A gripping tale: scientists claim to have discovered why skin wrinkles in water
Tom Smulders of Newcastle University Institute of Neuroscience explains his research suggesting that people are better at handling wet objects if their fingertips are wrinkled. Video: Guardian Royal Society of Publishing
Thousands of years after the invention of the bath, scientists have come up with a theory to explain why our fingers and toes wrinkle when steeped in water.
Puckered skin gives a better grip and may have helped our ancestors uproot wet plants when foraging for food, or be more sure-footed in a slippery, wet environment, they say.
The familiar wrinkles on wet fingers and toes may also have benefited early humans in their first forays into technology, said Tom Smulders, an evolutionary neurobiologist at Newcastle University.
"It might have helped handling tools in wet conditions," he said, such as fixing hunting weapons in the rain, or fishing with harpoons.
It is popularly believed that fingertips absorb water and swell, making the skin ripple with tiny folds. But this was ruled out by studies that showed the effect disappeared when nerves in the fingers were damaged.
Rather than swelling up, fingertips shrink when they wrinkle because the blood vessels inside them contract. The effect is controlled by the autonomic nervous system, which also governs breathing and heart rate.
Smulders investigated the benefits of wrinkled fingers after reading a paper by Mark Changizi, director of human cognition at 2AI Labs in Idaho. Changizi's report in the journal Brain, Behavior and Evolution suggested that wrinkles on fingers resemble car treads and the drainage networks seen on mountains.
In the latest study, Smulders had 20 people move 45 submerged marbles and fishing weights from one container to another. The objects were plucked one at a time, with the forefinger and thumb of the right hand, passed through a hole in a screen separating the containers, and into the thumb and forefinger of the left hand.
Smulder timed them on the task, once when they had dry and unwrinkled hands before starting, and again after they had soaked their hands in water for half an hour.
The task took between 90 and 150 seconds to complete, but those with wrinkled fingers moved the wet objects 15 seconds faster on average, compared with those who began with dry hands. Wrinkles made no difference to the time it took to do the task with dry objects, according to the study reported in Biology Letters.
"It could be working like treads on your car tyres, which give you a better grip," said Smulders.
The findings raise the question of how, and from which species, humans inherited their wrinkling skin. "My guess is that all primates have pruney fingers, but our only evidence at the moment beyond humans is from macaques," said Changizi.
At his lab in Idaho, Changizi has done a similar, though more rudimentary, experiment and reached the same conclusions as the Newcastle team.
"The obvious application here are biologically inspired rain treads for your shoes," Changizi said. "We'd ideally like to have shoe treads with the right wrinkle shapes for our foot topography. And we'd ideally like to have the treads flatten so that the entire shoe bottom grips the ground once the water is squirted out through the channels."
One question that remains is why fingers are not wrinkled all the time, even when they are not in water. The answer may be that wrinkling comes at a cost: the loss of sensitivity in our hands, Smulders said.