Episode 6 David Attenborough's Natural Curiosities

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 myth 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.

Animals are usually either male or female.

And, usually, they behave in a way

that is characteristic of their gender.

But in nature, there are always curious exceptions.

Female hyenas behave and look like males.

And male seahorses play mother

and physically give birth.

Only now are we beginning to understand

why these two animals seem to have swapped their sexual identities.

And also in this programme,

spiders spin intricate webs using their own silk.

And birds weave nests from strips of leaves.

I investigate the skill of these spinners and weavers

and the way they use such materials

to produce such truly complex structures.

Seahorses are fascinating.

Some are tiny and blend perfectly with their surroundings.

Others could grow to an impressive 35 centimetres in size.

They live in shallow waters, both tropical and temperate,

across much of the world,

and have even been found in the Thames Estuary near London.

Seeing one for the first time is a moment to remember.

They're magical creatures, with a truly fantastic appearance.

They have the head of a horse, eyes like a chameleon,

the prehensile tail of a monkey,

armour that can change colour

and, perhaps most strangely of all, a pouch.

Their unusual features inspired their name, Hippocampus,

a combination of two Greek words -

hippo, meaning "horse", and kampos, meaning "sea monster".

For centuries, they've been considered

animals of myth and legend,

and only today are we unravelling the true story

of males that give birth.

Seahorses baffled early naturalists.

Their unusual characteristics seemed to make them misfits.

But after much debate,

they were recognised as true bony fish.

But their breeding habits were hardly fishy.

Typically, female fish release large numbers of eggs into the sea

that males must quickly fertilise.

But a fish that kept its eggs in a pouch seemed scarcely believable.

The seahorse's striking appearance

has given it an almost magical status.

Images and stories of a creature, part horse, part fish,

have spanned the centuries across many cultures.

Among the most famous are those belonging to Poseidon.

This famous Greek god of the sea

lived below the waves,

and his golden chariot was pulled by a pair of giant hippocampi.

The seahorse's odd behaviour appeared mysterious, too.

As early as the third century BC,

Aristotle noted in his book on the history of animals

that pipefish, close relatives of the seahorse,

had a pouch that burst into two

to release the young.

These early observations

of the pipefish's strange breeding behaviour

help to reveal the true story

of the male seahorse's mysterious pouch.

Just like seahorses, pipefish carry their eggs around with them.

Some species simply stick the eggs to the outside of their bodies.

Others have a rudimentary pouch.

These simpler techniques provide some clues

as to how seahorses developed their more complex closed pouch.

But what Aristotle didn't know

when he spotted the pipefish giving birth,

was that he was actually looking at a male.

And this important detail

was to remain undiscovered for hundreds of years.

Although seahorses live in British waters,

until Victorian times few people apart from fishermen

had ever seen them.

In 1859, a Mr Pinto brought four live seahorses back to London

from the mouth of the River Tagus in Portugal.

Pinto endured a sleepless seven-day train journey through Europe,

waking himself frequently

to aerate the seahorse's water with a syringe.

His seahorses survived

and were installed in the new London Aquarium.

They were an instant hit.

Seahorses were headline news.

Mr Pinto's journey and their arrival made the front pages.

Now they could be seen in great detail,

and the study of their mysterious breeding began.

In that same year,

what was described as a "herd" of baby seahorses

was born in the British Midlands Aquarium.

This caused quite a stir, as did the discovery

that it was the male that gave birth to the young.

But why seahorses swapped parenting roles remained a mystery,

and we're still searching for the answers today.

Here at the London Zoo's aquarium,

over 150 years since the arrival of the first seahorses,

a detailed study is revealing more about their reproduction

and the usual role of the male.

These tanks are set like a seahorse dating centre,

the first port of call

is the courtship aquarium, or ballroom tank.

Here, a number of adult seahorses

spend time getting to know each other

as they look for compatible partners.

Breeding seahorses form lasting partnerships as mating pairs,

and their long, elaborate courtship dances

are a way of finding and securing a suitable mate.

Dances like those of this Australian species

can be complex and last several days.

They help the couple synchronise their bodies

so that the male's pouch is ready for the eggs.

They also help to establish the couple's joint territory.

