David Attenborough's Natural Curiosities (2013) s01e04 Episode Script

SIR DAVID ATTENBOROUGH: Nature has twisted the tusk of the narwhal and the shells of snails and their relatives.
But what is the purpose of the twist? I've met many fascinating animals in my time, but there are those I find particularly intriguing.
Some are very familiar- we've known them for centuries.
Others have been discovered only recently.
But all of them raise interesting questions, and in this series, I try to find some of the answers.
Spirals are common in the natural world.
We seldom pay attention to them.
But, in fact, they have remarkable characteristics which many animals exploit.
And some creatures, having developed a spiral, have re-worked it in many intriguing and beautiful ways.
In this programme, I'll try to discover why the spiral is so important to two very different kinds of animals.
The narwhal lives in the cold waters of the Arctic sea.
It's rarely seen and little is known about its life, even today.
But 400 years ago, it was the source of myths and tall tales that fooled everyone, including the royal households of Europe.
These tapestries hanging in Stirling Castle are modern, but they are accurate copies of medieval originals.
And they show several images of that most wonderful creature, the unicorn.
In the Middle Ages, the unicorn was thought to be a real animal and, what's more, one with magical powers.
So the King of Scotland incorporated one in his coat of arms, and that in due course was inherited by the British coat of arms and shown sitting opposite the English lion.
During the Middle Ages, it was believed that a unicorn horn could detect poison and neutralise it.
So it's not surprising that most of the kings of Europe wanted one of these wonderful and powerful objects.
Such treasures, however, weren't easy to come by.
But in the 16th century, an English seaman accidentally discovered one.
In 1576, Martin Frobisher sailed across the North Atlantic in search of a sea route to connect the Atlantic with the Pacific, and when he reached the chilly coast of Northern Canada he found, lying on the seashore, a "unicorn's horn".
He brought it back to Britain and soon found a buyer- Elizabeth I.
This is very like the object that Sir Martin Frobisher presented to Queen Elizabeth.
It's said that she paid £10,000 for it.
In today's money, that's about half a million or more.
Weight for weight, "unicorn horn" was worth more than gold.
But the object was not what Queen Elizabeth supposed it to be.
It was not the horn of a mythical animal, it was the tusk of a kind of whale that swam in the Arctic seas - the narwhal.
The first examples were brought south by the Vikings.
They almost certainly knew exactly what its origin was, but for 400 years, they maintained the story that it came from the mythical unicorn.
But further south in Europe, no-one knew about narwhals, and scholarly natural history books confidently described unicorns in detail, as if they were real.
Since unicorn horns were hard to come by, unscrupulous dealers met the demand by grinding up rhinoceros horn.
In fact, the horn of a rhino and a narwhal could hardly be more different.
You can see from this narwhal skull the hole where the horn would normally sit.
It grows outwards through the lip.
But whereas rhino horn is actually made of keratin, the same stuff as our fingernails are made of, the narwhal's great horn is actually made largely of dentine.
It's not a horn at all, it's an enormous canine tooth - a tusk.
Some female narwhals possess tusks, but by and large, male narwhals grow the long tusks, which can reach three metres in length.
It's been described as a cross between a corkscrew and a jousting lance.
But its true purpose has baffled scientists for centuries.
Very few creatures have tusks.
The most well-known, of course, are elephants.
Their tusks are in fact enlarged incisor teeth.
Both male and female elephants develop them and they're used in many ways, but primarily for getting food, digging into the ground, ripping up grass or pushing over trees.
The obvious difference between elephant and narwhal tusks is that the narwhal possesses just one whereas the elephant has two, but that may not always have been the case.
This is a rare curiosity indeed.
It's the skull of a narwhal with two tusks.
It's possible that such a rarity offers a window on the past.
Perhaps the ancient ancestors of the narwhals were once twin-tusked, but over time they lost one.
But what was it for? One early suggestion was that the narwhal used it to spear fish, though how it would manage to transfer its catch from the end of its tusk to its mouth was never explained.
Someone else suggested that the animal used its horn to stab holes through the Arctic ice.
That's not unreasonable, since narwhals spend a lot of time under ice and, being mammals, they have to get to air in order to breathe.
But it seemed strange that only males have a tusk.
