There are many theories about why earthworms come out when it rains, ranging from oxygen absorption to the need to move quickly for mating or migration.
On rainy days, it is common to see many earthworms crawling on sidewalks and roads. So what prompts them to leave their safe underground shelters when it rains?
Some believe that worms come to the surface to avoid drowning in their burrows. However, worms do not have lungs like humans, according to Thea Whitman, a soil scientist at the University of Wisconsin-Madison. Instead, they absorb oxygen through their skin and can take in oxygen from both water and air.
Earthworms often crawl up when it rains. (Photo: Roman Romanov/Alamy Stock Photo).
“I once kept earthworms in water for days, and they did not die,” said Kevin Butt, an earthworm ecologist at the University of Central Lancashire in Preston, England.
In fact, a study from 1956 found that five species of earthworms can survive 31 to 50 weeks in completely flooded soil, Whitman noted. “As long as there is oxygen in the water, these worms can still breathe,” she explained.
However, Whitman pointed out that a 2008 study on two species of earthworms indicated that some worms tend to come to the surface on rainy days, which may be partly related to oxygen consumption.
“Species that require higher oxygen levels are more likely to crawl out of their burrows when it rains. Species that do not need such high levels of oxygen are less likely to come to the surface. In summary, some worm species are more sensitive to low oxygen levels and leave their burrows during heavy rain, while others remain happily underground,” she said.
Another popular hypothesis is that worms sense vibrations from the rain, similar to the vibrations that warn a mole is approaching. Therefore, they may come up to escape from predators. However, Butt finds this hypothesis unlikely. The actions of a predator like a mole underground would not be rhythmic, so earthworms could differentiate between moles and 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 such as heavy metals are released, according to Whitman.
Although the 2008 study found no evidence that toxic compounds in rainwater drive worms out of the ground, Whitman noted that researchers sometimes use chemicals to collect worms. “Pouring a mixture of mustard powder and water into the ground will irritate the worms, causing them to come to the surface. There, researchers can collect them for analysis,” she explained.
Butt suggests that the most plausible explanation is that worms come out when it rains to move more quickly on wet ground, rather than sluggishly burrowing through the soil as they usually do. “They need a moist environment to survive, so rainy days may allow them to move on the surface,” Whitman said. Earthworms may make such trips to mate or migrate, she added.
According to Butt, this explanation may help clarify the behavior of “foot stamping” in some bird and reptile species. Rather than making noise while moving through the ground like moles, these animals stomp on the surface, creating vibrations similar to those caused by rain. “These vibrations may cause earthworms to come up and become prey for predators,” Butt said.
A practice known as “worm grunting” or “worm charming” takes advantage of this response of earthworms. Using a stick or saw, vibrations are created in the soil to lure worms to the surface for capture as bait. Additionally, worm charming is also featured in some competitions, such as the annual Worm Gruntin’ Festival held in Sopchoppy, Florida, USA.
