All language subtitles for 02z

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.