BioEd Online

Scientists now think they have answers to why our hands and feet shrivel up like old prunes when we soak in the bath. Wrinkly fingers improve our grip on wet objects or objects under water. Scientists at Newcastle University studied people taking objects out of water with wrinkled fingers and again without wrinkled fingers to explain why the effect occurs.

People often assume that wrinkling is the result of water passing into the outer layer of the skin, and making it swell up. But it has been known since the 1930s that the effect does not occur when there is nerve damage in the fingers, which points to it being the an involuntary reaction by the body's autonomic nervous system — the same that controls breathing, heart rate and perspiration. In fact, the distinctive wrinkling is caused by blood vessels constricting below the skin.

The rain-tread theory, proposed in 2011 by Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho, and colleagues suggested that wrinkling, being an active process, must have an evolutionary function. Changizi's team also showed that the pattern of wrinkling seemed optimised for providing a drainage network. But until now, there was no proof that wrinkly fingers did in fact offer an advantage.

In the study, people picked up marbles of different sizes with normal hands or with wrinkled fingers after having soaked their hands in warm water for 30 minutes. With the wet marbles, they were faster when their fingers were wrinkled. However, wrinkled fingers made no difference for moving dry objects. The results suggest that the wrinkles on fingers and toes serve the function of improving our grip on objects under water or maybe even wet objects in general.

"We have shown that wrinkled fingers give a better grip in wet conditions — it could be working like treads on your car tyres which allow more of the tyre to be in contact with the road and gives you a better grip," says Tom Smulders, whose team is publishing the results today in Biology Letters.

Wrinkled fingers could have helped our ancestors gather food from wet vegetation or streams, Smulder adds; the analogous effect in the toes could help us get a better footing in the rain.

Changizi says that the results provide behavioural evidence "that pruney fingers are rain treads," in addition to the morphological findings by his own team. What remains to be done is to check that similar wrinkling does happen in other animals for which it would provide the same advantages, he adds. "At this point we just don't know who has them, besides us and macaques.”

Also unknown is why our fingers would not be permanently wrinkled, something that would not seem to put us at a disadvantage when gripping dry objects, Smulders says. But, he has some ideas. “Our initial thoughts are that this could diminish the sensitivity in our fingertips or could increase the risk of damage through catching on objects."