The pruning up of fingertips when we bathe or swim is a phenomenon of the human body that is so common and yet little understood. Despite many decades of scientific inquiry, until recently, the reasoning remained elusive.

A common belief was that the wrinkling of wet skin was caused by osmosis. But a definitive study in 2011 turned the enigma of “wet-induced wrinkles” from a passive, purely chemical reaction of water molecules into an interesting evolutionary idea - what if pruney skin was an adaptation that gave our ancestors a better grip in wet conditions?

Let’s explore the experimental and historical reasoning that led to this understanding of what water does to fingers and toes.

The scientific reason why human skin develops wrinkles when it has been exposed to water is grounded in the anatomical structure of the skin. Skin is composed of three layers. The epidermis (the outermost layer) hydrates the body. The dermis (the middle layer) assists in thermoregulation. Finally, the hypodermis (the deepest layer) protects the body from harm. It is the epidermis that holds the key to understanding pruney skin. As Richard Gray reports in a June 2022 BBC article, citing the 2003-2004 work by ​​neurologists Einar Wilder-Smith and Adeline Chow, the pattern of skin wrinkling in water “depends on the way the outermost layer of skin - the epidermis - is anchored to the layers beneath it.”

The common misconception that osmosis-related swelling is the sole reason our skin wrinkles in water hinges on the idea that as the epidermis encounters water, water molecules pass through the epidermis from the watery environment into the skin cells. This happens most noticeably in fingers and toes because the outermost part of the epidermis contains a layer of dead keratin cells that is thickest on the hands and feet. (The keratin layer serves to protect the nerves and tissue below the skin from being harmed with their use during our regular, skin-abrasive activities such as scrapes and grazing.) But as Becky Summers writes in a 2013 Nature magazine article, the osmosis theory has been criticized since the 1930s, when researchers discovered that skin wrinkling “does not occur when there is nerve damage in the fingers.” In light of the discovery of the nervous system’s role, another factor besides osmosis must contribute to the reasoning behind the activity of wrinkling - perhaps, another line of thinking went, constriction of blood vessels underneath the surface of the skin was the primary cause of the wet-skin wrinkling effect.

Some scientists have suggested wrinkling to be an evolutionary function and one that our body has adapted over time. A leader in 21st-century thinking on the question is Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho. He and his team worked to prove that the pattern of skin wrinkling appeared to be optimized for providing a drainage network that improves grip. One of the helpful analogies they invoke is that of rain treads on a tire: the wrinkle patterns on wet fingertips similarly help with draining water and increasing surface area for a better grip.