David Attenborough examines animals with extreme young. The panda gives birth to the smallest baby of any mammal, while the kiwi lays an egg that is almost too large.
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The natural world is full of extraordinary animals
with amazing life histories.
Yet, certain stories are more intriguing than others.
The mysteries of a butterfly's life cycle,
or the strange biology of the emperor penguin.
Some of these creatures were surrounded by fantastic myths
Others have only recently revealed their secrets.
These are the creatures that stand out from the crowd,
the curiosities that I find particularly fascinating.
In this programme,
I explore the lives of two mothers
who give birth to unusually sized young.
The giant panda,
which, in relation to its size,
produces one of the smallest babies of any mammal.
And, the kiwi,
which lays one of the biggest eggs in the bird world.
Why do pandas and kiwis have babies of such extreme sizes?
Giant pandas are surely one of the most instantly recognisable
of all mammals.
Yet they're also one of the rarest.
Although they once lived over large parts of Central China,
today they're restricted to just six mountain ranges.
Once lowland creatures,
they now live in higher altitudes, in dense forests.
Very little was known about the wild lives of these elusive animals,
and their reproduction remained a mystery for centuries.
The earliest known ancestors of giant pandas
were small forest-dwelling creatures
that existed just over 11 million years ago.
Larger pandas have been around for about 3 million years.
The giant pandas we know today
evolved when bamboo forests were widespread.
With such an easy, reliable food source,
they abandoned their carnivorous ways,
and took to a plant-based diet.
Today, pandas are a huge attraction in our zoos,
but, persuading them to breed and care for their young in captivity,
has been historically very difficult.
Zookeepers were shocked to discover
that a newborn panda baby is one 900th of the parent's body weight.
The smallest of all percentile mammal babies.
But pandas have been a scientific enigma for a very long time.
a French missionary and naturalist called Abbe Armand David
set off on an expedition to China.
He was an expert horticulturist,
and had been commissioned by the Museum of Paris
to bring back plant specimens.
On the 21st of March, while collecting,
he was invited into a local hunter's house for tea and sweets.
He came across a strange, wiry-haired skin,
rather like this one.
He thought it must have come from an unknown species.
So he asked the hunters to bring him
a specimen of this mysterious creature.
After several days, they brought back one that Armand David described
as "a most excellent black and white bear".
Excitedly, he prepared the skin, and then he sent it off to Paris.
Knowing that it might take time to arrive,
he also wrote a letter to Parisian zoologist Milne-Edwards,
urging him to publish a brief description of the animal
for which David proposed the scientific name
of "Ursus Melanoleucus",
literally meaning "black and white bear".
From the very beginning, this new creature seemed odd for a bear.
It had the carnivorous appearance of other bears,
but it's diet was actually almost entirely vegetarian.
It spent up to ten hours a day
feeding on up to 20kg of bamboo.
And unlike other bears, the panda did not hibernate,
and its babies proved to be far smaller
than those of any other bear.
In fact, the panda was so different
that some doubted that it was a bear at all.
A creature called a "red panda" had been discovered some time before,
and it had striking similarities
to Armand David's new black and white bear.
It, too, fed mainly on plant matter,
about two-thirds of which was bamboo.
But this creature was classified as a relative of weasels,
skunks and raccoons, not bears.
Perhaps the giant panda was not a bear after all.
This could explain why its young was so small,
compared to most other bears.
Milne-Edwards, the Parisian biologist
who received the very first giant panda skin and bones,
compared them to his specimens of red panda.
He believed that the skull structure and the teeth were very similar.
This is the small red panda, and this is the giant panda.
He decided it was a new creature, which deserved a new name,
so he called it "Ailuropoda", meaning "panda foot".
Thus it became known as a panda, and not a bear.
Debate and confusion continued over the panda's identification
for nearly 100 years.
Few people had ever seen more than a fleeting glimpse of one,
and their wild behaviour remained a mystery.
Then, in the 1920s,
exploration became very popular amongst the wealthy.
And the race was on for the first foreigner to hunt and kill a panda.
It's said that Theodore Roosevelt Jr and Kermit Roosevelt
were the first Westerners to shoot a panda.
They persuaded the Field Museum in Chicago to foot the bill
for an expedition,
and were secretive about the "golden fleece" that they were hunting.
After six days of tracking in the same area where Armand David
had first found his panda, they saw nothing.
