CLOSE
Getty Images
Getty Images

Why Do We Have Fingerprints?

Getty Images
Getty Images

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.

nextArticle.image_alt|e
iStock
arrow
Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
iStock
iStock

In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
iStock

Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

nextArticle.image_alt|e
Mark Ralston/AFP/Getty Images
arrow
Big Questions
What Causes Sinkholes?
Mark Ralston/AFP/Getty Images
Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios