We’re swimming several times a week and the little girl is fascinated that she turns into a giant pink sultana. She has to check each finger and toe when we get out to see how wrinkly it is. So why do our fingers wrinkle?
I grew up with the old explanation, but I did some checking and it’s changed. Yay science! It used to be thought that the keratin protecting our skin absorbed water and got bigger, but because it’s tightly bonded to the deeper layers it couldn’t go anywhere and wrinkled. Our fingers and toes have more keratin to protect them so they absorbed more, which is why they were the places that wrinkled.
Controlled by nerves
However, doctors and surgeons noticed that people with nerve damage don’t get wrinkled fingers, which makes no sense at all if it’s just passive water absorption. In fact, what seems to be happening is the involuntary nervous system constricts the blood vessels in fingers when they are in water. Less blood in the fingertips makes the insides smaller, but the thick skin is still the same size so it wrinkles like a deflated balloon. This is actually used as a bedside test for nerve damage – put a patient’s hand in water and see what happens.
The way it works is quite interesting, showing how much our body is controlled by simple chemistry. Your skin absorbs water from around you. This changes the electrolyte balance, sort of like watering down a sports drink. This affects the way the nerves work, making them fire more and constricting the blood vessels.
The big question – why?
The big question is why does it happen? Is there a reason?
One explanation is that it acts like tread on a tyre. When we grab something in the wet the wrinkles form channels that allow the water trapped between our finger and the object to escape, then spread out and give us the maximum grip. A not insignificant advantage if you are climbing trees in a jungle! And water wrinkles have been confirmed in macaques so it’s possible this is their evolutionary purpose.
I have to admit I’m not convinced, mainly because I know how often we assume there must be a reason for everything. Biologists love Just So stories. But sometimes it’s just the way bodies work, it didn’t happen on purpose. Perhaps our fingers and toes have a leaky structure for another reason, like sensitivity or dexterity. Then water is going to be absorbed and chemistry takes over without any higher purpose.
In order to test the theory I would like to know more about the micro-anatomy of our fingers. The wrinkles form in a predictable pattern, so is there something that purposefully creates that pattern or is it just because that’s the way the muscles, tendons, tissue connections and blood vessels lie in a fingertip? And I’d like some practical tests – do wrinkles actually make you less likely to fall off the branch?
That’s the fun thing about science – you come up with a cool idea and then you get to do the experiments to test it out.
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