Crocodiles The Wonder of Animals

Planet Earth.

Millions of species.

But a few are special.

Thriving.

Dominating.

These are the opportunists.

The collaborators.

The survivors.

So, what makes these animals so successful?

In this series, we'll delve deep beneath the skin

to reveal the unique features that set some species apart.

New behaviour and the very latest scientific discoveries

will offer fresh insight into the wonder of animals.

Over the past 250 million years, continents have shifted.

ice ages have come and gone...

..and the climate has changed dramatically.

Throughout all of this, one animal group has survived.

It may look like a relic from a prehistoric world,

but I can tell you that appearances can be deceptive.

Beneath the crocodile's hard exterior

lies one of the most efficient and sophisticated

physiologies on the planet.

To see how that has allowed crocodiles to endure

and to succeed, we're going to follow them as they hunt.

From the warming up to the ambush.

And finally, to the kill.

Crocodiles are the largest reptiles on Earth.

And like all reptiles, these cold-blooded giants

use their environment to control their body temperature.

They need to be warm to operate effectively.

So to maintain their optimum temperature

of almost 30-degrees centigrade,

their heavily-armoured body is specialised

to capture energy from the sun.

Enlarged scales run along the back and tail,

maximising the surface area to be warmed.

Each bony scale is densely packed with a network of capillaries.

These tiny blood vessels carry the warmed blood from the scales

throughout the crocodile's body.

This heating system can be so effective

that the crocodiles have to find simple means to cool down.

Like gaping, or entering the water.

By carefully positioning themselves

to take advantage of the sun's warmth,

they reach their optimum temperature.

And this is when the crocodile's senses sharpen.

Colour vision...comes into focus.

Hearing is heightened.

It's now ready to hunt.

In harnessing the sun's heat to warm their bodies,

rather than generating their own, as mammals do,

crocodiles have lowered their energy demand.

This means they can afford to be patient

and wait for the very best feeding opportunities.

Crocodiles use the most efficient hunting method.

The ambush.

It's a strategy which is designed to conserve as much energy as possible.

There's no stalking, no chasing. There is, simply, waiting.

Now, I know it might sound basic, but to pull off the perfect ambush

requires some extraordinary physiology.

June, in the Serengeti.

This time of year marks the seasonal wildebeest migration.

A great herd of up to 2 million animals is on the move,

in search of new feeding grounds.

They stop to drink at watering holes like this as they pass through.

And these Nile crocodiles...are waiting.

With only their eyes, ears and nose above the surface,

they can track their prey's every move...

..without arousing suspicion.

The wildebeests are in their sights.

These stealthy hunters must now disappear completely.

But the very best place to launch an ambush from is beneath the surface.

Taking one last breath, it submerges.

If necessary, a crocodile can remain here for more than an hour.

This is where some of its remarkable physiology comes into play.

Allowing the crocodile to ration its oxygen.

Not just by controlling how quickly it's used...

..but by controlling WHERE in the body it's used.

The crocodile's heart is one of the most sophisticated

in the animal kingdom.

It has an extra aorta,

which allows a crocodile to control where its blood flows.

When submerged, oxygen-rich blood is pumped to the brain

and to the vital organs.

But as oxygen-depleted blood returns to the heart,

an unique valve closes,

preventing that blood from flowing to the lungs.

Underwater, those lungs contain no new oxygen.

Pumping it here would be a waste of energy.

So the heart bypasses the lungs

and recirculates this less-oxygen-rich blood

to the less important muscles.

Crocodiles can remain submerged for so long

because of this ability to pump oxygen where it's needed most.

But to make their single breath last even longer,

they also have specialised blood

which carries and releases more oxygen than any other animal.

Human blood keeps hold of a large percentage of the oxygen it carries.

Even when the body is in a state of hypoxia, or oxygen starvation.

But crocodile blood releases all of the oxygen molecules it carries.

As the crocodile uses up its breath, the blood chemistry changes.

