经济学人下载:爬行动物的排尿 可算是方便了

Science and technology
科学技术

Reptilian micturition
爬行动物的排尿

That's a relief
可算是方便了

Softshell turtles urinate through their mouths
鳖类通过其嘴部小便

WHY did the turtle stick its head in a bucket?
为什么乌龟会把它的脑袋扎到桶里呢？

sounds like the sort of riddle asked by ten-year-olds in school playgrounds.
这听起来像十岁小孩子在学校操场上问的那种谜题。

But it was also asked recently by Yuen Ip of the National University of Singapore.
但是新加坡国立大学的元艾坡最近也问了这个问题。

And his answer, it has to be said, is precisely the sort that would appeal to a ten-year-old.
不得不说，他的答案恰恰是能吸引十岁小孩子的那种。

It is that turtles pee through their mouths.
答案是乌龟在用嘴巴撒尿。

The turtles in question are a species known as the Chinese softshell.
问题中的乌龟是一种被称为中华鳖的龟种。

By preference, they live in brackish swamps.
它们偏爱生活在微咸水湿地中。

But such swamps are liable to dry up, and then the turtles often do something which has puzzled zoologists for centuries:
但是这类湿地容易干涸，然后中华鳖经常会做出下面的动作，这让动物学家们困惑了几个世纪：

they put their heads into those small pools of water which remain and hold them there for as long as two hours.
鳖将它们的头部扎入余留的小水坑里，一待就是2个钟头。

In 1886 a pair of American biologists called Gage, who were investigating this curious behaviour, dissected several turtles and found something strange in the animals' mouths, namely a set of structures that look like gills.
1886年，两名叫盖奇的美国生物学家研究了这一古怪的行为。他们解剖了数只鳖，在鳖的嘴巴里发现了些奇异之处，即一付长得像腮的结构。

That, they felt, explained how turtles were able to perform their head-in-a-pool trick, but not why. Softshells are not compelled to breathe through their gills; they have a perfectly adequate pair of lungs to do the job.
他们觉得这解释了鳖为何能做出把头扎入水坑的把戏，但无法解释为什么要这么做。鳖类并非必须要通过腮来呼吸，用它们的两个肺就完全足够了。

Nor did zoologists take seriously the hypothesis that the reptiles were pining for their aquatic homes and putting their heads in puddles to keep their spirits up.
还有一个动物学家不会当真的假设：这些爬行动物离不开它们的水之家，它们将头伸入水坑里是为了提提神。

So, with the aid of a dozen turtles purchased from a local market, Dr Ip decided he would try to solve the mystery once and for all.
所以艾坡博士从本地市场中买了十二只鳖，决定借助它们设法彻底的解开这个迷。

To do so, he strapped the animals down, to stop them moving, and provided each with a container of water into which it could dip its head if it chose.
他将这些鳖绑住，使它们无法移动，每一只前面都放了一个装有水的容器，以便鳖可以将头扎进去。

He also fitted each turtle with a plastic box that collected its urine, for he had a suspicion that therein lay the key to the mystery.
他还给每只鳖配了一个塑料桶来收集它们的尿液，他怀疑解开这个谜的钥匙就藏于其中。

This done, he monitored the animals for six days, paying particular attention to the chemistry of the water in the container.
准备好后，他对这些鳖监测了六天，并特别关注了容器中水的化学成分。

As expected, the turtles periodically submerged their heads in the water for between 20 and 100 minutes at a time.
正如所料，这些鳖定期将它们的头扎入水中，每次的时长在20到100分钟之间。

During these periods, Dr Ip noted that oxygen was being extracted from it.
在此期间，艾坡博士注意到水中的氧气在被提取。

This confirmed that turtles can, indeed, breathe underwater through their mouths.
这肯定了鳖确实可以在水下通过嘴部呼吸。

As the oxygen level fell, though, something else rose: the level of urea.
可是，随着氧含量下降，其他物质的含量上升了-尿素。

Clearly, the animals were disposing of this waste product by secreting it from their mouths into the water.
显然，鳖通过它们的嘴部悄悄的将这种废物排入了水中。

Indeed, a calculation Dr Ip performed, based on the volume and composition of the urine he collected in the plastic buckets, and of the head-dunking water at the end of each day, suggested that only 6% of the urea excreted by the turtles was leaving them in the traditional manner, via urine. The question was, why?
的确如此，艾坡博士根据每天结束后其在塑料桶中所收集尿液以及头部所扎入水的成分和体积进行了计算，表明鳖排泄的尿素中只有6%是以尿液这一传统方式排出的。问题是，为什么？

The most likely answer, as he explains in a paper in the Journal of Experimental Biology, is that urine, though cheap to produce when water is plentiful, is a physiologically costly product when water is scarce.
最可能的答案是，正如他在《实验生物学》上发表的论文中解释的：虽然在水量丰沛时，产生尿液很便宜，但当水稀缺之时，这就成了一种生理上很昂贵的产品了。

Turtle kidneys have difficulty processing salt water, so for an animal that lives in a brackish environment potable water is always scarce.
鳖的肾无法处理咸水，所以对于一个生活在微咸水环境中的动物来说，食水一直弥足珍贵。

One way round this for a swimming animal is to develop an outlet which allows urea to dissolve directly in the water as it swims, which is what the turtles seem to have done.
对游泳动物来讲，规避此点的一种方式是进化出一个可以在其游水时将尿素直接排到水中排泄器官，就像鳖类似乎已经做到的那样。

Dr Ip looked at which genes were active in the gill-like structures of the mouth and found one that appears, from its similarity to genes of known function from other species, to encode a urea transporter.
艾坡博士检查了嘴部这一类腮结构中有哪些基因在起作用，他发现一个基因，根据其和其他物种中已知功能基因的相似程度，会转译出一种尿素通道蛋白。

The turtle's gills, then, not only take in oxygen, they also excrete urea.
这样，鳖的腮不仅能吸取氧气，也能排出尿素。

If there is insufficient water to swim in, however, the turtles have to resort to desperate measures. The reason they stick their heads in puddles is in order to relieve themselves.
然而如果水量不足以供鳖游水时，它们会采取极端手段-将头扎入水坑是为了方便一下。