IELTS Academic Reading Practice Test 20: Passage 1

Every autumn, millions of orange and black butterflies leave their summer breeding grounds in southern Canada and the northern and eastern United States and set off towards the south-west. Some will fly more than 4,000 kilometres before they reach their destination: a small group of mountain forests in central Mexico, where they will spend the winter. The migration of the monarch butterfly is one of the most remarkable in the animal kingdom, not only because of the distances involved, but because the insects that make the journey have never made it before and have no older butterflies to follow.

The monarch's unusual life cycle helps to explain this. During the summer, a typical adult lives for only two to six weeks. In that time it mates, and the female lays her eggs, one at a time, on the leaves of milkweed, the only plant that the caterpillars can eat. Milkweed contains poisonous chemicals, which the caterpillars store in their bodies without being harmed. These make both the caterpillars and the adult butterflies unpleasant to eat, and their striking colours act as a warning to birds that might otherwise attack them. Three or four such short-lived generations follow one another each summer.

The butterflies that emerge from their pupae towards the end of the summer, however, are different. Shorter days, cooler temperatures and the declining quality of the milkweed plants cause them to enter a state known as diapause, in which their reproductive development is put on hold. Instead of mating, they devote their energy to flight and to building up reserves of fat, and they may live for eight or nine months, many times longer than their parents. As they travel south, they stop frequently to drink nectar from flowers, so that many arrive in Mexico heavier than when they set out. To save energy, they also make use of rising currents of warm air, which lift them upwards so that they can glide for long distances. In favourable conditions, they can cover more than 100 kilometres in a single day.

How the butterflies find their way has long puzzled biologists. Much of the answer lies in the sun. Monarchs use its position in the sky as a compass, but because the sun moves throughout the day, they must also know the time in order to interpret this information correctly. Research has shown that the internal clock that makes this possible is located partly in the antennae. In one experiment, monarchs whose antennae had been covered with black paint, so that no light could reach them, were unable to keep to a consistent direction, while those whose antennae had been coated with clear paint flew normally. There is also evidence that the butterflies can sense the Earth's magnetic field, which may help them when the sky is cloudy.

Although people living near the Mexican forests had known about the butterflies for generations, the location of the wintering sites remained a mystery to scientists until the 1970s. A Canadian zoologist, Fred Urquhart, had spent decades trying to trace the migration by attaching small labels to the wings of butterflies, relying on a large network of volunteers to report where labelled insects were found. Finally, in January 1975, two of his collaborators, Ken Brugger and Catalina Aguado, reached one of the sites, high in the mountains west of Mexico City. They found trees so thickly covered with butterflies that some branches bent under the weight.

The sites lie at around 3,000 metres above sea level, in forests dominated by a tree called the oyamel fir. Here the butterflies gather in enormous clusters on the trunks and branches. The mountain air is cold enough to keep them inactive, so that they use up their fat reserves slowly, but the dense canopy of the forest acts rather like a blanket, protecting them from freezing temperatures, snow and heavy rain. In late winter, the butterflies become active again and mate before beginning to fly north, and the females lay their eggs on young milkweed plants in the southern United States. It is their offspring, and the generations that follow, that complete the return journey to Canada, so no individual butterfly ever makes the full round trip.

Because the butterflies are so concentrated in winter, scientists estimate the size of the eastern population by measuring the area of forest that the colonies occupy. In the mid-1990s, this was around 18 hectares; in the winter of 2013–14, it was less than one hectare. Several causes have been identified. In Mexico, illegal logging has thinned parts of the forest, making it a less effective shelter, while in the United States, the widespread use of herbicides on farmland has removed much of the milkweed that once grew among crops. Extreme weather, which may become more frequent as the climate changes, is a further threat. The core of the wintering area is now protected as a biosphere reserve, and in 2008 it was declared a World Heritage Site. Across North America, people have also been encouraged to plant milkweed in gardens and on farms.