The surprising benefits of wrinkled fingers in water

The skin on our fingertips becomes wrinkled like a prune when we dip them in water for a few minutes. Could it be an evolutionary adaptation? And what can this reveal about our health today?

Just spend more than a few minutes soaking in a bathtub or swimming in a pool for our fingers to undergo a dramatic transformation. Where once there were the delicate whorls of slightly undulating epidermis, we now have swollen folds of unsightly, wrinkled skin.

This impressive shift is familiar, but also disconcerting. After all, only the skin on the fingers and toes is wrinkled when immersed in water, while other parts of the body, such as the forearms, chest, legs, and face, look the same as they did before being submerged.

This wrinkling of the skin of the fingers caused by water has been the subject of research by scientists for decades. Most of them did not understand what generated this phenomenon. Yet.

Recently, researchers have found new answers about the cause and possible purpose of this process.

But perhaps even more intriguing is what wrinkled fingers can reveal about our health.

the mechanism

It takes about 3.5 minutes in warm water – the ideal temperature is 40°C – for our fingertips to start to wrinkle. At lower temperatures, around 20°C, it can take up to 10 minutes. But most studies have concluded that it takes about 30 minutes in water to achieve maximum wrinkling.

It used to be believed that wrinkling of the fingertips was a passive reaction, in which the top layers of the skin swell as water invades the cells in a process known as osmosis. In it, water molecules move across a membrane to equalize the concentration of solutions on either side.

But in 1935, scientists suspected that the process must be more complex. That’s when doctors studying injured patients who had torn the median nerve – one of the main nerves that runs through the arm to the hand – discovered that their fingers didn’t wrinkle.

Among other functions, the median nerve helps control so-called sympathetic (sympathetic nervous system) activities, such as sweating and blood vessel constriction. This finding suggested that water-induced wrinkling of the fingertips is actually controlled by the nervous system.

Later medical studies in the 1970s provided further evidence on this process and proposed immersion of hands in water as an experiment to determine nerve damage that might affect the regulation of unconscious processes such as blood flow.

Until, in 2003, neurologists Einar Wilder-Smith and Adeline Chow, who at the time were working at the National University Hospital of Singapore, measured blood circulation in the hands of volunteers while they were immersed in water. They concluded that as the skin on the volunteers’ fingertips began to wrinkle, there was a significant drop in blood flow to the fingers.

And by applying a local anesthetic cream that temporarily constricted blood vessels in the fingers of healthy volunteers, doctors concluded that it produced levels of wrinkling similar to immersion in water.

“That makes sense when you look at your wrinkled fingers,” says Nick Davis, a neuroscientist and psychologist at Manchester Metropolitan University in the UK who has studied fingertip wrinkling. “The pads of the fingers are pale because blood flow is being restricted at the surface.”

Wilder-Smith and her colleagues imagined that when the hands are immersed in water, the sweat ducts in our fingers open up to allow water in, which causes an imbalance of salts in the skin. This change in salts triggers nerve fibers in the fingers, causing the blood vessels around the sweat ducts to constrict. This, in turn, causes loss of volume in the fleshy region of the fingertip, which pulls down the surface skin which then distorts, creating wrinkles.

The pattern of wrinkles depends on how the outermost layer of skin – the epidermis – is attached to the layers below it.

There have also been indications that the outer layers of the skin may swell a little to increase wrinkle formation. But, just by osmosis, our skin would need to swell by about 20% to reach the wrinkles that we see on our fingers, which would increase them enormously.

What happens is that when the upper layers of the skin are slightly swollen and the lower layers contract at the same time, wrinkles become pronounced much more quickly, according to Pablo Saez Viñas, a biomechanical engineer at the Technical University of Catalonia in Spain. , who used computer models to examine the mechanism.

“You need both factors to have normal levels of wrinkling,” he says. “If you don’t have this neurological reaction, as some people do, wrinkles don’t show up.”

But if wrinkling is controlled by our nerves, it means that our body actively reacts to immersion in water.

“Which is to say it happens for a reason,” according to Davis. “And that might be giving us some leverage.”

Advantage

A question from one of his children at bath time – why do fingers get wrinkled – prompted Davis to recently research what that advantage could be.

With the help of 500 volunteers who visited the Science Museum in London in 2020, Davis measured how much force it took for them to hold a plastic object.

Perhaps as expected, those with dry, wrinkle-free hands had to use less force than people with wet hands – so their grip on the object was better.

But when they dipped their hands in the water for a few minutes so that their hands were wrinkled, the necessary grip strength fell between the two groups, even if their hands were still wet.

