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ATTENBOROUGH: The natural world
is full of extraordinary animals

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with amazing life histories.

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Yet, certain stories
are more intriguing than most.

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The mysteries
of a butterfly's life-cycle

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or the strange biology
of the emperor penguin.

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some of these creatures were surrounded
by myths and misunderstandings

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for a very long time.

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And some have only recently
revealed their secrets.

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These are the animals
that stand out from the crowd.

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The curiosities 1 find
most fascinating of all.

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Female Komodo dragons can give birth
to live young

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without having contact with a male.

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And female aphids can clone themselves
to produce hundreds of copies.

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

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Most animals breed
by sexual reproduction.

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A male fertilises a female's eggs

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and both parent's genes
mix and produce young.

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But, in nature, a few animals stray
from this method

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and breed in a different way.

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In August, 2005, here in London Zoo,

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a female Komodo dragon called Sungai

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laid a clutch of eggs and, several
months later, four baby dragons hatched.

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That may not seem remarkable,
but it was.

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Because Sungai had had no contact
with a male Komodo dragon

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for more than two years.

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At first, keepers thought
that she had stored sperm

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from the male she had been
kept with previously in France.

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But genetic tests revealed that she had,
in fact, fertilised her own eggs

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and given birth
without any male involvement.

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This was an amazing discovery
about Komodo dragons.

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That they can breed by a process
called parthenogenesis.

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1t's a term derived from two Greek
words, parthenos, meaning virgin,

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and genesis, meaning birth.

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1ncredibly, the dragon's remarkable
reproductive abilities

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went unnoticed until just
a few years ago.

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

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Then stories started to circulate
in 1ndonesia

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of a strange reptilian monster living
on a tiny island

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lying far to the east of Bali.

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1t was said to be over six metres long

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and strong enough
to pull down a buffalo.

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In 1 91 0, two Europeans,
members of a Dutch pearling fleet

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

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Excited by this finding, photographs of
the skin were sent to Major Ouwens,

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director of the zoological museum
on Java.

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He was equally amazed and employed
an experienced Indonesian collector

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

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The land crocodile was identified as a
huge and new species of monitor lizard.

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He named it Varanus komodoensis.

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

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But World War 1 prevented
further visits to the island.

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And then, in 1 926, an expedition
was launched

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by an American called William Burden

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to find out more.

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His small team included his wife,

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Dr Emmett Reid Dunn, a reptile expert,

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and a newsreel cameraman from Pathe.

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Their film of this giant island creature
from a hidden world

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caused great excitement worldwide.

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Then, in 1 92 7, two living Komodo dragons
were sent to Europe.

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Although they clearly
could be dangerous,

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they proved to be more gentle
and intelligent than expected.

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But it would take 80 years before we
fully understood the way they reproduce.

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We know from other examples
that the reproduction of reptiles

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can be more varied than that of mammals.

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1n crocodiles, the sex of the eggs
is not genetically fixed,

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but is controlled instead
by temperature.

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Those incubated at warm temperatures
hatch as males

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and those in cooler conditions
as females.

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But the sex of an unhatched Komodo
dragon is determined in a different way.

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The fact that Komodo dragon eggs
can develop without fertilisation

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was a surprising and exciting discovery.

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

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Why should that be?

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Well, this is how it works.

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A female Komodo dragon has two
different sex chromosomes,

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a W and a Z.

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

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If there are no males, only the female,
WZ pair, remain.

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

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

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They then duplicate themselves
to form a WW and a ZZ.

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In the Komodo dragon, a WW combination
is not an operative pair.

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So only the male ZZ will hatch.

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Thus female Komodo dragons
can produce their own males.

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

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

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Komodo dragons are descended
from lizard-like ancestors

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that lived over
40 million years ago in Asia.

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They migrated to Australia and later
reached the islands of central 1ndonesia

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

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Parthenogenesis would enable
a single female arriving on an island

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to start a breeding population
all by herself.

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Nobody knew that Komodo dragons
could breed asexually

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before lone females
hatched fertile eggs in captivity.

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In the wild, it's virtually impossible
to know

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if a female has mated with a male

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and there are usually males around.

