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Ask Dr. Universe: Why do worms come out when it rains?

A robin pulls a worm from the ground on a rainy afternoon. (Scott Anderson / File/Associated Press)
A robin pulls a worm from the ground on a rainy afternoon. (Scott Anderson / File/Associated Press)
Washington State University

Q: Why do worms come out when it rains? – Courtney, Moscow, Idaho

Dear Courtney,

That’s a great observation. When it rains, worms sometimes leave their home in the soil and wiggle their way up to the surface where we see them on sidewalks and roads.

Worms come to the surface to move around, but exactly why they do it or where they are headed remains a bit of a mystery. Still, scientists have some interesting theories about it.

That’s what I found out from my friend Tarah Sullivan, a scientist at WSU who studies the living soil. Soil is very much alive, she reminds me.

It’s not only home to worms, but also all kinds of plants, insects and billions of tiny living things called microbes. The different kinds of life in the soil depend on water to survive, so rain is quite often a really good thing.

One theory for why worms come to the surface is so they don’t drown, Sullivan says. But worms breathe through their skin, and they can actually get some of their oxygen from water. They also need a wet environment to live and breathe, so it seems like there would need to be a lot of rain for this theory to work out, she adds.

Another idea scientists have is that rain creates some pretty good travel conditions for worms. Imagine you were trying to move through a big tunnel of soil. It would probably be hard to move around with all kinds of dirt clods, roots and other things in your way.

Yes, worms can wriggle and squirm through the soil. But it’s still going to be easier for them to travel, and faster, if they are moving around in the more open space on the surface. They may be able to find other worms easier, too. This is especially helpful if they happen to be looking for a mate or trying to find a new home.

Worms like to avoid light and don’t want to get fried in the sun, so moving when it’s rainy or cooler outside can make for safer travel conditions. If you see worms on the sidewalk, it’s best to not disturb them. They have some important work to do and have to continue on their journey.

Worms can help soil stay healthy, which is especially important to us because we need healthy soils to grow the food that we eat.

Maybe the next time it rains in your neighborhood you can go out and make some worm observations. How many worms can you count? How long was it raining and what were their travel conditions? Are they headed in a certain direction?

Tell me about it sometime at Keep asking great questions and maybe one day you’ll be helping us answer big questions about the living soil that supports life on earth.


Dr. Universe

Ask Dr. Universe is a project from Washington State University. Submit a question of your own at http://askDrUniverse.

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