Pruney Fingertips
We are all familiar with the prune-like fingers we get when we are in the bath. We assume it’s caused by water swelling up the skin in our fingers. In fact it could be an evolutionary trait caused by a reaction in our body’s central nervous system. Rather than swelling up, fingertips actually shrink when they wrinkle, because the blood vessels inside them contract. The creases could have evolved in our ancestors as they gathered food from wet vegetation or streams.

Dr Tom Smulders, an evolutionary neuroscientist at Newcastle University was inspired to test the hypothesis, previously proposed by Mark Changizi in 2011, that wrinkled fingers were an evolutionary adaptation. Like treads on tyres, Changizi had proposed that wrinkles served the function of removing water from between the fingers and objects, giving better grip.

Perfect Student Project
Smulders saw that it should be easy to establish that wrinkled fingers were better at gripping in moist conditions and it was an ideal student project. He explains, ‘we came up with the idea of timing how long it takes to move marbles from one container to another, between thumb and index finger. The subjects did that either underwater or not, with wrinkled fingers or not – so one person did all four conditions.’

They found that people are in fact better at moving objects from underwater with wrinkled fingers than with nonwrinkled fingers. Smulders points out how the original hypothesis was based on the wrinkling as an active process, controlled by the nervous system. People who have damage to their peripheral nerves in their fingers don’t get wrinkles any more.

Adaptation to Environment
Whether that means that wrinkled fingers are an adaptation is debatable, it could be a side effect of something that evolved for a different reason. As Smulders explains, ‘it all points towards that it may well be an adaptation for wet conditions, but you can’t prove that from a single experiment you have to bring many pieces of evidence to bear.’ The research suggests that there may be a loss of sensitivity in our hands, which explains why our fingers return to their normal state when not submerged.

Smulders hopes these findings will encourage colleagues to continue this research and understand what the evolutionary advantages were for having such a trait.