When you've been to the zoo have you noticed how some of the animals pace around a lot?

Scientists think the problem might be that the animals miss the freedom to roam. After all, a polar bear's natural range is about the size of South Carolina. And the typical zoo habitat is about one millionth of that.

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.

That also explains why stay-close-to-home species like snow leopards tend to thrive in zoos. So what happens now?

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.

The problem is that this is a double-edged sword: these animals' natural habitats are increasingly threatened as well.