Expandable Bodies 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 lifecycle

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 particularly fascinating.

The bodies of some animals stretch and shrink in extraordinary ways.

Constrictor snakes can swallow prey twice their own size.

While the camel's hump can almost double in weight,

giving it the energy to travel huge distances across deserts.

What is the secret behind such expandable bodies?

We've long been fascinated by the camel's ability

to live in the harshest of deserts.

Places where, during summer,

temperatures can soar up to 50-degrees Celsius.

While, in winter, they can drop to 30-degrees below freezing.

With little in the way of food or water,

camels can sometimes go without eating or drinking for over a week.

Most other animals couldn't survive conditions like this.

How does the camel do it?

The camel's secret was thought to lie in its hump.

In a healthy camel, it can be big and firm like this one

and can weigh as much as 30 kilos,

which is the weight of a ten-year-old child.

But if the camel goes without food and particularly water

for any length of time, then the hump can get floppy

and even droop over on one side, as that one has done.

So people used to think that the camel stored water in its hump.

In fact, there are two different kinds of camel.

The one-humped or dromedary and the two-humped or Bactrian.

Nearly all camels alive today

are the domesticated descendants of one or the other.

The wild dromedary almost certainly doesn't exist.

And only a few Bactrian camels remain roaming the deserts

of central Asia.

The camel is a very tough animal

but, in the wild today, it's rarer than the giant panda.

It's hard to say where the idea of a water-storing hump came from.

The Ancient Romans were the first to suggest

that the camel may have a built-in water reservoir.

And then, later on, people got the idea that it had two stomachs,

one for food and one for water.

In the 18th century, an eminent anatomist, John Hunter,

decided to investigate the truth behind these assertions

and he dissected a camel.

He found that the stomach consisted of three or four compartments,

similar to those of a cow or a sheep.

But inside one of those compartments,

he discovered these pocket-like structures,

which are not found in any other large mammal.

Hunter didn't know what the pockets were for.

But others after him proposed

that they were special water-storage cells.

And then, despite any kind of evidence to prove that this was true,

for another 250 years, books on natural history,

like this one, featured illustrations

of water-storage cells in the camel's stomach.

We now know that that's not true,

even though we don't know exactly what the strange pockets are for.

But the camel's hump is certainly not filled with water.

It's made entirely of fatty tissue.

It is, in fact, an energy reserve for times when food is scarce.

And it can expand to such a degree

that it makes up 80% of the camel's body fat.

This enables a camel to go for two weeks without feeding, if necessary.

But there's a twist to the story.

When fat is broken down in the body,

it produces not just energy but also water.

In fact, each gram of fat broken down during metabolism

produces one gram of water.

So could the camel's hump provide it with extra water after all?

A fatty hump that contains both food and water

would seem to be just what a desert animal needs.

But it's not as simple as that.

To consume its fat, an animal needs more oxygen,

so it has to breathe more.

So when living on the fat in its hump, the camel

actually loses more water through its airways than it gains.

So the camel doesn't have a secret store of water.

How then can it survive in a waterless desert?

Camels can go without drinking for more than a week because

they have an extraordinary ability to retain their body moisture.

We ourselves lose over a litre of water a day

through our moisture-laden breath.

But the camel has nostrils which it can shut tight

and that not only keeps out the sand

but retains the breath within the nose and there the moisture

can be reabsorbed by the linings of the nostrils.

Most mammals also lose a lot of water

when they cool their bodies by sweating.

But camels can endure a rise in body temperature

that would kill most other mammals without sweating.

If our temperature goes up by as little as one degree,

it's a sign of illness.

While three degrees causes vital organ damage and eventually death.

The camel can cope with as much as a six-degrees rise with no ill-effect.

This means that camels don't have to sweat

until conditions get very hot indeed and, if necessary,

they tolerate losing more of their body water than other mammals.

When animals become dehydrated, their blood becomes thicker

and more difficult to pump through the body.

If we lose 10% of our body water, we start to go dizzy and blind

and at 15% our internal organs start to fail.

