(Photo by Brenderous, click for full credit)
Almost everyone has experienced it. When you have been soaking in the bathtub, swimming, or just washing dishes for a long time, the skin on your fingers (and toes) wrinkles. From a scientific point of view, there are at least two questions to consider: (1) How does this happen? and (2) Why does this happen? Most textbooks explain (often incorrectly) the how, but I haven’t found any that explain the why. It seems that over the years, scientists have been studying this, and in the end, they have mostly answered both questions.
Let’s start with the “how.” A lot of textbooks and websites incorrectly explain this part. They say that it is the result of your skin absorbing water and swelling. It turns out that’s not true at all. More than 70 years ago, scientists showed that if certain nerves to the hand are damaged, its skin will not wrinkle, no matter how long it stays underwater.1 Over the years, other scientists have investigated water-induced wrinkling, and it seems to be the result of a process initiated by the nervous system.
Your skin is made of two layers: the epidermis (the layer you see) and the dermis (the layer underneath that contains blood vessels). When your hands and/or feet have been underwater for a long time, your nervous system tells the blood vessels in your dermis to constrict. This reduces the volume of the dermis, which in turn reduces the tension with which the epidermis is stretched. As a result, the epidermis “relaxes,” forming wrinkles.2
This answer is interesting enough, because I have long taught the incorrect explanation for how your skin wrinkles underwater. I am glad that I learned I was wrong on the point, and I will now start teaching the correct explanation. However, the “why” question is also very interesting, and a couple of recent studies have provided a good answer for that question as well.
Just a couple of years ago, some scientists from Boise, Idaho studied the water-induced wrinkling process in depth and showed that the wrinkles look a lot like the treads on a tire or shoe. Such treads allow the tire (or shoe) to grip a wet road (or floor) better, because the water that is on the surface is pushed into the treads, allowing the rest of the tire (or shoe) to have direct contact with the surface. They also point out:3
Wet-induced wrinkles may, in fact, be substantially superior to ‘rain treads’ on shoes, which maintain a tread even when under compression and thus have a surface area of contact that is reduced. Wet-induced wrinkle treads, on the other hand, are pliable, and the act of pressing a finger tip down on a wet surface ‘squeezes’ the fluid out from under the finger through the channels, and upon completion of this single pulsatile flow the entire finger’s skin contacts the surface.
While their study was very good, it didn’t provide any direct evidence that the wrinkles improved grip. It only showed the wrinkles have the properties that should allow them to improve grip. Now a different study has provided direct evidence for their conclusion.
A group of scientists from the UK tested how well people handle objects underwater. They constructed a vessel that was filled with water and had a barrier in the middle. The barrier had a small hole in it. Subjects were asked to put their hands underwater, one on each side of the barrier. Then, they had to pick up objects from an underwater bin with one hand and push them through the hole in the barrier to their other hand. Finally, they had to drop the objects through a hole into a box. They found that if the subjects had been soaking their hands in water so that they already had wrinkles, they completed the task (on average) about 12% faster.4
In order to make sure this was an effect similar to what is provided by the treads on a tire or shoe, the scientists repeated the experiment without the water. As a result, the objects were all dry. In that version of the experiment, subjects completed the task in roughly the same amount of time, regardless of whether or not their hands were water-wrinkled.
In the end, then, it seems that the wrinkles you get on your hands and feet act like the treads on tires or shoes. They improve the grip you experience when you are handling wet things. In addition, this seems to be a designed response, since it is initiated by the nervous system and is not the result of some passive effect such as the absorption of water. The more we study nature, the more we see how intricately it has been designed!
REFERENCES
1. Lewis, T and Pickering, GW, “Circulatory changes in fingers in some diseases of the nervous system, with special reference to digital atrophy of peripheral nerve lesions,” Clinical Science 2:149, 1936
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2. Wilder-Smith, EP and Chow, A, “Water-immersion wrinkling is due to vasoconstriction,” Muscle & Nerve 27:307–311, 2003
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3. Mark Changizi, Romann Weber, Ritesh Kotecha, and Joseph Palazzo, “Are Wet-Induced Wrinkled Fingers Primate Rain Treads?” Brain, Behavior, and Evolution 77(4):286-90, 2011
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4. Kyriacos Kareklas, Daniel Nettle, and Tom V. Smulders, “Water-induced finger wrinkles improve handling of wet objects,” Biology Letters doi:10.1098/rsbl.2012.0999, 2013
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It makes sense to me that it isn’t simply water absorption, otherwise it would occur all over the body, not just at the hands and feet. I just never thought about it before! Also, wouldn’t absorbing water result in MORE tension in the skin, not less? This is a great new discovery!
Thanks for always posting awesome topics Dr. Wile. You’re the best:-)
My pleasure, Inazuma. If the epidermis absorbs the water, it would swell to the point where it was much bigger, which would reduce the tension. Remember, since the epidermis and dermis are separate layers, there would be two ways to produce wrinkles: shrink the dermis or swell the epidermis. A lot of texts say it is the latter, when in fact, it is the former.
Haha, now that I have read your article it seems so obvious. Like many, I always assumed it was water absorption. It seemed a simple and obvious explanation.
I never thought to question why it was only my finger tips and toes that wrinkled and not my whole body. The water absorption theory sounds rather silly now.
