Original subtitles

A remote Colombian island...

...feared for its deadly wildlife...

...and home to a notorious prison.

A natural fortress,

surrounded by dangerous reefs

and shark-infested seas.

Covered with impenetrable forest,

teeming with venomous snakes.

For convicts, escape was futile.

But following in the footsteps

- of the few who tried... - [Screeching]

...we reveal another side to the island -

a wildlife haven.

A sanctuary where creatures thrive in remote isolation.

[Screeches]

Theirs is the real story of life on Gorgona:

the snake prison island.

Remote and isolated,

Gorgona is a tiny lump of volcanic rock

rising from the Pacific Ocean.

It lies 35 kilometers off the coast of Colombia.

Overrun by dense jungle,

it conceals a multitude of deadly animals.

[Rustling]

It was the stuff of nightmares

for early explorers

and has resulted in the island's sinister history.

Because of its isolation,

Gorgona was chosen by the Colombian government

as a site for a prison -

the country's own version of Alcatraz.

In 1959 it began housing the nation's

most notorious criminals.

Modeled on Nazi concentration camps,

claims of human rights abuse were rife.

Murders within its walls were common.

25 years it was a living hell

for the 2,500 inmates who did time here.

[Bird calls]

If the prison's thick concrete walls

weren't enough to prevent escape,

the jungle outside harbored a deadly deterrent.

There are 18 species of snake on the island,

from the skinny - but fast - sipo...

...to the powerful giant boa.

Of those 18, three carry deadly venom.

When Spanish conquistadors sailed to the island

in the 1500s and claimed it,

they learned the hard way

about the island's natural dangers.

87 sailors died from snake bites:

more than half the crew.

That snake legacy continues today.

The last prisoners left Gorgona in 1984.

30 years on, the island's natural inhabitants

have reclaimed the penitentiary for themselves.

Bats hang like the lightbulbs

that once lit the prison corridors.

Bees build cells to house their own colonies.

The island's rats now have the run of the prison.

[Chattering]

And the once imposing walls

have become a playground for monkeys.

Some of the prison's first inmates

were violent gang members...

...locked up for killing rivals in vicious turf wars.

Today, Gorgona still sees boundary disputes.

But they involve a very different primate.

White-faced capuchin monkeys

live in small troops led by an alpha male.

They watch each other's backs

and defend precious resources.

Females stay with the troop they are born into

for their entire lives.

But males must leave to find new families.

They seek out new groups every four years or so

to avoid inbreeding.

When they set out to find new troops,

they often band together.

There's safety in numbers.

But these roaming all-male gangs

can be bad news for a family troop.

[Chattering]

A gang has breached the prison perimeter -

climbing the walls and scaling the roof.

They're on the lookout for females to breed with.

The troop's resident alpha knows he's under threat.

When a new male takes over a group,

it will typically kill existing babies.

This brings the females into estrus more quickly,

allowing them to replace

the outgoing males' offspring with their own.

The alpha male calls his troop to arms.

[Screeching]

He and the subordinate males in his group

present a united front.

This is how the lower-ranking males

earn their place in the troop's hierarchy.

Providing back-up to the alpha

also earns them the chance to breed.

The invading males on the roof are visibly outnumbered.

It's not worth the risk of tackling this group.

[Screeches]

The male gang retreats.

With their patch defended,

the troop breaks out in search of food.

In its heyday, the prison was like a zoo.

But the people were behind bars,

while the animals ran free.

It must have been torturous for the inmates

to watch monkeys scaling the walls,

enjoying the freedom of the forest.

The monkeys may even have inspired

a few escape plans.

But if a convict did manage to break out,

it would have just been the start of his problems.

The fer-de-lance:

one of South America's most deadly snakes.

This two-meter pit viper

is capable of moving at lightning speed.

Highly aggressive, they're quick to strike.

[Hisses]

Its venom is so fast-acting,

it kills more people than any other poisonous animal

in South America.

One bite contains enough venom

to kill more than 30 people.

An escaped prisoner wouldn't have stood a chance,

suffering excruciating pain

as the venom destroys his body tissues

from the inside out.

The threat of a fer-de-lance bite alone

was enough to prevent most convicts

from attempting to escape.

[Chirruping]

But if a jail-breaker managed to avoid the deadly snakes, he'd soon be swallowed by the island's dense jungle.

Gorgona is two and a half kilometers across

and nine kilometers long -

about half the size of Manhattan.