After all, females need to breathe, too.
Charles Darwin had another explanation.
He likened the tusk to the antlers carried by male deer, stags.
Antlers help stags to establish hierarchies during the mating season.
The stag with the biggest antlers asserts his dominance by showing them off and occasionally fighting with them.
Darwin proposed that the long tusk of the narwhal functioned in just the same way - as a declaration of dominance and, if necessary, as a weapon.
That would explain why male narwhals possess the long tusks.
And why, when males meet, they sometimes cross tusks in what might be a ritualised form of combat.
Darwin's theory has long been accepted, but recently, scientists have been exploring other possibilities.
Our teeth are covered with a thick enamel layer that protects the softer material beneath.
If that erodes or is damaged, then it exposes the nerves within the tooth, which can make them extremely sensitive to temperature.
Narwhal tusks don't possess that external enamel covering, and high-magnification photography has revealed something very unusual about the exterior surface of this huge, elongated tooth.
The surface of the tusk is cratered with millions of tiny pits, called tubules.
Each tubule contains a fluid and, at its base, a nerve.
The fluid reacts to the narwhal's environment, so the tusk must be highly sensitive.
Tests on narwhals have shown that they can detect tiny changes in the temperature and salinity of water, key factors that govern the formation of ice.
Their migration is tied to the seasonal shrinking and expanding of the ice cap.
So perhaps the tusk plays a role in detecting ice or open water.
But its sensory powers could be even greater.
Perhaps the tusk is able to detect movement in the water, or even changes in the fertility of female narwhals.
These are theories yet to be tested.
If this is a sensory tool, then it would put a very different interpretation on the male jousting.
Perhaps males enjoy rubbing their tusks together.
There could be a third explanation, a more practical one.
Tusks from old narwhals often become coated with algae which might block the pores that lead to the nerves.
So, perhaps males rub their tusks together to help clean them.
Could this be not fighting but co-operative grooming? Why mainly male narwhals carry a sensory tool is still unexplained.
Rather than being a weapon, perhaps the highly sensitive tusk helps males to find female partners.
More than likely, the tusk serves many functions, but why should it be twisted? The twist increases the surface area so it's possible more nerve endings are exposed and this would increase its sensitivity.
There's another theory that suggests that the twist actually helps to keep the tusk straight.
That may sound counter-intuitive, but tusks of other large animals tend to curve down or up.
A spiral growth may actually help the tusk to keep pointing forwards and so reduce drag in the water.
There's another way in which the twist could help in swimming.
As the animal moves forward, the water around the tusk spirals away from it in a way that might reduce drag.
But at least today we know the true identity of the animals that produce these wonderful and spectacular ivory spears.
The myth that they came from the unicorn was finally exploded in 1638 by a Danish scientist, Ole Wurm, who gave a public lecture proving conclusively that they came from the narwhal.
So then, of course, their value plummeted.
Today, we no longer believe they have magical properties, but there's still quite a lot about them we don't fully understand.
Our second subject belongs to a group of animals that have taken the spiral and adapted it into a multitude of variations - snails.
When the first snails crawled out of the sea and up on to dry land, they carried with them the shells that were to be crucial to their survival out of water.
They themselves were distant relatives of other shelled creatures that had dominated the seas for millions of years.
They were the ammonites.
This is one of them, and this is about 160 million years old.
Although they experimented to some degree with the shape of the shell, nearly all of them are like this - flat, spiral and symmetrical.
In due course, the ammonites themselves became extinct, but since then, other creatures have developed the shell into a whole variety of different shapes and sizes.
This variety shows how successful the spiral can be as the basis for a shell's design and how it can be elaborated and decorated.
Snail shells, like the shells of birds' eggs, are made of calcium carbonate.
They appear at the very beginning of a young snail's life and they are never shed, but simply become enlarged as the animal grows.
But whatever their shape and size, they're almost always spiralled.
Spirals have been used by animals for a very long time.
We can trace them back to a group of sea creatures that first appeared around 500 million years ago, and some are still around today.
This is one - the nautilus.
Today, it's only found in the deep waters of the Indo-Pacific Ocean, but millions of years ago, animals like it were widespread.
Its earliest ancestors, however, had a very different shape.
