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Part 1
Read the text and answer questions 1–13.
For more than 100 years, scientists have argued over exactly what a panda is. Now, finally, with the help of DNA testing, the panda has been admitted to the ursidae (bear) family, and the spectacled bear of South America has been confirmed as its closest living relative.
In 1869, French Jesuit missionary Pere David first described the giant panda to western science. With just a pelt and reported sighting to go on, he classified it as a bear. However, the following year, zoologist Alphonse Milne Edwards dissected the first specimen and concluded that it had more in common with the red panda, a member of the raccoon family. For more than a century, scientists quarrelled over whether the giant panda belonged to the bear family, the raccoon family or a separate family of its own.
They had good reason to be confused. The giant panda shares many physical characteristics with the red panda. Both have evolved to feed on bamboo, grasping and eating it in the same way, with similar teeth, skulls and forepaws. They also both have a distinctive cry which they use to communicate with others in their group.
In the mid-1980s there were several studies involving DNA comparisons between the species. The first investigations linked the giant panda with bears, but in 1991 further tests contradicted these findings and placed it in the raccoon family with the red panda. By the year 2000, approximately twelve studies had been completed, and all except two placed the panda in the bear family. The data from these two studies was reanalysed by other researchers, who finally concluded that the giant panda was indeed a bear.
Today, there are eight species of bear. Along with dogs, their closest relatives, cats, raccoons and weasels, they belong to the order Carnivora, a group of meat-eating predators that evolved some 57 million years ago. The ancestors of modern bears split from this group about 34 million years ago, and today the panda is our oldest living bear, followed by the spectacled bear. Both are survivors of an ancient lineage dating back 18 million years. The rest — the brown, black, polar, Asiatic black, sloth and sun bears — are relatively modern, dating back four to five million years.
Researchers have found that the spectacled bear and the panda have several physical features in common. The spectacled bear’s muzzle is comparatively short and it has blunt molar teeth and large jaw muscles, which are good for grinding fibrous vegetation — vegetation such as bamboo. Indeed, scientists in Venezuela have found that bamboo makes up 70% of the diet of some spectacled bear populations. For most spectacled bears, however, the bromeliad, a tropical plant with fleshy leaves, is their main food source. Most species of bromeliad grow in trees, and spectacled bears therefore have to be adept tree climbers because they spend their lives foraging for these plants, as well as fruits, in the cloud forest of the Andes.
The giant panda’s diet is famously dull, with bamboo representing 99% of its intake. This is rather strange given that its physiology is typical of a carnivore and it has no special adaptation for digesting cellulose, the main constituent of plant cell walls. A panda manages to digest only about 17% of the bamboo it eats (a deer living on grass achieves 80% efficiency). It typically feeds for 14 hours a day, consuming 20 kg or more of bamboo. Unable to store fat effectively, it continues eating in the bitterly cold winter, at a time when many other bears hibernate.
With such a specialised diet, the giant panda has evolved a sixth digit, a prehensile elongated wrist bone called the radial sesamoid. They use this ‘false thumb’ to roll bamboo leaves into fat, cigar-shaped wads which they then sever using their powerful jaws. They feed mainly on the ground but are capable of climbing trees as well. The spectacled bear is a more frequent climber and will even climb spiky cacti plants to reach fruit at the top. They also construct tree nests to act as a bed as well as a platform to feed from fruit-laden branches.
Very occasionally, the giant panda supplements its diet with meat which it scavenges. Spectacled bears eat carrion, too, and some have been known to kill small calves. Spectacled bears are highly adaptable and are found in a wide range of habitats including rainforest, dry forest and coastal scrub desert. In contrast, the giant pandas live at an altitude of between 1,200 and 3,500 metres in mountain forests that are characterised by dense forests of bamboo.
There have been many theories as to why the panda has such a distinctive coat, but the most convincing argument is that of George Schaller, one of the first western scientists to study wild pandas. He believes the contrasting coat may help prevent close encounters with other pandas. ‘In pandas, a stare is a threat,’ Schaller says. ‘The eye patches enlarge the panda’s small, dark eyes tenfold, making the stare more powerful. A staring panda will hold its head low, so presenting the eye patches. To show lack of aggressive intent, a panda will avert its head, cover its eye patches with its paws or hide its face.’ Interestingly, the spectacled bear is the only other bear with comparably obvious markings around the eye.
Part 2
Read the text and answer questions 14–26.
