From watching coconut crabs in the wild, Shin-ichiro Oka knew their claws were amazingly strong. And the crabs are known to be able to lift as much as 28 kilograms — seven times their top body weight of 4 kg.
But no one had ever measured how hard coconut crabs can pinch.
This is understandable considering that their claws can cut through coconut shells. Indeed, when Oka and his colleagues measured the claw pinching force, they suffered in the name of science.
“When I was pinched, I couldn’t do anything until they unfastened their claws,” recalls Oka, a researcher at the Okinawa Churashima Foundation, Japan. “Although it was only few minutes, it felt like an eternal hell.”
Undaunted, the researchers measured the claw pinching force of 29 coconut crabs on Okinawa Island, Japan. In a new study in PLOS ONE, they report that pinch force increased with body mass. Based on the crabs’ maximum weight, the maximum pinch force was projected to be about 337 kg-force (1 kg-force is just what it sounds like: the force exerted by 1 kilogram).
“We expected the force would be very strong, but the actual power exceeded our expectations,” Oka says. “We were surprised that their pinching force was approximately 90 times their body weight — if I was a coconut crab weighing 65 kg, I could crush something with about 6 tons-force!”
Coconut crabs pinch harder than all other crustaceans. And the crabs even pinch harder than nearly all land predators can bite. The exception is alligators, which can snap their jaws with about 1,300 kg-force.
Their “mighty claws” let coconut crabs monopolize coconuts and hunt a variety of prey, which is key to maintaining their huge size. At up to a meter across, coconut crabs are the largest land arthropods in the world.
Research Article: Oka S-i, Tomita T, Miyamoto K (2016) A Mighty Claw: Pinching Force of the Coconut Crab, the Largest Terrestrial Crustacean. PLoS ONE 11(11): e0166108. doi:10.1371/journal.pone.0166108
Image Credit: Shin-ichiro Oka