NARRATOR: Somewhere in the far reaches of the Indian Ocean,
a remote island withstands the elements:
Christmas Island.
For those who travel across the ocean, it provides a safe resting place.
For others, it has become a life-giving home.
Christmas Island is famous for its red land crabs,
which migrate in their millions to spawn.
But its true king is of a different kind.
The robber crab.
A giant, living up to a hundred years old,
with a leg-span reaching over three feet long.
Their path to power is an epic journey.
♪ ♪
Christmas Island is the peak of a volcanic seamount
which rose three miles high from the ocean floor.
The island was named by the British captain William Mynors,
who discovered it on Christmas Day in 1643.
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It belongs to Australia,
although that continent lays more than 600 miles further southeast.
The island's 45-mile coastline is an almost continuous sea cliff.
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The shallow waters bordering Christmas Island
attract large numbers of marine inhabitants.
Coral reefs provide shelter for many species.
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This is where robber crabs begin their lives,
as planktonic larvae.
Only a few will make it through the first weeks.
There are many mouths on this reef.
Including the corals themselves.
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Of the young that survive the coral,
many are lost to strong sea currents and storms that batter the rugged coastline.
After two months at sea,
the few remaining larvae crawl ashore,
ready to begin their life on land.
♪ ♪
This young robber is about three years old,
far from being grown up.
He's hungry and can detect food from long distances.
This coconut's irresistible fragrance
hasn't only caught the attention of this young robber.
But of a giant...
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...whose claws could crush him in an instant.
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At up to eight pounds,
male robbers are the largest land invertebrates on Earth.
But he should be safe in here.
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A grown robber can open a coconut with ease.
It's only the fibrous husk that slows them down.
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This obsession with coconuts has earned them the nickname coconut crabs.
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Hidden away, the little robber still isn't safe.
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This neighborhood's just too rough!
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Leaving the coast behind, the forest becomes his new home.
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Strange creatures share this foreign land filled with dense undergrowth.
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He'll have to get used to the sight of one species.
Red crabs.
They're everywhere!
Fifty million of them roam the island.
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Red crabs are generally considered harmless vegetarians.
However, they do like to complement their menu with small animals.
The young robber would be ideal.
He should stay away from those claws, at least while he's small.
♪ ♪
Hunger drives the young robber forward.
Finally, about 66 feet above, a breadfruit.
For him, out of reach.
A pack of flying foxes have already discovered the delicacy.
These messy eaters turn out to be a stroke of luck.
But beware, not only food falls from the sky.
The flying foxes are the last remaining mammals on Christmas Island.
The other bats, native rodents and shrews that once existed here
couldn't survive with the arrival of humans
and the creatures they brought with them.
For the longest time, the island remained uninhabited.
It was only when phosphate was discovered in the soil that a mine opened.
Today the island has about 1,800 residents.
With just a few crops that survive here, so most of the food comes by boat.
And unloading it can sometimes get a little shaky...
...without the security of a safe harbor.
♪ ♪
(bird squawking)
Before people came to the island,
only creatures with the ability to cross oceans could settle here.
Most notably, birds.
Abbott's boobies nest in the highest rain forest trees.
Red-footed boobies are attentive parents, and chicks grow quickly on a diet of fish.
But it can be a hazardous home.
♪ ♪
The Pisonia tree requires more of its feathered friends
than only the nutrient-rich bird droppings.
Once a year, the Pisonia tree produces sticky, barbed seeds
that cover the forest floor in a dense carpet.
Harmless to crabs,
a death trap for others.
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Their minute hooks stick to feathers,
and seabirds become unwitting couriers, spreading the seeds far and wide.
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But the Pisonia's strategy doesn't always pay off.
This young booby is so smothered in seeds, it's impossible to take flight.
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A second young bird lands on the forest floor.
She finds herself in a mine field of sticky seeds.
Every step could lead to disaster.
She needs to get off the ground as fast as possible.
