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After a heavy rain storm, you've probably seen worms popping out of the soil, getting stranded all over sidewalks and parking lots. But between birds, clumsy humans and the sun it's dangerous for a worm to climb out of its safe, cosy burrow. And yet they suddenly abandon ship whenever it starts raining.

Scientists have come up with plenty of possible reasons why rain might trigger this behaviour but they don't quite agree on all of them. The oldest hypothesis, and one you might have heard, is that earthworms surface to avoid drowning.

That's because worms take in oxygen through their skin in a passive process called "diffusion," where oxygen moves from the higher concentration outside their bodies to the lower concentration inside. When soil floods after a rainstorm, worms can usually still breathe if there's enough oxygen dissolved in the water. But water is a lot denser than air, so diffusion is much slower - like, up to 1000 times slower.

Which means that for a worm it could be a lot harder to breathe after a downpour, which could make them rush to the surface. But some biologists have pointed out that plenty of worms can actually live for days submerged in water, although some of that may depend on the species. One study, published in 2008, found that two different species of worm use different amounts of oxygen. A common tropical worm uses relatively little so it doesn't come up to the surface after it rains. But another worm, the Alabama jumper, needs more oxygen, especially at night, and it does surface.

So there may be worms who do need to surface to breathe, or at least prefer to while others don't. Other experts think worms might surface for different reasons. They argue that worms flee their burrows because rain gives them great travel conditions. Earthworms move faster above ground and they can crawl more quickly and safely when it's wet because they are less likely to dry out.

Some scientists have also proposed that worms take flooded soil as a cue to mate, possibly increasing their chances of finding a partner. But that seems to be limited to nightcrawlers and a few other species.

One last hypothesis is that raindrops landing on the soil create vibrations similar to those made by one of the earthworm's biggest predators: the mole. It's definitely true that earthworms will book it to the nearest exit if they sense certain tremors. People actually exploit this by going "worm grunting," where they catch worms to use as bait by driving stakes into the ground and vibrating them. But experiments suggest that worms are only sensitive to vibrations that mimic moles and that falling rain doesn't do the trick.

So we may not know for sure why earthworms grace us with their presence after a storm but at least we know what to do if we want to go fishing.

Thanks to Patreon patron Miranda McCarthy for asking this question, and thanks to all of our patrons who keep these answers coming. If you'd like to submit a question to be answered, you can go to patreon.com/scishow. And don't forget to go to youtube.com/scishow and subscribe.

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