Attenbobough's Life That Glows

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As dusk gives way to twilight,

the encroaching darkness is lit by life.

These dancing lights around me are produced by fireflies -

creatures that have the strange ability to produce light.

They bioluminesce.

And fireflies are not alone.

Scientists are finding ever more strange and wonderful

glowing life forms all around the world.

Living light has always fascinated me.

And the discovery of more and more luminous creatures raises more

and more questions.

Why? What is the light for? And how is it made?

In recent years,

scientists have begun to find answers to those questions.

And in doing so,

they've taken us into a world that is utterly unlike our own.

However astonishing these images look, they are all real.

With help from new cameras, one designed just for this film,

we can reveal this extraordinary phenomenon

as it has never been seen before.

Bioluminescence holds many mysteries.

But we do know that fireflies use it to attract the opposite sex.

Each species has its own flash code and WE can join in the conversation.

I'm going to use this rod to fish for fireflies.

It's the actual rod used by the scientist who was the first

to decipher the various call signs of fireflies.

And there are 15 different species, at least, around here.

Each with its own signal.

Biologist Jim Lloyd used the rod to imitate male fireflies

and so decode their various light patterns.

He discovered that the call sign consisted partly in

the actual flight path of the species concerned.

There are, for example,

some fireflies which move steadily horizontally, like that.

And there are others which

turn their light on as they climb, like that.

But in addition to the flight path, they flash a particular signal.

It's rather like Morse code.

So I should be able to use this light myself.

There is a female amongst these leaves here,

which will emit a single flash.

And the male of her species waits for precisely four seconds,

and then answers back with a flash.

Whereupon she immediately gives another flash, like that.

And the male then knows that he is going to be a welcome visitor.

But the message has recently been shown to be more than

a simple signal for sex.

A female judges the quality of a male's genes

by the precision of his timing and the brightness of his light.

She encourages her chosen suitor by directing her lanterns towards him.

And it seems this male sent out all the right signals.

We are now discovering that

this language of light even has local dialects.

Throughout the summer months, from Florida to southern Canada,

gardens, fields and forests sparkle with these mating messages.

Time-lapse photography reveals

the extraordinary extent of this courtship.

Some species flash only at dusk.

Others prefer the forest canopy for their light show.

Some species make their flashes more conspicuous by choosing

the very darkest places in which to display.

I can see virtually nothing here, except the flashes.

And this particular species has another trick, too.

It synchronises the displays.

Individuals flash together.

Each individual is triggered by its neighbour,

and soon waves of light pulse through the woods.

Speeded up, the wave becomes clearer.

Between the waves,

an impressed female can respond with two flashes of her own.

And the males home in on her.

But she can only choose one.

These displays peak for just a few nights in June,

which could explain why they were only recently discovered.

Why they all flash together is still a mystery.

It's surprising how little we know about bioluminescence.

Fireflies are perhaps the best understood

but some living light is still very perplexing indeed.

With dawn, the sexual signals of the fireflies are drowned

by the increasing flood of light.

The flies take refuge in the undergrowth,

away from the sharp-eyed predators of the day.

But right now,

light is being produced by life in the soil under my feet.

The threads of certain fungi form a glowing underground network.

But why would a fungus shine in the permanent darkness of the soil?

We simply don't know.

And for years, fungus bioluminescence,

like much other living light, was written off as a beautiful

by-product of evolution with no function.

But some species only glow above ground and only at night,

when their intense green light is very obvious.

If it was just a biochemical accident then surely

they would shine all the time.

The glow certainly attracts insects

and the theory is that these visitors spread the fungal spores.

So here, too, just as with fireflies,

we're learning new things all the time.

But much living light remains a beautiful enigma.

And throughout history,

stories of bioluminescence were often thought to be pure fiction.

In the 1870s, Jules Verne, the French science-fiction novelist,

wrote this in his book, 20,000 Leagues Under The Sea.

"At seven o'clock in the evening, our ship, half-immersed,

"was sailing in a sea of milk.

"At first sight, the ocean seemed lactified.

"The whole sky seemed black by contrast with

"the whiteness of the waters."

Jules Verne may have based this story

on a myth told to him by sailors.

But in 1995, the captain of a British vessel wrote

a real-life account in his ship's log.

"At 18:
00 hours on a clear moonless night,

"while 150 miles east of the Somalian coast,

"a whitish glow was observed on the horizon.

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