They gather together on lawns to feed because the group dynamic gives them protection from cats and hawks. Generally, however, earthworms are loathe to come to the surface as they rapidly lose moisture when exposed to UV rays. Your driveway in the summertime is probably a graveyard of crusty worms, a good example of the dangers of leaving their natural surroundings. When it rains heavily, though, the worms have little choice.
Earthworms get their oxygen from the soil in which they live. On the sidewalk, in puddles, on the street. As is the case with so many nature stories, the real reason is likely more complicated than we imagine. This is still taught to schoolkids, and you can find a lot of detailed explanation online. It makes sense. Most researchers, though, dispute this explanation. As Chris Lowe, a researcher at the University of Central Lancashire, points out in Scientific American , earthworms breathe through their skin and require moisture to do so.
Humans drown when their lungs fill with water. This is not possible for earthworms as they lack lungs. Multiple studies have also shown that most earthworm species can survive being submerged in water for two weeks or more. Research published in the journal Invertebrate Biology , for instance, found that worm behavior following rain depended on the species. Two earthworm species with different life histories were investigated by the Taiwanese researchers.
One species consumed more oxygen at night than during the day, and the other consumed lower amounts of oxygen equally during a hour period. The species that consumed more oxygen at night tolerated water immersion poorly. At night, when it needed oxygen the most, it was especially intolerant of rainfall and came to the surface.
The other species never came to the surface, even during the rain. Its consistent, lower intake of oxygen enabled it to better stand water immersion.
So it may well be that the earthworms you see on sidewalks and streets are those that need more oxygen. But many researchers doubt this is the entire explanation. Another common explanation for worm emergence is that rain sounds like predators, so the worms come to the surface to escape. Moles, common earthworm predators, make vibrations in the soil as they hunt.
This brings worms to the top that are then harvested for bait. Essentially, humans are mimicking the sounds of hunting moles. I admit I have tried this without much luck. The predation-escape theory suggests that the patter of raindrops is also similar to the sound of moles. University of Wisconsin professor Thea Whitman notes that this theory has never been demonstrated in laboratory tests.
It always seemed like an unconvincing explanation to me. If worms were dying due to moisture, or fleeing predators, you might expect to see a broad cross-section of a given worm population. After a rainstorm, not all of the worms of a given species come to the surface.
When worms are brought to the surface with electricity, worm-grunting, or chemicals, many more worms are present.
It is obvious that only a small percentage of the adult red worms are emerging during heavy rains. Think about this. You are not seeing young earthworms in your damp backyard. Many earthworm and soil experts have settled on a version of this explanation : Moving around the soil is relatively slow and difficult, even for earthworms. They can cover a lot more ground on the surface.
The problem is, earthworms need to stay moist. Most of the time, they would dehydrate if they were above ground. But when it rains, the surface is moist enough for worms to survive and remain hydrated. For a few species, they can more easily move about and find mates. For other earthworms, it may well just be a way to disperse and move into new territory.
They can migrate much longer distances than they ever could underground. Of course, coming to the surface poses hazards. The earthworms are easy pickings for birds and other predators. In our urban and suburban world, they also get trapped on sidewalks, patios and roads. The worms I encountered this morning gave the impression that their migration strategy was a failed one. But most worms survive. They move across the lawn or park and burrow back into the soil without any problems.
There undoubtedly will be new research and new revelations about the lives and habits of earthworms. The common creatures that live around us still hold surprises. Take a moment and observe the phenomenon happening in the natural world, right now, where we live. The surprises and wonders are right under our feet. Please note that all comments are moderated and may take some time to appear. This article is very timely.
We had several days of rain in Santa Barbara. Most of these were already dried out. I was more surprised on two mornings to find worms that had crawled about three feet into our living room. They must have that entered the house by crawling under the door at night. The answer to this question is not what you may think. In fact, because they require moisture to breathe through the skin, worms can survive underwater for several days.
Another possibility is that the raindrops create vibrations similar to predator vibrations. Another earthworm predator that uses vibrations to unearth worms? According to AccuWeather. So, next time a storm rolls through Indiana, be sure to take a close look at the ground afterwards and count!
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