Why do fingers and toes turn into "prunes" when soaked in water, but other skin doesn't?

© Popular Science

Leviathan Press:

Every time I take a shower or swim, I find that my fingertips are wrinkled because of being in the water for a long time, but other parts of my body are not. This phenomenon has really bothered me for many years. After all, those wrinkles... are hard to be pleasing to the eye. But on the other hand, thinking that our brains do not need to be immersed in water to have such a view (it's just that we rarely have the opportunity to see it), it seems to be a little more acceptable.

Spend a few extra soaks in the tub or laps in the pool, and your fingers will start to change. If the ridges on your fingers are slightly threaded, they can become saturated with water and take on an unsightly, prune-like appearance.

We are used to seeing this "dramatic change" in fingers, but we are puzzled. When people immerse their bodies in water, only the skin on their fingers and toes shrinks, while the skin on other parts of their bodies, such as their forearms, torsos, legs and faces, remains the same as if they were not immersed in water.

For decades, scientists have studied the phenomenon of wrinkling the skin on our fingers and toes due to water exposure. Most scientists have been puzzled as to what causes it in the first place. Recently, researchers have been interested in understanding the purpose of this phenomenon. Perhaps more interestingly, wrinkled finger skin can reflect problems with your health.

(www.bmj.com/content/1/6060/551.2)

(jamanetwork.com/journals/jamaneurology/article-abstract/577244)

It is believed that warm water at 40°C is most conducive to skin wrinkling, and at this temperature, skin wrinkling only takes about 3.5 minutes. If the temperature is lowered to 20°C, the process can be extended to 10 minutes. However, most studies have found that the degree of skin wrinkling will reach its peak when immersed in water for 30 minutes.

(link.springer.com/article/10.1007/s11249-015-0515-4)

© Science ABC

Osmosis means that water molecules can pass through the cell membrane to balance the osmotic pressure of the liquid inside and outside the skin. The wrinkling of the skin on the fingertips is a passive reaction. Water enters the cell through osmosis, and the upper layer of the finger skin absorbs water and swells. As early as 1935, scientists began to speculate that there was more to the process.

(link.springer.com/article/10.1007/s10439-016-1764-6)

The median nerve is one of the main nerves that runs along the arm to the hand. Among its many functions, the median nerve controls so-called sympathetic activities, such as sweating and vasoconstriction. Doctors studying patients with damaged median nerves found that their fingers did not wrinkle when soaked in water. The finding suggests that whether the skin on the fingertips wrinkles when immersed in water is determined by the nervous system.

(onlinelibrary.wiley.com/doi/epdf/10.1002/mus.10323)

(pubmed.ncbi.nlm.nih.gov/6875230/)

Skin from 13 fingers shrinking after soaking in a bathtub, while other body parts remained unchanged. © ResearchGate

Doctors in the 1970s provided further evidence for this idea. They proposed that people could use a small test like placing their hands in water to assess the extent of nerve damage, as some nerve damage affects unconscious processes such as the nervous system's control of blood flow.

In 2003, neurologists Einar Wilder-Smith and Adeline Chow at the National University Hospital in Singapore conducted a study on volunteers. They asked the volunteers to place their hands in water and monitored their blood circulation. They found that when the volunteers wrinkled their fingertips, their finger blood flow decreased significantly.

(onlinelibrary.wiley.com/doi/epdf/10.1002/mus.10323)

Neuroscientists applied a local anesthetic cream that temporarily constricts blood vessels to the fingers of healthy volunteers. They found that wrinkles appeared on the volunteers' fingers, and the degree of wrinkles was similar to that when they were soaked in water.

(linkinghub.elsevier.com/retrieve/pii/S0026286203000207)

“There’s a reason why your fingers wrinkle,” says Nick Davis, a neuroscientist and psychologist at Manchester Metropolitan University who has studied the phenomenon. “The blood supply to the surface of the finger is reduced, so the pads become pale.”

