2012雅思开年大考雅思阅读原文

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来源：育路山东教育网发布时间：2012-01-11

WHEN Cindy Engel notices that her cat, Darwin, has vomited on her carpet, she doesn't worry. Vomiting, she says, is “a healthy response to sickness”. What is more, the habit that some cats and dogs have of eating plants and then vomiting could be related to self-medicating behaviour by their ancestors. Wild cats and dogs would probably have eaten rough grasses as emetics or purgatives. Denied such material today, a cat will attack houseplants or chew wool jumpers. Dogs might also be doctoring themselves when they chew grass, eat or lick sand, soil and clay, or eat rocks or socks.

For the past decade Dr Engel, a lecturer in environmental sciences at Britain's Open University, has been collating examples of self-medicating behaviour in wild animals. She recently published a book* on the subject. In a talk at the Edinburgh Science Festival earlier this month, she explained that the idea that animals can treat themselves has been regarded with some scepticism by her colleagues in the past. But a growing number of animal behaviourists now think that wild animals can and do deal with their own medical needs.

William Karesh, of the Wildlife Conservation Society, in New York, for example, has studied the health of a wide range of wild animals, including anaconda snakes, macaws, penguins, guanacos (South American beasts related to camels), impala and buffalo. The animals were mostly in good physical condition, which is not surprising, since the weak quickly go to the wall in the wild. But blood tests showed that many had encountered nasty viral and bacterial diseases in the past—including diseases that are often fatal in captive animals, even when treated by vets. Moreover, if healthy wild animals are brought into captivity, their health often deteriorates unless great care is taken over their living conditions. Such observations suggest that wild animals can do something to keep themselves healthy that captive animals cannot.

Hearty animals

One example of self-medication was discovered in 1987. Michael Huffman and Mohamedi Seifu, working in the Mahale Mountains National Park in Tanzania, noticed that local chimpanzees suffering from intestinal worms would dose themselves with the pith of a plant called Veronia. This plant produces poisonous chemicals called terpenes. Its pith contains a strong enough concentration to kill gut parasites, but not so strong as to kill chimps (nor people, for that matter; locals use the pith for the same purpose). Given that the plant is known locally as “goat-killer”, however, it seems that not all animals are as smart as chimps and humans. Some consume it indiscriminately, and succumb.

Since the Veronia-eating chimps were discovered, more evidence has emerged suggesting that animals often eat things for medical rather than nutritional reasons. Many species, for example, consume dirt—a behaviour known as geophagy. Historically, the preferred explanation was that soil supplies minerals such as salt. But geophagy occurs in areas where the earth is not a useful source of minerals, and also in places where minerals can be more easily obtained from certain plants that are known to be rich in them. Clearly, the animals must be getting something else out of eating earth.

The current belief is that soil—and particularly the clay in it—helps to detoxify the defensive poisons that some plants produce in an attempt to prevent themselves from being eaten. Evidence for the detoxifying nature of clay came in 1999, from an experiment carried out on macaws by James Gilardi and his colleagues at the University of California, Davis.

Macaws eat seeds containing alkaloids, a group of chemicals that has some notoriously toxic members, such as strychnine. In the wild, the birds are frequently seen perched on eroding riverbanks eating clay. Dr Gilardi fed one group of macaws a mixture of a harmless alkaloid and clay, and a second group just the alkaloid. Several hours later, the macaws that had eaten the clay had 60% less alkaloid in their bloodstreams than those that had not, suggesting that the hypothesis is correct.

Other observations also support the idea that clay is detoxifying. Towards the tropics the amount of toxic compounds in plants increases—and so does the amount of earth eaten by herbivores. Elephants lick clay from mud holes all year round, except in September when they are bingeing on fruit which, because it has evolved to be eaten, is not toxic. And the addition of clay to the diets of domestic cattle increases the amount of nutrients that they can absorb from their food by 10-20%.

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