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Why your fingers and toes get pruney in water

By Charles Choi, LiveScience 

Fingers may wrinkle when wet to help people grip wet objects, find researchers, who say the pruney feature may have helped human ancestors do the same in wet conditions.

When a person's hands and feet are soaked in water, wrinkles eventually develop on the tips of fingers and toes. Scientists once thought this puckering resulted from the outermost layer of skin absorbing and swelling with water, but recent studies revealed the nervous system actively controlled this wrinkling by constricting blood vessels below the skin.

That the nervous system controls this behavior suggests these wrinkles served a purpose. Now researchers find these wrinkles could help fingers and toes grip wet surfaces.

"A phenomenon that everybody is familiar with is not just some kind of side effect of the nature of the skin on fingers and toes, but a functional feature that has very likely been selected for by evolution," researcher Tom Smulders, an evolutionary biologist at Newcastle University in England, told LiveScience.

Smulders and his colleagues had 20 volunteers pick up wet marbles and small lead weights of different sizes. Volunteers attempted the task either with normal, dry hands or after their fingers had wrinkled following a 30-minute soak in warm water. The participants picked up wet items 12 percent faster with wrinkled fingers. [ 10 Odd Facts About the Human Body ]

"We have shown that wrinkled fingers give a better grip in wet conditions," Smulders said. "It could be working like treads on your car tires, which allow more of the tire to be in contact with the road, [which] gives you a better grip." Another possibility is that wrinkling causes changes in skin properties, such as its flexibility or stickiness, which help the fingers and toes perform better when wet.

"The most surprising thing to me was how the effect was there in all 20 participants, independent of how fast they were on average," Smulders said. "I never expected the effect to be so strong and obvious."

As a potential explanation of why this effect might have evolved, Smulders said, "it could have helped with gathering food from wet vegetation or streams. And as we see the effect in our toes, too, this may have been an advantage as it may have meant our ancestors were able to get a better footing in the rain."

Wrinkled fingers apparently made no difference when it came to picking up dry objects.

"This raises the question of why we don't have permanently wrinkled fingers, and we'd like to examine this further," Smulders said. "Our initial thoughts are that this could diminish the sensitivity in our fingertips or could increase the risk of damage through catching on objects."

Future research should analyze what other primates or non-primate animals might also show this phenomenon to shed light on when and why it evolved, Smulders said.

"Which other animals share this trait? And is the link among species that share it phylogenetic — that is, they're all related — or environmental — that is, they all deal with submerged objects, for example?" Smulders said.

The scientists detailed their findings online Jan. 9 in the journal Biology Letters.

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