The Mystery of Pruned Fingers and Toes

It's happened to all of us: the pruned wrinkly fingers from taking too long of a bath, too much time in the hot tub, or, worst case scenario, too long doing dishes without gloves.

Today, in the post-long-run shower, I noticed that my fingers immediately pruned severely.  I recalled that this had happened to me a few times after long runs, and I had no idea why.  I was staring at my hands in fascination and thought to myself, "Huh.  I wonder why that happens.  I'll look it up."

So I did.

I expected the Osmosis Explanation.  I'd been sweating, so my tissue was likely highly dehydrated and the concentration of solutes should be higher than normal.  But, no.  Osmosis is not generally accepted as the explanation because the wrinkling in water effect is only observed on fingers, hands, and feet, not over the entire epidermis.

I read the explanation that loss of the protective layer of Sebum may be a precursor to pruning with interest.  This ties nicely with the long run as the constant sweating for multiple hours likely removed much of the sebum that would ordinarily be on my fingers before I even entered the shower. 

But sebum is where the general agreement seemed to stop. Everyone seems to agree that sebum is supposed to function as a water barrier and that after a while in water, it would wash off and be unable to do its job. But, I couldn't actually find a consensus on what happens after the sebum is gone to cause the wrinkles.

Some people (like the folks at the Library of Congress) hypothesize that the attachments between the live and dead layers of the epidermis are the valleys in the wrinkles and that the raised areas are the dead areas where the water is free to flow and be absorbed in the swelling. They claim that since the dead layers of the epidermis are the largest on the hands and feet, the swelling and valleys are only observed in those areas.

Okay, I can believe that might be the explanation.  In fact, if you couple it with the sebum as being a water barrier to absorption, it seems like I’ve found a reasonable answer.

But, wait. 

Apparently, fingers and toes don’t wrinkle under water exposure if the person has nerve damage to the hand or foot. This goes against a purely chemical/mechanical explanation like simple absorption and biological structure.

The nerve piece and the fact that the wet wrinkling response has only been observed in humans and macauques caused the fine folks at 2AI Labs to publish a paper in Brain, Behavior, and Evolution that hypothesizes that the wrinkles in our fingers and toes are actually *rain treads.*

But, not everyone accepts this explanation either.  Some claim that the waterlogged fingers and feet are much less able to grip and navigate in watery environments because they are so soft, supple, likely to deform under pressure, and likely to be injured.

So, for now.  I just don't know.  I guess I'll wait for 2AI Labs to publish their follow-up research on whether grip is actually improved by the so-called "rain treads."

It's nice to be reminded that every once in a while even Google doesn't know the answer