The Grunter Gets the Worm

The Eighth Annual Sopchoppy Worm Gruntin' Festival in Sopchoppy, Florida, hardly seems like the place to make an important scientific discovery. But that's what happened this year, when two teams of researchers descended on the event, intent on figuring out why grunting noises bring burrowing earthworms to the surface.

Dozens of Florida Panhandle residents earn their living by grunting. Rising before dawn, they head out to the pine forests, hammer a wooden stake about 30 cm into the ground, and rhythmically scrape its top with a long, smooth piece of steel called a rooping iron. The rasping noises, which sound like low-pitched grunts (see video), bring hundreds of earthworms above ground, and the grunters sell them as fish bait. Yet scientists have never successfully explained why the technique works.

Hoping to get to the bottom of the mystery, a team led by biologist Jayne Yack of Carleton University in Ottawa, Canada, and another led by biologist Kenneth Catania of Vanderbilt University in Nashville, Tennessee, dragged geophones and other seismic equipment used to detect underground vibrations to the Sopchoppy festival last April. Yack and colleagues confirmed that worm grunting actually works. With help from experienced grunters, the researchers took up their own rooping irons and scraped their stakes about 25 times in 30 seconds, producing vibrations in the ground of about 100 hertz. As they report online 15 October in Biology Letters, in less than 2 minutes, their efforts brought up scores of native Diplocardia mississipiensis earthworms within a few meters of the grunting stake.

Yack suspected that the behavior arose as a defense against burrowing moles, whose digging also produces sounds at about 100 hertz, or as a response to the vibrations caused by falling rain, which can saturate the soil and drown some worm species. But she lacked the data to test these hypotheses.

That's where Catania's efforts came in. After making several trips to Sopchoppy to document the worms' response to grunting, he buried 50 worms in a box of soil and allowed a mole to tunnel in through the side. Most of the worms reacted immediately to the mole's vibrations and surfaced at their top speed of about 50 cm per minute. "It's equivalent to what a flying fish does when it leaves the water to escape a predator," says Catania, whose team reported its findings online 14 October in PLoS ONE. "Moles don't come to the surface, so as long as the worm is above ground, it is safe from them." Simulated rainfall, on the other hand, brought only a couple worms to the surface--and very slowly; it took 15 minutes or more for them to emerge.

Catania also concluded that grunters are taking advantage of what evolutionary biologist Richard Dawkins dubbed the "rare enemy effect." Earthworms have evolved to flee subterranean moles, which can eat their weight in worms every day. But when the vibrations come from a less common enemy on the surface--like herring gulls or wood turtles that also tap the ground while hunting lunch--fleeing upward no longer gets a worm off the hook.

"The studies independently confirm for the first time that earthworms are sensitive to, and respond to, vibrations that travel through the earth," says biologist Peggy Hill of the University of Tulsa in Oklahoma. Scientists are just beginning to examine the importance of vibrational signaling in the lives of animals, from elephants (ScienceNOW, 31 May 2007) to frogs, she says. "Responding to vibrational cues in the environment is turning out to be as common in the natural world as responding to sight, sound, or smell."