Nature (1982–…): Season 32, Episode 19 - Fabulous Frogs - full transcript

There are more than 5,000 species
of frogs and toads,

and they come in all colours,
shapes and sizes.

Over the years, I've filmed
many different kinds of them,

from across the world.

This is the largest frog
in the world, the goliath frog.

And here in the leaf litter
of this Madagascan forest,

is the tiniest of them all.

They can hop and climb.

They can parachute
from the tree tops.

And burrow deep into the ground.

Some, seemingly,
can even walk on water.



Frogs, like newts
and salamanders, are amphibians,

cold blooded animals
that need water to survive.

But despite this, they have
colonised some of the hottest

and the coldest places on Earth.

Frogs are truly fascinating.

You many not have thought much
about them

and they don't necessarily
grab the headlines,

but there's more to frogs
than you might suppose.

Frogs were the first creatures
that I kept when I was a boy,

and I thought they were fascinating
and beautiful,

and I still think they are.

They were, of course, the first
creatures to move up onto land.

Today's frogs are descended
from a group of amphibians that

lived around 300 million years ago,



and they were very like
the modern salamander.

That is to say, they had a very,
very long spine

with about 30 vertebrae in it,
ending in a tail.

But then about
250 million years ago,

an intermediate form appeared,

like this fossil from Madagascar,

and already you can see
a difference.

Most importantly, the spine,

instead of being 30 vertebrae long,
is only about 15.

The tail has almost been lost
altogether.

The hind legs are very much bigger.

Compare that with today's amphibian.

Most significantly,
its spine is now, again halved.

But the pelvis has become
greatly elongated.

But that's nothing compared
with what has happened to the legs.

They are gigantic.

So you can see that
this animal is a leaper.

And today, some exploit these
spectacular legs very dramatically.

This little frog is
an amazing jumper.

It can leap 30 times
its own body length,

and there are some
that can go even farther.

One can leap 55 times
its body length,

equivalent to me jumping the length
of a football pitch.

In the 1930s, in the United States,

frog leaping became
something of a craze.

People bet on how far
a frog could jump.

Even beauty queens took part
in frog-jumping contests,

each with her own pet.

In the past, people couldn't
understand how a frog could leap

so far, but a little scientific
research revealed how it does it.

When a frog is
in its sitting position,

its leg muscles are contracted.

This stretches the leg tendons.

Tendons are elastic
like rubber bands.

Stretching them
stores energy within them.

And then, when the frog jumps,
the tendons release that energy,

like firing a catapult.

And the frog is propelled
into the air.

When this skittering frog
from India jumps,

it simultaneously
extends its webbed toes

so that it virtually bounces
across the surface of the water.

The webs between the toes,
so useful for swimming,

can also help
when moving around on land.

The gliding leaf frog
has such extensive webbing that

when it jumps, the outstretched toes
of its feet act like parachutes

and keep it in the air long enough
to travel considerable distances.

When the first amphibians
emerged from the water, the only

animal sounds to be heard on land
were the whirrs and hums of insects.

CROAKING

But the amphibians also needed
to communicate with one another,

and soon they added their own croaks
and whistles.

CROAKING AND WHISTLING

Frogs blow air from their lungs,
through vocal chords

and so produce a croak.

But the muscles that do that are
comparatively weak, so many species

amplify the sound with resonators,
cheek pouches or throat pouches.

FROG CROAKS

One purpose for calling
is to find a mate.

A female gliding leaf frog
in the jungles of South America

is listening to all the males
calling around her.

Then she sets off in the direction
of the loudest voice,

because the owner is likely to be
the strongest male, the best mate.

It's a long haul.

On the way,
she has to avoid the weaker males.

It's not all that easy.

She has to fight off
several at a time.

Eventually,
the strongest male gets his reward.

It's a case of
he who shouts loudest.

This is called
the splendid leaf frog,

and quite right, too.

Most frogs communicate
with their voice,

by croaking or indeed squeaking.

And this one does, too,

but it has rather a quiet voice

and it also communicates
by using its legs.

What he'll do is to use them
to wave to other frogs.

And its legs are specially
adapted for the purpose.

They have flaps on them

so they appear to be specially wide
and prominent.

This waving technique is something
I once filmed with another

beautiful frog.

