Why do your fingers go wrinkly in the bath?

Your fingers wrinkle in the bath because your nervous system tells them to. Water seeping in through the sweat ducts probably upsets the balance of salts in the skin, which makes nerves fire and tells the tiny blood vessels under your fingertips to tighten. With less fluid underneath, the skin loses its tension and folds into ridges, like a balloon that has lost a bit of air.

For a long time people assumed the opposite: that the outer skin simply soaked up water and swelled until it had to crumple. Then, in 1935, two researchers, Lewis and Pickering, noticed something odd in patients with a damaged median nerve in the hand. The skin supplied by that nerve stayed smooth in water. Soggy skin can't explain that. A switched-off nerve can. Doctors later turned the discovery into a simple bedside test for nerve damage.

So why would the body bother? The leading idea is grip. In a 2013 experiment, people with wrinkled fingers moved marbles and fishing weights out of water 12 percent faster than with smooth fingers, while wrinkles made no difference for dry objects. A 2014 study couldn't repeat that result, but a 2021 study found wrinkles cut the force needed to hold wet objects by 20 percent. Your pruney fingers may be built-in rain tires.