Seems this would be good information in a forensic investigation. If a body found in the water didn’t have wrinkled fingertips or toes then it would seem the body entered the water post mortem. Does it work that way?
That’s a great thought, Calvin, but I am not sure. Even if the body died after being submerged, it would start to bloat up after being dead for a while, because the bacteria in the body would still live, and they would migrate into areas they aren’t supposed to be. This produces a lot of gas, which bloats up the body. I am not sure of the timescale, however. It could be that if the body has only been in the water a few hours, the lack of wrinkling would indicate it was dead before being in the water. Once the bacteria take over, however, the lack of wrinkles wouldn’t indicate anything.
I thought I would just add some thoughts here since I had a protracted discussion about this paper in one of my classes a month ago. After some discussion and looking up some critiques of the study we came away with the strong impression that this wrinkling finger thing was just a catchy publication but didn’t mean much. I think that this page sums up the reasons pretty well: http://www.genomicron.evolverzone.com/2013/01/the-great-wrinkled-finger-debate/ What I pointed out to the class was that many scientists think that just because something does something that there must be an adaptive reason for it. In other words they think that sectionalist forces had to somehow cause this feature to form. I think that you could rightly criticize this study as trying to use darwinian selection to explain everything. Ironically, the authors who are trying to find an adaptionist explanation for finger wrinkling aren’t really evolutionists by training and its evolutionists who are the biggest critiques of the study. The most interesting data that suggests that finger wrinkle is not adaptive is that there was a study on cadavers that showed the same wrinkling occurs even on fingers of the deceased which speaks to Calvin’s question. I also think that that blog I referred to makes a good point that if wrinkling were so useful why does it take so long for wrinkles to form when you get your fingers wet? As I said to the class, if I asked, will you expect to find differences before you even did the tests in gripping ability. Of course we would. How could there not be a difference really. what surprised me was how little difference there was. had wrinkled fingers been important I would have expected a far larger difference. Another way of looking at this would be, if I tested people that had 6 fingers vs 5 fingers for their ability to perform some task, I would expect to find differences but finding differences wouldn’t prove to me that the differences were due to selection of 6 over 5 fingers. In other words, why one person has 6 vs 5 fingers might have nothing to do with the differences in their function at all. I think this is the case for skin wrinkling, the fact the skin wrinkles does effect the behavior of the skin and so changes the function but that doesn’t have to be the reason the skin wrinkles. These function differences might just be secondary consequences of the mechanism that causes wrinkling.
The finger wrinkling is a fun study to think about but I’m not convinced they discovered much at all. I would be more interested in knowing more about the mechanism.
Thanks for your perspective and the blog post, Joel. I especially appreciate the photo that shows a dead hand can also wrinkle. I note that it took a long time (2 hours) in cold water, which suggests a different mechanism from the one that produces it in live fingers. However, it is an interesting result.
I certainly agree with you that some people try to fit anything into an adaptionist framework, and that doesn’t always work. However, I will have to disagree with you (and the post). I think reference 2 and the references cited therein show that the mechanism that occurs in live fingers is based on nervous-system-induced vasoconstriction. Also, I think the paper in reference 3 answers the timescale question, as well as the salinity question raised in the blog article you linked:
You ask, “How could there not be a difference really.” I agree. However, the difference is exactly what you would expect for the purpose being suggested. If there were a reduction in the ability to handle the objects underwater, then that would argue against the hypothesis. Obviously, then, an improvement in grip supports the hypothesis.
Finally, you say you were surprised by how little a difference there was. I wasn’t surprised by that at all. The experiment used one kind of task, probably not the kind that would be typical of people in real conditions. I suspect if someone devised a wider range of tests, he or she would find that the wrinkles benefit certain tasks a lot more.
Thanks for the thought provoking response. I’ll have to modify what I said before. Those data are definitely worth considering. I agree the winkling in the dead hand surely must be a different mechanism since I am convinced of the data that there is never response. I put it in the category of goose bumps. I’m not sure there is a great benefit in having the bumps themselves but I am sure there is a reason why the skin tightens and the bumps are the end result. This wrinkling them, maybe there is more of a benefit than I am giving it credit for. Not sure what I think now but it is an interesting question to ponder.
Thank you Dr. Wile for this intriguing post, complimenting your ability to use the scientific method as well as you teach it. As a homeschool student myself, I will continue to learn and be surprised and inspired to discover more of creation by your courses in General, Physical, Chemical, Physics and Biological science. I have recently found your science blog and fully enjoy it. Thank you for your invaluable contribution and time in the field of sciences.
Personally, I understood the process of skin-wrinkling in the bathtub or in a dishwashing venture to be caused by the water and/or soap drying up the skin. In this case, the removing the oil on your skin. But then again thats the how, not the why. I find myself asking, “Why does the skin on the hand return to an almost perfectly normal state shorty after being removed from the water? ” Why does a slightly oiled surface in a dry environment provide more grip or friction? (Take trying to put on clean, dry socks on moist feet for example.) Or why does a dry surface in a oiled environment give less friction? (Take an ring gymnast for example. Sweaty hands made to offer less friction with the use of powder.) Its these sort of question that following you courses has taught me to ask. Not only how to ask and answer them; But why as well.
Thank you so much, Ryan! I am very glad to see that my courses have helped you learn how to think through issues and ask the right questions!