All language subtitles for Natural Curiosities S01E03 - Reproduction and Preservation CC HDTV x264 AC3 1080p

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.

Female Komodo dragons can give birth
to live young

without having contact with a male.

And female aphids can clone
themselves to produce

hundreds of copies.

How and why do these very different
creatures reproduce by virgin birth?

And also in this programme, some
animals live in conditions so cold

that they seem to defy the rules of
nature.

The emperor penguin is the only
animal able

to raise its young in the harsh
Antarctic winter.

And the tiny wood frog faces
freezing conditions

that would kill any other amphibian.

How do they do it?

Most animals breed by sexual
reproduction.

A male fertilises a female's eggs

and both parents' genes mix
and produce young.

But in nature a few animals
stray from this method

and breed in a different way.

In August 2005, here in London zoo,

a female Komodo dragon called Sungai
laid a clutch of eggs

and several months later
four baby dragons hatched.

That may not seem remarkable,
but it was.

Because Sungai had had no contact
with a male Komodo dragon

for more than two years.

At first, keepers thought that she
had stored sperm from the male

she'd been kept with
previously in France,

but genetic tests reveal that
she had in fact fertilised

her own eggs and given birth without
any male involvement.

This was an amazing discovery about
Komodo dragons,

that they can breed by a process
called parthenogenesis.

It's a term derived from two
Greek words,

"partheno", meaning virgin,
and "genesis", meaning birth.

Incredibly, the dragon's remarkable
reproductive abilities

went unnoticed until just a few
years ago.

But the species itself had remained
unknown well into the 20th century.

Then stories started to circulate in
Indonesia of a strange

reptilian monster living on a tiny
island

lying far to the east of Bali.

It was said to be over
six metres long

and strong enough to pull down
a buffalo.

In 1910, two Europeans,

members of a Dutch pearling fleet,
finally confirmed

the existence of these great dragons
on the island of Komodo.

Excited by this finding,
photographs of the skin were sent

to Major Owens, director of the
zoological museum in Java.

He was equally amazed, and employed
an experienced Indonesian collector,

who captured two live adults
and two youngsters for his zoo.

The land crocodile was identified as
a huge

and new species of monitor lizard.

He named it Varanus komodoensis.

The discovery of this living monster
caused a flurry of excitement,

but World War I prevented further
visits to the island.

And then, in 1926, an expedition was
launched by an American

called William Burton
to find out more.

His small team included his wife,
Dr Emmett Reid-Dunne,

a reptile expert, and a newsreel
cameraman from Pathe.

Their film of this giant island
creature from a hidden world

caused great excitement worldwide.

Then, in 1927, two living
Komodo dragons were sent to Europe.

Although they clearly could be
dangerous,

they proved to be more gentle and
intelligent than expected.

But it would take 80 years before
we fully understood

the way they reproduce.

We know from other examples that

the reproduction of reptiles can be
more varied than that of mammals.

In crocodiles, the sex of the eggs
is not genetically fixed,

but is controlled instead by
temperature.

Those incubated at warm temperatures
hatch as males

and those in cooler conditions
as females.

But the sex of an unhatched
Komodo dragon

is determined in a different way.

The fact that Komodo dragon eggs can
develop without fertilisation

was a surprising and exciting
discovery.

But, interestingly, all the babies
that hatched were males.

Why should that be?

Well, this is how it works.

The female Komodo dragon has two
different sex chromosomes,

a "W" and a "Z".

And the male has two similar
chromosomes, a "Z" and a "Z".

If there are no males, only the
female W-Z pair remain.

In such a case, the female divides
her own egg-cell into two halves,

one of which has a W chromosome and
the other a single Z.

They then duplicate themselves to
form a W-W and a Z-Z.

In the Komodo dragon, the W-W
combination is not

an operative pair, so only the male,
Z-Z, will hatch.

Thus, female Komodo dragons can
produce their own males.

This seems almost unbelievable, but
when you come to think about it,

it's a very useful ability for an
animal that lives on a small island.

Komodo dragons are descended from
lizard-like ancestors

that lived over 40 million years ago
in Asia.

They migrated to Australia

and later reached the islands of
central Indonesia either

by swimming or by drifting across
the ocean on floating vegetation.

Parthenogenesis would enable a
single female

arriving on an island to start a
breeding population all by herself.

Nobody knew that Komodo dragons
could breed asexually

before lone females hatched fertile
eggs in captivity.

