All language subtitles for Earthsounds.S01E10.ATVP.WEB-DL.DDP5.1.H.264-NTb.en[cc]

[elephants rumbling]

[Hiddleston] The Okavango Delta.

Africa's water world.

Covering over 6,000 square miles,

this mix of wetland and islands
is awash with sound.

[birds chirping]

To thrive here,
animals must master life both above...

and below the surface.

The warm waters are rich
with the sound of flitting fish.

Over 80 different species.

An all-you-can-eat feast
for the birds here.

But catching them means overcoming

the acoustic challenge
of hunting in water.

The splash of a strike...

alerts the shoals
and gives them a head start.

[birds squawking]

But one high-speed hunter
has found an ingenious way around this.

[chirping]

The pied kingfisher.

A small bird with a big beak,

a third of its body length.

[chirping continues]

When defending their territories,
they produce a stream of shrill chirps.

But when hunting,
they are masters of stealth.

Wings beating eight times a second

as they hover 30 feet above the water.

Setting their sights, they dive.

As they hit the water,

kingfishers use their sonic secret weapon...

to catch fish unawares.

Their long cone-shaped beaks

can enter the water
without making a sound...

or even a ripple.

Giving the birds
a 15-millisecond head start on their prey.

The shape of their beaks
makes them so stealthy,

it's influenced
the design of high-speed trains.

The long, pointed nose
preventing sonic booms

as the engines rocket through tunnels.

The kingfishers' ability to minimize sound

make them one of the Delta's
most successful anglers.

From small silent hunters...

[rumbling]

...to noisy giants...

the Delta is one of the world's
most diverse wetlands.

And every animal here
has its own signature sound.

Whistling ducks are just
one of over 400 bird species.

[ducks whistling]

Aerial predators like the fish eagle

spark alarm calls in chacma baboons...

[baboons grunting, howling]

...which trigger panic
in herds of lechwe antelope.

With so many creatures
gathered in such proximity...

[elephant trumpeting, rumbling]

...this water world
is constantly buzzing with life.

[birds chirping, shrieking]

But even in Africa's great wetlands
there are areas of higher dry land...

[grunting]

...that reverberate with astonishing sounds.

[animals twittering]

These high-frequency twitters
don't belong to any bird.

[twittering continues]

This is the magnificent dialect
of African wild dogs...

and their playful pups.

[twittering, whining]

[sniffing]

For the pack, it's all about keeping
the little ones safe and fed.

[twittering]

To do that,

they've evolved a complex language
of nearly 30 different calls.

[twittering continues]

[whining]

Despite their constant chatter,

remarkably, the high-frequency sounds
are unlikely to be heard from afar...

as they are easily broken up by bushes.

But the most bizarre thing
about wild dog communication

is how the pack decides to start hunting.

Recent studies show they signal...

[sneezes]

...with a sneeze.

Each one is equivalent to a vote.

The more sneezes,
the more likely they are to hunt.

[sneezes]

- [sneezing]
- This strangest of voting systems

is unique to African wild dogs.

When enough of them are on board
the pack sets out,

leaving the pups to practice.

Relentless chatter is now exchanged
for near silence.

Because noise not only
alerts potential prey...

but larger predators too.

[panting]

Lions or hyena
will often steal their kills.

Today's target... wildebeest.

[grunting, snorting]

[snorting]

Wildebeest are faster than the dogs.

But the pack's phenomenal endurance...

and ability to work as a team...

makes them
Africa's most successful large predators.

[wildebeest grunting]

[chirps]

Back at the den...

another of the dogs'
complex communications is revealed.

[squeaking]

The returning hunters
have stored meat from the kill.

And when the little ones
make this particular call...

[squeaking]

...it triggers the adults
to share their spoils.

[squeaking continues]

It's one of the first calls
pups must learn.

Part of a rich and complex language,

the most sophisticated of any dog species.

[squeaking]

For prey that gather
on the Delta's islands,

paying close attention to sound
is a key part of staying off the menu.

Here, an army of lookouts

creates a neighborhood watch scheme
like no other.

Dozens of eyes and ears on the alert.

Useful when there's a leopard about.

One of Africa's stealthiest hunters.

But not all alarm calls
are treated equally.

Studies have shown,
the smaller the species...

[squawking]

...the less likely it is
that others will react to their warnings.

For little animals are easy targets...

[monkeys chattering]

...and that makes them jumpy.

[chattering continues]

[barks]

A quarter of barks from
the lightweight impala are false alarms.

They're even spooked
by grass blowing in the wind.

[barks]

These skittish antelope...

[antelope barks]

...are often the first to sound the alarm

while everyone else ignores them.

But if larger and less vulnerable zebra...

- [snorts]
- ...or kudu...

[kudu grunting]

...give a warning...

[grunts]

...everyone scarpers.

[panting]

But there's one small animal
whose alarm call...

[animal chittering]

...prompts a different response.

The African tree squirrel.

[purring]

A leopard's nemesis.

[squeaking]

Their persistent calls
can be heard from 500 feet away.

[squeaking continues]

On hearing a warning...

[squeaking]

...the family members rush together
and mob the hunter.

Their system clearly pays off,

as the squirrels make up less than
half a percent of a leopard's diet.

[squeaking]

Their warning calls
not only alert the neighbors,

they let the predator know...

they've been spotted and should move on.

His tail lifted,
the leopard signals he's given up.

