Why do fingers get pruney when wet?

A common explanation of pruney digits has the outer layer of skin absorbing water. The outer layer swells, causing wrinkles. Another theory posits that wrinkling is caused by the narrowing of blood vessels: when exposed to water, nerve fibers shrink, and this causes the skin to prune. But to what purpose? One hypothesis suggests these wrinkles are adaptive. Wet-induced wrinkles enhance the grip in wet conditions by channeling water away. Think the treads in tires.