Spend more than a few minutes in the shower or pool and your fingers will undergo a dramatic transformation.
Where once there were delicate rolls of slightly stiff epidermis, there are now plump folds of sultana skin.
The shocking change is familiar, but still unnerving.
Only the skin of the fingers and toes wrinkles when they are submerged in the water, while other parts such as the forearms, torso, legs and face remain as smooth as when they entered the water.
This wrinkling of the skin of the fingers and toes has occupied the minds and work of scientists for decades.
Many people have tried to understand what causes wrinkles, but more recently the question of why and what they are for has attracted the attention of researchers.
But what our wrinkled fingers can reveal about our health is perhaps even more interesting.
It takes about 3.5 seconds in hot water (40 degrees Celsius is considered the optimum temperature) for your fingers to start wrinkling, while in cooler temperatures, around 20 degrees Celsius, it can take up to 10 minutes.
Most studies have shown that it takes about 30 minutes in the water for maximum wrinkles to appear.
Finger wrinkles were commonly thought to be a passive response in which the upper layers of the skin swell as water floods into cells through a process known as osmosis: a process by which water molecules pass through through a membrane to equalize the concentration of the solutions on each side. of the membrane itself.
But as early as 1935, scientists suspected that the process was more complex than that.
Doctors who have studied patients with injuries in which the median nerve (one of the main nerves that runs through the arm and down to the hand) has been severed have noted that the fingers of these patients do not bend in water.
Among its many functions, the median nerve helps control so-called sympathetic activities, such as sweating and blood vessel constriction.
This finding suggests that the action of wrinkling the fingers in the water is controlled by the nervous system.
Further studies by physicians in the 1970s provided further evidence for this finding and proposed submerging the hands in water as a simple test to assess the extent of a type of nerve damage that can affect the regulation of unconscious processes such as blood circulation.
Then, in 2003, neurologists Einar Wilder-Smith and Adeline Chow, then working at the National University Hospital in Singapore, measured blood flow in the hands of volunteers immersed in water.
They found that when the skin on the fingertips began to wrinkle, blood flow to the fingers decreased dramatically.
When they used a local anesthetic cream that causes blood vessels in the fingers to temporarily constrict, they found that similar levels of wrinkling were produced as seen with immersion in water.
“It makes sense if you pay attention to your fingers when they wrinkle,” says Nick Davis, a neuroscientist and psychologist at Manchester Metropolitan University who has studied how fingers wrinkle.
“The tips of the fingers turn pale and this is due to the constriction of the vessels removing the blood supply from the surface.”
But why ?
But if the wrinkles are controlled by the nerves, it means that our body is actively reacting to being in the water.
“It means it’s happening for a reason,” says Davis. “And that means it could give us an advantage.”
With the help of 500 volunteers who visited the Science Museum in London in 2020, Davis measured the force needed to hold a plastic object.
As expected, those with dry, wrinkle-free hands had to use less force than those with wet hands, as their grip on the object was better.
But when they immersed their hands in water for a few minutes, until their hands wrinkled, both groups’ grip strength decreased, even though their hands were still wet.
“The results were incredibly clear,” says Davis.
“The creases increased the friction between the fingers and the object. What’s particularly interesting is that our fingers are sensitive to this change in surface friction, and we use this information to apply less force when gripping an object safely.”
Their findings are in line with those of other researchers, who have found that wrinkles in our fingers make it easier for us to grip wet objects.
Some scientists have suggested that the wrinkles on our fingers may act like the grooves in tires or the soles of shoes.
The channels that produce the wrinkles help water flow away from the point of contact between the fingers and the object.
This suggests that finger wrinkles may have evolved at some point in our past to help us grip wet objects and surfaces.
And because ?
“Since it seems to give us a better grip underwater, I suspect it has something to do with locomotion in very wet conditions or the potential manipulation of objects underwater,” says neuroscientist Tom Smulders of Newcastle University, who led the 2013 study.
It could have given our ancestors a decisive advantage when they had to walk on rocks or hold on to branches, for example.
It could also have helped us catch food like shellfish.
“If the explanation is the latter, it would imply that it is a feature unique to humans, whereas if not, we would expect to see it in other primates as well,” says Smulders.
We have yet to observe wrinkling of the fingers of the closest species to us in the primate world, such as chimpanzees, but we have also seen wrinkling of the fingers of Japanese macaques, which are known to bathe in hot water for long periods. being submerged in water.
But just because there’s no evidence doesn’t mean it’s not happening, it could just be that no one has looked closely enough, says Smulders.
“We don’t know the answer to that question yet.”
There are other interesting clues as to when this adaptation might have developed in our species.
Finger wrinkling is less noticeable in salt water and takes longer than in fresh water. This is probably because the salt gradient between the skin and the surrounding environment is less in salt water, so the salt imbalance that alerts nerve fibers is less dramatic.
It may therefore be an adaptation that allowed our ancestors to live more in fresh water than in coastal areas.
But there are no definitive answers, and some think it might just be a physiological response that arose by coincidence, without any adaptive function.
Interestingly, there are other mysteries: women take longer to develop wrinkles than men, for example.
And why does our skin return to its normal state, usually within 10 to 20 minutes, if there is no harm in picking up dry objects if our fingers are wrinkled?
If having wrinkled fingers can improve our grip in the water and doesn’t hurt when dry, why don’t our fingers always stay wrinkled?
One of the reasons may be the change in sensation caused by wrinkles. The tips of our fingers are full of nerve endings, and the wrinkles on our fingers change the way we feel about things we touch (although a study has shown that this does not affect our ability to differentiate objects based on touch).
But wrinkling our toes in the water can also reveal important information about our health in surprising ways.
Wrinkles also form slowly in people with skin conditions like psoriasis and vitiligo, for example.
Cystic fibrosis patients have excessive wrinkling of the palms and fingers. Type 2 diabetes patients also report a noticeable reduction in the number of wrinkles when their hands are immersed in water.
Similarly, a reduction in wrinkles has been observed in people with heart failure, possibly due to altered control of their cardiovascular system.
Even asymmetrical wrinkling of the fingers, where one hand bends less than the other despite equal time in the water, has been suggested as an early sign of Parkinson’s disease, as it indicates that the sympathetic nervous system is not working correctly on one side of the body. .
So while the question of why our fingers and toes started to wrinkle still remains, our aging fingers and toes turn out to be useful in medicine in surprising ways.
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