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{\an8}The Okavango Delta.
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Africa's water world.
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Covering over 6,000 square miles,
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this mix of wetland and islands
is awash with sound.
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To thrive here,
animals must master life both above...
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and below the surface.
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The warm waters are rich
with the sound of flitting fish.
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Over 80 different species.
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An all-you-can-eat feast
for the birds here.
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But catching them means overcoming
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the acoustic challenge
of hunting in water.
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The splash of a strike...
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alerts the shoals
and gives them a head start.
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But one high-speed hunter
has found an ingenious way around this.
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The pied kingfisher.
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A small bird with a big beak,
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a third of its body length.
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When defending their territories,
they produce a stream of shrill chirps.
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But when hunting,
they are masters of stealth.
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Wings beating eight times a second
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as they hover 30 feet above the water.
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Setting their sights, they dive.
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As they hit the water,
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kingfishers use their sonic secret weapon...
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to catch fish unawares.
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Their long cone-shaped beaks
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can enter the water
without making a sound...
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or even a ripple.
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Giving the birds
a 15-millisecond head start on their prey.
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The shape of their beaks
makes them so stealthy,
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it's influenced
the design of high-speed trains.
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The long, pointed nose
preventing sonic booms
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as the engines rocket through tunnels.
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The kingfishers' ability to minimize sound
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make them one of the Delta's
most successful anglers.
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From small silent hunters...
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to noisy giants...
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the Delta is one of the world's
most diverse wetlands.
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And every animal here
has its own signature sound.
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Whistling ducks are just
one of over 400 bird species.
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Aerial predators like the fish eagle
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spark alarm calls in chacma baboons...
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which trigger panic
in herds of lechwe antelope.
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With so many creatures
gathered in such proximity...
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this water world
is constantly buzzing with life.
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But even in Africa's great wetlands
there are areas of higher dry land...
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that reverberate with astonishing sounds.
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These high-frequency twitters
don't belong to any bird.
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This is the magnificent dialect
of African wild dogs...
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and their playful pups.
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For the pack, it's all about keeping
the little ones safe and fed.
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To do that,
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they've evolved a complex language
of nearly 30 different calls.
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Despite their constant chatter,
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remarkably, the high-frequency sounds
are unlikely to be heard from afar...
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as they are easily broken up by bushes.
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But the most bizarre thing
about wild dog communication
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is how the pack decides to start hunting.
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Recent studies show they signal...
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with a sneeze.
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Each one is equivalent to a vote.
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The more sneezes,
the more likely they are to hunt.
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This strangest of voting systems
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is unique to African wild dogs.
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When enough of them are on board
the pack sets out,
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leaving the pups to practice.
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Relentless chatter is now exchanged
for near silence.
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Because noise not only
alerts potential prey...
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but larger predators too.
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Lions or hyena
will often steal their kills.
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Today's target... wildebeest.
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Wildebeest are faster than the dogs.
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But the pack's phenomenal endurance...
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and ability to work as a team...
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makes them
Africa's most successful large predators.
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Back at the den...
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another of the dogs'
complex communications is revealed.
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The returning hunters
have stored meat from the kill.
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And when the little ones
make this particular call...
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it triggers the adults
to share their spoils.
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It's one of the first calls
pups must learn.
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Part of a rich and complex language,
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the most sophisticated of any dog species.
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For prey that gather
on the Delta's islands,
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paying close attention to sound
is a key part of staying off the menu.
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Here, an army of lookouts
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creates a neighborhood watch scheme
like no other.
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Dozens of eyes and ears on the alert.
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Useful when there's a leopard about.
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One of Africa's stealthiest hunters.
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But not all alarm calls
are treated equally.
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Studies have shown,
the smaller the species...
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the less likely it is
that others will react to their warnings.
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For little animals are easy targets...
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and that makes them jumpy.
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A quarter of barks from
the lightweight impala are false alarms.
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They're even spooked
by grass blowing in the wind.
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These skittish antelope
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are often the first to sound the alarm
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while everyone else ignores them.
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But if larger and less vulnerable zebra...
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or kudu...
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give a warning...
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everyone scarpers.
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But there's one small animal
whose alarm call...
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prompts a different response.
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The African tree squirrel.
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A leopard's nemesis.
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Their persistent calls
can be heard from 500 feet away.
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On hearing a warning...
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the family members rush together
and mob the hunter.
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Their system clearly pays off,
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as the squirrels make up less than
half a percent of a leopard's diet.
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Their warning calls
not only alert the neighbors,
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they let the predator know...
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they've been spotted and should move on.
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His tail lifted,
the leopard signals he's given up.
