Pruney digits help people get a grip

Scientists have an answer to the pressing question of why hands and feet get wrinkled after too much time in the bath: Pruniness may have evolved to make it easier to handle wet objects.

The smooth skin of human hands and feet becomes furrowed after extended periods in water. Though often assumed to be a result of water passively seeping into the skin, the phenomenon is actually caused by the nervous system constricting blood vessels. As early as the 1930s, surgeons noticed that no wrinkling occurred if a finger nerve had been severed, so furrowing has been used as a medical indicator of nerve function. But what evolutionary purpose wrinkling serves, if any, remained a mystery.

In 2011, a team of researchers proposed that the grooves in wet fingers might function as “rain treads” that improve grip by channeling water away, much like car tires on a wet road do. Now, researchers at Newcastle University in England have tested that theory.

The researchers had 20 volunteers manipulate objects with smooth fingers or digits shriveled by immersion in warm water for 30 minutes. The experimenters measured how long it took people to transfer the objects between a water-filled container and a dry one, or between two dry ones, with wrinkled versus unwrinkled fingers. The objects were glass marbles and fishing weights of various sizes.

All the participants transferred the wet objects (but not the dry ones) faster when their hands were pruney. The results suggest furrowed fingertips make it easier to handle moist items more efficiently, the scientists report online January 8 in Biology Letters.

The amount of time it took to move the objects varied from person to person. What was surprising was that every person transferred the wet objects faster with wrinkly hands, says study leader Tom Smulders.

The findings don’t explain exactly how the wrinkles improve grip, however. Although the results provide “converging behavioral evidence for the conclusion that wrinkled fingers are rain treads, this could also be true for other reasons, such as stickiness properties or oiliness,” says theoretical neurobiologist Mark Changizi of 2AI Labs in Boise, Idaho. Changizi led the team that put forward the rain tread hypothesis.

The fleshy grooves might also be more flexible or increase friction compared with smooth skin, scientists speculate. Toes wrinkle too, which the scientists say could have evolved to provide surer footing on slick surfaces.

But there may be some evolutionary cost to having pruney skin all the time, the researchers say, or else even dry skin would be wrinkled: Perhaps the ridges snag on things more easily, or impair sensitivity to touch. More studies are needed to test these theories.

If humans evolved this curious trait, other animals might possess it too — but evidence is scant. Although no one has reported observing it directly, Changizi claims to have seen finger wrinkling in a photo of a macaque monkey relaxing in hot springs in Japan. No word on whether wrinkled fingers help the monkeys pick up dinner afterward, though.