Academic Reading — Test 4

Reading passage

The platypus, a semi-aquatic mammal native to the rivers and streams of eastern Australia, is among the most unusual creatures in the animal kingdom. It lays eggs rather than giving birth to live young, possesses a broad tail reminiscent of a beaver, and feeds beneath the surface of freshwater systems that are frequently clouded with sediment. Because much of its foraging takes place in water so murky that vision is of little use, the platypus has evolved a remarkable alternative means of locating prey. Rather than relying on its eyes, it detects the faint electrical signals produced by the small invertebrates on which it feeds, a sensory ability known as electroreception. When a platypus dives, it characteristically closes its eyes, its ears and its nostrils, effectively shutting down three of the senses on which most mammals depend. This would seem to leave the animal almost helpless underwater, yet in practice it hunts with great precision. The key lies in its distinctive bill, which is soft and pliable rather than hard like a duck's beak, despite a superficial resemblance. The surface of the bill is densely covered with tens of thousands of tiny sensory receptors. These fall into two broad categories: electroreceptors, which respond to electrical fields, and mechanoreceptors, which respond to pressure and movement in the surrounding water. Working together, these two systems allow the platypus to build a detailed picture of its environment without seeing it. The electroreceptors function because all living animals generate weak electrical currents whenever their muscles and nerves are active. When a shrimp, insect larva or other small creature moves, flexes or even merely twitches, it emits a minute electrical signal into the water around it. The platypus is able to register these signals through the receptors in its bill. By sweeping its head from side to side as it swims along the riverbed, the animal scans the water much as a person might wave a metal detector across the ground. Prey that would otherwise be invisible in the gloom is thereby betrayed by its own biological activity. Detecting an electrical signal is only part of the challenge; the platypus must also work out where the signal is coming from. Researchers believe the animal achieves this by combining information from its two separate sensory systems. The electrical signal from moving prey travels through water almost instantaneously, whereas the pressure wave created by the same movement travels considerably more slowly. The platypus appears to measure the tiny delay between the arrival of the electrical signal and the arrival of the mechanical disturbance. The size of this gap indicates how far away the prey is: a longer delay means a more distant target. In this way the animal can estimate both the direction and the distance of its next meal, a feat of sensory calculation that is performed entirely underwater and in darkness. This dual sensory system is exceptionally sensitive. Laboratory experiments have shown that a platypus can respond to electrical fields far weaker than those a human being could ever perceive unaided. Such sensitivity is essential, because the signals given off by small aquatic invertebrates are extremely faint and are easily lost amid background interference. The receptors are not spread evenly across the bill; they are concentrated most heavily towards the front and along the edges, the regions that first encounter prey as the animal sweeps its head through the water. This arrangement helps the platypus to pinpoint a target quickly once a signal has been detected. Electroreception is comparatively rare among mammals, and the platypus is one of the very few that possess it. The ability is far more common in fish and amphibians, many of which live permanently in water where electrical signals travel well. Among land-dwelling mammals it is virtually unknown, partly because air, unlike water, conducts electricity poorly and so cannot carry such signals over any useful distance. The platypus therefore represents an unusual case of a warm-blooded, air-breathing animal that has adapted a sense more typical of cold-blooded aquatic life. Scientists regard it as a striking example of how evolution can equip a species with specialised tools suited precisely to the conditions in which it lives, allowing the platypus to thrive in an environment that would defeat most hunters relying on sight alone.