Seahorses were thought to be monogamous,

but we now know that some are only exclusive couples

for the duration of the breeding season.

The female must choose the right male

because she's going to pass over her precious eggs to him.

Female seahorses do not have a pouch,

so a strong pair-bond with a male is very important,

as he will care for her eggs.

This is the honeymoon tank.

Seahorses that have shown an attraction for each other

in the courtship tank

are removed as a couple

and given their own private space.

In the wild, each pair has its own territory

and these smaller tanks make captive breeding more successful.

Here, the pair can synchronise their courtship.

Timing is crucial.

The female's eggs must be fully developed

at exactly the same time

that the male's pouch is ready to receive them.

Once the female's eggs are ready,

she hydrates them with seawater.

They must then be laid within 24 hours.

She transfers her eggs to her partner

by inserting her egg-laying tube, or ovipositor,

into the male's pouch.

Once pregnant, the male attaches himself to one spot

and the female visits him every day.

She checks to see when he'll be ready for her next batch of eggs.

One theory suggests that because the male is incubating the eggs,

the female has more time to feed

and can put energy into making new eggs more quickly.

Swapping roles may be a smart way

to use their resources more efficiently.

What goes on inside the pouch is still a mystery.

The male may simply provide a closed incubator.

Or the inner skin may develop extra blood vessels

to give a more placenta-like connection.

It's not clear.

During pregnancy and birth

the male's metabolism increases,

but that's little wonder,

for he may have up to 1,500 eggs in his pouch.

The male seahorse gives birth to dozens of miniature babies,

perfect in every detail.

The free-swimming young are put into separate creche tanks

where they can be fed and cared for.

The parent seahorses in this biological hotel

remain in the honeymoon suite

ready to mate again.

These are some of last year's youngsters

and they've grown enormously.

Next year, they'll be breeding themselves.

Swapping the parental roles

seems to work well for seahorses.

In warm conditions, a male can give birth every 28-30 days.

But of the thousands of fry produced each year, only a few survive.

There is no safe creche in the open sea.

To succeed, seahorse parents must work well together,

yet in this partnership,

the female seems to have the freedom

to swim, feed, and patrol the territory,

which is normally the prerogative of the male.

So, is the male seahorse a slave to a gallivanting female?

Well, latest research suggests not,

and shows that some males may have more control over breeding

than first thought.

If small or poor-quality eggs are deposited into their pouches,

some males will absorb them.

Such males appear to be choosy

about how they invest their time and energy.

And some females, in entrusting their eggs to males,

are being cheated.

But the male seahorse can't be duped,

as having a pouch means that he can always be certain

that all the baby seahorses he gives birth to are his own.

So male and female seahorses

have swapped their roles.

The male is the mother and he gives birth to the babies.

Another animal with unusual parenting habits is the hyena.

Here, it's the female that looks and behaves more like a male.

Why have female hyenas becomes so masculine?

These are African spotted hyenas,

creatures that have an undeservedly bad reputation

and a very strange biology.

In the wild, they live in clans of up to 80 individuals

and the females dominate the males.

The females are big, aggressive,

and look physically almost exactly like males.

Unravelling why the female is like this has not been easy,

as it's difficult to tell the difference between the sexes.

The female's male appearance

is made all the more convincing by her reproductive organs -

they're external and very similar to a male's.

HYENAS SQUEAK

Understanding hyena biology

has helped to explain the female's masculinity

and the species' reputation as aggressive scavengers.

But in the past, these strange traits

gave hyenas a very bad image.

In the first century, Pliny the Elder described the hyenas

and did them a great disservice.

This is what he wrote.

"Hyenas are like a cross between a dog and a wolf.

"They break everything with their teeth,

"swallow it as a gulp

"and masticate it in the belly.

"They are believed to become male and female in alternate years.

"They can imitate the human voice,

"calling a shepherd by name

"so that he comes outside, where they tear him to pieces.

"Any animal that a hyena looks at three times

"will be unable to move."

That tainted image of hyenas

was perpetuated for many years to come,

and they were branded as evil, dangerous creatures.

Hyenas are not, of course, evil,

but their competitive nature and unusual eating habits

make them appear fearsome.

They're specialist feeders.

They crush, eat and digest bones

that other creatures can't tackle

and so leave behind.

