Original subtitles

On a remote island off the coast of Colombia...

...a small crab embarks on an epic adventure.

She's bearing a precious cargo:

85,000 fertilized eggs.

She's now confronted

with the biggest challenge of her life -

to get them to water.

She begins a massive journey,

from her mountain home to the coast.

[Bird screeches]

She must face hostile forests...

...a barrage of crab killers...

...and deadly crossings.

Time is running out.

She has just three days to reach the water

and release her eggs.

Across the island, thousands of crabs emerge -

an army of females laden with eggs.

This is Providencia, home to the March of the Crabs.

A small island rises out of the Caribbean Sea -

Providencia.

Just 22 square kilometers of rocky hills.

At its heart, 300 meters above sea level,

a normally nocturnal resident emerges from her burrow:

a female black land crab.

Her rocky home,

650 kilometers from Colombia's Atlantic coast,

was thrown up by volcanic activity

14 million years ago.

Its highest peak is 360 meters above sea level.

Just below the summit,

this crab has chosen to make her home.

She settled here four years ago.

Since that time, she's been slowly growing.

She's been through more than a dozen molts,

each time shedding her shell

as her body expands and matures.

She's now five centimeters across her carapace,

halfway to being fully grown.

Ten days ago, she mated for the first time.

Now she faces a daunting mission.

In just three days, this tiny crab must make her way

back to a beach on the southwest corner

of the island to release her eggs.

This is where her own story first began.

She was one of thousands of eggs released by her mother

off the island's southwest beach.

For the first 20 days of her life,

she was a tiny larva, no bigger than a grain of rice.

Carried by the ocean's currents,

she drifted into deeper water,

joining simple, microscopic animals

and the young larvae of other species.

This is the base of the ocean's food chain:

plankton.

The life of a planktonic baby crab is fraught with danger.

Many marine animals feed on plankton,

from small fish

to some of the biggest appetites in the ocean -

whale sharks.

Unlike most other sharks,

whale sharks are filter feeders

and sieve tiny creatures from the water

through modified gills.

They can scoop up whole shoals of larvae

in a single mouthful.

The biggest fish in the sea, over 12 meters long,

they can eat up to 21 kilos of plankton a day.

Migrating through the Caribbean,

some stop in the waters off Providencia

to take advantage of the annual black land crab spawn

and feast on the crab larvae.

Some years none of the larvae survive to return to shore.

This female defied the odds

and was one of the lucky few to avoid being eaten.

After 20 days growing at sea,

the currents were favorable

and brought her back to shore.

She arrived an ant-sized juvenile crab.

Having made landfall, instinct drove her inland.

But there are three million crabs on this island.

Trying to find a patch of unclaimed territory is tough.

And larger crabs sometimes kill intruding youngsters.

So slowly she made her way up into the rocky high country

and found some available real estate with plenty of food

and a safe shelter among the rocks.

The perilous journey will have taken the small crab

several weeks, as she dodged the numerous predators

that inhabit the island.

She's been hiding out here ever since.

As a land crab, she normally spends her days

hiding in her burrow, safe from predators,

and out of the drying rays of the sun.

At night, she emerges to feed on plant material

that she brings back to her burrow to eat.

It's a simple routine, day and night,

unchanging for four years.

But now it's time to begin the cycle of life again.

Having mated just over a week ago,

she's now the bearer of 85,000 fertilized eggs.

They're still developing, but the clock is ticking.

In three days, they'll be ready to hatch.

Once she starts her march,

they'll be exposed to the elements.

She'll travel both day and night.

If she can't complete her journey in that time,

they'll dry out and die.

So she must leave the safety of her home

and quickly make her way down to the beach.

She begins her quest alone.

But it won't be for long.

Across the island, millions of female black land crabs

are starting journeys of their own,

which they might repeat up to five times in their life.

This incredible seasonal mating and migration

is triggered only when moisture levels

rise sufficiently during the rainy season,

which can happen any time between April and July.

The rain prevents their eggs from drying out

during their journey.

This year, the rains are light,

but they're enough to have triggered the mating season.

Guardedly, she slowly descends.

She's exposed out in the open to predators

and the fierce Caribbean sun.

She looks for the brightest spot in the sky

or reflecting on the ground,

using the location of the sun or moon to orient herself.

