Why Do Fingers Prune in the Water

If youve had a long soak in a bathtub or pool, youve noticed your fingers and toes wrinkle (prune up), while the rest of the skin on your body seems unaffected. Have you ever wondered how it happens or whether it serves a purpose? Scientists have an explanation for the phenomenon and have proposed a possible reason for why it happens. Why Skin Prunes in Water The prune effect is different from true wrinkling of skin because the latter results from the degradation of collagen and elastin, making the skin less resilient. Fingers and toes prune, in part, because the layers of the skin dont absorb water evenly. This is because the tips of your fingers and your toes are covered with a thicker outer skin layer (the epidermis) than other body parts. However, most of the wrinkling effect is due to blood vessel constriction just below the skin. Nerve-damaged skin doesnt wrinkle, even though it has the same composition, so the effect may be a reaction to water by the autonomic nervous system. However, the hypothesis that wrinkling is under autonomic nervous system control doesnt account for the fact pruning occurs in cold water as well as warm water. How the Epidermis Reacts to Water The outer layer of your skin protects the underlying tissue from pathogens and radiation. Its also fairly waterproof. The keratinocytes at the base of the epidermis divide to produce a layer of cells rich in the protein keratin. As new cells are formed, the old ones are pushed upward and die and form a layer called the stratum corneum. Upon death, the nucleus of a keratinocyte cell involutes, resulting in layers of a hydrophobic,  lipid-rich cell membrane alternating with layers of hydrophilic keratin. When skin soaks in water, the keratin layers absorb water and swell, while the lipid layers repel water. The stratum corneum puffs up, but its still attached to the underlying layer, which doesnt change size. The stratum corneum bunches up to form wrinkles. While the water hydrates skin, its only temporary. Bathing and dish soap removes natural oils that would trap the water. Applying lotion can help lock in some of the water.   Hair and Nails Get Soft in Water Your fingernails and toenails also consist of keratin, so they absorb water. This makes them softer and more flexible after doing the dishes or bathing. Similarly, hair absorbs water, so its easier to over-stretch and break hair while its damp. Why Do Fingers and Toes Wrinkle? If pruning up is under nervous system control, it makes sense that the process serves a function. Researchers Mark Changizi and his colleagues at 2AI Labs in Boise, Idaho, demonstrated that wrinkled fingertips provide improved grip on wet objects and that the wrinkles are effective at draining away excess water under damp conditions. In one study, published in Biology Letters, subjects were asked to pick up wet and dry objects either with dry hands or after soaking them in warm water for half an hour. Wrinkles didnt affect the participants ability to pick up dry objects, but the subjects picked up wet objects better when they had pruned hands. Why would humans have this adaptation? Ancestors who got wrinkled fingers would have been better able to gather wet food, such as from streams or beaches. Having wrinkled toes would have made barefoot travel over wet rocks and moss less risky. Do other primates get pruney fingers and toes? Changizi e-mailed primate labs to find out, eventually discovering a photograph of a bathing Japanese macaque monkey that had wrinkled fingers. Why Arent Fingers Always Pruned? Since wrinkled skin offered an advantage manipulating damp objects yet didnt hinder abilities with dry ones, you may be wondering why our skin isnt always pruned. One possible reason might be that wrinkled skin is more likely to snag on objects. Its also possible that wrinkles diminish skin sensitivity. More research could give us additional answers. Sources Changizi, M.,  Weber, R.,  Kotecha, R.  Ã‚  Palazzo, J.  Brain Behav. Evol.  77,  286–290. 2011.Kareklas, K., et al. â€Å"‘Water-Induced Finger Wrinkles Improve Handling of Wet Objects.’†Ã‚  Biology Letters, The Royal Society.