After a heavy rain, earthworms suddenly appear on sidewalks, driveways, roads, and lawns.
The usual explanation is that they are escaping because they would otherwise drown underground.
That can happen in severely waterlogged soil, but it is probably not the whole story.
Earthworms breathe through their skin
Earthworms do not have lungs.
Oxygen moves through their moist skin and into their bodies.
As long as enough dissolved oxygen is available, many earthworms can survive in very wet soil for quite a while.
So a rainstorm does not automatically turn every worm tunnel into a death trap.
Wet ground creates a travel opportunity
Earthworms lose water easily when exposed to dry air.
That makes crossing the surface risky during ordinary weather.
Rain changes the situation.
A wet lawn or pavement keeps the worm’s skin moist, allowing it to move above ground without drying out as quickly.
Traveling across the surface can be much faster than tunneling through soil.
Why would a worm want to travel?
A worm may be looking for food, mates, or a new area with better conditions.
Surface travel gives it access to much more ground in a short time.
A rainstorm can therefore open a temporary highway.
Unfortunately, humans built sidewalks across the highway.
Can saturated soil still push worms upward?
Yes.
If soil becomes badly waterlogged and oxygen levels drop, moving toward the surface can help.
Different species and conditions can produce different reasons for coming up.
There is no need for one explanation to cover every worm after every rainstorm.
Do rain vibrations matter?
Possibly.
Vibrations caused by rain may resemble disturbances made by digging predators such as moles.
Some worms may respond by moving toward the surface.
People have even used vibrations to deliberately bring worms out of the ground.
Why do worms get stranded on pavement?
A worm may move onto pavement while conditions are wet.
Later the rain stops.
The surface dries or heats up.
Now the worm may be too far from soil to return safely.
That is why sidewalks can become dangerous even though the rain originally made surface travel possible.
So the sidewalk after a storm is not necessarily covered with worms that barely escaped drowning.
Some of them may simply be taking advantage of excellent travel weather.
They picked a route with bicycles.
A related animal question is Why do flies rub their hands together?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: Why do squirrels bury nuts?, Why do flies rub their hands together?, and Why does rain smell so good?