0:00:02 > 0:00:06Nature has twisted the tusk of the narwhal and the shells of snails and
0:00:06 > 0:00:10their relatives, but what is the purpose of the twist?
0:00:12 > 0:00:17I've met many fascinating animals in my time, but there are those
0:00:17 > 0:00:20I find particularly intriguing.
0:00:20 > 0:00:23Some are very familiar.
0:00:23 > 0:00:25We've known them for centuries.
0:00:27 > 0:00:29Others have been discovered only recently.
0:00:32 > 0:00:36But all of them raise interesting questions and, in this series,
0:00:36 > 0:00:38I try to find some of the answers.
0:00:47 > 0:00:50Spirals are common in the natural world.
0:00:50 > 0:00:53We seldom pay attention to them, but, in fact,
0:00:53 > 0:00:57they have remarkable characteristics which many animals exploit.
0:00:58 > 0:01:01And some creatures, having developed a spiral,
0:01:01 > 0:01:05have reworked it in many intriguing and beautiful ways.
0:01:07 > 0:01:12In this programme, I'll try to discover why the spiral is so
0:01:12 > 0:01:14important to two very different kinds of animals.
0:01:22 > 0:01:26The narwhal lives in the cold waters of the Arctic Sea.
0:01:27 > 0:01:31It's rarely seen and little is known about its life, even today.
0:01:31 > 0:01:32But 400 years ago,
0:01:32 > 0:01:36it was the source of myths and tall tales that fooled everyone,
0:01:36 > 0:01:39including the royal households of Europe.
0:01:40 > 0:01:44These tapestries, hanging in Stirling Castle,
0:01:44 > 0:01:49are modern, but they are accurate copies of medieval originals.
0:01:49 > 0:01:54And they show several images of that most wonderful creature,
0:01:54 > 0:01:56the unicorn.
0:01:56 > 0:02:01In the Middle Ages, the unicorn was thought to be a real animal and,
0:02:01 > 0:02:04what's more, one with magical powers.
0:02:04 > 0:02:08So the King of Scotland incorporated one in his coat of arms and that,
0:02:08 > 0:02:10in due course,
0:02:10 > 0:02:14was inherited by the British coat of arms and shown sitting
0:02:14 > 0:02:16opposite the English lion.
0:02:19 > 0:02:23During the Middle Ages, it was believed that a unicorn horn
0:02:23 > 0:02:25could detect poison and neutralise it.
0:02:25 > 0:02:29So it's not surprising that most of the kings of Europe wanted one of
0:02:29 > 0:02:32these wonderful and powerful objects.
0:02:32 > 0:02:34Such treasures, however, weren't easy to come by.
0:02:36 > 0:02:41But in the 16th century, an English seaman accidentally discovered one.
0:02:41 > 0:02:46In 1576, Martin Frobisher sailed across the North Atlantic
0:02:46 > 0:02:47in search of a sea route
0:02:47 > 0:02:50to connect the Atlantic with the Pacific.
0:02:50 > 0:02:54And when he reached the chilly coast of northern Canada, he found,
0:02:54 > 0:02:58lying on the seashore, a unicorn's horn.
0:02:58 > 0:03:00He brought it back to Britain and soon found a buyer...
0:03:01 > 0:03:03..Elizabeth I.
0:03:05 > 0:03:08This is very like the object that Sir Martin Frobisher
0:03:08 > 0:03:10presented to Queen Elizabeth.
0:03:10 > 0:03:14It's said that she paid £10,000 for it.
0:03:14 > 0:03:18In today's money, that's about half a million or more.
0:03:18 > 0:03:22Weight for weight, unicorn horn was worth more than gold.
0:03:24 > 0:03:28But the object was not what Queen Elizabeth supposed it to be.
0:03:28 > 0:03:31It was not the horn of a mythical animal.
0:03:31 > 0:03:36It was the tusk of a kind of whale that swam in the Arctic seas,
0:03:36 > 0:03:42the narwhal. The first examples were brought south by the Vikings.
0:03:42 > 0:03:47They almost certainly knew exactly what its origin was,
0:03:47 > 0:03:48but, for 400 years,
0:03:48 > 0:03:52they maintained the story that it came from the mythical unicorn.
0:03:54 > 0:03:56But farther south in Europe,
0:03:56 > 0:04:00no-one new about narwhals and scholarly naturalist books confidently
0:04:00 > 0:04:04described unicorns in detail, as if they were real.
