How some of the animals pace around a lot?
动物也爱散步?
Scientists think the problem might be that the animals miss the freedom to roam. That make sense. After all, a polar bear's natural range is about the size of South Carolina. Right. And the typical zoo habitat is about one millionth of that.
科学家认为这可能是因为它们被剥夺了散步的自由。有道理。一只北极熊自然分布范围为整个南卡罗莱纳州。对。而一般动物园的大小只是南卡罗莱纳州百万分之一。

Researchers in Oxford studied the pacing behavior of thirty-five captive species using data from over one thousand scientific articles published since the 1960s. When they analyzed this data–which represented more than five hundred zoos worldwide–they determined that the deciding factor for pacing was range size.
牛津的研究人员调查了从20世纪60年代发表了一千多篇科学论文里的数据，是关于35个圈养物种的散步习性。他们对这些数据研究后发现：这些数据代表了世界上500多个动物园，他们因此确定散步习性的决定因素是动物的分布范围。
That also explains why stay-close-to-home species like snow leopards tend to thrive in zoos. So what happens now?
那就可以解释：为什么留守家园附近的物种像雪豹在动物园很多了。现在呢？
Well, one option is for zoos to build larger, more varied habitats, and switch enclosures periodically to simulate roaming. But a better option might be to phase animals like polar bears out of zoos, and focus instead on those animals that do well in captivity.
一个选择就是把动物园建大一点，更多样化和定期变换围墙促进动物们散步。但还有一个更好的选择就是将那些像北极熊的动物走出动物园，多关注那些能够圈养的动物。
The problem is that this is a double-edged sword: these animals' natural habitats are increasingly threatened as well.
问题在于这是把双刃剑：这些动物的自然习性会受到很大的影响。