0:00:36 > 0:00:41A giant sperm whale preparing to dive a mile into the abyss.
0:00:51 > 0:00:53On its epic dive,
0:00:53 > 0:00:57it will hold its breath for longer than this programme lasts.
0:01:05 > 0:01:11And it will hunt using the loudest sounds made by any animal on the planet.
0:01:12 > 0:01:15CLICKING
0:01:19 > 0:01:26Hidden beneath the waves, whales and dolphins lead extraordinary lives.
0:01:41 > 0:01:46Travelling the world's oceans, we follow the scientists
0:01:46 > 0:01:50who have dedicated their lives to these incredible animals.
0:01:50 > 0:01:52They live in a world that's so foreign to us
0:01:52 > 0:01:55and give us just a glimpse of their lives.
0:01:55 > 0:01:57SQUEAKING
0:01:57 > 0:02:00SHE MIMICS DOLPHIN SCREECH
0:02:02 > 0:02:07I think we could talk with the dolphins within five years.
0:02:09 > 0:02:12Sharing these breathtaking encounters,
0:02:12 > 0:02:15two of the world's top underwater cameramen.
0:02:15 > 0:02:18It was certainly the biggest whale that I've ever seen.
0:02:18 > 0:02:20It just looked enormous underneath me.
0:02:23 > 0:02:26Fantastic! Today is the best day of my life.
0:02:26 > 0:02:30To survive in this alien world,
0:02:30 > 0:02:33whales and dolphins have developed unique powers.
0:02:36 > 0:02:39Killer whales use sound as a weapon.
0:02:41 > 0:02:45Humpback whales sing their hearts out to the world.
0:02:48 > 0:02:52And these strange creatures even see with sound.
0:02:59 > 0:03:01They are the voices of the sea.
0:03:14 > 0:03:17An estuary in Tasmania, Australia.
0:03:21 > 0:03:25Mighty sperm whales lie stranded.
0:03:37 > 0:03:41Something's gone wrong with their powers of navigation...
0:03:42 > 0:03:45..and the consequences are deadly.
0:03:55 > 0:04:00Disorientated and without sufficient water to support its bulk,
0:04:00 > 0:04:04this great whale is slowly dying.
0:04:13 > 0:04:16Luckily, help is at hand.
0:04:18 > 0:04:23His rescuers use their boat to create a wave,
0:04:23 > 0:04:25trying to wash him into deeper water.
0:04:40 > 0:04:43They carefully cover up his sunburned skin.
0:04:46 > 0:04:51And finally they use a net to tow him to safety.
0:05:09 > 0:05:11Free at last.
0:05:18 > 0:05:24Once clear of the shallows, he starts sending out loud clicks,
0:05:24 > 0:05:29using the echoes to form a mental picture of the world around him.
0:05:29 > 0:05:32CLICKING
0:05:34 > 0:05:38Soon, he is back in the safety of deep water.
0:06:04 > 0:06:07But sperm whales don't just use sound to navigate.
0:06:09 > 0:06:12Their voices are central to their very existence.
0:06:21 > 0:06:25Sperm whales are regular visitors to the very deep waters
0:06:25 > 0:06:27of the Caribbean island of Dominica.
0:06:37 > 0:06:41Here, scientists are trying to understand how sperm whales
0:06:41 > 0:06:42use sound underwater.
0:06:52 > 0:06:57It's a source of endless fascination for biologist Shane Gero.
0:06:57 > 0:07:02I think I like the sperm whales in particular because they're deep divers.
0:07:02 > 0:07:04They live in a world that's so foreign to us
0:07:04 > 0:07:07and give us just a glimpse of their lives.
0:07:07 > 0:07:11It's almost as if they have this big secret that they're not willing
0:07:11 > 0:07:14to share and I just have to dig away and try and figure out what that is.
0:07:18 > 0:07:23The team head out into deeper waters, home of the sperm whales.
0:07:32 > 0:07:34But finding them won't be easy.