0:04:04 > 0:04:07Since unicorn horn were hard to come by,
0:04:07 > 0:04:12unscrupulous dealers met the demand by grinding up rhinoceros horn.
0:04:12 > 0:04:16In fact, the horn of a rhino and a narwhal could hardly be more different.
0:04:17 > 0:04:22You can see from this narwhal skull the hole where the horn would normally sit.
0:04:22 > 0:04:27It grows outwards, through the lip.
0:04:27 > 0:04:29But whereas rhino horn is actually made of keratin,
0:04:29 > 0:04:32the same stuff as our fingernails are made of,
0:04:32 > 0:04:37the narwhal's great horn is actually made largely of dentine.
0:04:38 > 0:04:39It's not a horn at all.
0:04:39 > 0:04:43It's an enormous canine tooth, a tusk.
0:04:45 > 0:04:49Some female narwhals possess tusks, but by and large,
0:04:49 > 0:04:51male narwhals grow the long tusks,
0:04:51 > 0:04:54which can reach three metres in length.
0:04:55 > 0:05:00It's been described as a cross between a corkscrew and a jousting lance.
0:05:00 > 0:05:03But its true purpose has baffled scientists for centuries.
0:05:05 > 0:05:07Very few creatures have tusks.
0:05:07 > 0:05:10The most well-known, of course, are elephants.
0:05:11 > 0:05:15Their tusks are in fact enlarged incisor teeth.
0:05:15 > 0:05:19Both male and female elephants develop them and they are used in
0:05:19 > 0:05:23many ways, but primarily for getting food, digging into the ground,
0:05:23 > 0:05:25ripping up grass or pushing over trees.
0:05:28 > 0:05:31The obvious difference between elephant and narwhal tusks is that
0:05:31 > 0:05:35the narwhal possesses just one while the elephant has two.
0:05:35 > 0:05:38But that may not always have been the case.
0:05:38 > 0:05:40This is a rare curiosity indeed.
0:05:40 > 0:05:44It's the skull of a narwhal with two tusks.
0:05:44 > 0:05:48It's possible that such a rarity offers a window on the past.
0:05:48 > 0:05:52Perhaps the ancient ancestors of the narwhals were once twin tusked,
0:05:52 > 0:05:54but over time, they lost one.
0:05:56 > 0:05:58But what was it for?
0:05:58 > 0:06:02One early suggestion was that the narwhal used it to spear fish,
0:06:02 > 0:06:05though how it would manage to transfer its catch from the end of
0:06:05 > 0:06:08its tusks to its mouth was never explained.
0:06:09 > 0:06:13Someone else suggested that the animal used its horn to stab holes
0:06:13 > 0:06:14through the Arctic ice.
0:06:14 > 0:06:18That's not unreasonable, since narwhals spend lots of time under ice and,
0:06:18 > 0:06:22being mammals, they have to get to air in order to breathe.
0:06:22 > 0:06:26But it seems strange that only males have the tusk.
0:06:26 > 0:06:28After all, females need to breathe, too.
0:06:29 > 0:06:32Charles Darwin had another explanation.
0:06:32 > 0:06:38He likened the tusk to the antlers carried by male deer, stags.
0:06:39 > 0:06:44Antlers help stags to establish hierarchies during the mating season.
0:06:44 > 0:06:48The stag with the biggest antlers asserts his dominance by showing
0:06:48 > 0:06:50them off and occasionally fighting with them.
0:06:55 > 0:07:00Darwin proposed that the long tusk of the narwhal functioned in just
0:07:00 > 0:07:05the same way. As a declaration of dominance and, if necessary, as a weapon.
0:07:05 > 0:07:09That would explain why male narwhals possess the long tusks.
0:07:11 > 0:07:12And why, when males meet,
0:07:12 > 0:07:17they sometimes cross tusks in what might be ritualised form of combat.
0:07:22 > 0:07:26Darwin's theory has long been accepted, but recently,
0:07:26 > 0:07:29scientists have been exploring other possibilities.
0:07:30 > 0:07:34Our teeth are covered with a thick enamel layer that protects the
0:07:34 > 0:07:36softer material beneath.
0:07:36 > 0:07:38If that erodes or is damaged,
0:07:38 > 0:07:42then it exposes the nerves within the tooth which can make them
0:07:42 > 0:07:45extremely sensitive to temperature.
0:07:45 > 0:07:51Narwhal tusks don't possess that external enamel covering and high
0:07:51 > 0:07:56magnification photography has revealed something very unusual about the
0:07:56 > 0:07:59exterior surface of this huge, elongated tooth.