For once, nature might help.

The thick forest could conceal a prisoner,

if he could stay alive.

He'd need to find food.

Following the island's primates

might give him clues.

The capuchins snack on buds,

seeds, and even small animals.

But the bulk of their diet is fruit.

Foraging keeps them constantly on the move,

seeking out trees with the ripest treats.

They have an unusual way of making group decisions,

more like the behavior of bees and ants

than other social mammals.

[Monkey chatters]

Although they have an alpha,

anyone in the troop - male or female -

can have their say.

A monkey sets out in one direction.

It's his suggestion to the others.

But the troop won't blindly follow.

They carefully consider his proposal.

If they think he's likely to find food,

they'll follow him.

If not, he'll be forced to return to the group.

In the future, his suggestions

won't carry much weight.

But this capuchin male sticks to his guns,

hoping he can find two or three others

to share his vision.

So far, they aren't buying it.

Finally, a few decide to join him.

It's enough to tip the balance

and the rest of the troop is persuaded to follow.

But it could be a short-lived triumph.

Their movements are being closely monitored from below.

A giant boa constrictor is lying in wait.

Female boas are bigger than males.

And this one's a whopper, nearly four meters long.

Her powerful body's about 30 centimeters in diameter.

She's a formidable predator.

She may not mate every year,

and will only reproduce if she's in prime condition.

So, her priority is fattening herself up.

She'll eat anything she can swallow.

With her expandable jaws, a capuchin monkey,

about the size of a domestic cat,

would be an easy mouthful.

She doesn't have fangs,

but she does have more than 70 teeth

to lock into and anchor her prey.

Like alligators and sharks,

damaged or lost teeth are constantly replaced,

keeping them needle-sharp.

She can't hear the approaching capuchins.

Instead, she follows their movement

via vibrations through her jaw bone.

But as soon as she makes her move, her cover's blown.

[Low screeching]

[Low screeching]

The capuchins screech their alarm calls.

[Screeching]

The monkeys mob her, hoping to scare her away.

- [Screeches] - It doesn't work.

The capuchins have only one option: retreat.

They must start the decision making process from scratch.

The monkeys' movements are dictated by

the fruiting trees throughout their home range.

For Gorgona's convicts, forced to eat basic rations,

the fruit was a tantalizing but forbidden luxury.

The monkeys are picky,

looking for the ripest fruits.

As they browse, they dislodge fruit and leaves

that fall to the forest floor.

For escaped prisoners, the monkeys' leftovers

could have been a life-saving free meal.

The fallen fruit is manna from heaven for insects

and other creatures in the undergrowth.

One animal cashing in on the castoffs

is the millipede.

This species is toxic, releasing a chemical defense

through glands along the sides of its body.

In some species, the toxins, known as Benzoquinones,

are powerful enough to dissolve the bodies of ants

and other insects.

A wise prisoner on the run

would give millipedes a wide berth.

Even just touching them

can cause irritation of the eyes and skin.

But the capuchin alpha is actively trying to find one.

He isn't going to eat the millipede.

He wants it for protection.

The millipede's toxic chemicals keep pests at bay.

Wiping the insect over his fur

provides an excellent monkey mosquito repellent.

Despite being a walking chemistry set,

the millipede is a friend of the forest.

It breaks down decaying leaf litter,

returning nutrients to the soil.

Millipedes were among the first land animals

to appear on Earth, around 400 million years ago.

While they've evolved to survive

in almost any habitat,

they thrive in humid tropical forests like Gorgona's.

The island's location close to the equator

means there's an average temperature of 27 degrees.

Humidity rarely drops below 95 per cent.

Molds and fungi thrive in these conditions.

While leaves, bark and fallen fruits quickly rot.

This would have made it challenging

for any prison escapee.

The high levels of humidity

added to the tough conditions

for those imprisoned on the island.

A prisoner on the run wouldn't get much relief

from any sea breezes.

Here, trade winds laden with moisture

from the ocean's surface add to the humidity.

Moisture in the air has a snowballing effect,

acting like a magnet, attracting yet more humidity.

But there's only so much water the air can hold.

Eventually, the inevitable happens.

[Storm rumbles, wind whistles]

[Rain pattering]

For most of the year,

torrential downpours are a daily event.

The combination of heavy rain and high temperatures

would have added to the misery of the prisoners.

But for an escaped convict hiding out in the forest,

it could prove deadly.

The moist, warm environment

is the perfect breeding ground for infection.

