Coconut crab’s bone-crushing grip is 10 times stronger than ours

Packs a painful pinch

Shin-ichoro Oka

By Alice Klein

Its handshake could crush your fingers. A giant crab from the Asia-Pacific region can lift the weight of a small child and has the most powerful claw strength of any crustacean.

The coconut crab – Birgus latro – lives on islands in the Indian and Pacific oceans, and can reach a weight of 4 kilograms, a length of 40 centimetres and a leg span of almost a metre.

Its large claws are strong enough to lift up to 28 kilograms and crack open hard coconuts – hence its name. However, the squeezing force of its claws has never been precisely measured until now.

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Shin-ichiro Oka at the Okinawa Churashima Foundation, Japan, and his colleagues recorded the claw strength of 29 wild coconut crabs weighing between 30 grams and 2 kilograms from Okinawa Island in southern Japan.

Capturing the mighty beasts was tricky because they launched into offensive mode, says Oka. “I was pinched two times and felt eternal hell,” he says.

Get a grip

After the researchers managed to hold the crabs down by their backs, they gave them a force sensor to squeeze. Claw strength was found to increase proportionally with body weight, and the highest reading reached almost 1800 newtons.

A maximum-sized coconut crab weighing 4 kilograms could thus be expected to exert a crushing force of more than 3000 newtons, says Oka. This significantly out-muscles all other crustaceans, including lobsters, which have claw strengths of about 250 newtons.

Coconut crab claws are substantially stronger than human hands, which have an average grip strength of about 300 newtons. But they cannot squeeze as hard as crocodile jaws, which bite down with a whopping 16,000 newtons – the strongest grip force known in the animal kingdom.

On Okinawa Island, where there are no coconut trees, the crabs crack open nuts and hard fruit from pandanus palms. They also eat the remains of dead animals, using their claws to break the bones. Alternative names for the species include “robber crab” and “palm thief”, due to their tendency to steal food.

Jakob Krieger at the University of Greifswald in Germany has studied coconut crabs on Christmas Island in the Indian Ocean, and has found that they hunt other land crab species, such as red crabs (Gecarcoidea natalis). “It makes sense in the light of the robber crab’s dietary demands to evolve strong claws,” he says.

Another reason for powerful claws is defence, Oka says. The adult crabs do not have shells to shield them and instead rely on a hard, calcified outer body, which is less protective. As a result, they need their claws to ward off attackers.

The crabs lead solitary lifestyles and fight aggressively with their claws if they encounter each other, probably due to competition for food. “I’ve never seen them hanging out in groups,” Oka says.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0166108

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