0:08:00 > 0:08:05The surface of the tusk is cratered with millions of tiny pits
0:08:05 > 0:08:07called tubules.
0:08:07 > 0:08:11Each tubules contains a fluid and, at its base, a nerve.
0:08:11 > 0:08:14The fluid reacts to the narwhal's environment,
0:08:14 > 0:08:17so the tusk must be highly sensitive.
0:08:19 > 0:08:23Tests on narwhals have shown that they can detect tiny changes in the
0:08:23 > 0:08:26temperature and salinity of water,
0:08:26 > 0:08:28key factors that govern the formation of ice.
0:08:30 > 0:08:34Their migration is tied to the seasonal shrinking and expanding of
0:08:34 > 0:08:40the ice cap. So perhaps the tusk plays a role in detecting ice or open water.
0:08:42 > 0:08:46But its sensory powers could be even greater.
0:08:46 > 0:08:51Perhaps the tusk is able to detect movement in the water or even changes
0:08:51 > 0:08:54in the fertility of female narwhals.
0:08:54 > 0:08:56These are theories yet to be tested.
0:08:58 > 0:09:00If this is a sensory tool,
0:09:00 > 0:09:05then it would put a very different interpretation on the male jousting.
0:09:05 > 0:09:08Perhaps males enjoy rubbing their tusks together.
0:09:09 > 0:09:13There could be a third explanation, a more practical one.
0:09:13 > 0:09:18Tusks from old narwhals often become coated in algae which might block
0:09:18 > 0:09:20the pores that led to the nerves.
0:09:21 > 0:09:25So perhaps males rub their tusks together to help clean them.
0:09:26 > 0:09:30Could this be not fighting but cooperative grooming?
0:09:32 > 0:09:37Why mainly male narwhals carry a sensory tool is still unexplained.
0:09:38 > 0:09:39Rather than being a weapon,
0:09:39 > 0:09:45perhaps the highly sensitive tusk helps males to find female partners.
0:09:45 > 0:09:49More than likely, the tusk serves many functions.
0:09:49 > 0:09:50But why should it be twisted?
0:09:52 > 0:09:55The twist increases the surface area,
0:09:55 > 0:10:00so it's possible more nerve endings are exposed and this would increase
0:10:00 > 0:10:02its sensitivity.
0:10:02 > 0:10:06There's another theory that suggests that the twist actually helps to
0:10:06 > 0:10:08keep the tusk straight.
0:10:08 > 0:10:13That may sound counterintuitive, but tusks of other large animals tend to
0:10:13 > 0:10:15curve down or up.
0:10:15 > 0:10:20A spiral growth may actually help the tusk to keep pointing forwards
0:10:20 > 0:10:22and so reduce drag in the water.
0:10:24 > 0:10:27There is another way in which the twist could help in swimming.
0:10:28 > 0:10:30As the animal moves forward,
0:10:30 > 0:10:34the water around the tusk spirals away from it in a way that might
0:10:34 > 0:10:36reduce drag.
0:10:37 > 0:10:41But at least today we know the true identity of the animals that produce
0:10:41 > 0:10:45these wonderful and spectacular ivory spears.
0:10:51 > 0:10:56The myth that they came from the unicorn was finally exploded in 1638
0:10:56 > 0:10:59by a Danish scientist, Ole Worm,
0:10:59 > 0:11:03who gave a public lecture proving conclusively that they came from the
0:11:03 > 0:11:08narwhal. So then, of course, their value plummeted.
0:11:08 > 0:11:11Today, we no longer believe they have magical properties,
0:11:11 > 0:11:15but there's still quite a lot about them we don't fully understand.
0:11:21 > 0:11:25Our second subject belongs to a group of animals that have taken the
0:11:25 > 0:11:29spiral and adapted it into a multitude of variations.
0:11:30 > 0:11:33Snails.
0:11:37 > 0:11:42When the first snails crawled out of the sea and up onto dry land,
0:11:42 > 0:11:47they carried with them the shells that were to be crucial to their
0:11:47 > 0:11:52survival out of water. They themselves were distant relatives of
0:11:52 > 0:11:57other shelled creatures that had dominated the seas for millions of years.
0:11:57 > 0:11:59They were the ammonites.
0:11:59 > 0:12:04This is one of them and this is about 160 million years old.
0:12:04 > 0:12:08Although they experimented to some degree with the shape of the shell,
0:12:08 > 0:12:11nearly all of them are like this.
0:12:11 > 0:12:15Flat, spiral and symmetrical.