Even the smallest wound quickly festers.

[Rain gently splashing]

When the day's torrential downpour stops,

a brown-throated three-toed sloth emerges

to take advantage of the sun's warming rays.

Gorgona's sloths are unique.

The island used to be attached to the mainland.

At the end of the last Ice Age,

sea levels rose and cut it off,

leaving the sloths behind.

They have been isolated from mainland Colombia

for so long, they have become a distinct sub-species.

Just like the convicts, they are now marooned prisoners,

cut off from the outside world.

After the heavy rain,

the sloth hangs himself out to dry.

In most mammals, hair parts along the spine

and flows down the back to the belly.

But because of his upside-down life in the trees

the sloth's fur grows the other way,

belly to back, allowing him to drip-dry.

He's in no rush, and might hang here for hours.

The leaves of the rainforest don't offer great nutrition,

but he gets by thanks to a slow metabolism.

And to save energy, he spends a lot of time

just hanging around.

He keeps still for so long,

that his fur has developed its own mini-ecosystem.

Algae is the first organism

to colonize his hollow hair follicles.

The simple single-celled organisms

give his fur a greenish tinge,

perfect camouflage in the canopy.

The algae promote the growth of a fungus

that has scientists excited.

For sloths, the fungus seems to inhibit infections

and keeps parasites at bay.

For humans, scientists think it may have disease-

and even cancer-resistant properties.

The algae also feeds a species of moth

unique to the sloth.

When the sloth climbs to the ground to defecate,

the moth lays its eggs on the droppings.

Their larvae then feed on the dung.

Every member of this community benefits the other.

The moths fertilize the sloth's fur

with their nutrient-rich droppings.

That, in turn, encourages more algae to grow,

which helps to protect the sloth through camouflage.

It's an amazing, self-contained ecosystem,

all on the sloth's back.

As the sloth and his lodgers dry out,

other forest-dwellers

are making the most of the moisture.

It rains all year round on the island.

But the heavy downpours of the rainy season

trigger a male marbled poison frog

to spring into action.

His mate is an absent mother.

She laid her eggs directly onto the forest floor,

then left them.

Once the eggs hatch,

it's Dad who does all the heavy lifting.

Like all tadpoles,

his offspring breathe through gills.

They need water to survive.

So it's down to the male

to find them a more permanent pool.

He piggy-backs his tadpoles across the forest floor.

A layer of sticky mucous

makes sure his precious cargo is safe.

The hunt for a suitable nursery

is fraught with danger.

Even tiny ants could make a meal of his tadpoles,

so he needs to find somewhere secure.

Eventually he spots the ideal location -

a prison sink.

The bathrooms here were once a terrifying place.

There were so many murderous attacks on the inmates,

the bathroom walls had to be lowered

to allow constant monitoring.

Now the bathroom's a sanctuary,

an old basin creating the perfect nursery

for the young tadpoles.

It's sheltered, secluded,

and a breeding ground for the insect larvae

the tadpoles can feed on.

His precious load delivered,

the male's parental contribution is done.

Now his tadpoles must go it alone.

[Birds chirruping]

As the skies clear, the warmth of the sun

tempts other prison residents from their cells.

Stingless bees.

[Buzzing]

There could be up to 10,000 of them

living in this cavity in the wall.

Worker bees are strengthening the entrance.

They use their jaws to pulp wood,

using sticky saliva to cement it into place.

The hive is their fortress,

their honey store and a nursery for their young.

This is teamwork at its best.

Every individual has a specific role...

...from bringing back pollen, to construction,

to taking out the trash.

Because stingless bees don't pack the punch

of their well-armed cousins, they need to be careful.

Insects are low on the food chain.

[Bird cries]

As the sun begins to set over the island,

the bees' nemesis stirs.

Little big-eared bats.

Nocturnal hunters.

They use echolocation to home in on prey,

their large ears picking up the returning echoes

of their high-pitched squeaks.

[Squeaking]

Like a security camera,

they scan the room for signs of prey.

They have a special fondness for crunchy food,

especially beetles, crickets and cockroaches.

They're not the only bats in the ruins

but there's no competition for food.

The lesser long-tongued bats

are their vegetarian cousins.

At night they visit flowers, lapping up pollen and nectar,

and pollinating the forest's plants.

Their tongue is half the length of their body.

So long, it permanently protrudes

through a gap in their front teeth.

