After a long bath, your fingertips get wrinkly. Many people think this is because the skin takes in water. But that is only part of the story.

Doctors noticed that people with damaged finger nerves do not get wrinkles. This means the nerves tell the blood vessels in the fingertips to get narrower. Why? Wrinkles may help us hold wet things, like tires in the rain. But not all studies agree.

After a long bath, your fingertips look like raisins. Most people think this happens because the skin soaks up water. But as early as the 1930s, doctors noticed something odd. Fingers with cut nerves did not wrinkle in water.

So the wrinkling must be controlled by the autonomic nervous system. In water, nerves make the blood vessels in the fingertips narrow, and the skin on top folds into wrinkles.

Why would the body do this? Nobody is sure. In 2011, researchers suggested that the wrinkles work like tire treads that move water away. A 2014 study with 40 people found no difference, but a 2013 experiment had shown that people moved wet marbles faster with wrinkled fingers.

Wrinkly fingertips after a long bath are usually explained as waterlogged skin, but the real mechanism is more active. As early as the 1930s, doctors observed that fingers whose nerves had been cut did not wrinkle when soaked. Wrinkling depends on the autonomic nervous system: when skin is immersed, nerves cause the blood vessels in the fingertips to constrict, reducing the volume beneath the surface so that the skin folds. This is why doctors sometimes use a simple soaking test to check whether those nerves are working.

The more puzzling question is why the response exists. In 2011, Changizi and colleagues proposed that the wrinkles act like tire treads, channeling water away to improve grip in wet conditions. A 2013 study of 20 participants supported this, finding that wrinkled fingers transferred submerged objects faster, while performance with dry objects did not differ.

However, a 2014 German study of 40 participants failed to replicate the effect, and a 2021 British study found benefits in a different measure, grip efficiency. So the tire-tread idea is attractive, but the evidence for it is mixed.