The common wisdom is that your fingers wrinkle when they're wet because they absorb water. But Mark Changizi thinks there's more to it than that. According to him, pruney fingers are an adaptation to help humans, and probably other primates, get a better grip during wet conditions. They act like the rain treads on tyres. Mark lays out his hypothesis in a wonderful paper that I wrote up as a news story for Nature News. Here's the start; click through for the whole thing.

The wrinkles that develop on wet fingers could be an adaptation to give us better grip in slippery conditions, the latest theory suggests. The hypothesis, from Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho, and his colleagues goes against the common belief that fingers turn prune-like simply because they absorb water. Changizi thinks that the wrinkles act like rain treads on tyres. They create channels that allow water to drain away as we press our fingertips on to wet surfaces. This allows the fingers to make greater contact with a wet surface, giving them a better grip. Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed, implying that they are controlled by the nervous system. "I stumbled upon these nearly century-old papers and they immediately suggested to me that pruney fingers are functional," says Changizi. "I discussed the mystery with my student Romann Weber, who said, 'Could they be rain treads?' 'Brilliant!' was my reply."

Reference: Changizi, Weber, Kotecha, & Palazzo. Brain Behav. Evol. http://dx.doi.org/10.1159/000328223 (2011).