0:00:02 > 0:00:06The natural world is full of extraordinary animals
0:00:06 > 0:00:09with amazing life histories.
0:00:09 > 0:00:13Yet certain stories are more intriguing than most.
0:00:15 > 0:00:19The mysteries of a butterfly's life cycle
0:00:19 > 0:00:22or the strange biology of the Emperor penguin.
0:00:23 > 0:00:26Some of these creatures were surrounded by
0:00:26 > 0:00:28myths and misunderstandings for
0:00:28 > 0:00:29a very long time...
0:00:30 > 0:00:34..and some have only recently revealed their secrets.
0:00:34 > 0:00:38These are the animals that stand out from the crowd,
0:00:38 > 0:00:43the curiosities I find most fascinating of all.
0:00:52 > 0:00:56Female Komodo dragons can give birth to live young
0:00:56 > 0:00:59without having contact with the male.
0:00:59 > 0:01:03And female aphids can clone themselves
0:01:03 > 0:01:05to produce hundreds of copies.
0:01:05 > 0:01:12How and why do these very different creatures reproduce by virgin birth?
0:01:16 > 0:01:20Most animals breed by sexual reproduction.
0:01:20 > 0:01:22A male fertilises a female's eggs
0:01:22 > 0:01:27and both parents' genes mix and produce young.
0:01:27 > 0:01:30But in nature, a few animals stray from this method
0:01:30 > 0:01:32and breed in a different way.
0:01:34 > 0:01:38In August 2005, here in London Zoo,
0:01:38 > 0:01:43a female Komodo dragon called Sungai laid a clutch of eggs
0:01:43 > 0:01:48and several months later, four baby dragons hatched.
0:01:48 > 0:01:51That may not seem remarkable but it was,
0:01:51 > 0:01:53because Sungai had had no contact
0:01:53 > 0:01:57with a male Komodo dragon for more than two years.
0:01:57 > 0:02:00At first, keepers thought that she had stored sperm
0:02:00 > 0:02:04from the male she had been kept with previously in France,
0:02:04 > 0:02:06but genetic tests revealed that she
0:02:06 > 0:02:09had, in fact, fertilised her own eggs
0:02:09 > 0:02:12and given birth without any male involvement.
0:02:13 > 0:02:17This was an amazing discovery about Komodo dragons,
0:02:17 > 0:02:21that they can breed by a process called parthenogenesis.
0:02:21 > 0:02:26It's a term derived from two Greek words, partheno, meaning virgin
0:02:26 > 0:02:29and genesis, meaning birth.
0:02:29 > 0:02:33Incredibly, the dragon's remarkable reproductive abilities
0:02:33 > 0:02:37went unnoticed, until just a few years ago.
0:02:37 > 0:02:42But the species itself had remained unknown well into the 20th century.
0:02:42 > 0:02:45Then stories started to circulate in Indonesia
0:02:45 > 0:02:47of a strange reptilian monster
0:02:47 > 0:02:52living on a tiny island lying far to the east of Bali.
0:02:52 > 0:02:55It was said to be over six metres long
0:02:55 > 0:02:57and strong enough to pull down a buffalo.
0:03:00 > 0:03:05In 1910, two Europeans, members of a Dutch pearling fleet,
0:03:05 > 0:03:09finally confirmed the existence of these great dragons
0:03:09 > 0:03:11on the island of Komodo.
0:03:11 > 0:03:12Excited by this finding,
0:03:12 > 0:03:16photographs of the skin were sent to Major Owens,
0:03:16 > 0:03:19director of the zoological museum on Java.
0:03:19 > 0:03:23He was equally amazed, and employed an experienced
0:03:23 > 0:03:26Indonesian collector who captured two live adults
0:03:26 > 0:03:29and two youngsters for his zoo.
0:03:29 > 0:03:32The land crocodile was identified as
0:03:32 > 0:03:35a huge new species of Monitor lizard.
0:03:35 > 0:03:38He named it Varanus komodoensis.
