What is the point of pruney fingers?

By Ed Yong | June 28, 2011 10:03 am

The common wisdom is that your fingers wrinkle when they’re wet because they absorb water. But Mark Changizi thinks there’s more to it than that. According to him, pruney fingers are an adaptation to help humans, and probably other primates, get a better grip during wet conditions. They act like the rain treads on tyres. Mark lays out his hypothesis in a wonderful paper that I wrote up as a news story for Nature News.

Here’s the start; click through for the whole thing.

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.”

Reference: Changizi, Weber, Kotecha, & Palazzo. Brain Behav. Evol. http://dx.doi.org/10.1159/000328223 (2011).

Comments (62)

  1. mark

    Why does the doi not work?

  2. And raccoons will put their hands in water in order to increase their sense of touch, which is reportedly their most valuable sense. Human’s sense of sight has always been promoted as the most important, but now I wonder if our sense of touch is similar in importance – after all, we do have opposable thumbs.

  3. I’ve found that my skin is more likely to tear if it’s water-logged like that.

  4. shadegem

    @amoebaMike Hey, every adaptation can have a down-side.

  5. Brian Too

    Pruney fingers can point as well as the pruneless variety. Therefore there is no point!

    You can catch me debating down at the Roxy, Thursdays after darts. There I will also prove:

    - fusion is better as music (or food) than as physics;
    - dark matter: Why so racist?;
    - pirates were the first to discover the Planck!

    Thank you! Good night!! :-)

  6. Jaf

    how would this adaptation be selected for?

  7. CraisinFingers

    @Jaf Those whose fingers don’t ‘prune up’ slip, lose their grip, and drown :-P

  8. Pulak Prasad

    This is the kind of extreme adaptationist regime that the late Stephen Jay Gould warned us against, and George C Williams wrote about in his great book ‘Adaptation and Natural Selection’. Why does everything in every organism have to be adaptation? Why do we have to look for adaptation as an explanation for everything going on in the natural world? Why can’t fingers become ‘pruny’ for no reason at all, or for some other biological or mechanical phenomenon that we haven’t yet explored?

    In order to claim that pruny fingers is an adaptation, someone will need to prove that the selection coefficient (s) is high enough for it to be selected (instead of ‘non-pruny’ fingers) over thousands of generations. Is the author implying that the ability to hold on to surfaces in wet conditions was so highly advantageous that it was chosen over alternatives, and that the parents with ‘non-pruny’ fingers produced less offspring than the ones with ‘pruny fingers’? What proof do we have? In fact, the ‘savanna hypothesis’ would militate against this view – the hypothesis that claims why we became bi-pedal. The hypothesis briefly states that trees disappeared about 2.5-3 million years ago, and our ancestors were then forced to travel long distances for food over land thereby leading to an advantage for anyone who could be bi-pedal. If this is true (and I don’t know if it is, but it is the most well accepted hypothesis), then where were the wet surfaces to hold on to?

    In any event, my only point is that for us to accept adaptation as an explanation of any body part or phenomenon, the standards need to very very high. This explanation just does not pass the muster. What are we going to say next? We have a nose so that we can wear eye glasses?

  9. Pulak,

    You mean, someone will have to find biological evidence as to why a feature might be an adaptation, propose a hypothesis to explain this possible adaptation and and set out several ways in which that hypothesis might be tested?

    Kind of like the paper in question?

  10. To get a better grip during wet condition? What were they gripping that gave them a selective advantage? Wouldn’t it be better to suggest (common sense) that the greater surface area increases the speed of the skin drying?

  11. fintin

    Pulak,
    Have you ever, once in your entire life wondered why our fingers get pruny? That’s what these scientists are trying to figure out. Pure and simple. Not everything biological has a purpose. Just look at eyebrows. They once had a purpose, but not anymore. But we still kept them.

  12. Ian Govey

    fintin – yes, eyebrows perform a purpose, at least for me. I’m a runner. When I’m running in the summer and sweating profusely, they do a great job intercepting most of the sweat that would run into my eyes otherwise. And that stings, let me tell you.

  13. Ian

    In regards to means for this selective feature why has no one brought up fishing? Communities that were able to fish in greater or sufficient amounts stand a greater chance of survival. Proteins tie in nicely as they are considered to be a large part of our ability to help develop our brain mass (although that might be in question).

