Why do your fingers wrinkle in water?

The old explanation was that skin swells up like a sponge. It cannot be the whole story, because people with nerve damage in a finger do not get wrinkles in that finger. Doctors have known that since the 1930s. Something has to be switching the wrinkles on, and that something is the nervous system shrinking blood vessels beneath the skin.

In 2013 researchers at Newcastle University asked volunteers to pick up marbles out of water and pass them through a small hole, first with ordinary fingers and then after soaking their hands for half an hour. Wrinkled fingers moved the wet marbles about 12 percent faster. With dry marbles, wrinkles made no difference at all.

That is exactly how treads on a tyre behave: useless on a dry road, essential on a wet one. The researchers think our ancestors benefited when gathering food from streams and wet plants. Why we are not permanently wrinkled is still an open question. The best guess is that ridged fingertips would feel less and snag more.