There's evidence that the nautiloids started out more or less straight, like this one, just a little curl at the beginning and then running straight like that, with the separate chambers running along there.
But as millions of years passed, they began to coil, until they became species like this one.
And then, millions of years later, another group adopted the symmetrical coil.
These were called ammonites.
But why did these animals coil their shells? Well, if their shells remained straight as they increased in size, they would inevitably become somewhat cumbersome.
Coiling them made them more compact and perhaps more mobile.
Whatever the reason, the change in shell shape was a great success.
Thousands of new species appeared, all with coiled shells.
These fossilised shells tell us little about the soft-bodied creatures that lived in them, but the living nautilus can give us some clues about that.
At the start of its life, the shell consists of just a few chambers but by the time it's mature, there may be as many as 30.
Richard Owen, the founding director of London's Natural History Museum, wrote the first full description of the nautilus.
This is Owen's own personal copy, and it's full of exquisite sketches.
His drawings show just how the animal is placed inside a shell.
Almost all the soft tissues of its body and tentacles are held in the outermost chamber, and a long tube called a siphuncle runs through the chambers through which the animal can pump in water or remove it, and so regulates its buoyancy.
So, the nautilus' spiral shell not only protects its soft body from enemies, but enables it to cruise around, and it's so strong that the nautilus can descend as deep as 700 metres, where pressure would kill a human being.
At the peak of their success, there were thousands of different kinds of nautiloids, but their cousins, the ammonites, were even more varied and diverse.
Their buoyant shells allowed some of these creatures to grow to a huge size.
Some were as big as a human being.
But it would be impossible for such a creature to move out of water with a shell like this.
It would be far too heavy and too cumbersome.
Nonetheless, something was about to happen to the molluscs that would allow them to leave the water and move up onto land.
The ammonite dynasties were developing different shapes to their shells, uncoiling them in all sorts of ways.
Some of these new forms fed on the sea floor, and therefore had less need to be mobile.
But other shelled relatives of the ammonites were going even further, changing both their shell shape and twisting their soft bodies.
And these are their descendants - snails.
The problem with a symmetrical shell is that each whorl has to grow on the outside of the other one, so that the shell very quickly becomes very big.
But by coming asymmetrical and offsetting each whorl to the side, the shell can remain much more compact and rounded and easier to manipulate.
The shift in the snail's symmetry seems to have been triggered by the action of a single gene.
But this change can bring complications.
Because of their asymmetric shape, snails have to position themselves carefully during mating.
In most snails, this is not a problem, as the body plan of snails is usually the same.
But not all.
Just like humans who are either right-handed or left-handed, snail shells can twist to the left or the right.
The vast majority of snail shells are right-spiralling, but in one particular area of Japan, the left-handed form of this particular species has a clear advantage.
That is all because of this creature - a snail-eating snake.
It's so specialised for eating snails that its jaws have evolved to become asymmetrical, just like its prey.
The right side of its lower jaw has more teeth than the left.
Recently, scientists in Japan filmed the hunting behaviour of this snake.
When it attacks a snail with a right-spiralled shell, its row of extra teeth dig into the snail's flesh and, by moving its jaws back and forth, it separates the snail's body from its shell.
But attacking a snail with a left-spiralled shell is not so straightforward.
The position of the shell means that the snake can't use its specialised jaws so effectively and it gives up.
Shells help land-living snails to conserve moisture and also protect them from their enemies.
The snails' soft bodies are, of course, welcome meals to any predator that can crack their shells.
Some snails have strengthened their shells.
Some have protected them with spines.
Others have become very thick indeed, and almost uncrackable.
Some scientists believe that this could be the golden age of the snail.
They've never been more diverse in terms of species or indeed the variety of their shells.
But while the snails are more varied, that is not the case with the nautilus.
The oceans were once dominated by creatures like this, and today, just a handful of different types exist.
While snails have taken the spiral and modified it endlessly, the modern nautilus has stuck with a symmetrical spiral that's hardly changed for hundreds of millions of years.
So, it's fair to say that the nautilus shell is a window on the distant past, to a time when the simple, but symmetrical spiral dominated the seas.
So, both whales and snails have benefited from the twist, a design that first appeared 500 million years ago and is still widespread today.