IELTS Academic Reading full test 4

How honeybees find their way

A A honeybee may fly several kilometres from its hive in search of flowers and then return directly home, often carrying a load of nectar or pollen. For an insect with a brain smaller than a grain of rice, this is a remarkable achievement. Over the past century, researchers have gradually discovered that bees do not rely on a single method of navigation. Instead, they combine several sources of information, and they can switch between them when one becomes unreliable. Much of what is known comes from simple but carefully designed field experiments, in which researchers mark individual bees with small dots of paint, move feeding stations to new positions, or release bees at unfamiliar places and watch the direction in which they fly.

B The most important of these is the sun. Bees use it as a compass, keeping track of the angle between their direction of flight and the position of the sun. Because the sun moves across the sky during the day, a fixed angle would soon lead a bee in the wrong direction. Experiments have shown, however, that bees adjust for this movement: they have an internal sense of time that allows them to compensate for the sun's changing position. Even when the sun is hidden behind clouds, bees can often still find their direction, because their eyes can detect patterns of polarised light in patches of blue sky.

C Bees also learn the appearance of the landscape around the hive. On their first flights, young foragers make short orientation flights, circling and turning to face the hive so that they can memorise nearby landmarks such as trees, buildings and the edges of fields. Later, these landmarks help them to follow familiar routes and to find the entrance to the hive at the end of a journey. If a hive is moved only a short distance, returning bees may fly to its old position and search there, which suggests how strongly they depend on the scene they have learnt.

D To judge distance, bees appear to measure how much the visual world moves past their eyes as they fly, a process sometimes described as an optical odometer. This idea was supported by an unusual experiment in which bees were trained to fly through a narrow tunnel lined with a detailed pattern. Because the pattern passed close to their eyes, it created a strong sense of movement. When they returned to the hive, the bees signalled that the food was much further away than it really was. In other words, the bees were not measuring distance directly, or counting the energy they used, but estimating it from what they saw. Over open water, where there is little visual detail, the same method would be expected to make bees underestimate how far they have travelled.

E That signal is perhaps the best-known part of bee behaviour. In the 1940s, the Austrian scientist Karl von Frisch described how a returning forager performs a 'waggle dance' on the vertical surface of the honeycomb. The bee runs in a straight line while shaking its body from side to side, then circles back and repeats the movement. The angle of the straight run, compared with the vertical, shows the direction of the food in relation to the sun, and the length of the waggle run shows the distance. Other bees follow the dancer closely and then leave the hive to search in the direction shown. Von Frisch's work was not accepted by everyone at first, but later studies confirmed his main conclusions, and he shared a Nobel Prize for his research in 1973.

F Understanding how bees navigate has practical value. Bees are important pollinators of many crops, and researchers want to know how changes to the landscape, such as the removal of hedges or the spread of large fields of a single crop, affect their ability to find food. Engineers have also drawn on bee navigation to design small flying robots that need to find their way using simple sensors and very little computing power. Perhaps the most important lesson, however, is a general one. The bee shows that reliable navigation does not require a large brain or a detailed map. By combining a few simple clues, checking them against each other and learning from experience, even a very small animal can solve a problem that challenged human travellers for thousands of years.

Turning seawater into drinking water

A About ninety-seven per cent of the water on Earth is in the oceans, and it is far too salty to drink or to use for most farming. For countries with long coastlines and little rainfall, removing the salt from seawater, a process known as desalination, has therefore long been an attractive idea. Today, desalination plants supply drinking water to millions of people, particularly in the Middle East, North Africa and parts of Australia, and the technology is attracting interest in many other regions affected by drought.

B There are two main approaches. The older one, thermal desalination, copies the natural water cycle: seawater is heated until it evaporates, and the vapour is then cooled so that it condenses as fresh water, leaving the salt behind. This method is reliable, but heating large volumes of water requires a great deal of energy, so it has been used mostly where fuel is cheap. The second approach, reverse osmosis, uses pressure rather than heat. Seawater is pumped at high pressure against a semi-permeable membrane, a thin barrier whose pores allow water molecules to pass through but hold back most of the dissolved salt. In a large plant, thousands of these membranes are rolled into tubes and arranged in long rows, and the water may pass through more than one stage to reach the quality needed for drinking. Because it generally uses less energy than thermal methods, reverse osmosis is now the most common technology in new plants.

C Reverse osmosis is not as simple as it may sound, however. The membranes are delicate and easily blocked by sand, algae and other particles, so the seawater must first go through pre-treatment, in which it is filtered and sometimes treated with chemicals. After the salt has been removed, minerals are often added back to the water, both to improve its taste and to stop it from corroding the pipes that carry it to homes. Each of these stages adds to the cost and complexity of running a plant.

