Whether you’re swimming or washing the dishes or just taking nice, long, well-deserved bath --- if you’re immersed in water for longer than 10 minutes, chances are your fingers and toes will emerge looking like raisins.   So what’s up with the wrinkled digits?  

For years, scientists thought the phenomenon was the result of a type of osmosis, caused by water passing into the dry outer layer of skin.    The influx of water, the thinking went, would expand the skin’s surface area, but not the tissue below it, so the skin would bunch up and wrinkle.  

But in 1935, a pair of doctors noticed that this effect didn’t happen in their patients with nerve damage.   One patient, for example, was a boy who had lost the feeling in three of his fingers. The researchers found that, when his hand got wet, the fingers that he could feel wrinkled as normal, but the ones that were numb remained smooth.   

It turned out that pruney digits weren’t caused just by the passive flow of water through the skin -- it was an active response of the nervous system to prolonged moisture.   The nervous system causes the wrinkling by constricting blood vessels below the skin, which causes the upper layers of skin to pucker.

  Since the phenomenon is caused by an involuntary nerve response, some biologists have thought that it must have some evolutionary function.    But what possible purpose could it serve?   One recent theory suggests that wrinkly skin may have given our ancestors a better grip while working in wet conditions -- like gathering food from a stream or damp vegetation. And it may also have given us better footing while walking across slippery landscapes in the rain.  

In a 2013 study, evolutionary biologists tested this theory by asking subjects with either wrinkly and non-wrinkly fingers to pick up a variety of wet and dry objects, like marbles.    They found that the subjects with wrinkly digits picked up the wet objects 12 percent faster than their counterparts. But there was no difference when it came to picking up dry objects.   The wrinkles apparently helped channel the water away, much like the treads on your car’s tires.  

But then this raises the question: If wrinkly skin gives us a better grip, then why isn’t our skin wrinkly ALL of the time?   Well, maybe because shriveled fingers and toes are less sensitive, which is no advantage at all.   Thanks for asking! And if you’d like to submit questions for us to answer, or get these Quick Questions a few days early, check out Patreon.com/SciShow. And don’t forget to go to youtube.com/scishow and subscribe!