Why do humans, other primates and koalas have fingerprints? All are, or have ancestors who were, tree dwellers, and it has been generally accepted that fingerprints help individuals grab onto things like tree limbs by increasing the friction between the skin and the object.
Biomechanist Roland Ennos of the University of Manchester teamed up with undergraduate student Peter Warman to test the idea that fingerprints improve grip friction. They produced a system for measuring the friction between a fingertip and a piece of acrylic glass. Using a weighted apparatus, they could vary the force between Warman’s finger and the acrylic and carry out the experiment with different fingers and at different angles. (The diagram in their paper in the Journal of Experimental Biology is hilarious—it looks like they had to cut off Warman’s fingers to do the test. I doubt that the university would have allowed this, though, even with an undergrad as a test subject.)
With normal solids, friction increases in proportion to the force between two objects. In the fingertip experiment, however, the friction increased less than expected. The fingertip behaved more like a rubbery surface. Friction was determined less by force than by how flat the fingers were; that is, flatter fingers increased the contact area between the finger and the acrylic and created more friction. But the ridges and valleys, Ennos and Warman found, actually work to reduce the contact area between the finger and the flat surface, which reduces the friction between the two. “These results force us to re-evaluate the role of fingerprints,” they wrote.
With increased friction ruled out, scientists are left with having to again hypothesize why we have them. Possible functions for fingerprints include:
- Increased friction on rougher surfaces, like tree branches, compared with flat skin. The ridges might “project into the depressions of such surfaces and provide a higher contact area.”
- Improved grip on wet surfaces by helping water to run off, like a car tire’s tread.
- Increased contact area and friction as gripping force is increased.
- Increased touch sensitivity.