We found it several years ago
in the rain forests of Panama.

This is a golden Panama male,
and he is looking for a mate.

But a rival stands in his way.

Just in case his call can't be
heard above the sound of water,

he reinforces his message
with a visual signal, a wave.

His rival waves back.

He repeats his signal
so there's no misunderstanding.

But now another male arrives.

He too is looking for a female,

and he isn't going to let anyone
stand in his way.

This kind of argument has to be
settled with a wrestling match.

That should teach him!

The loser submits,
lowering his head.

And the winner
continues his hunt for a mate.

Finally, a large golden female
arrives. Just the ticket!

HE CROAKS

He waves to show he's interested.

She waves back.

It's a success.
His waving courtship has worked.

Calling and waving are comparatively
subtle ways of attracting a female.

Other frogs use
a more macho approach.

Rains, on the African savannah.

The African male bullfrog,
one of the biggest of all frogs.

He fights for the right to mate.

The males assemble
in a newly-filled pond

and battle with each other to
establish who is the strongest.

The winner will mate
with most of the females here.

No such luck for the loser.

Courtship techniques vary widely,
but for one remarkable little

brown frog in Madagascar, the key
to mating rituals is skin colour.

Following a bout of heavy rain,
the frogs all gather at a waterhole.

The females are still brown, but the
males have turned a bright yellow.

This enables males
and females to tell each other

apart in the multiple mating
that is about to happen.

They have to judge the moment
very precisely.

There must be enough water
in which to lay their eggs,

but not so much that the
eggs are washed away.

CROAKING

After a few hours, the orgy is over,

and the males turn back
to brown again.

It will happen all over again
next year,

when the rains return
and more eggs are laid.

When speeded up, you can watch

the extraordinary way
in which eggs develop.

These eggs were laid by a leaf frog.

And inside the jelly of each one,
you can see a little tiny dot.

And if you look closely,
you may even see it move.

And over the next five
or seven days,

they will continue to develop
until they are strong enough

to emerge from the egg
and drop into water beneath.

And then over the next four
or five weeks,

an enormous transformation
takes place.

The gills, which enabled the tadpole
to breathe in water,

gradually disappear.

The tadpole's intestines,
accustomed to a vegetarian diet,

have to be completely remodelled to
allow them to digest animal tissues.

Its skull, which was made
of cartilage, turns to bone

and the backbone grows.

The tadpole's legs get larger as the
tail shrinks, absorbed by the body.

It's an astonishing

and radical transformation,
which takes around six weeks.

A pond in Madagascar.

Not a good place for eggs

because it's full of fish that would
eat frogs' eggs given the chance.

So this little frog lays her eggs
on the leaves of trees

that overhang the pond.

The tadpoles are maturing quickly,
but there is danger here, too.

A predatory wasp has found
the clumps of spawn.

It manages to slice its way
through the protective jelly.

And now it starts to chew up
the tadpoles.

But these tadpoles,

although only five days old,
can react to such attacks.

The vibrations, created by the wasp,
stimulate them to hatch prematurely.

The jelly liquefies and the tadpoles
travel down the leaf

and drop into the water below.

They may be underdeveloped,
but they can swim

and they stand a better chance
of survival in the pond than

they did with
the wasp on their leaf.

These youngsters had to survive
without parental care, but other

frog parents go to great lengths
to look after their offspring.

This little frog is tiny,
hardly bigger than my thumb nail.

It is a strawberry poison-dart frog.

It may be very small,

but when it comes to caring
for its young, it's a real champion.

It lives in the wet lowland
forests of Central America.

This female is guarding
a clump of newly-fertilised eggs.

She and her mate will keep watch,

making sure that their offspring
are safe from predators.

But the leaf litter is drying out
and the tadpoles need water.

She must move them, and fast.

So she encourages one of them
to jump on to her back.

But it's not a pond on the forest
floor that she's looking for now.

She wants a bromeliad, a vase plant,
and they grow up in the branches.

Climbing a tree as tall as this
is an immense journey for such

a tiny creature.

A bromeliad plant
has a tiny permanent

pool of water at its centre,

an excellent nursery for a tadpole.

And there she delivers it.

But she has several tadpoles.

So now she must rush back to
the others down on the ground.

She collects them one by one and
carries each to its own bromeliad.

