Episode 10 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 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.

The abilities of some plants and animals are so remarkable

that they seem to be almost supernatural.

In this programme, I investigate the shocking power of a fish that

advanced our understanding of electricity,

and plants with senses that are surprising modern science.

But how do these extraordinary powers help the organisms that

produce them?

The freshwater eel is surrounded by legends.

The first Europeans to explore the New World heard amazing

stories about it.

And when, in the 18th century, specimens of this strange fish

reached Europe, they created a sensation.

In 1776, Captain George Baker,

an American mariner and whaler,

made the long and difficult journey from South America across a

raging Atlantic Ocean to bring five live electric eels to London.

These are two of his actual eels.

Captain Baker and his five electric eels,

or gymnotas as they were known,

set up shop in the Haymarket and offered two shillings

and sixpence for a shock, or five shillings for a spark.

Baker's eels had come all the way from the lower

reaches of the Amazon and Orinoco rivers,

where he had heard tales from the locals about

their astonishing powers.

They called these fish "trembladores".

Humboldt, the famous naturalist and explorer, had described how he

had witnessed horses being killed by the repeated shocks from these fish.

And he himself accidentally stepped on one

and vividly described the effect.

"With each stroke, you feel an internal vibration that lasts

"two or three seconds, followed by a painful numbness.

"All day I felt strong pain in my knees and in all my joints."

I encountered this remarkable fish in its natural environment

when I filmed at the same rivers that Humboldt explored.

There was talk of me swimming with the eel,

but thankfully we had some technical difficulties with the diving

equipment that I was supposed to wear,

and so I stayed safely in a canoe and was able to demonstrate

another subtler, but equally remarkable, side to this fish.

The eels were constantly producing electric discharges.

Somehow they were generating a small, nonstop flowing current.

ELECTRIC DRONE

They were also able to sense electricity and were

attracted to electrical pulses emitted from my underwater detector,

suggesting that electricity plays a key role in their lives.

But at the time of their discovery,

no-one knew the full functions of their extraordinary abilities.

We now know that the shock was caused by electricity,

and I can demonstrate it by touching the animal with an electrode.

Watch.

There. You see?

The scope and the lights are flashing up and down.

Extraordinary.

But this is only a small indication of the real power of this fish.

If I were to try and pick it up,

I could get a jolt of an astonishing 600 volts,

which is quite enough to kill me.

This 1960s educational film illustrated the shock,

even though the equipment used prevented

the volunteers from getting its full power.

They were to join hands and then connected to a live eel.

WOMAN SCREAMS

Firm believers in electric eels. Thank you very much.

You can imagine how startling Baker's electric eels

were 200 years ago.

In the 18th century,

electricity was becoming one of the most fashionable areas

of scientific investigation, but it was still very poorly understood.

Very few advances had been made since its discovery 150 years

earlier by Elizabeth I's personal physician, William Gilbert.

Gilbert repeated a trick that had been known about since Greek times.

Rubbing a piece of amber with cat fur, that allowed the amber

to attract a small object like a feather. Let's give it a try.

Here is a bit of amber.

There.

It had always been assumed that this amber effect was caused

by magnetism but Gilbert showed that it was something different.

He named this new force after the Greek word for amber,

electron, and so electricity was born.

Londoners of the time developed a fascination for this magical force.

Showmen staged bizarre spectacles to demonstrate its properties.

In one, a young boy attached to a friction generator

attracted small pieces of paper to his hands.

In another, a gentleman kissed a lady and was repulsed

by the charge carried through her whalebone corset.

No-one knew what to do with electricity

but a better understanding of its nature was slowly emerging.

More and more ingenious ways were developed

to create what we now call static electricity.

And soon it became something more than just a quirk of rubbing amber,

it became visible as a spark.

The ability to produce this characteristic blue spark

along with its invigorating smell became the signature

of this new force and it prompted scientists to make

obvious comparisons with other natural phenomena.

THUNDER RUMBLES

In the American colonies, Benjamin Franklin bravely,

or perhaps foolishly, flew kites into thunderstorms and proved that

lightning and the electric spark were one and the same.

But there's another common property of lightning and static electricity.

That is the ability to shock.

It wasn't long before a comparison was made between the shock from

the early generators and the shock that could be delivered by a fish.

