Why Do People Get Pruney Fingers after Being in the Water a Long Time?

We've previously featured some of the creative ideas of evolutionary neurobiologist Mark Changizi, including his explanations for optical illusions, human blushing, and his notion that hospitals could use skin-colored gowns to track patient health.

Now Changizi is trying to explain why the surface of human fingers get pruney after prolonged exposure to the water. His hypothesis is that the marks help disperse water and improve grip:

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.[...]

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.


Link -via Geekosystem | Photo: Ever So Strange

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I'd just like to draw attention to the distinction between cause and effect. Changizi is looking for the "effect" explanation, the "what-for" or "selection pressure". Which is different than a sufficient cause (as for example provided by AJW).

In the stream, eddies form, and dissolve, but they never leave the stream. They are stream the whole time they are also eddies. The causality which is the why and what-for never separates so cleanly into cause and effect. To say; "The stream causes the eddy" is erroneous because the eddy is stream, it would amount to an explanation causu sui (cause of itself) Some explanation from Brownian motion would be required to explain how the eddy is formed within* the stream by the activity of individual water molecules.

The "selection pressure" is the what-for, but on close inspection it is also the why. All accounts of a sufficient cause demand a nullifier or predicate variable; "If P Then Q" is not sufficient argument form, one would need to include "If P (and if not-X) Then Q" where X is a set of circumstances that would nullify Q. Or X could be given as a set of background conditions necessary for Q; "If P (and if X) Then Q". Example: "If it rains then the side-walk will be wet." is an insufficient causal argument, it would have to be; "If it rains and there is nothing obstructing the rain's descent onto the side-walk (such as an awning or umbrella) then the side-walk is wet." or "If it rains and the space between rain and side-walk is clear, then the side-walk is wet." The first statement is a positive and the second a negative, but mean the same thing. Any sufficient cause must contain some requisite in all that exists; the background conditions must be just-so. Whether or not something is "selected for" by "evolution" may just be another way of saying whether or not it is causally sufficient, whether or not the background conditions necessary for Q have been met. Many things may be manifest because meeting the criterion for a sufficient cause, but they may be useless in evolutionary terms. One can work backwards through causation, but it is problematic, especially if one starts off on the wrong foot, which I think is the case when trying to account for animal traits by evolutionary selection.

Typically, reductive materialists, which comprise the vast majority of scientists, do not accept teleological explanations, and when they say "We evolved this for that" they are meaning to say "These were the background conditions (X) that made it possible and beneficial which lent to it's continued survival" but there is a real problem of slipping into wild teleological speculation in evolutionary theory, and of communicating that distinction to the masses.
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Xi Chen just made a wild guess, and got it wrong. Fingers go pruney in cold water too!

I have heard it said that the skin absorbs water slowly and after prolonged immersion this causes it to expand. Since it is then larger than the finger, it wrinkles. That sounds more feasible to me.
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Well it's true that I NEVER get any aquaplanning problems while walking on my fingertips on wet surfaces.
I can imagine all the death and suffering that would result otherwise, so I can clearly see why evolution would favour that trait.
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