“The result was surprisingly clear,” explains Davis. “The wrinkles increased the friction between the fingers and the object. What was particularly interesting is that our fingers are sensitive to this change in surface friction and we use that information to apply less force to hold an object securely.”

The object Davis’ volunteers were holding weighed less than a couple of coins, so the grip required was small. But to perform more difficult tasks in humid environments, this friction difference can gain importance.

“If you don’t have to press down on an object as hard to hold it, the muscles in your hands are less tired and you can hold on for longer,” he explains.

Their findings coincide with those of other researchers who have found that the wrinkling of our fingertips makes it easier to handle wet objects.

In 2013, a team of neuroscientists at the University of Newcastle in the UK asked volunteers to transfer marbles of various sizes and fishing weights from one container to another. In one case, the objects were dry, and in the other, they were at the bottom of a container filled with water.

Participants took 17% longer to transfer submerged objects with wrinkle-free fingers than dry objects. But when their fingers became wrinkled, they were able to transfer the pellets and submerged weights 12% faster than with their wet, unwrinkled fingers. Interestingly, there was no difference in the transfer of dry objects with wrinkled or wrinkled fingers.

Some scientists have suggested that the wrinkles on our fingertips can function like the rain treads on tires or the soles of shoes. The channels produced by the wrinkles help to draw water from the point of contact between the fingers and the object.

This indicates that humans may have evolved to wrinkle our fingers and toes at some point in history to help us hold objects and walk on damp surfaces.

“Since wrinkles seem to provide better grip underwater, I would consider some relationship to locomotion in very wet conditions or possibly handling objects underwater,” says Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study.

Wrinkles may have given our ancestors a key advantage in walking on wet rocks or holding onto branches, for example. Or it may have helped us catch or collect foods like clams.

“In the second case, this would indicate that it is unique to humans, but in the first case, we would expect this to happen in other primates as well,” says Smulders.

Wrinkling of the fingers has not yet been observed in our closest primate relatives such as chimpanzees, but it has been found that the toes of Japanese macaques – known to stay for long periods in hot water – also become wrinkled after being submerged in water.

But the lack of evidence in other primates doesn’t mean wrinkling doesn’t happen. It could simply be that no one has observed enough yet, according to Smulders. In fact, “we still don’t know the answer to that question.”

There are other interesting indications as to when this adaptation may have appeared in our species. Fingertip wrinkling is less pronounced in salt water and takes longer to appear than in fresh water, for example.

This is likely because the variation in salt level between the skin and the surrounding environment is less in salt water, so the salt imbalance that triggers nerve fibers is less pronounced. So this adaptation may have helped our ancestors live in freshwater environments rather than along the coast.

But there are no definitive answers and some people believe that this physiological reaction may just be a coincidence, with no adaptive function.

Incognitos

It’s strange to note that there are other mysteries surrounding finger wrinkling. Women take longer to develop wrinkles on their fingers than men, for example.

And what’s the exact reason why our skin returns to its normal state after 10-20 minutes, if having wrinkled fingertips doesn’t bring any clear downside to the grip of fingers on dry objects?

If having wrinkled fingers can increase our grip in wet environments but not harm us in dry environments, why aren’t our fingertips permanently wrinkled?

One reason could be the change in sensation caused by wrinkling. Our fingertips have many nerve endings, and wrinkling of the skin changes how we feel about objects we touch (although one study has shown that this does not affect our ability to differentiate objects by touch).

“Some people have a real aversion to wrinkles because picking something up with wrinkled fingers feels weird,” says Davis. “It could be due to the balance of receptors in the skin that have shifted, but there could also be a psychological issue. It would be fun to investigate the reasons. There could be other things we can’t do so well with wrinkled fingers.”

But the most surprising thing is that wrinkling fingers and toes in water can also reveal fundamental information about our health.

Wrinkles take longer to form in people with skin conditions like psoriasis and vitiligo, for example. Patients with cystic fibrosis suffer excessive wrinkling of the palms and fingers, which has already been observed in genetic carriers of the disease.

Patients suffering from type 2 diabetes also sometimes demonstrate markedly reduced levels of skin wrinkling when their hands are placed in water. Similar reductions in wrinkling have been seen in people who have suffered cardiac arrest, perhaps due to some disruption in the control of their cardiovascular system.

Asymmetrical wrinkling of the fingers – one hand wrinkles less than the other in the same immersion time – has even been indicated as an early sign of Parkinson’s disease, as it indicates that the sympathetic nervous system is not working properly on one side of the body. body.

All of this goes to show that the question of why fingers wrinkle in water remains unanswered, but our wrinkled fingers are being useful to doctors in other, equally unusual ways.

Read the original version of this report (in English) on the BBC Future website.

This text was originally published here