0:07:34 > 0:07:38Sperm whales spend most of their time at depth,
0:07:38 > 0:07:40only returning to the surface to breathe.
0:07:49 > 0:07:52Andrew Armour and his crew
0:07:52 > 0:07:55have been following these whales for over a decade.
0:07:57 > 0:08:03Today, they are joined by underwater cameramen Didier Noirot and Doug Allan.
0:08:04 > 0:08:07- Right, OK.- Ready?- Yep.
0:08:09 > 0:08:14The trick to finding sperm whales is to use an underwater microphone,
0:08:14 > 0:08:20or hydrophone, and catch them doing what they do best - making noise.
0:08:23 > 0:08:28Sound travels faster and further in water than in air,
0:08:28 > 0:08:33so the team will be able to hear any whales long before they see them.
0:08:35 > 0:08:39So what you want to hear is a series of clicks.
0:08:39 > 0:08:44The more you hear, the better. If it sounds like popcorn or bacon frying, we're going to have a good day.
0:08:46 > 0:08:48I'm afraid the frying pan's empty!
0:09:09 > 0:09:13Sperm whales are the largest predators on the planet.
0:09:13 > 0:09:16Here's the male. Wow, he's really well marked.
0:09:16 > 0:09:19But even they have enemies.
0:09:23 > 0:09:25Pilot whales.
0:09:25 > 0:09:29They will attack the sperms. They'll eat the babies and we've seen this interaction.
0:09:29 > 0:09:32- The pilot whales will attack the sperm whales?- Yeah!
0:09:35 > 0:09:39Confident that pilot whales don't usually attack humans,
0:09:39 > 0:09:42Didier is keen to take a closer look.
0:10:09 > 0:10:12Close cousins of the killer whale,
0:10:12 > 0:10:17pilot whales are only a third of the size of an adult sperm whale.
0:10:19 > 0:10:22But they have strength in numbers.
0:10:26 > 0:10:29Hunting in packs up to 100-strong,
0:10:29 > 0:10:34they too will be listening out for the sounds of sperm whales.
0:10:50 > 0:10:54There is a chance that if we keep following these pilots
0:10:54 > 0:10:58that we'll end up tracking them as they track the sperms.
0:11:12 > 0:11:17At last, the team begin to hear sperm whales.
0:11:17 > 0:11:20But these aren't clicks for navigation -
0:11:20 > 0:11:24these are clicks for communication, known as codas.
0:11:24 > 0:11:25Codas, codas, codas, codas!
0:11:25 > 0:11:28CLICKING
0:11:28 > 0:11:31Oh, yeah, I can definitely hear the clicking, yeah.
0:11:31 > 0:11:32Can you hear them, Shane?
0:11:34 > 0:11:38It gets hard to count more than three because after you get...
0:11:38 > 0:11:40It might be a click... Oh!
0:11:40 > 0:11:43There she blows! There she blows!
0:11:43 > 0:11:46There she blows!
0:12:06 > 0:12:11This is the chance to get close to the world's largest predator.
0:12:48 > 0:12:52Sperm whale families may contain several generations
0:12:52 > 0:12:59of daughters with their babies, overseen by a wise old grandmother.
0:13:07 > 0:13:11Shane believes that they communicate through a complex array
0:13:11 > 0:13:16of coda clicks which vary from family to family.
0:13:21 > 0:13:26But they can also put these clicks to far more lethal use.
0:13:26 > 0:13:28It's time for the adults to feed.
0:13:33 > 0:13:37For the last 15 minutes, the adult whales have been deep breathing
0:13:37 > 0:13:42to store as much oxygen as possible in their huge muscles.
0:13:44 > 0:13:47Now it's time to dive.
0:14:17 > 0:14:21Doug and Didier can only hold a lungful of air,
0:14:21 > 0:14:25so their depth limit is around 60 feet.
0:14:40 > 0:14:44But with oxygen stored in their muscles, the sperm whales
0:14:44 > 0:14:49can go 100 times deeper and stay down for over an hour.
