Academic Reading — Test 3

Reading passage

Few journeys in the natural world are as remarkable as the autumn migration of the North American monarch butterfly. Each year, as the days shorten and temperatures fall, hundreds of millions of these insects abandon their breeding grounds across the eastern United States and southern Canada and travel southwards to a handful of forested mountains in central Mexico. The distance covered can exceed four thousand kilometres. What makes the feat especially puzzling to scientists is that no single butterfly ever completes the round trip. The individuals that fly south in any given autumn are several generations removed from those that made the journey the year before. They have never visited the overwintering sites, yet they reach them with striking precision, settling in groves of oyamel fir that their distant ancestors once occupied. This multi-generation pattern lies at the heart of the mystery. During the warmer months, monarchs breed continuously, and most adults live only a few weeks. Three or four short-lived summer generations are produced as the population spreads northwards. The final generation of the season, however, is physiologically different. Triggered by the cooler temperatures and shorter daylight of late summer, these autumn monarchs enter a state known as reproductive diapause, in which their development of eggs and sperm is suspended. Freed from the demands of breeding, they can live for up to eight months and devote their energy to the long flight south and to surviving the winter. Because the migratory knowledge cannot be taught by a parent that has long since died, the route and the destination must be encoded in the insect's inherited biology rather than learned through experience. To hold a southward course, monarchs rely chiefly on the position of the sun. By keeping the sun at a consistent angle, an animal can in principle maintain a straight bearing across open country. The difficulty is that the sun moves across the sky throughout the day, so a fixed angle at dawn would point in quite a different direction by mid-afternoon. Monarchs solve this problem with a so-called time-compensated sun compass. An internal biological clock, located not in the brain but in the antennae, tracks the passage of time and continuously adjusts the butterfly's interpretation of the sun's position. In a celebrated experiment, researchers painted the antennae of captive monarchs black to block out light; the insects could still see the sun perfectly well, but their internal clocks were disrupted and they lost their sense of direction, demonstrating that the antennae are essential to the compass. The sun, of course, is frequently hidden behind cloud, and a navigation system that failed on overcast days would be of little use. Monarchs appear to carry a backup. Like a number of other migratory animals, they are sensitive to the Earth's magnetic field and can use it as an alternative source of directional information. Laboratory studies suggest that this magnetic sense depends on light at the blue end of the spectrum and may involve specialised molecules in the eyes. With both a sun compass and a magnetic compass available, the butterfly can keep travelling in roughly the right direction whether the sky is clear or grey. A compass alone, however, only indicates direction; it does not explain how the insects converge on such a small target. The overwintering colonies occupy only a few dozen sites, some no larger than a few hectares, in the Transvolcanic Belt of Mexico. Researchers suspect that, as the migrants near their goal, broad-scale orientation gives way to finer cues. The mountains offer cool but frost-free conditions, and the particular microclimate beneath the fir canopy may be detected through temperature, humidity or the lie of the land. Certain landmarks, such as mountain ridges, may funnel the descending butterflies towards the groves. Even so, exactly how millions of naive individuals pinpoint the same forests year after year remains incompletely understood. The monarch's migration is therefore best understood not as a single mechanism but as a layered system in which several tools reinforce one another. An inherited programme sets the southward goal; a time-compensated sun compass provides the primary heading; a magnetic sense supplies redundancy when the sun is obscured; and local environmental signals guide the final approach. The study of these butterflies has practical value as well as scientific interest, for the migratory population has declined sharply in recent decades. Loss of breeding habitat, the felling of overwintering forests and changes in climate all threaten a phenomenon that depends on a fragile chain of conditions stretching across an entire continent. Understanding how the insects find their way is a necessary step towards ensuring that they continue to do so.