Why does the skin on your fingers and toes wrinkle after immersion in water? A panselectionist answer.

June 22, 2022 • 9:45 am

All of us have noticed that after a period of immersion in water, the skin on both our fingers and toes wrinkles up, but not the skin anywhere else on our body. Here are two photos of the crenulated digits:

From The Conversation


This raises two questions:

a.) What is the mechanism for the wrinkling?

b.) Is there any usefulness or “adaptive significance” of the wrinkling? That is, did natural selection favor it because the wrinkles are useful. 

The two articles below, the first a new popular summary from the BBC and the second a year-old scientific paper discussing the “adaptive significance” of the wrinkling, suggest answers to both questions.

It turns out that we know the mechanism of wrinkling pretty well, but, despite the assurance of both articles, we still have no idea whether it’s an “adaptive” response to water or merely some epiphenomenon that makes no difference to our well being or reproductive output.  That both articles immediately look for an adaptive “reason” why natural selection promoted finger and toe wrinkling is an example of what Steve Gould called “naive pan-selectionism”: assuming that every feature has natural selection behind the evolution of that feature, and favoring the production of that feature—in this case, wrinkling.

Panselectionists often accept pretty scanty evidence as being supportive of their theory, and I think you can see that here.

Click on both screenshots to read the article; the pdf of the scientific article (in PLOS One; reference at bottom) can be downloaded here.



I’ll use facts from both articles, but quotes will be attributed to one or the other.

First, how long does it take to wrinkle up? It depends on the temperature, with 3.5 minutes in warm water to begin wrinkling (40º C or 104° F) and 10 minutes in tepid water (20º C or 68° F). But even in cool water we will wrinkle.

How does it happen? Scientists first thought that it was simple osmosis: the skin cells absorbed ambient water and that made the cells swell up, causing wrinkles. But then they noticed that if the median nerve in the arm is severed, there is no wrinkling. That rules out the osmosis theory as a complete explanation. Osmosis may contribute a bit to the wrinkling, but nerves and blood vessels are also involved. Author Davis of the PLOS ONE paper says this:

Explanations for the wrinkling of the skin include a passive response of the skin to immersion, or an active process that creates the wrinkles for a functional purpose. There is overwhelming evidence that finger-wrinkling is an active process. The small blood vessels of the fingertip constrict, which creates valleys in the skin surface, triggered by water entering sweat pores . Note that a passive explanation would usually assume that water absorbs into the skin, pushing up ridges. This vasoconstriction appears to occur most readily at a temperature of around 40° Celsius, or the temperature of a warm bath [2]. People with autonomic neurological conditions including Parkinson’s, cystic fibrosis, congestive heart failure or diabetic neuropathy may show abnormal or asymmetric wrinkling in the affected parts of the body.

Note that in the first sentence he conflates an “active process” with “an adaptation that has a functional purpose.” This isn’t necessarily true. We get wrinkles, gray hair, and liver spots with age, which are “active processes,” but that doesn’t mean those features are the direct products of natural selection. (What is the adaptive function of liver spots?). The BBC adds a bit more about the mechanism:

Wilder-Smith and his colleagues proposed that when our hands are immersed in water, the sweat ducts in our fingers open up to allow water in, which leads to an imbalance in the salts in our skin. This change in the salt balance triggers the firing of nerve fibres in the fingers, leading to the blood vessels around the sweat ducts to constrict. This in turn causes a loss of volume in the fleshy area of the fingertip, which pulls the overlying skin downwards so that it distorts into wrinkles. The pattern of the wrinkles depends on the way the outermost layer of skin – the epidermis – is anchored to the layers beneath it.

The involvement of nerves explains why some conditions that affect nerves (see first indented para above) affect skin wrinkling.

Let’s assume, then, that we have a pretty good idea of how this happens in fingers, though nobody says much about toes or the rest of the body. (Toes are also sorely neglected in the “adaptive” explanation.

Both sets of authors then set about explaining why natural selection would favor such wrinkling (again, they discuss only fingers, not toes). The experiment describe in the second link above, which gives results in line with previous studies, suggests that the wrinkled skin allows you to grab wet objects with more force than if your skin is unwrinkled and wet. And if your fingers are wrinkled, you’re likely to be in an environment where there are wet objects.  The purported mechanism for this is the same one for treads and valleys in tires: the “channels” in our finger wrinkles suposedly help squeeze out the water when we’re gripping a wet object, allowing better contact with the object. (But what about the toes?)

