Once a year tens of millions of red crabs pour out of the forest on a remote Australian island and march to the sea, so engineers built them bridges and tunnels over the roads
On Christmas Island in the Indian Ocean, the start of the wet season triggers one of the planet's strangest commutes. The red crab migration sends a living red tide across the whole island, and humans have rebuilt their roads around it.
During the red crab migration, the roads of Christmas Island vanish under a moving carpet of crabs. Illustration: Watts & Wild.
Picture a whole island turning red as the ground itself seems to crawl toward the sea.
That is what happens every year on Christmas Island, when tens of millions of land crabs set off together in one of the greatest animal migrations on Earth.
What is the Christmas Island red crab migration? It is the yearly mass movement of tens of millions of red crabs across Christmas Island, in the Indian Ocean, from the forest to the sea to breed. It is timed to the wet season and the Moon, and rangers close roads and use crab bridges and tunnels to protect the crabs.
A red tide on land
Christmas Island is a tiny Australian territory, and it is home to an estimated 40 to 50 million red crabs that normally live quietly on the rainforest floor.
Then the first rains of the wet season arrive, and almost as one, the red crabs leave the forest and begin marching toward the coast.
The red crab migration floods gardens, paths and highways with a moving sheet of bright red bodies, sometimes piling several deep.
The whole spectacle is tied to the lunar cycle, because the crabs must reach the sea to spawn on a precise tide.
It is so dramatic that wildlife filmmakers rank it among the natural wonders of the animal world.
Closing roads for crabs
For the people who live on Christmas Island, the red crab migration is both a marvel and a logistical headache.
Driving over the crabs would kill them by the million and leave roads slick and dangerous, so the national park simply shuts roads down for the crabs.
Rangers use rakes and leaf blowers to gently sweep crabs off the tarmac, and temporary barriers steer the red crabs where it is safe to cross.
For a few weeks, the island quietly reorganises human life around the needs of a crustacean.
A bridge built for crabs
The most charming response has been to build the crabs their own infrastructure.
Christmas Island now has dozens of crab underpasses, small tunnels dug beneath the roads so the animals can cross unseen and unsquashed.
It also has a striking arched crab bridge, a purpose-built span that lifts the migration up and over a busy road.
Low barriers act like guide rails, funnelling the red crabs toward each crab bridge and tunnel rather than into the traffic.
It is one of the clearest examples anywhere of engineering bent entirely around an animal, in the same spirit as the giant wildlife crossing built for mountain lions in Los Angeles.
The march to the sea
The journey is a carefully timed ritual, not a random scramble.
The males arrive at the coast first, dig burrows in the cliffs and terraces, and wait for the females to join them and mate.
The females then brood their eggs for about two weeks before releasing them into the sea at a precise turn of the tide near dawn.
Weeks later, if conditions are right, the surviving young crabs swarm back ashore in a second, smaller migration that turns the shoreline red all over again.
Most years the ocean eats almost all the larvae, but occasionally a huge wave of babies returns and refills the forest.
The honest catch
For all its wonder, the red crab migration is under serious threat from an enemy the size of a grain of rice: yellow crazy ants.
Invasive yellow crazy ants, accidentally brought to Christmas Island, have formed vast supercolonies that spray acid and have killed an estimated tens of millions of red crabs over the years, an invasion blamed for gutting parts of the island's ecosystem. Where the crazy ants swarm, they can wipe out almost every crab in a patch of forest, leaving an eerie dead zone.
Scientists have fought back with biological control, importing a tiny wasp to attack the scale insects whose sugary secretions feed the crazy ants. Holding the crazy ants back has become a permanent battle for the island's rangers.
On top of that, the migration depends on the rains arriving on time, so a shifting climate could scramble the signal that sets the whole red crab migration in motion, which is why Parks Australia closely manages and monitors the island and its crabs.
The red crab migration is proof that a small, overlooked creature can be the beating heart of an entire island, important enough that people will close their roads and build a crab bridge just to let it pass.
It is a cousin of the other places where humans have learned to engineer around the wild, from beehive fences that keep elephants and farmers apart to the dams torn down so salmon could migrate again.
Would you happily give up your roads for a few weeks each year so tens of millions of crabs could reach the sea, and what does it say about us when we build a bridge for a crab? Tell us in the comments.