Hermit crabs shake in their shells to ward off competitors who have designs on their homes, scientists have found.

Field tests conducted on a beach in Costa Rica showed Pacific hermit crabs are swiftly deterred from ousting an incumbent when they sense strong vibrations coming from inside.

Researchers at Dartmouth College in New Hampshire set out to investigate the role of vibrations after noticing that the land-dwelling crabs performed a shunting motion when others climbed on their backs.

Competition for desirable shells is so intense among Pacific hermit crabs that the animals have no qualms about clambering on those they pass to gauge if they can turf out the current owner. Most only move up the property ladder when another crab dies or is evicted.

Play Video 0:44 Hermit crabs shake shells to ward off competitors – video report

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“They are constantly looking at who is weakening enough so they can pull them out and move into that shell,” said Mark Laidre, who studies the social lives of hermit crabs. “When someone jumps on them, they duck inside their shell and perform this shunting movement to deter them.”

The sudden movement is not enough to throw an aggressor off, but the experiments by Laidre and his colleague, Louise Roberts, suggest that by shaking in their shells, the crabs signal that someone is home and intends to stay put.

In a series of field tests, Laidre and Roberts watched as hermit crabs approached a particularly well-appointed shell they had placed on the beach. Inside the shell was a device that mimicked the vibrations produced by small or large crabs.

From their vantage point behind an umbrella several metres away, the scientists looked on as hermit crabs approached the shell, which offered spacious open-plan living only a short walk from the sea. When a hopeful new tenant touched the shell, they hit a button which randomly selected whether the shell vibrated strongly, weakly, or not at all.

“When they feel the strong vibration they back off right away. But when it’s weak, they’re much more likely to try to flip the shell over, which is the first step in escalating the conflict because that’s the only way they can get in the door,” Laidre said. “If there’s no vibration, they keep at it for a long time, but the wires prevent them from turning the shell over.”

Writing in Biology Letters, the scientists explain that vibrations are a neglected aspect of animal communication. Next, they hope to investigate the unusual chirping sound that hermit crabs make when eviction is under way.