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Don’t mess with the coconut crab, one of the strongest pinchers on land

Coconut crab
Coconut crabs, like this one weighing about 4 1/2 pounds, can pinch with their claws harder than most other terrestrial animals can bite.
(Shin-ichiro Oka)

Coconut crabs might be the heavyweight champions of all crustaceans.

The largest land-dwelling crab on Earth, Birgus latro can lift about 66 pounds with its pincers and can pinch with about 750 pounds of force. That makes the coconut crab among the strongest terrestrial animals — only alligators and a few other species have a stronger bite force.

These intimidating findings come from a new study published in the journal PLOS One, in which researchers at Okinawa Churashima Foundation in Japan measured the pinching prowess of 29 coconut crabs on Okinawa Island.

The crabs varied significantly in weight, ranging from less than a pound to about 5 pounds. Their pinches exerted about 7 to 400 pounds of force (or 29 to 1,765 Newtons).

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During the challenging process of measuring and weighing the crabs, the researchers got pinched multiple times by the animals’ claws.

Since the the strength of the crabs’ claws was strongly correlated with body mass, the study authors were able to calculate the pinching force of the largest recorded coconut crab. This giant, 9-pound crustacean would have been able to pinch with about 750 pounds of force (or 3,300 Netwons).

To put that in perspective, a human’s bite (from the molar) exerts an average of 265 pounds of force. And an Olympic boxer’s average punch exerts around 770 pounds of force, although this is more of a push than a clamping force.

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Coconut crabs, or robber crabs, may have gained their tremendous claws as they lost the need to carry a shell during the course of their evolution. These crustaceans are descended from a hermit crab ancestor which, up to about 5 million years ago, would have scavenged a hard snail shell to carry on its back for protection.

Without their shells, the crabs were able to grow larger and protected themselves by developing a hard, calcified abdomen, the study suggests. (Young coconut crabs do carry a shell, but only while they’re very small). 

Using their super-crustacean strength, coconut crabs brandish their claws to ward off competitors and fight other animals for food and resources. But their claws aren’t just weapons, the authors wrote.

The crabs are found on islands across the Pacific and Indian oceans, and their powerful pincers give them access to all kinds of foods their competitors can’t get. Their menu options include other hard-bodied animals, carrion, fruit and the fallen insides of trees. 

Most important, though, they use their large claws to tear through the husks of their favorite foods: coconuts.

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