Why do fingers wrinkle after a long soak?

Answer: Nerves shrink the blood vessels under the skin, and the skin buckles

Nerves shrink the blood vessels under the skin, and the skin buckles ✓ — Wrinkling is an active response, not soaking. Water entering the skin triggers the sympathetic nervous system to tighten the blood vessels in the fingertip; the tissue underneath loses volume and the skin above buckles into ridges. The proof is that a finger with a damaged nerve does not wrinkle at all. Whether the ridges are for grip is still debated: one study found wet objects easier to handle with wrinkled fingers, another found no benefit, and a 2021 study found wrinkled fingers needed less force to hold a wet object.

The outer skin soaks up water and swells — The outer layer does absorb some water, but a swollen sponge would bulge, not fold, and a finger with a cut nerve does not wrinkle at all. The folds come from the tissue underneath shrinking.

Salt is drawn out of the skin and it shrinks — Osmosis is the old textbook story, but it would predict wrinkling all over the body, and only fingers and toes do it. The pattern is switched on by nerves.