0:03:41 > 0:03:46The discovery of this living monster caused a flurry of excitement,
0:03:46 > 0:03:49but World War I prevented further visits to the island.
0:03:51 > 0:03:54And then, in 1926, an expedition was launched
0:03:54 > 0:03:57by an American called William Burton, to find out more.
0:04:00 > 0:04:02His small team included his wife,
0:04:02 > 0:04:05Dr Emmett Reid Dunn, a reptile expert,
0:04:05 > 0:04:08and a newsreel cameraman from Pathe.
0:04:10 > 0:04:14Their film of this giant island creature from a hidden world
0:04:14 > 0:04:16caused great excitement worldwide.
0:04:20 > 0:04:25Then, in 1927, two living Komodo dragons were sent to Europe.
0:04:27 > 0:04:30Although they clearly could be dangerous,
0:04:30 > 0:04:33they proved to be more gentle and intelligent than expected.
0:04:35 > 0:04:37But it would take 80 years
0:04:37 > 0:04:40before we fully understood the way they reproduce.
0:04:44 > 0:04:48We know from other examples that the reproduction of reptiles
0:04:48 > 0:04:50can be more varied than that of mammals.
0:04:54 > 0:04:58In crocodiles, the sex of the eggs is not genetically fixed,
0:04:58 > 0:05:01but is controlled instead by temperature.
0:05:03 > 0:05:05Those incubated at warm temperatures
0:05:05 > 0:05:09hatch as males, and those in cooler conditions, as females.
0:05:15 > 0:05:18But the sex of an unhatched Komodo dragon
0:05:18 > 0:05:21is determined in a different way.
0:05:23 > 0:05:24The fact the Komodo dragon eggs
0:05:24 > 0:05:27can develop without fertilisation was a
0:05:27 > 0:05:29surprising and exciting discovery.
0:05:29 > 0:05:34But interestingly, all the babies that hatched were males.
0:05:34 > 0:05:36Why should that be?
0:05:36 > 0:05:38Well, this is how it works.
0:05:38 > 0:05:43A female Komodo dragon has two different sex chromosomes,
0:05:43 > 0:05:46a W and a Z.
0:05:47 > 0:05:53And the male has two similar chromosomes, a Z and a Z.
0:05:53 > 0:05:59If there are no males, only the female WZ pair remain.
0:05:59 > 0:06:03In such a case, the female divides her own egg cell
0:06:03 > 0:06:06into two halves, one of which
0:06:06 > 0:06:10has a W chromosome and the other, a single Z.
0:06:10 > 0:06:15They then duplicate themselves to form a WW
0:06:15 > 0:06:17and a ZZ.
0:06:17 > 0:06:19In the Komodo dragon,
0:06:19 > 0:06:23a WW combination is not an operative pair,
0:06:23 > 0:06:27so only the male, ZZ, will hatch.
0:06:27 > 0:06:32Thus, female Komodo dragons can produce their own males.
0:06:34 > 0:06:39This seems almost unbelievable, but when you come to think about it,
0:06:39 > 0:06:43it's a very useful ability for an animal that lives on a small island.
0:06:43 > 0:06:45Komodo dragons are descended from
0:06:45 > 0:06:50lizard-like ancestors that lived over 40 million years ago in Asia.
0:06:50 > 0:06:52They migrated to Australia
0:06:52 > 0:06:54and later reached the islands of central Indonesia,
0:06:54 > 0:06:57either by swimming, or by drifting
0:06:57 > 0:07:00across the ocean on floating vegetation.
0:07:00 > 0:07:02Parthenogenesis would enable a
0:07:02 > 0:07:04single female arriving on an island to start
0:07:04 > 0:07:07a breeding population all by herself.
0:07:10 > 0:07:12Nobody knew that Komodo dragons
0:07:12 > 0:07:15could breed asexually, before lone females
0:07:15 > 0:07:18hatched fertile eggs in captivity.
0:07:18 > 0:07:20In the wild, it's virtually impossible to know
0:07:20 > 0:07:22if a female has mated
0:07:22 > 0:07:25with a male and there are usually males around.
