The wrinkles that form on wet skin actually serve a function

From a biological point of view, what function do fingerprints (and palm, toe, and sole prints) serve?

A long-standing hypothesis holds that they aid in climbing and grasping, but this notion has been challenged.

When the friction between a fingertip and a smooth, dry surface was measured, it was found, counter to expectations, that fingerprints reduced grasping efficiency by about 30 per cent.

However, when a surface is wet or rough, fingerprints increase friction and stabilise grasp.

In that respect they resemble automobile tires: Race cars, which encounter only smooth, dry racetracks, are fitted with smooth tires to maximise the contact area between tire and roadway and thereby provide maximum grip.

Screen Shot 2015 02 06 at 10.26.27 AM_This story comes from ‘Touch: The Science of Hand, Heart, and Mind’ by David J. Linden.

I can still hear my mother’s voice: “Get out of the bath. You’re turning into a human prune!”

Many people believe that the wrinkly finger and toe pads one gets from prolonged exposure to water are the result of a passive process in which water is gradually absorbed by the dead skin cells of the stratum corneum.

However, this was shown to be false as early as 1936. The key observation regarding this phenomenon is that finger and toe wrinkling do not occur if the electrical signals flowing from the spinal cord to the skin are interrupted by cutting the nerve or applying a drug that blocks the nerve’s signals, manipulations that have no effect on the stratum corneum.

In particular, the wrinkling response requires a branch of the subconscious autonomic nervous system called sympathetic outflow.

So what, if anything, is the purpose of the wrinkling response? Mark Changizi and his colleagues at 2AI Labs have suggested that, like fingerprints, the wrinkles function as rain treads to increase traction on wet surfaces. They note that the wrinkling response is also found in macaque monkeys and chimps and suggest that it may be an adaptation of primates to wet, slippery conditions.

In support of this hypothesis, Kyriacos Kareklas and his colleagues at Newcastle University showed that subjects with wrinkled fingers were able to transfer wet marbles from one container to another at a significantly higher speed than subjects with unwrinkled fingers. Wrinkled fingers conferred no advantage in handling dry marbles, however.

From Touch by David J. Linden, published on January 29, 2015 by Viking, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright by David Linden, 2015.

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