Academic Reading — Test 104

Reading passage

For decades, one of the most enduring puzzles in animal behaviour has concerned the sea turtle. A female loggerhead turtle, having hatched on a particular stretch of sandy coast, may spend the next two or three decades roaming across an entire ocean basin before returning, as an adult, to lay her own eggs. Remarkably, she frequently comes back to the very beach where her life began, sometimes within a few kilometres of the precise spot. This phenomenon, known to scientists as natal homing, demands an explanation, for the open ocean offers few of the visual landmarks that guide navigation on land. The leading answer, supported by a growing body of evidence, is that turtles read the Earth's magnetic field. The planet behaves, in effect, like an enormous bar magnet. Its magnetic field varies in a predictable way across the surface of the globe. Two features of the field are especially useful to a migrating animal. The first is inclination, the angle at which the field lines meet the Earth; this angle is shallow near the equator and grows steeper towards the poles. The second is intensity, the overall strength of the field, which also changes with latitude. Because these two properties vary, most stretches of coastline carry a magnetic signature that is, to a close approximation, unique. In principle, an animal able to detect both inclination and intensity could determine not merely its direction of travel but also its approximate position, rather as a sailor once used instruments to fix a location at sea. Evidence that turtles exploit this information has come chiefly from controlled laboratory experiments. Researchers have placed young loggerheads in circular tanks surrounded by large coils of wire. By passing carefully regulated currents through these coils, the team could generate artificial magnetic fields that imitated the conditions found at distant points along a migratory route. When the turtles were exposed to a field matching a location to the north of their normal path, they tended to swim southward; when the field mimicked a southerly location, they swam north. In each case the animals oriented themselves in the direction that would, in nature, have carried them back towards their proper course. Crucially, the hatchlings had never made the journey before, which suggests that the basic responses are inherited rather than learned. How turtles actually sense magnetism remains uncertain, and two main hypotheses compete. One proposes that tiny crystals of a magnetic mineral within the animal's tissues physically respond to the field, transmitting a signal to the nervous system. The other suggests that the relevant mechanism is chemical, involving reactions in specialised cells that are sensitive to magnetic fields. Whichever proves correct, the behavioural results are not seriously disputed: turtles plainly obtain directional and positional information from the field, even if the underlying biology is still being unravelled. This gap between confident observation and uncertain mechanism is, in fact, common in the study of animal navigation. The magnetic explanation also accounts for an otherwise curious pattern in the nesting records. A subtle feature of the Earth's field is that it does not stay perfectly still; the magnetic signatures attached to particular beaches drift slowly over the years. If turtles were guided purely by an inherited magnetic target, then when two beaches happen to acquire very similar signatures, the turtles might confuse them. Studies of nesting data along certain coastlines appear to confirm this prediction. Where neighbouring beaches developed more alike magnetic signatures, the number of turtles nesting on them tended to rise; where the signatures of adjacent beaches diverged, nesting numbers fell. Such findings strengthen the case that the magnetic field is not a minor cue but a central organising principle of the turtles' behaviour. None of this means that magnetism is the only sense turtles employ. As an adult female approaches her destination after a journey of thousands of kilometres, the magnetic map may bring her to the general region, after which finer cues take over. Chemical traces in the water, the character of the local seabed, and possibly even particular sounds could help her settle on the final beach. Navigation in nature is rarely the product of a single faculty; rather, it combines several systems, each compensating for the limitations of the others. Yet the magnetic field appears to provide the foundation, the broad framework within which the other senses operate. Understanding this framework matters beyond pure curiosity, for sea turtles are threatened in many parts of the world, and conservation measures depend on knowing how and why these animals return to the beaches they do.