0:14:52 > 0:14:57Until recently, no-one knew how sperm whales hunt in the abyss,
0:14:57 > 0:15:02but scientists now believe it happens like this.
0:15:07 > 0:15:09The reason sperm whales dive so deep
0:15:09 > 0:15:13is that there's lots of food down there...
0:15:15 > 0:15:19..including giant squid over 30 feet long.
0:15:34 > 0:15:37A mile down, there's no light.
0:15:37 > 0:15:40The whale must find its prey through echolocation...
0:15:42 > 0:15:46..interpreting the echoes of the clicks it makes.
0:15:47 > 0:15:50But these are no ordinary clicks.
0:15:50 > 0:15:55These hunting clicks are the loudest sounds made by any living thing,
0:15:55 > 0:15:58- louder than a thunderclap. - THUNDERCLAP
0:15:58 > 0:16:00CLICKING
0:16:02 > 0:16:08Produced in its five-tonne nose, the whale's clicks travel backwards,
0:16:08 > 0:16:12then are reflected forwards through a series of oil-filled lenses
0:16:12 > 0:16:18which focus them into a narrow sound beam, like a sonic laser.
0:16:24 > 0:16:27For the giant squid, there's nowhere to hide.
0:16:42 > 0:16:44While the adults are hunting,
0:16:44 > 0:16:47their infants are left at the surface,
0:16:47 > 0:16:51baby-sat by an auntie or sister.
0:16:51 > 0:16:54But even the best baby-sitters
0:16:54 > 0:16:56can lose track of their charges at times.
0:17:19 > 0:17:22Out of the blue, a baby heads straight for the boat.
0:17:25 > 0:17:28Unusually, it's all alone.
0:17:38 > 0:17:40It takes refuge between the hulls,
0:17:40 > 0:17:43maybe mistaking them for other whales.
0:17:56 > 0:18:02As Doug approaches, it calls out with a stream of loud clicks.
0:18:02 > 0:18:04CLICKING
0:18:13 > 0:18:19It is said that when an adult sperm whale clicks at a diver
0:18:19 > 0:18:22it's like being kicked in the chest by a horse.
0:18:26 > 0:18:29While this infant's coda clicks may not be as powerful,
0:18:29 > 0:18:33they will still carry for more than a mile,
0:18:33 > 0:18:35down to where the adults are feeding.
0:18:55 > 0:18:59From out of the blue, the baby's mother appears.
0:19:13 > 0:19:16Ignoring Doug and Didier,
0:19:16 > 0:19:21she gently shepherds her baby back to the family.
0:19:30 > 0:19:32- That was great!- It's so cute!
0:19:32 > 0:19:38It was just great! For a first time, it was super!
0:19:42 > 0:19:46- We heard a lot of clicks. - A lot of clicks.- Calling clicks. A lot of calling clicks.
0:19:46 > 0:19:51- Well, he was clicking at me. - Yeah, he was clicking. He was calling Mum, you know.
0:19:51 > 0:19:53The mum came quickly to fetch him.
0:19:53 > 0:19:57- She came right up and took him away. - Yeah, yeah.
0:20:02 > 0:20:06Through their remarkable manipulation of sound,
0:20:06 > 0:20:09sperm whales are able to communicate,
0:20:09 > 0:20:15navigate and even hunt in the pitch-black abyss.
0:20:26 > 0:20:30For a whale, there's no tougher place to survive than here...
0:20:32 > 0:20:34..in the frozen Arctic Ocean.
0:20:53 > 0:20:58It's home to the most bizarre looking of all toothed whales...
0:21:06 > 0:21:07..the unicorn of the sea...
0:21:13 > 0:21:15..the narwhal.
0:21:31 > 0:21:35To reach new fishing grounds, these whales must migrate
0:21:35 > 0:21:40huge distances through constantly shifting fields of ice.
0:21:53 > 0:21:56Unable to hold their breath for more than 20 minutes,
0:21:56 > 0:22:01they must find regular breathing holes in the ice...
0:22:03 > 0:22:07..or they risk becoming trapped, starving or drowning.