    @Pulak I also have to disagree what would being bipedal really have to do with not coming in contact with water? I understand that some traits are simply the result of another gene expression changing but that certainly can’t mean that everything is the result of one change. Most features of organisms are shaped by nature to degrees unobtainable in artificial selection due to both time constraints and the need to compete. Though I would never try to definitively say that this trait is one or the other having a hypothesis and testing these ideas is the only way to prove/disprove them.

  14. The fact that the pruning happens under nervous control suggests that it’s not just a side effect of being wet. Why do they develop then? Someone should probably suggest a hypothesis. You know, sort of like what Changizi did…

    Also, if macaque fingers prune too, it’s unlikely that this is a unique human thing – it probably originated deeper in the primate family tree. So talking about bipedality and the savanna hypothesis is irrelevant – it’s the wrong evolutionary context.

  15. Cory

    To that effect, Ed, I might also look at the amount of pruning under different moisture conditions. If a primate in a tropical area increases the pruning, and thus traction, of his fingers as a reaction to humidity conditions, then this kind of sensory feedback loop is common.

  16. Georg

    I think Pulak is right, generally.
    The interpretation in analogy to grooves in tires is
    absolute nonsense. The velocities when tires have to
    displace water to “reach” street surface are several
    orders greater by magnitude.
    The main problem with biologists aside pupish or perish
    is their lack of solid physics or chemistry knowledge.
    And “brilliant” is a word worn out.

  17. There’s two key things here that suggest to me that pruney fingertips are adaptive. The first – and key – is that it doesn’t happen if nerves are severed. This means its not a passive process of water absorption. In other words, the water doesn’t do it to you, you do it to yourself. The second, which no-one seems to focus on, is that the REST of you DOESN’T get pruney, even when in water for the same amount of time.

    And Georg – try grabbing a wet fish sometime. They move mighty quickly and you’ll be grateful for every skerrick of additional grip you can get. It seems a perfectly reasonable hypothesis to me. If you think it’s nonsense, I suggest an experiment to prove it so rather than just sledging on “biologists” (as if they are a single type of person, now THAT is absolute nonsense)

    Of course, it’s possible that its a fixed trait leftover from an ancestor, but I doubt it.

  18. Tallgrass05

    There is a big difference between tires and tire tread shedding water and fingertips. This sounds like an example Stephen Jay Gould used to cite about why flamingos are pink….someone eloquently decided that was for camouflage when flying into the setting sun.

  19. Why does it take so long for it to happen then? If you want wet traction, I would think the affect would begin a lot sooner after exposure to wetness.

    Very interesting hypothesis. More research must be done! =)

  20. @Al – Spot on. The lack of pruning on other parts of the body is weird. Some people have tried to suggest that it’s the curved nature of fingers and toes that means they prune and other body parts do, but others have pointed out that by that logic, the penis ought to prune as well. And doesn’t.

    @Multiple people who dispute the tyre tread analogy – Er, it’s an analogy? Rebutting the analogy doesn’t rebut the thing the analogy is about.

    @Kevin Sweeney – Good question. This is covered in the paper and was cut out of the Nature piece. It happens in 5 mins. Changizi argues that this is more than quick enough to cope with wet conditions, but not so quick that finger prune up when wrinkles aren’t needed, e.g. if they get splashed with juice while holding fruit.

  21. bob henderson

    there seems to be something in the water that draws moisture from our fingers. or our bodies eg after a hot shower we seem to be dehydrated.maybe the government should put up a million to find out the real results.its starting to scare me that Ive even given in to ostentatious rhetoric such as this

  22. Eleanor

    Cool. Something completely mundane I’d never had the wit to question. I’m now wondering how the fingers detect wetness; having just one or two fingers in water doesn’t seem to be enough to trigger it. I wonder if wrapping a few fingers in cling-film before a bath will leave those fingers unwrinkled or not? And would things that seem wet, like vasline, trigger it too?

    Time to clear up my desk – it’s surprisingly hard to work with one hand in a cup of water.

  23. Dirty Harry

    I always thought it was a result of Osmosis…… :) the Capillary action taking care of the rest through the miniscule pores in the skin…Considering that skin Texture on Palms/Feet is slightly [allright a Lot more] different than elsewhere…this causes the water to push in and then change the Salt content in the Open Skin texture…My Two Bits worth of it…

  24. Erica

    Humans share this gripping trait with other primates, because all primates evolved from a common ancestor. Being able to grip tree branches better in wet conditions would have been extremely important for survival. I’ve always wondered about the pruney fingers, and this makes total sense. Very cool!