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An elevated launch pad is her only chance of escape.
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She's in luck!
But the fate of the other booby is sealed.
NARRATOR: A robber crab is after delicacies of a different kind.
His keen sense of smell has picked up the scent of arenga palm fruit.
(bird cooing)
The palm relies on birds like imperial pigeons
to spread its seeds around the island.
But robber crabs are thieves by name and nature.
Skilled at taking what doesn't belong to them.
At around 30 years of age,
this male is still far from fully grown.
He's late to the feast.
Competitors are already devouring the delicious berries.
Their pincers, capable of bending metal pipes,
carefully pick up the scattered fruits.
The crabs can barely see what they're grasping,
but the tips of their claws are extremely sensitive.
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It's not long before the palm thieves have completely stripped the forest floor.
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Latecomers are ruthlessly pushed away by the biggest crabs.
(bird cooing)
It's all a matter of size.
Those that don't want to risk an amputated limb
should keep their distance.
But a daring maneuver could push the newcomer
to the front of the line.
The palm has a near-vertical trunk.
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But he's still small enough and light enough to try climbing.
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His goal hangs about 50 feet up.
The smooth trunk makes it a challenge.
And to make matters worse, the palm sways in the wind.
The robber would not survive a fall from this height.
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But with great risk comes great reward.
The crafty male has the pick of the crop.
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His only problem now is how to get down again.
Cumulus clouds announce the beginning of the rainy season.
On average, more than six feet of rain falls on Christmas Island each year.
A welcome relief for frigate birds.
But a serious problem for animals whose nest is made of paper.
The wasps do their best to prevent water damage.
The eggs and larvae could drown inside their chambers.
Turning their bodies into miniature pumps,
the wasps dry their nest, one drop at a time.
(thunder rumbling)
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Eventually the rain stops.
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And the parched forests are revived.
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Mushrooms sprout from rotting wood on the forest floor.
Their fluorescent glow attracts insects that spread their spores.
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The penetrable soil stores very little rainwater.
Only in a few places do ponds and streams develop.
These are ideal conditions for the iconic red crabs.
NARRATOR: It's the start of the wet season
and the rain-soaked forest floor lures the red crabs out of their dens.
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One of the largest animal migrations on Earth is about to begin.
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The red crabs' destination is the coast, where they will mate and spawn.
On their way, they eat everything they can find.
The hungry hordes clean up as they go.
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Moist, shady valleys become migration passageways,
protecting the crabs from the sun.
But here, there are bandits waiting in ambush.
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More and more crabs gather in the valley.
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Like bears in a river full of salmon,
the robber crabs wait for their prey to come to them.
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But unlike bears,
they are hopeless hunters.
Slow...
and impatient.
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It's an awkward hunt.
But when an attack succeeds, there's no escape.
(shell cracks)
Although individuals pay a deadly price,
the loss to the red crab population is minor.
The migration soon meets its next obstacle.
Roads!
In the past, millions of crabs were run over.
Today they enjoy a more privileged status.
For a few days of the year, red has right of way.
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It's a luxury that would benefit robber crabs seeking their next coconut.
Even though robber crabs are protected on Christmas Island,
roadkill is still a major problem.
Up to 1,000 robbers die each year on the island's few roadways.
(flies buzzing)
♪ ♪
But one man is trying to make the roads safer for these ambling giants.
Park Ranger Jason Turl.
JASON: We are looking for robber crabs that have been hit by cars,
and, uh, we do a count on the road mortality of robber crabs
crossing the roads.
What we need to do is just take our time on the roads.
♪ ♪
NARRATOR: Unfortunately, dead crabs are a common sight for Jason.
It really, um, upsets me, um, personally seeing these old guys hit by cars,
because they have spent so long...
You get some guys living up to a hundred years,
and, you know, they have seen the world wars
and they have seen much more in their lifetime than I have,
and it's, you know, very sad to see something that's had such a long life
to be brought down by something like a car.
