When you get out of the pool, why do your fingers and toes wrinkle?

Consider this: when human beings step out of the pool, just two areas of their body wrinkle up, their fingers and their toes.  Consider further that scientists have little idea whether other mammals (save for the macaques) experience this same phenomenon. What’s the purpose?

Years ago, it was believed that wrinkled fingers and toes was caused by osmotic (think: osmosis) reactions. Two common theories were advanced: 1. Fingers wrinkle because water enters the tip, and seeks to balance the water content on both sides; and 2. Wrinkling is the result of water passing into the outer layer of the skin and making it swell up.  But in the 1930s, researchers discovered that if you sever the nerves in your finger (not recommended, by the way), the wrinkles won’t form. 

The ready conclusion? Wrinkling is an involuntary reaction by the body’s autonomic nervous system (the system that controls breathing, heart rate and perspiration). Specifically, wrinkling is caused by blood vessels constricting below the skin.

But why does it exist?  What’s the evolutionary purpose? Recently, neurobiologist Mark Changizi and colleagues developed a theory that wrinkling is designed to enhance human grip, and last year an independent research team out of the UK’s Newcastle University confirmed his hypothesis.  They found that, like rain treads on tires, pruney fingers “create channels that let water drain away, allowing them to make better contact with damp surfaces,” according to a piece authored by Ed Yong, for National Geographic’s Phenomena.

Becky Summers, author of an article written for Nature magazine, quoted Tom Smulders, an evolutionary biologist at Newcastle University, UK, and a co-author of the paper. Said Smulders: “We have shown that wrinkled fingers give a better grip in wet conditions — it could be working like treads on your car tires, which allow more of the tire to be in contact with the road and gives you a better grip.”  Summers, paraphrasing Smulders, explains that “wrinkled fingers could have helped our ancestors to gather food from wet vegetation or streams.” The analogous effect in the toes, the article adds, could help us to get a better footing in the rain.

Summers goes on:

“Given that wrinkles confer an advantage with wet objects but apparently no disadvantage with dry ones, it's not clear why our fingers are not permanently wrinkled, says Smulders. But he has some ideas. ‘Our initial thoughts are that this could diminish the sensitivity in our fingertips or could increase the risk of damage through catching on objects.’”

Some take issue with Changizi’s evolutionary hypothesis, raising the concern that testing human beings in their current form won’t necessarily help us explain evolutionary origins. Said Yong, in his piece for Phenomena: “The new study . . . raises some interesting questions about how to test evolutionary explanations. So far, all of the evidence for Changizi’s idea comes from looking at modern human fingers. . . . If modern human fingers grip wet marbles well, and form patterns that resemble rain treads, does that tell us anything about the origins of such patterns or are all such explanations merely just-so-stories?”

Steve Ferber is author of “21 Rules to Live By,” available either at Amazon.com or Island Expressions, located on Daniel Island. Reviews at www.21rules.com.