And this diet has a significant effect

on the female's appearance and her family relationships,

especially those with her cubs.

In the early 19th century,

an unusual discovery in Britain

excited one man to look more closely at the hyena's diet.

In 1822, a rather eccentric but very eminent geologist

called William Buckland

made a significant discovery

that was to further the modern understanding of hyenas.

Quarry workers in Kirkdale, Yorkshire,

had come across a cave that contained a large number of bones.

Buckland was very excited

and rushed to see the remains before they were disturbed any further.

And he found that mud deposits in the cave

had preserved the bones of over 22 different species of animals,

including tiger, bear, wolf, elephant

and, significantly, hyenas,

which Buckland described as "littering the cave

"like the bones in a dog kennel".

This is one of the actual hyena jaws that Buckland found.

It belonged to a young but ancient hyena.

There were also a lot of these on the cave floor.

They are coprolites,

or fossilised faeces from hyenas.

They contain bone fragments

that have passed through the hyena's digestive tract

and so showed that they were successful bone-crushers.

Buckland's discovery of so many bones

in what he believed to be a hyena's den

indicated that they were very successful hunters.

Contrary to popular belief,

they scavenge very little

and kill over 80% of their own food.

A lone hyena can easily kill a wildebeest or a topi,

and with teamwork they will tackle bigger animals,

like zebra and giraffe.

They do scavenge as well,

but it's more usual for lions to steal from hyenas

rather than the other way around.

Female hyenas have become big and strong

and compete for food with other members of their clan.

Nothing goes to waste -

they can eat even the thickest of bones.

Buckland was fascinated by the marks on the bones from the cave,

but found it hard to believe that hyenas had made them.

He wanted to be sure of his findings

and understand how their jaws,

with their strange, massive teeth, actually worked.

Hyenas are African or Asiatic animals,

so Buckland's discovery of hyena bones in an English cave

was strange, to put it mildly.

As a man of science, he wanted to confirm

that the skull he had collected from Kirkdale was definitely from a hyena

and that it had made the marks on the many fractured bones.

To try and prove his case,

he asked a friend, William Burchill, an African traveller,

to send a young hyena back to England from the Cape.

He planned to kill it

and compare its skull and teeth with the specimens in the cave.

The young hyena that arrived at the docks was already tame

and had become a great favourite with the sailors,

who christened him "Billy".

Billy became quite a celebrity

and was as tame as a pet dog.

No-one could bring themselves to sacrifice him

for the sake of science.

Instead, a search of British museums produced a hyena skull

and Billy's life was spared.

Buckland was then able to compare the new and old skulls,

and they matched.

Billy also helped to clarify the fractures on the bones.

He was fed ox bones, this was one.

And Buckland compared it with one that was found in the cave,

and they closely match.

This ability to crack massive bones

explains why female hyenas look like males.

It's also tied up intricately

with the relationship they have with their cubs.

Cubs are born underground

and are fed on their mother's rich milk.

At about three months of age,

they emerge from the den

and continue to suckle for almost another two years.

Their mother helps feed the youngsters,

as they can't yet crack and crush bones for themselves.

Even at almost a year in age,

when they're big enough to join the kill,

their teeth and jaws are still not sufficiently developed

to tackle big bones.

The skull of a young hyena

is quite different from that of an adult.

It's got a flat top, narrow cheeks

and relatively small teeth.

An animal with a skull like this

would not be able to crush and eat big bones.

It takes almost three years

for a young hyena's skull to grow to full size

and reach mechanical maturity.

And this is the result.

This skull has a large, vaulted forehead

that dissipates biting stress,

carrying it away from the face.

It's also got wide arches at its sides

for the attachment of powerful jaw muscles,

and robust premolars

that have specialised crack-resistant enamel.

Jaws like these

can crack the dense bones of zebra and even giraffe.

Developing this substantial specialised eating equipment

takes time.

So it may be several years

before a young hyena can feed independently.

This puts pressure on their mothers

to become dominant and aggressive.

They need to fight to get enough food for their cubs.

The female's status in the clan's hierarchy

will directly affect the survival of her young.

The biggest, oldest, most established females

are the most dominant and take a bigger share of the kill.

