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HOW COME? Finger wrinkles' advantage

How come if you keep your fingers underwater, they start to wrinkle? asks Brendon Allen, a student in Brookville, NY.

Could prune-y digits help us make our way across a wet surface -- say, a tiled bathroom floor? A recent study suggests that finger and toe wrinkles aren't just a random (weird) effect of soaking in the tub, but an evolutionary advantage.

What happens when you submerge your fingers and toes for at least five minutes? Whether in tub or pool, or so one theory goes, keratin protein in the skin's outer layer quickly soaks up six to 10 times its weight in water. Blood vessels in the fingers and toes constrict. As the vessels shrink, the negative pressure tugs down the plumped-up (but flexible) epidermis. Result: furrows.

However, some researchers say that absorbed water should cause fingers and toes to simply swell up, remaining wrinkle-free. And it's been known since the 1930s that when nerves to the fingers and toes are damaged by injury or disease, or severed completely, the wrinkling effect disappears. In a 2006 study, researchers found that fingers reattached after a nerve-severing accident remained smooth and wrinkle-free after submersion.

Evolutionary biologist Mark Changizi was intrigued by such studies. Since the prune effect is orchestrated by the body's nervous system, he said, there must be a reason for the wrinkles. Changizi and his colleagues note that most of the wrinkling occurs on the tips, which are the first part of our digits to make contact with a surface.

Studying images of wrinkled fingertips, the research team found that finger furrows closely resemble the branching channels carved by rivers running down the sides of mountains. This suggests that the wrinkles on our digits are channels, too -- the body's way of draining water off our human "gripping points."

Changizi says the wrinkles are like the rain treads on a tire. On a dry surface a treadless tire can function fairly well. But on a wet road a smooth tire will cause a car to hydroplane, dangerously skimming the surface rather than gripping the pavement.

Changizi thinks our built-in treads were an adaptation preserved from our evolutionary past. Like the ridges and furrows on our sneakers, ridges on fingers and toes keep us from slipping on a wet surface. When our bare feet and hands get wet enough, the gripping tips wrinkle up, channeling water away. And unlike shoe treads, Changizi says, our finger and toe treads are pliable, allowing more complete contact with a wet surface -- like the cement surrounding a pool.

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