IELTS Academic Reading Practice Test 10: Passage 1

Few animals are as closely associated with deserts as the camel. There are two domesticated species: the dromedary, which has one hump and lives mainly in the hot deserts of North Africa, the Middle East and South Asia, and the Bactrian camel, which has two humps and is found in the cold deserts and dry grasslands of Central Asia. A third species, the wild Bactrian camel, survives in small numbers in remote parts of China and Mongolia. The domesticated species have served people for thousands of years, carrying loads, providing milk, meat and wool, and making long journeys across terrain that few other animals could cross.

The camel's most famous feature is also its most misunderstood. A hump does not contain water, as many people believe, but fat, which the animal can break down for energy when food is scarce. When a camel has gone without food for a long time, its hump shrinks and may flop to one side. Storing fat in one place, rather than spreading it under the skin over the whole body, has a second advantage: it allows heat to escape more easily from the rest of the body, which helps the animal to stay cool.

Water, however, is the real challenge of desert life, and here the camel's abilities are remarkable. In hot weather, a camel can go for a week or more without drinking, and in cooler seasons, when the plants it eats contain more moisture, it may not need to drink for several months. Most mammals die if they lose more than about 15 per cent of their body weight in water, but a camel can survive the loss of around a quarter of its weight. When it does find water, it can drink more than 100 litres in a matter of minutes. Such a rapid intake would be dangerous for most animals, because the sudden flood of water into the blood would cause their blood cells to swell and burst. The camel's red blood cells, however, are oval rather than round, and can expand considerably without breaking. Their shape also helps them to keep flowing when the blood thickens as the animal dries out.

Camels also save water in less obvious ways. Most mammals keep their body temperature almost constant, and when the air is hot they sweat in order to cool down, losing large amounts of water in the process. A camel, by contrast, allows its body temperature to vary by as much as six degrees Celsius over a day. It warms up slowly during the hot daytime hours and begins sweating only when its temperature reaches about 41°C; then, during the cool desert night, it gets rid of the stored heat without using any water at all. Its kidneys produce highly concentrated urine, and its droppings are so dry that they have traditionally been used as fuel. Even the nose plays a part: as the animal breathes out, moisture is captured by the cool surfaces inside its nasal passages, so less water is lost to the dry atmosphere.

Other features protect the camel from heat and sand. Its thick coat may seem unsuited to the desert, but it acts as insulation, reducing the amount of heat that reaches the skin from the sun and the hot air; camels that have been shorn sweat considerably more than those that have not. Two rows of long eyelashes shield its eyes from blowing sand, and a thin third eyelid, through which the animal can still see, wipes away any grains that get past them. Its nostrils can be closed during sandstorms. Its broad feet, with two toes joined by a tough, leathery pad, spread out as they touch the ground, preventing the animal from sinking into soft sand, while thick patches of hardened skin on its chest and knees allow it to kneel on burning ground. Its mouth is tough enough to deal with thorny plants that most other grazing animals avoid.

Yet some of these features may not have evolved in deserts at all. Camels first appeared in North America around 40 million years ago and reached Asia only much later, by crossing a land bridge that once connected the two continents; their relatives in South America include the llama and the alpaca. In 2013 scientists described fossil bones from Ellesmere Island in the Canadian Arctic which showed that a giant camel had lived there about three and a half million years ago, when the region was warmer than today but still had long, freezing winters. Some researchers have suggested that features such as the hump and the broad feet originally helped camels to survive in cold, snowy forests, where food was scarce for much of the year, and only later proved useful in the deserts of Africa and Asia.