Buzz
For many years, scientists believed that the skin’s wrinkling after prolonged exposure to water was simply due to the skin becoming sponge-like. The outer layer of skin, known as the stratum corneum, consists mostly of corneocytes, cells packed with keratin, a protein that helps retain moisture by soaking up water and reducing evaporation. When you stay in the water for an extended period, the keratin absorbs large amounts of water, causing the cells to swell. Since the stratum corneum expands while the deeper layers beneath it don’t, the outer skin wrinkles and folds, much like a shirt that bunches up when it's too big for its fit.
A more modern theory suggests that the wrinkles result from vasoconstriction, the narrowing of blood vessels. According to this idea, warm water causes blood vessels in the fingers to constrict, causing the surrounding tissues to tighten, which results in skin folds.
However, the true explanation may be even more complex, particularly when considering the effect on individuals with nerve damage.
A Little Disconcerting
In the 1930s, two scientists studied a boy whose median nerve had been cut, leaving his thumb, index, and middle fingers numb. When they submerged his hand in water, they observed that while his ring and pinkie fingers wrinkled, the fingers with nerve damage remained smooth.
In 2001, researchers from Tel Aviv University discovered that nerve dysfunctions caused by Parkinson’s disease also impacted the skin's wrinkling response. In their study, Parkinson’s patients exhibited less wrinkling on one side of their body compared to the other, and their fingers wrinkled less overall than those of healthy individuals. While traditional explanations suggest the wrinkles were localized, the involvement of the nervous system hints at something more complex.
The Theory of Grip
Mark Changizi, a neuroscientist and Director of Human Cognition at 2AI Labs in Boise, Idaho, argues that the wrinkles’ neural connection indicates they might serve an adaptive purpose. Instead of being a simple result of soaking, he suggests that the wrinkles are a functional response to wet environments: They act like drainage channels or tire treads on our fingers and toes, directing water away and enhancing grip on slippery surfaces.
Studying the wrinkles on different soaked fingers, Changizi and his team noted similar shapes and patterns—specifically, channels that branched out as they moved away from the fingertip, resembling a drainage network. While this observation didn’t provide definitive proof for his theory, it sparked further exploration. (Update: 11/30/2012, 1:25 pm) Though it might not seem conclusive, Changizi emphasizes that the "morphology prediction is actually very strong."
"Out of the countless wrinkle patterns that could occur," he explains, "[the] drainage theory predicts [the] exact [pattern]."
Since presenting the theory and preliminary data last year, Changizi and his team have been investigating finger wrinkling in other primates living in wet habitats (they had already observed it in Japanese macaques). They are also conducting experiments to directly assess the impact of wrinkles on grip. Although the results are not yet ready for publication, initial findings suggest that pruney fingers do indeed enhance grip.
(Update: 11/30/2012, 1:25 pm) Changizi shared the pilot data with me. The experiment was conducted by Changizi and undergraduate student Joseph Palazzo. They had participants complete a timed task of moving various objects—bottles, stones, logs, and more—from one point to another on a table. The task was performed under four conditions: wet-pruney, dry-pruney (dry objects, with fingers dried after wrinkling), wet-nonpruney (wet fingers, but not yet wrinkled), and dry-nonpruney. Wet-pruney subjects outperformed wet-nonpruney, completing the task faster and with fewer errors.
Changizi is eager to see more behavioral studies like this conducted and to gather more data from other species for further experiments, but he likely won’t be the one to carry them out. "A more refined next experiment would be version-2.0 of this type of study, in my view," he says. "But that’s not my strength." He believes other researchers would be better suited for this kind of work.
"As for the types of tests," he notes, "there are three categories: morphology, behavior, and phylogeny. At this stage, we’ve completed the first, made initial attempts with the second, and only speculated about the third."