But after moving further south,
they had a dramatic encounter with a panda that they followed
and shot dead.
Sadly, then, the driving force to collect giant pandas
was money and fame, not biological revelation.
The only way to learn anything more about the giant panda
was to watch one in the wild, or to catch one alive.
In 1936, a baby panda was captured alive.
Named Su Lin, she was the first to be brought into captivity,
but sadly died soon after.
A craze for captive pandas followed.
And in the late 1950s, one arrived in Britain.
This particular individual
would help us to appreciate the complexities
of the giant panda's biology.
Perhaps the most famous and popular of all giant pandas was Chi-Chi,
who came to London Zoo in September 1958.
She was actually on her way to the United States,
but the US Customs refused to admit her
on the grounds that she was a communist,
or, at any rate, came from a communist country.
So, London Zoo was able to buy her for £12,000,
and she was very quickly extremely popular.
Desmond Morris, who was in charge of London Zoo's mammals at the time,
decided, however, that she was alone
and she really ought to be allowed to breed.
Don't you want to go to Moscow?
Here, at last, was a chance to learn more about panda reproduction.
Desmond Morris travelled to Russia with Chi-Chi
to introduce her to a potential mate, a male panda called An-An.
But when they were introduced, all did not go to plan.
Chi-Chi was in no mood to breed, and was sent back home.
Clearly, panda mating was not a simple affair,
and it was a rare sight in the wild, too.
Now we know that successful mating needs very precise timing.
Female pandas live a solitary life,
and are only ready to mate for just one or two days a year.
Even then, there is a window of 12 to 24 hours.
It's little wonder that Chi-Chi did not breed.
Males are attracted to the female's scent,
and will guard them until they're ready to mate.
A female in season is a rare thing,
and competition to mate is worth fighting for.
GROWLING, BARKING SOUND
The panda was gaining a reputation for having unusual and difficult
breeding habits, and its peculiar diet seemed to be responsible.
In the 1960s, biologists took a fresh look at the giant panda.
This time, they studied the panda's digestive system,
and discovered that it was exactly like that of a carnivorous bear.
So, the giant panda was reclassified,
and changed from being a relative of the red panda, to being a true bear.
This also revealed that the giant panda gut was unsuited
to its plant-based diet,
and that this oddity might affect its metabolism and breeding.
Female bears feed on rich food
to build up fat reserves for motherhood and hibernation.
They then give birth to up to four babies,
and produce enough milk to feed all of them.
The well-grown cubs emerge from the den in early spring.
Panda reproduction has significant differences.
They don't have enough fat reserves to hibernate,
and usually produce only one small baby at a time.
Their poor vegetarian diet
seems to have had an impact on their breeding.
Bamboo presents a lot of problems as a food.
To start with, it's very low in energy.
Secondly, the panda has to sit upright
in order to release its front paws, in order to handle the bamboo.
On top of that, the panda's gut is very short,
like that of a carnivore,
so that the food, when it's eaten,
passes through its body very quickly.
As a consequence of all those difficulties,
the panda only manages to extract about 20% of the little energy
that bamboo does contain.
So, the Panda's ancestors switched from being meat eaters
to plant eaters, and this compromised their digestive systems
and greatly affected their metabolism.
They became slow-moving,
and their breeding changed to cope with such a low-energy diet.
In the late 1960s,
efforts to understand panda reproduction became more crucial
as their numbers in the wild plummeted.
The Worldwide Fund for Nature was formed,
and their famous logo was a panda based on Chi-Chi.
The Chinese built a state-of-the-art reserve in Wolong,
leading to a new era of great progress in panda breeding.
Small babies weighing an average of just 100g
are now regularly born in captivity, and are fed on milk for many months.
On a poor diet of bamboo,
pandas are unable to grow bigger babies in the womb,
so they give birth to small young, and use their limited nutrition
to produce food for them after birth.
As with all mammals,
milk is essential to the baby's development,
and ensures even the tiniest babies grow up to be giants.
So, the giant panda is not a racoon, it's a bear.
A bear that spends nearly all its time eating vegetation,
and that's nearly always bamboo.
Which, although it can occasionally produce twins,
normally gives birth to just one baby at a time.
And that a very small one.
But those are the consequences if you are a bear
that has become adapted to living on a very low-energy diet.
The panda's tiny baby is an oddity,
but the only solution for a bamboo-eating bear.
