing in...

and want the feeding tree for themselves.

Dad challenges them to the jungle's loudest shouting contest.

[howling]

[howling continues]

As the competition builds, he swaps to shorter barks.

[barking]

A sign of increased aggression.

[howling]

But when the rivals still don't back down,

howlers have one final acoustic strategy.

Mum and Dad join forces.

[howling]

A duet that sends the intruders packing.

Howling uses huge amounts of energy,

but it's worth it, as these monkeys can size each other up through sound alone,

so rarely come to blows.

While in the treetops it may pay to be loud,

on the forest floor, it's a different story.

[creatures chirping]

[hissing quietly]

Deep in the jungle,

we can now eavesdrop on the Amazon's most elusive creatures...

using remote cameras and microphones.

[chirping]

Here, animals are drawn to clay licks, a vital source of minerals.

But surprisingly, no one makes a sound.

Even the howler monkeys are hushed.

Down here, they battle in silence.

A behavior never filmed before.

And there's a good reason for their stealth.

[snarling]

A jaguar.

The Amazon's apex predator.

As he prowls,

the alarm calls of birds follow his every move.

[birds chirping]

It's the rainforest's neighborhood watch.

[snake hissing]

Every animal is listening out for these warnings.

All it takes is one shrill alarm...

[bird calling]

[birds chirping]

...and everyone scarpers.

South America's biggest cat foiled by the rainforest's remarkable alarm system.

The soundscape of the Amazon not only changes from forest floor to canopy,

but also throughout the day.

Around dawn, the cool, still air means sound travels further.

[birds chirping]

The perfect time for primates to broadcast long-range calls.

[monkeys cackling]

It's why birds also sing in the cooler mornings.

[birds screeching, calling]

As humidity builds during the day, sound doesn't travel as far.

Larger animals stop calling, leaving the airwaves open for insects.

[insects chirping]

At dusk,

one of the last birds to call is the great tinamou.

[Great Tinamou calling]

At night, a new chorus takes over.

[frogs croaking]

Frogs call in the dark.

Some even sound like racing cars.

[frogs trilling]

Perfectly timed,

this is nature's most intricate animal orchestra.

Fine-tuned over millions of years.

[cawing]

But not all jungle dwellers stick to this carefully choreographed daily schedule.

The three-fingered sloth, the slowest mammal on the planet.

To avoid predators, they stay hidden and quiet most of the time.

But once a month, to attract a mate, female sloths break their silence...

with the rainforest's most piercing love song.

[squealing]

[squealing continues]

For male sloths, life is laid back.

But the female's shrill invitation is a real wake-up call.

[sloth squealing]

Irresistibly drawn towards her song, this male begins an epic journey...

descending from the safety of the treetops in pursuit of love.

On the ground, he's not the fastest...

[sloth squealing]

...and being shortsighted doesn't help.

[sloth squealing]

As he gets closer,

he ups his speed to nearly one mile an hour.

Not that you'd notice.

Finally, he makes it to the female's tree.

But now he's got competition.

Another male.

The jungle's slowest race is on.

The rivals are on a collision course.

[squealing]

Microphones capture the close-up sounds of the rare sloth fight.

[squealing]

[sloths panting]

Our male throws a fierce left hook

and his rival lets out a whimper.

[whimpers]

It's a sign that he's giving up,

leaving our male to complete his mission and join the female.

Their final coming together takes less than a minute.

It seems it's the only thing sloths do quickly.

United through sound,

the world's slowest mammals find love in the canopy.

But in the battle for the Amazon's airwaves,

there's one daily event that drowns out even the most piercing call.

[thunder rumbling]

Every afternoon,

the patter of each raindrop...

[patters]

...falling on different-sized leaves...

[patters]

...creates different notes...

[patters]

...and builds to a complex beat.

[pattering]

In a single storm,

over a trillion raindrops can fall...

producing a background din so loud...

[pattering]

...most animals simply stop calling.

[thunder rumbling]

After a storm, the birds soon restart their chorus.

[birds chirping]

And one little bird has come up with a unique way of making itself heard.

The club-winged manakin.

This handsome young male is keen to attract a partner.

But in the dense undergrowth, his bright feathers can be hard to spot.

So, he's come up with a nifty way

of getting the attention of any female nearby.

Instead of singing,

he uses his wings...

[whistles]

...as a musical instrument.

[whistles]

It's known as stridulation,

and manakins are the only birds that do it.

By slowing their sound down...

[wings vibrating]

...we can show the secret to his performance.

The feathers slide over each other like a bow playing the strings of a violin.

[vibrating continues]

But unfortunately for this youngster, he isn't the only one performing today.

[birds whistling]

Up to ten different males have gathered in this patch of forest.

