Wrinkled fingers and toes gain traction as an adaptation against slipping

Prunelike fingers and toes may be an unpleasant result of an extended swim, but scientists suggest they may serve an important purpose: to keep us from slipping.

A study published online last month by the journal Brain, Behavior and Evolution posits that the grooves that form after a few minutes of soaking may function like tire treads, helping to divert water and allowing for a better grip in wet environments. The common assumption is that wrinkling is a byproduct of osmosis.

Mark Changizi, a theoretical biologist and the study’s lead author, was intrigued by research discounting that belief: Fingers with nerve damage do not form water wrinkles, which suggests a more controlled bodily function. “If it’s neurologically modulated,” Changizi said, “there’s got to be a good reason for it.”

Changizi and his team sought an evolutionary explanation. Co-author Romann Weber came up with the tire analogy, which led the group to analyze images of 28 wet fingers from 13 hands. The fingers, as expected, formed a pattern of channels, similar to tire treads, in which water would be diverted from the fingertips.

The theory needs further research, however, before it holds water. Einar Wilder-Smith, a neurologist at the University of Heidelberg, served as a reviewer on the Changizi paper, and believes that wrinkling is a “byproduct of another function,” such as temperature regulation, in which the constriction of blood vessels might lead to shriveled skin. “I would be astonished if it actually improves grip.”

Another of Changizi’s co-authors, Joseph Palazzo, conducted a 12-person pilot study testing participants’ ability to quickly move pieces of wood, rocks and a smooth plastic bottle. Palazzo said that the people with wet, wrinkled fingers made fewer mistakes, on average, than those whose fingers were dry and smooth.

Changizi offered a similar experiment for the summer: Count the people who slip on their way into a pool and compare it with the number who slip on their way out, once their toes have formed wrinkles. Changizi had looked into collecting such data himself, but he quickly changed his mind.

“I thought it would be wrong to wait around for people to fall,” he said.

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