In New Zealand,
there's a very different creature that has just as curious a story.
The kiwi is one of the strangest of birds.
It sleeps underground, and usually only comes out at night.
It can't fly, and its brown feathers resemble a thick coat of fur.
Its small eyes are virtually useless and it finds its food
with its sensitive beak.
It's a peculiar lifestyle, more like that of a nocturnal mammal.
But most remarkable of all, it lays the biggest egg of any bird
in proportion to its body.
A kiwi is roughly the size of a chicken.
But its egg...
..is more than seven times as large as a chicken's egg.
And it can weigh half a kilo.
It's hard to imagine how this huge egg
could fit into a kiwi's small body.
And, yet, it does.
Just before the egg is laid,
it takes up so much room inside the female
that her belly almost touches the ground.
And when she lays it, it's equivalent, in terms of weight,
to a human mother giving birth to a four-year-old child.
Most birds only take around a day to produce an egg.
But because the kiwi's is so large, it takes almost ten days.
The female's inner organs become so compressed, she can't feed.
Expelling the monster egg is also a huge effort.
Why is the kiwi such a curiosity?
And why does it lay such a gigantic egg?
The kiwi didn't come to the attention of Europeans
until about 200 years ago, when a dried specimen, much like this one,
arrived in England on a merchant vessel.
It puzzled those who saw it.
It was clearly a bird, but it had no wings.
Its feathers were soft and hairy, more like mammalian fur.
And it had these strange, long whiskers
around the base of the beak.
The first specimen was examined and described by a naturalist
at the British Museum, a man called George Shaw,
who gave it the scientific name Apteryx, which, in Greek,
means "wingless creature".
Shaw studied the skin, together with his colleague John Latham,
but the two men disagreed as to what kind of bird it could be.
They knew it had come from New Zealand,
and Shaw thought it was probably related to the ratites,
a group of primitive flightless birds that includes the ostrich.
Latham, on the other hand,
was convinced that it was a kind of penguin.
When Shaw published his description in 1813,
it was accompanied by an artist's impression of the living bird.
This is it.
Clearly, the artist must have been swayed by Latham's argument,
rather than Shaw's.
He shows the kiwi standing bolt upright and very tall,
much like a penguin.
And so the kiwi was introduced to the scientific world.
Shaw's kiwi continued to provoke debate long after his death.
The most eminent zoologists of the time disagreed
over the nature of the strange creature
and, indeed, whether it actually existed.
It's not surprising that many wondered if the kiwi was a hoax.
It was a time when travellers were bringing back all kinds of strange
creatures from far-flung places, and many were frauds,
put together from parts of different animals.
Almost 20 years later,
and with only one specimen on which to make a judgment,
the Zoological Society of London made an appeal for more kiwi skins.
So, other specimens finally began to arrive in Britain.
European naturalists may have been mystified by the kiwi,
but the Maori people of New Zealand
had admired and respected the bird for a very long time.
According to Maori legend,
the kiwi lost its wings at the request of Tane,
the god of the forest.
Tane asked all birds to go down to live on the forest floor
and feed on the insects that were killing the trees.
But only the kiwi agreed, and gave up his wings and beautiful feathers.
So, the kiwi has always been sacred to the Maori.
Back in Europe, others now joined in the debate.
Professor Richard Owen,
the most powerful British zoologist of the time,
studied the anatomy of the kiwi in detail.
Comparing its features to those of other birds,
he concluded that it was most closely related
to that group of flightless birds called the ratites.
The ratites include the largest birds in the world -
the emu, the South American rhea,
the cassowary, and the ostrich.
All of them stand nearly as tall as a human being.
So, could the kiwi's large egg have anything to do
with its possible relationship to these larger birds?
To answer that, we need to look at its close relatives.
The emu lives nearby in Australia.
It has remnants of wings, but it can't fly.
And its feathers are similar to those of the kiwi,
hairy and plume-like.
They simply serve to protect the bird, and keep it warm.
So, how similar are the emu and the kiwi when it comes to their eggs?
This is the egg of a kiwi.
And this is the egg of an emu.
More or less the same size.
And, yet, the kiwi is the size of a chicken,
but an emu is almost as tall as I am.
Why should such a big egg come from such a small bird?
Well, for a long time,
it was argued that that was because the ancestors of the kiwi
were once as big as the emu and, over time,
they got smaller, but the egg remained the same size.
