阅读理解 Most people think of zoos as safe places for animals, where struggles such as difficulty finding food and avoiding predators(食肉动物)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 suffer from poor health．They develop diseases, joint problems and behavior changes．Sometimes, they even become infertile, or unable to have babies． To learn more about how captivity(监禁)affects elephants, a team of international scientists compared the life spans of female elephants born in zoos with female elephants living outdoors in their native lands．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 African and Asian elephants in zoos across Europe．The scientists compared the life spans of the zoo-born elephants with the life spans of thousands of female wild elephants in Africa and Asian elephants that work in logging camps, over approximately the same time period． The team found that female African elephants born in zoos lived an average of 16.9 years．Their wild counterparts who died of natural causes lived an average of 56 years-more than three times as long．Female Asian elephants followed a similar pattern．In zoos, they lived 18.9 years, while those in the logging camps lived 41.7 years． Scientists don’t yet know why wild elephants seem to fare so much better than their zoo-raised counterparts．Georgia Mason, a biologist at the University of Guelph in Canada who led the study, thinks stress and obesity may be to blame．Zoo elephants don't get the same kind of exercise they would 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 herds and family groups． Another finding from the study showed that Asian elephants born in zoos were more likely to die early than Asian 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 maintain healthy populations, that doesn't appear to be the case with elephants．"Currently, zoos are net consumers of elephants, not net producers," Mason says． | 