0:07:25 > 0:07:29In most circumstances, sexual reproduction is preferable.
0:07:29 > 0:07:34A mix of male and female genes can enable the repair of DNA
0:07:34 > 0:07:36and prevent unwanted mutations.
0:07:36 > 0:07:39Such genetic variation also helps animals
0:07:39 > 0:07:42to adapt to changing environments,
0:07:42 > 0:07:46so sexual reproduction seems to make more biological sense
0:07:46 > 0:07:50than parthenogenesis, and it should be rare in the wild,
0:07:50 > 0:07:52an extreme last resort.
0:07:52 > 0:07:54Strangely, that's not always so.
0:07:56 > 0:08:02In 2012, odd breeding behaviour was noticed in two species of snake,
0:08:02 > 0:08:04copperheads and cottonmouths.
0:08:04 > 0:08:08Some females were reproducing by parthenogenesis
0:08:08 > 0:08:11even though males were present.
0:08:11 > 0:08:15These females were often small, and overlooked by the males,
0:08:15 > 0:08:18so rather than not breed, they cloned themselves.
0:08:20 > 0:08:24But this kind of breeding is potentially a genetic dead-end.
0:08:24 > 0:08:27If individuals all have the same genes,
0:08:27 > 0:08:29the species can't react to a changing world.
0:08:32 > 0:08:36For whiptail lizards, which live in a harsh but very stable desert,
0:08:36 > 0:08:39being genetically the same was actually an advantage.
0:08:41 > 0:08:45For them, parthenogenesis is better than sexual reproduction
0:08:45 > 0:08:48as it prevents them from varying from their winning formula.
0:08:52 > 0:08:56Strangely, the females still go through the motions of mating.
0:08:59 > 0:09:01This stimulates their hormones,
0:09:01 > 0:09:03but these lizards are taking a gamble.
0:09:04 > 0:09:07If their environment changes for the worse,
0:09:07 > 0:09:11they'll be unable to adapt, and so they risk extinction.
0:09:13 > 0:09:16Clearly, the best survival technique
0:09:16 > 0:09:19is to be able to reproduce in either way.
0:09:21 > 0:09:25Parthenogenesis has enabled isolated dwellers like the Komodo dragon
0:09:25 > 0:09:28to survive by forming breeding populations
0:09:28 > 0:09:30from just a single female.
0:09:30 > 0:09:34More recently, studies of wild Komodo dragons
0:09:34 > 0:09:37have revealed that two thirds of the population is male,
0:09:37 > 0:09:41suggesting that even when both sexes are present,
0:09:41 > 0:09:44asexual breeding is still occurring.
0:09:44 > 0:09:48So, Komodo dragons keep their breeding options flexible.
0:09:48 > 0:09:52It's likely that many animals are breeding by parthenogenesis
0:09:52 > 0:09:56or have the potential to do so, but we just don't know about them.
0:09:56 > 0:10:00Parthenogenesis has been occurring unnoticed for millions of years.
0:10:00 > 0:10:05Here is a natural curiosity that is only just revealing its secrets.
0:10:10 > 0:10:12Next, we meet a tiny animal that
0:10:12 > 0:10:16uses parthenogenesis to be one of the fastest breeders in nature.
0:10:18 > 0:10:22Surprisingly, this lives in our own back gardens.
0:10:26 > 0:10:30In summer, this is not an uncommon sight,
0:10:30 > 0:10:34thousands of aphids mass together on a stem.
0:10:34 > 0:10:35At this time of the year,
0:10:35 > 0:10:40each of them can produce five to ten youngsters in a day,
0:10:40 > 0:10:43and each is the genetic copy of herself.
0:10:46 > 0:10:51So, vast numbers can suddenly appear within a day or so.
0:10:53 > 0:10:55Birds and other insects arrive and prey on them,
0:10:55 > 0:10:58but the aphids usually manage to keep ahead.
0:11:00 > 0:11:04This astonishing ability attracted the attention of early scholars.
