When it comes to physiology, theleatherback turtle is, in some ways, more like a reptilian whale than a turtle. It swims farther into the cold ofthe northern and southern oceans than any other sea turtle, and it deals withthe chilly waters in a way unique among reptiles.
A warm-blooded turtle may seem to bea contradiction in terms. Nonetheless, an adult leatherback can maintain a bodytemperature of between 25 and 26°C (77-79°F) in seawater that is only 8°C (46.4°F). Accomplishing this featrequires adaptations both to generate heat in the turtle’s body and to keep itfrom escaping into the surrounding waters. Leatherbacks apparently do notgenerate internal heat the way we do, or the way birds do, as a by-product ofcellular metabolism. A leatherback may be able to pick up some body heat bybasking at the surface; its dark, almost black body color may help it to absorbsolar radiation. However, most of its internal heat comes from the action ofits muscles.
Leatherbacks keep their body heat inthree different ways. The first, and simplest, is size. The bigger the animalis, the lower its surface-to-volume ratio; for every ounce of body mass, thereis proportionately less surface through which heat can escape. An adultleatherback is twice the size of the biggest cheloniid sea turtles and willtherefore take longer to cool off. Maintaining a high body temperature throughsheer 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 greenturtles can maintain their body temperature at a degree or two above that ofthe 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.
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 haveblubber, but they do have a reptilian equivalent: thick, oil-saturated skin,with a layer of fibrous, fatty tissue just beneath it. Insulation protects theleatherback everywhere but on its head and flippers. Because the flippers arecomparatively thin and blade-like, they are the one part of the leatherbackthat is likely to become chilled. There is not much that the turtle can doabout this without compromising the aerodynamic shape of the flipper. Theproblem is that as blood flows through the turtle’s flippers, it risks losingenough heat to lower the animal’s central body temperature when it returns. Thesolution is to allow the flippers to cool down without drawing heat away fromthe rest of the turtle’s body. The leatherback accomplishes this by arrangingthe blood vessels in the base of its flipper into a countercurrent exchangesystem.
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 thebody to pick up some heat from the warmer blood vessels; thus, the heat istransferred from the outgoing to the ingoing vessels before it reaches theflipper itself. This is the same arrangement found in an old-fashioned steamradiator, in which the coiled pipes pass heat back and forth as water coursesthrough them. The leatherback is certainly not the only animal with such anarrangement; gulls have a countercurrent exchange in their legs. That is why agull can stand on an ice floe without freezing.
All this applies, of course, only toan adult leatherback. Hatchlings are simply too small to conserve body heat, even with insulation and countercurrentexchange systems. We do not know how old, or how large, a leatherback has to bebefore it can switch from a cold-blooded to a warm-blooded mode of life. Leatherbacks reach their immensesize in a much shorter time than it takes other sea turtles to grow. Perhapstheir rush to adulthood is driven by a simple need to keep warm.
参考译文:温血海龟
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。在不损害鳍部气动外形的情况下很少有海龟可以做到这些。问题是血液流经海龟鳍部时，很容易损耗热量，血液回流时便降低了动物的中心体温。解决办法是在身体其余部分的热量还没有损耗前，允许鳍部降低温度。棱皮龟通过鳍下排列的血管流入逆流交换系统来实现这一点。
在逆流交换系统中，血管将鳍部冷却的血液与身体其他部位温热的血液进行交换。因此，热量在到达鳍部前就通过流入的血液和流出的血液完成了热量转移。人们发现老式蒸汽式暖气片有着与之类似的装置，当水流经这些盘绕的管子时热量进行了交换。当然并不只是棱皮龟有这种结构。海鸥的腿部也有一个逆流交换系统，这就是为什么海鸥可以站在冰川上而不被冻结。
当然，这些都仅适用于成年棱皮龟。刚孵化的棱皮龟太小，即使有绝缘层和逆流交换系统也不能保存体温。现在我们还不知道棱皮龟要达到多大年龄或者多大尺寸才能从冷血动物转变成温血动物。棱皮龟庞大体型的形成时间要比其它海龟短得多。可能它们是为了保暖才急着向成年过渡。
(选自TPO15)
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