Bay Nature magazineWinter 2019

Ask the Naturalist

Why Do Earthworms Surface After a Rain?

December 28, 2018

Why do earthworms emerge after a heavy rain? And how do earthworms in California survive the dry season in the desiccated soils?  —Max, Berkeley

You, me, worms, and Goldilocks like it just right! Not too hot, not too cold, not too dry, not too wet. Burrowing through the soil, earthworms can travel up, down, and sideways to find (hopefully) just the right humidity and temperature. They can actually stay submerged in water for days with no ill effect, and they require some amount of moisture to breathe through their skin. Available moisture at the surface makes it easier to travel, allowing worms to cover more distance than when below the surface. Also, the pounding of the raindrops is thought to mimic the sound of predators like moles, so the worms may also be fleeing imagined underground danger.

Earthworms are hermaphrodites; they each have both male and female parts. What is the advantage of being hermaphroditic? I know what you are thinking … you’ll always have a date on Saturday night. Wrong. Animals shouldn’t self-fertilize. That’s inbreeding and might result in a bunch of idiot worms. But for slow-moving creatures there is a real advantage to being both male and female. How many worms do you think one worm is going to meet in a night? Not many, but every one is always the right one. Two of them line up side by side, facing opposite directions. That fat part on earthworms is called the “saddle.” Each saddle secretes some slime that temporarily glues the animals together while they exchange sperm. When offspring from each worm eventually hatch, the baby worms feed on their protective cocoon and then slither on out into the soil. 

In California’s Mediterranean climate, a prolonged drought begins every summer and earthworms survive the dryness by digging down to where the soil is wetter. But as the dry season stretches on, a worm must estivate. It wraps itself into a tight ball within a chamber in the soil lined with mucus. Inside, the humidity is high, and the worm’s metabolism slows as it waits for the winter rains to begin anew.  

Worm fiddling; worm grunting; worm twanging—these are all another way of saying worm charming. Fishermen and other worm seekers through the ages have found that vibrating the ground through various techniques will bring these creatures scurrying to the surface. Just like with the pattering of rainfall, the vibrations presumably are similar to those made by worm-hungry moles. Not surprisingly, worm charming is a dying skill set. But in England (of course: where else?) there are annual competitions. The current world record is held by a 10-year-old English girl who in 2009 charmed 567 worms to emerge within a nine-foot square. No kidding: I could not make this up. 

However, during the dry months in the Bay Area, one would be hard-pressed to entice even one worm to the surface, unless it was on a well-watered golf course.

About the Author

Send your questions to Rosa-based naturalist Michael Ellis leads nature trips throughout the world with Footloose Forays (