Causing its haemoglobin, the oxygen-carrying molecule,

to give up all the oxygen.

Underwater, the crocodile's ears and nostrils are closed.

And their eyes are protected by an additional eyelid.

Whilst they can still detect shadows and shapes,

for a successful ambush, they need to precisely locate their prey.

And to do it, they have a rather unexpected sense.

It's perhaps somewhat surprising,

considering the heavy-armoured exterior,

but crocodiles have exceptionally sensitive skin.

Buried within each scale,

these tiny black dots are bundles of nerve endings.

They detect touch, pressure

and the most minute vibrations.

There are as many as 9,000 of these sensory organs

across a crocodile's body.

4,000 of them are concentrated around the jaws and teeth alone.

Making the crocodile's jaws 10 times more sensitive to touch

than our own fingertips.

These bundles of receptors are also grouped along

the outermost digits of the forearms.

So that in still water, a crocodile can detect

the vibrations of a wildebeest drinking from 20 metres away.

The crocodile's heart and blood

allow it to remain hidden underwater.

Whilst its skin detects what's going on above the water.

It's this combination of unusual physiology and acute senses

which allow the crocodile

to position itself perfectly for an attack.

Crocodiles have the strongest known bite of any animal on Earth.

More than 60 cone-shaped teeth

pierce and clamp down on their prey.

Exerting over 2,000 kilos of force.

And they don't let go.

With muscular legs and claws for grip,

the crocodile drags its kill into the water and drowns it.

Whilst crocodiles are accomplished predators

in the light and heat of the day...

..they can also turn their extraordinary senses

to ambush in complete darkness.

And capture a very different type of food.

Australia's Northern Territory.

At unusually high tides, this river breaches its barrage.

Introducing a deluge of saltwater into the river system.

With it, a whole tide of grey mullet.

It's a welcome feast for these saltwater crocodiles.

But the fish are fast and extremely difficult to catch.

To snare such fidgety prey in moving water,

they must draw on their sensory arsenal.

For a successful ambush, they each take their optimal position.

Now, saltwater crocodiles are the most aggressive

and territorial of all the crocodile species.

But faced with such a bounty of food,

they are willing to tolerate one another.

Just about.

With water temperatures of 28 degrees,

the crocodiles are warm and ready to hunt.

The darkness is not an issue either.

Because crocodiles have excellent night vision.

Their retinas contain extremely dense concentrations

of light-sensitive cells.

And to maximise the light reaching these receptors,

their normally-slit pupils open wide, into a full circle.

Their vision is also boosted by

a layer of tissue called the tapetum lucidum.

Sitting behind the retina, this reflects the light back

onto its light receptors,

giving these cells a second chance to react.

In the glare of a torch,

this layer glows like a cat's eyes caught in the headlights.

With their mouths gaping, they lie in wait.

The touch receptors on their jaws scanning for pressure waves.

Able to differentiate between the touch of a neighbour

and the movement of a fish.

And when these sensors fire,

the jaws clamp shut in just 15 milliseconds.

Six times faster than you can blink.

Physiology enables the crocodiles to hunt

large, slow-moving prey like wildebeest,

as well as smaller, faster animals like fish.

But it also lets them tackle a very different type of food.

One which poses new challenges

and allows them to feed in a truly surprising way.

Summer in Zambia.

For the predators living around this water hole,

this dead hippopotamus is a feast for the taking.

But suspiciously, it's been left untouched.

An outbreak of anthrax has killed this and many other hippos here.

Contaminated carrion like this would be hazardous to most other animals.

But not the crocodile.

They can eat rotten or diseased meat with little risk.

Proteins in the blood bind to pathogens such as anthrax...

..and kill them before they take hold.

Antibacterial blood is an unique trait,

which opens up a raft of easy feeding opportunities

for the crocodile.

Having smelled the rotting flesh,

more than 40 crocodiles move in for the feast.

Thrashing their powerful tails, they spin themselves in the water.

These death rolls swiftly dismember the carcass.

They tear into their meal.

