Most people regard zoos as safe places for animals, where struggles such as difficulty finding food and avoiding predators（捕食性动物） don't exist. Without such problems, animals in zoos should live to a ripe old age. But that may not be true for the largest land animals on Earth. Scientists have known that elephants in zoos often develop diseases, joint problems and behavior changes. Sometimes, they even become unable to have babies. To learn more about how captivity(监禁) affects elephants, a team of international scientists led by Mason, a biologist, compared the life spans of female elephants born in zoos with female elephants living outdoors in the wild. Zoos keep detailed records of all the animals in their care, documenting factors such as birth dates, illnesses, weight and death. These records made it possible for the researchers to analyze 40 years of data on 800 elephants in zoos. The team found that female elephants born in zoos lived an average of 16.9 years. Their wild counterparts（同类事物） who died of natural causes lived an average of 56 years——more than three times as long. Scientists don't yet know for sure why wild elephants seem to live so much better than their zoo-raised counterparts. Mason thinks stress and obesity(肥胖) may be to blame. Zoo elephants don't get the same kind of exercise as they do in the wild, and most are very fat. Elephant social lives are also much different in zoos than in the wild, where they live in large groups. Another finding from the study showed that elephants born in zoos were more likely to die earlier than elephants captured in the wild and brought to zoos. Mason suggests stress in the mothers in zoos might cause them to have babies that are less likely to survive. The study raises some questions about acquiring more elephants to keep in zoos. While some threatened and endangered species living in zoos reproduce successfully and keep healthy populations, that doesn't appear to be the case with elephants. 小题1:The text tells us that zoo elephants are different from other zoo animals because they________
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