There's a new theory on why our fingers develop prune-like channels when the skin gets soaked. NatureNews reports:
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 tyres. 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."
. . .
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.
. . .
... 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."
There's more at the link.
Earlier this year, Wired published an article titled 'Physics of Pruney Fingers Revealed'. It also suggests that the shape of the wrinkles on wet fingers isn't an accident, and describes the skin in terms of a mathematical model. It'd be interesting to put that study together with this one and see what comes out in the wash (you should pardon the expression).