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Fingerprints do more than bust us

By Bill Briggs

We are our fingerprints.

From the loops on our thumbs to the whorls on our pinky toes, no human shares the same delicate swoops on the skin of our palms and feet. But those inimitable wrinkles on our digits didn't develop just to let us access keyless doors, or bust us for being at the scene of a crime. What is the biological purpose of those tiny, raised crinkles?

It's long been thought that the distinct skin patterns reduce surface friction when we're grasping or holding objects — that the the friction improves our grip. However, British researchers have found that fingerprints actually make it more difficult for people and primates to grip and hold flat, smooth things.

Mary Altaffer / AP

"Because the skin is ridged, a lot of the fingertip isn't touching the surface," explains lead researcher Dr. Roland Ennos, a biomechanics expert at the University of Manchester in England's faculty of life sciences. "Think of the tires on a Formula 1 race car, or on an Indianapolis car. They want to have the best grip as possible, so they use flat tires. In just the same way, our fingertips – because they are ridged – don't have the same grip they would if they were perfectly flat."

To get a better grasp of the science, Ennos' team built a contraption that looks a little like a guillotine. A sheet of Perspex (a plastic also known as Plexiglas) was hung from a cross beam. The beam was then lowered and raised while student Peter Warman held the strip between the fingers and thumb on his right hand. While measuring the force of his clasp on the Perspex, the machine pulled the strip down via a weighted plastic cup. The team tried three different widths of Perspex as well as three different grip angles to mathematically separate Warman's own pressing force from the contact area, and to weed out any variables.
Their research, published in the June Journal of Experimental Biology, found the student's fingerprints added slip, not grip.

Humans aren't the only creatures with skin prints, either. Why do koalas have them? Why do monkeys in South America have similar prints on their tails? Ennos has a theory.

The padded sections of our hands and feet that do the toughest physical work – grabbing, twisting, pushing, pulling and thrusting – are laced with prints. The grooves and ridges give those swaths of our skin more elasticity. They allow the skin to stretch and distort as the labor takes place. The opposite is true of the smooth areas of our hands and feet. After withstanding excessive friction, they rip or collect fluid between the skin's various layers.

"When you do 'DIY' work or when you're walking with ill-fitting boots, it's the areas which haven't got the ridges that tend to get blisters," Ennos said. "My idea is that by having fingerprints, the skin is actually about twice as flexible. It struck me that having a fingerprint is part of a design package that strengthens the skin, allowing it to deform an awful long way without being damaged."

Ennos believes that fingerprints also allow us to more easily grab rougher surfaces and wet objects while they also add sensitivity to our feel. But touch cannot be their main function, he theorized, because the heel is covered with skin ridges yet "isn't used to discriminate anything" if, say, we're trying navigate a dark room.

The British researcher has applied for additional funding to conduct extra tests on the friction-prevention qualities of our fingerprints. But here's a warning to all interested student subjects: Blisters are part of the job.