Weird Science | Why our fingers and toes wrinkle when wet
The reason why toes wrinkle is still not fully understood, but it may be related to walking on slippery surfaces like stones in rivers.
If you have ever been in a bathtub or a swimming pool for a long time, you will have noticed wrinkles appearing across your fingers and toes. Scientists have long wondered why this happens, and there has been more than one explanation over the decades.
There are two aspects to the science behind this wrinkling. One is how it happens, or the mechanism that causes it. The other is why, or the reason that triggers such a mechanism.
A common theory about the mechanism, widely accepted at one time, is that wrinkling is caused simply by water soaking into the skin. The outermost layer of our skin contains dead keratin cells, which absorb water in the pool or bathtub, and swell. The layers underneath this do not swell, causing the outermost layer to puff out and wrinkle. The reason this happens only in the fingers and toes, and not elsewhere in the body, is that our digits have the thickest layer of dead keratin cells.
The current thinking, however, is that wrinkling happens by shrinking of the inside of the finger, rather than swelling of the skin.
“Swelling skin should make the skin tauter, not wrinkly. Instead, when the inside shrinks, just like a date or a raisin, the outside skin wrinkles as the inside is too small to fit properly,” Tom Smulders, an evolutionary neuroscientist at Newcastle University, said over email.
More than a side effect
The absorption theory would suggest that wrinkling is an entirely passive process, simply a side effect of the skin interacting with its wet surroundings, as opposed to an active process, or a function that the body is meant to perform. Evidence in recent years, in fact, suggests that wrinkling has evolved with a purpose: to help us grip wet objects better.
“If you ask most people why your fingers go wrinkly in the bath, they might say that water just soaks into the skin. This, to me, is a passive process. However, we know that this can’t be the whole story, because of a few different clues,” Dr Nick Davis, a psychologist and neuroscientist at Manchester Metropolitan University, said over email.
“The best evidence that this is an active process is that some people with nerve damage may wrinkle only on one side and not the other (people with Parkinson’s disease may notice this),” Davis said.
This would suggest that the nervous system plays a role, rather than the process being triggered by the wet surroundings alone. If a nerve to the finger is cut, in fact, no wrinkling takes place, a fact observed by surgeons as far back as 1936.
In recent years, a series of studies has suggested that wrinkling, rather than being an accidental side effect of wetness, has evolved to enhance grip in wet conditions. One study, published in the journal Brain, Behaviour and Evolution in 2011, found similarities between the wrinkle morphology and drainage networks.
In 2013, Smulders and colleagues published a study in Biology Letters, showing that picking up wet objects is faster with wrinkly fingers than with dry fingers. When they asked study participants to pick up dry objects, no difference was found between those with wrinkly fingers and those whose fingers were dry.
Then in 2021, Davis and colleagues showed that wrinkles allow a person to grip a wet object less tightly than with non-wrinkled fingers. “This tells us that we use less energy in our muscles when we have wrinkled fingers, which is useful for prolonged foraging in wet environments,” Davis said. This study was published in PLOS One.
Tyre treads and finger wrinkles
The tyres in our cars have grooves or treads for an obvious reason. These treads allow water to be expelled beneath the tyre, increasing friction between the tyre and the road. The 2011 study drew a parallel between treaded tyres and wrinkled fingers. So did Smulders and Davis.
“The hypothesis we were testing is that wrinkles essentially do the same job as rain treads on car tyres: The water that forms a layer between the object surface and the finger surface gets pushed into the channels between the ridges, allowing it to be pushed away from between the skin and the object, and therefore the tops of the ridges can make better contact with the surface of the object. And that improves grip,” Smulders said.
Davis drew an additional parallel: “If you are holding a glass of cool water on a hot day, you may see beads of water collecting on the glass. This water makes the glass slippery, which means you need to squeeze a little bit harder to stop the glass from slipping... The amount that you have to squeeze the glass relates to the weight of the glass, and to the friction of the surface. Less friction means you need to squeeze harder.”
If wrinkly fingers evolved to help us grip things better, how does that explain wrinkly toes? This is not fully clear.
Davis suggested two possible explanations. “The first is that it seems likely that finger-wrinkling evolved so that wet objects can be held while foraging. It is unlikely that you would ever have wet hands but dry feet, so this suggests that having grippy toes is important for walking on slippery stones in rivers. The other explanation is that there is one single genetic code that does both finger and toe wrinkling, rather than one set of genes for feet and one for hands. I don’t think we understand the biology well enough at the moment to be able to decide between each possibility,” he said.
And Smulders said: “We think that grasping objects may not be the original function for which wrinkling evolved. Given that it happens at all four limbs, it may well have evolved for better grip during locomotion, whether on the ground or through the trees (grabbing wet branches).”
Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but isn't it.