0:11:04 > 0:11:06In the mid-18th century,
0:11:06 > 0:11:09a new survey of insects was published in France.
0:11:09 > 0:11:13Its author, Rene Antoine Ferchault de Reaumur,
0:11:13 > 0:11:16expressed surprise that he'd never seen aphids mating.
0:11:16 > 0:11:19Neither had he seen a male.
0:11:19 > 0:11:21He made the revolutionary suggestion
0:11:21 > 0:11:24that they were reproducing without sex
0:11:24 > 0:11:27and invited his readers to help him prove it.
0:11:29 > 0:11:32In the spring of 1740,
0:11:32 > 0:11:36Charles Bonnet, then a young law student from Switzerland,
0:11:36 > 0:11:37took up that challenge.
0:11:40 > 0:11:45Charles Bonnet took a newborn female aphid from its mother
0:11:45 > 0:11:49immediately after birth, and put it in an isolation chamber.
0:11:49 > 0:11:54He placed the aphid on a leaf inside an upturned glass jar,
0:11:54 > 0:11:56and using a magnifying glass,
0:11:56 > 0:12:02watched it from early morning until night for 12 days.
0:12:02 > 0:12:07On the evening of June 1st, 1740, at 7.30pm,
0:12:07 > 0:12:12the female aphid gave birth to a brand-new baby aphid.
0:12:12 > 0:12:19Then, over the next 21 days, she had 94 more female offspring.
0:12:19 > 0:12:21Bonnet had no clue how this could happen,
0:12:21 > 0:12:25but he knew for sure that the aphid had bred
0:12:25 > 0:12:27without any male contact.
0:12:30 > 0:12:33He sent his findings to Reaumur in Paris
0:12:33 > 0:12:36who published this new and important discovery
0:12:36 > 0:12:38of sexless reproduction.
0:12:40 > 0:12:42But how this parthenogenesis worked,
0:12:42 > 0:12:45and why aphids use virgin birth in
0:12:45 > 0:12:48their life cycles was still a mystery.
0:12:48 > 0:12:52And entomologists puzzled over it for many years.
0:12:56 > 0:12:57In the 1830s,
0:12:57 > 0:13:01an entomologist called Francis Walker
0:13:01 > 0:13:02took a great interest in cataloguing
0:13:02 > 0:13:05various small insects including aphids.
0:13:05 > 0:13:09He made more than 13,000 slides.
0:13:09 > 0:13:11Walker collected hundreds of aphids,
0:13:11 > 0:13:14many from Southgate and the surrounding areas of London.
0:13:14 > 0:13:16Here we can see some of them.
0:13:16 > 0:13:20He made successive collections of the same species of aphid
0:13:20 > 0:13:23from the same locality across all the seasons.
0:13:23 > 0:13:26As a result, he found several different forms
0:13:26 > 0:13:29of each aphid throughout the breeding cycle.
0:13:29 > 0:13:33They varied in size and some were wingless.
0:13:33 > 0:13:35That suggested that female aphids
0:13:35 > 0:13:38had a rather extraordinary life cycle.
0:13:40 > 0:13:43It was clear from Walker's study
0:13:43 > 0:13:47that nearly all individual aphids are female.
0:13:47 > 0:13:50But they change in form over the seasons.
0:13:50 > 0:13:54In early spring, when plants are growing, most are without wings.
0:13:54 > 0:13:57With plenty of food on offer, they have no need to fly.
0:13:59 > 0:14:03Later in the season, when overcrowding becomes an issue,
0:14:03 > 0:14:08females are born with wings, so that they can travel to find new food.
0:14:08 > 0:14:11Aphids seem to be able to produce females
0:14:11 > 0:14:13that can exploit every situation.
0:14:16 > 0:14:21Although Walker was prolific, he wasn't always entirely accurate.
0:14:21 > 0:14:23He recorded many aspects of the aphids' life cycles
0:14:23 > 0:14:28but he didn't piece them together to produce the complete picture.
