冷知識 爲什麼洗澡時皮膚會變皺(上)

Some parts of human skin, better known as glabrous skin, have a unique response to water. Unlike the rest of the body, the skin of our fingers, palms, and toes, and soles wrinkles after becoming sufficiently wet. Five minutes or so will usually do the trick.

人體有些部位的皮膚（通常指無毛皮膚）對水有特別的反應。手指、手掌、腳趾和腳底在充分潤溼後會變皺，而其他部分的皮膚不會。浸水五分鐘左右就會出現這種現象。

But why do these patches of skin wrinkle? Some think that this is a biochemical reaction, an osmotic process in which water yanks a handful of compounds out of the skin, leaving an expanse of parched, pruney skin in their place.

但是，爲什麼這些部分的皮膚會變皺？一些人認爲這是生物化學反應，在滲透作用下，水吸走了皮膚中的一些化合物，導致皮膚乾癟，出現褶皺。

But a century ago, scientists already knew that this curious reaction wasn’t a simple reflex or the result of osmosis.

但是一個世紀前，科學家就已經知曉，這種奇怪的反應並不單純是一種條件反射或滲透作用的結果。

That’s because surgeons learned that if certain nerves to the fingers were cut, the wrinkling response would disappear. Wrinkled fingers, then, are signs of an intact nervous system. Indeed, the wrinkling response has been suggested as a means of determining whether the sympathetic nervous system is functional in patients that are otherwise unresponsive.

原因是，外科醫生發現，假如切除某些通向手指的神經，泡水起皺反應就會消失。由此看來，手指變皺是神經系統完整的標誌。的確，一直以來，起皺反應都被用來判斷患者的交感神經系統運作是否正常。

On all of this, the scientific community has consensus. But whether and how the wet-wrinkling response may have evolved, and if it even represents an adaptation in the first place, remains a contentious debate.

科學界已經對以上這些達成了共識。但是關於泡水起皺反應是否是進化得到的，是如何進化來的，以及它首先是否代表了對環境的適應，仍存在爭議。

Neurobiologist Mark Changizi of 2AI Labs thinks it’s got to be an adaptation. Consider tyre treads. In dry conditions, smooth tyres can best maintain their grip on the asphalt, which explains why race cars typically have smooth, featureless tires. But when driving in rain, treads are far safer.

2AI Labs的神經生物學家馬克•尚依茲(Mark Changizi)認爲，這一定是一種適應。以輪胎胎面爲例。在乾燥的環境下，平滑的輪胎在瀝青路面上的抓地力最好，所以賽車通常使用平滑、無特徵的輪胎。而在下雨的環境下，帶紋路的胎面的輪胎安全性更好。

Wrinkled fingers, then, could be optimally designed for gripping in both wet and dry conditions. It’s extremely difficult to prove that any biological feature is an adaptation, let alone why it may have evolved. But researchers can look for hints, tell-tale clues that indicate that a feature may indeed have evolved as an adaptation.

由此可見，手指變皺可能是爲了在乾溼兩種環境下都能抓握而優化的設計。但是，要想證明某種生物特徵是適應環境的結果是極其困難的，更不用說要弄清進化的原因了。不過研究者還是可以尋找蛛絲馬跡，證明一種生物特徵是適應環境的結果。

In 2011, Changizi and his colleagues found evidence that wrinkled fingers indeed act as rain treads, channelling water away from the fingers and toes during wet conditions, allowing primates – humans and macaques, to be specific – to maintain tighter grips.

2011年，尚依茲和他的同事們發現證據證明手指變皺確實和雨天使用帶紋路胎面的原理一樣。在潮溼的環境下，手指和腳趾的溝壑能夠引導水流，讓靈長類動物——具體來說是人類和獼猴——保持更強的抓握力。

In other words, the wrinkles that result from exposure to water can be thought of as tiny, manual river drainage systems. In a typical ‘watershed’, multiple streams combine into larger channels, which eventually dump into a central river, much like twigs combine to form branches, which flow into a tree’s trunk. The pieces of land between the streams and brooks, on the other hand, are disconnected.

換句話說，接觸水所帶來的皺紋可以被視作一套微型的人工排水系統。在一個典型的“分水嶺”上，多條溪流匯聚成較大的河流，最後匯入中央河流，就好像樹木的枝條匯合成枝幹，最後匯合成樹幹。而從另一個角度來看，溪流之間的小塊土地就被分離開來。