The skin on the tips of wet fingers wrinkles due to an involuntary brain reaction. The resulting surface is analogous to the tread on a car tire.
Mark Changizi, an evolutionary neuroscientist at the 2AI laboratory in Boise, Idaho, stumbled upon research from the 1930s that showed that deeply wet fingers do not fold when the nerves in the fingers are cut. One of Changizi's subordinates, with whom the professor discussed the find, suggested that the "potholes" on wet fingers are analogous to the pattern on the tires, which provides the car with better grip.
Changizi and his colleagues analyzed large photographs of 28 wet, wrinkled fingers that belonged to different people, and concluded that the location of the folds is everywhere and always the same: single channels diverge from the pads and do not intersect. The pressure exerted by the finger on the surface is mainly applied by the pad, and the wrinkles act as drains.
The scientific world took the idea differently: biomechanist from Columbia University Xi Chen believes that fingers wrinkle simply due to narrowing of blood vessels in hot water. However, that doesn't explain why wrinkles appear in cold weather, or why wrinkles don't appear on fingers with non-working nerves, says neurosurgeon Jin-Hua Xie of Changong Hospital in Kaohsiung, Taiwan.
Pilot tests have shown that folds in wet human fingers do improve grip.