Academic Reading — All-Types Test 26

Reading passage

The migration of the monarch butterfly A Every autumn, across the eastern half of North America, a remarkable event unfolds almost unnoticed above the heads of millions of people. Monarch butterflies, fragile insects weighing scarcely more than a paperclip, abandon the fields and gardens where they have spent the summer and set off on a journey of some four thousand kilometres to a cluster of mountain forests in central Mexico. There, in a handful of high, cool groves, they gather in their tens of millions, blanketing the trees so densely that whole branches sag beneath their collective weight. When spring returns, the survivors turn north again, and the cycle begins anew. That so slight a creature should undertake so vast a migration is one of the more astonishing feats in the natural world. B What makes the achievement stranger still is that no single butterfly completes the round trip. The monarchs that set out from Mexico in spring live only a few weeks; they travel north, breed, and die, leaving their offspring to carry on. It takes three or four such short-lived generations to repopulate the northern breeding grounds over the course of the summer. Then, as the days shorten and the weather cools, a very different generation is born. These late-summer monarchs do not breed at once. Instead they enter a state of suspended development, their bodies primed for endurance rather than reproduction, and it is they who make the entire southward flight and survive the winter, living as long as eight months. The insect that returns to Mexico has never been there before, nor has any butterfly it ever met. C How the monarchs find their way remains only partly understood, though decades of patient study have revealed some of the mechanism. The butterflies appear to rely on the position of the sun as a compass, adjusting for the fact that it moves across the sky as the day progresses. To make this correction they consult an internal clock, housed partly in their antennae, which keeps track of the time of day. Researchers demonstrated the antennae's role with a simple experiment: butterflies whose antennae had been coated with paint, blocking the light, could no longer hold a steady course, while those with clear antennae flew true. On overcast days, when the sun is hidden, the monarchs are thought to fall back on the Earth's magnetic field, giving them a second means of orientation when the first fails. D The destination itself is no accident. The Mexican groves where the monarchs overwinter sit at a particular altitude in the mountains, high enough to be cool but not freezing. The temperature there is critical. If the forest were warmer, the butterflies would burn through their limited fat reserves too quickly and starve before spring; if it were colder, they would simply freeze. The dense canopy of the fir trees acts as a kind of blanket and umbrella at once, trapping warmth on cold nights and sheltering the insects from rain and hail. Crowding together in enormous numbers, the monarchs further buffer themselves against the cold. A grove that meets these exacting conditions is a rare thing, which is one reason the entire eastern population depends on so small a patch of ground. E That dependence is also the source of the monarchs' peril. Because almost the whole population funnels into a few dozen hectares of forest each winter, any damage to those groves threatens the migration as a whole. Illegal logging has thinned the protective canopy in places, letting in the cold and the wet against which the trees are meant to guard. Far to the north, meanwhile, the wildflower on which monarch caterpillars exclusively feed has been cleared from vast stretches of farmland, leaving the butterflies with fewer places to breed along their route. The numbers reaching Mexico have fluctuated sharply from year to year, and conservationists warn that the migration, though not the species itself, could one day fail. Protecting it will require cooperation across a continent, from the Mexican forests to the fields of the far north.