Why do wet fingers wrinkle? Pruney fingers give better grip with rain treads!

in biology

Did you ever wonder why fingers get wrinkled when they are dipped in water for a while? A new study tells us that wrinkled fingers could be an adaptation to strengthen grip in wet conditions.

The wrinkling of wet fingers and toes is attributed to local osmotic movement of water. Osmosis is a process by which water moves from low salt concentration to high salt concentration through a semi-permeable membrane such as skin.

It is also known since long that wrinkling does not take place when certain nerves to the fingers are cut, though the exact mechanism of finger skin wrinkling is still subject to controversy.

Usually skin wrinkles appear as a result of aging, temporarily, wrinkles can appear on fingers and toes as the result of prolonged immersion in water.

The authors argue that the wrinkles on fingers function like rain treads on tires.
The authors studied 28 fingers of 13 hands and came to the conclusion that wrinkle structures on fingers not only have the signature of a drainage network from high ridge, but more specifically they are consistent with the pressure topography.


Image credit: Mark Changizi

The study was published recently in the journal Brain Behavior and Evolution. The authors Mark Changizi, Romann Weber, Ritesh Kotecha and Joseph Palazzo are from the 2AI Labs in Boise,Idaho.

Why should wrinkles develop? According to the authors, when a wet finger grips wet surface, the treads helps in draining fluid out of the contact surface, so that the finger skin gets maximum contact with the surface.

Changizi and colleagues think that the wet-induced wrinkles on fingers have a structure similar to that of drainage. They also argue that the time needed to wrinkle, about 5 min, has practical meaning, because your finger does not need such strong grip on casual contacts such as  when you eat a piece of apple.

According to a news article on this finding in the scholarly journal Nature, “Not everyone is gripped by the new theory.  “This hypothesis is unjustified,”  says Xi Chen, a biomedical engineer at Columbia University in New York.  Chen thinks that the wrinkles have a simpler cause: when fingers are immersed in hot water, the blood vessels tighten and the tissue shrinks relative to the overlying skin. This contraction causes the skin to buckle. “It is a classic mechanics problem,” he says.”

The authors are planning to investigate whether wrinkling is more prevalent among mammals in wet conditions. 

The authors meticulously compared the contours of finger wrinkles to convex promontory network,found similarity and came to the clever conclusion that this is an adaptation to strengthen the grip.  However, if this is an adaptation to strengthen the grip, it needs to be further investigated whether wrinkling leads to quantifiable increase in grip strength. Nevertheless the authors must be credited for coming up with an interesting reasoning for an age-old observation.

Source article: Are Wet-Induced Wrinkled Fingers Primate Rain Treads?  Mark Changizi, Romann Weber, Ritesh Kotecha, Joseph Palazzo. Brain Behav Evol. (2011) Published online: June 23, 2011 (DOI: 10.1159/000328223).

Link to Mark Changizi Lab


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