  25. bob henderson

    my first wife pregnant ,and with swollen hands wanted me to squeeze her hand harder because it felt so good and a bit later water and a small amount of blood dripped from our hands .with no open wounds the water released through the pores of her skin and she felt relief.our skin obviously releases water in heat or pressure.i am not sure of course about the pruney finger in cold water.maybe water softened hands reliese water much easier.

  26. Ryan

    Man, what a huge hypothesis to posit. How on Earth do you go about trying to prove this hypothesis as a theory? For millions of years, the ancestors of humans have supposedly not been tree-dwellers. It would then seem to me more advantageous a trait for human ancestors to have the fine articulation and coordination garnered by not having your fingers shrivel up, but maybe gripping wet weapons has put more evolutionary pressure on human populations than playing the piano with wet hands, in which case I suppose the shriveling would be a good thing.

  27. the fact that your whole body DOESN’T prune supports this hypothesis. makes tons of sense.

  28. Brian Too

    OK, seriously, I always thought this was a straightforward materials issue. The water is interacting with the skin, either taking something out or putting something in. Osmosis seems like a plausible mechanism.

    Now the question of why other skin does not react the same is a good one. Penile skin is awfully loose (most of the time, ahem!) and may not be the best comparison. However skin on the sides and backs of the fingers and arms would seem to be a good data point.

    Now what if the skin isn’t the same however? We know that the fingertips are among the most sensitive areas of the body. That’s due to the density of nerve endings there. Perhaps there are other differences as well?

    So here is a puzzler. Your mouth is a persistently wet environment, covered in skin. Probably specialized skin, especially on the tongue. So how come it does not pucker? Of course if different skin composition can cause pruney fingers, presumably water resistant skin in the mouth can prevent pruning too. Also, saliva does not have the same composition as ordinary tap water.

  29. Maybe the pruning makes water gather and drop off faster. We ‘hunker down’ in the rain, maybe this is a method of keeping the fingers and toes warmer by ditching the water faster.

  30. Brian too

    A long time ago (several decades to be sure) it was demonstrated that our skin is different depending on where it is located. As an example, soaking in a hot tub, nearly fully immersed, my fingers and toes ‘prune’ yet my legs and arms and ankles and wrists don’t, although they all were equally submersed.
    Why?
    Because different parts of our bodies have different skin characteristics. Our fingers and toes ‘prune up’ our legs, arms and torso’s do not. Obviously there was some reason for this pruning to happen. It happens on the parts of us that try(ied) to get a grip. So maybe it helped not slip. Or maybe it helped keep the extremities warmer by losing the water.

  31. Mutant Dragon

    On balance I agree with Pulak. Without more evidence to back up Changzi’s hypothesis, it’s not logical to _assume_ this is adaptive and would have any significant effect on fitness. And sure, Changzi’s just “proposing a hypothesis”, but I’d like to see more to back it up than this. No offense to anyone involved, but count me unconvinced and unimpressed.

  32. Wow, if this is true then my whole world has been rocked. So if the nerves are severed there can’t be any pruning?

  33. Colugo

    - Demonstrated: Pruning is not a passive reaction to water, but an active neurologically mediated response.

    - Possibility: Therefore, it might be an adaptation. Might. It could also be a quirk of the nervous system.

    - Possibility: If it is indeed an adaptation, that adaptation might be gripping ability – but it could be something else. We need more data.

    And to the Stephen Jay Gould fans: Gould was wrong about the kiwi's egg, the stupidity of birds, the size of of robust australopithecines, and a host of other things based on his constraintist Just So Stories. (You bet that the Just So Stories concept works both ways. The funny thing is that Kipling's original Just So Stories were not generally adaptationist but rather more 'contingent'.)

    To be sure, a lot of evolutionists – adaptationists and non-adaptationists alike (e.g. Gould's now-outmoded neoteny fable for human hairlessness and a ton of other features) like to wave their hands from their armchair, making things up rather than even suggesting how they could be subjected to rigorous hypothesis testing. Changizi's paper looks like more of the latter than the former. So I'm open to the idea. But for now, color me skeptical.

