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Wrinkled When Wet: Accidental or Adaptive?

Fingers wrinkled after an afternoon snorkeling. Photo by notanyron via Flickr Creative Commons

Beachgoers know it well. You soak in the sea or the tub long enough, and your waterlogged fingers get puckered and funny looking. But why do our fingers and toes wrinkle in water?

This pruning phenomenon is controlled by the nervous system. When nerves to the finger are severed, fingers no longer prune.

There are different theories on the mechanics of how it occurs. One common explanation is this: When fingers and toes are submerged, water absorbs into the outermost layer of the skin, which is made up of dead keratin cells. The skin swells as this happens, and the resulting extra surface area of the skin, which is tightly attached to the layer of skin underneath, compensates by wrinkling. (See this Library of Congress article.) Another theory says wrinkling is caused by the narrowing of blood vessels. When exposed to water, nerve fibers shrink, and this causes the skin to prune.

Evolutionary neurobiologist, Mark Changizi of 2AI labs, has a new hypothesis on why — but not how — this occurs. Wrinkles serve an adaptive purpose, he said. “Rather than being an accidental side effect of wetness, wet-induced wrinkles have been selected to enhance grip in wet conditions,” according to his latest article, published June 23 in the journal, Brain, Behavior and Evolution.

Primates adapted to develop this grip, possibly for tree climbing or clinging to their young, Changizi said.

He analyzed the wrinkles in 28 human fingers, and concluded that the morphology of wrinkled fingers has similar properties to drainage networks flowing from mountains. He calls it the ‘rain tread’ hypothesis. Wet-induced finger and toe wrinkles, he said, act as drainage networks that channel away water to allow for a stronger grip when it’s wet.

Patterns on the wrinkled fingers, which stem vertically from the tips of the fingers downward, share a similar pattern to drainage networks on convex landscapes, or mountains, he said. The channel-like wrinkles are disconnected from each other, and diverge away from one another as they get more distant from the ‘peak’ – in this case, the fingertip.

“So here, when you grip something…you want it so that the drainage networks are designed to most efficiently channel water away from where they were, channel it away from where fingertips are touching,” Changizi said. From his paper: “The act of pressing a finger tip down on a wet surface ‘squeezes’ the fluid out from under the finger through the channels, and upon completion of this single pulsatile flow, the entire finger’s skin contacts the surface.”

Changizi’s theory has not been proven. Aside from one picture of a Japanese macaque with pruning fingers, he has no direct evidence on how many other primates share this ability.

And this article in Nature News quotes Xi Chen, a biomechanical engineer at Columbia University, who calls the theory “unjustified”:

Chen thinks that the wrinkles have a simpler cause: when fingers are immersed in hot water, the blood vessels tighten and the tissue shrinks relative to the overlying skin. This contraction causes the skin to buckle. “It’s a classic mechanics problem,” he says.

Changizi agrees that more research is needed. A future step, he said, will be to carry out behavioral studies to determine whether wrinkled fingers indeed grip better than nonwrinkled fingers in wet conditions.

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