When she's on easy terrain,

this little crab can cover 50 meters an hour,

but the rocks slow her down.

After half a day's difficult descent,

the terrain starts to level out.

Gradually her rocky home is replaced by jungle.

This is dry tropical forest,

populated by deciduous broad-leaved trees.

It's one of the most threatened

environments on the planet.

It's been decimated by farming

across many Caribbean islands.

Providencia is one of the few places

where the forest is still intact.

The deep leaf litter makes it slow going for the crab.

The trees' long taproots

are an added obstacle to her already slow progress.

But there are far worse things than roots

to contend with in the forest.

A black iguana.

At over one meter long,

this male is an intimidating sight.

His bold black stripes are prominent

as he heads into the canopy to sunbathe.

He's cold-blooded, or ectothermic,

and needs the sun's warmth to heat his body.

The darker his stripes, the more heat he'll absorb.

Once he's reached his optimum temperature,

the contrast will fade and he'll be fully charged.

This is when he's most dangerous to the crab.

Although mainly a vegetarian,

he won't pass up the opportunity

of a protein-packed meal.

Especially one wandering right under his doorstep.

Being nocturnal normally minimizes the crab's chances

of running into predators like this.

But traveling by day, she's taking a terrible risk.

If she's killed,

it's not just one life lost from the crab population.

It's also the 85,000 eggs she carries with her.

Another black iguana notices the crab.

Her path is blocked.

This could be the end of the road

for the young female and her brood.

At the last minute, the two male iguanas spot each other.

Black iguanas are highly territorial.

One of them is gate-crashing.

The sun-bathing male stakes his claim

and signals his annoyance with the intruder

by head bobbing.

He flexes small bones in his throat outwards

to extend the skin,

and make himself look bigger and more intimidating.

While the iguanas are distracted

by their territorial argument,

the crab makes good her escape.

As she descends, the environment changes again,

bringing new dangers.

Open pockets are colonized by thorn-laden bullhorns,

a type of acacia tree.

Many acacias have high levels of toxins in their leaves

to dissuade plant eaters.

But bullhorns don't.

Instead they employ vicious bodyguards -

acacia ants.

The ants are attracted to the tree's nectar

and protein-rich oils,

and make their homes inside the thorns.

But once they've established a colony,

the ants can never leave.

The tree demands loyalty by secreting an enzyme

that prevents the ants from digesting sugar

from any other source.

To survive, the colony must defend its food source,

and ironically, the ants will fight to the death to do so.

They will destroy other saplings growing too close

and muscling in on the acacia's light and nutrients.

And they will rapidly descend on invading insects,

stinging them into submission,

and will even attack much larger animals.

A lone crab wouldn't stand a chance

against a horde of thousands of stinging acacia ants.

But the female has no choice.

Her route takes her straight through the acacia thicket.

Fortunately, she's not alone.

As she descends, her path converges with those

of other crabs going from the high-country to the coast.

Tentatively, they creep forwards.

The first crab makes its move.

It instinctively moves from one shady hideout to another.

But this is not the tree to stop under.

The ants quickly pick up the unfamiliar scent

of the invaders and launch their attack.

A crab this size is no real threat to the acacia,

but defense is so hard-wired into the ants

that they go instantly into action.

For a human, an acacia ant's acidic sting

is like that of a bee.

The pain can be excruciating.

The ants search for chinks in the crab's thick armor.

They target its leg joints and eyes.

The ambushed black land crab instinctively

folds its eyes into protective grooves.

But there's no escape.

They pump a cocktail of chemicals,

including formic acid, into the crab.

The toxins dissolve its body tissues,

paralyzing its joints.

Soon it can't move.

Its fate is sealed.

As the colony focuses on the unlucky victim,

the female grabs her chance.

Shaking off a few ant soldiers,

she quickly makes her way through the acacia thicket.

The first day of her major migration

has seen her cover almost a kilometer.

It's a good start to her journey.

More importantly, she's still alive

and her eggs are safe.

But there's still a long way to go and she needs to rest.

She sinks into the leaf litter to recuperate.

[Leaves rustling in the wind]

Change is in the air.

- [Thunder rumbles] - A storm is brewing.

[Crash of thunder]

[Rain pouring]

The brief downpour is what the migrating crabs need.