A cat that's pound for pound
the strongest predator in Africa,

driven away by
the simple squeak of a squirrel.

[birds shrieking]

But not all calls on the Okavango Delta
are warnings.

Some are all about attraction.

[chirping]

Beneath the surface, a curious love song
travels through the lagoons.

[chirping continues]

Produced by tiny aquatic animals
less than a quarter of an inch long.

[chirping]

Water boatmen.

Surprisingly,
some produce high-frequency clicks

by rubbing their genitals
against their stomach.

It may be an unusual way of attracting
a mate, but it's highly effective.

Incredibly, relative to body size,

it's a water boatman that has
the loudest call of any animal on Earth.

[chirping]

But these freshwater lagoons echo
to an even weirder sound.

[hissing]

As aquatic plants photosynthesize
in the hot sun,

they release oxygen.

[hissing continues]

The air bubbles, squeezing out of pores
on the plant's surface,

creating the strange but reassuring buzz
of a healthy freshwater world.

Against this background hum...

[grunts]

...underwater microphones
pick up mysterious groans and grunts.

[groaning, grunting]

Not from small aquatic insects...

but amphibious giants.

[groaning]

Part of the secret language
of 11 different calls...

[grunts]

- ...produced by the hippopotamus.
- [hippopotamus grunts]

[snorts, honks]

Hippos live in chatty groups or pods.

This one is made up of around 20 females...

and young calves,
who spend their days on mum's back.

Well, that's the idea.

The pod is overseen by a huge,
dominant male.

[honking]

For hippo families,
staying in touch is crucial.

[honking continues]

And they do this
through a surprising use of sound.

Above the water,
their trademark wheeze honks...

[amplified honking]

...carry for over half a mile.

But hippos are one of the few animals

that can communicate above
and below the surface...

[honks]

...at the same time.

- [honking]
- [distorted honking]

[distorted honking continues]

When underwater,

their calls are believed to be
detected through sensitive jawbones,

which conduct sound waves
directly into the ear.

On hearing a sub-aquatic honk...

[distorted honking]

...everyone heads to the surface...

for a hippo head count.

It allows pods to check on each other...

as these animals
can roam for miles each day.

[honking]

[distorted honking]

But new science has revealed that
the animals have another acoustic ability.

[clicking]

Below the surface, hippos can emit
a series of rapid-fire clicks.

[clicking continues]

It's believed they listen to the sounds
bouncing back from objects around them...

[clicking]

...and use this information
to create a sonic map.

An incredible discovery
that suggests hippos can echolocate

much like their distant relatives, dolphins.

A skill which helps them navigate

and find each other
in the Delta's murky waters.

We now know 80% of hippo vocalizations
occur beneath the surface.

[grunting, honking]

By using sound both above and below,

hippos have mastered life
in Africa's water world.

To record the secret
underwater conversations of hippos,

the Earthsounds crew
had to come up with innovative ways

to get close to
one of Africa's most dangerous animals.

[honking]

I've worked with, uh, hippos a lot,
and, ja, quite a tricky customer.

[honking]

They've killed more people
than any other mammal on the continent,

so to try and capture the way
they communicate underwater

is very difficult.

[Hiddleston] It's far too dangerous
to get in the water,

so the crew must try and float cameras
and microphones towards the pod.

Well, they look like
they're on the right track.

See if this works. [chuckles]

[honks]

[Hiddleston] It's a promising start,

but relying on water currents
doesn't always go to plan.

Damn it. Looks like, uh, the cameras
are stuck right next to hippos,

so we gonna have to go pretty close
to fetch them. [sighs]

[Hiddleston]
The crew need to approach with care,

particularly when the males are around.

Just watch this boy 'cause he was...
He was coming up to us snorting, so...

[Hiddleston] Weighing up to four tons,
bull males are highly territorial.

Gotta keep an eye on that bull.

[honking]

He's coming towards us.

[Hiddleston]
If the hippo gets underneath the boat,

he could easily capsize it.

[Amos] Take your seats.

[Hiddleston] This time...

[exhales sharply]
That was close. [chuckles]

[Hiddleston] ...he moves off.

Yoh! That was a bit tidy.

[Hiddleston] Getting
the underwater footage is hard enough,

but sound recordist Phil Bax
wants to record hippo calls

above and below the surface
simultaneously.

[honking]

I've got a microphone
sticking up out of the water,

and then another microphone
called a hydrophone,

which records sound in the water.

So it'll record both above and below
at the same time,

and, hopefully,
let us hear exactly what the hippos hear.

[distorted groaning]

[honking, clicking, groaning]

[Bax] Oh, that's good.

They've got a whole language of clicks
and croaks and moaning sounds

going on underneath the water.

[clicking]

[Hiddleston]
Phil's recordings back up the new science

that suggests hippos can echolocate.

The clicking noises that we're getting
underneath the water make sense to me...

[clicking]

...'cause the water's quite murky
at the moment,

and they wouldn't be able to
see particularly well.

They're using sound
as a way of finding each other

or finding the things
that they're looking for.

[Hiddleston] And this sixth sense
is becoming more important than ever.

In recent years, changes brought about
by our heating climate

have reduced visibility in the Delta...

meaning hippos could become increasingly
reliant on this skill to navigate.

It's just one way
that animals are using sound...

[clicking]

...to adapt to our rapidly changing planet.

[snorts, honking]