Academic Reading — Test 178

Reading passage

Most animals rely on light to build a picture of their surroundings, but a remarkable minority have evolved an entirely different solution. Instead of seeing the world, they listen to it. By emitting bursts of sound and interpreting the echoes that return, these creatures construct a detailed map of objects, obstacles and prey, even in complete darkness. This ability, known as echolocation or biological sonar, has arisen independently in several unrelated groups of animals, a striking example of how nature can converge on the same answer to a shared problem. The principle behind echolocation is straightforward, even if the underlying mechanics are sophisticated. An animal produces a sound and waits for it to strike a surface and bounce back. Because sound travels at a known and relatively constant speed through air or water, the time that passes between the call and the returning echo reveals how far away an object lies. A short delay indicates something nearby, while a longer delay points to a more distant surface. The strength and quality of the echo carry further information: a faint return may suggest a small or soft object, whereas a sharp, strong echo often signals something large or hard. By repeating this process many times each second and comparing the results, the animal can judge not only distance but also the size, shape, texture and movement of whatever lies ahead. Bats are the best-known practitioners of this skill. Many species hunt flying insects at night, producing rapid clicks and chirps through the mouth or, in some cases, the nose. These calls are typically pitched far above the range of human hearing, in the ultrasonic band, which allows the animal to detect very small targets with great precision. As a hunting bat closes in on its prey, it dramatically increases the rate of its calls, a burst sometimes described as a feeding buzz, giving it an almost continuous stream of updates during the final moments of pursuit. Crucially, bats do not merely hear their own echoes; their brains must separate the faint returning sound from the much louder outgoing call, a feat of timing and neural processing that scientists still find astonishing. In the oceans, toothed whales and dolphins have developed their own form of biological sonar, suited to a medium in which sound travels far more efficiently than light. Dolphins generate clicks not with the larynx but by passing air through structures in the nasal passages. These clicks are focused and directed forward by a fatty organ in the forehead called the melon, which acts somewhat like a lens for sound. The returning echoes are received mainly through the lower jaw, which channels the vibrations to the inner ear. Using this system, a dolphin can distinguish between objects of different materials and detect prey buried beneath sand on the sea floor. Some researchers believe dolphins can even gain information about the internal structure of an object, a capability that has no direct equivalent among land-based echolocators. Echolocation is not confined to bats and marine mammals. A handful of birds that nest deep in dark caves, such as certain swiftlets and the oilbird of South America, produce audible clicks to navigate where no light reaches. Their system is far less refined than that of bats, allowing them to avoid cave walls rather than to capture tiny prey, but it serves their needs well. Perhaps most surprisingly, some blind humans have taught themselves a simple form of echolocation by making clicking sounds with the tongue and listening to the reflections, demonstrating that the human brain retains a latent capacity to process such information. Studies suggest that, with practice, these individuals can detect the presence and rough position of nearby objects. The repeated, independent evolution of echolocation across such different animals underlines its value as a survival strategy. It permits hunting and movement in conditions where vision is useless, opening up niches that would otherwise remain closed. Yet the ability also illustrates the limits of any single sense, for echolocation has its own weaknesses: it can be disrupted by background noise, and some prey species have evolved countermeasures, including the capacity to hear approaching calls and take evasive action. The ongoing contest between predators that listen and prey that listen back continues to shape both groups, a quiet reminder that sound, no less than sight, has been a powerful force in the history of life.