David Attenborough's Natural Curiosities (2013) s02e03 Episode Script

ATTENBOROUGH: The natural world is full of extraordinary animals with amazing life histories.
Yet, certain stories are more intriguing than most.
The mysteries of a butterfly's life-cycle or the strange biology of the emperor penguin.
Some of these creatures were surrounded by myths and misunderstandings for a very long time.
And some have only recently revealed their secrets.
These are the animals that stand out from the crowd.
The curiosities I find most fascinating of all.
In this programme, I examine the remarkable lives of two animals that have mastered the problems of life in the dark.
The giant squid, which lives in the deepest oceans, and owls, highly-specialised hunters that seek their prey at night.
When you think of animals of the night, owls tend to come to mind.
In fact, not all owls are nocturnal.
But those that are have a very similar shaped face.
Round and flat.
And the most prominent facial features are the large forward-facing eyes.
These give them a seemingly wise look.
And, in fact, owls have often been revered for their wisdom.
But they have also been linked with legends of death and evil.
They are birds of the night.
To many, they seem eerie and mysterious.
But how good is an owl's eyesight? Can they really see what we can't? The colour picture that forms at the back of our eyes is very much like that that forms in the eyes of a bird.
We have roughly the same number of colour receptors.
But when day changes to night, the picture changes.
Then different receptors come into play called rods.
And owls have a much higher proportion of rods in their eyes than we do.
So they are extremely good at seeing at low light levels.
Aren't you? The barn owl sets off to hunt shortly after dusk.
As the light fades, we struggle to see.
But the owl has no such problem.
Flying low, it keeps its eyes trained on the ground, looking for any movement of the grass.
Its eyes now give it the edge over its prey.
And it can hunt at a time when few other birds can.
And there's another important difference between an owl's eye and ours.
The pupil in front of the eye, the hole, is very much bigger than ours.
Ours measures around 8 millimetres across.
An owl's, like this tawny owl, is around 13.
That means very much more light can get into the eyes, so the picture formed on the retina is very much brighter.
In fact, it's about three times as bright.
- (HOOTING) - Oh! - (HOOTING) - Oh! So unlike other birds, which cannot see so well in the dark, the owl can remain active throughout the night.
But the specialist eyes create problems.
Squeezing a large eyeball into a relatively small skull requires changes.
The shape of the owl eye is more tubular than round.
And this may help to increase the size of the image on the retina at the back.
But the owl eye shape and size presents certain problems.
It doesn't fit snugly into the skull and there's no room in the socket for muscles to move it.
And there's another problem.
A closer look at an owl's skull shows that its ear openings are very big.
So the only way for the tubular eyes to fit into the skull is for them to be placed in the middle of the face in a forward-looking position.
And this limits the owl's field of view.
But owls have a trick that allows them to dramatically increase their field of view.
They can rotate their heads nearly all the way round.
Folklore has it that you can kill an owl by walking in circles round a tree in which one is perched and so make it twist its head off.
That, of course, is not true.
But owls can certainly turn their heads through 270 degrees in either direction.
If we tried to do that, we'd tear our arteries and break our necks.
So how do owls do it? Recently, scientists have discovered that it's due to a remarkable adaptation of their bones.
Owls' necks, as you can see in this skeleton of an eagle owl, have 14 vertebrae.
That's twice the number that we have.
And this gives them greater flexibility.
But only recently, CT scans have shown researchers how the owl can rotate its head without passing out.
Cavities within the neck bones are 10 times larger in an owl's neck than in ours, giving more room for vital blood vessels that run up to the owl's head.
What's more, the carotid arteries enter the head much higher up the neck and are centrally positioned.
And this may help avoid damage during twisting.
And the owl's arteries seem to widen below the brain, allowing blood to pool.
This may create a vital blood reservoir that guarantees blood flow to the brain should the vessels below be squeezed while the head is turning.
So the owl can turn its head almost all the way round without risk of injury.
So owls have successfully dealt with the problems created by having large eyes.
(HOOTING) But are these eyes really all they seem? It was long thought that owls can see perfectly even on the darkest of nights.
But that is not the case.
On cloudy nights and beneath trees with dense canopies, they can only discern the faintest silhouettes.
It's nowhere near detailed enough to hunt for prey.
But the owl has another sense to help it.
Acute hearing.
