Though fingerprints are handy for identifying perps, biologically, scientists still aren't quite sure what our fingerprints are for. But as they test different hypotheses, they're getting closer to the answer—and learning some pretty cool stuff in the process.
Is it to improve our sense of touch?
In a 2009 study, researchers from the Ecole Normale Superieure in Paris built two biomimetic tactile sensors, which mimic the human ability to touch and perceive texture. One had grooves that mimicked fingerprints; the other was flat like smooth skin. When these faux fingers moved across roughly-textured surfaces, the fingerprinted sensors produced vibrations up to 100 times stronger than the smooth ones. These vibrations, the scientists found, were dominated by a frequency in the optimal range of sensitivity of the Pacinian corpuscles, receptors in our skin that detect pressure changes and vibrations. These researchers think that our fingerprints' job might be to amplify certain tactile information so that it's more easily processed by the nervous system. They also suggest that the swirling patterns of fingerprints ensure that some of the ridges are always brushing sideways across a surface, no matter which way the finger is moving, to better generate vibrations.
Is it to improve our grip?
Humans, apes, monkeys and koalas all have fingerprints. Some New World monkeys even have ridged pads on their tree-gripping tails. Fingerprints’ design, and their presence in all these animals, has led people to think that they’re an adaptation for improved grip while climbing trees and manipulating objects, but there isn’t much experimental evidence for that. Research by biomechanicists at the University of Manchester, who tested the idea in 2009, suggests that a good grip isn’t fingerprints’ forte. Dr. Roland Ennos and his student Peter Warman tested the grip of Warman's fingers at different angles on strips of acrylic glass sheets similar to Plexiglas. While many solid objects obey Amonton's law and friction between them is proportional to the force between them, the friction between finger and glass increased less than Ennos expected when more pressure was applied. The pair inked Warman's fingers to measure the contact area between them and the sheets and found that friction did increase when the contact area increased, but also noted that the grooves between fingerprint ridges reduce the fingers' contact surface with the glass by about one third, compared with smooth skin, and actually reduced friction and ability to grip.
What are some other possibilities?
Ennos and Warman throw out a few other plausible explanations for fingerprints at the end of their paper: that they allow our skin more to more easily comply with and deform to objects we're touching or holding, reducing shear stress and preventing blister formation; that they increase friction on rough surfaces compared with flat skin because the ridges project into the depressions on these surfaces and provide a higher contact area; that they facilitate runoff of water like tire treads. Ennos says his lab is testing all of these hypotheses, but hasn’t published any results yet.