Mock Test 20.2 | Academic Reading

READING PASSAGE 2

You should spend about 20 minutes on Questions 14-28, which are based on Reading Passage 2 below.

The Leatherback Turtle

A.    When it comes to physiology, the leatherback turtle is, in some ways, more like a reptilian whale than a turtle. While all other sea turtles have hard, bony shells, the inky-blue carapace of the leatherback is somewhat flexible and almost rubbery to the touch. 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.

B.    Q23A 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. Q14A leatherback may be able to pick up somebody heat by basking at the surface; its dark, almost black body color may help it to absorb solar radiation. However, Q18most of its internal heat comes from the Q24action of its muscles.

C.   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. Leatherbacks are the largest turtles on Earth, growing up to seven feet (two meters) long and exceeding 2,000 pounds (900 kilograms). [Crack IELTS with Rob] 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. However, these animals have additional means of staying warm. Muscular activity helps, too, and an actively swimming green turtle may be 7°C (12.6°F) warmer than the waters it swims through.

D.  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 Q25insulating blubber (fat).  [Crack IELTS with Rob] 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. Q20The 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.

E.  In a countercurrent exchange system, the blood vessels carrying cooled blood from the Q26flippers run close enough to the blood vessels carrying warm blood from the body to pick up some heat from the warmer blood vessels; [Crack IELTS with Rob] thus, the heat is transferred from the outgoing to the ingoing vessels before it reaches the flipper itself. Q16This 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. Q21The 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.

F.   All this applies, of course, Q17only to an adult leatherback. Hatchlings are simply too small to conserve body heat, even with insulation and countercurrent exchange systems. We do not know Q27how 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.

G.  The ability to maintain warm body temperatures in cold water allows leatherbacks to have the widest global distribution of all reptile species, and possibly of any vertebrate. [Crack IELTS with Rob] They can be found in the tropic and temperate waters of the Atlantic, Pacific, and Indian Oceans, as well as the Mediterranean Sea. Adult leatherbacks also traverse as far north as Canada and Norway and as far south as New Zealand and South America. Leatherbacks also undertake the longest migrations between breeding and feeding areas of any sea turtle, averaging 3,700 miles (6,000 kilometers) each way. After mating at sea, females come ashore during the breeding season to nest. The nighttime ritual involves excavating a hole in the sand, depositing around 80 eggs, filling the nest, leaving a large, disturbed area of sand that makes detection by predators difficult, and finally returning to the sea.

H.  Although their distribution is wide, the number of leatherback turtles has seriously declined during the last century. Now the species that has survived for more than a hundred million years is facing extinction. [Crack IELTS with Rob] The Q28Pacific population of leatherback sea turtles is falling at an alarming rate Q15due to egg harvest, fishery bycatch, coastal development, and highly variable food availability: as few as 2,300 adult females now remain, making the Pacific leatherback the world’s most endangered marine turtle population. Some Pacific populations have disappeared entirely from certain areas, such as Malaysia. The number of leatherbacks in the Atlantic appears to be stable, but scientists believe that it, too, will decline due to the large numbers of adults being killed accidentally by fishing fleets. Scientists around the world are tracking and studying leatherbacks to learn more about these reptilian giants and how they can be saved.