Fishermen have long known that you can drive earthworms to the surface with a strange technique called worm grunting.

The trick involves driving a wooden stake into the ground and then rubbing the top of the stake with a long piece of steel called a rooping iron.

It generates a peculiar grunting sound that drives nearby earthworms to the surface where they can be easily collected for fish bait.

Charles Darwin described it long ago: "It is often said that if the ground is beaten or otherwise made to tremble worms will believe that they are pursued by a mole and leave their burrows."

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But in truth, worm grunters (the people who do this) still don’t know why the technique works.

Ken Catania, an associate professor of biological sciences at Vanderbilt University in Tennessee who studies moles, thought Darwin might be right — perhaps earthworm aim to escape from perceived predators upon sensing the vibrations.

Catania compared the vibrations produced by worm grunting and those of a mole burrowing.

Analyzing the geophone recordings of the two types of sound, he found that the worm grunting vibrations were more uniform and concentrated near 80 Hz whereas the moles produce a wider range of vibrations that peak at around 200 Hz.

Nevertheless, there is a considerable overlap between the two.

"The moles are quite noisy. Often you can hear the sounds of a mole digging in the wild from a few feet away," he said.

Worm grunting is practiced in a number of parts of the southeastern United States under various names including worm fiddling, snoring and charming, but it reached its apex in popularity the 1960s in the Florida Panhandle.

Catania's conclusion, that worm grunters drive the annelids to the surface by unknowingly mimicking the sound of digging moles, is detailed in the Oct. 14 issue of the Public Library of Science ONE.

The research was funded by a MacArthur Foundation fellowship and a National Science Foundation CAREER award.

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