If you’ve ever been to a pool or just taken a long shower/bath, you’ve probably gotten all wrinkly after getting out and toweling off. You might think it’s just you getting kind of soggy for a bit, but when you remember the human body is around 60% water you probably believe the “soggy person” a bit less. Also only our fingertips and feet really prune. So what’s up with that, why do we wrinkly in water?
Are You Actually Getting Soggy?
Conventional wisdom for the mechanism of finger pruning has been that we’re getting soggy. Well it’s phrased a lot more diplomatically than that. The idea is that water gets into the outer layers of your skin. This, in theory, meant that water floods those cells and they inflate a bit. The swelling of your skin, then, causes wrinkles.
Except here’s the thing. We know that’s misinformation. For starters, this proposition is a little problematic since you only really get wrinkly in your extremities. If we wrinkled because we got soggy, why wouldn’t our like… forearms and biceps prune? There’s more than intuitive logic at play here. People who have suffered nerve damage to their fingertips actually do not prune. We’ve actually known this since the 1930s. This tells us that the pruning is actually a response from the nervous system, and not just a property of physics and osmosis or something. Specifically, because the wrinkling is an involuntary response, it’s likely governed by the autonomic (sympathetic) nervous system. That’s the same part of you that governs your heartbeat or even breathing. We can narrow this down further, because the nerve damage we discussed earlier is specifically damage to the median nerve—a long nerve in your arm that governs sympathetic (automatic) nervous responses.
Other primates also don’t seem to wrinkle the same way we do, with the exception of Japanese macaque monkeys which spend a lot of time bathing.
What’s the Mechanism?
The actual mechanism behind your pruning is the constriction of the blood vessels in your skin. It takes anywhere between a minute and three minutes for your fingers to begin wrinkling, and you can hit full wrinkles within 30. Blood flow also drops demonstrably when we begin wrinkling. At the same time, anesthetic applied to fingertips that causes the blood vessels in our fingers to constrict also causes a similar wrinkling effect.
Salt seems to play a role in our pruning, since the wrinkling process takes longer in saltwater than freshwater. This has led to the conjecture that the pruning process might be aided by osmosis, and our bodies letting water in so your fingers swell a bit (facilitating the wrinkling). Why would salt be important? Well the transfer of stuff into and out of our cells is largely due to the difference of salt on the inside and outside of our cells. Generally, the proportion of salt within a cell and the area around it wants to be equal, so when there’s no salt outside the cell, there’s a stronger tendency for water to flood in. Vice versa when there’s salt in the water.
But, why?
There are actually benefits to the wrinkling of our fingers and toes in water. It’s not just shenanigans, even though it might feel a little less than convenient.
Turns out, when we prune, the wrinkles in our fingers actually improves our ability to handle wet objects. So next time you get all wrinkly in the shower, you can thank your body for trying to make it easier to hold stuff. Because we all know dropping something in the shower might as well be as loud blowing something up indoors.
Here’s a finger-themed word ladder.