In the wild, it's virtually
impossible to know if a female

has mated with a male, and there are
usually males around.

In most circumstances, sexual
reproduction is preferable.

A mix of male and female genes can
enable the repair of DNA

and prevent unwanted mutations.

Such genetic variation also helps
animals to adapt to changing

environments, so sexual reproduction
seems to make more biological sense

than parthenogenesis and it
should be rare in the wild,

an extreme last resort.

Strangely, that's not always so.

In 2012, odd breeding behaviour was
noticed in two species of snake,

copperheads and cottonmouths.

Some females were reproducing by
parthenogenesis

even though males were present.

These females were often small and
overlooked by the males,

so, rather than not breed,
they cloned themselves.

But this kind of breeding is
potentially a genetic dead-end.

If individuals all have the same
genes,

the species can't react to
a changing world.

For whiptail lizards, which live
in a harsh but very stable desert,

being genetically the same is
actually an advantage.

For them, parthenogenesis is better
than sexual reproduction,

as it prevents them from varying
from their winning formula.

Strangely, the females still go
through the motions of mating.

This stimulates their hormones, but
these lizards are taking a gamble.

If their environment changes for
the worse,

they'll be unable to adapt and so
they risk extinction.

Clearly, the best survival technique
is to be able to

reproduce in either way.

Parthenogenesis has enabled isolated
dwellers like the Komodo dragon

to survive by forming breeding
populations from just

a single female.

More recently, studies of wild
Komodo dragons have revealed

that two thirds of the population
is male,

suggesting that even when both sexes
are present

asexual breeding is still occurring.

So Komodo dragons keep
their breeding options flexible.

It's likely that many animals are
breeding by parthenogenesis

or have the potential to do so, but
we just don't know about them.

Parthenogenesis has been occurring
unnoticed for millions of years.

Here is a natural curiosity that's
only just revealing its secrets.

Next, we meet a tiny animal that
uses parthenogenesis to be

one of the fastest breeders in
nature.

Surprisingly, this lives in our own
back gardens.

In summer, this is not an uncommon
sight.

Thousands of aphids massed together
on a stem.

At this time of the year, each of
them can produce five to ten

youngsters in a day, and each is a
genetic copy of herself.

So vast numbers can suddenly appear
within a day or so.

Birds and other insects arrive and
prey on them,

but the aphids usually manage to
keep ahead.

This astonishing ability attracted
the attention of early scholars.

In the mid-18th century,

a new survey of insects was
published in France.

Its author, Rene Antoine
Ferchault de Reaumur,

expressed surprise that he'd never
seen aphids mating.

Neither had he seen a male.

He made the revolutionary suggestion
that they were reproducing

without sex and invited his readers
to help prove it.

In the spring of 1740,
Charles Bonnet,

then a young law student from
Switzerland, took up that challenge.

Charles Bonnet took a newborn
female aphid from its mother

immediately after birth and put it
in an isolation chamber.

He placed the aphid on a leaf
inside an upturned glass jar

and, using a magnifying glass,
watched it from early morning

until night for 12 days.

On the evening of June the 1st,
1740, at 7.30pm,

the female aphid gave birth to a
brand-new baby aphid.

Then, over the next 21 days,
she had 94 more female offspring.

Bonnet had no clue how this
could happen,

but he knew for sure that the aphid
had bred without any male contact.

He sent his findings to Reaumur in
Paris, who published this

new and important discovery of
sexless reproduction.

But how this parthenogenesis worked

and why aphids used virgin birth
in their life cycles

was still a mystery

and entomologists puzzled over it
for many years.

In the 1830s, an entomologist called
Francis Walker took a great

interest in cataloguing various
small insects, including aphids.

He made more than 13,000 slides.

Walker collected hundreds of aphids,
many from Southgate

and the surrounding areas of London.

Here we can see some of them.

He made successive collections of
the same species

of aphid from the same locality
across all the seasons.

As a result, he found several
different forms of each aphid

throughout the breeding cycle.

They varied in size and some were
wingless.

That suggested that female aphids
had a rather extraordinary
life cycle.

It was clear from Walker's study
that nearly all individual

aphids are female,

but they change in form over
the seasons.

In early spring, when plants are
growing, most are without wings.

With plenty of food on offer,
they have no need to fly.

