By Sid Perkins
A long soak in the tub can wreak havoc on your fingertips, transforming your smooth digits into wrinkly eyesores. But this rumply skin may actually serve a purpose, according to a new study. It helps us get a stronger grip on slippery objects, especially those underwater.
Scientists long thought that wrinkly fingers were caused by osmosis—swelling of the outer layer of the skin as water seeped into cells. But experiments conducted in the past few years—as well as observations that water-induced wrinkles don't form on the tips of previously severed but subsequently reattached fingers—suggest that the wrinkles are instead produced by nerves that automatically trigger constriction of the blood vessels beneath the skin, reducing the volume of the tissues there.
Having something under the direct control of a nerve, even an involuntary one, suggests it serves an evolutionary purpose. But that begged the question of what function finger wrinkles have. In 2011, a team of neuroscientists proposed that the folds improved our grip on wet or submerged objects, just as the treads on tires help improve traction. "That seemed like a clever hypothesis that would be easy to test," says Tom Smulders, an evolutionary biologist at Newcastle University in the United Kingdom.
So, he and his colleagues designed an experiment where volunteers picked up 45 submerged objects such as glass marbles and lead fishing weights from a bin one at a time with their right hand, passed them through a postage stamp-sized hole in a barrier to their left hand, and then dropped them through another hole into a box. When test subjects had wrinkly fingertips—induced by soaking their hands in 40°C water for 30 minutes—they completed the task about 12% faster than they had when their fingers hadn't been soaked, the team reports online today in Biology Letters. When performing the same task with dry objects, wrinkly fingertips didn't provide a performance advantage.
The team's results are "very interesting," says Xi Chen, a biomechanical engineer at Columbia University who has analyzed how the skin on fingertips buckles when vasoconstriction causes underlying tissues to shrink. "They show that the wrinkles have a biological function."
"I'm happy to see the team has investigated this," adds Romann Weber, a mathematician at Rensselaer Polytechnic Institute in Troy, New York, and a member of the group that proposed the wrinkles-as-rain-treads hypothesis in 2011. The results mirror those of the limited number of similar preliminary tests Weber and his colleagues had conducted and "are a good practical demonstration of the benefits that wrinkles provide," he notes. Smulders's results "are encouraging, and consistent with our rain-tread hypothesis," adds Mark Changizi, a theoretical neurobiologist at 2AI Labs in Boise and senior co-author with Weber of the 2011 journal paper.
Yet neither the original research nor that of Smulders's team pin down precisely how the wrinkling enhances grip performance, Changizi says. Besides channeling water away from the fingertips—and, in essence, preventing small-scale hydroplaning of skin across slick objects—it's possible that long-term exposure to water temporarily robs the skin of body oils, thereby boosting the skin's friction and enhancing grip.
Another possibility, Chen says, is that wrinkled skin spreads to provide a larger contact area when fingers are touching an object. Further experiments may discern whether one or more of these factors are responsible for improvements in grip performance.
It's also unclear whether wrinkles evolved to help us grasp underwater objects, or whether they're simply a byproduct of a nervous system quirk. Weber says finding out whether such puckering occurs in other primates might shed light on the evolutionary origins of the phenomenon.
Despite their grip-enhancing benefits, wrinkles don't persist indefinitely once a hand is no longer submerged—a sign that under most circumstances there may be a downside to water-induced puckering, Smulders says. Maybe wrinkled skin is less sensitive, or maybe it's more prone to be snagged and injured. Neither notion has been tested. Wrinkled toes and feet may provide surer footing in wet conditions, his team contends, but that, too, hasn't been studied. Perhaps this is one experiment you can try at home.