So food and the need to fight for it

has made females look and behave like aggressive males.

But it has also had a strange side effect.

Female hyenas have large amounts of the male hormone testosterone

and, consequently, develop male-like reproductive organs.

This can be a problem.

Having a long, thin birth canal

makes mating very difficult,

and both mothers and cubs sometimes die during birth.

The female's strange gender swap

is one of the most unusual in the animal kingdom.

And new science has now made sense of the old clues

and solved this mystery.

Hyenas are very intriguing animals.

William Buckland's early observations of their bones

in his hyena experiments

started a study of these creatures

that was to reveal their fascinating biology.

Hyenas may have a frightening reputation,

but their odd characteristics all have a reason.

The story of their aggression and bizarre bodies

is intimately tied up with their food and the survival of their cubs.

They've evolved a perfectly formed bone-breaking jaw,

but the time it takes to grow

has resulted in one of the most unusual but dedicated mothers

in the animal kingdom.

So, to become the best parents,

female hyenas have become more male,

and male seahorses more motherly.

Birds build a variety of nests,

each with a design that is characteristic of their species.

The simplest nests are just sticks wedged into position,

but some are more complicated.

The long-tailed tit builds a delicate nest

from plant material and spider silk.

And weaverbirds do, literally, weave with leaves.

But are such skills learned or instinctive?

In 1905, Eugene Marais,

a South African writer and scientist,

was intrigued by the complexity of weaverbird nests.

He wanted to understand more about their nest building skills

and performed a rigorous, but simple, experiment

to see if they learnt how to make nests

or if they built them using what he called "cultural instinct".

He took eggs from a pair of wild weaverbirds

and put them into a canary's nest to hatch.

Then he encouraged the next three generations of weaverbirds to breed,

but gave them no nest material

and hatched their eggs, once again, under canaries.

When nesting time came for the fourth generation of weaverbirds,

he gave them natural nest materials

and, without hesitation,

they vigorously set about constructing perfect wild nests.

So nest-building is largely under genetic control,

but it is influenced by experience and the environment.

Nests of the same kind of weaverbird are not always exactly the same,

and the birds of necessity must have some flexibility in how they build.

Nests that hang are particularly difficult to make,

as the birds have to work against gravity with no support from below.

Weaverbirds solve part of this problem

with a skill none others have.

They're the only birds that can tie knots.

These knots vary and are worked on until the weaver succeeds

in attaching several strands of grass to a suitable branch or stem.

These first fastenings are crucial,

as the whole of the completed nest will hang from them.

Once the birds have secured the foundation,

they can start to weave.

Weaving is just one way of binding leaves together.

There are others.

These are tailorbird nests.

They consist of folded leaves stuffed with soft material

and stitched together using spider's silk.

The tailorbird pierces the leaves with its sharp beak

and then binds them together by pulling silk through the holes.

The complete operation involves a number of different skills.

Making the holes is like riveting.

Two leaves are placed together

and then pierced to create matching holes above and below.

Then the edges are sewn up.

The upper surface of the leaf is kept to the outside

to help the nest look unobtrusive.

The result is a secure pocket, which is then stuffed with a soft lining.

The materials the birds choose to sew up their nest can vary.

At the turn of the century,

there was a report in The Common Birds Of Bombay

of weaverbirds watching carpet makers and tailors

as they worked on verandas.

When the coast was clear,

the birds flew down and stole tiny pieces of thread

with which to sew up their nests.

Birds search with a clear idea

of what will be suitable nest material.

Many use sticks and twigs.

They will, however, occasionally use other material

that does the same job.

And their choices are sometimes surprising.

This nest was found in an aircraft hangar in the 1950s

and it's made entirely of twisted wire.

When it was discovered, it contained two blackbird eggs.

It's an unusual nest for a blackbird,

but similar nests have been found belonging to crows and pigeons.

Weaverbirds work with natural material

and, like the tailorbird,

they have to solve the problem of joining leaves together.

After making a knot to secure the basic framework,

they begin their weaving.

They construct the main egg chamber

and then add a small entrance

around the first securely knotted ring of leaves.

The male, as he works, is under intense scrutiny.

Females are looking for mates,

and males that build firm, well-positioned nests

are favoured as fathers.

