We know you've asked yourself that question in the headline and common knowledge dictates that the reason your fingers look pruney when you're in water for too long is that the skin absorbs water.
Well, Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho, thinks he has a better theory. Nature reports on the study, which was published in the journal Brain, Behavior and Evolution:
Changizi thinks that the wrinkles act like rain treads on [tires]. 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."