From Brain, Behavior, and Evolution, a suggestion that the wrinkles induced in fingers by water serve a function similar to that of rain treads on vehicular tires.
Here we provide evidence that, rather than being an accidental side effect of wetness, wet-induced wrinkles have been selected to enhance grip in wet conditions. We show that their morphology has the signature properties of drainage networks, enabling efficient removal of water from the gripped surface..
One can immediately see that they possess the signature structure of drainage networks on convex promontories… Namely, the channel-like wrinkle depressions tend to be disconnected from one another and diverge away from one another as they get more distant from the ‘peak’ near the fingertip; the divides (or borders between the channels) are connected, forming a tree with its root near the fingertip…
Wet-induced wrinkle treads, on the other hand, are pliable, and the act of pressing a finger tip down on a wet surface ‘squeezes’ the fluid out from under the finger through the channels, and upon completion of this single pulsatile flow the entire finger’s skin contacts the surface. In addition to wet-induced finger wrinkles having the signature morphology of drainage networks,the time scale at which they appear (around 5 min) [Cales and Weber, 1997] is plausibly appropriate for natural wet conditions; it is fast enough to be relevant for dew or rainy conditions but not so quick that casual contact with water (like when eating fruit) will elicit it.