D Desalination also produces a by-product. For every litre of fresh water, a plant creates a quantity of brine, a liquid that is considerably saltier than ordinary seawater. Most coastal plants return the brine to the sea. To protect marine life near the outlet, engineers often release it through a series of diffusers, which spread it out so that it mixes quickly with the surrounding water. Environmental scientists continue to monitor the areas around outlets, since a high concentration of salt in one place could harm organisms living on the sea floor.

E Energy remains the most debated issue. Even with efficient membranes, pumping seawater at high pressure uses far more electricity than collecting rainwater in a dam. If that electricity comes from fossil fuels, desalination adds to greenhouse gas emissions, the very problem that is making some regions drier. For this reason, some operators buy electricity from wind or solar farms to match the energy their plants use, and researchers are developing membranes that require lower pressures.

F In Australia, several state capitals built large desalination plants during and after the Millennium Drought, a long and severe drought that ended in 2009. Some of these plants have run continuously, while others have been used mainly as insurance and switched on only when dam levels fall. Keeping a plant on standby is not free, however: equipment must be maintained and staff kept ready, so the cost of a plant continues even in years when little water is produced. Water planners generally see desalination as one option among several, alongside reducing leaks, encouraging households to use water efficiently and recycling treated wastewater. Each option has its supporters, and the right mix depends on local conditions such as population growth, the reliability of rainfall and the distance between the coast and the people who need the water. Desalination's great advantage is that it does not depend on rainfall; its main disadvantages are cost and energy use.

Seeing with sound: how bats find their way in the dark

A Bats are the only mammals capable of true, powered flight, and with well over a thousand known species, they make up a large share of all mammal species on Earth. They are found on every continent except Antarctica, in habitats ranging from deserts to rainforests and city centres. Many of them are active at night, and a large number feed on flying insects, which they catch in complete darkness while flying quickly between trees, buildings and cave walls. How they managed to do this without crashing puzzled naturalists for a very long time. The answer, when it finally came, revealed a sense that humans do not have at all.

B The first important clues came from experiments in the 1790s by the Italian scientist Lazzaro Spallanzani. He found that bats which could not see were still able to fly around a room without hitting obstacles, but that bats whose ears had been blocked flew clumsily and collided with objects. Other researchers repeated and extended this work and reached a similar conclusion: hearing, not sight, seemed to be the key. Yet nobody could hear the bats making any sound while they flew, and many scientists doubted the conclusion. Some argued instead that bats felt their way using a sense of touch in their wings, which could detect small changes in the air.

C The mystery was solved more than a century later. In the late 1930s, Donald Griffin, then a student at Harvard University in the United States, took some bats to the physicist George Washington Pierce, who had built a device that could detect sounds too high for humans to hear. The device showed that the bats were producing a constant stream of high-pitched calls. In further experiments with Robert Galambos, Griffin showed that bats use the echoes of these calls to avoid obstacles. In 1944, Griffin introduced a word to describe the ability: echolocation.

D Echolocation works by comparing a sound with its echo. A bat produces a call, through its mouth or, in some species, through its nose, and the sound bounces back from any object in its path. The time between the call and the echo tells the bat how far away the object is, while small differences between what reaches each ear help it to judge direction. Very high-pitched sounds are especially useful because they reflect well from small objects, such as insects. As a bat closes in on its prey, it calls more and more rapidly, producing a fast series of calls known as a 'feeding buzz' just before it makes the catch. Bats also change the type of call to suit their surroundings: in open air, many species use longer calls, while close to trees, walls and other obstacles they switch to shorter ones, so that each call has finished before its echo returns from nearby objects. Some calls are extremely loud, so to avoid being deafened, many bats briefly contract small muscles in the middle ear each time they call, which reduces their sensitivity to their own voice while leaving them ready to hear the returning echo.

E Bats' prey have not stood still in this contest. Many moths have simple ears that are sensitive to the frequencies used by hunting bats. When a moth hears a bat approaching, it may change direction suddenly, spiral or drop towards the ground, making it much harder to catch. Some tiger moths go further and produce clicking sounds of their own. Researchers believe that these clicks may warn bats that the moth tastes unpleasant, or in some species may interfere with the bat's ability to process its echoes.

F Not all bats rely on echolocation in the same way. Most of the large fruit bats of Africa, Asia and Australia, including the flying foxes, do not echolocate with their voices at all; they have large eyes and a strong sense of smell, which they use to find fruit and flowers. A few fruit bats produce simple clicks with their tongues instead. Bats are also not the only animals with this ability. Dolphins and other toothed whales use echolocation under water, and a small number of birds that nest in dark caves, such as the oilbird, use clicks that people can hear. Ships' sonar systems work on a similar principle, although they were developed separately, without help from the study of bats.