0:22:18 > 0:22:22So how do they find these tiny air holes
0:22:22 > 0:22:25dotted across the vast sheets of ice?
0:22:37 > 0:22:40Like sperm whales, narwhals can echolocate.
0:22:44 > 0:22:48They fire off a series of sonic pulses and use the echoes
0:22:48 > 0:22:50to map the world around them.
0:22:53 > 0:22:58In this way, they can locate vital breathing holes
0:22:58 > 0:23:01from hundreds of feet away.
0:23:21 > 0:23:24As winter closes in,
0:23:24 > 0:23:28the sea around the narwhals begins to freeze over,
0:23:28 > 0:23:33leaving them with fewer and fewer holes in the ice.
0:24:24 > 0:24:28Only by using their sophisticated powers of echolocation
0:24:28 > 0:24:32can the narwhals find enough air holes to survive.
0:24:36 > 0:24:39But at least narwhals can see the fish they're after.
0:24:42 > 0:24:46One group of dolphins is not so lucky.
0:24:46 > 0:24:51They live in the last place you might expect to find a dolphin...
0:24:54 > 0:24:57..3,000 miles inland...
0:24:59 > 0:25:03..in the heart of the Brazilian rainforest.
0:25:04 > 0:25:08Flowing through it, the mighty Amazon River -
0:25:08 > 0:25:12the planet's largest fresh water system.
0:25:20 > 0:25:24In some ways, it makes perfect sense for a dolphin to make its home here.
0:25:27 > 0:25:31There are more species of fish in the Amazon
0:25:31 > 0:25:33than in the whole Atlantic Ocean.
0:25:36 > 0:25:40But how can a dolphin catch fish in water
0:25:40 > 0:25:44so muddy that it can't see a thing?
0:25:55 > 0:25:58500 kilometres we've done, just...
0:25:58 > 0:26:03Cameraman Doug and Brazilian scientist Vera da Silva are on their way to investigate.
0:26:03 > 0:26:07It's very, very beautiful. I'm looking forward to going down there.
0:26:07 > 0:26:12- Will we be travelling everywhere by boat?- Just rivers and water.
0:26:12 > 0:26:15- And lots of dolphins. - Lots of dolphins.
0:26:22 > 0:26:25This is the creature Vera is studying,
0:26:25 > 0:26:29perhaps the strangest dolphin of all.
0:26:29 > 0:26:33The boto, or Amazon river dolphin.
0:26:51 > 0:26:55Doug is having trouble finding any botos.
0:26:55 > 0:27:01For a better view, Vera must take Doug to a very special place.
0:27:05 > 0:27:08In the wet season, one of the Amazon's tributaries
0:27:08 > 0:27:11the Rio Negro, breaks its banks,
0:27:11 > 0:27:16and floods an area of jungle the size of France.
0:27:17 > 0:27:21This unique water world is known as the flooded forest.
0:27:32 > 0:27:37The water is clearer here than in the main river, but it's choked
0:27:37 > 0:27:42with branches and leaves and as dark as well-brewed tea.
0:27:54 > 0:27:58Finding these strange dolphins hidden in the forest
0:27:58 > 0:27:59would be impossible
0:27:59 > 0:28:02if it were not for their natural curiosity.
0:28:02 > 0:28:07- Come on, botos!- No, they are there. - Yeah, they are there.
0:28:07 > 0:28:10- Look, look, look.- Oh, yeah.
0:28:10 > 0:28:14Right here. Another one there. They start showing up now.
0:28:18 > 0:28:20Having spotted some botos,
0:28:20 > 0:28:23Vera uses her hydrophone to listen to their calls.
0:28:23 > 0:28:28- Let's see how much noise these guys are doing now.- Oh, OK.
0:28:35 > 0:28:38- Do you want to listen? - Yes, a little bit. Thank you.
0:28:41 > 0:28:47Yeah, now I hear it. Like a series of "trrrrrrrrrr".
0:28:47 > 0:28:50- Oh, now... - SHE SQUEAKS
0:28:50 > 0:28:52You can have it.