Later in the season, when
overcrowding becomes an issue,

females are born with wings so that
they can travel to find new food.

Aphids seem to be able to produce
females that can exploit

every situation.

Although Walker was prolific, he
wasn't always entirely accurate.

He recorded many aspects of
the aphids' life cycles,

but he didn't piece them together to
produce the complete picture.

And then aphid research was taken up
by another entomologist,

called George Buckton.

He chronicled every detail of
the complex aphid life cycle.

In 1883, George Buckton published a
monograph

of British aphids in four volumes.

He wanted to share his passion for
these tiny insects

in books that he hoped would not be
too dry academically.

Buckton corresponded with many
leading naturalists of his day

to pull together every possible
specimen and record of behaviour.

He was an accomplished artist
and produced beautiful,

accurate drawings from live
specimens

and they interestingly show
a distinct absence of male aphids.

"The sexual forms of aphides,"
he wrote,

"are in many species
very rarely met."

Buckton's drawings confirmed that
aphid populations are commonly

all-female and the males have
been almost entirely

eliminated from the species.

For most of the breeding season,

females only give birth to
daughters.

They don't waste time producing
males which can't by themselves
produce offspring.

So do aphids need males at all?

The life cycle of another insect
would seem to suggest not.

This wonderful creature is
a Phyllium giganteum,

a giant leaf insect.

It's the largest species of its
group and it lives wild in Malaysia.

Nearly all individuals are female.

In fact, the male of this species
wasn't discovered until 1994.

They're extremely rare.

The species for the most part
reproduces itself by
parthenogenesis.

They lay unfertilised eggs that
hatch into more females

and this method of reproduction has
enabled it

to extend its range dramatically.

Much like a single female
Komodo dragon arriving on an island,

a lone female stick insect can
start a breeding colony

in a new area even if males never
arrive.

And that's what happened in southern
England in 1903,

when a different species of stick
insect arrived on vegetation

imported from New Zealand.

Now, all female populations survive
thousands of miles

away from their native home.

These populations have no males and
don't appear to need them.

The females produce fertile eggs
that survive the cold winters

and new females hatch out
in the spring.

But, without males, the population
could become dangerously inbred.

Aphid populations face the same
problems, but most species

have a twist in their life cycle
that freshens up their gene pool.

In the autumn, the aphid production
line switches from producing

just asexual females to producing
sexual males and sexual females.

At the end of the season,
as the food supply wanes

and the temperature drops,

there's a phase of sexual
reproduction that produces eggs.

These eggs will overwinter to
produce next spring's new aphid
generation.

Aphids don't produce their eggs
until the autumn.

However, most populations survive
until then, because in many cases

they form a relationship with
another insect, ants.

An aphid feeds by piercing
the stems of plants

and drinking the sugary sap.

But sap contains far more sugar
than the aphids can use,

so they excrete the excess as
honeydew.

This is perfect food for the ants

and they keenly farm the aphids to
harvest the rich liquid.

And in return
the ants protect the aphids

from insects that try to
prey on them.

So, with ants guarding them,
the aphids have a good chance

of surviving until the end of the
year, when they produce their eggs.

In the spring, new females will
emerge from the eggs and start

once more to produce new versions of
themselves over and over again.

And aphids have a final, almost
unbelievable twist in their
life cycles

that greatly speeds up their
breeding.

They do something truly astounding.

Even before they're born, they have
embryos

developing inside their bodies.

Parthenogenesis, combined with this
telescoping of generations,

give aphids an extremely rapid
turnover of generations.

Like tiny Russian dolls,

they just keep popping out
smaller copies of themselves.

A newly born summer aphid has
inside her body

her own developing daughters,
who in turn contain her

fully formed unborn
granddaughters.

So several generations of aphid
overlap in time and space

and in one season a single female
can produce

thousand upon thousand
of cloned females.

Aphids' lives are varied, often
complicated and truly amazing.

They can change plant host, change
their form

and alter their method of
reproduction.

In the spring,
females hatch from eggs and

produce several generations of
wingless females.

Their numbers grow, and they produce
winged females that can fly to

new food and rapidly produce
even more females.

In the autumn, the sexual forms
of both males and female appear,

which mate and lay eggs, which then
can survive the winter.

The ability to breed
by parthenogenesis seems almost

magical to us. But in nature
virgin birth is not uncommon.

Having the ability to produce
daughter clones or more males

can save a species or
create a new one.

