Nobody Pinches Harder Than a Coconut Crab

Behold the coconut crab and its magnificent clampers. Scientists in Japan say the strength of the crab’s “mighty claw” is greater than most predators’ jaws. They published a report of their crab tests in the journal PLOS One.

Coconut crabs (Birgus latro) are strange, strange critters. They’re crustaceans, but they live on land. They climb trees, as if that’s a thing that crabs should be doing. They’re huge—adults can easily reach 3 feet across. They can lift more than 60 pounds at a time. They’ll feed on dead and decaying animals when they have to, but they prefer tropical fruit, especially the coconuts that gave them their name.

But as anyone who’s ever been stranded on a desert island knows, coconuts are not a convenience food. Getting inside requires desperation, persistence, and a very tenacious claw.

Oka et al 2016. PLOS One

 
The scientists rounded up 29 wild crabs, weighed them, and took measurements of their bodies, legs, and claws. Then they dangled stick-like metal sensors in front of the crabs, who predictably grabbed the instruments and clamped down as hard as they could. Once the test was over, the crabs were released … which must have been something to see.

The results of the pinch tests were impressive. The larger a crab was, the harder it could pinch. Using this formula, the researchers determined that a large crab could exert up to 3300 Newtons (N) of force with a single claw.

That’s a lot of Newtons. To put it in context: scientists estimate that human jaws have a bite strength of about 754 N. Wolves have 1267, black bears 1747 [PDF]. Pound for pound, a coconut crab's claws come down almost twice as hard as a black bear’s jaws. The authors estimate that there’s only one land predator that can outclamp the coconut crab, and that’s an alligator.

The crab’s “mighty claw” is what’s allowed it to survive, the authors write. Not only is it a weapon, but it’s also a tool, allowing the crab to get inside just about any organism, plant or animal, it wants.

So, kids, for your fingers' sake: Please don’t tease the coconut crabs. Leave that to the professionals.