单选题 Koalas, an Australian tree-climbing animal, are very particular about what they eat, devoting themselves entirely to a diet of the leaves of eucalyptus trees. But there are problems associated with an exclusive diet of leaves, especially if, like the koala, you happen to be a relatively small animal. One of these problems is that the leaves of trees are rich in fibre, and so resist digestion. Eucalyptus leaves are worse than most, for they contain large amounts of lignin, the indigestible, woody material found in the cell walls of many plants.
But there is another drawback for the koala. The ratio of an animal's gut volume to its energy requirements depends on body mass; the smaller it is, the lower the ratio. So tiny leaf-eaters are likely to have difficulty processing sufficient quantities of their poor-quality food to meet their metabolic needs.
S. J. Cork and T. J. Dawson of the University of New South Wales and I. D. Hume of the University of New England have made a study of the koala's digestion. They have identified three major factors that allow koalas to exploit its fibre-laden diet.
In the first place, the koala has a discerning digestive system; like the rabbit, it can regulate the passage of food through its gut in a way that discriminates between particles of different sizes. The alimentary canal retains and solutes smaller, more digestible particles, while expelling unwanted, coarser matter. This is probably a space-saving exercise; it has the effect of increasing the rate at which raw material can be fed into the system.
The second factor behind the koala's success is that it has a low overall requirement for metabolic energy, compared to other Australian animals of similar size. So it saves on its fuel needs. In this respect, the koala is not dissimilar to another slow-moving, leaf eating mammal, the three-toed sloth.
Thirdly, eucalyptus leaves have hidden qualities. Despite the large quantity of lignin, such leaves are rich in digestible energy -especially in the form of fatty substances. Not all such resources are available to the koala's metabolic machinery; essential oils are passed out, for example. But some fatty substances are available, as are sugar and starch. It is these compounds that satisfy the bulk of the koala's energy needs.
Surprisingly, constituents of the eucalyptus's cell walls, such as cellulose, are less important. Some cellulose is digested, but the koala's accomplishments in this field do not rival those of other animals that reshow.

单选题 One of the problems of eating too much of tree leaves is that

单选题 The purpose of Cork, Dawson and Hume study is to

单选题 The word "discerning" in Line I, paragraph 4 is closest in meaning to

单选题 Eucalyptus leaves are different from other tree leaves in that they have

单选题 The title of the text might be