Original subtitles

(Hisses)

They've been on Earth for millions of years.

Crocodiles...

..the largest of all living reptiles...

..and among the most dangerous animals

in Australia.

Superbly adapted to aquatic life,

crocodiles are formidable predators...

..with skull-crushing jaws and lightning speed.

They're skilled hunters:

fishermen,

scavengers,

and even cannibals.

And they're survivors.

Once rare and in decline...

..these powerful carnivores

are in the ascendant once more.

They dominate the waterways and wetlands

of far North Australia...

..a vast island continent,

home to some of the planet's

most unusual and fascinating animals.

These are the Secrets of the Crocodile.

Far North Australia.

The climate is tropical...

..and features some of the most exotic landscapes

in the country.

The coastline is fractured by multiple river systems

that snake their way towards the sea.

In these brackish waters and along these wooded riverbanks

lurks an awe-inspiring creature...

..the saltwater or estuarine crocodile.

Known locally as "salties,"

these are the country's largest land-based predators.

Freshwater crocs also live here.

They have narrower snouts,

better suited to catching the aquatic life they eat.

They often live alongside the salties,

in fresher waters further upstream.

These amphibious reptiles are everywhere.

Whether they're visible or not,

if there's water in this part of Australia,

crocs are there...

..solitary and extremely aggressive...

..in rivers and creeks...

..mangrove swamps...

..waterholes...

..even the open sea.

This is Boris,

one of the crocodiles on this stretch of river

named by researchers studying crocodile behavior.

He's a large saltie,

at around 16 and a half feet long

and weighing over 1,500 pounds.

At this time of year, he spends time

soaking up the sun at the water's edge.

It's October, and the dry season is drawing to a close.

Up here in the far north,

there are just two main seasons:

the wet and the dry.

When the rains come, everything will change,

and it will be time to mate.

Boris is already moving in

on Amber.

At just under ten feet,

she's one of the largest females around.

But there's a problem.

Amber's in the territory of Ted,

the dominant male of the area.

Crocodiles have a fierce dominance hierarchy,

and, as top croc,

one of Ted's privileges is to hold on to mating rights

with any females on his patch.

If Boris wants Amber,

he'll have to get past Ted.

Dominance is partially based on size,

and - at 17 feet long

and weighing nearly 1,800 pounds -

Ted is the largest croc on this stretch of river.

Most males are between ten and 16 feet,

but salties have been known to grow

as long as 21 feet

and weigh in at over 2,800 pounds.

Size is also a marker of age -

and Ted is thought to be an impressive 100 years old.

Ted's gnarled and deformed head

are evidence of years of battling with rival males.

He's also blind, and has lost many of his teeth.

These fall out and grow again up to 45 times

as the crocodile increases in size.

But Ted's run out of replacements.

Crocs swallow their prey whole,

so he can still eat,

but his hold on power may be starting to wane.

When breeding season begins,

Boris could challenge Ted for mating rights with Amber.

In the meantime, Boris and the other crocodiles

must get on with the everyday business of surviving.

It's something they're perfectly adapted to do.

Crocodiles were here long before the dinosaurs,

and have outlived them by millions of years.

Crocodile fossils have been found dating as far back

as 200 million years...

..and they appear in ancient Aboriginal rock art.

Salties are found in parts of southeast Asia and New Guinea too.

In other parts of the world, their numbers are threatened.

But they've been protected in Australia since the 1970s,

which has led to a huge surge in their numbers.

It's thought that the saltwater crocodile population now stands

at between 150,000 and 200,000.

It's been dry season now for over five months.

There hasn't been rain in a long time.

Waterholes are developing thick mud at their edges.

Rivers are low.

And animals coming down to the water's edge to drink

can be excellent quarry for the stealthy crocodile.

Salties are hyper-carnivores,

meaning their diet is more than 70 per cent meat.

They'll eat just about any animal they can catch and overpower,

including fish, birds, small animals...

..even humans.

Like all estuarine crocodiles,

Boris is mostly nocturnal...

..but will hunt during the day, when he gets a chance.

He gets into position along the riverbank,

to watch for anything

that comes within striking distance.

Sweeps of his powerful tail

propel him silently through the water.

His short limbs with clawed, webbed feet...

..are ideal for swimming.

His eyes, ears, and nostrils

are located on top of his head,

allowing him to lie low,

while keeping an eye on his prey.