Davis, then, did a study estimating the strength it took to grip a small and initially DRY plastic disk under three conditions:

a. dry unwrinkled fingers

b. wet wrinkled fingers (note: they apparently didn’t use dry wrinkled fingers, but it’s not clear from the paper. In fact, if they used dry unwrinkled fingers, it would make the adaptive explanation less credible.)

c. wet unwrinkled fingers

Not did they use wet objects, which is crucial for their adaptive hypothesis, though of course gripping a plastic disk with wet wrinkled fingers will make the object wet. Note also that the object is small and light (the BBC says it weighed as much as a couple of coins).

I won’t go into the detail to measure force, but they had an apparatus that measured both grip strength and the ability of the subject to lift up the object and hold it sufficiently tightly so it could be manipulated to follow a computer track. Here’s a photo from the paper:

(From paper): Fig 1. Picture of the equipment in use. The participant is gripping a load cell between finger and thumb. The participant’s task is to pull up on the second load cell to match a force trace that appears on the laptop monitor. The current load force is shown as a red circle, and the history of the participant’s force is shown as a trail of green dots.

The results: people with wet wrinkled fingers and those with dry fingers had similar grip forces, but those with wet, unwrinkled fingers needed significantly more force to grip the disk. Here’s one graph (just look at the top three lines) showing no significant difference between wrinkled-finger force (red) and dry-fingered force (purple), but significantly more force needed using wet, unwrinkled fingers. (The paper give statistics). This shows no real benefit of wet, wrinkled fingers over dry fingers when gripping the disk, but if your fingers are wet and unwrinkled, it’s harder to grip (the plastic get slippery).

(From paper): Fig 2. Comparison of performance across conditions. Mean grip force (thinner traces) and load force (thicker traces) when participants tracked a load weight target (black line). Participants with wrinkled fingers produced a grip force that did not differ from that used by people with dry fingers in the static hold phase, however people with wet but non-wrinkly fingers used a significantly higher amount of grip. The shaded area indicates the pointwise ±1 standard error of each mean trace. Lines below the trace indicate the attack phase (A) of the trial, the static phase (S) and the decay phase (D).

Here’s another graph that shows pretty much the same thing, but showing the grip force needed to sustain the load of the plastic disk under the same three conditions but with varying “load force” (weight, which could be manipulated). Green is wet, unwrinkled fingers, red is wet, wrinkled fingers, and blue is dry unwrinkled (normal) fingers:

(From paper): Fig 4. Relationship between grip and load force in Dry, Wet and Wrinkly conditions. This illustrates the grand mean of the grip and load forces for the whole duration of the trail, minus the first 1000 ms. The target force is shown as a dashed line. The three grip force traces lie above this line, indicating the safety margin. The ‘easiest’ condition, Dry (blue trace) follows the target force most closely. The ‘hardest’, Wet (green trace), shows a higher safety margin, and looser coordination. Participants with Wrinkly fingers (red trace) lie between the two.

Wet unwrinkled fingers require more force to hold the disk than do dry ones. Wet, wrinkled fingers aren’t superior to either, but intermediate between them. (No statistics are given, but another graph implies that none of the differences between the lines in the plot right above are significant.)

The overall conclusion is not strong. Clearly, wet unwrinkled fingers make it harder to grip a smooth plastic object than either dry fingers or wet wrinkled fingers (DUH), but wet wrinkled fingers don’t make it easier to grasp an object than dry unwrinkled fingers. In other words, any advantage of wrinkling is only when it’s compared to wet unwrinkled fingers. Otherwise, dry fingers grasping a dry object are marginally (and nonsignificantly) better than wet, wrinkled fingers.

What can you conclude from this? I’d say, “not much”, but the author of both the BBC article and of the paper seem to think that wrinkling is an adaptation that evolved in our ancestors to enable them to grip objects under wet conditions:


This suggests that humans may have evolved fingertip and toe wrinkling at some point in our past to help us grip wet objects and surfaces.

“Since it seems to give better grip under water, I would assume that it has to do with either locomotion in very wet conditions or potentially with manipulating objects under water,” says Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study. It could have given our ancestors a key advantage when it came to walking over wet rocks or gripping branches, for example. Alternatively, it could have helped us when catching or foraging for food such as shellfish.

From the paper:

Grip and load force coordination is an important aspect of object handling. The ability to generate the correct amount of grip force for a given load means that the minimum necessary amount of energy is used by the muscles that control the fingers and hands, and means that objects are less likely to be dropped or to be crushed. Efficient grip force coordination is seen in many extant primates, and is likely to have evolved early in the primate lineage [13]. The grip force required to stabilise a wet object is greater than the force required for a dry object, since the coefficient of friction of the digit-object interface is reduced [8]. It would therefore benefit an animal to gain an advantage in handling wet objects, as this would increase success in hunting and foraging in watery environments. The skin of the fingertip is already adapted for regulation of moisture at the contact surface [14]. Fingertip wrinkles would seem to afford an enhanced advantage in object handling, and may plausibly aid travel and clambering in wet areas, especially if combined with wrinkled toes.