Flexible ways of breeding have
allowed creatures

to colonise new areas

and survive in small communities,
like those on islands.

The Komodo dragon has certainly
survived for many centuries.

And aphids have been around for
more than 200 million years.

So parthenogenesis is a breeding
strategy that is a real life-saver.

These eggs were collected
more than 100 years ago

during an expedition to
the Antarctic.

The conditions were so cold that
the man that collected them

never made it back to England alive.

He perished alongside Captain Scott
during the ill-fated journey

to reach the South Pole.

The eggs were laid by an emperor
penguin, a bird whose life history

would surprise and confound
those early polar explorers.

At the end of the 19th century,
the Antarctic was an unfamiliar

and mysterious place.

Only a handful of explorers had
ventured this far south

and there was still a huge blank
in the world map.

But then, in 1901,

a British expedition set off on a
purpose-built ship, the Discovery,

to explore this most southerly land.

In charge was
Commander Robert Falcon Scott.

Scott took on board with him
a young man named Edward Wilson,

who would serve as the ship's doctor
and naturalist.

Wilson had only just
qualified as a surgeon

and had no formal training
in scientific research.

But the young man's
passion for natural history and art

would prove to be an
invaluable asset to the expedition.

Wilson's job was to draw and record

any plants
and animals that they encountered.

But from the start there was
one creature that fascinated him

more than any other -
the emperor penguin.

This largest of all penguins

had only been discovered
60 years earlier.

But, as yet, nothing was known
about its habits or where it breeds.

The expedition was
an opportunity to find out more.

When the Discovery reached
the southern continent,

they put up a hut in which they
would spend the long, dark winter.

Then, as the sun started to
appear again in spring,

the sledge teams started to explore,

and one returned with some
tantalising news.

They had discovered a breeding
colony of emperor penguins

in a place called Cape Crozier.

It was the first colony any
human being had ever seen

and, much to their surprise,
the birds were breeding on sea ice.

It was a truly astonishing
discovery.

No other bird breeds on ice,

and Wilson was keen to find out
more about this remarkable creature.

Very little was known about emperor
penguins but there was another bird

which could give Wilson some
insights into their lives -
the king penguin.

Adult king penguins look very much
the same as adult emperors.

The main difference is in size.

These kings are only about
half as big as an emperor,

and they live in
the northern part of Antarctica.

They breed in the middle
of the Antarctic summer -

November, December - and incubation
takes about seven weeks.

Wilson thought that emperors
would do very much the same.

But he was about to discover
otherwise.

The following spring, with the hope
of collecting some penguin eggs,

Wilson left for Cape Crozier
as early as he dared.

When he got there, however,
much to his surprise,

he found only well-grown chicks.

After repeated calculations,
he finally concluded that these

penguins must lay their eggs in
the middle of the Antarctic winter.

That emperors should start
breeding at the coldest

and bleakest time of the year
was an astonishing discovery.

It seemed to defy
all the rules of nature,

and Wilson was indeed amazed.

But it seems that this strange
lifestyle does, in fact, make sense.

Emperor penguins are big birds
and the chicks take more than

a year to grow large enough
to be independent.

By laying the eggs earlier in
winter,

emperors give their chicks
a head start

so that they first go to sea
in the summer months

when food is plentiful.

But how do emperor penguins
protect their eggs

and chicks from the bitter cold?

Neither kings nor emperors
make a nest

or lay their eggs on the ground.

If they did, the eggs
would freeze within minutes.

Instead, they keep their eggs
on the top of their feet

and cover them with a feathered
fold of skin from the abdomen,

and inside that pouch

the temperature is about 70 degrees
warmer than it is outside.

With temperatures
of minus-60 degrees Celsius,

and winds gusting at 200km/h, the
birds huddle together for warmth.

Even under these extremely
difficult conditions,

Wilson recorded everything he saw.

WIND ROARS

Able to work for only a few
minutes at a time,

he still managed to produce detailed
notes and drawings that give us

a first insight into
the southern continent.

This is the expedition's
scientific report.

And it contains most of Wilson's
observations on the Antarctic.

At a time when illustrations
of animals were often

drawn from dead specimens,

Wilson drew his subjects live
in the field wherever possible,

to capture the true nature
of the animal.

Despite the extreme conditions
under which he had to work,

he made over 900 detailed
drawings in the Antarctic.