She continues to make
her long, arduous journeys,

while the male guards the remaining
eggs on the forest floor.

But the little bromeliad pools
don't have any food in them.

So she lays an unfertilised egg
in each one for each tadpole to eat.

A single egg won't sustain
a developing tadpole for very long.

So she has to return
to each pool every few days

and deliver another food parcel.

This continues
for the next two weeks,

during which time, she will have
travelled over half a mile.

An astonishing distance
for such a tiny creature.

Eventually, the tadpole develops
into a froglet large enough

to leave the pool
and fend for itself.

Mum has done her job well.

But it isn't always the female
who takes on the task of rearing

the young.

Sometimes, the male does.

And one of those lives
in the mountains of northern Spain.

This little creature is called a
toad because its skin is rather dry.

But the names frog and toad
are largely interchangeable,

for all amphibians with this shape
are very closely related.

This one is known
as the midwife toad.

When they mate, the male clasps
the female with his arms

and squeezes
until she releases her eggs.

They emerge in a long chain
of jelly,

and as they do so,
he fertilises them.

He then hitches them up
around his hind legs

and then the pair
will then separate.

She leaves
and he carries them around

until the time comes
to deposit them in water.

The midwife toad is not the only
species in which the male

takes care of the young.

The African bullfrog
is quite a character.

It's the biggest frog in Africa
and it's very aggressive.

It's got a very powerful bite,
for one thing.

But in spite of that,
it's a devoted father.

Bullfrogs spawn in little pools
around the margins of a larger pond.

And after mating is over, one male
stays to watch over them all.

But the water is evaporating
and the tadpoles are now crowded

together in a single pool,
and that too is now drying up.

The tadpoles will be dead
within an hour,

unless the male can do
something to save them.

And he starts doing just that.

He begins to dig a channel
to connect the tadpoles'

little pool to the main pond.

He must be quick.

It's a major task,
but he is determined.

Down they swim.

The tadpoles are saved!

But perhaps the prize for fatherly
care should go to a rare

little frog that lives
in the forests of Chile.

This is Darwin's frog.

After the female has laid
her eggs on the moist ground

and the male has fertilised them,
he, apparently, eats them.

But they go, not into his stomach
but into his throat pouch.

And there they develop, and wriggle.

And when they're ready...

..one jumps out.

And another.

Giving birth from your mouth
is pretty odd,

but what about producing babies
from your back?

The Surinam toad is an aquatic
species from South America.

The female produces
around 100 eggs at a time.

The male collects them and steers
them on to the female's back.

And there they stick.

The skin on the female's back
then begins to swell around them.

A membrane grows over them and
eventually completely encloses them.

After a couple of days,
they've virtually disappeared.

A few weeks later,
the young hatch as tadpoles.

In some Surinam toads, the young
remain within their mothers'

back so long that they emerge
not as tadpoles but little froglets.

It's certainly an odd way
of producing young, but it works.

All adult amphibians are hunters.

To help them catch their prey,
they have a secret weapon -

a special kind of tongue.

The extendable tongue is
an amphibian invention.

No fish ever had one.

And the tongue is not attached
to the back of the mouth like ours,

but to the front.

Frogs eat worms and insects.

And the larger the frog,
the bigger prey it will tackle.

And some species,
like these cane toads,

will eat almost anything,
including one another.

It's recently been discovered
that way back in prehistory,

there were some frogs
big enough to catch mammals.

A few years ago,
fragments were found in Madagascar

of a really giant frog dating back
from about 65 million years ago.

This is part of its skull.

Here is the orbit of the eye.

And this is where its spine...

The spine would have run down here.

And this is the right cheek.

So the animal's head, when complete,
would have been about that wide.

So this really was a monster.

And here is a computer
reconstruction

of the complete skeleton.

The scientists who worked on it

referred to it among themselves
as a devil frog.

And when the time came
to give it a scientific name,

they called it Beelzebufo.

65 million years ago
was just towards

the end of the reign
of the dinosaurs.

So maybe this giant frog actually
ate hatchling dinosaurs,

as been shown in this artist's
imaginative reconstruction.

Some frogs today also have teeth,
but they don't chew with them.

The teeth are used
either for defence

or as a way of gripping prey.

