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Just Ask Us: Why do worms come out onto the pavement after it rains?
JUST ASK US

Just Ask Us: Why do worms come out onto the pavement after it rains?

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Q: Why do worms come out on the pavement after it rains?

A: Scientists don’t know the exact reason worms come out of the soil during rain showers, but UW-Madison professor Thea Whitman said there are a few theories.

“Worms are usually more active on the soil surface at night, when predators or UV (rays) from the sun are less likely to harm them,” Whitman said. “However, right after a rain, it is common to see them all over sidewalks during the day.”

One theory is that worms would drown in the water-saturated soil if they stayed below ground. Whitman said that theory isn’t as easy as it sounds. Worms don’t have lungs, and instead absorb oxygen through their skin, which is possible even in water. Many worms can live in soil that is completely saturated for months at a time.

Whitman said some species of worms may be more susceptible to the lower oxygen levels that are found in water-saturated soil, so those may be the ones that come to the surface during rainy weather.

Another theory is that the vibrations of the rain coax the worms to surface, Whitman said. Worms travel to the surface when they feel vibrations because it could be a signal that a mole is nearby. However, Whitman said this theory did not hold up in experiments as researchers didn’t see the habit mimicked in tests.

Worms may also take advantage of a rainy day to mate or migrate, Whitman said. Some, but not all, worms travel to the surface to mate, Whitman said, and moist environments are necessary to their survival. The wet surroundings also give them a chance to travel on the surface rather than burrowing through the dirt.

— Shelley K. Mesch

Send questions to: justaskus@madison.com; Just Ask Us, P.O. Box 8058, Madison, WI 53708.

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