Hey! It’s Friday Question time! Every fortnight, I’ll be answering an everyday question about science: from the ridiculous to the fascinating, from wonderful to bizarre. So, if there’s a question that’s always bugged you, or something you just don’t understand, drop me an email at firstname.lastname@example.org (I am always mega excited to receive mail). For now, onto this week’s question…
My Dad actually asked me this question a while ago, and I was actually completely stumped. He asked me “if our skin is waterproof, why does skin wrinkle in the bath? And if it isn’t waterproof, why don’t we swell up with water in the bath?” The thing is I thought I knew that, although our skin is mostly waterproof, water wrinkles were caused by water being absorbed into keratin buried in layers of skin, particularly in areas where there were lots of absorbent skin layers, like your hands and feet. These absorbent layers are attached to other layers, so they can’t swell up properly, and instead they wrinkle. It turns out, though, that I was apparently wrong. This is a common myth, but it seems to be untrue.
In fact, water-wrinkling is more likely to be controlled by the autonomic nervous system (ANS). The ANS is responsible for all the things you do without ever having to think about doing them, like breathing and keeping your heart beating, and digesting food, and generally staying alive. And, apparently, wrinkling your skin in water. What happens is that your ANS causes all the tiny blood vessels in the ends of your fingers and toes to constrict; tightening them up and making them narrower.
Once your blood vessels have narrowed, there is a decrease in the volume of fluid surrounding them; the shrinkage of your blood vessels leaves room in the ends of your fingers and toes. This causes the skin to pull backwards, to fill the space that was previously full of nice plump baby blood vessels and fluid. It cannot, however, pull backwards entirely; or you’d simply shrink in the wash like that hand-knitted cardigan you accidentally shoved into the washing machine. Instead, the skin distorts, causing wrinkles on the surface.
Of course, we don’t wrinkle when we’re just standing around at a bus-stop, so there must be something about being in water that causes all the crinkling. Water does enter the skin, via sweat ducts, and in doing so changes the carefully controlled balance of water and electrolytes in the area. This change is noticed by your neurotic neurons, which panic wildly, and scream at the blood vessels to constrict, quickly. And they do.
Voila, wrinkly you.
This still isn’t the whole story though, because your nervous system isn’t actually THAT neurotic, and if it tells your body to do something, it usually has a fairly valid reason. So WHY does our ANS make our fingers and toes wrinkle in water? The question is always WHY?! We don’t know for certain, sadly, but there IS now a theory. The theory is that wrinkly toes and wrinkly fingers give us better grip when we’re in a wet and slippery environment. This was tested by asking people to perform certain tasks involving moving marbles and weights around in water; on average people were faster when they had submerged their hands in water first, and got them good and wrinkly. It gave them better grip, and made them better at completing tricky dextrous tasks underwater. If this theory proves to be true, it effectively means that we’ve evolved to have watery tyre treads on our hands and feet to help us grip. I find this SO COOL; evolution really is amazing, just when you think it’s stopped blowing you away, you discover that something as seemingly bizarre and pointless and bath wrinkles are actually evolutionarily awesome. Good times!