On a rainy day, a common sight is the emergence of earthworms onto sidewalks and roads, crawling away from the apparent safety of their underground burrows. But why? The popular belief that they’re escaping drowning is surprisingly incorrect. Unlike humans who breathe through lungs, earthworms absorb oxygen through their skin, a process that works equally well in water and air. In fact, studies have shown that earthworms can survive for weeks submerged in saturated soil, as long as oxygen is present.
While the drowning theory is debunked, oxygen levels may still play a role, albeit a nuanced one. A 2008 study revealed that earthworm species with higher oxygen requirements were more prone to surfacing during rain compared to those with lower needs. This suggests that for some species, heavy rainfall may decrease oxygen levels in their burrows, prompting them to seek better conditions above ground. The species’ oxygen needs, therefore, determine their response to rainfall.
Another intriguing theory posits that the vibrations from raindrops mimic those caused by moles, a common earthworm predator. The idea is that worms interpret the rhythmic vibrations as a threat and surface to escape. However, experts like earthworm ecologist Kevin Butt find this less plausible. The inconsistent nature of mole activity in the soil would likely be distinguishable from the regular pattern of rainfall vibrations.
The possibility of rainwater harming worms through acidity or the release of harmful chemicals like heavy metals has also been considered. While a 2008 study didn’t support this, the use of chemicals by scientists to collect worms supports the idea that certain substances can irritate them to the surface. A common practice involves pouring a mustard powder and water mixture, effectively driving the worms out for analysis. This highlights the sensitivity of earthworms to certain substances in their environment.
The most compelling explanation, however, suggests that the wet surface provides earthworms with a more efficient means of travel. Instead of slowly burrowing through the soil, they can move rapidly across the damp ground. This mobility could facilitate mating or migration, vital processes in their life cycle. The improved locomotion on wet surfaces allows for more efficient movement and longer journeys for mating or migration.
This theory aligns with a practice called “foot trembling,” observed in some birds and reptiles. These animals create vibrations by stomping on the ground, potentially mimicking rain vibrations to lure earthworms to the surface and become easy prey. This predatory behavior highlights the impact of vibrations on earthworm behavior.
Furthermore, the human practice of “worm charming” or “worm grunting” exploits this vulnerability. By creating vibrations using sticks or saws, people entice worms to the surface, often harvesting them for fishing bait. This centuries-old practice has even evolved into competitive events, like the annual Worm Gruntin’ Festival in Sopchoppy, Florida. This cultural practice underscores the long-standing understanding, albeit an exploitative one, of the earthworms’ sensitivity to vibrations.
In conclusion, while the simple explanation of avoiding drowning is false, the reason why earthworms surface during rainfall is likely a multifaceted phenomenon, involving oxygen requirements, potential threat perception from vibrations, and the sheer practicality of easier movement on wet surfaces. The interplay of these factors contributes to this fascinating natural behavior, a testament to the complex lives of these often-overlooked creatures.