This large monkey frog is in the
process of swallowing a cricket.

And, strangely perhaps,
it uses its eyes to help it do so.

We're waiting for it to burp.

LAUGHTER

As a frog swallows,
it pulls its bulging eyes downwards

so that they help to push
the food down its throat.

Most frogs have very big eyes.

They have to be big because frogs,
since they don't have necks,

can't turn their heads
to look to one side.

Instead, their two eyes,
between them,

give a frog
an almost 360-degree vision.

They also have a kind
of see-through third eye-lid,

which protects their eyes underwater
without blocking their sight.

The red-eyed tree frog's
third eyelid has a distinctive

green pattern, which, when out
of water, helps with camouflage.

Frogs have managed to adapt to
a surprising range of environments.

The little red-eyed tree frog
lives up in the forest canopy,

catching insects.

And to do that, of course,
it has to be an expert climber.

So, not surprisingly, they have
very special hands and feet.

Its toe-pads are highly
complex structures.

Each has a large surface area,
which helps it get a good grip.

Between the six-sided skin cells,

there are small channels
which fill with a sticky mucus.

This acts as a continuously renewed
glue, but one that allows

the frog to peel off its foot
and re-attach it as it climbs.

Some frogs have evolved
a way of using their hands

and feet in a quite remarkable way.

In the remote rain forests
of South America,

there's a small amphibian
known as a waterfall toad.

It, like so many frogs,
is a favourite meal of snakes.

Many frogs avoid
their enemies by hopping,

but this little toad can't hop
more than an inch or so. Instead,

when a swift retreat is needed,
it has other techniques.

Free-falling, which is easy,

and stopping,
which is more difficult.

Above the rain forests, there
are mountains so drenched with rain

that the rocks are bare, except
for a coat of slimy black algae.

And here you can find
the pebble toad.

But there are predators here, too.

This is a toad-eating tarantula,
an expert in ambushing its prey.

The pebble toad can't hop
very far either,

but it has a different defence.

It clenches its muscles so tightly

that it becomes as bouncy
as a rubber ball.

Danger averted and no damage done.

But hunters are everywhere - not
only on the ground, but in the sky.

Camouflage is an excellent defence.

Blink and you could easily miss
this Darwin's frog.

Most frogs are beautifully
camouflaged

so that it's very difficult
sometimes to spot them.

But this one,

which is the tiger-striped
monkey frog from South America...

..has tiger stripes -
orange and black -

on the inside of its legs.

So when it's sitting like that,
it looks green.

But if they're threatened
by a predator,

they can suddenly open their legs
and reveal

that orange and black underbelly,

which, people think,
puts off a predator.

The great majority of frogs
rely on camouflage.

But a few take the other option -
conspicuousness.

Bright colouring can be a warning
that an animal is

unpleasant to eat. But some are
more than merely unpalatable.

This is the golden poison-dart frog
from Colombia.

And its skin contains enough poison
to kill ten human beings,

which is why I'm taking no chances
and wearing gloves.

Traditionally, the people in
Colombia used that poison

to tip their blow-pipe darts.

But of course, for the frog,

the poison serves as a defence
against predators.

And many poison-dart frogs are
very brightly coloured, just to

warn predators of what would await
them if they did take a mouthful.

The toxicity of frog skin
has traditionally been exploited

by local people, but modern medicine
has also found ways of using it.

One chemical compound from the skin

of the tri-coloured
poison-dart frog...

..is being used in the development
of a groundbreaking pain-killer...

..several hundred times
more powerful than morphine.

A frog doesn't drink,

it absorbs all the water
it needs through its skin.

It also gets most of its oxygen
in the same way.

But a permeable skin that allows
water to flow in also allows

it to flow out,
and for some that can be a problem.

If there's a dry spell,

this giant monkey frog
from South America

produces a kind of ointment
from glands

in its skin,
which it uses as sun cream.

At the beginning of the dry season,

it takes steps to make sure that it
doesn't get sun-burnt or dry out.

And with their supple joints,
frogs can manage to reach all those

odd places that the rest of us
find a little tricky.

The dry season can last for weeks.

So it's best to be really thorough.

Some frogs however,
never leave water at all.

This is the highest lake
in the world - Lake Titicaca

in the Peruvian Andes.

And the frog that lives here
has very different problems.