0:28:53 > 0:28:55Oh, yeah, yeah.
0:28:57 > 0:29:00The dolphins are very noisy.
0:29:00 > 0:29:02But what are these sounds for?
0:29:05 > 0:29:06To find out,
0:29:06 > 0:29:11Vera and Doug first need to attract the dolphins close to the boat.
0:29:11 > 0:29:15So is there a secret to bringing them closer?
0:29:15 > 0:29:20Yes, sometimes they will react to the noise or to some fish.
0:29:20 > 0:29:23If we give them some fish, they will come quickly.
0:29:25 > 0:29:28- See?- Oh!- Three at the same time!
0:29:34 > 0:29:40The bizarre-looking boto shows off its slender toothy snout...
0:29:42 > 0:29:47..and the enormous bulge on its forehead called the melon.
0:29:49 > 0:29:53But to find out how it uses them underwater,
0:29:53 > 0:29:56Doug will need to dunk himself in the tea.
0:30:25 > 0:30:30The botos are using two completely separate sound systems.
0:30:33 > 0:30:38The chattering noises are the sound of them talking to each other.
0:30:38 > 0:30:40CHATTERING
0:30:42 > 0:30:49But they also appear to be scanning Doug with strange clicks and buzzes.
0:30:49 > 0:30:51BUZZING AND CLICKING
0:30:57 > 0:31:01This is their echolocation in action.
0:31:21 > 0:31:24He came up, took the fish and he swam backwards!
0:31:24 > 0:31:27I've never seen a dolphin swim backwards before.
0:31:36 > 0:31:41The botos lead Doug deeper into their underwater forest...
0:31:47 > 0:31:49..where echolocation allows them
0:31:49 > 0:31:53to navigate safely through this tangle of submerged vegetation.
0:32:06 > 0:32:10The boto's clicks and buzzes are focused into a sound beam
0:32:10 > 0:32:14by its bulging forehead melon.
0:32:18 > 0:32:23The beam works like a sonic head-torch,
0:32:23 > 0:32:28allowing the dolphin to build up a clear picture of its surroundings.
0:32:33 > 0:32:39It's as tangled and spiny underwater as it is on the surface.
0:32:39 > 0:32:42Those botos are going to need all their manoeuvrability
0:32:42 > 0:32:44and all the sonar skills they possess
0:32:44 > 0:32:47to operate in this sort of underwater environment.
0:32:52 > 0:32:56While all dolphins can echolocate,
0:32:56 > 0:32:59botos' clicks have a higher frequency
0:32:59 > 0:33:02and better resolution than any other dolphin,
0:33:02 > 0:33:06allowing them to distinguish objects as small as a pin.
0:33:15 > 0:33:21Even in zero visibility, catching fish is child's play.
0:33:33 > 0:33:36These are the weirdest dolphins that I've ever seen.
0:33:36 > 0:33:39They're absolutely... They're just fantastic.
0:33:43 > 0:33:50The world of botos is a noisy one, full of clicks and buzzes.
0:33:50 > 0:33:55But there are times in life when it pays to be silent.
0:34:02 > 0:34:04Killer whales.
0:34:06 > 0:34:08The ocean's top predator.
0:34:16 > 0:34:19From the tropics to the poles,
0:34:19 > 0:34:22they use an extensive vocabulary of clicks
0:34:22 > 0:34:25to co-ordinate their deadly attacks.
0:34:29 > 0:34:33But sometimes, they become ominously quiet.
0:34:49 > 0:34:54Each spring, on the Patagonian beaches of South America,
0:34:54 > 0:34:56fur seals give birth to their pups.
0:35:02 > 0:35:03Three months later,
0:35:03 > 0:35:07the naive pups must leave the safety of the colony
0:35:07 > 0:35:10and venture into the sea for the first time.
0:35:16 > 0:35:20It's the moment the killers have been waiting for.
0:35:32 > 0:35:35But hunting seals isn't easy.
0:35:35 > 0:35:40Like all marine mammals, seals have excellent hearing.