In the 18th century, the great French naturalist Count de Buffon wrote, "Their sense of hearing seems to be superior to that of other birds.
"And perhaps, to that of every other animal, "for the drum of the ear is proportionately "larger than in quadrupeds.
"And besides, they can open and shut this organ at pleasure, "a power possessed by no other animal.
" Well, we know today that that's true of some owls, though not all.
But Buffon was quite right to draw our attention to the remarkable hearing of owls.
(HOOTING) The owl's large ear openings are not visible because they're hidden beneath the face feathers.
And, unlike other birds, they have fleshy outer ears like our own.
In many owls, they are positioned at slightly different levels on either side of the head.
And it's these features that help them to accurately pinpoint their prey.
Most owls have very similar shaped faces, flat and round.
It's called a facial ruff.
And it's formed from feathers that are particularly dense and bristly.
And they lie flat on either side of the face, just behind the opening to the ears.
It's thought that they deflect the sound into the ears.
In fact, the facial ruff seems to be a kind of sound amplifier.
The barn owl has a distinctive heart-shaped ruff and its face acts like a satellite dish, focusing the sounds from below into the ears.
Its soft flight feathers enable it to move through the air in all most complete silence, so that it can hear the slightest rustle and approach its prey undetected.
But few have as large a facial ruff as the great grey owl.
Although it hunts during the day, its prey is hidden under a cover of snow.
So it has to rely entirely on its ears.
Studies have shown that owls' hearing is particularly acute for very quiet sounds.
In fact, part of an owl's brain that detects sound has three times as many neurons as its equivalent, in say, a crow's brain.
And the hairs of the inner ear, which detect the vibrations of sound, are particularly abundant in an owl.
And not only that.
Whereas, the equivalent hairs in my ear degrade with age, in an owl's, they are regrown.
So whereas my hearing gets worse as I get older, an owl's always remains very acute.
The owl's ears may, in fact, be more crucial to its nocturnal lifestyle than its eyes.
But by combining all its senses, it has solved the problems of living in the dark.
So it seems that the shape of the face helps both the owl's sight and its hearing.
So whether or not you think the owl is wise, it certainly has a head for life in the dark.
Next, we journey into the darkest of places to try and unravel the life of a creature that has long captured our imagination.
Here, in the Natural History Museum, is a specimen of an animal that has fascinated humanity for thousands of years.
It's a giant squid.
This particular one was netted off the Falkland Islands, immediately put on ice and then brought here to the museum in London.
Few museums have complete, or as perfectly-preserved specimen as this one.
This one measures about eight metres, the length of a London bus.
But others have been caught even bigger.
One about twice the length, that weighed around a tonne.
Very few people have ever seen one of these creatures alive.
And that's because they live at depths of around 1,000 metres.
And down there, it's pitch black.
So how do these animals manage to hunt in such conditions? That's a question that's proved exceedingly difficult to answer.
Sailors, a long time ago, told stories of having seen a gigantic squid-like creature known as the kraken.
It was said to have huge tentacles, strong enough to grip and sink a ship.
The tales seemed unlikely and far-fetched.
But could the giant squid perhaps have been the source of these extraordinary reports? The first clues to this creature may, in fact, be real came from the tales of sailors on whaling ships in the 18th and 19th centuries.
Some of them reported in their ships' logs that they often noticed strange circular scars on the heads and jaws of captured sperm whales.
The scars suggested a fierce wrestling match with some enormous beast.
But what creature could take on a 70-tonne whale? Inside of the stomachs of the whales were clues.
A number of hard indigestible objects, like this one.
It looks a bit like a beak of a parrot.
But, in fact, it belongs to an entirely different kind of animal.
To a cephalopod.
Cephalopods are marine animals that include the octopus, the squid and the cuttlefish.
This beak is a mouth part of one such creature, and is used to tear its prey into small pieces.
The sailors on the whaling ships immediately recognised the beak as being from a cephalopod.
But its size suggested a creature many times bigger than any known species.
Cephalopods have a ring of eight or ten arms, or tentacles, which they use to push food into their mouth in the centre of the ring.
The arms are equipped with round suckers to help hold onto their prey.
And it's the marks from these that were found by sailors on the bodies of sperm whales.
Could a gigantic squid have caused such injuries? And how massive must it be to tackle a sperm whale, one of the biggest animals on the planet? And then, in 1873, fishermen caught what they called a sea monster off the coast of Newfoundland in Canada.