I used to think that ants knew what they were doing. The ones marching across my kitchen bench looked so confident that I figured they had a plan, knew where they were going and what needed to be done. How else could ants organise highways, build elaborate nests, stage epic raids and do all of the other things ants do? But it turns out I was wrong. Ants aren’t clever little engineers, architects or warriors after all – at least not as individuals. When it comes to deciding what to do next, most ants don’t have a clue. ‘If you watch an ant trying to accomplish something, you’ll be impressed by how inept it is,’ says Deborah M Gordon, a biologist at Stanford University. How do we explain, then, the success of Earth’s 12,000 or so known ant species? They must have learned something in 140 million years.
‘Ants aren’t smart,’ Gordon says. ‘Ant colonies are.’ A colony can solve problems unthinkable to individual ants, such as finding the shortest path to the best food source, allocating workers to different tasks, or defending territory from neighbours. As individuals, ants might be tiny dummies, but as colonies they respond quickly and effectively to their environment. They do this with something called swarm intelligence. Where this intelligence comes from raises a fundamental question in nature: how do the simple actions of individuals add up to the complex behaviour of a group? How do hundreds of honeybees make a critical decision about the hive if many of them disagree? What enables a school of herring to coordinate its movements so precisely it can change direction in a flash, like a single organism? One key to an ant colony is that no one’s in charge. No generals command ant warriors. No managers boss ant workers. The queen plays no role except to lay eggs. Even with half a million ants, a colony functions just fine with no management at all – at least none that we would recognise. It relies instead upon countless interactions between individual ants, each of which is following simple rules of thumb. Scientists describe such a system as ‘self-organising’.
Consider the problem of job allocation. In the Arizona desert, where Deborah Gordon studies red harvester ants, a colony calculates each morning how many workers to send out foraging for food. The number can change, depending on conditions. Have foragers recently discovered a bonanza of tasty seeds? More ants may be needed to haul the bounty home. Was the nest damaged by a storm last night? Additional maintenance workers may be held back to make repairs. An ant might be a nest worker one day, a trash collector the next. But how does a colony make such adjustments if no one’s in charge? Gordon has a theory.
Ants communicate by touch and smell. When one ant bumps into another, it sniffs with its antennae to find out if the other belongs to the same nest and where it has been working. Ants that work outside the nest smell different to those that stay inside. Before they leave the nest each day, foragers normally wait for early morning patrollers to return. As patrollers enter the nest, they touch antennae briefly with foragers. ‘When a forager has contact with a patroller, then ten seconds apart before it will go out.’ To see how this works, Gordon and her team captured patroller ants as they left a nest one morning. After waiting half an hour, they simulated the ants’ return by dropping glass beads into the nest entrance at regular intervals – some coated with patroller scent, some with maintenance worker scent, some with no scent. Only the beads coated with patroller scent stimulated foragers to leave the nest. Their conclusion: foragers use the rate of their encounters with patrollers to tell if it’s safe to go out. (If you bump into patrollers at the right rate, it is time to go foraging. If not, it’s better to wait. It might be too windy, or there might be a hungry lizard out there.) Once the ants start foraging and bringing back food, other ants join the effort, depending on the rate at which they encounter returning foragers. ‘So nobody’s deciding whether it’s a good day to forage. The collective is, but no particular ant is.’ That’s how swarm intelligence works: simple creatures following simple rules, each one acting on local information.
When it comes to swarm intelligence, ants aren’t the only insects with something useful to teach us. Thomas Seeley, a biologist at Cornell University, has been looking into life among honeybees for the past 30 years to work out decisions. With as many as 50,000 workers in a single hive, honeybees have evolved ways to work through individual differences of opinion to do what’s best for the colony. Seeley and others have been studying colonies of honeybees to see how they choose a new home. To find out, Seeley’s team applied paint dots and tiny plastic tags to all 4,000 bees in each of several swarms that they ferried to Appledore Island. There, they released each swarm to locate new homes by had placed on one side of the island. In one test, they put five nest boxes. Scout bees soon appeared at all five boxes. When they returned to the swarm, each performed a dance urging other scouts to go and have a look. These dances include a code to give directions to a box’s location. The strength of each dance reflected the scout’s enthusiasm for the site.
After a while, a small cloud of bees was buzzing around each box. As soon as the number of scouts visible near the entrance to a box reached about 15, the bees at that box sensed that a decision had been reached and returned to the swarm with the news. The bees’ rules for decision-making – seek a diversity of opinions, encourage a free competition among ideas, and use effective mechanisms to narrow choices – so impressed Seeley that he now uses them at Cornell in his role as chairman of his department.
Part 3
Read the text and answer questions 27–40.
