Giant coconut crab sneaks up on a sleeping bird and kills it
A giant coconut crab has been filmed stalking, killing and devouring a seabird. It is the first time these whopping crustaceans have been seen actively hunting large, back-boned animals, and suggests they might dominate their island ecosystems.
Coconut crabs (Birgus latro), also known as robber crabs, are an imposing sight. They can weigh up to 4 kilograms, as much as a house cat, and sport legs that span almost a metre. This makes them the largest invertebrates – animals without backbones – on land. The crabs live on coral atolls in the tropical Indian and Pacific oceans.
They are renowned for their tree-climbing abilities and taste for coconuts, which they crack open with their powerful claws. They do sometimes eat meat, but until now it was thought that they only obtained it by opportunistic scavenging.
Between January and March 2016, Mark Laidre of Dartmouth College in New Hampshire visited the Chagos Archipelago, a remote series of atolls in the Indian Ocean. Chagos is ideal for studying coconut crabs: it is in pristine condition, is surrounded by one of the largest marine reserves on Earth and has lots of coconut crabs, making them easier to find and observe.
Attack of the killer crab
“The claws of coconut crabs can generate a force 80 to 100 times the mass of their body,” says Oka. “The crab in the video seems to be about 2 kilograms, so it would be able to easily break the bird’s bones.”
After the first attack, the crab slowly descended and followed the wounded bird, breaking the other wing with its claws. “At that point, when both its wings were broken and it was on the ground, it couldn’t go anywhere,” says Laidre.
Before long, five other coconut crabs ambled onto the scene, perhaps lured by the commotion and scent of blood. They proceeded to tear the bird apart and eat it.
“It was pretty gruesome,” says Laidre.
An island ruled by crabs
In line with this, surveys Laidre carried out showed that if coconut crabs were living on an island, birds were less likely to, and vice versa.
“In areas where these guys are present and abundant, it would be a smart move, especially among ground-nesting birds, not to place eggs there,” he says.
There may be a flip side to this called a “priority effect”. If an island already has lots of seabirds, coconut crabs will find it hard to colonise, as they start off small and vulnerable.
“If you have a bunch of birds there, it’s going to be very hard to get bigger because they are going to eat you,” says Laidre.
He now plans to set up remotely activated cameras at entrances to the crabs’ burrows. This should reveal what the crabs drag back to eat and how often they hunt birds.