The electric eel wasn't the only kind of fish

known to give humans a powerful jolt.

The ancient Egyptians knew that the electric catfish could also

give shocks and they called it the "Thunderer of the Nile".

And in the nearby Mediterranean lives the torpedo ray.

Its muscle batteries make it so bulky

it can't undulate its body like other rays

but has to propel itself by waving its tail.

Like the electric eel,

it uses its discharge to stun the other fish on which it preys.

Sadly, the pressure of celebrity and having to produce shocks

and sparks to order exhausted Baker's long-suffering eels

and they didn't last the winter.

But two were preserved and expertly dissected by John Hunter,

a very distinguished Scottish surgeon of the time

and he found a great number of striped muscular layers

that proved to be where the electricity was generated.

They are now referred to as Hunter's organs.

He found these muscles along the tail and sides of the eels

arranged in stacks.

One scientist called Galvani believed that animals

had their own natural electricity even without these electric organs

and he tried to prove this by connecting wires to frogs' legs

and making them twitch.

He called this phenomenon animal electricity.

But another scientist called Volta had other ideas.

He proved that the frog was merely a conductor for electricity

with a simple experiment.

Volta replaced Galvani's frog with discs of cloth

soaked in saltwater or acid

and sandwiched them between two different metals.

I can do the same thing with filter paper,

copper two-pence pieces and these simple galvanised zinc washers.

Watch.

Tuppenny piece.

Filter.

And washer.

There, nearly 0.6 of a volt.

But the amount of electricity generated was tiny.

Certainly not enough to make the sparks seen from eels.

Unlike Galvani, Volta saw no distinction

between animal electricity and his new electricity from metals

so he now looked at animals to see how he might amplify his new device.

Was it significant that the muscles

producing the electric power in the eels were arranged in stacks?

Volta decided to add more stacks to his electric pile.

We call this way of connecting electric cells together

"in series", and we now know that it increases the voltage.

But Volta was about to find this out for the first time.

He piled up his tiny cells like the bands of muscle

in an electric fish.

Here I've got ten pairs.

And just watch.

Nearly six volts.

Wonderful.

Volta could now produce heat, shocks and even sparks

from electricity in a continuous never-ending stream.

He had made the first battery, partly inspired by the electric eel.

The pieces of the puzzle had come together and the eel's example

had helped to advance our understanding of electricity.

Eels, in fact, contain natural batteries -

stacks of special muscles.

It's amazing to think when electricity is so much

a part of our lives today that before Volta

the only source of electricity was lightning,

a few static generators

and fish like this incredible electric eel.

Understanding how electric eels managed to find their way around

revealed a hitherto unknown animal sense.

But it's not just animals that have surprised us.

We're now discovering that plants too

have intriguing abilities that are still mysterious.

We think of plants as passive, still and silent,

but they may have more in common with animals than you might think.

New research suggests that they have surprising abilities.

It depends on how you look at them.

I first started seeing plants in a different light

when making a series called The Private Life of Plants.

We used time-lapse photography to reveal the way they move.

The bramble spreads aggressively - seemingly unstoppable.

Other plants pulse to the rhythms of day and night,

and flower buds explode like fireworks.

So, with speeded-up film, we've been able to translate

their time into ours

and to realise that they're constantly on the move.

200 years ago, one plant that moved very quickly indeed

attracted the attention of a great scientific mind.

It appeared to behave like an animal

and could move fast enough to catch its own food.

Charles Darwin was fascinated by the Venus flytrap.

He called it one of the most wonderful plants in the world.

He recognised that it could move in a very different way

to that of plant growth.

This movement was not only fast but also repeatable.

Darwin experimented and found that the traps

are not triggered by raindrops

but only by a very particular stimulation of the leaf hairs,

such as an insect might make.

But what intrigued him most was the speed of the reaction.

He sent one of these flytraps to a friend, Dr Burdon-Sanderson,

who was performing ground-breaking work on muscles and electricity.

His tests confirmed that the tiny electrical discharge

caused by an animal muscle cell contracting was almost identical

to those signals obtained by attaching electrodes to the flytrap

when it was shutting.

Although plants have no muscles,

electrical stimulation enables them to move in a similar way to animals.

Electrical signals cause cells to change the pressure of sap

in their leaves, so creating movement.

As a result, some plants, like animals,

can actively catch their prey.

