National Geographic

Debate: did wrinkled fingers evolve for better grip?

Stick your fingers in water and your fingertips will soon start to wrinkle. There’s a common belief that this happens because the tips absorb water, but that can’t be right. Since the 1930s, we’ve known that the wrinkling process is under nervous control—if you sever the nerves in your finger (don’t try this at home, kids), the wrinkles won’t form.

But neurobiologist Mark Changizi has an intriguing hypothesis about the origin of pruney fingers—they’re an adaptation that allows us to grip wet surfaces. Like the rain treads on tyres, when pressed down, pruney fingers create channels that let water drain away, allowing them to make better contact with damp surfaces.

I wrote about this hypothesis back in 2011, when Changizi first proposed it. Now, an independent team  from Newcastle University has found some support for it. In a simple experiment, they showed that people can pick up wet marbles more quickly if their fingers are wrinkled after a 30-minute soak in warm water than if they are dry. And this advantage only applied to the wet marbles—both wet and dry fingers were equally good at picking up dry marbles.

The new study has picked up a lot of coverage, but it also raises some interesting questions about how to test evolutionary explanations. So far, all of the evidence for Changizi’s idea comes from looking at modern human fingers. How much can such structural studies tell us about function? How much, if anything, can the what tell us about the why? If modern human fingers grip wet marbles well, and form patterns that resemble rain treads, does that tell us anything about the origins of such patterns or are all such explanations merely just-so-stories?

T. Ryan Gregory, an evolutionary biologist from the University of Guelph, has been a vocal critic of studies like these, including the dubious paper on fists and fighting that I covered recently (here’s his post). I engineered a debate between Gregory and Changizi over the pruney fingers hypothesis on Twitter, and I think it’s a fascinating case study in how to think about evolutionary hypotheses.

Here’s a Storify of their exchange, with some commentary from me for people who aren’t familiar with the issues being raised.

 


There are 17 Comments. Add Yours.

  1. Reginald Selkirk
    January 11, 2013

    Humans would have had to spend a lot of time in the water for this to be an adaptation. Time to rehash teh ‘aquatic ape’ debacle?
    .
    Why don’t the extremities of humans in high latitudes wrinkle to resemble snow tires?

  2. Dale
    January 11, 2013

    Is 30 minutes of trying to hold on to a wet tree branch in the rain enough to trigger the response?

    (I’d perform this experiment myself, except my precipitation is currently snow.)

  3. Brian Schmidt
    January 11, 2013

    I’ve often been out in the rain long enough to get prune fingers, and I’ve likely had more chances to avoid that situation than our ancestors did.

  4. amphiox
    January 11, 2013

    Before you can even begin to suggest evolutionary/adaptive hypotheses for traits, you have to first provide at least some evidence that the trait is even heritable in the first place.

  5. TR Gregory
    January 11, 2013

    This was an interesting discussion, and I’m glad you captured it (and could make sense of the overlapping threads in the exchange!). I decided to write up some more details points here:

    http://www.genomicron.evolverzone.com/2013/01/the-great-wrinkled-finger-debate/

  6. Mark Houston
    January 12, 2013

    I would say that a water environment, past and present, in a natural world, would be a very dangerous environment and wonder how long our ancestors would have had to spend, submerged, in order for this type of Darwinian Adaptive Selection to have occurred.

  7. Rafael
    January 13, 2013

    Even scientist don’t understand that evolution is a messy process with no objetive. Every time I hear this kind of argument is think about what kind of pressure would select these genes. I could just be a end project of other genes selection or a bottleneck effect.

    Unfortunately the journalist cheery pick just these kind of article. And NEVER heard the consensus of science on that moment. Every science news just come with a comments of a specialist on that subject.

  8. W. Benson
    January 13, 2013

    Both Gregory and Changizi seem to be unaware that it rains, even over ‘the savanna’. Sad commentary on lab scientists.

  9. Julian D.
    January 14, 2013

    Does pruning occur similarly among other primate species? I could easily see this sort of adaptation being useful in dense, humid jungles.

  10. TR Gregory
    January 16, 2013

    W. Benson — The point is, pruning takes several minutes even when soaked in hot water. No one has done any work on how it might be relevant in the rain.

    Incidentally, my students and I do plenty of field work, including in Africa.

  11. J. Snow
    January 16, 2013

    And on the opposite side…in the winter my fingers get so “dry” that I can’t pick up things with just my fingertips if they are glass slick and too heavy. It requires a more substantial grip. I guess that means that my ancestors never got to a colder climate.

  12. T Swinfield
    January 18, 2013

    I’m in favour of a mechanistic explanation. I had always assumed that wet skin simply absorbed more water than the flesh below and so expanded to a greater extent, causing the wrinkling.

    A counter argument to an adaptive explanation is that if finger skin preferentially becomes wet in order to wrinkle and aid grip, this is likely to trade off against an increased potential for damage in soggy skin. This is going to be particularly pronounced in tasks requiring large amounts of friction. Any climber knows this.

  13. Shane Shariff
    February 25, 2013

    Fingers prune for heat dispersion. This also occurs under heavy sweating. Greater surface area PLUS, movement of the hands, increase evaporation. This enhances the ability of the fingers to cool the blood.

  14. Ian Gallagher
    July 13, 2013

    Why would wrinkled fingers have greater surface area than non-wrinkled fingers? You still have the same amount of skin when your fingers wrinkle. Unless you’re suggesting that the skin stretches.

  15. Kieran Ward
    November 17, 2013

    Loving that people can do hours of research and then I can post a comment because I obviously know better after five mins…obviously not but my guess is biological as on the toes and fingers the sweat pores are raised to their highest and most isolated point on the body. Excess water hinders the process of sweating which results in puckering as we can no longer regulate our temperature whether hot or cold

  16. Cee Dvistani
    January 12, 2014

    A recent PLOS ONE study “Water-Induced Finger Wrinkles Do Not Affect Touch Acuity or Dexterity in Handling Wet Objects” – used a manual dexterity task (the time it took participants to transfer 52 objects from a source container into a target container with a 5×5 cm opening was measured), a tactile acuity task (grating orientation determination) and measured vibration detection threshold. See http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0084949 – the title tells the results

  17. Rosemary White
    August 9, 2014

    So, do raccoons have wrinkly fingers? Or those japanese macaques who loll about in hot springs?

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