IELTS Academic Reading Practice Test 417

A visitor walking quietly through the rainforest of north-eastern Queensland may hear a low, booming rumble that seems to come from the earth rather than from any animal. The sound belongs to the southern cassowary (Casuarius casuarius), a flightless bird that can stand almost two metres tall and is among the heaviest birds alive. Its body is covered in glossy black feathers that hang like coarse hair, its head and neck are bare and coloured in vivid blue and red, and a helmet-like structure of hard keratin rises from the top of its skull. Few Australians have seen one in the wild, because the bird keeps to dense forest and avoids people.

Ornithologists recognise three species: the southern cassowary, the northern cassowary of New Guinea and the much smaller dwarf cassowary of the New Guinea highlands. All three belong to an ancient group of flightless birds known as ratites, which also includes the ostrich, the emu and the kiwi. Ratites share a flat breastbone, without the keel to which flight muscles are attached in other birds, and their feathers lack the tiny hooks that lock ordinary feathers into a smooth surface. Genetic studies place the cassowaries closest to the emu, and the two are usually grouped together in the same family.

The function of the casque, as the structure on the head is called, has been argued over for more than a century. It is not a horn in the sense that a cow's is: a sheath of keratin covers a light, spongy core, the whole structure grows slowly throughout the bird's life, and it is not used as a weapon. Suggested purposes include shielding the skull from thorns and falling branches, pushing aside vegetation as the bird moves through the undergrowth, and signalling age or status to other cassowaries. Because the casque contains blood vessels, some researchers have also proposed that it helps to regulate the bird's temperature, although no single explanation has yet won general agreement.

Cassowaries are fruit-eaters, though they also take fungi, insects, snails and the occasional small vertebrate. They feed largely on fruit that has fallen to the forest floor, and they can swallow fruit far larger than most other birds can manage. This habit makes the southern cassowary one of the most important seed dispersers in Australian rainforests. Some trees appear to depend on the bird almost entirely, and there is evidence that the seeds of certain species germinate more readily after passing through a cassowary's gut. Because a single bird may wander over several square kilometres, it carries seeds well away from the parent tree.

Females are larger than males and, unusually among birds, take no part in raising the young. A female lays three or four eggs in a shallow scrape on the ground, lined with leaves and grass, and then leaves the site. The male incubates the eggs for about seven weeks and afterwards guards the striped brown chicks for up to nine months, showing them where to find food and driving off intruders. Cassowaries are long-lived birds, and individuals have survived for more than thirty years in captivity.

The cassowary's reputation for danger comes from its legs. Each foot carries three forward-pointing toes, and on the inner toe of each foot is a claw shaped like a dagger, which may be more than ten centimetres long. When cornered or defending chicks, the bird kicks out with both feet at once. Serious attacks are rare but well documented, and a few have been fatal: the last fatal attack in Australia took place in 1926, when a teenager near Mossman was knocked down and struck in the neck. In 2019 a man in Florida, where cassowaries are kept in private collections, was also killed by one of his own birds.

The bird's voice is as remarkable as its appearance. Its main call is a booming note so low that much of its energy lies below the range of human hearing. It can travel a long way through dense forest, since low frequencies are less easily scattered by trunks and foliage than high ones. Cassowaries also produce hisses and other sounds, but the boom is the call most often heard. Outside the breeding season the birds are usually solitary, and they call mainly to advertise their presence to neighbours.

For all its formidable appearance, the southern cassowary is in trouble. Its Australian range is confined to the Wet Tropics, a strip of forest between Townsville and Cooktown, and much of that forest has been cleared for farming and housing at some time. What remains is fragmented, and roads cut through it. Collisions with vehicles are among the commonest causes of recorded deaths; dogs are another, and hand-feeding by residents and tourists encourages birds to approach roads and houses. The Australian population is thought to number only a few thousand individuals, and it is officially listed as endangered.