Ergo, it helped us “hunt and forage in watery environments.” But this raises a number of questions:

a.) If you’re hunting or foraging in a watery environment, but your hands have been immersed for fewer than 20 minutes so they’re unwrinkled, you’re better off gripping a dry object with dry hands instead of wet ones. You have an advantage with wrinkled fingers only if they’ve been underwater long enough to get wrinkled, and that advantage is only over unwrinkled wet fingers so long as you’re gripping an object that is itself wet, like a plastic disk that your fingers have wetted.  If you’re trying to grab a dry object when your hands are wet and wrinkled, you’re worse off than when using dry hands.

b.) They did not test the three conditions when gripping large dry objects like a tree branch or an animal, which may not behave like plastic disks! This is essential if you think that either grabbing dry objects was important for our ancestors even when our fingers were wrinkled from having been immersed in water.

b.) We did not evolve in a watery environment; the “aquatic ape” hypothesis has long been dispelled. As for our relatives, the BBC article says “only one other primate has so far been found to have water-induced wrinkling of the fingers—Japanese macaques.” (Naturally, they show a photo of a macaque sitting in water.) I’m not sure if other primates have eve been tested (no such tests are referenced), but if chimps, bonobos, and orangs show finger wrinkling, that would imply that it did NOT evolve to enhance grip strength in watery environments. These primates don’t live in those environments!

d) What about doing the study with dry wrinkled fingers? (You quickly dry them before grasping the object.) The adaptive hypothesis would predict that there would be no grasping advantage of dry wrinkled fingers over dry unwrinkled fingers. They didn’t do that experiment (as far as I can see).

e.) What about the TOES? They get wrinkled too. The paper posit that wrinkled toes would aid “travel and clambering in wet areas”, but that is pure speculation—not even a hypothesis. It could be fairly easily tested, but wasn’t.

f.) If wrinkly skin is pretty much as good as dry skin for gripping almost anything, why don’t we have permanently wrinkled skin? Author Davis has an answer:

A previous study of object manipulation with wrinkled fingers found that wet objects were moved more quickly when the fingers were wrinkly compared to dry [15]. Importantly, there is no difference in tactile sensitivity in wrinkled fingers compared to dry [16], meaning that people are not trading off acuity for friction at the fingertip. It is therefore reasonable to wonder why healthy people do not have permanently wrinkled fingers. The answer presumably lies in the changes in the mechanical properties of the finger tissues, where there may be lower shear resistance when the finger is wrinkled [17]. Previous studies have also suggested differences in manipulation across the lifespan [1820]; the present results show age-related effects, although they are rather weak in this sample. Our journey through life leads us to develop strategies for handling familiar and unfamiliar objects, so it seems likely that strategic changes, along with sensory and motor changes, will affect how children and adults perform tasks with handheld objects [21].

Here we have ultimate pan-selectionism: if your hypothesis fails to explain another phenomenon, you simply make up a reason why that’s also adaptive. In this case, Davis posits “lower shear resistance” for wrinkled fingers, which for a reason he fails to specify must confer a disadvantage (presumably because you can’t hold onto an object as tightly).

I’m not at all convinced by this explanation or the supporting data, as they’re contradicted by evolutionary observations and by the absence of data on wrinkled toes. As the BBC says, some believe that wrinkling “could just be a coincidental physiological response with no adaptive function.” (Go have a look at that link!). I am one of those skeptics. What surprises me is that that statement is the sing caveat (and doesn’t reprise what’s at the link) in a whole article pushing the “adaptive wrinkling in wet environments” hypothesis.

Other venues have also picked up this result, and I guess they are either overly credulous or didn’t read the paper carefully enough. Or they didn’t ask probing questions.

h/t: Peter


Davis NJ (2021) Water-immersion finger-wrinkling improves grip efficiency in handling wet objects. PLOS ONE 16(7): e0253185. https://doi.org/10.1371/journal.pone.0253185

25 thoughts on “Why does the skin on your fingers and toes wrinkle after immersion in water? A panselectionist answer.

  1. Although I agree that this study does not give a definite answer, it should be noted that the results do not dispel the adaptation hypothesis either: the question re. Dry fingers maybe better but you cannot always have dry fingers . I’ve been hiking with some minor climbing, and I can assure your fingers will wrinkle. With feet in boots I would not know, but i guess they would if barefoot.
    I don’t think the ‘aquatic ape’ hypothesis has been ‘dispelled’, it has rather morphed into the ‘coastal ape’ hypothesis, with still quite a bit of evidence going for it.

    1. With the difference that Kipling’s ‘Just So’ stories did not really pertain to evolutionary adaptation.
      I always thought that Gould was a bit quick to dispel adaptation. I mean, one needs good evidence to accept a trait is adaptive, but one needs equally good evidence to accept it is not.
      No, this is not the same as: you make a wild statement, so you have to prove it.
      I feel justified in keeping agnostic about wrinkled toes being adaptive, as I am about the ‘coastal ape’ hypothesis.

  2. It is indeed more complicated than I thought. I did not know about the role of the nervous system in the response.

    Now another thing that wrinkles up in water (cold water in particular) are male genitals. I would argue that is mainly a reaction to retain heat.

  3. What a fun post! This quirky trait would definitely benefit from a comprehensive phylogenetic analysis on other primates. So it has been observed in Japanese macaques? If so, I would guess it occurs more widely. It would also be useful to know if it happens in the prehensile tails of the new world monkeys, where the fleshy epidermis of the tail is similar to the fleshy ridges and pads of fingers and toes.

  4. As a young man , long ago, probably brimming with testosterone ( 😁), I actually had erections in cold water. But the stimulus was great.

  5. It would be interesting to see some multi-mammal data on the prevalence of feet/pad wrinkles (or not) and its impact on locomotion in wet conditions – akin to tire treads for driving in the rain. As to why they are not always wrinkled – more rubber on the ground when it is dry.

    I just looked up whether bears/apes have wrinkled pads, and they both seem to – does anyone know if cats’ paws wrinkle when wet?

    My Bayesian update: I knew and considered zero on this topic prior to reading this post, but now I’m willing to offer 1.8:1 odds on it being an adaptation geared for enhanced traction -:)

    1. Just a guess: cats with their retractable claws don’t need wrinkling of their pads. If slippery they have needle sharp claws. But it would be interesting to find out.

  6. As a rock climber I’m somewhat obsessed with fingers gripping.
    Climbers chalk (magnesium carbonate) is a drying agent and in my experience makes a big difference in sweaty situations. Sadly a lot of climbers seem to feel the need to (in my opionion) overchalk which can leave the rock looking a mess with the excess left behind.
    As a lots of my climbing has been in North Wales I have lots of experience of climbing in the rain, way beyond when chalk would be of any use, and have frequently been climbing on rock running with water using wrinkled soaked fingers.
    So far I haven’t fallen to my death and climbing in those conditions is not as hard as you might think so maybe I was benefiting from super special wrinkly grippiness, or then again maybe not 🙂

  7. I would put my money on non-adaptive. Immersed in water, skin cells take some up, thereby increasing in size. Since the deeper tissues have no similar increase in size, the skin cells are forced to wrinkle to accommodate their extra size.

    1. I think it was pointed out that that was not the mechanism, nerve cells are involved. Did you even read the post?

        1. So?
          You actually comment on a post that you did not read ? Because Jerry’s posts are too stupid to comment on?
          Did I get that right?

  8. Why are elephants big, grey, and wrinkly?

    Because if they were small, white, and smooth they would be aspirins.

    1. Mary Danby’s Book of Fun – I hope that is where you got this.
      I have been telling elephant jokes my whole life – and this was always my favourite (because of it’s absurdity).
      This always made my daughter laugh/groan.
      Thanks for the memory!

  9. I can certainly see an advantage to having enough added traction when crossing wet, algae covered rocks, of not losing one’s footing and being swept downstream.
    I think it would be hard to test because almost everyone wears shoes most of the time, which changes the shape of your feet, and the way you walk.

  10. Is it known when wrinkly fingers appeared in humans? If it coincided more or less with when humans were spreading around the globe, often by water to distant islands by, presumably some sort of raft, maybe some had wrinkly fingers that coped with the mechanics of rafting and being wet full time better than the smoothies, they got to the island(s), went forth and multiplied – ergo – a species with wrinkly digits. Do ALL in the human species get wrinkles?

  11. The examples where wrinkles help to seem nonspecific. General “foraging”.

    Would it not matter more if specific objects like a wooden spear – for a high-calorie high value food – were argued? And hunting conditions – long periods of time, possibly sweating, or hands otherwise in an unusual conditions, maybe even very cold – how far back are mittens known in the historical record?

    That would count more, I think, than mere walking on wet rocks. Could natural selection be so precise – a specific behavior like hunting with manual weapons like spears or bows?

    Fascinating, and unsolved, puzzle!

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