0:12:17 > 0:12:22In due course, the ammonites themselves became extinct, but since then,
0:12:22 > 0:12:27other creatures have developed a shell into a whole variety of
0:12:27 > 0:12:30different shapes and sizes.
0:12:33 > 0:12:39This variety shows how successful the spiral can be as the basis for a
0:12:39 > 0:12:42shell's design.
0:12:43 > 0:12:45And how it can be elaborated
0:12:46 > 0:12:48and decorated.
0:12:51 > 0:12:54Snail shells, like the shells of birds' eggs,
0:12:54 > 0:12:56are made of calcium carbonate.
0:12:57 > 0:13:01They appear at the very beginning of a young snail's life and they are
0:13:01 > 0:13:05never shed, but simply become enlarged as the animal grows.
0:13:07 > 0:13:11But whatever their shape and size, they are almost always spiralled.
0:13:13 > 0:13:16Spirals have been used by animals for a very long time.
0:13:16 > 0:13:21We can trace them back to a group of sea creatures that first appeared
0:13:21 > 0:13:25around 500 million years ago and some are still around today.
0:13:25 > 0:13:28This is one, the nautilus.
0:13:28 > 0:13:33Today, it is only found in the deep waters of the Indo-Pacific Ocean,
0:13:33 > 0:13:37but millions of years ago, animals like it were widespread.
0:13:37 > 0:13:40Its earliest ancestors, however, had a very different shape.
0:13:42 > 0:13:46There's evidence that the nautiloids started out more or less straight,
0:13:46 > 0:13:51like this one. Just a little curl at the beginning and then running
0:13:51 > 0:13:55straight like that with the separate chambers running along there.
0:13:55 > 0:13:59But as millions of years passed, they began to coil
0:13:59 > 0:14:02until they became species like this one.
0:14:03 > 0:14:05And then, millions of years later,
0:14:05 > 0:14:09another group adopted the symmetrical coil.
0:14:09 > 0:14:12These were called ammonites.
0:14:13 > 0:14:16But why did these animals coil their shells?
0:14:17 > 0:14:21Well, if their shells remained straight, as they increased in size,
0:14:21 > 0:14:24they would inevitably become somewhat cumbersome.
0:14:26 > 0:14:30Coiling them made them more compact and perhaps more mobile.
0:14:33 > 0:14:37Whatever the reason, the change in shell shape was a great success.
0:14:38 > 0:14:43Thousands of new species appeared, all with coiled shells.
0:14:44 > 0:14:48These fossilised shells tell us little about the soft-bodied
0:14:48 > 0:14:52creatures that lived in them, but the living nautilus can give us some
0:14:52 > 0:14:54clues about that.
0:14:56 > 0:15:01At the start of its life, the shell consists of just a few chambers,
0:15:01 > 0:15:05but by the time it's mature, there may be as many as 30.
0:15:07 > 0:15:11Richard Owen, the founding director of London's Natural History Museum,
0:15:11 > 0:15:15wrote the first full description of the nautilus.
0:15:16 > 0:15:22This is Owen's own personal copy and it's full of exquisite sketches.
0:15:22 > 0:15:28His drawings show just how the animal was placed inside a shell.
0:15:28 > 0:15:32Almost all the soft tissues of its body and tentacles are held in the
0:15:32 > 0:15:37outermost chamber and a long tube called a siphuncle runs through the
0:15:37 > 0:15:43chambers, through which the animal can pump in water or remove it and
0:15:43 > 0:15:46so regulate its buoyancy.
0:15:46 > 0:15:51So, the nautilus' spiral shell not only protects its soft body from
0:15:51 > 0:15:54enemies, but enables it to cruise around.
0:15:54 > 0:15:59And it's so strong that the nautilus can descend as deep as 700 metres,
0:15:59 > 0:16:01where pressure would kill a human being.
0:16:03 > 0:16:04At the peak of their success,
0:16:04 > 0:16:08there were thousands of different kinds of nautiloids.
0:16:08 > 0:16:12But their cousins, the ammonites, were even more varied and diverse.
0:16:14 > 0:16:18Their buoyant shells allowed some of these creatures to grow to
0:16:18 > 0:16:20a huge size.
0:16:25 > 0:16:28Some were as big as a human being.
0:16:28 > 0:16:32But it would be impossible for such a creature to move out of water with
0:16:32 > 0:16:34a shell like this.
0:16:34 > 0:16:36It would be far too heavy and too cumbersome.
0:16:37 > 0:16:41Nonetheless, something was about to happen to the molluscs that would
0:16:41 > 0:16:45allow them to leave the water and move up onto land.
