Episode 8 David Attenborough's Natural Curiosities


Episode 8

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'The natural world is full of extraordinary animals

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'with amazing life histories.

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'Yet certain stories are more intriguing than most.'

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The mysteries of a butterfly's life cycle.

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Or the strange biology of the Emperor penguin.

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Some of these creatures were surrounded by myth

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and misunderstandings for a very long time.

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And some have only recently revealed their secrets.

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These are the animals that stand out from the crowd,

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the curiosities I find most fascinating of all.

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'In this programme,

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'I examine the remarkable lives of two animals

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'that have mastered the problems of life in the dark.'

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'The giant squid, which lives in the deepest oceans...'

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..and owls.

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Highly specialised hunters that seek their prey at night.

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When we think of animals of the night, owls tend to come to mind.

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In fact, not all owls are nocturnal,

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but those that are have a very similar-shaped face,

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round and flat.

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And their most prominent facial features

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are the large, forward-facing eyes.

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These give them a seemingly wise look and in fact,

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owls have often been revered for their wisdom.

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But they have also been linked with legends of death and evil.

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They are birds of the night.

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To many, they seem eerie and mysterious.

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'But how good is an owl's eyesight?

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'Can they really see what we can't?'

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The colour picture that forms at the back of our eyes

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is very much like that that forms in the eyes of a bird.

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We have roughly the same number of colour receptors.

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But when day changes to night, the picture changes.

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Then, different receptors come into play, called rods.

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And owls have a much higher proportion of rods

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in their eyes than we do.

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So they're extremely good at seeing at low light levels.

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Aren't you?

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The barn owl sets off to hunt shortly after dusk.

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As the light fades, we struggle to see.

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But the owl has no such problem.

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Flying low, it keeps its eyes trained on the ground,

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looking for any movement in the grass.

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Its eyes now give it the edge over its prey,

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and it can hunt at a time when few other birds can.

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And there's another important difference

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between an owl's eye and ours.

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The pupil in the front of the eye, the hole,

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is very much bigger in an owl's.

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Ours measures around eight millimetres across.

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An owl's, like this tawny owl, is around 13.

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That means very much more light can get into the eye,

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so the picture formed on the retina is very much brighter.

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In fact, it's about three times as bright.

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OWL SQUEAKS Aw...

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OWL SQUEAKS Aw...

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So, unlike other birds, which cannot see so well in the dark,

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the owl can remain active throughout the night.

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But specialist eyes create problems.

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Squeezing a large eyeball

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into a relatively small skull requires changes.

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The shape of the owl eye is more tubular than round.

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This may help to increase the size of the image on the retina

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at the back.

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But the owl's eye shape and size presents certain problems.

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It doesn't fit snugly into the skull

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and there's no room in the socket for muscles to move it.

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And there's another problem.

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A closer look at an owl's skull

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shows that its ear openings are very big.

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So the only way for the tubular eyes to fit into the skull is for them

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to be placed in the middle of the face in a forward-looking position.

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This limits the owl's field of view.

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But owls have a trick that allows them

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to dramatically increase their field of view.

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They can rotate their heads nearly all the way round.

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Folklore has it that you can kill an owl

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by walking in circles round a tree in which one is perched

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and so make it twist its head off.

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That, of course, is not true.

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But owls can certainly turn their heads

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through 270 degrees in either direction.

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If we tried to do that, we'd tear our arteries and break our necks.

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So, how do owls do it?

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Recently, scientists have discovered that it's due

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to a remarkable adaptation of their bones.

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Owls' necks, as you can see in this skeleton of an eagle owl,

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have 14 vertebrae. That's twice the number that we have.

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This gives them greater flexibility.

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But only recently, CT scans have shown researchers

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how the owl can rotate its head without passing out.

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Cavities within the neck bones are ten times larger

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in an owl's neck than in ours,

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giving more room for vital blood vessels

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that run up to the owl's head.

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What's more, the carotid arteries enter the head

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much higher up the neck and are centrally positioned,

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and this may help avoid damage during twisting.

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And the owl's arteries seem to widen below the brain,

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allowing blood to pool.

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This may create a vital blood reservoir that guarantees blood flow

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to the brain, should the vessels below be squeezed

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while the head is turning.

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So the owl can turn its head almost all the way round

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without risk of injury.

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So, owls have successfully dealt with the problems

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created by having large eyes.

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OWL HOOTS

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But are these eyes really all they seem?

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It was long thought that owls can see perfectly,

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even on the darkest of nights.

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But that is not the case.

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On cloudy nights and beneath trees with dense canopies,

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they can only discern the faintest silhouettes.

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It's nowhere near detailed enough to hunt for prey.