Camels, however, can lose a third of their body water with no ill-effect,

something that would kill most other animals.

How do they do it?

Well, some of the answers may lie in the shape of their blood cells.

These are the red blood cells from a human being,

which are disc-shaped like that of most mammals.

These, on the other hand, are from a camel

and are slimmer and more oval in shape.

It may be that the oval streamlined shape

makes it easier for the blood to flow when the animal is dehydrated.

Certainly, a camel's blood is less thick and sticky than ours.

The cells also have particularly strong walls.

This prevents them from rupturing

when the animal suddenly drinks large amounts of water.

And when they do find water,

camels have the ability to drink it very quickly.

A single camel can take the contents of all these bottles,

that's 100 litres, in a mere ten minutes.

For any other animal to do that, it would be extremely dangerous.

But the camel has the ability to hold the water in the stomach

and only release it into the bloodstream very slowly

in a way that does no damage.

We now understand how camels can survive harsh desert conditions

and yet, surprisingly, new research suggests

that they may first have evolved to live in the cold Arctic.

Scientists have recently discovered the fossil bones

of giant shaggy camels that roamed the forests of the Canadian Arctic

some three-and-a-half million years ago.

The Arctic camel was a third larger than the modern Bactrian.

But, otherwise, looked very similar and that may be no coincidence.

The wide flat feet that stop the camel from sinking into desert sand

could also have helped its ancestors walk in deep snow.

And a fatty hump provided the food reserve

a camel would need to survive long cold winters.

We may never fully understand the mysteries of the camel's hump,

whether it evolved first as a way of keeping warm or staying cool.

But we have unravelled many other mysteries of the animal's body

that enable it to endure conditions

that few other animals would be able to withstand.

The camel's expandable hump was a mystery to us for centuries.

Our second curiosity can stretch its body in even more extraordinary ways

and devour prey many times its own size.

This is a green anaconda, one of the largest snakes in the world.

It's about four-metres long and weighs 70 kilos

and it's only a half grown.

They can grow to a length of six metres

and weigh twice as much as this one.

But it's their ability to be able to swallow enormous prey

that's really grabbed our imagination.

Could one of these really bite a man

and swallow him whole and alive?

In the 16th century, European explorers

venturing into the Amazon jungle

were fascinated by tales of a huge river monster.

It was said to devour cattle and deer

and to spit out water like shot from a cannon,

knocking animals out of trees.

These fantastic stories led people to go in search

of this marvellous beast.

In 1907, a British explorer, Colonel Percy Fawcett,

claimed to have encountered an enormous snake on the Amazon River.

"A huge head," he said, "rose up from the water

"dangerously close to his canoe and a colossal anaconda emerged."

Greatly alarmed, he shot the snake dead.

He claimed that, when measured, it proved to be nearly 19 metres,

over 60-feet long.

But Fawcett's account was met with disbelief

and he never provided convincing proof because, soon after that,

he vanished into the Brazilian jungle and was never seen again.

The creature that Fawcett encountered

was almost certainly a green anaconda.

Despite their massive proportions, these huge snakes are seldom seen

because they spend most of their time in water

waiting in ambush for their prey.

In this murky world, they're certainly well camouflaged

and so some people believed that somewhere

another real monster might still be lurking unseen.

In the 1960s, a snake was brought to the Museum of Zoology

at the University College London. This is it.

It had lived in London Zoo for some years before it died

and it was five-metres long.

A lot of work went into preparing the skeleton,

it had to be carried out onto the flat roof of the museum.

And it was finally displayed in this rather unusual way,

wrapped around the branch of a tree.

For years, the museum displayed it as an anaconda.

But, in 2012, a member of the public saw an old photo of the snake

on the museum's website and pointed out that it looked like

an African rock python and not an anaconda.

It's unclear how the mistake came about.

The markings on the two snakes are quite different.

But both are giants. And there is much controversy

as to which species is the largest snake of all.

