Why Do Your Fingers Wrinkle in the Bath?

Stay in the bath long enough and your fingertips turn into tiny prunes. The smooth pads crinkle into ridges and valleys, your toes do the same, and for a few minutes you have the hands of a very old wizard. It seems like the most obvious thing in the world. Skin sits in water, skin soaks up water, skin goes wrinkly. For a long time, that is exactly what people believed. The truth is stranger, and a good deal cleverer.

The first clue is where it happens. The wrinkles appear on the palms of your hands and fingers and the soles of your feet, the hairless skin that does the gripping and standing, and hardly anywhere else. If it were just soggy skin, you might expect your elbows and knees to wrinkle too. They do not. The second clue came from doctors, who noticed that fingers whose nerves had been damaged do not wrinkle in water at all.

That means the wrinkling is not passive. It is an active process, switched on by the autonomic nervous system, the part of your nervous system that runs things you never think about, such as heart rate, breathing and sweating. After a while in water, it tells the small blood vessels under the skin of your fingertips to narrow. The soft pad beneath the skin loses a little volume, and the skin on top, which stays the same size, buckles into those familiar ridges.

The connection to the nerves is so reliable that a soak in warm water has been used by doctors as a simple test of nerve function in the hand. Which raises the real question. If the body goes to the trouble of making wrinkles deliberately, what are they for? Something that takes effort is usually doing a job.

In 2013, researchers at Newcastle University in England tested one idea. Volunteers soaked their hands in warm water for thirty minutes, and then moved glass marbles and small weights from one container to another, some taken out of water and some dry. With wrinkled fingers, they moved the wet objects about twelve percent faster. With dry objects, the wrinkles made no difference at all. The researchers compared the effect to the treads on a car tyre, which help channel water away and keep a grip on a wet road.

That suggests a neat story about our ancestors. Wrinkly fingers could have helped with gathering food from wet plants or streams, and wrinkly toes might have given a steadier footing in the rain. It would also explain why the wrinkles only appear when things get wet. The Newcastle team suggested that permanent wrinkles might cost us some sensitivity in our fingertips, or catch more easily on things.

The story is not completely settled. Not every later experiment has found the same grip advantage, and nobody yet knows for certain how long ago the response evolved or which job it evolved for. But the old idea, that wrinkles are simply skin swelling like a sponge, has been set aside. The next time you look at your pruney fingers in the bath, you are looking at your nervous system quietly getting you ready to hold on.