Ask Smithsonian: Why Do We Get Prune Fingers?

Some researchers say that, like tire treads, our fingers and toes could get better traction in wet conditions

It’s pretty much a given that fingers and toes become wrinkly after being immersed in water, but the reasons for the wrinkling weren’t fully explained until five years ago, and even now, some would say it’s still not a settled matter.

The phenomenon has been explained away over the centuries as “an accidental side effect of wetness,” according to Mark Changizi, a theoretical neurobiologist and director of human cognition at 2AI Labs in Boise, Idaho. That is, osmosis caused the skin to become waterlogged.

But Changizi—a big thinker—wasn’t satisfied with that answer. As he was investigating the shape, function and structure of primate hands, he came across a bunch of studies—starting in the 1930s and going through the 2000s—that showed that if the nerves that fed the hands had been damaged, fingers would not wrinkle after being soaked in water.

It was eventually determined that the wrinkling was caused by the constriction of multiple blood vessels beneath the skin and is the result of an involuntary nervous system reaction. While that explains how it happens, it didn’t offer the why.

And if wrinkling was only due to osmosis, why didn’t it occur everywhere on the body? The fact that wrinkling was linked to the sympathetic nervous system (part of our innate fight-or-flight mechanism) led Changizi and his colleagues to wonder if it was an evolutionary adaptation to the wet conditions that made up the habitats of many primates and some of our ancestors.

They began with the notion that the wrinkles were potentially channels, or “rain treads,” designed to drain water off the fingertips and toes to allow a better grip—just as the tread on tires allows cars to grip the asphalt in spite of water on the roadway. Changizi decided to reverse-engineer the answer to the question, starting with trying to replicate the wrinkle patterns. It took a year to figure out the best theoretical shape, he says.

They found it by looking at the topography of mountains. Rivers bunch up at the peaks and flow down, with the divides in between acting as drainage channels. Changizi and his colleagues saw the same thing on prune fingers—the divides channeled water away, allowing for a be better grip. And it was functional: the channeling didn’t happen until at least five minutes after immersion—fast enough to be of use when it’s truly wet, but not so fast that casual contact with a liquid would kick in the extra grip.

Changizi and his colleagues published their findings in 2011, but said that more research was needed to validate their grip theory.

About a year later, Kyriacos Kareklas and his colleagues at the Centre for Behaviour and Evolution at Newcastle University tested whether people with and without wrinkly fingers could pick up and move wet marbles from one box to another. Sure enough, prune fingers were more efficient. Score one for Changizi.

But in early 2014, scientists at the Max Delbrück Center for Molecular Medicine in Berlin-Buch, Germany tried to repeat the experiment and found that having wrinkly fingers made no difference in how well or poorly someone could grip a wet or dry object.

“I don’t think either study was good,” says Changizi, who hasn’t gone back to studying pruney fingers again, but says that someone could probably do a better job of proving his theory.

A big stumbling block, however, is that no one knows whether any animal—aside from humans and macaques—gets pruney fingers.

Answers will have to come from more studies of how humans use their wrinkly fingers and toes. Changizi has the perfect subject group in mind: parkour athletes who freestyle run, roll, tumble and climb outside of gyms. Give some of them prune fingers and toes and others dry digits, he says.

Changizi predicts that those with the dry hands and feet will inevitably slip and crash. Any volunteers?

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