Anacondas, pythons and boas, like this one, don't kill with venom,

they're constrictors, they squeeze their prey to death.

And their coils can exert a very strong pressure indeed,

as I can feel with this one on my arm.

But a big anaconda can squeeze with a force of around 4,000 kilos,

that's like having a bus on your chest and that can certainly

crush the spine of a deer or a capybara.

And yet, constrictor snakes don't usually crush their prey.

In most cases, they simply squeeze it so hard

that the animal can't breathe.

Every time its prey tries to inhale,

the snake's powerful muscles squeeze harder.

The unfortunate victim then either dies because

its blood can no longer circulate or suffocates.

An anaconda or a python can kill prey that is not only twice

its own body size but many times bigger than its head.

So how does it manage to swallow its victim whole?

Popular folklore has it that anacondas and pythons unhinge

or dislocate their jaws to swallow large prey.

That is not true. They do, however,

have the ability to open their mouths wider than most animals.

Pythons and anacondas have this additional

bone attached to the back of their jaws.

This provides a double hinge at the joint and allows them

to open their jaws extremely wide both downwards and sideways.

In addition, the two sides of the lower jaw are not fused

together but joined by an elastic ligament.

This gives the jaws a lot of stretch

and they can even move apart when the snake is swallowing large prey.

It also allows each side of the jaw to move independently of the other.

When eating a meal, particularly one that is much larger than itself,

the snake can alternately move its jaws on either side of its head

and "walk" its prey into its mouth, even while its victim is still alive.

As the jaws open wide, the snake's elastic skin stretches.

But the mobility of the skull comes with a price.

Many of the joints that in other snakes are solid

have been replaced by mobile ones.

So the skull has less crushing power. As a consequence,

the snake has to use its entire body to overpower its prey.

Getting large prey into the mouth is one problem but how does the snake

push it all the way down the length of its body into its stomach?

This is a Burmese python and it hasn't fed for a long time.

So I'm hoping to give it a little breakfast with a dead rat.

What about that?

Saliva from the salivary glands in the mouth has moistened the prey

so it's easier to swallow.

And now it's moving its jaws, drawing the rat farther down its throat,

until, eventually, the muscles of the flanks take over,

squeezing the prey and pushing against the ribs,

so that it looks as though the snake is, as it were,

crawling around the rat.

And that will continue for some time as the prey is worked down

into the snake's body, until, eventually, it reaches the stomach,

which is around the middle here.

Equally remarkable is what happens inside the snake.

After months of fasting, it has to restart its digestive system quickly.

Within a day, some of the internal organs double in size.

The heart expands, pumping greater volumes of blood around the body.

And special cells in the lining of the stomach produce powerful enzymes

that break down flesh and bones.

And when the prey is entirely digested,

the python's organs return to normal again.

Anacondas and pythons are able to take in

enormous meals in a single mouthful.

But how do they then survive fasting for months on end?

Like all cold-blooded animals, snakes get much of their heat from the sun,

so they need less food to fuel their bodies

and most of what they eat is converted directly into body mass.

Snakes continue to grow throughout their lives

and anacondas get bigger than any other species

because they live mostly in water.

Their massive bodies supported by its buoyancy.

So it's certainly possible that an anaconda

could grow to an enormous size.

But how large can a snake really get?

In 2009, further light was shed on this question

with the discovery of the fossils of a super-snake.

It was given the name Titanoboa

and it suggests that snakes can get very large indeed.

Titanoboa was nearly 13-metres long,

the length of a bus, and must have weighed over a tonne.

It lived around 60-million years ago,

shortly after the extinction of the dinosaurs.

We don't know for sure but it may be that the warmer climate of the earth

at the time allowed cold-blooded snakes to grow much larger in size.

What is certain is that, for at least ten-million years,

Titanoboa was the largest predator on the planet.

Both the camel and the anaconda can withstand extreme periods of fasting.

But it's only by looking inside the camel's hump

and the anaconda's stomach that we've discovered the truth

behind their amazing expandable bodies.