IELTS Academic Reading Practice Test 24: Passage 1

Few animals have a worse reputation than the sloth. Its very name in English means laziness, and when European naturalists first described the animal, they were far from complimentary. In the eighteenth century, the French naturalist Georges-Louis Leclerc, Comte de Buffon, regarded it as a badly designed creature, suggesting that one more defect would have made its existence impossible. Modern biologists take a very different view. The sloth's slowness, they argue, is not a failure but a highly successful strategy for living on one of the poorest diets in the animal kingdom.

There are six living species of sloth, all found in the tropical forests of Central and South America. They fall into two groups, known as two-toed and three-toed sloths, although the names are misleading: all sloths have three toes on their back feet, and the difference lies in the number of fingers on the front limbs. The two groups look and behave so alike that they were long assumed to be close relatives. Genetic studies, however, have shown that their ancestors separated tens of millions of years ago, and that their tree-dwelling way of life evolved independently in each line. Their nearest living relatives are anteaters and armadillos, and among their extinct cousins were giant ground sloths, some of which were as large as elephants.

Three-toed sloths feed almost entirely on leaves, while two-toed sloths have a more varied diet that also includes fruit and flowers. Leaves are plentiful in a rainforest, but they are tough, low in energy and often contain chemicals that are mildly poisonous. To cope with them, the sloth has a large stomach divided into several chambers, in which bacteria slowly break down the plant material. The process is remarkably slow: a single meal may take weeks to pass through the animal's body, and at any one time, the contents of its stomach can make up a substantial part of its weight.

Because its food provides so little energy, the sloth must spend as little as possible. Its metabolism runs at a much lower rate than that of most mammals of a similar size, and it has relatively little muscle, which is expensive to maintain. Unlike most mammals, three-toed sloths do not keep their bodies at a constant temperature. Instead, their temperature rises and falls with that of their surroundings, and on cool mornings they often climb into the upper branches to bask in the sun, warming themselves without using energy from food. Even their movements are economical. A sloth may travel less than 40 metres in a day, and it moves so slowly and steadily that it rarely draws attention to itself.

The sloth's body is also adapted to a life spent hanging upside down. Its long, curved claws act as hooks, so that it can hang from a branch with little muscular effort, and many of its daily activities, including eating and sleeping, take place in this position. Three-toed sloths have more bones in their necks than almost any other mammal, which allows them to turn their heads through around 270 degrees and reach leaves without moving their bodies. Surprisingly, sloths are also capable swimmers, and they sometimes cross rivers in this way.

A sloth's fur is an ecosystem in its own right. Each hair of a three-toed sloth has cracks in which tiny green algae grow, giving the animal a greenish tint that helps to hide it among the leaves from predators such as harpy eagles, which hunt by sight. The fur is also home to a variety of insects, including several species of moth found nowhere else. About once a week, the three-toed sloth climbs down to the ground to empty its bowels at the base of its tree, where it is far more vulnerable to attack. The moths lay their eggs in the sloth's dung, and when the young moths become adults, they fly up into the canopy to find a sloth of their own. Why the sloth takes this risk is still debated. One suggestion is that the moths carry nutrients into the fur that help the algae to grow, and that the sloth gains nutrients by eating the algae, but this has not been proved.

Another long-standing belief about sloths has also been questioned. They were once said to sleep for up to 20 hours a day, but these figures came from animals kept in captivity. When researchers fitted wild three-toed sloths in Panama with devices that recorded their brain activity, they found that the animals slept for under ten hours a day, not much more than humans. Today, most sloth species are not considered to be at risk, but they face growing threats from forest clearance, roads, power lines and domestic dogs, and the pygmy three-toed sloth, which lives only on a single small island off the coast of Panama, is critically endangered.