Though they're land-based,

like all crustaceans, they still breathe through gills,

which they need to keep moist.

The rains bring out more crabs.

Every hour, new females emerge to join the march.

Some crustaceans have a long fleshy abdomen

that they use to swim.

In prawns and crayfish, this is the part that humans eat.

In crabs, the abdomen is a short flap called a pleon,

which wraps under the carapace

and holds the bundle of eggs in place.

Gravid females are said to be ".berried".

Our female's cluster of ".berries"

are quickly maturing and getting bigger.

She needs to get her timing right

and deliver them into the sea

just as they're ready to hatch.

The low clouds quickly evaporate

as the temperature climbs once more across the island.

Being so close to the equator,

it usually reaches the high 20s

as a steady year-round high.

Providencia's lizard population

comes out to bask in the sun.

Just like the much bigger black iguanas,

these crab cay anoles are territorial.

As the female crab continues her migration

to the southwest corner of the island,

she passes under an eight-centimeter male anole

asserting his dominance.

From his vantage point,

he aggressively gestures to his neighbors.

He's making sure his property boundaries are observed.

He flashes his dewlap - an elongated throat pouch -

to make himself look more menacing.

His neighbors, safe in their own territories, flash back.

They're no threat to the passing crab.

These tiny reptiles feed on small insects and fruit.

But their preoccupation with territorial disputes

puts the anoles themselves at risk.

The dense, tangled jungle

growing at these lower elevations

is the perfect hideout for predators.

A boa is hunting for breakfast.

It detects its prey with heat signatures

and the vibrations created by their movements.

The crab freezes as the two meter snake slithers past.

Spotted.

She quickly rises up in defense,

showing off her powerful pincers.

It's enough to put the reptile off

and it resumes its hunt elsewhere.

The crab scuttles on.

Snakes are nervous climbers, even species like the boa

that spend a lot of time in the trees.

The boa moves its body like an accordion.

While some of its coils extend its reach,

others hold on tight.

So tight, in fact, that it grips the branch

with 20 times the force needed to hold it in place.

It's this same strength that enables it

to squeeze the life out of its prey

and ensures that the snake won't fall.

But it requires a lot of energy.

This unnecessary expenditure has puzzled scientists.

Under normal circumstances,

snakes lead very energy-efficient lifestyles,

spending long periods without movement to conserve energy.

Perhaps the benefit of exerting the extra energy

needed to climb outweighs the cost

of a potentially damaging fall.

The boa's arrival in the canopy

isn't welcomed by other residents.

It's buzzed by bananaquits -

small song birds with distinctive eye stripes -

as well as a rare pair of vireos.

These tiny flycatchers are unique to Providencia

and its neighboring islands.

Screeching and flicking their tails,

the birds mob the potential threat.

They're letting all other animals in the area

know of the danger.

A stealth hunter, there's not much point in the boa

continuing its hunt with all the commotion.

But resting brings a new challenge,

even to this strong predator.

A mosquito searches for a weak spot

in the snake's scaly covering to drink its blood.

Now, perhaps, the whistle-blowing birds

might prove advantageous.

They're fly catchers

and help keep the insect population down.

Once the birds shift their focus from the snake

to chasing mosquitos, the boa moves on.

Despite the mobbing and the opportunistic bloodsucker,

the climb may have been worth it.

The boa spots the male anole,

distracted with its territorial display.

But the anole's keen eyesight spots movement.

It freezes. Perhaps the snake won't see it.

Time for a leap of faith!

The passing of the large snake stirs up insects,

inadvertently helping another forest predator -

a golden orb web spider.

She's set traps throughout the branches.

Along the lines of silk are tiny drops of glue

secreted by glands on her abdomen.

[Insect buzzing]

Once the mosquito makes contact, there's no escape.

The spider feels the vibrations

as her trap is sprung.

She pumps tissue-dissolving venom into the bug

and wraps it in silk.

She'll let the venom dissolve it from the inside,

then come back later to suck up the insect smoothie.

Below, the female black land crab hurries on.

She's now more than halfway through her journey.

But she can't afford any delays.

If she's to deliver her eggs at exactly the right time,

she has just 36 hours left to complete her mission.

She's already faced numerous dangers.

But the biggest hurdle is yet to come.

The entire island is encompassed by a road.

Around 6,000 people live on Providencia.

