Why Does Skin Get Pruney in the Tub?
For a long time, scientists thought that pruning of the skin after spending time in the water was simply a matter of fingers being a little spongey. The outermost layer (the stratum corneum) of the outermost layer (the epidermis) of our skin is mostly made up of cells called corneocytes. These cells are filled with keratin, a protein that helps keep the skin hydrated by absorbing water and preventing its evaporation. When you hang out in the pool or the bathtub for a while, the keratin absorbs a lot of water, and the cells swell up. While the thin stratum corneum swells with water, the lower layers of skin that it is attached to don’t, so that outermost layer has to buckle and bend to accommodate its relatively larger size, sort of like a too-big shirt that wrinkles and bunches together when it's tucked in.
Another, more recent explanation is that the wrinkles come from vasoconstriction, or the narrowing of blood vessels. The idea is that hot water makes the blood vessels in the fingers tighten and the surrounding tissue contract, causing the skin to fold.
But the explanation might be more complicated than either one of those potential causes—especially when you consider how the phenomenon occurs in people with nerve damage.
In the 1930s, two scientists examined a boy whose median nerve was severed, leaving his thumb, index, and middle fingers numb. When they soaked his hand in water, the ring and pinkie fingers wrinkled but the fingers affected by the damaged nerve stayed smooth.
And in 2001, researchers at Tel Aviv University found that nervous system malfunctions caused by Parkinson’s disease also interfered with finger wrinkling. In their study, Parkinson’s patients’ fingers wrinkled less on one side of the body than the other, and wrinkled less overall than the fingers of healthy subjects. Going by the common explanations, the wrinkles were a local phenomenon happening in very small bits of flesh. The involvement of the nervous system, though, suggests that something else is going on.
Getting a Grip
Mark Changizi, neuroscientist and the Director of Human Cognition at 2AI Labs in Boise, Idaho, thinks that the wrinkles’ neural factor is a clue that they’re adaptive. Rather than being a mere side effect of water-logged digits, he says, they’re a functional response to wet conditions: The wrinkles act like drainage networks or tire treads on our fingers and toes, channeling water away and giving skin more contact with, and a better grip on, wet surfaces.
Analyzing the wrinkles on various soaked fingers, Changizi and his team found that they all had similar shapes and characteristics—with disconnected channels that moved away from each other as they got farther from the fingertip—consistent with what is expected in a drainage network. That wasn’t much evidence for Changizi’s hypothesis, but it got the ball rolling. (Update: 11/30/2012, 1:25 pm) While that doesn't seem like much, Changizi points out that the "morphology prediction is actually very strong."
"Of the infinitely many wrinkle patterns that are possible," he says, "[the] drainage hypothesis predicts [the] actual [pattern]."
Since publishing the idea and the initial data last year, he and his team have been looking for evidence of finger-wrinkling in other primates that live in wet environments (they’d already found it happens in Japanese macaques) and are setting up experiments to directly test the wrinkles' effects on grips, While the results aren't ready to be published yet and the pilot studies so far suggest that pruney fingers do help improve grip.
(Update: 11/30/2012, 1:25 pm) Changizi has filled me in on that pilot data. The experiment was conducted Changizi and undergraduate student Joseph Palazzo. They had subjects carry out a timed task of moving objects, including bottles, stones, logs and other items, from one place on a table top to another, and back again. They did this in wet-pruney, dry-pruney (dry objects, and fingers dried after they had wrinkled), wet-nonpruney (wet fingers, but not yet wrinkled) and dry-nonpruney conditions. Wet-pruney performance was better than wet-nonpruney, with the subjects being faster and making fewer mistakes.
Changizi would like to see more behavioral studies like this carried out, and see more data from other species for further tests, but probably won’t carry out any of these studies himself. "A more sophisticated next experiment would be version-2.0 of this sort of thing, in my mind," he says. "But not my forte." He thinks that other scientists would be much better at that kind of experiment.
"In terms of the categories of test, then," he says. "There's morphology, behavior, and phylogeny, and at this point we've done the first, poked at the second, and only wondered about the third."