指紋は「すべり止め」じゃなかった？ 物にふれる面積が減り、摩擦はむしろ下がる方向。役目は手ざわりを感じることかも

Many people think fingerprints help us hold things without slipping. But in 2009, two scientists measured this. They found that fingerprints made the finger touch less of a smooth surface, so there should be less friction, not more.

Another team suggested a different job for fingerprints. When we move a finger over something rough, the lines make small vibrations. These may help us feel tiny differences in texture.

Fingerprints are often said to work like the grooves on a tire, helping us grip objects. In 2009, however, Warman and Ennos pressed people's fingertips against a smooth acrylic surface and measured friction. They found that the skin behaved like rubber, where friction depends on contact area, and that the ridges reduced the contact area by about a third.

As a result, fingerprints probably lowered friction. That same year, a French team built a sensor that copied a fingertip. When it had ridges like fingerprints, sliding it over a rough surface produced strong vibrations at frequencies that nerves deep in the skin detect best. This suggests fingerprints may help us sense texture.

Like tire treads, fingerprints were long assumed to improve grip. In 2009, Warman and Ennos at the University of Manchester pressed fingertips against smooth acrylic while recording friction and contact area. The fingerpad behaved like rubber, whose friction rises with contact area. Compared with flat skin, the ridges cut that area by roughly a third, so on dry, smooth surfaces fingerprints should lower friction.

The authors allowed that ridges might still help on wet or soft surfaces. A more convincing role emerged the same year. Scheibert and colleagues built smooth and ridged versions of a biomimetic sensor. When the ridged sensor slid across a textured surface, it produced vibrations at a frequency set by sliding speed and ridge spacing. That frequency falls within the range where Pacinian receptors, deep touch sensors, respond best.

So fingerprints may work as a filter that turns texture into signals the nervous system can process. People born without fingerprints manage everyday tasks, which also argues against a vital gripping role. Developmental research in 2023 showed that ridges arise from interacting chemical signals that form a Turing pattern, a process sensitive to tiny differences, which helps explain why even identical twins have different prints.