Crocodiles and alligators are among the world's great survivors.
They've been around in one form or another for over 200 million years.
And in that time they've weathered the worst the planet could throw at them.
These ancient animals seem almost indestructible.
But where do they get their extraordinary staying power?
What makes crocodilians such phenomenal survivors?
The crocodilians are the most deadly inland water predators on Earth,
the best known of the reptilian giants.
Crocodilians are built to last.
Clues to their success lie in their history, their behaviour and the way their bodies work.
How has King Croc held his throne so long?
The crocodilians dominate the world's tropical freshwaters and tidal estuaries.
They include the gharials, crocodiles, alligators, and cayman.
23 species rule the swamps, rivers and lakes of Asia,
Africa, Australia and the Americas.
They come in many shapes and sizes,
but are modelled on the same basic and highly durable design.
Around 100 million years ago the ancestors of living crocodilians
hit on the ideal blueprint for a water-based stealth predator -
a winning formula that their descendants still exploit today.
Their elongated, streamlined body can slide silently and smoothly through the water.
Their huge muscular flattened tail provides a powerful thrust for swimming.
A stealth hunter relies on keeping a low profile.
A croc can travel fast without revealing its presence.
Short legs can be tucked in tight against the body to minimise drag,
or splayed out to act as stabilisers.
Beneath their primitive-looking exterior
lies a crucial mix of flexibility and strength.
The vertebrae in the spine are shaped for maximum mobility.
Special interlocking processes in the neck vertebrae increase their strength and rigidity,
helping the croc to hold on to large, powerful prey.
The shape of their head and back allows them to break the water surface with barely a ripple.
Most of the body remains concealed underwater
but eyes set high on the head are a periscope on the world around.
As aquatic predators, crocodilians need to be able to open their mouth underwater
without risk of drowning.
When their head is submerged, a valve at the back of the tongue
completely seals the throat and stops water pouring down into the lungs.
In addition, air tubes open behind this valve and run to the nostrils, enabling crocodilians to breathe
as they lie concealed below the water surface.
Crocodilians combine protective armour with great sensitivity.
Their scales are studded with tiny pressure receptors
that pick up the minutest disturbance in the water.
Even a falling seedpod can trigger a response.
At over six metres long, this is the largest of all modern crocodilians,
the Australian saltwater crocodile.
For a water-based predator fruit bats are tricky prey.
The crocodile's best tactic is an ambush attack.
In the murky waters, it repositions
and waits for the right moment to strike.
It can afford to be patient,
thanks to one of the most sophisticated elements of its design.
During a dive, crocodilian blood is more efficient
than that of any other animal at delivering oxygen to the tissues,
and a special valve in the croc's heart can divert blood from non-essential body areas
to where it's needed most.
By reducing its heart rate and rationing its oxygen,
the croc can hold its breath for an hour or more,
and play a waiting game that would kill us.
Underwater, a transparent third eyelid closes to protect the eye.
Back at the surface, full sight is restored.
130 degrees of widescreen vision
means the croc can monitor the slightest movement.
And a binocular view lets it judge distance accurately.
All these adaptations help to make the crocodilians masters of the surprise attack.
The muscular tail provides power and drive to launch the all-important strike.
Crocodilians are at the pinnacle of amphibious predator design.
But this alone is not enough to explain their enduring success.
Something else has helped crocodilians flourish
in the largely poor and unpredictable habitats in which they live -
engine design.
South America's freshwater rivers and lakes are home to many kinds of animal.
Like all living creatures, they're powered by internal flesh and blood combustion engines.
But while some engines are slow,
others have a fast idling speed and are tuned for high performance.
Black cayman and giant otters share the same space
but their very different body engines dictate very different ways of life.
The basic mechanics, however, are the same.
Oxygen breathed in with air is carried in the blood to all the body's cells.
Inside each cell this oxygen reacts with digested food,
the body's fuel, releasing energy.
Some energy is spent on growing and getting around.
Some is stored inside muscles for future use
And some is converted into heat.
The faster, high performance engines of the giant otters
burn a lot of fuel quickly, and generate a lot of heat.
So much heat that the otters can run an internal heating system,
which keeps their bodies permanently warm.
It's why they, like all mammals, are called warm-blooded.
The caymans' engines work in a similar way,
but with one vital difference.
Their slow idling engines just don't generate as much heat...
..certainly not enough to keep their bodies permanently warm,
which is why cayman, like all reptiles, are referred to as being cold-blooded.
One of the benefits of a high performance engine,
a high metabolism, is a constant, warm body temperature.
Being warm-blooded means that you can live almost anywhere.
You can stay warm whatever the temperature outside.
It's why mammals and birds are able to live anywhere in the world, even in the coldest places.
Cold-blooded reptiles like crocodilians, on the other hand,
have to rely on an external source of heat to rev up their bodies and keep them running properly.
They're solar-powered.
Special bony discs along the crocodilian's back act like solar panels,
absorbing heat and downloading it into the bloodstream.
If a crocodilian's body temperature falls too low it becomes sluggish.
Too high and it risks heat exhaustion, even death.
To warm up and cool down as their body demands, they have to move
between sunlight and shade and from land to water.
Being so dependent on the sun's heat means crocodilians are restricted to the warmer regions of the world.
They simply can't survive anywhere else.
It's the downside of having a low performance engine, a low metabolism, but there is an upside -
fuel economy.
A slow, idling engine consumes less, so fuel needs are slashed.
