“Raisin Fingers” May Be an Evolutionary Advantage
By Kyle Kurpinski
If you have ever gone swimming for several hours or taken a really long bath, then you are probably familiar with the phenomenon referred to as “pruney fingers” or “raisin fingers.” A common explanation for this wrinkling of the hands (and feet) is that the skin absorbs excess water when submerged, which causes it to expand. This expansion increases the surface area of the skin, thereby resulting in wrinkles. So while these pruney fingertips may have the appearance of being shrunken or shriveled, they are actually larger than they were when dry. It has been suggested that fingers and toes are more susceptible to this effect than other areas of the body due at least in part to a lack of hair follicles, which produce sebum – an oily secreted substance that may act as a temporary protective barrier against water absorption.
But water uptake is only part of the story. As early as the 1930s, scientists noted that patients with palsy-related nerve damage in their hands showed no signs of water-induced wrinkling in the areas specifically affected by the impaired nerves. This insight suggested that the nervous system is actively involved in the wrinkling process, and additional research has shown that vasoconstriction – narrowing of the blood vessels – plays a role as well. A modern view of raisin fingers goes something like this: prolonged immersion leads to excess water uptake by the skin; the resulting electrolyte imbalance causes neurons to fire more rapidly, which causes blood vessels to constrict, thereby reducing blood flow underneath the skin and leading to a decrease in skin tension, thus causing wrinkling. This process is clearly more complicated than simple water-induced swelling, which is probably why most people have never heard about it. The complete physiological mechanism of action is likely a combination of water-induced swelling and vasoconstriction.
But none of this explains why fingers that look like little dried fruits should be a part of our biology in the first place. A new theory (put forth in a recent article in the journal Brain, Behavior and Evolution) proposes that wrinkles on the fingers and toes may actually act as miniature drainage channels to displace water and increase grip in wet conditions, similar to the rain treads on the tires of a car. In other words, when your fingers and toes get really wet, they wrinkle up to counteract the slickness of the water. Additional work still needs to be done for the theory to become more widely accepted, but it appears that the raisiny morphology may be an evolutionary adaptation for life in slippery environments.
As a professional bioengineer I feel inclined to run my own experiments, but I’m still waiting for my hot tub/climbing wall grant to go through.
Image: Brian Zambrano (cc)