Wilder-Smith and his colleagues believe that when the hands are soaked in water, the sweat glands in the hands begin to dilate, and water enters the skin, eventually causing an imbalance in the salt balance of the skin. The salt imbalance causes the nerve fibers in the fingers to fire, causing the sweat glands around the blood vessels to contract, and the contraction of the sweat glands causes the fleshy areas of the fingertips to decrease in volume, pulling down the surface skin and eventually causing it to distort into wrinkles. The pattern of wrinkles depends on how the outermost layer of skin, the epidermis, is fixed to the layers below.

(link.springer.com/article/10.1007/s10286-004-0172-4)

Pablo Saez Viñas, a biomechanical engineer at the Technical University of Catalonia who used computer modeling to test the mechanism, said there were also signs that the outer layer of the skin could swell slightly, thereby exacerbating the wrinkles. However, the skin would need to swell 20% to form visible wrinkles just by osmosis, which is a shocking expansion. But when the upper layer of the skin swells slightly while the lower layer contracts, the wrinkles become more obvious.

“You need to get the wrinkles to be at a normal level for both,” he said. “Some people don’t have that neural response and don’t wrinkle.”

If wrinkling is indeed neurally controlled, it means our bodies are actively responding to our presence in water. “That means the wrinkling must be happening for a reason and it’s giving us a benefit,” Davis said.

The wrinkles caused by water may have provided our ancestors with increased grip when walking on slippery rocks or foraging for shellfish. © Alamy

Davis is currently investigating the question, and its potential benefits, of her children's bath time - why fingers wrinkle. In 2020, with the help of 500 volunteers who visited the Science Museum in London, Davis measured the force required to grasp plastic objects.

The results were as expected. When the palms were not wrinkled, people with wet hands needed to use more force than those with dry hands. Obviously, dry hands had better grip strength, but when these people soaked their hands in water for a few minutes and their hands became wrinkled, even if their hands were wet, their grip strength would be between the first two.

(journals.plos.org/plosone/article?id=10.1371/journal.pone.0253185#sec008)

"The conclusion is obvious: the wrinkles increase the friction between the fingers and the object," Davis said. "It's worth noting that our fingers are very sensitive to this change in surface friction, which allows us to grasp objects with less force."

The objects Davis had the volunteers grasp weighed only a few coins, and the grip force required was very small, but if you're doing physical labor in a wet environment, this change in friction can be significant.

"So you don't have to squeeze as hard to grab the object, the muscles in your hand relax a little bit, and you can work longer," Davis said.

© OFM

Other researchers have found that people find it easier to pick up objects when their fingertips are wrinkled, which coincides with Davis's findings. In 2013, a team of neuroscientists at Newcastle University in the UK organized volunteers to conduct an experiment in which they moved glass balls and fishing weights from one container to another.

The experiments were divided into two categories: one was to place the object in a dry environment, and the other was to place the object at the bottom of a container filled with water. The results showed that when the volunteers' hands were not wrinkled, it took 17% longer to pass the wet object than to pass the dry object. But when the hands were wrinkled, it took 12% less time to pass the items than when their hands were wet but not wrinkled. Interestingly, whether the fingers were wrinkled or not, there was no difference in the time it took the volunteers to pass the dry object.

(royalsocietypublishing.org/doi/full/10.1098/rsbl.2012.0999#d3e279)

There is still a mystery about wrinkling caused by soaking in water, which is really confusing - women take longer than men to wrinkle after soaking in water.

Some scientists say that the wrinkles on fingertips and toes may function like the grooves on a tire or shoe sole: The wrinkles create "grooves" that help water drain away from the point of contact with the fingertips and toes.

This suggests that humans may have evolved wrinkling mechanisms in our fingers and toes at some point in the past to help us grip slippery objects and increase surface friction.

“Wrinkling seems to improve underwater grip, and I think there are two main reasons for this: one is locomotion in very wet environments, and the other is moving objects underwater,” said Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study. Wrinkling would have been important for our ancestors, for example, when they were navigating slippery rocks or holding onto branches. It would also have made it easier for our ancestors to forage for food, such as shellfish.