As well as a good sense of smell,

he has sensory pits along his entire body,

which detect vibrations.

Boris senses movement.

He dives, keeping any exposure to the minimum.

When Boris is underwater, his scales -

known as osteoderms -

create a low-pressure system around him,

resulting in a counter current around his body.

The water surface won't ripple,

as he swims up to his prey.

He's invisible.

The little moorhen in the reed beds

is oblivious to his presence.

But Boris is after a bigger prize.

He moves on silently.

An unsuspecting wallaby has come to the water to drink...

..unaware of what may lurk nearby.

A crocodile can lunge incredibly fast,

using both feet and tail

to propel himself from the water.

Boris clamps the wallaby with his long, powerful jaws.

A special valve at the back of his throat

allows him to open his mouth to catch and hold prey

under the water, without any entering his throat.

But to eat the wallaby, his head needs to be above water.

A crocodile can eat

an incredible 23 per cent of its body weight in one go.

He can secrete gastric acid faster than any other animal,

liquefying bone matter in just a few hours.

The only thing Boris can't digest is keratin,

so he'll cough up the fur later.

Anything he can't manage now,

he'll conceal, and return for snacks when he's hungry.

While crocodiles are opportunistic hunters...

..it's harder for them to move around

to other waterways during the dry season.

This can limit their feeding options.

But the drier, less humid weather

allows the crocs to stock up on energy in other ways.

From the sun.

Saltwater and freshwater crocodiles are ectotherms,

meaning they're cold-blooded.

Unlike humans and other mammals, they're unable

to internally regulate their body temperature.

To keep warm, they need to sunbathe.

They stay very still,

and their metabolism shuts right down

to conserve energy,

whilst they absorb the sun's warmth.

The scales on a crocodile's back

act rather like their own solar panels.

Tiny blood vessels carry blood

right to the surface of the scales,

where it's quickly warmed by the sun's rays.

As the blood circulates,

the heat is transferred to the rest of the body.

This regular need for warmth explains why crocodiles

are only suited to living in the tropics.

Crocodiles don't need to expend any energy

generating body heat, which is why they can go

for long stretches without feeding.

Basking to absorb heat

takes up at least a few hours of each day,

depending on the weather.

Now, at the end of the dry season,

there's still plenty of sunshine.

Crocodiles often like to sunbathe

with their mouths open.

It may show off this croc's superbly sharp teeth,

but it's actually a way of helping him

regulate his body temperature

when it gets hotter.

The Australian sun is fierce,

and temperatures may rise still further

when the wet season arrives.

Like a panting dog, the crocodile produces saliva,

which will cool him as it evaporates.

But if his temperature rises above 95 degrees Fahrenheit,

he'll shoot back into the cool of the water.

While Amber basks on the banks,

she may look asleep...

..but she's actually always half awake.

Crocodiles are able to shut down

one side of their brains at a time,

while keeping one eye open.

This is known as unihemispheric sleep.

Scientists have discovered that crocodiles can deploy

unilateral eye closure while dozing...

..enabling them to keep one eye out

for potential threats or prey

at all times.

Crocodiles also have three eyelids:

two ordinary ones, made of keratin,

and a third underneath, which covers the eye sideways,

called the nictitating membrane.

This acts as a protective layer against detritus

while underwater.

As the dry season draws to a close...

..many crocodiles take to the water

to make the most of an unusual feeding opportunity.

The East Alligator River in the Northern Territory.

It's 99 miles long,

and, despite its name, is full of crocodiles.

50 miles from the river's mouth

is a causeway known as Cahills Crossing.

It's in Kakadu National Park,

a popular tourist destination.

Cars can only cross here

when the river's lower in dry season.

But at the end of the season, that changes.

This year, the change of season

coincides with the highest tide in weeks.

And the crocodiles seem to know exactly when it's due.

As the waters start rising,

Boris...

..Amber

and numerous other crocodiles begin to appear.

They converge on the area

from all over the river system.

It's not known precisely what triggers the crocs

to arrive at the same time.

They may have deeper communication abilities

than are currently understood.

Crocodiles have extraordinary powers of hearing.

Their middle ear can detect sounds

that are inaudible to the human ear.

In between either side of their cranial plate and ears

is a small muscular flap,

which allows them to navigate magnetically, like birds.

It's also known that crocs can track

the migratory routes of their prey, as the seasons change.

The causeway usually prevents fish from migrating upstream.

But as the spring tides

boost the water levels in the river,

they bring mullet, barramundi

and even turtles from downstream,

attracted by the weed beds and nutrients

of the fresher water.

The arrival of the tide means that the river's flow

switches direction to upstream,

making movement easier for the fish,

as they travel towards the source.

As the water levels steadily rise,

the crossing is still used by road traffic.

The big males are first in line

for the migrating fish feast, and take up

the best positions just over the causeway.

One of the largest crocodiles here,

a 14-foot male named Nigel,

has been fitted with a GPS tracking device

by scientists following his movements.

Studies of crocodile navigation

have found that, when they're on the move,

salties can travel great distances,

up and down the river systems and out to sea.

Males of between ten to 12 feet

have been observed traveling furthest.

They aren't big enough to have their own territories,

but are large enough to defend themselves

from sharks and other crocodiles.

Nigel has been tracked traveling 75 miles

down the East Alligator River,

into the sea, and back up another river.

Another study tracked a crocodile

covering nearly 370 miles in less than a month.

As the waters begin to engulf the causeway,

motorists - keen to make the crossing -

take greater risks.

The crocodiles use the surge created by the cars

to give them extra propulsion.

Local police advise drivers

to always give way to crocodiles.

Salties may not actively hunt humans,

but - if one crosses their path -

they're more than happy to take the opportunity.

During the 1980s,

a crocodile knocked a car off the crossing with its tail.

And a fisherman was also taken by a crocodile here,

when he fell into the water.

Over the past 45 years in Australia,

there have been 99 crocodile attacks -

27 of them fatal.

The fish are jumping.

The crocodiles wait with open mouths...

..and their front limbs stretched out wide.

It's a technique known as cross-posture fishing.

The crocs have sensory receptors in their scales,

called ISOs,

which are more sensitive than a human fingertip.

Scientists believe that, by holding their arms wide,

the crocodiles create an arc

with the receptors located around their jaws.

This increases their ability

to determine any movement around them.

And as more and more fish cross the causeway,

the better the opportunities for feasting.

There are as many as 40 crocodiles

at the crossing now.

What makes this event so extraordinary

is that it's the only time of year

when big males like Boris will tolerate one another

in such close proximity.

Some experts believe

the crocodiles may be cooperating:

working together in close line formations

to herd schools of fish, so they can all feed.

The blockade effect of the crocodiles

makes it harder for the fish to get through.

Further upstream,

any fish that have made it past the big males

come up against a second line of hungry jaws.

Females like Amber -

and the smaller males - wait at a slight distance

from the front-line feeders.

When crocs are able to feast like this,

they make the most of it.

This meal can last them some time,

and they may even store fish under rocks to eat later.

The salties swallow the fish whole.

The crocodiles' tongues don't move easily,

so they manipulate the food into position for swallowing.

But if a fish isn't pointing head first,

the scales can get stuck in their throats.

So this croc flips his fish like a pancake

to get it the right way up.

But sometimes a misjudged throw

lets the fish make a lucky escape.

After just two hours, the tidal waters start to recede.

As quickly as they arrived,

the crocodiles silently disappear...

..and drift apart once again

to return to their more solitary existences...

..well stocked up for the season ahead.

(Thunder rumbles)

The wet season finally arrives.

Humidity goes up...

..storms erupt...

..and heavy rain falls in blasting showers.

90 per cent of the entire annual rainfall

will occur in the next four months.

Away from the unusual feeding event,

the crocs are back to their more typical hierarchy.

Crocodile society is run

on highly combative domination tactics.

Head clashes will establish and reinforce dominance,

and can even happen between females.

Wet season means that the time is approaching

for the crocodiles to mate.

Ted may be old,

but he's still the dominant crocodile here.

Keeping his grip on the top spot is vital,

as it means more chances to breed.

As temperatures rise, intense courtship begins...

..a challenge for Boris, who has his sights set on Amber.

Boris needs to broach Ted's territorial dominance

to get her.

For crocodiles,

body posture is a key means of communication.

Boris approaches Amber.

She raises her head:

a sign of submission.

These are the first overtures in crocodile courtship.

Amber starts blowing bubbles in the water.

This indicates that she's receptive.

(Rasping breaths)

She makes encouraging noises,

with chirpy rasps from her nostrils.

Boris starts to circle her -

the next move in the ritual.

But just as things are hotting up...

..Ted moves in,

and Boris is forced to retreat.

