“Raisin Fingers” May Be an Evolutionary Advantage

December 28, 2011 at 2:00 am 11 comments

By Kyle Kurpinski

If you have ever gone swimming for several hours or taken a really long bath, then you are probably familiar with the phenomenon referred to as “pruney fingers” or “raisin fingers.” A common explanation for this wrinkling of the hands (and feet) is that the skin absorbs excess water when submerged, which causes it to expand. This expansion increases the surface area of the skin, thereby resulting in wrinkles. So while these pruney fingertips may have the appearance of being shrunken or shriveled, they are actually larger than they were when dry. It has been suggested that fingers and toes are more susceptible to this effect than other areas of the body due at least in part to a lack of hair follicles, which produce sebum – an oily secreted substance that may act as a temporary protective barrier against water absorption.

But water uptake is only part of the story. As early as the 1930s, scientists noted that patients with palsy-related nerve damage in their hands showed no signs of water-induced wrinkling in the areas specifically affected by the impaired nerves. This insight suggested that the nervous system is actively involved in the wrinkling process, and additional research has shown that vasoconstriction – narrowing of the blood vessels – plays a role as well. A modern view of raisin fingers goes something like this: prolonged immersion leads to excess water uptake by the skin; the resulting electrolyte imbalance causes neurons to fire more rapidly, which causes blood vessels to constrict, thereby reducing blood flow underneath the skin and leading to a decrease in skin tension, thus causing wrinkling. This process is clearly more complicated than simple water-induced swelling, which is probably why most people have never heard about it. The complete physiological mechanism of action is likely a combination of water-induced swelling and vasoconstriction.

But none of this explains why fingers that look like little dried fruits should be a part of our biology in the first place. A new theory (put forth in a recent article in the journal Brain, Behavior and Evolution) proposes that wrinkles on the fingers and toes may actually act as miniature drainage channels to displace water and increase grip in wet conditions, similar to the rain treads on the tires of a car. In other words, when your fingers and toes get really wet, they wrinkle up to counteract the slickness of the water. Additional work still needs to be done for the theory to become more widely accepted, but it appears that the raisiny morphology may be an evolutionary adaptation for life in slippery environments.

As a professional bioengineer I feel inclined to run my own experiments, but I’m still waiting for my hot tub/climbing wall grant to go through.

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Sources: Amazing Health Trivia, WonderQuest, Wikipedia, Nature, 1Mark Changizi, et al

Image: Brian Zambrano (cc)

Entry filed under: Demystified, Health and Beauty. Tags: , , , , , .

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11 Comments Add your own

  • 1. Jen  |  December 28, 2011 at 9:19 am

    I heard about this. Very interesting!

    Reply
  • 2. Wendy  |  December 28, 2011 at 9:41 am

    Very interesting. Makes sense. I’ll join the believer’s.
    Now, my daughter recently posed this question:
    Why is it that grapes become raisins?
    Plums become prunes? (I know there are more.)
    Yet Apricots and apples as well as many others, when dried and shrivelled up, stay the same name?

    Reply
    • 3. Kyle  |  December 28, 2011 at 5:00 pm

      Based on a very limited web search, it looks like the prune and raisin nomenclature are a function of etymology, with both words being derived from their French counterparts. The French word for grape is “raisin,” while the French word for a dried grape is “raisin sec.” Similarly, the French word for plum is “prune,” while the French word for a dried plum is “pruneau.” So, my totally speculative answer is that someone in history decided to use the French word variants to refer to the dried forms of these fruits. That said, I didn’t find any info on why we don’t call a dried apple a “pomme” or a dried apricot an “abricot,” etc., etc. Perhaps we can chalk it up to the fact that, in many ways, English is a mish-mash of other languages assembled haphazardly over time. That’s the best I’ve got on 5 minutes of internet research.

      Reply
      • 4. Wendy  |  December 28, 2011 at 9:17 pm

        Well, you are very kind!
        Hogmanay:)

  • 5. Dan the Man  |  February 27, 2012 at 10:57 am

    I was interested until I remembered that evolution is pseudo seance. Darwin himself admitted he got it wrong and that ‘his’ ideas (they weren’t even his) were turned into a religion and an extremely well funded one at that to destroy our society as part of the depopulation program.

    This is quite funny. It is freemason ‘darkwins’ vs truth

    Reply
    • 6. Kyle  |  February 27, 2012 at 3:02 pm

      I totally agree with your assessment of evolution as a “pseudo seance.” It is definitely not a seance (which would make no sense, because communication with spirits is not involved), and much more of a scientific theory, really. Glad we got that sorted out.

      I also find debates with Richard Dawkins to be quite amusing, though for very different reasons than some people.

      Thanks for reading my article!

      Reply
      • 7. Jen  |  February 27, 2012 at 4:06 pm

        I considered writing a similar reply, but didn’t. You made my day, thanks. :)

  • 8. Hungry Hungry Hippo  |  November 26, 2012 at 9:45 pm

    In biology I learned that the wrinkling occurs as water leaves the skin by osmosis. The skin doesn’t absorb more, the cells begin to collapse as water is lost.

    Reply
    • 9. Kyle  |  November 27, 2012 at 12:10 pm

      Osmosis works in both directions depending on the osmotic pressure of the solution (i.e., the water outside the skin). In general, if the concentration of solutes outside the skin is relatively high compared to the concentration inside, then water would be lost from the skin. Conversely, if the concentration of the solutes outside was relatively low, then water would be absorbed by the skin. The articles I have referenced all suggest that water is absorbed by the skin rather than being lost, but in either case the potential evolutionary advantage of wrinkling would remain the same.

      Reply
  • […] residue is interfering with the scan or your finger has absorbed too much water, giving you “raisin fingers” (and a slightly different-looking print than the recorded data on your […]

    Reply
  • […] residue is interfering with the scan or your finger has absorbed too much water, giving you ‘raisin fingers‘ (and a slightly different-looking print than the recorded data on your […]

    Reply

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