Academic Reading — All-Types Test 11

Reading passage

The Great Journey of the Monarch A. Few insects have captured the public imagination as thoroughly as the monarch butterfly. Recognisable by the deep orange of its wings, veined and bordered in black and flecked with white, the monarch is celebrated above all for a feat of endurance that seems improbable for so fragile a creature. Each autumn, monarchs living east of the Rocky Mountains abandon the cooling meadows of Canada and the northern United States and travel south, sometimes covering more than four thousand kilometres, to a small cluster of mountain forests in central Mexico. There they wait out the winter, packed in their millions onto the branches of oyamel firs, before beginning the long return the following spring. B. What makes the migration so remarkable is that no single butterfly completes the round trip. The monarchs that fly south in autumn are a special generation, sometimes called the Methuselah generation because they live for as long as eight months, far outlasting the four or five weeks granted to their summer relatives. These long-lived insects reach Mexico, overwinter, and start north again, but they die along the way, and it falls to their descendants to finish the journey. Three or four short-lived generations may be born and die over a single summer before autumn once more produces the travellers. The insects that arrive in Mexico have therefore never been there; the route exists only in their inheritance. C. How they find their way has occupied biologists for decades. The prevailing explanation involves a kind of internal compass. Monarchs appear to read the position of the sun, adjusting for the time of day by means of a biological clock housed, unusually, in their antennae rather than their brains. When the sky is overcast and the sun is hidden, they are thought to fall back on the Earth's magnetic field, sensing its inclination to hold a southward heading. Neither mechanism alone would suffice, and it is the combination that keeps a scattered population converging, over weeks, on a few hectares of forest. Experiments in which the antennae of captive insects were painted over, blocking the light that reaches the clock, left the butterflies disoriented, unable to hold a steady course; the finding did much to establish where the timing device actually sits. D. The oyamel forests themselves are not chosen at random. At an altitude of around three thousand metres, they occupy a narrow band of conditions that suits an overwintering butterfly. The dense canopy acts as a blanket and an umbrella at once: it traps enough warmth to keep the insects from freezing, yet shields them from the rain that would otherwise soak their bodies and make them fatally vulnerable to frost. Temperatures cool enough to slow the butterflies' metabolism, so that they burn their fat reserves slowly and survive until spring. Remove the trees, and this delicate equilibrium collapses. E. It is here that the monarch's story turns anxious. The forests have been thinned by illegal logging, and although protection has tightened, the wider threat now comes from far beyond Mexico. Across the breeding grounds of North America, the milkweed on which monarchs depend has been retreating. Female monarchs lay their eggs only on milkweed, and the emerging caterpillars eat nothing else; the plant supplies the toxins that render the adult butterfly distasteful to predators. The spread of herbicide-tolerant crops has allowed farmers to eliminate milkweed from vast areas of former grassland, and the loss of this single plant may do the migration more harm than any chainsaw. F. Conservationists have responded by encouraging the planting of milkweed in gardens, on roadsides and along the margins of fields, hoping to stitch together a corridor of habitat spanning the continent. The results are still uncertain, and counts of the overwintering colonies swing sharply from one year to the next, driven by weather as much as by policy. Yet the monarch endures as a rare emblem of a natural process that crosses national borders and depends, for its survival, on the cooperation of the three countries its wings pass over.