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But the owl has another sense to help it...

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acute hearing.

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In the 18th century,

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the great French naturalist Count de Buffon wrote,

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"Their sense of hearing seems to be superior

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"to that of other birds and perhaps to that of every other animal,

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"for the drum of the ear

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"is proportionately larger than in quadrupeds

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"and besides, they can open and shut this organ at pleasure,

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"a power possessed by no other animal."

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Well, we know today that that's true of some owls,

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though not all, but Buffon was quite right

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to draw our attention to the remarkable hearing of owls.

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OWL HOOTS

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The owl's large ear openings are not visible

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because they're hidden beneath the face feathers.

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And unlike other birds, they have fleshy outer ears like our own.

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In many owls, they're positioned at slightly different levels

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on either side of the head.

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And it's these features that help them

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to accurately pinpoint their prey.

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Most owls have very similar shaped faces, flat and round.

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It's called a facial ruff.

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It's formed from feathers that are particularly dense and bristly,

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and they lie flat on either side of the face,

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just behind the opening to the ears.

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It's thought that they deflect the sound into the ears.

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In fact, the facial ruff seems to be a kind of sound amplifier.

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The barn owl has a distinctive, heart-shaped ruff and its face

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acts like a satellite dish, focusing the sounds from below into the ears.

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Its soft flight feathers enable it to move through the air

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in almost complete silence so that it can hear

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the slightest rustle and approach its prey undetected.

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But few have as large a facial ruff as the great grey owl.

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Although it hunts during the day, its prey is hidden under

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a cover of snow, so it has to rely entirely on its ears.

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Studies have shown that owls' hearing is particularly

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acute for very quiet sounds.

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In fact, part of an owl's brain that detects sound has three times

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as many neurones as its equivalent in, say, a crow's brain.

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The hairs of the inner ear which detect the vibrations

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of sound are particularly abundant in an owl.

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Not only that, whereas the equivalent hairs in my ear

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degrade with age, in an owl's they are regrown.

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So whereas my hearing gets worse as I get older,

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an owl's always remains very acute.

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The owl's ears may in fact be more crucial to its nocturnal

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lifestyle than its eyes.

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But by combining all its senses,

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it has solved the problems of living in the dark.

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So it seems that the shape of the face helps both the owl's sight

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and its hearing.

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So whether or not you think the owl is wise,

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it certainly has a head for life in the dark.

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Next we journey into the darkest of places to try and unravel

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the life of a creature that has long captured our imagination.

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Here in the Natural History Museum is a specimen of an animal

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that has fascinated humanity for thousands of years.

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It's a giant squid.

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This particular one was netted off the Falkland Islands,

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immediately put on ice,

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and then brought here to the museum in London.

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Few museums have complete or as perfectly preserved

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specimens as this one.

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This one measures about eight metres, the length of a London bus.

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But others have been caught even bigger,

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one about twice the length that weighed around a tonne.

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Very few people have ever seen one of these creatures alive.

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That's because they live at depths of around 1,000 metres

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and down there, it's pitch-black.

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So how do these animals manage to hunt in such conditions?

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That's a question that has proved exceedingly difficult to answer.

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Sailors a long time ago told stories of having seen a gigantic,

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squid-like creature known as the Kraken.

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It was said to have huge tentacles strong enough to grip

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and sink a ship.

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The tales seemed unlikely and far-fetched, but could the giant

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squid perhaps have been the source of these extraordinary reports?

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The first clues that this creature may in fact be real came from

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the tales of sailors on whaling ships

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in the 18th and 19th centuries.

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Some of them reported in their ships' logs that they often noticed

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strange, circular scars on the heads and jaws of captured sperm whales.

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The scars suggested a fierce wrestling match with

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some enormous beast.

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But what creature could take on a 70-tonne whale?

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Inside the stomachs of the whales were clues.

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A number of hard, indigestible objects like this one.

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It looks a bit like the beak of a parrot.

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But in fact, it belongs to an entirely different

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kind of animal - to a cephalopod.

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Cephalopods are marine animals that include the octopus,

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the squid and the cuttlefish.

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This beak is the mouth part of one such creature

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and is used to tear its prey into small pieces.

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The sailors on the whaling ships immediately recognised the beak

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as being from a cephalopod.

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But its size suggested a creature

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many times bigger than any known species.

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Cephalopods have a ring of eight or ten arms, or tentacles, which they

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use to push food into their mouth in the centre of the ring.

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The arms are equipped with round suckers to help

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hold on to their prey.

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It is the marks from these that were found by sailors on the bodies

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of sperm whales.

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Could a gigantic squid have caused such injuries,

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and how massive must it be to tackle a sperm whale,

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one of the biggest animals on the planet?