At 4,000 metres above sea level,

there's very little oxygen
in the atmosphere,

and therefore in the water.

So the frog has developed
bizarre-looking flaps

and folds that increase
the skin's surface area,

and therefore its ability
to absorb oxygen.

The frog also increases the flow
of water across its skin by doing

what looks like press-ups.

Because their skin is
so sensitive to their surroundings,

frogs are important
biological indicators.

If there are environmental problems,

they're among the first creatures
to be affected.

But that permeability has also led,

in recent years,
to alarming declines.

This, crouched on a leaf,

is one of the rarest frogs
in the world.

It's called the lemur leaf frog

and it lives in Costa Rica.

Once it was widespread there,
but today,

it's been reduced
to a very small area.

It's nocturnal and lives in the
humid rainforest of the lowlands.

Unusually for a leaf frog,
it has no webbing between its toes

and its stick-like legs give it
a very distinctive walk.

What a lovely little creature.

Here in the Manchester Museum,
they're studying the species

and doing their best to conserve it,

both by breeding it in captivity

and partly by going out to Costa
Rica and studying it in the wild.

The cause for its loss in numbers
is three-fold -

loss of habitat, pesticides

and a particularly lethal kind
of fungus called a chytrid.

The fungus causes the cells
in a frog's skin

to suddenly multiply,

so that the outer layers thicken.

That blocks the flow of essential
salts through the skin.

The muscles then
can't function properly...

..and eventually the heart
simply stops beating.

Scientists think that chytrid
fungus started to spread

back in the 1940s,
when African clawed frogs,

which were probably
carrying the fungus,

were shipped all over the world

for medical research,
including pregnancy testing.

Today, chytrid fungus
is spreading uncontrollably,

causing the extinction
of some frog species

and the severe decline
of many others.

It's the worst infectious
disease ever recorded

in terms of the number
of species affected.

Almost a third of all amphibians
are now threatened with extinction,

including the lemur leaf frog.

The future of this little frog
is still hanging in the balance,

but hopefully the work that's being
done here in Manchester

will prevent it
from becoming extinct.

But other frogs
have not been so lucky.

Over the years, I've filmed
a number of different species,

some of which are now
extinct in the wild.

The waving golden frogs
we filmed in Panama

belonged to one of the last
remaining populations.

The chytrid fungus was already
spreading up from South America.

So when we had finished, scientists
collected all they could find

and took them to specially
sterilised breeding centres

to keep them in safety
until such time, if ever,

the fungus disappears

and they can be reintroduced
to their original home.

But despite the decline, the fact
that frogs are such adaptable

creatures does offer some hope.

Some species, after all, manage
to survive in places that might

seem to spell certain death
for creatures with moist skins.

This is part of the Australian
desert where several years

can pass without rain.

It's a great relief
when at last the drought breaks.

And then, amazingly,
little toads emerge from the sand.

Numbers of them appear
almost simultaneously.

They have specially large legs
to help them dig,

which gives them their name -
spade-foots.

Now they must mate,
if possible, before the sun rises.

The desert dries very quickly,
even after the heaviest of storms.

Temperatures rise
to 50 degrees Centigrade.

Now any water on the surface
will evaporate instantly.

But the toads are already retreating
and will soon be back underground.

It really is a miracle
that they're here at all.

The spade-foot toad is not the only
frog species to adapt

to extreme environmental conditions.

Of all the frogs in the world, this
perhaps is the most extraordinary.

It's called the wood frog
and it lives in America,

north of the Arctic Circle.

And it survives some of the coldest

temperatures on Earth,

and it does so
by becoming frozen solid.

As the winter frost starts to bite,

ice begins to form
on the frog's skin.

The liver goes into overdrive,

producing glucose which is pumped
around the body by the heart.

This glucose acts like an
anti-freeze within the cells,

preventing them from freezing.

Instead, ice forms around them.

The blood, however, is frozen and
all the organs are encased in ice.

And the heart stops.

But then, months later,
spring at last returns.

And the ice around the wood frog
begins to melt.

The wood frog's ability to survive
is truly extraordinary,

and that does give us the hope
that maybe, in spite of the threats

that face them today,

frogs as a whole will continue
to live on this planet.

It would be truly sad
if we lost them.

That's it.