Why do earthworms appear more when it rains?

There are many theories as to why earthworms emerge during rain, ranging from oxygen absorption to the need to move quickly for mating or migration.

On rainy days, people often see many earthworms crawling on sidewalks and roads. So what makes them crawl out of their safe underground shelter when it rains?

Some people think worms crawl to the surface to avoid drowning in their burrows. However, worms do not have lungs like humans, says Thea Whitman, a soil scientist at the University of Wisconsin-Madison. Instead, they absorb oxygen through their skin, and can get it from both water and air.

Earthworms often come out when it rains. (Photo: Roman Romanov/Alamy Stock Photo).

'I have left earthworms in water for days and they did not die ,' says Kevin Butt, an earthworm ecologist at the University of Central Lancashire in Preston, UK.

In fact, a 1956 study found that five species of earthworms could live 31 to 50 weeks in completely submerged soil, Whitman says. 'As long as there was oxygen in the water, these worms could breathe,' she explains.

However, Whitman notes that a 2008 study of two species of earthworms found that some of their tendency to crawl to the surface on rainy days may be partly related to oxygen consumption.

" Species that need higher oxygen levels are more likely to come out of their burrows when it rains. Species that don't need such high oxygen levels are less likely to come to the surface. In short, some worms are more sensitive to low oxygen levels and leave their burrows during heavy rains while others remain happily underground ," she said.

Another popular theory is that the worms sense the vibrations from the rain in the same way that a mole is alerted to approaching. So they might crawl up to escape a predator. Butt says this theory is unlikely. The movements of a predator like a mole in the soil would not be rhythmic, so earthworms could distinguish the mole from the rain.

Another possibility is that rainwater in the soil harms worms in some way . For example, when the rain is acidic, or when harmful chemicals like heavy metals are released, according to Whitman.

While the 2008 study found no evidence that toxic compounds in rainwater drive worms out of the ground, Whitman notes that researchers sometimes use chemicals to collect worms. 'Pour a mixture of mustard powder and water on the ground and the worms will be irritated, causing them to crawl to the surface, where researchers can collect them and analyze them ,' she explains.

Butt thinks the most likely explanation is that the worms crawl up during rains to move faster across wet ground, instead of slowly burrowing through the soil as they normally do. ' They need wet environments to survive, so rainy days may allow them to move across the ground ,' Whitman says. Earthworms may make such trips to mate or migrate, she adds.

This explanation could help explain the ' foot-shaking' behavior of some birds and reptiles, Butt says. Instead of making a sound as they move through the soil, like moles, these animals stomp their feet on the surface, creating vibrations similar to those caused by rain. 'These vibrations could cause earthworms to crawl up and become prey for predators,' Butt says.

A practice called 'worm grunting' or 'worm charming' takes advantage of this response. Using sticks or saws, people create vibrations in the soil to lure worms to the surface and capture them as bait. In addition, worm charming is now also used in some competitions, for example in the annual Worm Gruntin' Festival in Sopchoppy, Florida, USA.