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.