Conservation efforts have concentrated on keeping patches of forest connected and on reducing the number of birds killed on the roads. Crossings have been built beneath or over the highway at places where cassowaries are frequently struck, warning signs have been erected, and residents have been asked not to feed the birds. None of this addresses the underlying problem of a shrinking habitat, but it has reduced deaths in some areas.

On a warm summer evening in many parts of the world, the darkness above a meadow begins to blink. The insects responsible are not flies at all but beetles, and the family to which they belong, Lampyridae, contains roughly two thousand described species. Most live in warm, humid regions, where their larvae find damp soil and plenty to eat. Not every adult glows: in some species the adults are active by day and find their mates by smell, while in others the females are wingless and never leave the ground.

The light comes from a specialised organ in the rear segments of the abdomen. It consists of layers of cells known as photocytes, packed with the substances the reaction requires, and it is threaded with a dense network of fine air tubes that deliver oxygen. Behind the photocytes lies a layer of crystals that reflect light outwards, and the cuticle above them is largely transparent, so that little of the light is lost inside the body. In some species the organ is divided into two parts, and the pattern of bright and dark patches differs between males and females.

The reaction itself is a form of chemiluminescence. Two molecules are central to it: luciferin, which actually emits the light, and luciferase, an enzyme that brings luciferin together with oxygen. In the presence of oxygen and of ATP, the molecule that carries energy within cells, luciferase converts luciferin into a compound in an excited state. The compound immediately releases the surplus energy as a photon of visible light. The colour depends on the structure of the luciferin and on the shape of the enzyme's active site; most species emit a yellow-green light with a wavelength of about 560 nanometres, but green and orange occur too.

The reaction is remarkably efficient. An incandescent bulb wastes most of its energy as heat, whereas a large share of the energy released in the firefly's light organ emerges as light and very little as heat, so the insect's glow is cool to the touch. This efficiency has attracted interest far beyond biology. Because the reaction consumes ATP, the enzyme can be used in the laboratory to reveal very small quantities of it, and tests based on that principle are employed to check whether surfaces in food factories and hospitals are really clean.

A glow that cannot be switched off would be of little use for signalling, and the insect controls its light with gas. Oxygen reaches the photocytes through the fine tubes of the tracheal system, and the supply is regulated by nitric oxide, a small molecule that interferes with the ability of the cells' mitochondria to consume oxygen. While nitric oxide is present, oxygen is left for the light-producing reaction and the organ shines; when the gas disperses, the mitochondria take the oxygen again and the light fades. This can happen within a fraction of a second, so the insect produces a precise flash rather than a steady glow.

In most species that glow as adults, light is a courtship signal. Males fly and flash in a pattern characteristic of their species, with a particular number of pulses at a particular interval, and females waiting in the vegetation answer after a fixed delay. A male that receives the right answer closes in, and the exchange continues until pairing takes place. Because each pattern is specific to its species, the signals prevent wasted encounters between close relatives that share the same habitat, and in some species the brightness of a male's flash appears to influence the female's choice.

Not every flash is a courtship signal. Females of the genus Photuris are predators that exploit the code of their relatives. When a male of another species flashes, a Photuris female answers with the reply he expects, lures him within reach and eats him. The deception brings her more than a meal. Males of the prey species carry defensive steroids known as lucibufagins, which make them unpleasant to predators such as spiders and birds, and a female that has swallowed several of them passes the protection on to her eggs.

The young of many species glow as well, though they have no mate to attract. Larvae live in leaf litter and damp soil, where they hunt snails and worms, and their light is generally interpreted as a warning that advertises the same defensive chemicals. Some species even lay eggs that glow. Among the European glow-worms, which belong to the same family, it is the wingless adult female, not the larva, that shines most strongly, climbing a stem at dusk to display a steady green light.

Several questions remain open. The metabolic cost of the display, once assumed to be high, has in some experiments proved modest, which is puzzling given how conspicuous light makes an insect to predators. The forces that cause flash patterns to diverge between populations are also debated: sexual selection is clearly involved, but whether it drives the formation of new species or merely accompanies it is unresolved. And although many researchers believe that artificial light, pesticides and the loss of habitat are reducing numbers, the evidence is patchy, because few places hold long-term records.