  34. crecsky

    It’s also theorized that apes lived near the sea and hunted fish, and they also use tools. They have the breath reflex when they dive into water that suggests that they’ve lived in/around water.

    Grip in wet conditions like that could be advantageous. Just hasn’t adapted out because nothing about wrinkling fingers is a hindrance.

  35. JOSH

    the human body has evolved to live on land therefore it starts to fall apart in the water. The fingers and toes are most susceptible to heat, cold, water, etc, thus they start to fall apart first. proof of this is trench foot. Long exposure to water and the flesh starts to fall off. Pruney fingers is the first sign that the flesh is going to fall off. It takes days or weeks for that to happen so most peoples skin does not fall off. So I guess trench foot helps my foot get a better grip on my boots? Someone is not using their brain correctly. I used to read this magazine as a child and it had some great ideas but this is not one of them.

  36. Renee

    Ugh, it’s like no one who reads this magazine thinks scientifically.

    Take JOSH, for instance ” Pruney fingers is the first sign that the flesh is going to fall off. It takes days or weeks for that to happen so most peoples skin does not fall off. So I guess trench foot helps my foot get a better grip on my boots? ”

    He argues that it cannot be adaptive because it’s the “first sign” before the flesh falls off, because everyone who has trench foot has wrinkled feet before getting it. But whether or not wrinkled toes is adaptive OR NOT, it would precede trench foot, because trench foot involves feet becoming wet. The two hypotheses are not at all mutually exclusive.

    Furthermore, if there is nerve damage, the feet will not wrinkle, and you’ll still get trenchfoot. The two are simply only related because having wet feet both make your feet wrinkle, and if prolonged, suffer from necrosis.

    Simply put, wrinkled feet would precede trench foot even if it was adaptive, so trenchfoot has absolutely no bearing on this issue at all.

  37. Sparkie Arbuckle

    I agree one hundred percent with Pulak’s first paragraph:

    “This is the kind of extreme adaptationist regime that the late Stephen Jay Gould warned us against, and George C Williams wrote about in his great book ‘Adaptation and Natural Selection’. Why does everything in every organism have to be adaptation? Why do we have to look for adaptation as an explanation for everything going on in the natural world? Why can’t fingers become ‘pruny’ for no reason at all, or for some other biological or mechanical phenomenon that we haven’t yet explored?”

    Evolutionary biology’s ‘deep history’ is a bunch of just so stories which are hopelessly vague.

  38. Abbo

    I think it’s viable to think that it was adaptation if we talk about other primates (they are longer here than us [and by longer I mean reeeeally longer]. Why do everyone goes only for a humans?). On a wet trees there are lot of cases when primate gets advantage if it can grip in all weather not only when it’s sunny. But that needs additional evaluation to check if other primates have pruney on their extremities and if it is really strengthens the grip. So if it can be confirmed it heavily reinforce this hypothesis weight.

  39. Abbo

    By the way if someone want to know how exactly they are formed there is research on that already: http://www.wired.co.uk/news/archive/2011-03/09/physics-of-pruney-fingers

    But from that research it’s easier to suggest that pruning is caused by exposing skin on the extremities of primates to more contact with water so it needed to save rigidity while in contact with water than other skin.

  40. JOSH

    So let me get this straight you are calling flesh that is falling apart because of water damage adaptive? that’s good logic.

  41. Abbo

    He says that it is not(!) falling apart at that stage. You need a lot of hours in water to actually damage flesh. For example if you have a rubber padding that doesn’t mean that you won’t break it by over-stretching, but that doesn’t mean that rubber not supposed to stretch either.

  42. Dru

    What really bugs me about this article is the use of the terms “hypothesis” and “theory.”

    In the first paragraph we hear that the idea behind pruny fingers is a theory; in the second, a hypothesis. I am aware that the word “theory” has many definitions. This situation would apply to one of those definitions in the literal sense. But in science, shouldn’t the word be reserved for overarching ideas about how the universe operates? Darwin’s Theory of Evolution, Einstein’s Theory of Relativity, or for any other idea which has substantial evidentiary backing? Not have its use given to an idea not rigorously vetted by the scientific community? At least in the science journalism community, where the general public is exposed to scientific ideas.

