
Waterlogged fingers aren't simply skin swelling like a sponge. The characteristic wrinkles are produced when the nervous system causes blood vessels beneath the skin to constrict. Research suggests the resulting grooves may improve grip on wet objects, although their evolutionary role is still being investigated.
People with certain nerve injuries may not develop normal water-induced wrinkling in affected fingers. It's an active biological response rather than just water soaking into the skin. Those bathtub wrinkles are controlled from inside the body.
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