What is certain is that the light carries information as well as illumination. It tells a mate which species is flashing, warns a hunter that the meal would be bitter, and, in the case of Photuris, invites a victim to come closer.

In 2020 California became the first American state to require solar panels on most new homes, and since April 2025 Tokyo has obliged the largest housing companies to fit them to new houses. Several German states have adopted comparable rules, and a European Union directive agreed in 2024 sets deadlines for installing solar on new and heavily renovated buildings. In the same period the price of a panel has fallen sharply, which is one reason the idea has moved from the margins to the centre of energy policy. Such laws remain unusual, but the direction of travel is clear. The question is whether compulsion is the right instrument, or whether it achieves less, at greater cost, than other ways of encouraging the same technology.

The strongest argument for a mandate is that it is cheaper to fit panels while a house is being built than to add them afterwards. Scaffolding is already in place, the roof is accessible, and the wiring can be planned together with the rest of the electrical system rather than threaded through finished walls. Because the cost can be included in the mortgage, it is spread over decades instead of being met in a single payment by an owner who may have just spent everything on moving in. Installers also tend to favour mandates, since a predictable flow of work allows them to train staff and buy equipment in advance rather than chasing bursts of demand.

Environmental groups make a different case. Buildings account for roughly a third of the energy used in Europe and North America, and a house built this year will still be standing in the 2060s, so the panels fitted to it will go on displacing fossil generation for decades. From this point of view the relevant calculation is not the price the first owner pays but the emissions avoided over the life of the building. Campaigners add that a roof is the one power station almost every household already owns, and that a rule is the quickest way to bring solar to people who would never have considered it.

Housing economists are less enthusiastic. A requirement raises the price of a new home, and in cities where housing is already expensive that increase falls hardest on the buyers with the least room to absorb it. Their objection is not that panels are worthless but that the purchaser is compelled to buy them, when the same money might have paid for better insulation, a more efficient heating system or simply a smaller mortgage. A subsidy, they argue, would leave the choice with the household; a mandate removes it and calls the result progress. Some also contend that a rule transfers money from young buyers to an industry that is already expanding quickly. The same sum, they say, would cut more emissions if it were spent on the grid.

There is also the awkward fact that not every roof is suitable. Panels on a roof shaded by trees, or facing away from the sun, may generate disappointingly little, and a compulsory rule must either grant exemptions, which weaken it, or compel installations that make little sense. Nor is orientation the only constraint: some designs leave too little space, and some roofs will need strengthening before they can carry the extra weight. A rule that ignores such details produces the appearance of progress without its substance, and it gives solar technology a reputation for being imposed rather than chosen.

Network operators, for their part, welcome clean generation but worry about its timing. Rooftop systems produce most of their output in the middle of the day, when many households are out, and in sunny regions with a great deal of solar the wholesale price of electricity now falls towards zero, or below it, at exactly those hours. In parts of California and Australia, so much solar is generated at midday that some of it is wasted. Additional panels are therefore worth less than their owners expect unless the power can be stored in a battery or exported through a connection with spare capacity. Where local cables cannot carry more power, the operator may cap what a household is allowed to export, which lowers the value of an installation still further.

Evidence about house prices is more encouraging than the critics allow. Researchers at the Lawrence Berkeley National Laboratory, who have examined thousands of sales across the United States, have repeatedly found that homes with solar systems sell for more than comparable homes without them. The premium varies widely between markets and depends heavily on local electricity prices. That does not prove that the panels are worth what they cost to install, but it does suggest that buyers do not regard them as a burden.

My own view is that compulsion is defensible only where it is designed with care. A blanket rule will produce installations that are expensive and useless on unsuitable roofs, and it will be resented by the households that pay for them. A better policy would oblige builders to make every new home ready for solar panels — with the wiring, the roof strength and the orientation that will eventually be needed — and would then rely on incentives to encourage installation. Preparing a roof costs little and can be checked at the planning stage, whereas the decision to fit the panels themselves should depend on how long a household expects to remain in the house.