And all of them use this road -

the only route to anywhere.

But it's an unavoidable obstacle

for egg-bearing crabs trying to reach the coast.

All of them will have to cross it.

It takes a bigger toll on the crab migration

than any other threat.

The Tarmac is already littered

with the evidence of failed crossings.

As the second day of her journey comes to a close,

the female hunkers down for a rest,

before making the perilous crossing.

But other crabs, desperate to deliver their eggs in time,

march on.

Caught in the headlights, crabs must dodge the wheels.

For many, it's the last thing they'll see.

In a macabre twist,

the squashed crabs help those that survive.

To fuel the journey, some crabs turn cannibal,

picking over the remains of their fallen comrades.

Were crabs to have nightmares,

it would surely be this.

In the early hours of the morning,

our female makes her move.

Even at this time, the road is surprisingly busy.

Long before the sun's up,

fishermen are heading to the docks to ready their boats.

Busy with their own journeys,

they ignore the crabs making theirs.

Another close shave.

Some cannibal crabs linger to feed but, in doing so,

they risk becoming another course.

Our female, like all the others,

is on a tight schedule and keeps going.

Success.

No sooner has the female got over one hurdle,

she's faced with another.

The early morning light

reveals a mob blocking her path.

More crabs - but these ones are different.

Blue land crabs are abundant in the lowlands

closer to the coast.

They're often bigger than the migrating black land crabs,

with bodies that can span up to 15 centimeters across.

The males are equipped with an enlarged claw.

This one brandishes its weapon

as the female black land crab approaches.

The blue males are fiercely protective

of the small patch of land around their burrows

and the sudden influx of black land crabs

has them all worked up.

Like the black land crab, blues are mostly plant eaters

collecting fruit and leaves that fall close to home.

But they have been known to turn cannibal.

Undeterred, the female muscles through.

As with the black land crabs,

the wet season has triggered the breeding of the blues.

The males have already played their part.

They used pheromones to draw females

into their burrows for mating.

Now, two weeks on, their females

are also heading to the coast to release their eggs.

So they too join the march.

As the sun climbs, penetrating the vegetation,

waking animals bring the forests to life.

Finches forage and reptiles emerge from the leaf litter

to bask in the morning sun.

The tiny anoles stay out of the way

of the larger lizards foraging for breakfast.

A 30-centimeter rainbow whiptail

searches for insects.

But it's an opportunist,

and wouldn't hesitate to snatch a smaller lizard.

The crabs hurry on, but they're being watched.

Yellow-crowned night herons have gathered

to take advantage of the traveling seafood buffet.

Known locally as ".crab crushers"

these herons are solitary and patient hunters.

The heron's beak easily smashes through

the tough crab carapace.

Then it swallows its victim whole.

Luckily, there are only so many crabs

a heron can stomach,

and, with its belly full, this one flies off to digest.

Both blue and back land crabs

that escaped the crab crusher press on.

They're less than a kilometer from the coast,

but the path ahead is a rich mosaic of habitats.

A creek snakes through the forest.

Although there's water here, it's brackish,

a mix of freshwater flowing down from the hills

and saltwater from the sea.

It's not salty enough for the crabs to spawn.

Crabs evolved in the ocean and remain tied to it.

Their young cannot survive freshwater,

so they must press on.

The creek supports another landscape

that the crabs must negotiate -

mangrove.

This will slow down the progress of our female crab.

She hasn't had to deal with terrain like this

since she was a baby, four years ago.

She cautiously picks her way

through the dense tangle of roots.

These are a mangrove trademark.

The trees are saltwater specialists.

They live in dense mud that's low in oxygen.

Specialized roots protrude like periscopes,

absorbing oxygen directly from the air.

Desperate to reach the water,

the crab gingerly picks her way

through the maze of vertical spires.

Her slow progress is being carefully watched

by a mangrove resident - a red-footed tortoise.

But he's no threat.

He spends at least half the day sleeping,

so his energy demands are low.

His food needs are minimal and he's not a fussy eater.

The cooler morning hours are the perfect time

for him to forage.

Most plant matter, fungi and even insects

are all on the menu, though he'll also scavenge.

The occasional bit of carrion provides calcium

that's lacking in the mangrove leaves.

It's now day three, the crab's deadline looms.

