By: Helen Beilinson
Summer has finally settled upon Connecticut after a long winter. The undergrads are gone from campus, grad students are finding every excuse to go outside, outdoor seating at local breweries is constantly packed, and festivals are in full swing. Festivals in the United States are reaching far beyond the classic food and/or music type and into bizarre territories. There are road kill cook-off fests to days devoted to cow cake (read: dung) throwing contests. However, one fascinating festival, biologically speaking, happens every April in Sopchoppy, Florida: the Annual Worm Gruntin’ Festival.
To win this annual worm grunting contest, all you need to do is charm the greatest amount of earth worms out of the ground as possible, but you can only do so by making the ground vibratev. Using either hand tools or power equipment (although traditionalists prefer the former) one can easily cause earthworms to exit the ground by the thousands, making them very easy to collect. The most popular, and some argue most effective, way of creating the vibrations is using a stack of wood inserted into the ground that is vibrated by a flat iron slab being rubbed across the top. Although it has been known since the 1800’s that beating the ground forces earthworms to above ground, worm grunting as it is done today with wood and iron slabs originated in the 60’s and 70’s, first as a personal means to get worms for fishing, and then used on a more industrial scale for selling of worms. Over the years, the technique has been passed down and is still frequently being used for obtaining live earthworms as bait. Animals, such as herring gulls and wood turtles, have also been observed to use ground vibration to bring worms to the surface.
Despite its long history, the reason vibrating the ground charmed worms from the ground was largely a mystery before 2008. Earthworms live, well, in the earth. Leaving the ground poses two problems for worms. The first is aboveground predators. Birds and other small animals feast on worms. Second, earthworms have to continuously be in a moist environment, as they breathe air through their skin and must stay wet in order for oxygen to be exchanged through their skin (a reason why they come above ground at night or after a rainstorm—they can move faster on the cool, wet soil, while still being able to breathe).
In his last scientific book, The Formation of Vegetable Mould through the Action of Worms, Charles Darwin, knowing of the early observations that ground vibrations stirred earthworms (although he noted that he was personally unable to replicate them) commented on the strangeness of the worms’ ascending migration and offered a hypothesis that has for the last century and a half been the predominant theory for the worms’ movement. Although we are mostly familiar with worms’ above ground predators, they have underground predators as well – chiefly, moles. One of the situations in which worms escape to the ground’s surface, presumably, is to escape the jaws of moles. When moles dig, the ground around them vibrates. Darwin proposed that the vibrations made by humans or other creatures mimic the vibrations caused by mole digging, inducing fear in the worms, making them surface. It has also been proposed that the vibrations mimic heavy rainfall, which also makes worm surface, as to prevent drowning.
One hundred and twenty seven years after the publication of Darwin’s Worms, Kenneth C. Catania, a scientist from Vanderbilt University, produced a study proving that the former’s hypothesis was, indeed, correct. Catania recorded the vibrations made by the traditional technique of driving a wooden stake into the ground and rubbing a flat iron slab lengthwise across the top of the stake. Vibrations were measured at intervals away from the stake and from multiple stake locations. The magnitude of the vibrations decreased as one got further away from the stake, as to be expected, and the intensity of vibrations depended on the soil composition, as different stake locations produced different vibration intensities. Accordingly, the number of worms that emerged also decreased with increasing distance from the stake. A year after Catania’s original paper, a Canadian research group published a study recapitulating the results showing ground vibrations cause earthworms to emerge from the soil.
Catania was collecting these data in Sopchoppy, FL, home of the worm grunting contest. The only mole living in Sopchoppy is the eastern American mole. These moles eat the equivalent of their body weight every day, with a diet consisting predominantly of earthworms mixed in with some vegetable matter. To test Darwin’s original hypothesis, Catania first confirmed that the presence of these moles in the forest in Sopchoppy. By studying mole tunnels, Catania showed that the moles are abundant in the area and that there is a clear overlap in the populations of earthworms and moles.
Then, studies were performed to test whether earthworms responded to simulated rain or to digging moles. To do these studies, fifty earthworms were placed in a large, container containing soil soil and allowed to burrow into the ground. Once the worms entered the soil, Catania studied their movements, specifically, how often the worms would exit the soil. The number of exiting worms was negligible, essentially one or zero over the course of a few hours. First, Catania studied the effects of simulated rain by placing the soil boxes under a sprinkler system. The number of exiting worms remained unchanged. However, when a mole was introduced into the bottom of the container and allowed to burrow through it, nearly half of the worms, on average 24, rapidly exited the soil. Catania even noted that many worms “crawl[ed] over the container walls”. Similar observations were seen when the same experiment was conducted in a much larger area, where rain did not influence the number of worms exiting the soil, but moles digging drove them out quickly.
A human observer can hear moles digging underground when standing several feet away because they dig so powerfully, particularly if they are digging through a root-filled environment. To study these vibrations and sounds more carefully, Catania recorded their digging vibrations and found that the amplitude of the vibrations was highly similar to that of the worm grunters. One difference was that the vibrations of the worm grunter were much more consistent throughout the vibrations, as opposed to the moles. This is unsurprising, as moles do not continuously burrow, often changing directions to avoid roots or stopping to munch on a worm. To test the worms’ responses to the mole’s vibrations, Catania took the recording of the mole’s digging and modified the sound file to simulate how the digging would sound if the mole were approaching the worm, such that it progressively grew louder. This experiment was conducted in the aforementioned container containing fifty earthworms. Fascinatingly, after the recording was played, an average of 16 worms exited the container compared to the one or zero that would exit with a nonspecific recording playing, mimicking the previous results. This finding was surprising as merely the sound of the vibrations, without any physical perturbations to the soil itself, was a strong enough force to drive many worms to the surface. The difference in the number of worms leaving the soil (a difference of about ten worms between the mole vs mole sound experiements) is probably due to the worms being able to detect compression of the soil. Without the mole digging, there is no change in compression. With only one of the two (or more) “predator approaching” signals, some worms probably don’t feel enough fear to leave the soil.
This study was conducted with one species of earthworm, Diplocardia mississippiensis, and one species of mole, Scalopus aquaticus. It is unknown whether this phenomenon is carried out throughout various worm and mole species or if this effect is specific for this predator-prey pair. However, detection of predators approaching is not uncommon in the animal kingdom. In the discussion section of this paper, Catania makes an interesting comparison between the vibration-avoiding mechanism of the worms to the ultrasound-avoiding mechanism used by flying insects preyed on by bats. As bats use ultrasound for echolocation, their prey have evolved the means to sense ultrasound and have learned to fly from it to avoid predation.