To outplay the competition,

our male needs to put on a virtuoso performance.

With a female nearby, now is his time to shine.

[chirping]

[whistles]

[chirps, whistles]

[chirps, whistles]

[chirping]

It seems to be going well.

[chirps, whistles]

[chirps, whistles]

[chirps]

[vibrating]

But when he plays off-key...

[whistles]

...there are no second chances.

Practice makes perfect.

[whistles]

In time, he'll become a musical maestro.

But of all the unusual ways animals make themselves heard in the Amazon,

surely the strangest takes place deep in the undergrowth...

where there's a hidden acoustic world beyond our hearing.

A treehopper.

These insects make sounds in a highly unusual way.

[vibrating]

By quivering their abdomen,

they send out a wave of vibrations

that travel down through their legs

and along the stems of the plants like their own telephone lines.

[vibrating]

Now using technology that transforms vibrations into sound,

we can eavesdrop on their secret conversations.

[vibrating]

And treehoppers sound as weird as they look.

[chirping]

They're calling for mates

and to let each other know they found food.

[warbling, vibrating]

[warbling]

[vibrating, chirping]

Communication in some treehoppers

has reached even more remarkable levels of complexity.

This green treehopper is a caring mother.

She looks after her nursery of nymphs.

Each only the size of a grain of rice.

Treehoppers are some of the only insects that show maternal care.

And it's just as well, because the forest is full of dangers.

An assassin bug, a venomous predator.

As soon as the nymphs sense its footsteps,

they send out an alarm.

[vibrating]

A series of vibrations that ripple through the nursery.

[vibrating]

Mum detects their alarm call.

The nymphs' Mexican wave...

[vibrating]

...alerts her to exactly where the danger is.

She follows their signals.

Mum now has a fight on her hands.

But those legs that are so good at sending and receiving messages

are also pretty handy at a spot of kung fu.

[vibrating]

With the assassin bug defeated,

Mum returns to the nursery and sends soothing vibrations to her young.

[vibrating]

The secret language of treehoppers is so complex,

scientists believe it rivals that of some vertebrates.

A hidden world of sound deep within the forest.

Just one of the thousands of amazing voices

that make the Amazon the richest soundscape on the planet.

To capture the complex sounds of the world's largest rainforest,

sound recordist Ellie Williams traveled deep into the jungles of Peru.

To experience the Amazon at its best, she would have to get up high.

[speaking indistinctly]

[Williams] My sound world is very much the level of my own ears

and there is so much that goes on up in the canopy,

so I was really keen to get up.

[Hiddleston] With her microphone dangling below,

Ellie headed upwards.

But this is not for the fainthearted,

especially when you find you're giving a lift

to the Amazon's most painful stinging insect.

[Williams] I'm not very good with heights.

- [person] And stop. - Wanna stop there?

[Williams] But the thing that scared me was I was about ten meters off the ground

and I noticed a bullet ant.

Uh, I've got a bullet ant on my lap right under my bag.

I can't see it. It's really... No, it is a bullet ant. It's massive.

I've been hearing about how you get eight hours worth of pain

from one bite.

Oh, I can see it.

Yeah. Oh.

Yeah, I knock... I knocked it off. I knocked it off. It was massive.

[chuckles] That's...

Oh, my God. That's way more scary than, um, big heights.

- [person] Are you ready to go up, Ellie? - [chuckles] Uh...

Oh, yeah.

Uh, do you think it was on its own or do you think it had a girlfriend?

Holy moly.

Oh, my God.

[Hiddleston] With no more stowaways,

Ellie could now focus on her recording,

and she soon discovered that the forest soundscape is made up of distinct layers.

[Williams] I was kind of going up a few meters and then stopping for a minute.

Can you go a little bit higher, Ollie?

[Ollie] We can, Ellie. Yeah.

[Williams] The sound seemed to change quite a lot

as I was moving up through the tree.

Great. Thanks. Amazing.

[Hiddleston] With the canopy towering 190 feet above the ground,

Ellie could hear a whole new world.

- [birds calling] - [insects chirping]

[bird caws]

[Williams] There seemed to be more going on higher up because I think...

because I was at the top of the canopy,

the sounds from all around were coming from further away

and I was able to hear them. So, it felt really rich up there.

[Hiddleston] And for Ellie, one canopy sound soon became her favorite.

[bird chirping]

[Williams] I love the crested oropendola,

which sounds like it's from Star Wars.

It sounds almost robotic and I just... I absolutely love it.

[bird chirping]

[Hiddleston] With up to 90% of all life here found in the canopy,

climbing high gave Ellie a chance

to record the noisiest part of the rainforest,

and the most biodiverse place on Earth.

- [insects chirping] - [monkey cackling]

[bird calling]