And the originator of that theory was, in fact, Richard Owen himself.
In 1839, Owen acquired the fragment of a strange bone from New Zealand.
After studying it closely, he suggested it came from a gigantic,
flightless bird that was probably extinct.
From this meagre evidence, he reconstructed the entire animal,
a giant moa.
Owen was ridiculed by other scientists at the time,
who considered such a deduction on one bone outrageous.
But in due course, other moa birds were found,
and he was proved to be correct.
Owen's discoveries seemed to confirm the idea
that the kiwi could have evolved from a big bird like the moa,
and that maybe its egg was a relic from a giant ancestor.
Large, flightless birds first appeared
when the dinosaurs became extinct.
This is a southern cassowary.
It's a native of northern Australia and New Guinea.
And the males, like this one,
are extremely territorial and, therefore, dangerous.
They will attack you, as I know to my cost.
So, I'm not going to get in there with him.
Instead, I'll see if I can tempt him
with a few grapes, which are one of his favourite foods.
Like the kiwi, the cassowary evolved
in an area where the adult birds have no ground predators.
As a consequence, they don't fly.
Flying is a very energy-demanding business.
If birds don't need to fly, birds don't fly.
Until recently, it was thought that all the ratites
had one common flightless ancestor.
This seemed possible because the places where they live today
were once part of a supercontinent called Gondwanaland.
When this continent split up around 150 million years ago,
the fragments drifted apart.
Each one might independently have evolved its own flightless species,
including New Zealand.
When Owen came to examine the skeleton of a kiwi,
he noticed something very strange about the skull.
Most bird skulls have two little tiny holes there
at the base of the beak, which accommodates the nostrils,
through which they smell.
But there are no such things here on the kiwi skull.
Instead, the nostrils are right at the tip of the beak.
Not only that, but these big spaces on either side the skull,
which in most birds hold the big eye,
are, in fact, filled by the olfactory organ, the smelling organ.
And Owen deduced from those two facts that this, therefore,
must belong to a bird that was nocturnal.
And he was quite right.
The kiwi is mostly active at night,
and uses both touch and smell to find its food.
The long whiskers allow it to feel its way in the dark,
and special sensory cells in the beak
detect the movement of prey underground.
But why did the kiwi choose this unusual lifestyle?
It's possible that the moas had already taken the role
of giant plant eaters during the day,
so the kiwi may have shrunk down to feed on small insects at night.
Owen had shed light on both the moa and kiwi,
but he was wrong about their true relationship.
Evidence from DNA has now revealed that the kiwi is, in fact,
more closely related to flightless birds of Africa and Australia.
This means that the moa and the kiwi had different ancestors,
and flightlessness must have evolved in New Zealand
on two separate occasions.
It's an extraordinary thought.
But another recent finding supports the idea.
Genetic techniques have shown that the closest relative of the ratites
is, in fact, a small flying bird, the tinamou.
Tinamous are partridge-like birds from South America
that spend much of their time on the ground,
but they can fly perfectly well.
So, it seems that birds like this may have flapped their way
between the continents, giving rise to the different ratites,
including the kiwi.
We've unravelled much of the mystery around the kiwi's curious lifestyle,
but one question remains.
What could be the reason for its huge egg?
Some think that the large egg may give the kiwi a competitive edge,
by allowing it to hatch a chick that is already very well developed.
It's like a miniature adult,
and the large yolk sac provides nourishment
until it becomes fully independent.
So, it seems that the kiwi's gigantic egg may have evolved
to suit its lifestyle and habitat.
Most birds have to lay their eggs as soon as possible
to avoid being weighed down when flying.
But the flightless kiwi has no such problem, and can, therefore,
keep the heavy egg in its body for longer, and let it grow bigger.
And in an environment with few predators, it may make sense to,
as it were, put all your eggs in one basket and raise a single chick
that is big and strong,
and therefore has the better chance of survival.
The kiwi and the panda both produce young that stand out
because of their size,
but are a perfect fit for the life choices of these curious creatures.
The giant panda gives birth to the smallest baby of any mammal and has to care for and protect it for many months. Why don't they give birth to more developed, robust young? The kiwi lays one of the largest eggs in the bird world, which produces a very well-developed chick. Why do kiwis produce a single egg that is a quarter of its body mass and almost too big to lay?