When he finishes, a male advertises his handiwork by fluttering.

But he may be forced to build several nests

before a female finally chooses him as a partner.

Weaverbirds' nests are very conspicuous.

Other birds, however, go to some trouble to conceal them.

We may not have tailorbirds or weaverbirds in Britain,

but we do have long-tailed tits.

Delicate little birds

that make intricate and finely constructed nests.

With tiny, repetitive movements,

they use loops of spider's silk

to fell together their mixture of wool and moss.

Both male and female work on the construction.

As the nest takes shape,

they decorate the outside

with several thousand tiny flakes of lichen.

The nest is then lined with hundreds of feathers

and provides a delicate but strong structure

to house the growing chicks.

And it's a nest that's particularly hard to find

because of its covering of lichen.

For years, it was believed that this acted as a sort of camouflage

to help hide the nest.

But the recent discovery of long-tailed tit nests

covered with small flakes of paper and polystyrene

have helped explain more clearly the reason for this decoration.

Rather than helping to blend the nest with its background,

these small flakes reflect light from it, making it almost invisible.

And it seems paper and polystyrene do the job just as well as lichen.

The largest and, perhaps, the most long-lasting nest of all

is made by the social weaverbird.

They live in the dry areas of southern Africa

and work together

to build what looks like a great haystack up in a tree.

New nest chambers are continually added,

as many as 100 pairs of birds may live together

under the one roof, as you might say.

The chambers provide shade during the day

and keep out the chill at night.

And the whole construction is so robust

that it may provide mass housing

for generation after generation of birds.

Recently, the biggest nest ever recorded was discovered

attached to telegraph poles in the Kalahari Desert.

It's more than seven metres across and three metres high.

So weaverbirds make their nests in many different ways

and it was once thought that they worked entirely by instinct,

but this is not so.

They are amongst the most expert nest-builders in the animal kingdom,

and this array of nests

shows the complex and elaborate designs

that they can produce.

Recent studies suggest

that weaverbirds may be using mental skills

that are not dissimilar to those required to make simple tools.

For weaverbirds, a well-built nest is a ticket to successful breeding.

Who would imagine that such complexity could be produced

using just a foot and a beak.

Weaverbirds make their elaborate nests

from simple materials they find around them.

Another of nature's extraordinary builders are the spiders.

They make their complex webs

from an incredible substance they produce themselves, silk.

Spider silk is unique.

It's very thin, very strong,

and has many exciting potential uses.

Spiders spin it with ease,

but scientists have been trying to copy it for many years.

To do that, we need to understand two of the spider's secrets -

the exact structure and nature of their silk,

and the way they transform it from a fluid into a thread.

Spider silk is a truly remarkable material.

It can withstand impact

and it can be strong, stretchy and sticky all at the same time.

Spiders produce it from special glands inside their bodies

and extrude it from tiny nipples called spinnerets

at the back end of their abdomens.

And what is more, they can produce up to seven different kinds,

each with its own purpose.

For centuries, it was the only silk known to man.

The Ancient Greeks used cobwebs to stop bleeding

and Australian Aborigines used it to catch small fish.

Then, in the Far East,

a different and mysterious new kind of silk started to appear,

and in much larger quantities.

According to Chinese legend,

the first person to weave silk into a fabric

was the Empress Leizu, back in the 27th century BC.

She was having tea in her garden under a mulberry tree,

when a cocoon fell from the branch above

and dropped into her cup

and started to unravel.

Whether that's true or not,

the Empress Leizu is now honoured as the goddess of silk.

And silk-moth farming dates back

to the beginning of Chinese civilisation.

The silk was traded right across the Near East and into the Roman Empire.

The Chinese traders were sworn to secrecy

about how this marvellous material was made.

But in the year 532,

the Roman emperor Justinian managed to find out

that it came not, as some suspected, from a spider's web,

but from the cocoon of a moth.

Silk moth caterpillars produce large quantities of silk

and they make it in a very different way to spiders.

The caterpillars feed voraciously on mulberry leaves,

and then, when they're full-grown and ready to transform into a moth,

they spin silken cocoons in which they will pupate.

Unlike spiders, which have specialised spinning organs,

silk moth caterpillars produce silk from their salivary glands.