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In most circumstances,
sexual reproduction is preferable.

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

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and prevent unwanted mutations.

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Such genetic variation
also helps animals

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to adapt to changing environments.

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So sexual reproduction

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seems to make more biological sense
than parthenogenesis.

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And it should be rare in the wild,
an extreme last resort.

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Strangely, that's not always so.

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1n 201 2, odd breeding behaviour was
noticed in two species of snake,

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copperheads and cottonmouths.

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some females were
reproducing by parthenogenesis

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even though males were present.

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These females were often small
and overlooked by the males.

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so, rather than not breed,
they cloned themselves.

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

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1f individuals all have the same genes,

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the species can't react
to a changing world.

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

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being genetically the same
is actually an advantage.

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For them, parthenogenesis is better
than sexual reproduction,

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as it prevents them from varying
from their winning formula.

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

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

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1f their environment changes
for the worse,

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they'll be unable to adapt

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and so they risk extinction.

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Clearly the best survival technique is
to be able to reproduce in either way.

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Parthenogenesis has enabled isolated
dwellers, like the Komodo dragon,

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to survive by forming breeding
populations from just a single female.

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More recently, studies
of wild Komodo dragons

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have revealed that two-thirds
of the population is male,

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suggesting that, even when
both sexes are present,

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asexual breeding is still occurring.

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So, Komodo dragons keep their
breeding options flexible.

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It's likely that many animals
are breeding by parthenogenesis,

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

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Parthenogenesis has been occurring
unnoticed for millions of years.

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

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Next, we meet a tiny animal
that uses parthenogenesis

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to be one of the fastest
breeders in nature.

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surprisingly, this lives
in our own back gardens.

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In summer, this is not
an uncommon sight.

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Thousands of aphids
massed together on a stem.

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At this time of the year,

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each of them can produce
five to ten youngsters in a day,

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and each is a genetic copy of herself.

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

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Birds and other insects arrive
and prey on them,

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but the aphids usually
manage to keep ahead.

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This astonishing ability attracted
the attention of early scholars.

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1n the mid-1 8th century, a new survey
of insects was published in France.

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1ts author, Rene-Antoine
Ferchault de Reaumur,

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expressed surprise
that he'd never seen aphids mating.

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Neither had he seen a male.

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He made the revolutionary suggestion
that they were reproducing without sex

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and invited his readers
to help him prove it.

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1n the spring of 1 740, Charles Bonnet,

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then a young law student
from switzerland,

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took up that challenge.

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Charles Bonnet took a newborn
female aphid from its mother

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immediately after birth

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and put it in an isolation chamber.

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

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using a magnifying glass,
watched it from early morning

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until night for 1 2 days.

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On the evening of june the 1 st, 1 740
at 7:30 pm,

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

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Then, over the next 2 1 days,
she had 94 more female offspring.

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Bonnet had no clue how this
could happen, but he knew for sure

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that the aphid had bred
without any male contact.

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He sent his findings
to Reaumur in Paris,

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who published this new and important
discovery of sexless reproduction.

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But how this parthenogenesis worked,

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and why aphids use virgin birth
in their lifecycles

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was still a mystery.

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And entomologists puzzled over it
for many years.

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In the 1 830s, an entomologist
called Francis Walker

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took a great interest in cataloguing
various small insects, including aphids.

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He made more than 1 3,000 slides.

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Walker collected hundreds of aphids,

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many from Southgate
and the surrounding areas of London.

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Here we can see some of them.

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He made successive collections
of the same species of aphid

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from the same locality

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across all the seasons.

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

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throughout the breeding cycle.

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They varied in size
and some were wingless.

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That suggested that female aphids
had a rather extraordinary lifecycle.

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1t was clear from Walker's study that
nearly all individual aphids are female.

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But they change in form
over the seasons.

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1n early spring, when plants are
growing, most are without wings.

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

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Later in the season, when
overcrowding becomes an issue,

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

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Aphids seem to be able
to produce females

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that can exploit every situation.

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

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He recorded many aspects
of the aphids' lifecycles,

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

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And then aphid research was taken up

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by another entomologist
called George Buckton.