0:35:46 > 0:35:50A single call could betray the hunters' presence.
0:35:56 > 0:36:02So they keep radio silence as they close in for the kill.
0:36:27 > 0:36:30The young victims never heard them coming.
0:37:04 > 0:37:08While seal-hunting killer whales prefer silence,
0:37:08 > 0:37:12there are others that use sound as a sophisticated weapon.
0:37:17 > 0:37:20Including a remarkable population found off Iceland,
0:37:20 > 0:37:22in the north Atlantic Ocean.
0:37:26 > 0:37:27On the Westman Islands,
0:37:27 > 0:37:32a team of whale scientists is joined by cameraman Didier Noirot.
0:38:17 > 0:38:20Large numbers of killer whales have gathered
0:38:20 > 0:38:25to hunt the vast shoals of herring that come here to spawn each spring.
0:38:37 > 0:38:41But the coordinated and fluid movements of the shoals
0:38:41 > 0:38:44make them exceptionally hard to catch.
0:38:51 > 0:38:56So how do these huge predators manage to catch the herring,
0:38:56 > 0:38:59and in sufficient numbers to make it worthwhile?
0:39:24 > 0:39:31Scientist Volker Deecke is an expert in killer whale acoustics.
0:39:31 > 0:39:35- Where do you think we'll find a killer whale?- We've had really good success right in this area...
0:39:35 > 0:39:39Recently, Volker has been investigating how killer whales
0:39:39 > 0:39:42break through the herring's defences,
0:39:42 > 0:39:45and he thinks it is to do with sound.
0:39:55 > 0:39:57To prove his theory,
0:39:57 > 0:40:01he and Didier need to get right in close to the action.
0:40:15 > 0:40:18Having reached the area where the whales were last seen,
0:40:18 > 0:40:22Volker deploys his underwater microphones.
0:40:33 > 0:40:37A gathering of sea birds means the herring shoals
0:40:37 > 0:40:39must be close to the surface.
0:40:39 > 0:40:42Something must have spooked them.
0:40:42 > 0:40:44Oh!
0:40:44 > 0:40:49Seven or eight together at the top of the waves.
0:40:49 > 0:40:51Killer whales.
0:40:52 > 0:40:56Volker is perfectly positioned to record
0:40:56 > 0:40:58the sounds of the hunting whales.
0:40:58 > 0:41:01Three, four.
0:41:03 > 0:41:07Be great to see what the whales are doing underwater
0:41:07 > 0:41:09- when they're making all these sounds.- Sure.
0:41:09 > 0:41:14To capture the video evidence, Didier uses a pole camera.
0:41:14 > 0:41:17You are the ears, I'm the eye.
0:41:17 > 0:41:20Getting in the water with feeding killer whales
0:41:20 > 0:41:24would be too dangerous in this poor visibility.
0:41:47 > 0:41:49Volker gathers his evidence.
0:41:49 > 0:41:52WHALES CALL ON TAPE
0:41:52 > 0:41:55Yeah, that's the call.
0:41:55 > 0:41:58That was a killer whale, no doubt about it, yeah.
0:42:03 > 0:42:06After two hours of intensive feeding,
0:42:06 > 0:42:10the killers have fallen silent.
0:42:11 > 0:42:14The attack is over.
0:42:16 > 0:42:18Back at HQ,
0:42:18 > 0:42:21the team analyse the killers' calls
0:42:21 > 0:42:24and reveal the whales' secret weapon.
0:42:24 > 0:42:26What you'll hear is a lot of echolocation clicks,
0:42:26 > 0:42:29but then you'll hear something special. Here we go.
0:42:29 > 0:42:31HUMMING AND CLICKING
0:42:31 > 0:42:33Whistles, echolocation.
0:42:33 > 0:42:35CLICKING
0:42:35 > 0:42:37And now have a good listen.
0:42:37 > 0:42:40HIGHPITCHED WHISTLE
0:42:45 > 0:42:49This high-pitched note is what's known as the herding call.