Wilson was an exceptional artist
and a meticulous scientist

and most of his observations have
stood the test of time.

But some things puzzled him
more than others.

He noted, for example, that
the brooding of the chick was not

just carried out by one bird
or even by a single pair.

It appeared as if numerous birds
were taking turns in looking after

the chick. Today, of course, we know
that this is not quite correct.

It's only the parents who care for
both the egg and then the chick.

RAPI D STACCATO CAWI NG

We now have a much better
understanding of how

emperor penguins breed,
but Wilson's confusion as to

who cares for the chicks is in fact
quite understandable.

He observed numerous occasions

when a youngster was accidentally
dropped by its parent.

In his report, he writes,

"what we actually saw again and
again was the wild dash made by
adults,

"each weighing anything up to
90 pounds, to take possession

"of any chick that happened to
find itself deserted on the ice.

"It can be compared to nothing
better than a football scrimmage."

The birds Wilson had observed

were in fact females who had lost
their own egg or chick

and were trying to adopt or kidnap
any unattended youngsters.

What he couldn't know was that these
adoptions are never successful.

A new parent rarely
feeds its foster chick

and simply broods it for a few days.

After that, the youngster is
abandoned again

or dies of starvation.

It's likely that the female
eventually recognises that

the adopted chick is not her own.

Although Wilson had been the first
man to find an emperor penguin
colony,

he had not been able to obtain
any freshly laid eggs.

These were particularly sought-after
by scientists of the day.

It was thought at that time
that the emperor penguin was

one of the most primitive birds

and possibly a missing evolutionary
link with dinosaurs.

If embryos could be obtained
at an early enough stage then maybe

one would see reptilian scales
or some other dinosaur features.

So the emperor penguin egg was
regarded as a great scientific
prize.

A few years later,
Scott and Wilson

planned a second
expedition to the Antarctic.

The main objective was
to reach the South Pole,

but Wilson was determined to
bring back

newly laid emperor penguin eggs.

This time, he made plans to travel
to Cape Crozier even earlier,

so as not to miss the birds on eggs.

BIRD CAWS

He picked two men to accompany him,
Bowers and Cherry-Garrard,

and they set off in
the pitch black of the winter.

It was a journey of over 70 miles
and they had to cover it on foot.

For six painful weeks,

the three men pulled their heavy
sledges in complete darkness

and howling gales at temperatures
of minus-40 degrees centigrade.

Never before had anyone travelled
in such bitter cold

or in such difficult conditions.

They sometimes barely
covered a mile a day.

It was what Cherry-Garrard
would later call

"the worst journey in the world".

Their clothes were iced up
and their breath

and sweat froze on their bodies.

Each night, it took them an hour to
chip into their sleeping bags,

which were frozen solid.

When they finally reached
the penguin colony,
they collected five eggs,

with great difficulty, and put them
inside their mittens for safety.

The men staggered back to
base camp close to death

and only three eggs
survived the journey.

These are two of them.

It was an extraordinary
feat of determination

by Wilson and his companions.

The precious eggs were supposed to
reveal the evolutionary links

between reptiles and birds,

but getting them had nearly killed
the collectors.

A few months later,

Scott led his party on the final
push to reach the South Pole.

His team consisted of just five men,

and Wilson was amongst them.

On their return journey, all five
men perished, succumbing to the cold

and starvation just a few kilometres
from their nearest food depot.

In the end, Wilson's eggs didn't
contribute as much

to our understanding of the
development of the penguin chick

as he had hoped,
but his beautiful drawings

and meticulous observations
are quite a different matter.

They helped to unravel
the biology of a bird that is able

to rear its young in the depths
of the polar winter.

The emperor penguin amazes us

by raising its chicks in the
most inhospitable place on earth.

But a small frog has a way of coping
with the cold

that seems to be beyond belief.

This is a North American
wood frog,

and it lives as far north
as the Arctic Circle,

but, like all cold-blooded
creatures,

it can't generate its own heat
and its body temperature rises

and falls with the surroundings.

So when conditions drop below zero
the frog risks freezing.

How does a creature like this
survive the harsh winters?

The skin of amphibians is thin
and moist and this makes them

particularly vulnerable to the cold.

Any contact with ice can instantly
trigger freezing within

their bodies and, for most animals,
this means almost certain death.

When water freezes, it expands,

and the sharp ice crystals can
puncture blood vessels

and break cell walls,
causing irreparable damage.

