Pointless evolution – Why do we go wrinkly in the bath?

 One to think about while you soak…

Happy New Year! I hope you all had relaxing holidays!

I thought I would start off the year by asking a soothing question… one to contemplate while you soak in the bath perhaps…

The next time you have a good long soak in the tub, I’d like you to look at your finger tips (or your toes if you are that flexible!). You will probably notice that your fingertips have ?pruned? or gone wrinkly. Why is this? What is causing the wrinkling and what scientific process is driving it? An intelligent suggestion (relating to your keen knowledge of the GCSE syllabus) could be that water is moving into our skin epidermal layers through the process of osmosis, causing the skin to swell and bulge. However, this is not the case and we will revisit the reason why at the end of this blog.

Before we answer that question, let’s think even more high level. Why should our fingertips (and toe-tips!) only go wrinkly? Could there be an advantage to this? There has been much spirited debate about the Aquatic Ape Hypothesis which posits that the ancestors of modern humans spent a period of time adapting to a semi-aquatic existence. While this hypothesis has never gained widespread approval, it is worth watching the following TED video by one of the hypotheses main proponents, Elaine Morgan. 

So back to the wrinkly fingers – it was claimed in early 2013 that pruniness of the fingers may in fact have evolved to make it easier to handle wet objects. In 2011, a team of researchers proposed that the grooves in wet fingers might function as ?rain treads? that improve grip by channelling water away, much like car tires on a wet road do. In 2013, researchers at Newcastle University in England tested the theory.

The researchers had 20 volunteers manipulate objects with smooth fingers or digits shrivelled by immersion in warm water for 30 minutes. The experimenters measured how long it took people to transfer the objects between a water-filled container and a dry one, or between two dry ones, with wrinkled versus unwrinkled fingers. The objects were glass marbles and fishing weights of various sizes. All the participants transferred the wet objects (but not the dry ones) faster when their hands were pruney.

However, in a classic example of the back and forth nature of scientific findings, a new study led by Gary Lewin at the Max Delbrück Center for Molecular Medicine in Berlin-Buch, Germany, conducted a very similar study in which the watery advantage washed up.

So why do our fingers go pruney? Well, that mystery remains unsolved for now. However, we do know the physiological reasons for the wrinkles and it is not osmosis. Though often assumed to be a result of water passively seeping into the skin, the phenomenon is actually caused by the nervous system constricting blood vessels. As early as the 1930s, surgeons noticed that no wrinkling occurred if a finger nerve had been severed, so furrowing has been used as a medical indicator of nerve function. For now, what evolutionary purpose wrinkling serves, if any, remains a mystery.