    I know that in every-day speech the word “theory” is tossed around like a rag doll. Even in the lab. But the public gets confused when they hear “theory” and “hypothesis” used interchangeably; especially when, in science classes, they are taught that a hypothesis is an educated guess. They begin to think the same of the word “theory.” This fuels the fire of the creationists. Let’s agree to make the distinction, be conscious of the differences, and use them in the appropriate scientific sense. Shall we?

  43. JOSH

    “the fact that your whole body DOESN’T prune supports this hypothesis. makes tons of sense.”
    the whole body doesn’t prune because the whole body wasn’t specifically build for tactile function like the hands.
    Amoeba Mike is the only one that seems to make any sense here.
    “I’ve found that my skin is more likely to tear if it’s water-logged like that.”

  44. Hazy

    When my hands get pruney it’s a rather uncomfortable phenomena. I would prefer if they stayed nice and taut like the rest of my skin. Some adaptive evolutionary trait.

  45. Sherelene

    @ Brian: “plank” is spelt wrong…

  46. SarahC

    Josh – You’re trolling – surely?

    Are you even aware of the Scientific Method?

  47. JOSH

    @SarahC OK I will explain to you something about the scientific method. If you propose a theory or hypothesis, the burden of proof falls on you, not the other way around. People are acting as if he has already proven his idea and stating it as a matter of fact. It must be so because there is an article about it. A real scientist would question every part of it. If you want to pretend you are a real scientist then go ahead but no matter what degree you have if you don’t know how to use logic properly then you are not a real scientist. “The fingers look like tire treads, it must be an evolutionary development.” There is a huge gap in logic there.

  48. Ian

    @Sherelene:

    I’d hazard a guess that @Brian was referring to http://en.wikipedia.org/wiki/Planck_constant by his tongue-in-cheek tone (and capitalization of Planck), going for the fun play on words homophones can offer from time to time.

  49. MPT

    There’s something odd about using attributes of an artificial, man-made object in order to explain the evolution of a biological trait. Biomimicry is cool, topical, and useful, but the other way around? I didn’t know that applying ideas hopelessly entangled in human perception to long term evolutionary development would be useful in the pursuit of knowledge.

    In the end, I guess it’s all we got.

  50. Jennifer

    @nutritionist brisbane

    That would make sense if pruning actually increased the surface area of the skin. Instead, it just changes the shape of the skin, and it then goes right back. If I pull on the skin of my arm, it stretches out, but the surface area doesn’t change. You don’t make new skin when you prune up or tug on some skin. You just change the shape of it.

  51. davem

    When I get pruney fingers, I also get less sensitive fingers. Is this an adaptation to make you grip harder in the wet, to get the same feel?

  52. Brian Too

    @49. Ian,

    Thank you, I own you one. Explaining the joke, it just sucks all the fun out of it, you know?

  53. Bahb

    @Emma #2 – If you really want to see something funny, put out some sugar cubes next to a bowl of water. The racoon will grab a cube and then try to “wash” it in the water only to have it slowly disappear. The look on the racoons face is priceless!

  54. JOSH

    @Bahb OMG, that sounds awesome, I tried googling it but to no avail. is there a link to a video somewhere? you would be a youtube star if you made that video.

  55. Kevin N

    I don’t think I buy it either. If it is at all advantageous, it seems to be so little advantageous as to provide no selective force at all, in comparison with all the other important things that need to be selected for. I have trouble that some pruney proto-primates, who probably spent little time in water, were able to produce more offspring that their brethren who didn’t get pruney fingers.

  56. Joe

    Funny thing is Changizi and Palazzo both have Physics degrees. So why are we talking about Biologists here?

  57. Colin McGinn

    Century old papers my ass! I posted this theory on Wikipedia years ago. Seriously. Look at it’s edit history. User name puzzlerf. Discover, I’m a 14 year subscriber, please contact me as you now have my email.

  58. Colin McGinn

    Disregard last comment, I’m in contact with Mr. Changizi, and all is well.

  59. CMH

    NYT is JUST reporting on this: http://well.blogs.nytimes.com/2011/09/12/really-the-claim-fingers-wrinkle-because-of-water-absorption/

    I couldn’t easily figure out if it is from the same research paper or not…

  60. lemmy caution

    The discovery (or rediscovery) that fingers with nerve damage do not prune up in water was made by a mother of a kid who underwent hand surgery:

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1586908/pdf/brmedj01576-0025.pdf

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