Sunday, 16 February 2014

Why do your fingers wrinkle in the bath?

I don't really like having a bath. It's boring and takes far too long for my liking. I'm more of a shower person. However, there is one use for baths, the opportunity to observe a bodily quirk that it would appear has been with us since Homo sapiens left the trees.
Finger and toe wrinkling takes about 5 minutes of full immersion for the effect to begin, but if you want maximum, scientifically proven wrinkle then soak in salty water at 40ºC for 30 minutes. If you seek out an explanation for this you will overwhelmingly find the following science, and spoiler alert - it's rubbish.

<rubbish>The outermost layer of your skin is called the stratum corneum. It protects us from cuts, abrasion and general wear and tear. According to standard finger wrinkling theory, water soaks into the stratum corneum, swelling the cells and making this top layer of skin expand. Immediately beneath this layer is the stratum granulosum that is full of water resistant fats. This blocks the water from soaking in any further. So, the outer layer expands and the inner layers stay the same. To accommodate this growth, the surface of your skin puckers up. Furthermore, only your finger tips and toes wrinkle since they lack sweat glands that secrete water proofing. Finger wrinkling is a by-product of our biology, nothing more.</rubbish>

It's a simple, elegant theory that is unfortunately completely wrong in pretty much every aspect. Which is a shame as I like simple elegance. However, it turns out that the real explanation for wrinkles is far more surprising, rather useful and may tell us something about out pre-human ancestry.

In 1936 two researcher were studying patients with paralysis in their arms. Thomas Lewis and George Pickering knew that the paralysis was caused by damage to the main nerve that runs down the arm all the way to the fingertips (ref 1). What came as a surprise was that the fingers of their patients did not wrinkle when soaked in water. The patients' non-paralysed hands and their feet showed normal wrinkling, but the wrinkles were gone from the paralysed hand. Nobody took much notice at the time and the story goes quiet until 1973 when an Irish plastic surgeon, Seamus O'Riain noticed essentially the same thing, or at least the mother of one of his child patients noticed smooth fingers where once there had been wrinkles (ref 2). It turns out that your fingers wrinkle because your body tells them to wrinkle. It's not a passive effect, but an active decision that your body takes. We aren't conscious of making that decision, because it's controlled by the automatic part of your nervous system that controls things like breathing, heart rate and perspiration. This can lead to some weird effects. For example, people who have severed fingers stitched back on loose the wrinkles from the reattached fingers, until the nerves regrow and feeling and wrinkling returns. You can also mess around with this using drugs that shut down parts of the nervous system, they also turn off finger wrinkling. The evidence is conclusive, finger wrinkling is nothing to do with water soaking into the skin and everything to do with an active control, albeit one we are not conscious of.

How then does our body turn on and off finger wrinkling? This is where the science becomes a bit more fuzzy and uncertain. Your fingertips do have sweat glands, despite what the commonly believed theory states. When sat in a bath, water runs up these sweat glands and somehow signals to the body that we are soaking in water, and this is why it takes a little while to get going. How this sets off the wrinkle signal is not known. One suggestion is that the water dilutes the sweat in the sweat gland and this change triggers the nearby nerves. What happens next has been carefully looked at. The automatic part of our nervous system is very good at controlling how much blood flows to various parts of the body (ref 3) . In the case of wrinkling the nerves tell the blood vessels to shrink, lowering blood flow especially to strange little balls of blood vessels in your toes and fingertips called glomus bodies. These are normally used to help reduce heat loss and keep your hands and toes warm, although clearly in some people, my wife included, they don't work very well. When the glomus blood vessels constrict, the whole glomus gets smaller and the flesh under your skin shrinks a bit. The skin on top, the stratum corneum stays the same size, but since the flesh has shrunk, the skin has to pucker up to accommodate this underlying change.

So, it has nothing to do with water soaking into the skin, the flesh under your skin gets smaller and the skin on top stays the same.