Chunks of meat are swallowed whole.

With such a powerful grip on the flesh and bone,

crocodiles often lose their teeth.

But it's not a problem. They're quickly replaced.

Up to 3,000 new teeth grow in a lifetime.

Unaffected by this hazardous meal, the crocodiles gorge themselves.

Following a feed, crocodiles' stomachs can contain

up to 25 kilos of prey.

Which is great, but then they've got to digest it.

Once again, the crocodiles have got some ingenious physiology

to meet the challenge.

With their stomachs full,

they return to the banks to find a basking spot.

The first task is breaking down the heavy load

into smaller, digestible pieces.

And for this, crocodiles use an unusual digestive aid.

Throughout their lives, they ingest small stones.

Known as gastroliths, these stones remain

in the first section of the crocodile's stomach.

And they're thought to work in the same way as a bird's gizzard,

to mechanically grind up the contents.

This shared feature may seem odd,

but birds are the closest living relatives of crocodiles.

And the two groups still share common physiology.

Another relative of the crocodiles, the dinosaurs,

also used gastroliths to process their food.

This theropod dinosaur also displays these distinctive stomach stones.

The more-manageable load now passes to the second stomach chamber.

Here, bone, horn and hooves are chemically broken down.

Basking speeds up this stage of digestion.

The hotter the crocodile gets, the more efficient the breakdown.

But the process can still take days.

And whilst the contents sit in the stomach,

they're at risk of putrefying.

Fortunately, this stage of digestion doesn't just rely on the sun.

Crocodiles have the most acidic stomach in the animal kingdom.

And it's all down to their unique heart.

To boost digestion,

the heart directs deoxygenated blood to the stomach.

This blood is rich in acidic carbon dioxide

and stimulates the production of extremely-corrosive gastric juices.

With this remarkable system, crocodiles can secrete stomach acid

10 times faster than any other animal.

Not only does this keep bacteria at bay,

combined with heat, it enables crocodiles

to digest a wildebeest in just a matter of days.

Unique physiology has not only enabled the crocodile to ambush,

kill and eat in the most efficient way possible,

its high-performance digestive system saves them energy, too.

Being cold-blooded and not needing to generate body heat,

crocodiles use energy stores so sparingly

that they can go as long as a year between feeds.

They can do this because they are the most efficient vertebrates

when it comes to converting food into body mass.

In India, it's the height of the dry season.

Lakes are disappearing

and feeding opportunities are few and far between.

Drought conditions are enough to kill even the most hardy animals.

But the crocodile can draw on systems

which enable it to survive where others can't.

This mugger crocodile is not on the hunt for food,

it's on the hunt for shelter.

Crocodiles exploit the world around them to heat up,

but they also use it to cool down.

It's searching for a refuge.

A cool, sheltered den.

The crocodile's final and most unexpected adaptation

is that it can shut its body down

through the hottest times of the year.

This period of dormancy, similar to hibernation in mammals,

is known as aestivation.

Whilst warm-blooded animals hibernate through the cold months,

crocodiles use the same physiological process

to survive the hottest months.

But where mammals struggle to lower their body temperature

and slow their metabolic rate,

crocodiles have no such problems.

They're already cold-blooded and have a low metabolism,

so they can easily conserve resources.

Drawing on its fat reserves which are stored mainly in the tail,

it can lie here for five months without food and water.

Only once the monsoon arrives to replenish the lakes and rivers

will it once more come out to hunt.

This ability to shut down when times are hard

is thought to be the key to the crocodile's exceptional longevity.

Enabling it to avoid the shifts in climate

that have wiped out so many other species over time.

But crocodiles haven't endured unchanged.

They've evolved some of the most surprising physiology

in the animal kingdom.

Physiology which allows them to stay submerged.

To sense the world around them.

And eat and digest practically anything.

And it's these unexpected qualities,

hidden beneath the primitive exterior,

which allow these remarkable predators not only to survive,

but also, to prosper.

And that is the wonder of crocodiles.