Spinners and Weavers 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.

Spiders spin intricate webs using their own silk

and birds weave nests from strips of leaves.

In this programme, I investigate the skill of these spinners and weavers

and the way they use such materials

to produce such truly complex structures.

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 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 silk

to felt together a 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.