托福阅读真题第271篇Temperature Regulation in Turtles

Temperature Regulation in Turtles

While turtle species vary widely in size and habitat, all must control their body temperatures for essential functions. Turtles can achieve a considerable degree of stability in body temperature by regulating their exchange of heat energy with the environment. Turtles exposing themselves to the sun on a log in a pond are a familiar sight in many parts of the world, because few pond turtles are large enough to maintain body temperatures higher than the temperature of the water surrounding them. Emerging from the water to bask is the only way most pond turtles can raise their body temperatures to speed digestion, growth, and the production of eggs. In addition, basking may help aquatic turtles to rid themselves of algae and leeches. Exposure to ultraviolet light may activate vitamin D, which is involved in controlling calcium deposition in their bones and shell. A few turtles spend quite a lot of time in trees; these turtles have small shells that allow considerable freedom of movement for limbs. The big-headed turtle from Southeast Asia lives in fast-flowing streams at high altitudes and is said to climb on rocks and trees to bask. In North America musk turtles bask on overhanging branches and drop into the water when they are disturbed.

Small terrestrial turtles, such as box turtles and small species of tortoises, can thermoregulate by moving between sunlight and shade. Small tortoises warm and cool quite rapidly, and they appear to behave very much like other small reptiles in selecting suitable microclimates for thermoregulation. Familiarity with a home range may assist this type of thermoregulation. A study conducted in Italy compared the thermoregulation of Hermann’s tortoises living in their own home range with individuals that were brought to the study site and tested before they had learned their way around. The resident tortoises warmed faster and maintained more stable shell temperatures than did the strangers.

Turtles are unusual among reptiles in having a substantial number of species that reach large body sizes. The giant tortoises of Aldabra Atoll, which weigh 60 kilograms or more, allow their body temperatures to rise to 32 to 33C on sunny days and cool to 28 to 30C overnight. Although large body size slows the rate of heating and cooling, it can make temperature regulation more difficult. Small turtles can find shade beside a bush or even a clump of grass. Giant tortoises, however, need a bigger object – a tree, for example. Thus, in open, sunny habitats overheating can be a problem for giant tortoises. The difficulty is particularly acute for some tortoises on Grande Terre, an island in the Indian Ocean. During the rainy season some of the turtles on the island move from the center of the island to the coast. This movement has direct benefits, because the migrant turtles gain access to a seasonal flush of plant growth on the coast. The extra food allows migrant females to lay more eggs than females that remain inland. There are risks to migrating, however, because shade is scarce on the coast and the rainy season is the hottest time of the year. Tortoises on the coast must limit their activity to the vicinity of patches of shade, which may be no more than a single tree in the midst of a grassy plain. As the day grows hotter, tortoises try to get into the deepest shade, and the biggest individuals do this most successfully. As the big tortoises (which are mostly males) into the shade, they force smaller individuals (mostly females) out into the sunlight, and some of these tortoises die of overheating.

Marine turtles are large enough to achieve a considerable degree of endothermy (the maintenance of a relatively stable body temperature by internal means). A body temperature of 37C was recorded from a green turtle swimming in water that was 20C. The leatherback turtle is the largest living turtle; adults weigh up to 1000 kilograms. Leatherbacks range far from warm equatorial regions and in the summer can be found off the coasts of New England and Nova Scotia in water as cool as 8 to 15C. Body temperatures of these turtles appear to be 18C or more above water temperatures, and a countercurrent arrangement of blood vessels in the flippers may contribute to retaining heat produced by muscular activity.

►While turtle species vary widely in size and habitat, all must control their body temperatures for essential functions. Turtles can achieve a considerable degree of stability in body temperature by regulating their exchange of heat energy with the environment. Turtles exposing themselves to the sun on a log in a pond are a familiar sight in many parts of the world, because few pond turtles are large enough to maintain body temperatures higher than the temperature of the water surrounding them. Emerging from the water to bask is the only way most pond turtles can raise their body temperatures to speed digestion, growth, and the production of eggs. In addition, basking may help aquatic turtles to rid themselves of algae and leeches. Exposure to ultraviolet light may activate vitamin D, which is involved in controlling calcium deposition in their bones and shell. A few turtles spend quite a lot of time in trees; these turtles have small shells that allow considerable freedom of movement for limbs. The big-headed turtle from Southeast Asia lives in fast-flowing streams at high altitudes and is said to climb on rocks and trees to bask. In North America musk turtles bask on overhanging branches and drop into the water when they are disturbed.

