Why do our fingers wrinkle in water? It’s much deeper than skin.

“Why have my fingers gone all pruney?” It’s a question that has puzzled children at bath time, teens at swimming lessons, and adults after long hot tub sessions. Scientists once thought that these wrinkles were due to fingers swelling up like a sponge after being soaked with water. However, this theory washes off quickly. Our digits would have to increase drastically in size to produce such wrinkles. Instead, the answer has more to do with our nerves than skin.

The secret of hairless skin

The skin on the palms of our hands and soles of our feet is unique, says Danilo Del Campo, a dermatologist at the Chicago Skin Clinic. It’s called glabrous skin, a name from the Latin word glaber, meaning “bald.” This hairless surface plays an essential role in sensing our environment. It is packed with receptors that transmit signals from our skin into electrical impulses that our brain can read. Glabrous skin is not unique to humans. It makes up the unusual protuberances that the star-nosed mole uses to sense its environment and the upper bill of the platypus. An analysis of glabrous skin in rats found that the ratio of fast-conducting to slow-conducting nerves was three times higher in glabrous skin than in hairy skin. 

Clinicians first realized that the uniquely sensor-packed structure of glabrous skin might have something to do with wrinkly skin in a study nearly a century ago, says Lauren Taglia, a dermatologist at Northwestern Medicine. In a 1936 paper, Sir George White Pickering and Thomas Lewis reported case studies of people with polio-induced nerve damage who had remarkably wrinkle-proof digits. Regardless of how long they spent in water, their fingers remained smooth as marble. 

These patients had sustained damage to their median nerve, which runs down the arm and provides motor and sensory links to the hand and forearm. This nerve is an important connection in the sympathetic nervous system. 

Failing fight or flight

The sympathetic nervous system gets our body ready for physical action. In the heart, it increases pump rate and contraction force. In the eye, the same system dilates the pupil, allowing more light to enter. These functions are often referred to as our “fight-or-flight” responses. Research in the 1970s identified that it was damage to these sympathetic connections in the median nerve that stopped wrinkling. Dipping a hand in warm water was recognized as a valuable test for this type of nerve damage. 

It wasn’t until 2003 that a study identified how the sympathetic nervous system made fingers wrinkle. Here, researchers looked at how blood flow to the hands changed after water immersion. Finger wrinkling was accompanied by a sudden drop in blood flow. This decrease was caused by the sympathetic nervous system narrowing the diameter of the blood vessels in the hand, a process called vasoconstriction. 

These studies finally mapped out how finger wrinkling works. There was just one remaining question: why does it happen in the first place? 

Getting a grip on finger wrinkles

Scientists settled this question in a 2021 study. Nick Davis, a researcher at Manchester Metropolitan University, organized an experiment at the British Science Museum. Davis assessed museum visitors’ ability to grip wet and dry objects. Then, he asked them to immerse their hands in warm water. Once their fingers had wrinkled, he tested their grip again. Anyone who has fought to grab a bottle of shampoo during a shower will know that picking up wet objects requires more force than dry objects. Davis’s analysis showed wrinkled fingers gripped wet objects more easily than wet but unwrinkled fingers. “Evolutionarily speaking,” says Taglia, “perhaps there’s an advantage for our ancestors that they could grab fish from bodies of water.” Our shoeless forefathers would also have benefited from being able to ford streams or rivers without slipping through their wrinkly feet, she says. 

It’s a fortunate coincidence that a feature carved by evolution millennia ago now benefits diagnostic medicine today. “I like to say that the skin is the window to your entire body,” concludes Del Campo.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.

 

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