It sounds a little like snake charming. You drive a wooden stake into the ground and draw a flat metal rod across the top, creating an bullfrog-like grunting sound. Within minutes, hundreds of earthworms come to the surface, where they meet their doom in the buckets of fishing bait collectors.
Now, researcher Ken Catania of Vanderbilt University in Nashville, Tenn. has an explanation for the worms' hightailing it to the surface: Worm grunting mimics the vibrations made by burrowing moles, a voracious worm predator.
Catania, who studies moles, had read Charles Darwin's statement in a book he wrote on worms: "It has often been said that if the ground is beaten or otherwise made to tremble, worms believe that they are pursued by a mole and leave their burrows."
Others have observed certain turtles and birds tapping the ground to bring worms to the surface to eat.
"Then I heard about this 'worm grunting'," Catania said.
Worm grunting has been going on in Florida's Apalachicola National Forest for decades. In the 1960s and 70s, hundreds of people earned a living by grunting for worms to sell as fishing bait. Today, there are only a handful of professional grunters, but an annual worm grunting festival happens each year in Sopchoppy, Fla.
"Basically, the question was, 'Why do earthworms come up?' They should go the other direction." Catania said. "Lots of things would eat an earthworm, so why would they come to the surface?"
In addition to the idea that mole-mimicry sends the worms scooting upward, another idea was that grunting mimics the vibrations of rain. It's known that worms appear on sidewalks during a rainstorm, presumably to keep from drowning in the soil, so some suspected this triggered the same response.
But nobody had done the tests.
So Catania contacted Gary and Audrey Revell, two of the remaining professional grunters in Apalachicola. They worked with Catania on his experiments.
At one point, Catania was having iced tea with the Revells, who had a bucket full of worm-laden dirt. Catania had a mole he had caught. "I said, 'Should I put him in the bucket?'" He did. "Up bolted the worms. When the worms came up, they looked like they were doing the equivalent of a worm run, if there is such a thing."
From there, Catania repeated the observation in more controlled environments — boxes filled with dirt and a known number of worms, into which he put a mole and recorded the worms' behavior. In trials in an outdoor enclosure containing 300 worms and one mole, one third of the worms came to the surface within an hour.
In contrast, Catania measured the worms' response to a rainstorm. Only six earthworms came to the surface over three trials, even though water was standing on the soil surface by the end of the trial. Afterward, the worms appeared healthy; they had not drowned.
Catania also made recordings of both grunting and mole sounds and showed that the sound spectra overlapped.
All of this points to moles, not rain, as the trigger to flee.
(But why, then, do worms come to the surface during rainstorms, if they don't drown? Catania suggests that some species of worm may be more sensitive to oxygen, and therefore to rain, than others.)
He published his results this week in the open-access journal PLoS ONE.
In another paper published this week in Biology Letters, a team including Mac Callaham at the U.S. Geological Survey in Athens, Ga., showed that recordings of the worm grunting vibrations will bring worms to the surface, and that more worms surface nearer to the source of the vibrations, where they are stronger.
"It seems remarkable that they would give up their safe haven and expose themselves to dry air and other predators," Bill Kristan, a neurobiologist at the University of California, San Diego told Discovery News. "It must be a really strong motivator. It's a very strong response. You don't have to do a video and look at subtle little movements. These guys are running out of their holes."
"I think it's a wonderful combination of folk science and historical science and up-to-date science," Kristan said. "Darwin had it right 150 years ago."
"I'm jealous as hell that somebody could have that much fun and make a really interesting insight," he added. "I think it's one of the most enjoyable papers I've ever read."