The fast, gas-guzzling engines of the giant otters burn huge quantities of fuel and oxygen
and they need constant refuelling.
A huge 80% of all the food the otters eat
is turned straight into heat to keep them warm.
So they spend much of their time hunting just to stay alive
and support their high octane, full throttle lifestyle.
But the slow-performance crocodilians focus on fuel economy.
They don't need so much food and they only expend energy when absolutely necessary.
It means they can often afford to wait for food to come to them.
All crocodilians are capable of a last-minute burst of energy to nail their victim,
but unlike a mammal they simply can't sustain such activity for long.
A few minutes and they're completely exhausted.
Crocodilians don't use the majority of their food for heating,
they turn it into more body mass, more croc.
One good kill will last for months.
This strategy has served crocs well in the unpredictable environments in which they live.
In parts of India, rivers, lakes and swamps can swing between extremes of drought and flood.
To survive here you need to be able to make the most of good times and endure the bad.
It's something crocodilians,
like the great marsh mugger crocodiles of India and Sri Lanka, have evolved to do extremely well.
As drought approaches, water levels start to drop.
As lakes and swamps dry up, not only are the crocs unable to hunt for food,
they also lose the cool water that helps them regulate their body temperature.
As his kingdom literally vanishes before his eyes, how will King Croc cope now?
When the situation reaches crisis point,
the muggers leave their shrinking pools and strike out on their own,
on a life and death expedition over land.
It's thought that mugger crocodiles can travel up to 10km each night in search of water.
If necessary, they can lie up in shade during the day.
This is the mugger at its most vulnerable.
Without access to water, it could easily bake to death before its journey's end.
But with no water to be found anywhere, rescue comes in another form -
underground.
Sometimes muggers have to excavate new burrows,
but often, they revisit ones dug in previous years.
Down here it's cooler,
and after an arduous time on the road, the crocodile can, at last, rest.
But with no food available, the mugger is facing starvation.
Its body must begin to adapt.
The mugger crocodile sinks into a state of dormancy called aestivation.
Its breathing begins to slow and its heart rate falls.
Even so, its body engine continues to tick over,
using up water and fat reserves and generating toxic waste products.
Entombed underground,
this sleeping giant has to overcome its body's biological demands.
In ways not yet fully understood,
it copes with not eating and drinking or releasing waste for several months.
It plays the trademark crocodilian waiting game.
Eventually, the rains return.
The mugger senses that the world outside is reviving.
It's time to emerge.
This ability to withdraw from the world when food is scarce has served crocodilians well.
Thanks to their slow performance engine and their low metabolic rate,
they can simply stop eating and live off their reserves.
This biological endurance has helped crocodilians reign supreme
over some of the most disturbed and unpredictable environments on the planet.
Because crocs need so little to survive,
they're able to maintain breeding populations
in places too poor for similar sized fuel-hungry mammals.
Crocodilians also have a mechanism that helps them recover from setbacks fast.
When numbers are low, a higher percentage of females reproduce
and they do it more often.
In Africa, a female Nile crocodile is laying a batch of up to 60 eggs.
She's dug a shallow scrape along the riverbank
and this soft, sandy soil will be a natural incubator for her eggs.
For seven weeks, she'll guard them against predators,
but her babies' lives will also depend on what happens inside the nest.
They'll need a fairly stable temperature if they are to develop properly.
Across the world, in the Florida Everglades,
another crocodilian mother lays the foundations of the next generation.
She's built a mound of dirt and vegetation to incubate her eggs,
a clutch of baby North American alligators.
Each egg contains enough food to take the baby through its whole development,
but each new life depends on more than just this.
The young gators will only survive if they're incubated between 26 and 35 degrees centigrade.
Within this range, specific temperatures will determine their sex -
those in the warmer eggs will become male, and those in the cooler eggs, female.
Temperature will also determine their rate of growth and help set
the idling speed or metabolic rate of their cold-blooded body engine.
Hatchlings that develop at the warmer end of the range
will have a slightly higher metabolic rate than siblings incubated at the cooler end.
They'll grow faster and will have a head start in a warmer environment with plenty of food.
But in cooler conditions,
the advantage will shift to hatchlings incubated at cooler temperatures.
The newly-emerged baby alligators call to their mother from inside the nest.
She's an attentive parent and jaws that can crush bones gently carry the babies down to water.
The alligator hatchlings stay near their mother for several months and she'll do her best to protect them.
This kind of care is unique amongst reptiles.
A crocodilian mother has no way of feeding her young,
so they must learn how to feed themselves right from the start.
It's a tough world, and even their own kind may not always be on their side.
But Nile crocodile hatchlings, barely out of the egg,
already show all the killer instincts of an adult.
It's another key to crocodilian success.
Whatever the environment, it seems that some of King Croc's offspring will always survive.
So the crocodilians' unique survival kit has served them well in nature,
but what about the man-made world?
We're probably the greatest threat they've ever had to face.
We've polluted their homes, slaughtered them in massive numbers,
built our cities on their ancestral lands.
We've pushed some of their kind to the very edge of survival.
In the wild world, it would be far too dangerous for people and crocodilians to get this close.
But there are places where crocs and people are learning to share space.
With a little help from us, even the more vulnerable crocodilians may be able to stage a comeback.
Crocodilians have proved they're tough,
and truth is, some will probably still be here long after we've gone.
Long may King Croc, supreme survivor, reign.
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