Having descended the mountain,

traversed the forest

and navigated her way through the tangled mangroves,

the crab is almost finally at the water's edge.

She's surrounded by crabs, both blue and black,

all laden with eggs.

They've come from all over the island

and arrived en masse in perfect sync

to hit the beach at nightfall.

They're all aiming for this same point.

And it is here that the crabs

will finally complete their journey.

Most of the coastline is rocky.

But here there is a gentle slope

that will allow the crabs to paddle

and safely release their eggs.

The ideal conditions are all thanks

to the fine sand on the beach.

The crabs have this ideal spawning site

thanks to the work of industrious fish

living just offshore.

Parrotfish.

Their home is the third-longest

barrier reef on Earth,

one that stretches 32 kilometers along the coast,

covering 250 square kilometers.

The parrotfish spend up to 90 per cent of their day

feeding on algae.

They use powerful, bird-like beaks

and a set of strong grinding teeth

to scrape the algae off the rock and break up the coral.

The result is a cleaner, healthier reef.

And the by-product is sand.

A large parrotfish can poop out

up to 90 kilos of sand a year,

much of which is washed onto the beach

two kilometers away by the currents.

This creates the perfect spawning environment

for the black and blue land crabs.

Night falls on day three.

Under the cover of darkness,

the female emerges,

at last sensing the proximity of the sea.

She's one of the first to arrive.

It's been an arduous journey

but her mission isn't over yet.

There's one last hurdle before she can spawn.

In the dense mangrove,

she couldn't see the light that's her guide.

She's misjudged her exit and come out on a boulder pile

at the end of the bay.

Heavy with eggs, she's forced to carefully pick her way

down the steep slope.

She's not the only one to make the mistake.

Behind her, thousands of crabs

begin to pour down the rock face.

It's not the gently sloping sandy shore they need.

Waves breaking around the rocks

have carved out deeper channels.

But the pull of the water's strong.

Finally, she can dip her feet in the ocean.

She isn't here for a swim.

Though land crabs are tolerant of salt water,

they're not strong swimmers.

Each crab searches for a suitable place to spawn

without being swept away.

They scramble off the rocks to the gently sloping beach.

But having reached her destination,

the female black land crab suddenly stops.

She's right to hang back.

Many of the first wave of crabs

have misjudged the gentle surf.

Small as they are, the waves create a strong undercurrent

around the rocks that sweeps them out

into the deeper water close to the reef.

Night is a dangerous time to go swimming.

Tawny nurse sharks.

These sleek hunters stalk the shallow waters of the reef.

Sharks are able to hunt in the dark

thanks to electro-receptors on their skin.

These specialized organs allow them to detect

the tiniest electrical signals

generated by the muscles of moving prey.

Any land crabs that have been swept out to sea

don't stand a chance...

...vacuumed up by enlarged gill chambers.

The exhausted female crabs desperately try

to release their precious broods

while dodging the surf.

Still the spawning goes on.

Female crabs are now swarming the beaches.

Those who have released their eggs

are already making their way back

to the safety of the forest.

But our female black land crab is still holding back.

Perhaps she's mistimed it

and her eggs aren't quite ready to hatch.

She climbs to the safety of a rock and waits.

[Waves lapping]

As the first hint of dawn lights the sky,

a new threat appears overhead -

frigatebirds, known locally as man-of-war,

aerial fighters famed for mobbing other sea birds

to steal their catch.

They breed on the cliffs above the beach.

And each morning patrol Providencia's coastline

in search of an easy meal.

The spawning crabs are an easy target.

But for the female, it's now or never.

Her eggs are ready to hatch.

As soon as they make contact with water,

the young larvae start to emerge.

She pumps her abdomen,

releasing them into the current.

More than 85,000 chances for her to propagate her genes.

Perhaps only one or two will survive to adulthood

to breed themselves,

but that's enough to make all her efforts worthwhile.

In two or three weeks, the lucky few

will clamber ashore as baby crabs,

ready to take to the hills to mature.

Her legacy fulfilled,

the exhausted female begins her journey home.

Ahead, she faces three kilometers of thick forest

and a scramble up 300 meters of rocks,

with predators lurking at every turn.

But at least now the pressure's off.

She's egg-free and can take her time.

She'll recover in the mountains, until next spring,

when the clouds build & the march begins again.