0:16:47 > 0:16:51The ammonite dynasties were developing different shapes to their shells,
0:16:51 > 0:16:55uncoiling them in all sorts of ways.
0:16:55 > 0:16:59Some of these new forms fed on the sea floor and therefore had less
0:16:59 > 0:17:01need to be mobile.
0:17:01 > 0:17:06But other shelled relatives of the ammonites were going even further,
0:17:06 > 0:17:09changing both their shell shape and twisting their soft bodies.
0:17:10 > 0:17:13And these are their descendants.
0:17:13 > 0:17:15Snails.
0:17:16 > 0:17:21The problem with asymmetrical shells is that each whirl has to grow on
0:17:21 > 0:17:28the outside of the other one, so the shell very quickly becomes very big.
0:17:28 > 0:17:33But by becoming asymmetrical and offsetting each whirl to the side,
0:17:33 > 0:17:37the shell can remain much more compact and rounded and easier
0:17:37 > 0:17:39to manipulate.
0:17:41 > 0:17:45The shift in the snail's symmetry seems to have been triggered by the
0:17:45 > 0:17:47action of a single gene.
0:17:49 > 0:17:52But this change can bring complications.
0:17:54 > 0:17:56Because of their asymmetric shape,
0:17:56 > 0:18:00snails have to position themselves carefully during mating.
0:18:02 > 0:18:04In most snails, this is not a problem,
0:18:04 > 0:18:07as the body plan of snails is usually the same.
0:18:08 > 0:18:10But not all.
0:18:12 > 0:18:15Just like humans, who are either right-handed or left-handed,
0:18:15 > 0:18:19snail shells can twist to the left
0:18:20 > 0:18:22or the right.
0:18:22 > 0:18:26The vast majority of snail shells are right spiralling,
0:18:26 > 0:18:29but in one particular area of Japan,
0:18:29 > 0:18:33the left-handed form of this particular species has a clear advantage.
0:18:35 > 0:18:40That is all because of this creature, a snail-eating snake.
0:18:40 > 0:18:44It's so specialised for eating snails that its jaws have become evolved
0:18:44 > 0:18:47to become asymmetrical, just like its prey.
0:18:47 > 0:18:50The right side of its lower jaw has more teeth than the left.
0:18:52 > 0:18:57Recently, scientists in Japan filmed the hunting behaviour of this snake.
0:18:57 > 0:19:01When it attacks a snail with a right spiral shell,
0:19:01 > 0:19:06its row of extra teeth dig into the snail's flesh and by moving its jaws
0:19:06 > 0:19:10back and forth, it separates the snail's body from its shell.
0:19:12 > 0:19:16But attacking the snail with a left spiral shell is not so
0:19:16 > 0:19:21straightforward. The position of the shell means that the snake can't use
0:19:21 > 0:19:24its specialised jaws so effectively and it gives up.
0:19:31 > 0:19:36Shells help land-living snails to conserve moisture and also protect
0:19:36 > 0:19:38them from their enemies.
0:19:40 > 0:19:43The snails soft bodies are, of course,
0:19:43 > 0:19:46welcome meals to any predator that can crack their shells.
0:19:52 > 0:19:54Some snails have strengthened their shells.
0:19:56 > 0:19:58Some have protected them with spines.
0:20:01 > 0:20:05Others have become very thick indeed and almost uncrackable.
0:20:08 > 0:20:12Some scientists believe that this could be the golden age of the snail.
0:20:13 > 0:20:18They've never been more diverse in terms of species or indeed the
0:20:18 > 0:20:19variety of their shells.
0:20:21 > 0:20:23But while the snails are more varied,
0:20:23 > 0:20:26that is not the case with the nautilus.
0:20:26 > 0:20:30The oceans were once dominated by creatures like this and today,
0:20:30 > 0:20:33just a handful of different types exist.
0:20:35 > 0:20:39While snails have taken the spiral and modified it endlessly,
0:20:39 > 0:20:43the modern nautilus has stuck with a symmetrical spiral that's hardly
0:20:43 > 0:20:47changed for hundreds of millions of years.
0:20:47 > 0:20:51So it's fair to say that the nautilus shell is a window on the
0:20:51 > 0:20:58distant past, to a time when the simple but symmetrical spiral dominated the seas.
0:21:00 > 0:21:04So both whales and snails have benefited from the twist,
0:21:04 > 0:21:08a design that first appeared 500 million years ago and is still
0:21:08 > 0:21:10widespread today.