Episode 6 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.

Female Komodo dragons can give birth to live young

without having contact with the male.

And female aphids can clone themselves

to produce hundreds of copies.

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

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 had been kept with previously in France,

but genetic tests revealed 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 on 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 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 Dunn, 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 the 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.

A 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 WZ 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 WW

and a ZZ.

In the Komodo dragon,

a WW combination is not an operative pair,

so only the male, ZZ, 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 was 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 is 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 mass 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 the 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 him 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 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 use 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 pool 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 aphideas," he wrote,

"are in many species very rarely met."

Buckton's drawings confirm that aphid populations

are commonly all-female,

and that 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 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, a 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 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 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 are 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

thousands upon thousands 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 some 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 over 200 million years.

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