Each cocoon is made from a single, unbroken filament,

that can be over 500 metres long.

This silk is plentiful and easy to spin commercially,

but it isn't as tough as spider silk.

And spider silk also has more exciting potential uses.

An orb web like this

is constructed over a Y-shaped scaffold of silk threads,

which are extremely strong.

Unlike silkworms, the female spiders, which spin the webs,

are very territorial and aggressive.

So farming and collecting spider silk is very difficult,

but it has been done.

In 1762, a Spanish missionary called Termeyer

made a machine that held a single spider,

from which he pulled a silken thread.

In London, Daniel Rolt, a factory worker,

attached spiders to a small steam machine

and succeeded in reeling out 18 metres of silk a minute.

That led to machines that were able to milk several spiders at a time.

Experiments then stopped, until 2004,

when two textile artists in Madagascar

built a machine based on these early designs,

with which they made something very special indeed.

The golden colour of this stunningly beautiful spider silk shawl

is completely natural.

The silk from which it was made

was produced by 1,063,000 spiders,

like this one, over four years.

Local people collected 3,000 spiders a day

and trained handlers extracted silk

from groups of 24 at a time.

After being milked, the spiders were released back into the wild.

The individual silk strands were then twisted into a thread

which was woven into this intricately patterned fabric on looms.

Now, this kind of silk fabric production

couldn't work commercially.

Apart from being hard work to make in quantity,

spider silk isn't really a very suitable thread for fabric.

As a cloth it reacts badly to moisture and heat,

but in its natural state, as a single thread,

it has physical qualities

that could be exploited medically.

These special characteristics

are a consequence of the molecular structure of spider silk.

It consists of two large protein molecules.

One is stretchy and spaghetti-like,

and the other has a harder, crystalline structure.

Combined, these two proteins

give silk unique qualities of strength and flexibility.

Spiders store these proteins as a gel-like liquid in their bodies.

And when they need to make silk,

they extrude it through the spinnerets,

combining the molecules in a special way.

If we hold down a spider without harming it

we can see this process in more detail.

Normally, the spider would attach the end of the silk filament

to an object and then move away,

so that the filament is pulled from the spinnerets.

We can produce the same reaction,

by gently pulling the end of the filament itself.

Internally, the silk liquid is passing down a long duct

in which stretchy elements within the protein molecules

are lined with harder crystalline ones,

to create an extremely strong and tough thread.

Scanning electron microscopes

reveal how the liquid emerges from the spinnerets.

Incredibly, spiders can convert liquid proteins

into a hardened thread at room temperature

with very little energy.

If we could understand and copy this process,

it would be a major scientific breakthrough.

Scientists have, in fact, spent many years

trying to replicate the spider's liquid silk and the way it's spun.

Recently, the genes of spider-silk proteins were cloned

and put into goats

to try and produce silk in their milk.

It worked, and when the goats had kids

silk proteins were extracted from the mother's milk.

But none of these processes

have yet produced silk that is as tough as natural spider silk.

This machine is called a tensile tester

and it shows how strong and stretchy spider silk can be.

This dragline silk is being pulled apart,

and a graph shows the force the fibre is taking

and at what point it breaks.

A steel thread of similar diameter

would have broken by now.

There, it's broken.

Spider silk is the toughest natural material known to man.

A single thread of web silk,

less than a millimetre thick,

can absorb the impact of fast-moving prey

and bring it to a halt without breaking.

Complete webs can stretch enormously

and then return to their original shape

with a minimum of damage.

Incredibly, spiders can make this complex material

from just fresh air, flies and water.

The best we can do in making a material like it

requires oil, chemicals and a great deal of energy.

Although we now better understand the structure of spider silk

and the natural spinning process,

we still can't perform the spider's magic

and copy this extraordinary substance.

But using small amounts of natural spider silk in clever ways

has, nonetheless, a very exciting future.

A sumptuous golden cloth

is just one possible product.

This is a dream that has become a reality,

and shows just how lovely spider silk can be.

But it also has the potential

to make other dreams come true.

It's a biodegradable material

that we're now using to make artificial joints,

and it may even help repair damaged spinal tissue.

This curiosity of nature could, eventually, save lives.