A In the jungle of scientific debate, you cannot always see the wood for the trees. But in climate change, the wood itself sometimes holds the key. Imagine an annual register of a year’s sunshine and rainfall and frost, kept up to date with perfect accuracy almost everywhere south of the tundra and north of the tropics, and available for inspection not just at any time in life but, quite often, for centuries after death. The register is, of course, the annual growth rings of trees. Match the rings from young trees with those from old forest giants and you have a centuries-long measure of the march of the seasons. Match the rings from old trees with old cathedral rafters and you have a still longer chronology — and a science called dendrochronology.
B Dendrochronologists, scientists who study the growth of rings in trees, have successfully constructed long tree-ring records by overlapping the patterns of wide and narrow rings in successively older timber specimens. There are now a dozen or so chronologies in the world that date back more than 5,000 years. These records, normally constructed in a restricted area, using a single species of tree, are year-by- year records of how the trees reacted to their growth conditions — an environmental history from the trees’ point of view.
C Because tree-ring chronologies are constructed on a regional basis, there has, in the past, been a tendency for dendrochronologists to think local. However, the success of dendrochronology as an international research topic means that there are now quite a lot of chronologies available for study. As the chronologies are dated absolutely, it is possible to compare the records from different areas year by year. Recently, an analysis of 383 modern chronologies, drawn from a vast area across Europe, northern Eurasia and North America was published. The authors, Keith Briffa and colleagues, observed that the maximum late-wood density of the growth rings in each year was related to the temperature in the growing season. Their analysis spanned 600 years, back to AD 1400, and presented a summer temperature record reconstructed from the huge grid of precisely dated ring densities. What they noticed was that the years of really low density — 0the cool summers — were directly associated with large explosive eruptions, as known from historical sources and from dated layers of acid in the Greenland ice record. Greenland ice is kilometres thick and is made up of the compressed snowfall of tens of thousands of years, so the ice record can be read in almost the same way as tree-rings. I shall use this study as an example of what else tree-rings can tell us.
D The study provides a year-by-year estimate of temperatures, together with the dates of some major volcanoes. It is a nice clean story — volcanoes load the atmosphere with dust and aerosol and reflect back sunlight, cooling the earth’s surface. This cooling leads to variations in the density of growth rings in northern conifers. Because there are a lot of other records, it is possible to test the findings from the conifer density record.
E We can, for example, look at what European oak was doing across the same 600-year period. Was oak responding in the same way as the conifers? The ‘oak chronology’ is the mean of eight regional oak chronologies across a strip of land from Ireland to Poland. It represents how, on average, hundreds of millions of oaks grew. What we see from this comparison is that the oaks clearly do respond to the volcanoes in some cases (in 1602, 1740 and 1816, for instance), but nothing like so clearly in others. Immediately it becomes apparent that the conifers tell only part of the story. There are many downturns in oak growth, and only a few are related to the conifer record. The oaks were quite capable of being more stressed in years where the conifers were not affected. The point of this, however, is not to argue about the quality of global cooling; the point is to show what dendrochronology can do.
F Take the case of 1816, called the ‘year without a summer’ because of the terrible unseasonable cold and the crop failures that ensued. It has long been known that the primary cause of the cooling was the massive eruption of Tambora, east of Java, in 1815. However, there was a lot more going on in the run-up to 1816. Bald cypress trees in Tennessee show a major growth anomaly, with rings up to 400 per cent wider than normal, in the years following a huge earthquake in 1811-12 in Eastern America. But there is a volcanic acid layer in several Greenland and Antarctic ice cores in 1809-10, as well as in 18 15-16. So here we have a combination of a highly unusual quake in an area of the USA not normally affected by earthquakes, and at least two volcanic eruptions, including Tambora, which is widely regarded as the largest in the last 10,000 years. According to Briffa, the period 1810-20 was the coldest in the last millennium, so we begin to see a combination of three unusual elements in less than ten years – exceptional earthquake, exceptional volcanic eruption, and exceptional cold. Given that the defeat of Napoleon’s invasion of Russia in 1812 was famously attributed to ‘General Winter’, one wonders whether a natural series of events actually helped to change the course of modern history.
G Obviously, the case of 1816 and the years just before and after it is relatively recent and well documented. However, dendrochronology allows us to investigate the effects of such events geographically, indeed globally. We can interrogate the trees in areas where there is no historical or instrumental record. Further back in time, dendrochronology is almost the only way to reconstruct abrupt environmental events and perhaps throw new light on far darker moments in human history. Were there just political forces at work in the Dark Ages, or did violent natural events also take a hand, tipping the balance by darkening the skies and lowering the temperature? The trees were there too, and kept a record. The wood hewn from them and preserved through the centuries is slowly beginning to yield at least circumstantial evidence that could support some of the stories – think of the Arthurian wasteland, or the plagues of Egypt – so far told only in enigmatic artefacts, or in legends, epics, and religious chronicles.