0:43:04 > 0:43:07The idea is that the whales may be using the sound
0:43:07 > 0:43:11to essentially cause resonance in the herring's swim bladder.
0:43:11 > 0:43:13That makes the herring's guts vibrate
0:43:13 > 0:43:15and that makes the herring very uncomfortable.
0:43:19 > 0:43:22Panicked by the gut-wrenching noise,
0:43:22 > 0:43:25the herring bunch so tightly together,
0:43:25 > 0:43:27they have no room for manoeuvre.
0:43:27 > 0:43:29They school together, then bang!
0:43:34 > 0:43:38The killer whales then deliver devastating tail-swipes
0:43:38 > 0:43:41with the force of an exploding grenade.
0:43:54 > 0:44:00Job done, the whales can relax over a leisurely feast of stunned fish.
0:44:10 > 0:44:16Whales and dolphins use their voices to keep the family group together
0:44:16 > 0:44:20but there are times when talking just isn't enough.
0:44:32 > 0:44:34In the rich waters off New Zealand,
0:44:34 > 0:44:38hundreds of dolphins travel together in super-pods.
0:44:48 > 0:44:53Moving in such large groups requires clear communication.
0:44:57 > 0:45:01But clicks and whistles only have a limited range.
0:45:06 > 0:45:10The only way to broadcast messages across the group
0:45:10 > 0:45:13is by generating louder, more explosive sounds.
0:45:18 > 0:45:22Different body smacks convey different meanings.
0:45:25 > 0:45:29It's thought dolphins may use such acrobatic leaps
0:45:29 > 0:45:33to signal the discovery of large shoals of fish.
0:46:03 > 0:46:08Body percussion is a key ingredient in dolphin communication,
0:46:08 > 0:46:13and they will use it even to signal to us.
0:46:17 > 0:46:20On Brazil's Atlantic coast,
0:46:20 > 0:46:25one remarkable group of dolphins is in cahoots with the locals.
0:46:46 > 0:46:50Fishermen from the town of Laguna have discovered they do better
0:46:50 > 0:46:52when they don't fish alone.
0:47:00 > 0:47:04Each dawn, they gather in the murky shallows of the estuary
0:47:04 > 0:47:07and wait for help to arrive.
0:47:21 > 0:47:26Their fishing partners are a group of bottlenose dolphins.
0:47:41 > 0:47:47Scientist Arnaldo Russo claims that this local dolphin pod
0:47:47 > 0:47:49have learned to communicate with us.
0:47:56 > 0:47:59The fishermen know when they can throw their nets
0:47:59 > 0:48:03because the dolphin's making special signs.
0:48:03 > 0:48:06But what are these special signs
0:48:06 > 0:48:08that the dolphins give the fishermen?
0:48:08 > 0:48:14They can give a head-slap or a tail-slap into the water.
0:48:14 > 0:48:18- When the fishermen see those special signs, they throw their nets.- Yeah.
0:48:18 > 0:48:20It's like a conversation.
0:48:20 > 0:48:25A conversation that starts with the dolphins letting the fishermen know
0:48:25 > 0:48:31where the mullet are by slapping the water loudly with their tails.
0:48:41 > 0:48:42Go, go, go.
0:49:02 > 0:49:06The dolphins do the hard work herding the mullet.
0:49:13 > 0:49:16The fish are handed to the fisherman on a plate.
0:49:19 > 0:49:23But what the dolphins stand to gain is less obvious.
0:49:26 > 0:49:29Since they detect prey by echolocation,
0:49:29 > 0:49:34they have no problem finding fish in the murky water.
0:49:34 > 0:49:37But catching them is difficult.
0:49:37 > 0:49:42By casting their nets, the fishermen scatter the shoal,
0:49:42 > 0:49:46making it easier for the dolphins to snap up individual fish.
0:50:07 > 0:50:10This culture of co-operation
0:50:10 > 0:50:13has been going on at Laguna for over a century.
0:50:16 > 0:50:20Dolphin mothers teach their calves how to slap the water
0:50:20 > 0:50:23to signal to the fishermen.
