Why do my fingers absorb water and become wrinkled?

Category: Biology
Published: February 18, 2015

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Human finger tips do not become wrinkled because they absorb water. Skin is a fairly good barrier that keeps out most of the external water. If our fingers did absorb a significant amount of water after staying in the pool or bath for several minutes, then our fingers would swell to round, plump shapes, which is the opposite of wrinkling. The wrinkling is actually caused by a reduction of fluid inside the finger tips.

Public Domain Image. Source: Christopher S. Baird.

How can our finger tips experience reduced fluid internally when exposed to increased fluid externally? From a physical perspective, this effect seems to be a paradox. The answer is that the wrinkling of wet finger tips is an active biological response, rather than a passive physical effect. Scientists have known for almost a hundred years that nerve damage in the hand can result in a person no longer being able to get wet-induced finger wrinkles. Finger wrinkling is therefore controlled by the nervous system. Nerve signals cause blood vessels in the finger tips to constrict, reducing the amount of fluid in the finger tips. Just as drying out a grape causes it to turn into a wrinkled raisin, the reduction of fluid in the finger tips causes them to shrivel and wrinkle.

Since finger wrinkling is an active neurological response, it is likely that evolution selected for it because this response confers some type of survival advantage. But how could wrinkles ever help one survive? This question has long been wondered and the answer is still far from settled. However, research done in 2011 seems to hint at an answer. As reported in the journal Brains, Behavior and Evolution, Mark Changizi and his associates at the 2AI Labs found evidence that finger tip wrinkles are shaped to improve the grip of wet fingers. They state, "We show that their morphology has the signature properties of drainage networks, enabling efficient removal of water from the gripped surface...Wet-induced wrinkles may, in fact, be substantially superior to 'rain treads' on shoes, which maintain a tread even when under compression and thus have a surface area of contact that is reduced. Wet-induced wrinkle treads, on the other hand, are pliable, and the act of pressing a finger tip down on a wet surface 'squeezes' the fluid out from under the finger through the channels, and upon completion of this single pulsatile flow the entire finger's skin contacts the surface." A human with an improved grip can better handle tools and weapons in the rain, as well as retrieve food from rivers and streams. In this way, the wrinkling of fingers in response to water could have provided a significant survival advantage to our ancestors.

Additional research carried out in 2013 by Kyriacos Kareklas and associates, as reported in Biology Letters, found that participants were able to transfer wet marbles and fishing weights between containers significantly faster if they had wet-induced finger wrinkles. They therefore showed that finger wrinkles indeed improve a human's ability to grip and manipulate wet objects. While more research is needed to confirm these results, these studies suggest that the improved-grip hypothesis is a plausible explanation to wet-induced finger wrinkling.

Topics: evolution, finger, finger wrinkles, fingers, grip, water, wet