NARRATOR: Jason removes the dead animals,
so that the scent of decay doesn't lure other robbers on the road.
Finally, a pink marker reminds drivers to be more careful in the future.
For the red crabs migrating to the coast, one last obstacle awaits.
The forest suddenly gives way to manicured grass.
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The open terrain is unfamiliar territory.
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And who are these two-legged creatures chasing little white balls?
Normally, the males would now be digging courtship burrows.
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It's impossible in this traffic.
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For the golfers, the crab migration is nothing more than an additional handicap,
which soon drifts on towards the sea.
NARRATOR: Once the fairways have been successfully navigated,
the hard work really begins for the males.
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Only those with perfect breeding dens will secure the chance to mate.
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The forest floor resembles a construction site,
as soil is tilled and excavated.
For a few days the usual peaceful crabs will become aggressive fighters.
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The brawling sessions end suddenly, with the arrival of the females.
Mating is tender, face to face, with 20 legs coiled.
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A public display of affection attracts unwanted attention.
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Mating complete, the females retreat into the new dens,
until their eggs have matured.
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In the human world, there's also reason to celebrate.
It's the birthday of the Taoist deity Kang Tian Tai Di.
♪ ♪
(indistinct chattering)
But not all guests are welcome.
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Only when night falls and all is quiet
does this burglar make his move.
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Coconut cakes have been left as an offering.
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And that's just what this thief is after.
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Getting up onto the altar is a challenge in itself.
This smash-and-grab isn't going the way he'd planned.
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Well, he already knows one way down.
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It's the perfect crime, robber crab style.
A month has passed since the red crabs mated.
On the coast, one of nature's greatest spectacles is about to unfold.
The females now leave the safety of the forest.
Each animal carries up to 100,000 miniature eggs.
More and more females gather on the coast,
ready to release their eggs into the ocean.
Spawning is risky business.
Being land dwellers,
red crabs have lost the ability to breathe underwater
and can easily drown.
The safest time to spawn is on the gentle neap tide
determined by the moon phase.
It's the calmest sea of the month.
Time is short.
Those who wait too long risk being caught by the rising sun.
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Most crabs spawn in just one night.
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The first females brave the treacherous waters.
It's time to get jiggy.
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(muffled music)
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Hands in the air and wave them like you just don't care.
The girls shake their booties until every last egg is free.
♪ ♪
(muffled music)
The eggs hatch the moment they touch seawater,
freeing the tiny larvae inside.
For the next three to four weeks, they'll drift on the current as plankton.
Only a few larvae survive the hungry reef
and its many mouths.
But some years, a miracle happens.
The beach is swamped by a gelatinous red tide,
millions upon millions of tiny crabs.
The babies have returned home, ready to begin their life on land.
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There are so many this year that the entire coast turns red.
A marvel of nature, on a level that's rarely ever seen.
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NARRATOR: The small crabs don't quite look like their parents yet.
(birds chirping)
But that's about to change,
as the babies seek shelter in small crevices.
They molt into their adult form,
and their old skin is eaten to recover the nutrients.
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With tough new armor, they're now ready to face the challenges of the world.
Like a red invasion, they cover trees and roots.
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The forest-dwellers are overwhelmed by this living flood.
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For the first time, they encounter their largest enemy.
But the greatest danger is from animals even smaller than themselves,
ants.
Yellow crazy ants arrived years ago as stowaways on ships
and have since spread across most of the island.
It only takes a single ant to kill a baby crab.
But by working together, the ants can kill with deadly efficiency.
It's an unfair fight with only one winner.
Some years, almost all the baby crabs are killed.
But this time, their numbers are so overwhelming
that even the ants can't stop them.
The ants shouldn't pose much danger to a fully grown robber crab.
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He's about 800,000 times larger than a crazy ant,
and he's just passing by.
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Yet size means nothing to ants defending their colony from an intruder.