After killing it with their knives, they lost the body.
But they brought the head and tentacles to the local clergyman.
The clergyman bought it off the fishermen for $10 and displayed it in his living room by carefully draping it over a bath stand to show off its many arms and tentacles.
The photograph clearly proved that here was a gigantic squid with its beak at the top and over seven-metre-long tentacles.
Here, at last, was the evidence that the monster of the deep, the kraken, really does exist.
But the giant squid itself continued to evade scientists even after its discovery.
It's only since the invention of submersibles that we've been able to follow it down into its deep sea home.
Even so, we seem to have had little success in finding the elusive giant.
So scientists are now trying to piece together its biology by looking at other closely-related animals.
This is an octopus.
And it uses both its eyes and its tentacles to explore its surroundings.
The octopus's brain is distributed throughout its body so that its arms can control much of their own movement.
It also has highly complex eyes and sees in much the same way as we do, with a lens projecting an image onto the retina behind.
But while our eyes focus by squeezing the lens to change its shape, the octopus' eyes focus like a camera, with a lens moving in and out.
The giant squid's eyes have much the same structure as those of an octopus.
But when it comes to size, it has the biggest eye in the animal kingdom, as large as a football.
For seeing in dim light, a large eye is better than a small one.
So many animals of the deep have exceptionally big eyes.
But, in order to see at all, there has to be some light.
And the giant squid lives at depths of 1,000 metres.
Although very little sunlight reaches the deeper parts of the ocean, there is another kind of light there.
It's produced by the deep sea animals and is called bioluminescence.
The light is produced by a chemical reaction in the same way as that in a glow stick does.
When I shake and snap the stick, two chemicals called luciferin and luciferase react together to produce a bioluminescent glow.
Like this.
There.
Some deep sea animals use their own luciferants to produce light, while in others it's produced by bacteria, living within special light organs.
A flashing light can act as a lure, or confuse a predator.
It's thought that about 90% of deep sea creatures produce bioluminescence.
And they use it in a number of different ways.
All these fish come from the deep sea, and they all produce light in one way or another.
This is the football anglerfish.
And it has a modified ray from its dorsal fin which has a lot of little tentacles on the top.
And the tip of each tentacle produces a little green light.
So it looks as though there's a little shoal of small creatures, maybe shrimps, hovering above it in the blackness.
And when some other shrimp thinks it might join some friends and come along that way, the angler fish simply tilts up, opens this immense jaw and has its breakfast.
This, on the other hand, is a stoplight loose jaw.
And it operates in a different way.
It produces red light from two little organs at the front.
Hardly any other species of fish in the sea can see red light.
So it can hunt that way and find its prey.
And when it does it opens this immense loose jaw and engulfs it.
There you are.
Back you go.
But what about the giant squid? Could it also be producing bioluminescence? Some of its close relatives apparently can.
This is the vampire squid.
It has eight arms lined with tooth-like projections.
And when threatened, it turns itself inside out, wrapping its body in a dark cloak.
If that doesn't work, the squid has another trick.
Small lights at the end of its arms flash like eyes to distract the predator.
With so many creatures of the deep producing light, you might think that the giant squid would do so as well.
But scientists studying their carcasses have not been able to find any evidence of light-producing bacteria or pigments in their bodies.
So it seems that the ocean's elusive giant truly hides in the dark.
Although it may not produce its own light, the giant squid can surely see the bioluminescence of others.
And this may help it to locate its prey.
With no sightings of a living giant squid since it was first discovered, we seem to be no closer to discovering the truth.
But, in 2004,Japanese scientists finally made a breakthrough.
Using small squid as bait, they were able to attract a live giant squid.
These first images are tantalising.
But they still reveal little of the animal's true behaviour.
Where does it live and how does it feed? Questions such as these remain unanswered.
In spite of its great size, the giant squid has proved remarkably difficult to find.
No doubt scientists will continue to search for it and discover more about it.
But my guess is that the giant squid is likely to remain ahead of the game.
That this natural curiosity is likely to see us before we see it.
Both the owl and the giant squid live in a world with little light and both have evolved large eyes, the better to see the world around them.
But while we've unravelled the owl's ways of surviving in the dark, much about giant squid still remains a mystery.