It's four centimeters long,

tipped with bristles to help gather up their sugary diet.

As the bats prepare to head out on the hunt,

other nocturnal inmates emerge.

[Chirruping]

Rats were a permanent fixture of prison life,

adding to the misery and squalor

of the living conditions.

The people may have gone,

but the prison's ruins

still offer hiding places to rodents.

Under the cover of darkness,

spiny rats vacuum up any fallen fruit.

Their teeth never stop growing.

They rub against each other, wearing down the edges,

forming chisel-sharp blades

that can cut through the toughest nut shells.

The rodents perform an important role

in the island's ecology.

Many of the forest's trees

rely on the spiny rats for propagation.

Cracking the shells allows the seeds to germinate.

And, by caching their finds,

the rodents help to distribute the seeds

around the forest.

But these gardeners are themselves

on the forest menu.

They're prey for pretty much every large predator

on the island.

Having failed to snare a capuchin,

the female boa is on the hunt again.

Like many snakes, she has a sixth sense

when it comes to hunting.

She's able to detect the warmth of her prey

even if she can't see them.

And with every flick of her tongue,

the boa picks up scent molecules

from the surrounding environment.

Guided by heat and scent,

she can track her quarry even on the darkest night.

She's locked on to her target.

A master of stealthy stalking,

she's got the perfect camouflage for an ambush.

She lies in wait for the rat to get within striking range.

Her sharp teeth lock into the rat.

Before it has the chance to fight back,

she throws her coils around it.

She kills by crushing,

squeezing the rat so hard it stops the blood flow.

Death comes quickly.

The rat is swallowed whole.

[Soft squelching]

It will now take her at least a week

to digest her meal.

Gorgona's forest at night

would have been a terrifying place

for an escaped convict.

Dawn would have brought immense relief.

[Birdsong]

The early morning sun lures a ten-centimeter-long

female blue anole from the shadows.

Today, she's on a mission.

But she's not going anywhere until she warms up.

Blue anoles only exist on Gorgona.

Once she's absorbed the sun's energy,

she heads down to the forest floor.

She's going to lay her eggs.

Her forest home wasn't always like this.

Before the jail first opened its doors,

Gorgona suffered mass clearance of its trees

for shipping and farming.

Although the building of the prison

allowed the forest to grow back, the damage was done.

The blue anole was left critically endangered.

The female's precious cargo - her eggs-

are vital for the future of her species.

But going to the ground is a risky business.

Today the biggest threat

to the survival of this endangered species

is a non-indigenous invader...

[Clicking]

...the Western basilisk.

Introduced from the mainland,

the meter-long lizard is wreaking havoc on Gorgona.

It eats anything from insects to fruit

but is especially fond of other lizards.

This crested male has the pregnant female anole

in its sights.

Just when it looks like the end of the road

for the anole,

the hunter becomes the hunted.

A Boddaert's racer.

Three-quarters of its diet is made up of lizards.

And the basilisk looks like a convenient meal.

Despite being no bigger than the basilisk,

the racer snake is more than up for the challenge.

But the basilisk has a clever trick up its sleeve.

Named the Jesus Christ lizard,

the basilisk is a master of escape.

It uses its long, broad, scaly toes

to trap air pockets under its feet

and walk or run on water.

It can cover one-and-a-half meters a second.

But eventually gravity catches up,

and the lizard sinks.

It's still been enough to escape the snake.

Just to be sure, the basilisk stays submerged.

It's capable of hiding under water for up to half an hour.

But this time he might have jumped out of the frying pan,

and into the fire.

Spectacled caiman can reach

two-and-a-half meters in length.

At just a meter long, this is a youngster.

But it still has dagger-sharp teeth and powerful jaws...

...more than enough to make short work of the basilisk.

Time for another miracle.

Gorgona's waterways are full of dangerous predators,

but to a prisoner on the run,

they'd be a useful navigational tool.

Walking in the water wouldn't leave footprints

for the guards to track.

Following the rivers downstream

would lead the escapee to the coast.

But the island wouldn't release

its captive inhabitants quite so easily.

Before reaching the coastline,

an escapee would be faced

with the island's mangrove swamps.

The soft mud would act like quicksand,

while the tangle of roots presents a labyrinth:

a spider's web of living plants...

...so dense it's almost impossible to penetrate.

Mangrove trees are specialists,

adapted to cope with brackish water.

They're vital to the island,

protecting its coastline from storm surges.