It seems like the tiny insects are fighting against the odds.
But the crazy ants' true strength lies in numbers.
The first attackers emit a scent that signals for backup.
The reinforcements spray formic acid from their abdomens,
targeting the leg joints and eyes.
Increasingly blind, the robber crab loses orientation.
Wherever he goes, ants are waiting.
He's stuck in the middle of a super colony.
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His fate is sealed.
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(flies buzzing)
The proud king becomes a feast for an insatiable army.
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Adult red crabs are killed in even larger numbers.
Tanya Detto from the National Park is trying to stop the crazy ant invasion.
Red crabs play a huge role in the health of the whole ecosystem,
so as soon as the red crabs are wiped out,
it has a big impact on the forest structure.
NARRATOR: Without crabs to eat seeds and seedlings,
the forest grows wild.
The ants lived here for a long time in relative peace,
but that changed with the arrival of another stowaway.
Scale insects, resembling small buds,
feast on plant sap and excrete the excess as sugary honeydew.
These sweet secretions fuel the crazy ants' rampage,
allowing them to form their immense super colonies.
♪ ♪
Tanya and her colleagues
believe they've found an effective weapon against the ants.
(gunshot)
We're not shooting ants, we're shooting down some branches,
where we find these scale insects, which are what the ants feed on.
So we're hoping to try and control the ants by controlling these scales.
NARRATOR: But how do you combat tiny insects in a jungle this dense?
NARRATOR: Inside dozens of small vessels are microscopic wasps.
♪ ♪
In the coming months and years, Tanya will breed them in her lab.
The wasps have a parasitic lifestyle.
Using a needle-like ovipositor,
females inject their eggs into the scale insects.
Like a sci-fi horror film, the baby wasps hatch within
and eat their host from the inside out, killing the scale in the process.
To ensure a successful release,
Tanya has bred thousands of wasps in a special greenhouse.
Now, the time has come.
The baby wasps are transferred inside their scale hosts,
and raised high into the canopy.
Here, they'll hatch and find new scale insects to infect.
The health of the entire ecosystem depends on the success of this project.
A great responsibility for a tiny wasp.
Meanwhile, the robber crabs have retreated underground.
It's breeding season.
A female carefully approaches a much larger male.
In the shelter of the cave, they'll mate,
and she will spend the next few weeks underground, until her eggs mature.
The high humidity and the constant temperatures
are ideal for the development of robber crab eggs.
♪ ♪
Embryos are growing, dutifully protected by their mother.
Others also use the cave to raise their young.
At first glance they look like bats.
But they're actually birds.
Christmas Island swiftlets use sticky saliva
to attach their palm fiber nests to the cave ceiling.
They remain faithful to their nesting sites for generations.
Mosquitoes hide in the dark,
stealing the blood of nesting adults and vulnerable chicks,
to in turn invest this energy into their own offspring.
♪ ♪
For the mother robber crab, it's time to leave the cave.
It's been five weeks, and her eggs are ready to hatch.
Like the red crabs, she'll entrust her offspring to the sea.
But her journey is far more dangerous.
An army of razor-sharp rocks block her way to the water,
and the sun beats down mercilessly.
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The ocean's energy violently explodes from cracks in the limestone,
filling the air with salt.
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To cross now would be too risky.
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Only when the sun loses its power
will the female begin her mission.
She navigates her way with ease through the maze of stones.
Their sharp edges don't seem to bother her.
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At last, she reaches the edge of the island.
For her, the very edge of the world.
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Risking everything, she begins the death-defying descent to the water.
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In constant danger of being swallowed by the waves,
she starts to loosen the egg packages.
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Pounding swells do the rest.
This is the most dangerous moment of a female robber crab's life.
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Her offspring are now at the mercy of the sea.
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The cycle begins anew.
Only a few young crabs will make it back to Christmas Island.
Their safe home in the expanse of the Indian Ocean.