0:14:28 > 0:14:30And then aphid research was taken up
0:14:30 > 0:14:34by another entomologist called George Buckton.
0:14:34 > 0:14:39He chronicled every detail of the complex aphid life cycle.
0:14:39 > 0:14:44In 1883, George Buckton published a monograph
0:14:44 > 0:14:47of British aphids in four volumes.
0:14:47 > 0:14:52He wanted to share his passion for these tiny insects in books
0:14:52 > 0:14:55that he hoped would not be too dry academically.
0:14:55 > 0:14:59Buckton corresponded with many leading naturalists of his day
0:14:59 > 0:15:03to pool together every possible specimen and record of behaviour.
0:15:03 > 0:15:06He was an accomplished artist and produced beautiful,
0:15:06 > 0:15:09accurate drawings from live specimens
0:15:09 > 0:15:13and they, interestingly, show a distinct absence of male aphids.
0:15:13 > 0:15:16"The sexual forms of aphideas," he wrote,
0:15:16 > 0:15:20"are in many species very rarely met."
0:15:22 > 0:15:25Buckton's drawings confirm that aphid populations
0:15:25 > 0:15:27are commonly all-female,
0:15:27 > 0:15:29and that males have been
0:15:29 > 0:15:31almost entirely eliminated from the species.
0:15:33 > 0:15:34For most of the breeding season,
0:15:34 > 0:15:37females only give birth to daughters.
0:15:37 > 0:15:39They don't waste time producing males
0:15:39 > 0:15:41which can't, by themselves, produce offspring.
0:15:44 > 0:15:46So, do aphids need males at all?
0:15:48 > 0:15:50The life cycle of another insect
0:15:50 > 0:15:52would seem to suggest not.
0:15:53 > 0:15:59This wonderful creature is Phyllium giganteum,
0:15:59 > 0:16:01a giant leaf insect.
0:16:01 > 0:16:03It's the largest species of its group,
0:16:03 > 0:16:06and it lives wild in Malaysia.
0:16:07 > 0:16:09Nearly all individuals are female.
0:16:09 > 0:16:13In fact, a male of this species wasn't discovered until 1994.
0:16:13 > 0:16:15They're extremely rare.
0:16:15 > 0:16:17The species, for the most part,
0:16:17 > 0:16:21reproduces itself by parthenogenesis.
0:16:21 > 0:16:25They lay unfertilised eggs that hatch into more females.
0:16:25 > 0:16:28And this method of reproduction has enabled it
0:16:28 > 0:16:30to extend its range dramatically.
0:16:33 > 0:16:37Much like a single female Komodo dragon arriving on an island,
0:16:37 > 0:16:41a lone female stick insect can start a breeding colony
0:16:41 > 0:16:45in a new area even if males never arrive.
0:16:45 > 0:16:48And that's what happened in southern England in 1903,
0:16:48 > 0:16:51when a different species of stick insect arrived
0:16:51 > 0:16:55on vegetation imported from New Zealand.
0:16:55 > 0:16:58Now all-female populations survive
0:16:58 > 0:17:01thousands of miles away from their native home.
0:17:01 > 0:17:04These populations have no males
0:17:04 > 0:17:06and don't appear to need them.
0:17:08 > 0:17:10The females produce fertile eggs
0:17:10 > 0:17:12that survive the cold winters and
0:17:12 > 0:17:14new females hatch out in spring.
0:17:16 > 0:17:21But without males, the population could become dangerously inbred.
0:17:23 > 0:17:26Aphid populations face the same problems,
0:17:26 > 0:17:29but most species have a twist in their life cycle
0:17:29 > 0:17:31that freshens up their gene pool.
0:17:32 > 0:17:35In the autumn, the aphid production line switches from producing just
0:17:35 > 0:17:41asexual females to producing sexual males and sexual females.
0:17:41 > 0:17:43At the end of the season,
0:17:43 > 0:17:46as the food supply wanes and the temperature drops,
0:17:46 > 0:17:50there's a phase of sexual reproduction that produces eggs.
