Why Do Earthworms Come Out in the Rain?

Earthworms are some of the most common creatures below the earth’s soil, and yet humans rarely see them day to day. In fact, it’s relatively uncommon to see a living earthworm wriggling around on the surface of the ground in fair weather, unless they were dug out of the ground by someone working in a yard. The one time humans expect to see earthworms, and quite a few of them all at once, is right after it rains.

Earthworms are covered with a mucous secretion that comes from the cells of the creature. Not only does this mucous allow worms to slither slimily through the dirt as they search for food, it also serves to help them “breathe.” Because they are relatively simple creatures, earthworms lack the respiratory systems that many animals have, and they rely on their moistness in order to gather oxygen.

Conventional Wisdom

For years, it was thought that earthworms surfaced during the rain because the porous ground quickly fills up with water during a storm. Because earthworms breathe oxygen just like other creatures, it was assumed that earthworms come up to keep from drowning. If they remained underground, it was reasoned, they would be suffocated by the deluge of water from above, quickly killing off any earthworm population just below the surface.

On the face of it, this seems like a reasonable conclusion to draw. It has been speculated for centuries that earthworms surface when their survival instincts kick in. However, modern science has proven quite differently. Earthworms do tend to surface in the rain, but the reasons why are very different than speculation has led many to believe.

Healthy Environment

In truth, worms surface during the rain for nearly the opposite reason supposed by most people. In dry, hot weather, a worm will quickly shrivel and dry in the sun. In the rain, earthworms can come up to the ground and not worry about the harmful effects of the sun. Because the ground is so wet, it is easy for the worms to stay moist and travel along the ground at the surface.

Meeting a Mate

In spite of this scientific logic, we see far more dead earthworms above the ground after it rains, suggesting that the worms actually did drown due to the amount of water falling on them from the sky. Why do they surface at all? The reason for that is that the open surface makes it easier for them to scope out a mate.

The burrows of earthworms underground look similar to what you’d expect. They’re very narrow, very dark, and very cramped. It is rather difficult for a worm to find a mate underground.

Conditions on the surface when it rains are much more conducive to these worms (which are incidentally hermaphroditic) exchanging sperm to fertilize eggs. Have you ever noticed groups of worms spending time together on the surface after a rain? Now you know why.

Coming to the Surface

Worms may surface for reasons other than the rain. When it is dark outside, the atmosphere (and the ground) is usually far more moist than it is during the day time. Large groups of worms also surface and congregate in the evening hours and stay out during the night, a time during which they may also meet mates. For this reason, worms are often known as nightcrawlers. It’s true—the early bird gets the worm.

In addition to surfacing and mating, worms do serve a purpose to an ecosystem. They make the soil more fertile for growing due to aerating and digesting the ground in which they move. Instead of stomping that earthworm that comes to the surface, move it to the grass. It’s doing you and your lawn a great service.