As with any good bit of science, it turns out to be a rather useful observation. It was the Irish surgeon, Seamus O'Riain who first suggested using finger wrinkling as a test of the health of the nervous system. There is now a standard protocol for soaking a patient's hand and checking how finger wrinkles develop over time. It is not a universally used test as it has a few problems. Firstly, when is a wrinkled finger wrinkled? How do you measure finger wrinkle in an objective way? Several routine drugs and smoking can inhibit your ability to wrinkle. These problems aside, being able to test a patient's nerve health by just popping their hand in a bowl of water turns out to be jolly handy, if you will pardon the pun.

So, that settles the big why do your fingers wrinkle debate, but it leaves me with one even more interesting question. Why does our body go to this effort on our behalf? Why would we have developed this strange ability to make wrinkles on our fingertips? We don't really know the answer to this yet, but the only theory at the moment is that it gives us better grip.

The idea first surfaced when scientists in Boise, Idaho noticed that wrinkles on wrinkled fingers looked similar to the patterns of water flow in river deltas or even to the tread pattern on modern car tyres (ref 4). In both cases the pattern, whether natural or designed, is there for a reason, to most effectively move water away from an area to the edge.

To test this a group of volunteers in Newcastle were asked to move a pile of marbles and fishing weights from a water filled bowl to a box (ref 5). They had to pick up each object with their right hand thumb and forefinger, pass it through a hole in a small screen to their left thumb and forefinger and then place it in a box. Each volunteer had to do this both with and without wrinkled fingers. So, quite a fiddly task and one that I imagine the poor volunteers found deeply pointless, although they did get paid £5 for their time. The results were pretty clear, if you had wrinkled fingers you could move the objects quicker than if you had smooth fingers. Wrinkled fingers do appear to give you more grip in the wet.

But this does not answer my question of why our bodies go to this effort. This is where we can dive into the wooly depths of informed guess work. Possibly our ancient, pre-human primate ancestors evolved this trait to help them cope better in wet conditions. Imagine the daily torrential downpour in a rain forest, the branches of trees become suddenly slippery and slick with wet moss and lichens. Our primate ancestor turns on their special wet-weather grip and hangs on more effectively. That said a 2013 study failed to find an improvement in grip in the wet, so the case is far from clear (ref 6).

Finger wrinkling seems to be a quirk of our deep evolutionary past made present for us whenever we take a bath. The real explanation of how and why may not be the simple, elegant theory that you see so often used, but for my money it is a far more exciting and interesting one. Now I think on it, it may even be enough to get me into a bath more often.

1) Clin. Sci 2: 1936, 149-175
2) BMJ: 1973, 3, 615-616
3) Muscle and Nerve: 2003, vol 3, 307-311
4) Brain, Behaviour and Evolution: 2011, 77, 286-290
5) Biol. Lett. 2013 9, 20120999
6) PLoS ONE: 2013, 9(1), e84949


  1. Superb article, and with the evolutionary link, appropriately published on Darwin Day. :-)

    1. Ashamed to say that the Darwin Day link was entirely coincidental...

  2. Hiện nay, nhu cầu đi lại của người dân tăng cao, thị trường xe máy đa dạng, chính về thế làm cho không ít người phân vân để mua cho mình chiếc xe máy thích hợp nhất Nên mua xe tay ga nào tốt nhất hiện nay

    Tìm hiểu chi tiết về Sau 49 ngày người chết đi về đâu trong thế giới tâm linh. Nhện là một loại động vật gần gủi với con người, hơn nũa nhên là một loại động vật linh thiêng thường được xuất hiện nhiều trong những câu truyện xa xưa Nhện sa trước mặt là điềm gì

    Một số điều kiêng cử trong tự nhiên khiến không ít người lo lắng. Như Làm vỡ bát là điềm gìCó con nhỏ có nên đi đám ma không. Nếu bạn quan tâm có thể tham khảo chi tiết.
    Bàn thờ là nơi linh thiêng, thờ cúng tổ tiên, ông bà, do vậy bạn không nên chưng những loại đồ giả. Vậy Có nên cắm hoa giả trên bàn thờ không. Tìm hiểu chi tiết Dơi bay vào nhà báo điềm gì


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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.