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And then in 1873, fishermen caught what

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they called a sea monster off the coast of Newfoundland in Canada.

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After killing it with their knives, they lost the body,

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but they brought the head and tentacles to the local clergyman.

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The clergyman bought it off the fishermen for 10

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and displayed it in his living room by carefully draping it over

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a bath stand, to show off

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its many arms and tentacles.

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The photograph clearly proved that

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here was a gigantic squid with

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its beak at the top and over seven metre-long tentacles.

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Here last was the evidence that the monster of the deep,

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the Kraken, really does exist.

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But the giant squid itself continued to evade scientists,

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even after its discovery.

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It's only since the invention of submersibles that we have

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been able to follow it down into its deep sea home.

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Even so, we seem to have had little

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success in finding the elusive giant.

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So scientists are now trying to piece together its biology

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by looking at other closely-related animals.

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This is an octopus.

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It uses both its eyes and tentacles to explore its surroundings.

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The octopus's brain is distributed throughout its body

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so that its arms can control much of their own movement.

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It also has a highly complex eyes and sees in much the same way

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as we do, with the lens projecting an image onto the retina behind.

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But while our eyes focus by squeezing the lens to

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change its shape, the octopus's eyes

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focus like a camera, with the lens moving in and out.

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The giant squid's eyes have much the same

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structure as those of an octopus, but when it comes to size, it has

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the biggest eye in the animal kingdom, as large as a football.

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For seeing in dim light, a large eye is better than the small one.

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So many animals of the deep have exceptionally big eyes.

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But in order to see at all, there has to be some light,

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and the giant squid lives at depths of 1,000 metres.

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Although very little sunlight reaches the deeper parts

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of the ocean, there is another kind of light there.

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It's produced by the deep sea animals

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and it's called bioluminescence.

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The light is produced by a chemical reaction in the same

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way as that in a glow stick does.

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When I shake and snap the stick,

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two chemicals called luciferin and luciferase react together to produce

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a bioluminescent glow like this...

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There.

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Some deep sea animals use their own luciferins to produce light, while

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in others it's produced by bacteria living within special light organs.

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A flashing light can act as a lure or confuse a predator.

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It's thought about 90% of deep sea creatures produce

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bioluminescence and they use it in a number of different ways.

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All these fish come from the deep sea.

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They all produce light in one way or another.

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This is the football angler fish and it has a modified

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ray from its dorsal fin which has lots of little tentacles on top.

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The tip of each tentacle produces a little green light

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so it looks as though there is a little shoal of small creatures,

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maybe shrimps, hovering above it in the blackness.

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When another shrimp thinks it might join some friends

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and come along that way, the angler fish simply tilts up,

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opens its immense jaw and has its breakfast.

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This, on the other hand,

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is a stoplight loosejaw, which operates in a different way.

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It produces red light from two little organs at the front.

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Hardly any other species of fish in the sea can see red light,

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so it can hunt that way and find its prey.

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When it does, it opens this immense loose jaw and engulfs it.

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There you are. Back you go.

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But what about the giant squid?

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Could it also be producing bioluminescence?

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Some of its close relatives apparently can.

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This is the vampire squid.

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It has eight arms lined with tooth-like projections.

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When threatened, it turns itself inside out,

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wrapping its body in a dark cloak.

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If that doesn't work, the squid has another trick.

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Small lights at the end of its arms

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flash like eyes to distract the predator.

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With so many creatures of the deep producing light, you might think

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that the giant squid would do so as well.

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But scientists studying their carcasses have not been

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able to find any evidence of light-producing bacteria or

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pigments in their bodies.

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So it seems that the ocean's elusive giant truly hides in the dark.

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Although it may not produce its own light, the giant squid can surely

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see the bioluminescence of others

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and this may help it to locate its prey.

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With no sightings of a living giant squid since it was

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first discovered, we seem to be no closer to discovering the truth.

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But in 2004, Japanese scientists finally made a breakthrough.

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Using small squid as bait,

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they were able to attract a live giant squid.

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These first images are tantalising,

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but they still reveal little of the animal's true behaviour.

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Where does it live and how does it feed?

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Questions such as these remain unanswered.

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In spite of its great size,

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the giant squid has proved remarkably difficult to find.

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No doubt scientists will continue to search for it

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and discover more about it.

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But my guess is that the giant squid is likely to remain ahead of

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the game, that this natural curiosity

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is likely to see us before we see it.

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Both the owl and the giant squid live in a world with little light

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and both have evolved large eyes, the better to

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see the world around them.

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But while we've unravelled the owl's ways of surviving in the dark,

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much about giant squid still remains a mystery.

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