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He chronicled every detail
of the complex aphid lifecycle.

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In 1 893, George Buckton published
a Monograph of British Aphides

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in four volumes.

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He wanted to share his passion
for these tiny insects in books

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that he hoped would not be
too dry academically.

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Buckton corresponded with many
leading naturalists of his day

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to pull together every possible specimen
and record of behaviour.

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He was an accomplished artist

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and produced beautiful, accurate
drawings from live specimens.

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And they interestingly show
a distinct absence of male aphids.

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''The sexual forms of aphides,'' he wrote,
''are in many species, very rarely met.''

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Buckton's drawings confirm that aphid
populations are commonly all female

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and that males have been almost
entirely eliminated from the species.

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For most of the breeding season,
females only give birth to daughters.

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They don't waste time producing males,

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which can't by themselves
produce offspring.

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so, do aphids need males at all?

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The lifecycle of another insect
would seem to suggest not.

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This wonderful creature
is a Phyllium giganteum,

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a giant leaf insect.

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

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Nearly all individuals are female.

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In fact, a male of this species
wasn't discovered until 1 994.

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They're extremely rare.

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The species for the most part
reproduces itself by parthenogenesis.

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They lay unfertilised eggs that hatch
into more females.

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And this method of reproduction

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has enabled it
to extend its range dramatically.

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

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a lone female stick insect can start
a breeding colony in a new area

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even if males never arrive.

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And that's what happened in
southern England in 1 903

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when a different species of stick insect

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arrived on vegetation
imported from New Zealand.

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Now, all female populations survive

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thousands of miles away
from their native home.

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

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The females produce fertile eggs
that survive the cold winters

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and new females hatch out in the spring.

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

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Aphid populations
face the same problems.

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But most species have
a twist in their lifecycle

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that freshens up their gene pool.

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In the autumn, the aphid production line

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switches from producing
just asexual females

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to producing sexual males
and sexual females.

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At the end of the season,

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as the food supply wanes
and the temperature drops,

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

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These eggs will over winter

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to produce next spring's
new aphid generation.

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Aphids don't produce their eggs
until the autumn.

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However, most populations
survive until then

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because, in many cases, they form a
relationship with another insect, ants.

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An aphid feeds by piercing the stems
of plants and drinking the sugary sap.

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

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so they excrete the excess as honeydew.

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This is perfect food for the ants

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

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And in return, the ants
protect the aphids

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from insects that try to prey on them.

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so, with ants guarding them,

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the aphids have a good chance of
surviving until the end of the year,

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when they produce their eggs.

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1n spring, new females will emerge
from the eggs

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and start once more to produce
new versions of themselves,

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over and over again.

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And aphids have a final, almost
unbelievable, twist in their lifecycles

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that greatly speeds up their breeding.

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They do something truly astounding.

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Even before they're born, they have
embryos developing inside their bodies.

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Parthenogenesis combined with
this telescoping of generations

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give aphids an extremely rapid turnover
of generations.

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Like tiny Russian dolls,

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they just keep popping out
smaller copies of themselves.

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A newly-born summer aphid has, inside
her body, her own developing daughters,

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who in turn contain her fully-formed
unborn granddaughters.

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so several generations of aphid overlap
in time and space

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and, in one season,
a single female can produce

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thousand upon thousand
of cloned females.

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

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They can change plant host,

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change their form,
and alter their method of reproduction.

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1n the spring, females hatch from eggs

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00:20:35,007 --> 00:20:37,760
and produce several generation
of wingless females.

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Their numbers grow and
they produce winged females,

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that can fly to new food and rapidly
produce even more females.

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1n the autumn, the sexual forms
of both males and females appear,

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

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The ability to breed by parthenogenesis
seems almost magical to us,

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but in nature virgin birth
is not uncommon.

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Having the ability to produce
daughter clones or more males

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can save a species or create a new one.

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Flexible ways of breeding have allowed
creatures to colonise new areas

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and survive in small communities,
like those on islands.

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The Komodo dragon has certainly survived
for many centuries

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and aphids have been around
for over 200 million years.

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