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A cat that's pound for pound
the strongest predator in Africa,
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driven away by
the simple squeak of a squirrel.
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But not all calls on the Okavango Delta
are warnings.
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Some are all about attraction.
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Beneath the surface, a curious love song
travels through the lagoons.
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Produced by tiny aquatic animals
less than a quarter of an inch long.
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Water boatmen.
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Surprisingly,
some produce high-frequency clicks
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by rubbing their genitals
against their stomach.
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It may be an unusual way of attracting
a mate, but it's highly effective.
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Incredibly, relative to body size,
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it's a water boatman that has
the loudest call of any animal on Earth.
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But these freshwater lagoons echo
to an even weirder sound.
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As aquatic plants photosynthesize
in the hot sun,
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they release oxygen.
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The air bubbles, squeezing out of pores
on the plant's surface,
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creating the strange but reassuring buzz
of a healthy freshwater world.
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Against this background hum,
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underwater microphones
pick up mysterious groans and grunts.
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Not from small aquatic insects...
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but amphibious giants.
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Part of the secret language
of 11 different calls...
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produced by the hippopotamus.
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Hippos live in chatty groups or pods.
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This one is made up of around 20 females...
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and young calves,
who spend their days on mum's back.
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Well, that's the idea.
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The pod is overseen by a huge,
dominant male.
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For hippo families,
staying in touch is crucial.
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And they do this
through a surprising use of sound.
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Above the water,
their trademark wheeze honks...
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carry for over half a mile.
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But hippos are one of the few animals
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that can communicate above
and below the surface...
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at the same time.
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When underwater,
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their calls are believed to be
detected through sensitive jawbones,
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which conduct sound waves
directly into the ear.
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On hearing a sub-aquatic honk...
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everyone heads to the surface...
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for a hippo head count.
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It allows pods to check on each other...
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as these animals
can roam for miles each day.
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But new science has revealed that
the animals have another acoustic ability.
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Below the surface, hippos can emit
a series of rapid-fire clicks.
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It's believed they listen to the sounds
bouncing back from objects around them...
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and use this information
to create a sonic map.
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An incredible discovery
that suggests hippos can echolocate
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much like their distant relatives, dolphins.
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A skill which helps them navigate
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and find each other
in the Delta's murky waters.
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We now know 80% of hippo vocalizations
occur beneath the surface.
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By using sound both above and below,
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hippos have mastered life
in Africa's water world.
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To record the secret
underwater conversations of hippos,
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the Earthsounds crew
had to come up with innovative ways
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to get close to
one of Africa's most dangerous animals.
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{\an8}I've worked with hippos a lot,
and, ja, quite a tricky customer.
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They've killed more people
than any other mammal on the continent,
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so to try and capture the way
they communicate underwater
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is very difficult.
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It's far too dangerous
to get in the water,
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so the crew must try and float cameras
and microphones towards the pod.
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Well, they look like
they're on the right track.
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See if this works.
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It's a promising start,
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but relying on water currents
doesn't always go to plan.
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Damn it. Looks like the cameras
are stuck right next to hippos,
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so we gonna have to go pretty close
to fetch them.
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The crew need to approach with care,
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particularly when the males are around.
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Just watch this boy 'cause
he was coming up to us snorting, so...
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Weighing up to four tons,
bull males are highly territorial.
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Gotta keep an eye on that bull.
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He's coming towards us.
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If the hippo gets underneath the boat,
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he could easily capsize it.
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Take your seats.
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This time...
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That was close.
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...he moves off.
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Yoh! That was a bit tidy.
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Getting
the underwater footage is hard enough,
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but sound recordist Phil Bax
wants to record hippo calls
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above and below the surface
simultaneously.
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{\an8}I've got a microphone
sticking up out of the water,
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and then another microphone
called a hydrophone,
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which records sound in the water.
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So it'll record both above and below
at the same time,
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and, hopefully,
let us hear exactly what the hippos hear.
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That's good.
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They've got a whole language of clicks
and croaks and moaning sounds
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going on underneath the water.
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Phil's recordings back up the new science
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that suggests hippos can echolocate.
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The clicking noises that we're getting
underneath the water make sense to me
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'cause the water's quite murky
at the moment,
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and they wouldn't be able to
see particularly well.
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They're using sound
as a way of finding each other
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or finding the things
that they're looking for.
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And this sixth sense
is becoming more important than ever.
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In recent years, changes brought about
by our heating climate
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have reduced visibility in the Delta...
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meaning hippos could become increasingly
reliant on this skill to navigate.
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It's just one way
that animals are using sound
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to adapt to our rapidly changing planet.