0:50:23 > 0:50:28And the fishermen teach their sons to recognise the dolphins' signals.
0:50:40 > 0:50:43I've never seen this anywhere else in the world.
0:50:43 > 0:50:46- No, it's beautiful. It's amazing. - It is.
0:51:05 > 0:51:09But is there more that creatures like these are trying to communicate
0:51:09 > 0:51:10that we are simply missing?
0:51:17 > 0:51:19One enduring mystery
0:51:19 > 0:51:22surrounds the most celebrated musical event in nature...
0:51:23 > 0:51:27..the songs of the humpback whale,
0:51:27 > 0:51:30currently playing off the islands of Hawaii,
0:51:30 > 0:51:32in the middle of the Pacific Ocean.
0:51:46 > 0:51:48Before the singing can begin,
0:51:48 > 0:51:52the performers must give a master-class in percussion.
0:51:59 > 0:52:0440-tonne bodies hit the sea with an ear-splitting crash.
0:52:08 > 0:52:14And giant tail flukes thunder out a message to rival males.
0:52:24 > 0:52:28Didier makes sure to get a front row seat.
0:52:31 > 0:52:33Neutral!
0:52:44 > 0:52:47The ocean reverberates with explosions of bubbles.
0:53:20 > 0:53:24And then the opera begins.
0:53:24 > 0:53:27WHALE SONG
0:53:54 > 0:53:57The song of the humpback, it's something really beautiful.
0:53:57 > 0:54:01The most beautiful sound you can hear in the sea.
0:54:14 > 0:54:16You're the scientist.
0:54:16 > 0:54:21Jim Darling has spent the last 30 years trying to work out
0:54:21 > 0:54:24the meaning of these extraordinary songs.
0:54:34 > 0:54:36WHALE SONG
0:54:36 > 0:54:37Wow!
0:54:37 > 0:54:40- There are a few of them, huh? - Yeah, there's a few.
0:54:40 > 0:54:44- There's one close and three or four in the background.- OK.
0:54:44 > 0:54:45Yeah, they are a beautiful sound.
0:54:45 > 0:54:49WHALE SONG
0:54:58 > 0:55:03With his mouth shut tight, shunting air back and forth through a larynx
0:55:03 > 0:55:05the size of a phone-box -
0:55:05 > 0:55:09technically, he's humming, not singing.
0:55:12 > 0:55:17Whatever it is, it's a voice that can travel thousands of miles
0:55:17 > 0:55:19across entire oceans.
0:55:41 > 0:55:46The male's recital may include half a dozen distinct themes
0:55:46 > 0:55:48which he's learned from other males.
0:55:52 > 0:55:56He may even add his own improvised variations.
0:56:01 > 0:56:03By copying sequences from each other,
0:56:03 > 0:56:06males constantly evolve their repertoire.
0:56:13 > 0:56:15But does it have a purpose?
0:56:16 > 0:56:21For a long time, it was presumed that the singers were singing
0:56:21 > 0:56:23to attract a female.
0:56:23 > 0:56:24And I think people like that idea,
0:56:24 > 0:56:27because it's been written about a lot.
0:56:27 > 0:56:30But there's not a shred of evidence for it.
0:56:30 > 0:56:32We've never seen a female approach a male.
0:56:32 > 0:56:38If they aren't calling a mate, then why are they singing?
0:56:39 > 0:56:43Is it a way for males to size each other up?
0:56:44 > 0:56:48Could they even be making music for pleasure?
0:56:52 > 0:56:55The truth is that nobody knows.
0:57:11 > 0:57:14The enigmatic voices of whales and dolphins
0:57:14 > 0:57:18have captured our imagination for centuries.
0:57:22 > 0:57:25Scientists may one day find out the whole truth
0:57:25 > 0:57:29behind these extraordinary voices of the sea,
0:57:29 > 0:57:33but, for now, the private life of these ocean giants
0:57:33 > 0:57:36remains wonderfully mysterious.