Episode 3 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.

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.

RAPID STACCATO CAWING

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.