Recently it's been discovered that other plants use electricity too

but for a very different purpose.

Plants are rooted to the ground and have a small negative charge.

The higher up the plant you go, the greater the electric charge.

This creates an electric field around the flower.

We can't see it but these electrodes are picking up the energy

of this tiny field and converting it into the sound that we can hear.

Bees, on the other hand, have a positive charge.

Friction whilst flying causes them

to lose electrons, leaving them electrically charged.

As a bee approaches a flower, the charge fields around the flower

and the bee interact, and the sound changes...

FALTERING ELECTRONIC BUZZ ..there.

And when it lands, the positive

and negative fields immediately cancel each other out.

As this happens, there are two very surprising consequences.

Firstly, the plant's negatively charged pollen actually

jumps across onto the positively charged bee.

Secondly, the plant has a changed electrical field

and when another bee comes along, it detects this altered

electrical signature and avoids the flower.

The plant is, in effect, telling the bee that it has no nectar

and to come back later.

When the flower has refilled its stores of nectar, it creates

a new electric charge which attracts another passing bee.

This simple on/off signal benefits both the bee and the flower,

but it does have its limitations.

The electrical field is tiny,

so insects can only detect it at close quarters.

But flowers can also draw attention to themselves over much

greater distances and they do this by floating messages in the air.

The perfume of a flower is not just a pleasant smell,

it's also the primary way in which plants communicate with insects.

A rose can contain over 400 chemical compounds and a bee

can recognise a particular combination from over a mile away.

The very latest research has discovered

that 90% of the chemicals made by plants, are also

produced by insects and that is no coincidence.

Most flowers produce scent to persuade insects to visit them,

but others use it in a more sophisticated way...

for protection.

Cabbages communicate with each other using smell.

When the leaves of one plant are being attacked by caterpillars,

it releases a scent which warns its neighbours.

They then produce chemicals in their leaves that caterpillars

don't like and so they avoid being eaten.

And scent also serves to call in the cavalry.

Leaves that are under attack give off a chemical alarm signal that

attracts wasps which obligingly pick off the caterpillar attackers.

So, vegetables, fruits, leaves and flowers are constantly

communicating with each other using touch, vision and smell.

They seem to exploit all the senses, apart, that is, from hearing.

But there are old stories that one particular plant is able to

produce a very strange sound.

Hundreds of years ago, a plant with a root that was thought to

resemble a human body was said to emit a sound that could kill.

The root was known to have strong anaesthetic

and hallucinogenic properties. And in the first century AD,

it was called a mandragora or mandrake as it's now known.

It was associated with magic and the supernatural

and was thought to derive power from a demon that emitted a dreadful

and fatal shriek if the plant was uprooted.

Fortunately, there were creative ways of avoiding

death from the killer sound.

One account advised plugging one's ears

and then tying a starving dog to the mandrake plant.

And then, as the dog lunged for food, the plant would be uprooted.

The dog would tragically die from the mandrake's shriek

but the man would survive.

This particular story may have arisen because drinks made with

the mandrake root can produce hallucinations.

But we're just beginning to realise that the sensory abilities

of a root could be as sophisticated as the rest of the plant.

Latest research suggests that roots are communicating underground.

And we now have the technology to eavesdrop on the roots' world.

Believe it or not, the roots of these corn seedlings can make

and sense sound.

The noise is very quiet but we can hear it with this equipment,

if I place a corn seedling in front of a laser beam Like this.

Now the sound vibration can be detected

and we can hear it through a speaker...

CRACKLING

..there.

That strange crackling is the sound of corn roots growing.

It can be seen as pulses on the screen.

It's been shown, too, that the corn roots respond to the sound

when it's played back to them.

Time-lapse footage shot over just a few hours clearly shows

the roots growing towards the tiny speakers that emit the sound.

There is much speculation

about the purpose of this curious phenomenon.

Perhaps it helps roots avoid growing into hard objects or being too

close to competing plants.

It could act like simple echolocation,

we just don't know, but it's the first clear evidence that

plants have a rudimentary form of hearing

and might even be communicating underground using sound.

Sensitive equipment is creating a new window into the plant world

and it seems that, like animals, they have a sophisticated

sense of their environment and possess abilities that

not so long ago, we would have thought of as supernatural.