The mangroves' aerial roots

are an adaptation to their soggy lifestyle.

They act like snorkels, taking in oxygen from the air

when the rest of the roots are submerged.

The forest's dropped leaves feed mangrove fiddler crabs.

Which, in turn, aerate the waterlogged, anaerobic soil.

The crabs may be adept at maneuvering

through the thick sticky mud of the swamps.

But it would slow down any prison break.

By following the rivers through the swamp,

the exhausted prisoner would finally break free

of the mangroves.

Only to come to the ultimate dead end...

the Pacific Ocean.

Remote and isolated,

the nearest coastline is over 30 kilometers away.

Marooned on the beach,

the prisoner could have done little

but watch the receding tide.

As the water retreats,

painted ghost crabs emerge to feed.

For a fugitive, the crabs might have become

a life-saving meal.

Left to their own devices,

the aggressive crustaceans scavenge for treasure

washed up by the sea.

Each fiercely defends its patch of sand.

In the crab world,

trespassing's a risky business.

They take no prisoners, often cannibalizing intruders.

But right now, this male has other things on his mind.

He's busy excavating his burrow.

Each crab digs a burrow above the waterline.

It needs constant maintenance

and every crab has its own technique.

Some crabs just kick the sand to one side.

But this male is piling up a spoil heap.

This isn't just good housekeeping.

The mound of sand serves another purpose.

Female ghost crabs release their eggs

under the new moon,

when tides are higher and the darker nights

make it harder for predators to spot their offspring.

To get the timing right,

they need to breed two weeks earlier,

signaled by the full moon,

which brings the males out to perform.

[Tapping]

They bang their drums

pounding the mounds with their pincers.

They're serenading potential mates.

A colony of drumming crabs

might have been a wondrous sight

to a convict camping out on the beach.

But it would not have helped his escape.

[Squawking]

The birds flying overhead

might have offered inspiration.

[Squawking]

Every morning, blue-footed boobies

preen their feathers,

coating them in oil to waterproof them

before setting out to feed.

Studying the way the seabirds used the waves

might have helped him to understand

the currents offshore, and formulate a plan.

In the island's entire history as a penitentiary,

only three prisoners ever got past the beach,

making the ultimate prison break by building rafts.

They may have used the large Cecropia trees

that are common on the island.

Its bark can be twisted to make excellent rope

that could have strung a raft together.

Even if a convict got this far,

he was far from home free.

Dragging the raft out through the shallows

would have possibly been the most dangerous part

of this prison break.

Huge tiger sharks patrol the beach.

They're one of the most dangerous sharks

in the world.

More than half a tonne of muscle,

with teeth shaped to slice through flesh and bone.

The tiger shark is a highly effective predator,

hunting in shallow water.

Isolated from the mainland,

filled with venomous snakes,

enclosed by a web of mangroves,

and patrolled by sharks...

...it's obvious why Gorgona was chosen

for a maximum security jail.

But even if a prisoner managed to safely launch

a raft into the water,

the island still has one more natural barrier to breach -

a coral reef surrounding the whole island.

The seabed is littered with the wrecks of ships

the reef's claimed over the years.

Reefs have always represented a threat to mariners.

The vast structures funnel water into eddies and rifts

that can drive boats

against the hull-destroying jagged edges.

The razor-edged coral

can tear through rafts and human flesh alike.

Passengers thrown overboard

would be churned in the breakers

and dashed against the rocks.

If they didn't drown right away,

they'd face marine predators

or be swept out into the Pacific Ocean.

The reef is the final barrier

to escape from the natural fortress -

a prison with no escape.

Just like the prison ruins,

today the shipwrecks scattered around Gorgona

provide a sanctuary for wildlife.

During its operation,

the prison took a heavy toll on the reef.

Fish were harvested to feed the inmates.

Goliath grouper was heavily hunted.

This species has now been pushed to the brink.

It's one of the top ten most endangered fish on Earth.

But, ironically, the island that once led

to the grouper's downfall might now be its savior.

Gorgona's prison ran from 1959 to 1984.

But in the same year that its doors finally closed,

the island was declared a national park.

The goliath grouper,

the blue anole,

the brown-throated sloth -

all the wildlife that has evolved here

and lives nowhere else on Earth is now fully protected.

And at its heart, the prison itself -

a reminder of the island's brutal past -

is slowly returning to the jungle.

A once impregnable fortress, Gorgona has become a refuge.

Snake Prison Island

is a prison no more.