Academic Reading — Test 101

Reading passage

The wanderer butterfly, known in much of the world as the monarch, is a relatively recent arrival to the Australian continent. Although its bold orange-and-black colouring is now a familiar sight in suburban gardens and along country roads, the species is generally believed to have reached Australia only in the second half of the nineteenth century. It is thought to have crossed the Pacific Ocean by a combination of natural dispersal and accidental human assistance, establishing itself once a suitable supply of its preferred food plants, the milkweeds, had spread across the eastern states. Unlike many native Australian insects, the wanderer therefore offers researchers an unusual opportunity to study how a migratory species adapts its behaviour to an entirely new landmass. In its North American homeland, the monarch is celebrated for one of the most remarkable migrations in the animal kingdom, with populations travelling thousands of kilometres between Canada and central Mexico each year. The Australian wanderer does not undertake journeys on anything like this scale. Instead, populations in the cooler south-eastern regions move shorter distances to escape the worst of the winter, gathering in sheltered coastal valleys where the climate remains comparatively mild. In these overwintering sites, sometimes only a few hectares in extent, thousands of butterflies may cluster together on the trunks and branches of trees, their folded wings forming dense, drab-coloured curtains that open into brilliant colour when the sun warms them. The precise locations of these clusters can remain remarkably stable from one year to the next, even though the individual butterflies arriving each autumn have never visited the site before. This last point lies at the heart of what makes the wanderer's navigation so puzzling. The butterfly is a short-lived insect, and no single individual completes a full annual cycle of departure and return. The generation that flies to the overwintering grounds is not the same generation that left them in spring; several generations may separate the two. Consequently, the knowledge of where to go cannot be passed directly from parent to offspring through learning or example. The route and the destination must instead be encoded in some way that each new generation can follow without instruction, a feat that continues to challenge scientists seeking to explain it. A central component of the wanderer's navigational toolkit is a so-called sun compass. By using the position of the sun in the sky, the butterfly can hold a consistent heading over long distances. However, because the sun moves steadily across the sky during the day, a fixed orientation towards it would send the insect in a curving and ultimately useless path. To correct for this, the butterfly relies on an internal clock, a biological timekeeping mechanism that allows it to compensate for the changing position of the sun as the hours pass. The combination of clock and compass enables the insect to maintain a stable direction from morning until evening. Researchers have shown that interfering with this internal clock, for instance by exposing captive butterflies to artificial light cycles, can predictably shift the direction in which they attempt to fly, demonstrating just how tightly the two systems are linked. The sun, of course, is not always visible. On overcast days, when thick cloud obscures the sun's disc, the butterfly appears to fall back on a secondary cue: the pattern of polarised light in the sky. Sunlight scattered by the atmosphere produces a distinctive pattern that the human eye cannot detect but which many insects can perceive, and this pattern persists even when the sun itself is hidden. There is also evidence from related species that butterflies may be sensitive to the Earth's magnetic field, providing a further backup that does not depend on the sky being clear at all. Whether the Australian wanderer makes significant use of such a magnetic sense remains a matter for continuing investigation, and firm conclusions have yet to be reached. What emerges from this research is a picture of an insect that does not depend on any single method of finding its way. Rather, the wanderer seems to draw on a hierarchy of cues, turning to whichever is most reliable under the prevailing conditions. This flexibility may help to explain how the species has been able to colonise a continent so different from the one in which its migratory instincts first evolved. For Australian scientists, the wanderer remains a valuable subject precisely because it is a newcomer: by watching how an immigrant species fits ancient navigational machinery to unfamiliar terrain, they hope to learn something general about how animal migration arises and how it might be reshaped as climates and landscapes change.