Why Do Your Fingers Prune Up After a Bath?

If you’ve ever spent too much time in the pool, or if you like to unwind at the end of a hard day with an hours-long soak in the tub, you’re probably familiar with the “pruney fingers” phenomenon. Believe it or not, scientists have been studying this raisin-like effect for decades, trying to figure out why your hands (and sometimes your feet) get so wrinkly when wet.

One popular idea is that your skin simply becomes waterlogged. According to the Library of Congress, the epidermis, or outer layer of skin, is made up of dead keratin cells that absorb moisture when immersed in water for an extended period of time. This absorption causes the cells to swell, but because they’re still connected to the underlying tissue, which does not expand, the outer skin has to wrinkle to compensate for its larger surface area. It’s a little like having a king-size sheet for a queen-size mattress: The extra material has nowhere to go, so it bunches up unevenly on top.

But why are only your fingers and toes affected — why doesn’t your entire body wrinkle? Scientists say it’s because our hands and feet have the thickest epidermis and thus more keratin cells to absorb water. (Your nails also contain keratin, which is why they may feel softer after you do the dishes.)

Is There a Purpose to Pruning?

The problem with this hypothesis is that it doesn’t really account for the fact that fingers and toes don’t wrinkle when their nerve endings have been severed, as by injury or complications from diabetes. This can be explained by a different theory, which proposes that the prune-like effect is due not to skin saturation but to a reaction in the central nervous system — a “classic mechanics problem,” as Columbia University biomechanical engineer Xi Chen, PhD, explained it to Nature News.

The mechanics theory is based on the idea of vasoconstriction, or the narrowing of blood vessels. Basically, when fingers and toes are immersed in hot or cold water, nerve endings fire off signals that cause your blood vessels to constrict and the tissue below your skin’s surface to contract. This, in turn, forces the outermost layer of skin to buckle, resulting in wrinkling.

But recent research suggests there may be even more to wrinkly skin than that. Evolutionary neurobiologist Mark Changizi, PhD, and his team at 2AI Labs in Boise, Idaho, are working to prove their theory that pruney fingers and toes are not just the products of a dermatological quirk; rather, they’re the genetic equivalent of rain treads — those functional grooves on all-weather tires and shoes that help with traction in wet conditions. Changizi believes that these so-called treads on hands and feet may have been built into DNA over the ages to improve your grip.

“Our pruney fingers may be a crucial part of our primate repertoire,” he wrote in an article for Forbes.com. “Once primates went the way of finger nails rather than claws, treads were needed where claws may have sufficed before.”

To support this theory, Changizi and his team analyzed pattern similarities in 28 photographs of pruney fingers. All 28 had wrinkles that formed vertical channels, which work to drain water away from fingertips.

“In order for a hand to reach out and grip a wet surface without hydroplaning, it needs a way to efficiently remove the water between the skin and the surface it is trying to grip,” he explained. “The best way to quickly move water tends to be via channels, the stuff of arteries and rivers.”

More research is needed to prove Changizi’s hypothesis, but he’s already hard at work on follow-up studies. Next on his list, according to Nature News: testing whether wrinkles actually allow for better grip and looking into whether mammals in wet climates are more likely to get pruney fingers than those in dry habitats. If the theory holds up, he says, we may be able to use pruney fingers to improve our existing tire and shoe rain-tread technology.

“Far from an embarrassing mistake,” he wrote, “wet wrinkled fingers are yet another testament to biology’s brilliance.”

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