0:17:50 > 0:17:52These eggs will overwinter,
0:17:52 > 0:17:55to produce next spring's new aphid generation.
0:18:00 > 0:18:05Aphids don't produce their eggs until the autumn.
0:18:05 > 0:18:08However, most populations survive until then
0:18:08 > 0:18:09because in many cases,
0:18:09 > 0:18:13they form a relationship with another insect - ants.
0:18:15 > 0:18:18An aphid feeds by piercing the stems of plants
0:18:18 > 0:18:21and drinking the sugary sap.
0:18:21 > 0:18:25But sap contains far more sugar than the aphids can use,
0:18:25 > 0:18:29so they excrete the excess as honeydew.
0:18:29 > 0:18:33This is perfect food for the ants and they keenly farm the aphids
0:18:33 > 0:18:35to harvest the rich liquid.
0:18:36 > 0:18:39And in return, the ants protect the aphids
0:18:39 > 0:18:42from insects that try to prey on them.
0:18:43 > 0:18:45So, with ants guarding them,
0:18:45 > 0:18:47the aphids have a good chance of surviving
0:18:47 > 0:18:49until the end of the year
0:18:49 > 0:18:51when they produce their eggs.
0:18:59 > 0:19:04In spring, new females will emerge from the eggs and start once more
0:19:04 > 0:19:08to produce new versions of themselves over and over again.
0:19:20 > 0:19:24And aphids have a final almost unbelievable twist
0:19:24 > 0:19:26in their life cycles
0:19:26 > 0:19:29that greatly speeds up their breeding.
0:19:29 > 0:19:31They do something truly astounding.
0:19:36 > 0:19:37Even before they are born,
0:19:37 > 0:19:41they have embryos developing inside their bodies.
0:19:41 > 0:19:45Parthenogenesis, combined with this telescoping of generations,
0:19:45 > 0:19:49give aphids an extremely rapid turnover of generations.
0:19:49 > 0:19:50Like tiny Russian dolls,
0:19:50 > 0:19:54they just keep popping out smaller copies of themselves.
0:19:55 > 0:19:58A newly-born summer aphid has, inside her body,
0:19:58 > 0:20:01her own developing daughters
0:20:01 > 0:20:06who in turn contain her fully formed unborn granddaughters.
0:20:06 > 0:20:11So, several generations of aphid overlap in time and space,
0:20:11 > 0:20:13and in one season, a single female can produce
0:20:13 > 0:20:17thousands upon thousands of cloned females.
0:20:19 > 0:20:24Aphids' lives are varied, often complicated and truly amazing.
0:20:24 > 0:20:26They can change plant host,
0:20:26 > 0:20:30change their form and alter their method of reproduction.
0:20:30 > 0:20:34In the spring, females hatch some eggs and produce
0:20:34 > 0:20:37several generations of wingless females.
0:20:37 > 0:20:40Their numbers grow and they produce winged females that can fly to
0:20:40 > 0:20:45new food and rapidly produce even more females.
0:20:45 > 0:20:49In the autumn, the sexual forms of both males and female appear,
0:20:49 > 0:20:53which mate and lay eggs, which then can survive the winter.
0:20:59 > 0:21:01The ability to breed by parthenogenesis
0:21:01 > 0:21:04seems almost magical to us,
0:21:04 > 0:21:07but in nature, virgin birth is not uncommon.
0:21:07 > 0:21:09Having the ability to produce daughter clones
0:21:09 > 0:21:12or more males can save a species
0:21:12 > 0:21:14or create a new one.
0:21:14 > 0:21:17Flexible ways of breeding have allowed creatures
0:21:17 > 0:21:19to colonise new areas and
0:21:19 > 0:21:23survive in small communities like those on islands.
0:21:23 > 0:21:27The Komodo dragon has certainly survived for many centuries,
0:21:27 > 0:21:31and aphids have been around for over 200 million years.
0:21:31 > 0:21:36So, parthenogenesis is a breeding strategy that is a real life-saver.