Are wet-induced wrinkled fingers primate rain treads?

Wet fingers and toes eventually wrinkle, and this is commonly attributed by lay opinion to local osmotic reactions. However, nearly a century ago surgeons observed that no wrinkling occurs if a nerve to the finger has been cut. 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.