But Boris hasn't given up.

Later that night,

he uses the cover of darkness to continue wooing Amber...

..giving out deep rasping noises of his own.

(Rasping breaths)

They swim around together for hours,

before they eventually mate.

Copulation can last up to 15 minutes,

and takes place while fully submerged.

Ted's reign as the dominant male has been challenged.

Amber's behavior is not unusual.

Females will often allow subordinate males

to mate with them on the sly,

to ensure all of their eggs are fertilized.

Researchers analyzing crocodile eggs

have found that, in some cases,

only a few belonged to the dominant male.

The next day, Ted's regime is threatened once more,

when large male Chopper makes his way into his territory.

Saltwater crocodiles are keen ambush predators,

but - if a chance arises - they'll also scavenge.

And the wet season presents new opportunities.

As water levels fluctuate with the rain,

the riverbanks become littered with all sorts of debris,

including carrion.

Domestic livestock can end up here,

as well as wild animals,

like this large feral pig.

It may be in Ted's territory,

but whoever gets to the carcass first

will consider it theirs.

When there's a ready food source like this,

Chopper doesn't hold back.

Salties don't mind whether food is fresh or rotting.

The carcass quickly attracts other crocodiles.

Soon Ted, Boris and several females

are fighting it out for the free meal.

Chopper keeps trying to oust Boris and Ted...

..by smashing down onto their snouts.

Chopper continues to dominate.

He fights off the others...

..and sticks close to the carcass, hissing.

(Hisses)

Crocodiles have no vocal chords.

He makes these sounds

by forcing air out of his nostrils.

A study of the saltie's bite found that theirs

is the strongest of any creature in the world.

With a maximum bite force

of 4,000 pounds per square inch,

their muscles are ranged for clamping down tightly.

Once Chopper gets close enough

to get his teeth around part of the pig,

he needs to find a way of taking bites from it.

His teeth interlock,

so, when they bite into an animal,

it creates a serration,

better for gripping prey, rather than for chewing.

To shear meat off, he needs to shake.

So to do that,

he grips, rolls, and rips.

This behavior is often referred to

as the death roll.

A dominant male will let the females feed

and chase away the smaller males.

But Ted doesn't manage to see off Chopper.

Chopper manages to drag most of the carcass away.

Now it's clear that Ted's rule is on the decline.

It's December...

..and Amber has produced a small clutch

of 16 hard-shelled eggs.

Females can lay up to 80 eggs in a season.

She's scratched a nest almost three feet high,

using vegetation and mud.

The eggs will be incubated both by heat

generated from the rotting leaves

and by solar radiation.

Her nest is close to the water.

She's placed it carefully.

Too close,

and rising waters could flood the nest,

and kill the embryos.

Amber will stay here day and night

for the three months it takes for the eggs to hatch.

She's fiercely protective.

She'll react aggressively

to defend her eggs against would-be predators,

like wild dogs, lizards,

or feral pigs,

as well as other crocodiles.

For Amber's babies,

the struggle for survival has begun.

The exact timing of hatching

will depend on an optimal temperature

of 87 to 89 degrees.

Any higher or lower

and the hatchlings could be deformed or die.

It's estimated that up to 75 per cent of eggs

laid in a season won't hatch at all.

When Amber's hatchlings emerge,

they're highly vulnerable.

Baby crocodiles are only ten to 12 inches long at hatching,

an easily edible snack for many predators.

Carnivorous fish, like barramundi,

will eat a young crocodile...

..while on the land, goanna lizards may prey on them.

From the air, birds of prey,

like the white-bellied sea eagle, can attack.

Crocodile mothers protect their young

during the first months of life.

But as the young saltie grows,

an even greater threat will come from his own species.

Big males, like Ted or Boris,

will do anything they can to get rid of young rivals.

And that extends to cannibalism.

They'll eat both hatchlings and young juveniles,

if they get a chance.

A dominant male can pick up the scent

of smaller males in his territory...

..and get rid of them.

Young females can also be eaten,

but have a better chance

of being saved for future breeding.

It's thought that only one per cent of male hatchlings

make it to adulthood.

So the odds are that only one or two of Amber's hatchlings

may reach maturity.

If they do, there's a chance

that they could still be alive 100 years from now...

..long after their likely father -

Ted or Boris - has departed.

They could become the next alpha crocodiles,

aiding the survival of their extraordinary species

for many more generations to come.