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新托福TPO15阅读原文(一)：A Warm-Blooded Turtle

TPO15-1：A Warm-Blooded Turtle

When it comes to physiology, the leatherback turtle is, in some ways, more like a reptilian whale than a turtle. It swims farther into the cold of the northern and southern oceans than any other sea turtle, and it deals with the chilly waters in a way unique among reptiles.

A warm-blooded turtle may seem to be a contradiction in terms. Nonetheless, an adult leatherback can maintain a body temperature of between 25 and 26°C (77-79°F) in seawater that is only 8°C (46.4°F). Accomplishing this feat requires adaptations both to generate heat in the turtle’s body and to keep it from escaping into the surrounding waters. Leatherbacks apparently do not generate internal heat the way we do, or the way birds do, as a by-product of cellular metabolism. A leatherback may be able to pick up some body heat by basking at the surface; its dark, almost black body color may help it to absorb solar radiation. However, most of its internal heat comes from the action of its muscles.

Leatherbacks keep their body heat in three different ways. The first, and simplest, is size. The bigger the animal is, the lower its surface-to-volume ratio; for every ounce of body mass, there is proportionately less surface through which heat can escape. An adult leatherback is twice the size of the biggest cheloniid sea turtles and will therefore take longer to cool off. Maintaining a high body temperature through sheer bulk is called gigantothermy. It works for elephants, for whales, and, perhaps, it worked for many of the larger dinosaurs. It apparently works, in a smaller way, for some other sea turtles. Large loggerhead and green turtles can maintain their body temperature at a degree or two above that of the surrounding water, and gigantothermy is probably the way they do it. Muscular activity helps, too, and an actively swimming green turtle may be 7°C (12.6°F) warmer than the waters it swims through.

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Gigantothermy, though, would not be enough to keep a leatherback warm in cold northern waters. It is not enough for whales, which supplement it with a thick layer of insulating blubber (fat). Leatherbacks do not have blubber, but they do have a reptilian equivalent: thick, oil-saturated skin, with a layer of fibrous, fatty tissue just beneath it. Insulation protects the leatherback everywhere but on its head and flippers. Because the flippers are comparatively thin and blade-like, they are the one part of the leatherback that is likely to become chilled. There is not much that the turtle can do about this without compromising the aerodynamic shape of the flipper. The problem is that as blood flows through the turtle’s flippers, it risks losing enough heat to lower the animal’s central body temperature when it returns. The solution is to allow the flippers to cool down without drawing heat away from the rest of the turtle’s body. The leatherback accomplishes this by arranging the blood vessels in the base of its flipper into a countercurrent exchange system.

In a countercurrent exchange system, the blood vessels carrying cooled blood from the flippers run close enough to the blood vessels carrying warm blood from the body to pick up some heat from the warmer blood vessels; thus, the heat is transferred from the outgoing to the ingoing vessels before it reaches the flipper itself. This is the same arrangement found in an old-fashioned steam radiator, in which the coiled pipes pass heat back and forth as water courses through them. The leatherback is certainly not the only animal with such an arrangement; gulls have a countercurrent exchange in their legs. That is why a gull can stand on an ice floe without freezing.

All this applies, of course, only to an adult leatherback. Hatchlings are simply too small to conserve body heat, even with insulation and countercurrent exchange systems. We do not know how old, or how large, a leatherback has to be before it can switch from a cold-blooded to a warm-blooded mode of life. Leatherbacks reach their immense size in a much shorter time than it takes other sea turtles to grow. Perhaps their rush to adulthood is driven by a simple need to keep warm.

TPO15-1译文：温血海龟

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从生理学上讲，棱皮龟在某些方面上更像爬行的鲸鱼。跟其他海龟相比，它们能够游入更寒冷的北部和南部海洋，并且和其他爬行类动物相比，它们在应对寒冷水域时有其独特的方式。

温血海龟似乎是一个自相矛盾的术语。尽管如此，成年棱皮龟能够在仅8摄氏度(46.4华氏度)的海水中将体温维持在25～26摄氏度(77-79华氏度)之间。棱皮龟要做到这一点就必须调节其自身的体温，还要防止温度散失到周围水域。很显然，棱皮龟产生体内热量的方式与我们或者鸟类不同，并非细胞代谢的副产物。棱皮龟可能会通过晒太阳来收集身体所需的热量。其深色近乎黑色的体色有助于吸收太阳辐射。然而，它的大部分体热来自于肌肉运动。

棱皮龟通过三种方式保持体温。第一种也是最简单的方式就是体型大小。动物体型越大，表面和体积的比例越小。体重每增加一盎司，相应的容易流失热量的表面就越少。成年棱皮龟是最大的海龟的两倍，因此它变凉就需要更久的时间。完全依靠庞大体积维持体温的方法叫巨温性。大象、鲸鱼也许包括很多恐龙也是通过这种方法保持体温的。其它海龟或多或少也存在这种现象。红海龟和绿甲海龟可以维持与周围水温略高1摄氏度或2摄氏度或的体温，可能就是利用的巨温性。肌肉运动也有助于维持体温，一个活跃游水的绿海龟体温可能比它所游水域温度高7摄氏度(12.6华氏度)。

然而，在寒冷的北部水域巨温性不足让棱皮龟保暖。同样，对于通过厚厚的绝缘脂(脂肪)来维持体温的鲸鱼来说也是不够的。棱皮龟没有鲸鱼那样的脂肪，但是它们和爬行类的动物有着相似的结构：厚且含油的皮肤，皮肤下有一层纤维，而脂肪组织就在这个纤维层下面。除了头部和鳍，这个“绝缘”结构可以保护它们的所有部位。因为棱皮龟的鳍相对较薄且呈叶片状，这一部位很有可能会被冻僵。