The animal's internal organs may
never function properly again.

So, how do frogs avoid freezing?

Many sit out the winter by
hibernating at the bottom of a pond.

The surface may freeze but
underneath the ice

the temperature remains
just above freezing.

And most land-living amphibians
seek out a sheltered spot

on the ground to avoid
the deadly frost.

But, in the 18th century, Arctic
travellers came back with tales

so extraordinary
they were scarcely believable.

A British explorer called
Samuel Hearne reported seeing

frozen frogs among the piles of
leaves in Arctic Canada.

He went on to make
an extraordinary claim.

"Frogs of various colours
are numerous in these parts.

"I have frequently seen them
dug up with moss,

"frozen as hard as ice,

"in which state the legs are
as easily broken off as a pipe stem,

"without giving the least sensation
to the animals.

"But, by wrapping them up
in warm skins and exposing them

"to a slow fire,
they soon recover life

"and the mutilated animal
gains its usual activity."

Frozen frogs that, if gently warmed
by a fire, would come back to life.

What truth could there be
in this account?

Well...

..this is a marsh frog

and it's found in ponds
and marshes throughout

central and northern Europe.

It's lying completely immobile on
my hand because it's frozen solid.

From the outside,
it feels much like a rock.

And you might be forgiven
for thinking it was dead.

Well, watch what happens when
I put it into a bowl of warm water.

Although it appears dead
and has in fact stopped breathing,

the frog's heart is still beating.

Only the outer layer has frozen.

The vital organs inside are still
undamaged.

Lab experiments have shown that,
in this state, the marsh frog

can survive temperatures
of two degrees below freezing.

Yes! It's lifted itself up,
it's moving.

Look at this.

There, it's moving its right leg.

Within a few minutes the frog has
awakened to life once again.

This is surely one of the most
extraordinary miracles of nature.

Nonetheless, the marsh frog can only
survive a few hours of freezing.

Anything more would mean
certain death.

Where it lives,
it rarely faces extreme winters

and is protected from the worst
by the insulating water.

So what about Samuel Hearne's story?

Could some frogs survive
longer periods of freezing?

Another account from North America
would seem to suggest so.

In the 19th century, a naturalist
called John Burroughs

found a wood frog underneath
the leaf litter

at the beginning of the winter.

Burroughs was surprised,

but reasoned that the frog must know
that a mild winter was on the way

and had therefore not bothered
to bury itself deeper.

In fact,
a very severe winter followed.

Wondering about his frog,
Burroughs went back to the same spot

in spring and found the animal
seemingly unharmed.

The wood frog must have spent
the entire winter above ground

and survived temperatures
that should have killed it.

How did the tiny frog do it?

The wood frog is not
strong and large enough
to dig itself into the ground,

so it has to sit out the winter
beneath the leaf litter.

But this doesn't provide sufficient
protection against the cold.

So, how does this small frog
survive?

Today, we know the truth,

and if Burroughs had done so
he would have been astounded.

It's only recently that we've
discovered just how the wood frog

avoids the usually fatal
consequences of freezing.

As winter sets in, the frog prepares
for an extraordinary change.

First, it draws water out of
its cells into spaces where it

will do less damage if it freezes.

At the same time, its liver produces
large amounts of sugar

that act as antifreeze.

This is pumped through the body to
slow down the freezing.

Now the entire frog slowly
freezes from the outside inwards.

And finally, the heart stops.

The frog isn't dead,

but it's probably about as close
as you can get.

70% of its body is frozen.

And it can remain like this
for several weeks on end.

Then, as the air warms up again,

a miraculous
transformation takes place.

The ice melts and the frog's body
thaws and suddenly

the heart sprouts back to life.

Unlike the marsh frog,
the deeply frozen wood frog needs

several hours before it can
resume normal activity.

The wood frog's ability to survive
in a frozen state

has fascinated scientists.

Could this one day help enhance
our own medical understanding?

We still don't understand
completely how the wood frogs
survive

something that would kill
most animals.

What we do know is that,
when freezing occurs slowly

and in the right places,
it appears to do less damage.

This little frog seems to have
mastered the problem

by controlling how and where
ice forms in its body.

The emperor penguin's ability
to breed during the Antarctic winter

is a remarkable
feat of endurance,

but for a small frog to freeze solid
and come back to life

must surely be one of the most
astonishing curiosities of nature.