Japanese macaques are the only primates to have wrinkled fingers. © Planet Zoo Wiki

"It's unique to humans to move objects underwater, but other primates may also move in wet environments," Smulders said. Chimpanzees and other animals, our closest relatives, have not been found to have wrinkled fingers, but Japanese macaque monkeys (Japanese macaque monkeys) will wrinkle their fingers after "bathing" in water for a long time. At present, people have not observed other primates in detail enough to prove how different they are, but that doesn't mean that wrinkles don't occur in these animals. "We are still confused about this," Smulders said.

(www.karger.com/Article/FullText/328223)

There are a few interesting points about when wrinkling occurred in humans. In salt water, the wrinkles on the fingertips are less severe and take longer to develop than in fresh water. Perhaps this is because in seawater, the salt gradient between the skin and the surrounding environment is lower, so the salt imbalance that causes the nerve fibers to fire is lower. This suggests that wrinkling is an adaptation that helped our ancestors live in freshwater environments rather than along coastlines.

(www.sciencedirect.com/science/article/abs/pii/S0266768105000021?via%3Dihub)

(www.sciencedirect.com/science/article/abs/pii/S0266768105002895)

This question remains unresolved, and some people believe that this is just a coincidental physiological reaction, not an adaptive mechanism.

Astonishingly, there are other mysteries surrounding this question that are puzzling. For example, it takes women longer to wrinkle than men. Why does it normally take 10-20 minutes for wrinkled skin to return to normal? Do wrinkled fingertips significantly affect our ability to grip dry objects? It is certain that wrinkles on fingers can increase grip when hands are wet, but do wrinkles affect grip when hands are dry? Why don't fingers remain wrinkled all the time?

(juniperpublishers.com/jojdc/JOJDC.MS.ID.555588.php)

© Andrii Biletskyi/Alamy

One reason could be that wrinkles also cause changes in our sense of touch. Our fingertips are full of nerves, and these bumps and grooves in the skin can change the way we feel the touch of an object (although one study suggests this doesn’t affect our ability to distinguish objects by touch).

(www.ncbi.nlm.nih.gov/pmc/articles/PMC3885627/)

“Some people are disgusted by this and find it weird to pick things up with wrinkled fingers,” Davis said. “This may be because the balance of skin receptors has changed, but there may also be a psychological aspect. It may be interesting to investigate why, but wrinkled fingers are not always helpful.”

Yet, the wrinkling of our fingers and toes can reveal some key information about our health, often in unexpected ways. For example, people with skin conditions such as psoriasis and vitiligo take longer to wrinkle. People with cystic fibrosis have extremely high and low wrinkles on their palms and fingers when soaked in water. This has also been noted in people with the cystic fibrosis gene. People with type 2 diabetes sometimes experience significantly less wrinkling when they put their hands in water. Similarly, people with heart failure also experience less wrinkling when soaked in water, perhaps due to cardiovascular disorders.

(jamanetwork.com/journals/jamadermatology/fullarticle/394464)

(diabetesjournals.org/care/article/7/3/224/32740/Decreased-Skin-Wrinkling-in-Diabetes-Mellitus)

(link.springer.com/article/10.1007/s10286-010-0109-z)

Asymmetric wrinkling of the fingers—one hand wrinkles less than the other, despite being in the water for the same amount of time—is thought to indicate that the sympathetic nervous system on one side of the body is not functioning properly and is an early sign of Parkinson's disease.

(www.sciencedirect.com/science/article/abs/pii/S0753332201000889?via%3Dihub)

Why fingers and toes wrinkle first in water remains an unsolved question, but our prune-like digits and toes may have a surprising amount of information to offer doctors.

By Richard Gray

Translated by Zhao Hang

Proofreading/Rabbit's Light Footsteps

Original article/www.bbc.com/future/article/20220620-why-humans-evolved-to-have-fingers-that-wrinkle-in-the-bath

This article is based on the Creative Commons License (BY-NC) and is published by Zhao Hang on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan