It’s a pole, but it sounds like a mole.
That’s the conclusion of the first scientific study of the old art called worm grunting. In the southeastern United States, bait collectors hunt earthworms by rubbing a piece of iron across a stake in the ground, a technique that creates vibrations that sound like grunting noises. The serenade brings earthworms to the surface, where collectors grab them.
Those grunting vibrations probably trigger the worms’ urge to flee from moles, says Kenneth Catania of VanderbiltUniversity in Nashville, Tenn. An alternative explanation, that grunting mimics the patter of rain, doesn’t fit his evidence, he reported online October 13 in PLoS ONE.
A worm grunter could be a new example of what theorists call a rare enemy, a classic twist in predator-prey dynamics, Catania says. Moles can eat a lot of earthworms so he’s not surprised that worms have evolved an urge to rush out of the ground at the first twang of any, even remotely mole-like vibration.
Compared to moles, though, people haven’t played such an important role in earthworm evolutionary history. Thus they count as rare enemies. Their habits don’t have the punch of the big mole menace, and people can easily exploit worms’ defensive reactions — such as fleeing vibrations, the reaction that works against the more important predator.
Catania started his quest to understand worm grunting by visiting the 2008 Sopchoppy Worm Gruntin’ Festival in Florida. There, expert practitioners Gary and Audrey Revell, who run a local bait shop, wowed Catania with their speed at catching the hefty Diplocardia mississippiensis earthworms.
These earthworms are native to the region, as is the eastern American mole, Scalopus aquaticus. Home to two long-time antagonists, the locale seemed the place for testing ideas about how worm grunting works, Catania says.
In his 1881 treatise on “vegetable mould” and worms, Darwin had suggested that when the ground trembles, earthworms flee as if from a mole. He’d tried pounding on the ground but got no reaction from worms, possibly, he wrote, because he wasn’t doing it right.
Catania followed the Revells around the ApalachicolaNational Forest, where a permit allows them to collect the worms. Moles abound there and have a taste for the worm species: A mole in Catania’s lab ate its weight in Florida worms day after day.
When the Revells started grunting, worms burst to the ground at top speed, about 50 centimeters per minute, but then slowed down. “They kind of come out running,” Catania says, as if in mad flight from danger.
To see if moles evoked the same flight, Catania observed worms in buckets of soil and in his soil-filled worm enclosures. When he released a mole onto the soil surface, the mole burrowed down and worms crawled up. Surfacing earthworms headed in just about every direction except toward the mole. In the enclosures, broadcasting a recording of a digging mole produced the same response.
Worms also crawl above ground after a rain, so Catania drenched his enclosures with a sprinkler. And for a more natural effect, he waited for big thunderstorms to see if pounding raindrops prompted worms to surface. “I was out there in the rain with a flashlight,” he says. In both cases, a few worms eventually surfaced, but nothing like the numbers boiling up from a mole.
Catania’s evidence provides some support for the mole hypothesis, agrees Jayne Yack of CarletonUniversity in Ottawa, Canada. She and her colleagues published a paper online October 14 in Biology Letters documenting the success of grunting in bringing earthworms out of the soil.
Among the new questions, she says, is why some of the other known species of earthworms, such as the common night crawler, don’t appear to respond to grunting even though they contend with burrowing predators.
Still, Darwin’s hunch about moles is looking good. And maybe he would have been soothed to hear Catania say that grunting actually is tricky to learn.