►Small terrestrial turtles, such as box turtles and small species of tortoises, can thermoregulate by moving between sunlight and shade. Small tortoises warm and cool quite rapidly, and they appear to behave very much like other small reptiles in selecting suitable microclimates for thermoregulation. Familiarity with a home range may assist this type of thermoregulation. A study conducted in Italy compared the thermoregulation of Hermann’s tortoises living in their own home range with individuals that were brought to the study site and tested before they had learned their way around. The resident tortoises warmed faster and maintained more stable shell temperatures than did the strangers.

►Turtles are unusual among reptiles in having a substantial number of species that reach large body sizes. The giant tortoises of Aldabra Atoll, which weigh 60 kilograms or more, allow their body temperatures to rise to 32 to 33C on sunny days and cool to 28 to 30C overnight. Although large body size slows the rate of heating and cooling, it can make temperature regulation more difficult. Small turtles can find shade beside a bush or even a clump of grass. Giant tortoises, however, need a bigger object – a tree, for example. Thus, in open, sunny habitats overheating can be a problem for giant tortoises. The difficulty is particularly acute for some tortoises on Grande Terre, an island in the Indian Ocean. During the rainy season some of the turtles on the island move from the center of the island to the coast. This movement has direct benefits, because the migrant turtles gain access to a seasonal flush of plant growth on the coast. The extra food allows migrant females to lay more eggs than females that remain inland. There are risks to migrating, however, because shade is scarce on the coast and the rainy season is the hottest time of the year. Tortoises on the coast must limit their activity to the vicinity of patches of shade, which may be no more than a single tree in the midst of a grassy plain. As the day grows hotter, tortoises try to get into the deepest shade, and the biggest individuals do this most successfully. As the big tortoises (which are mostly males) into the shade, they force smaller individuals (mostly females) out into the sunlight, and some of these tortoises die of overheating.

►Marine turtles are large enough to achieve a considerable degree of endothermy (the maintenance of a relatively stable body temperature by internal means). A body temperature of 37C was recorded from a green turtle swimming in water that was 20C. The leatherback turtle is the largest living turtle; adults weigh up to 1000 kilograms. Leatherbacks range far from warm equatorial regions and in the summer can be found off the coasts of New England and Nova Scotia in water as cool as 8 to 15C. Body temperatures of these turtles appear to be 18C or more above water temperatures, and a countercurrent arrangement of blood vessels in the flippers may contribute to retaining heat produced by muscular activity.

Turtles are unusual among reptiles in having a substantial number of species that reach large body sizes. The giant tortoises of Aldabra Atoll, which weigh 60 kilograms or more, allow their body temperatures to rise to 32 to 33C on sunny days and cool to 28 to 30C overnight. Although large body size slows the rate of heating and cooling, it can make temperature regulation more difficult. ⬛ Small turtles can find shade beside a bush or even a clump of grass. ⬛ Giant tortoises, however, need a bigger object – a tree, for example. ⬛ Thus, in open, sunny habitats overheating can be a problem for giant tortoises. ⬛ The difficulty is particularly acute for some tortoises on Grande Terre, an island in the Indian Ocean. During the rainy season some of the turtles on the island move from the center of the island to the coast. This movement has direct benefits, because the migrant turtles gain access to a seasonal flush of plant growth on the coast. The extra food allows migrant females to lay more eggs than females that remain inland. There are risks to migrating, however, because shade is scarce on the coast and the rainy season is the hottest time of the year. Tortoises on the coast must limit their activity to the vicinity of patches of shade, which may be no more than a single tree in the midst of a grassy plain. As the day grows hotter, tortoises try to get into the deepest shade, and the biggest individuals do this most successfully. As the big tortoises (which are mostly males) into the shade, they force smaller individuals (mostly females) out into the sunlight, and some of these tortoises die of overheating.

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