填空题

It was a sorry end. Cut down in his prime, the cunning thief lay on the slab, his cold body offering pathologist Brett Gartrell no outward sign of how he. had met his maker. Once Gartrell had wielded his scalpel, however, the cause became, clear: a belly stuffed with sticky brown gunk. Diagnosis Death by chocolate.
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If you’re reading this after scoffing your fifteenth chocolate Santa, don’t panic: We humans have been safely enjoying the beans of the cacao plant, Theobroma cacao, for millennia. Theobroma is Greek for "food of the gods", reflecting the Mayan belief that cocoa had divine origins. Every April, they sacrificed a dog with cacao-coloured markings in honour of Ek Chuah, the god of cacao.
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It was methylxanthines that did for the kea too. Gartrell, a wildlife pathologist at Massey University in Palmerston North, New Zealand, is wearily familiar with keas’ propensity m poison themselves. Besides being arguably the world’s smartest birds, keas are extraordinarily inquisitive foragers, using their beaks to rip open tents and backpacks, open garbage bins and even pry pieces off cars in their quest for food. "They’ll try anything that is vaguely edible, which is part of the reason they get into trouble," says Gartrell.
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The reason humans don’t turn up their toes after bingeing on chocolate is largely down to the speed at which our bodies metabolise theobromine, the most abundant methylxanthine in chocolate. Rats metabolise it much more slowly than humans, and dogs are slower still. There are no reliable figures for theobromine toxicity in humans, but based on caffeine toxicity an average adult would have to gorge on around 50 kilograms of milk chocolate in a single sitting to get anywhere near a lethal dose.
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Coyotes are a serious pest in the US, killing $44 million worth of livestock each year, damaging property and attacking people and pets. Measures such as fences are often ineffective. Sometimes culling them is the only option but unfortunately the poisons now used, such as sodium cyanide, are toxic to humans and most other animals too. "If we can come up with something that is more selective, it offers an advantage," says Johnston. "It’s a more responsible approach."
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Methylxanthines are also shaping up as a way to dispatch other pests. Earl Campbell of the US Pacific Basin Agricultural Research Center in Hilo, Hawaii, discovered that caffeine sprays could kill two species of noisy and ecologically damaging Caribbean tree frogs that have plagued the island since they were accidentally introduced in the 1980s. Campbell noticed that the spray also killed slugs. His colleague Robert Hollingsworth then found that caffeine spray made snails kick the bucket too. Hollingsworth is now developing caffeine as an alternative to conventional pesticides, such as those used in slug pellets. "There’s a huge amount of interest in using botanical extracts," he says. "People are more comfortable with things that are natural."
A. Knife-wielding priests aside, chocolate is still bad news for many animals. Cocoa beans are naturally rich in caffeine and its chemical relatives theobromine and theophylline, collectively called methylxanthines. To humans these are little more than benign stimulants, but to a number of animals they are highly toxic. Just 240 grams of unsweetened dark chocolate contains enough methylxanthines to kill a 40-kilogram dog, about the size of a German shepherd.
B. The methylxanthines are just a start. "Cocoa is a real gold .mine of different components," says Herwig Bernaert, research manager at Barry Callebaut, a chocolate manufacturer in Zurich, Switzerland. Cocoa contains more than 700 compounds and there is a great deal of research on which of these can affect people or other creatures.
C. Methylxanthines looked as though they might fit the bill. After testing the toxicity of several different types of chocolate, Johnson came up with a mixture of theobromine and caffeine that killed coyotes quickly and with minimal distress. The mixture can be hidden in bait and is currently undergoing field tests.
D. The observation that methylxanthines are highly toxic to animals, with dogs being especially vulnerable, prompted John Johnston, a chemist at the US Department of Agriculture in Fort Collins, Colorado, to investigate chocolate as a more selective way of controlling coyotes.
E. Divine—yes. Delicious--absolutely. But deadly For some it certainly is. The corpse on Gartrell’s slab belonged not to a human but to a kea, an endangered New Zealand parrot. Like many animals, keas are acutely sensitive to chemicals in chocolate that are harmless to humans in all but huge doses. Scientists are now studying these chemicals, along with other substances in cocoa, hoping to exploit their toxic effects to control pests or microbes.
F. The dead kea was found outside a hotel kitchen in the holiday resort of Mount Cook Village in the Southern Alps. It had eaten more than 20 grams of dark chocolate, presumably pilfered from the kitchen garbage. "He’d really pigged out," says Gartrell. The ill-fated kea was by no means alone in its folly. Veterinary journals are peppered with stories of dogs, cats, parrots, foxes, badgers and other animals dropping dead after finding chocolate or being fed it by well- meaning humans.