The Okavango Delta.
Africa's water world.
Covering over 6,000 square miles,
this mix of wetland and islands is awash with sound.
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.
But one high-speed hunter has found an ingenious way around this.
The pied kingfisher.
A small bird with a big beak,
a third of its body length.
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...
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.
Aerial predators like the fish eagle
spark alarm calls in chacma baboons...
which trigger panic in herds of lechwe antelope.
With so many creatures gathered in such proximity...
this water world is constantly buzzing with life.
But even in Africa's great wetlands there are areas of higher dry land...
that reverberate with astonishing sounds.
These high-frequency twitters don't belong to any bird.
This is the magnificent dialect of African wild dogs...
and their playful pups.
For the pack, it's all about keeping the little ones safe and fed.
To do that,
they've evolved a complex language of nearly 30 different calls.
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...
with a sneeze.
Each one is equivalent to a vote.
The more sneezes, the more likely they are to hunt.
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.
Lions or hyena will often steal their kills.
Today's target... wildebeest.
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.
Back at the den...
another of the dogs' complex communications is revealed.
The returning hunters have stored meat from the kill.
And when the little ones make this particular call...
it triggers the adults to share their spoils.
It's one of the first calls pups must learn.
Part of a rich and complex language,
the most sophisticated of any dog species.
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...
the less likely it is that others will react to their warnings.
For little animals are easy targets...
and that makes them jumpy.
A quarter of barks from the lightweight impala are false alarms.
They're even spooked by grass blowing in the wind.
These skittish antelope
are often the first to sound the alarm
while everyone else ignores them.
But if larger and less vulnerable zebra...
or kudu...
give a warning...
everyone scarpers.
But there's one small animal whose alarm call...
prompts a different response.
The African tree squirrel.
A leopard's nemesis.
Their persistent calls can be heard from 500 feet away.
On hearing a warning...
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.
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.
But not all calls on the Okavango Delta are warnings.
Some are all about attraction.
Beneath the surface, a curious love song travels through the lagoons.
Produced by tiny aquatic animals less than a quarter of an inch long.
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.
But these freshwater lagoons echo to an even weirder sound.
As aquatic plants photosynthesize in the hot sun,
they release oxygen.
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,
underwater microphones pick up mysterious groans and grunts.
Not from small aquatic insects...
but amphibious giants.
Part of the secret language of 11 different calls...
produced by the hippopotamus.
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.
For hippo families, staying in touch is crucial.
And they do this through a surprising use of sound.
Above the water, their trademark wheeze honks...
carry for over half a mile.
But hippos are one of the few animals
that can communicate above and below the surface...
at the same time.
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...
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.
But new science has revealed that the animals have another acoustic ability.
Below the surface, hippos can emit a series of rapid-fire clicks.
It's believed they listen to the sounds bouncing back from objects around them...
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.
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.
I've worked with hippos a lot, and, ja, quite a tricky customer.
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.
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.
It's a promising start,
but relying on water currents doesn't always go to plan.
Damn it. Looks like the cameras are stuck right next to hippos,
so we gonna have to go pretty close to fetch them.
The crew need to approach with care,
particularly when the males are around.
Just watch this boy 'cause he was coming up to us snorting, so...
Weighing up to four tons, bull males are highly territorial.
Gotta keep an eye on that bull.
He's coming towards us.
If the hippo gets underneath the boat,
he could easily capsize it.
Take your seats.
This time...
That was close.
...he moves off.
Yoh! That was a bit tidy.
Getting the underwater footage is hard enough,
but sound recordist Phil Bax wants to record hippo calls
above and below the surface simultaneously.
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.
That's good.
They've got a whole language of clicks and croaks and moaning sounds
going on underneath the water.
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
'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.
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
to adapt to our rapidly changing planet.