By Ed Yong of Nature magazine
The wrinkles that develop on wet fingers could be an adaptation to give us better grip in slippery conditions, the latest theory suggests.
The hypothesis, from Mark Changizi, an evolutionary neurobiologist at 2AI Labs in Boise, Idaho, and his colleagues goes against the common belief that fingers turn prune-like simply because they absorb water.
Changizi thinks that the wrinkles act like rain treads on tires. They create channels that allow water to drain away as we press our fingertips on to wet surfaces. This allows the fingers to make greater contact with a wet surface, giving them a better grip.
Scientists have known since the mid-1930s that water wrinkles do not form if the nerves in a finger are severed, implying that they are controlled by the nervous system.
"I stumbled upon these nearly century-old papers and they immediately suggested to me that pruney fingers are functional," says Changizi. "I discussed the mystery with my student Romann Weber, who said, 'Could they be rain treads?' 'Brilliant!' was my reply."
In a study published online in the journal Brain, Behavior and Evolution, Changizi and his team studied photos of 28 wrinkled fingers. The team saw that all of them had the same pattern--long, unconnected channels branching from a point at the top of the finger.
When we press down with a finger, we apply pressure from the tip backwards. The sides of the finger are like cliffs where water can easily fall away, but the flat part is more like a plateau where water can pool. Wrinkles form on the plateau because "that's where all the work has to be done to channel the water away", Changizi explains.
Not everyone is gripped by the new theory. "This hypothesis is unjustified," says Xi Chen, a biomechanical 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's a classic mechanics problem," he says.
But neurosurgeon Ching-Hua Hsieh of the Chang Gung Memorial Hospital in Kaohsiung, Taiwan, says that the process Chen describes does not account for the fact that fingers wrinkle even in cold water, or that they do not wrinkle when their blood supply is cut off. He thinks people should be looking for more explanations of water wrinkling. "I really appreciate the new hypothesis," he says.
Hsieh and Changizi both note that water wrinkles appear only on the fingers and feet, and that the most prominent wrinkles develop at the ends of digits, which are the first parts to touch a surface.
Changizi now wants to see if mammals that live in wet habitats are more likely to develop wrinkled fingers. "E-mails to a couple dozen primate labs led to a couple dozen 'gosh-I-don't-knows'," he says. "It occurred to me to look at the bathing [Japanese] macaques, and I finally found one photograph [of a monkey] with pruney fingers. So it's at least us and macaques, and surely many others."
The ultimate test of the hypothesis will be to see if people with wrinkled fingers are better at gripping in wet conditions. "We began pilot experiments," says Changizi. "The results thus far suggest that, yes, being pruney helps."
This article is reproduced with permission from the magazine Nature. The article was first published on June 28, 2011.