It’s a warm summer day. You’re chillaxing in the gentle waves of Lake Ontario. The soft ebb and flow of the water lulls you into a state of deep relaxation. This is the life.
You lose all concept of time. You look down at your water-logged hands, but instead of fingers, you see prunes. PRUNES!
Well, not literally, but your hands are now so wrinkly that you don’t even recognize them. It takes upwards of 5 minutes for your fingers and toes to get wrinkly. This process speeds up if your hands and feet are in warmer water.
Have you ever stopped to wonder why or how this happens? Why is it that our fingers and toes wrinkle when wet? Why doesn’t the rest of our body go wrinkly? There’s a saying about “knowing something like the back of your hand”. You would think that we know all there is to know about our own limbs. But we actually don’t!
Scientists aren’t entirely sure why our digits turn to prunes, but they do have some pretty cool theories…
I am so glad you asked. I am going to go all science nerd on you here. Are you ready? It’s not boring, I promise. It’s actually pretty cool.
First of all, you need to understand your skin. Your skin is made up of a bunch of layers (*insert shrek-onion reference here*). Those three main layers are called the epidermis, the dermis, and the hypodermis. We are going to focus on the top layer, the epidermis.
The top layer (the epidermis) is itself made up of a bunch of layers. Each layer is composed of special keratin cells. As you’re going about your day, new keratin cells are constantly being produced. These cells divide rapidly at the bottom of your epidermis and migrate up towards the surface. Once they near the very top, they flatten out and create the protective outer layer of your skin.
The original pruney-finger theory was that our fingers wrinkled due to a process called osmosis (the movement of water through the semi-permeable layers of your skin). The idea was that the dead keratin cells would absorb water, causing the outer layer of your skin to swell and bunch, like the gathering of a skirt. Makes sense, right?
Scientists have known since the 1930s that feet and hands don’t get pruney if someone has nerve damage. What does this mean? Well, it means that there is something more than simple osmosis going on here.
The wrinkling is actually an active process brought on by your sympathetic nervous system (the one that controls your fight or flight responses). One of the things that your sympathetic nervous system controls are the sweat glands on your hands and feet (among other areas).
It has been suggested that when your hands and feet get wet, water can travel into the tiny sweat ducts in your skin to reach your sweat glands. This triggers your nerves to restrict blood flow to the tiny blood vessels in your fingers and toes. This tightening, or constricting, of your blood vessels is also known as vasoconstriction.
This wrinkling process only appears to happen to the skin on our hands and feet. The skin on your digits is “glabrous” (meaning that it has no hair). It is also much thicker than the skin on the rest of your body.
So now you *sorta* understand how your fingers get wrinkly, but why does this happen? What’s the point of having prunes for hands?
Since this response is triggered by the nervous system, and since the rest of our body does not wrinkle when wet, it suggests that there is an evolutionary reason for our pruney-ness. Could it be that our ancestors benefitted from wrinkly digits?
Some studies have found that these added wrinkles might actually help us grip wet surfaces. Think of a car tire on a road. A racing tire on a dry road will grip better if it is completely flat, whereas a grooved tire on a wet and uneven road is preferred.
It could be that the wrinkles act as channels to drain the water (much like the tire analogy). It might also improve adhesion to the object as your finger squishes down and releases the water in the grooves. A few studies have found that wrinkly fingers help us move wet objects faster, but it’s clear that more studies are needed before we can reach any solid conclusions.
You might be wondering, ‘if pruney fingers are so advantageous, why aren’t they pruney all the time?’. Scientists believe that permanently pruney fingers might reduce their sensitivity, making our little digits more vulnerable. We need to be able to take in those tactile cues from our environment.
The next time you go for a swim, take a good look at your fingers and toes. Try picking up wet objects and running your own experiment. Maybe you will be the one to discover why we really get wrinkly when wet.
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