Buzz
Reader Bruce asked, 'Why do worms emerge onto the pavement after a heavy downpour, only to perish when the rain stops? Is it some kind of suicidal behavior?'
Worms have long been thought to come to the surface during and after rain simply to avoid drowning in saturated soil. However, as scientists studied annelids (which include earthworms, ragworms, and leeches), it became clear that these creatures breathe differently from most land animals and are not as susceptible to drowning as we might be in water-filled soil.
Earthworms breathe by absorbing oxygen and expelling carbon dioxide through their skin. To ensure proper oxygen transfer, they need to maintain moisture on their skin, so they produce mucus to stay slippery. Moist soil helps prevent the mucus from drying out, making it easier for them to breathe. Even when submerged in water or when the soil is extremely saturated, earthworms can survive as long as there's enough oxygen available to absorb through their skin.
Drowning during a rainstorm isn't typically a major concern—at least for most worms. Different species consume oxygen at varying rates, sometimes even at different times of the day. This means some species are more vulnerable to oxygen depletion in waterlogged soil than others. In 2008, Taiwanese zoologists studied two worm species, one that surfaces during rainfall and one that doesn't. They found that the worm that comes up during rain consumes oxygen more rapidly, particularly at night, and doesn't tolerate full submersion in water well. This species could stay underground for a while when it rains during the day but had to surface sooner if it rained at night. The other species, however, consumed oxygen more slowly and could survive in oxygen-depleted soil for longer periods.
So, while some earthworms do face the risk of drowning in saturated soil, it's not a universal concern. But is there something else that draws them to the surface?
Some biologists suggest that worms come up to the surface to mate and travel, both of which are easier to accomplish aboveground than below. Since they need to stay moist to breathe, they wait for rain to ensure the surface is wet, which reduces the risk of dehydration.
The hypothesis of mating and movement is supported by the types of worms seen aboveground during and after rainfall. As Phil Nixon from the University of Illinois explains:
“If other factors were the main reason, we would expect to see large numbers of juvenile worms, but instead, the majority that surface are adult red worms, with a few adult nightcrawlers. When worms are brought to the surface through electricity, worm-grunting, or chemicals, many more worms emerge. It’s clear that only a small percentage of adult red worms come up during heavy rainfall. Perhaps this small group reflects those with a colonial spirit.* This is somewhat similar to the small group of people with a colonial spirit who immigrated to this continent and became our ancestors.”
Emerging from the ground can be risky. There are predators like hungry birds, oblivious humans, and the danger of getting stuck on a sidewalk where the worm could dry out. However, if worms are surfacing to mate, the urge to reproduce might outweigh the potential dangers. Surfacing isn't necessarily an act of self-destruction. The worms don’t perish as soon as the rain stops, as Bruce believed. Many of them manage to return underground before they face any real threat. If death was inevitable, there wouldn't be any worms left to observe surfacing behaviors.
*Or perhaps those that are capable and prepared to reproduce.
