A trip to the zoo allows many people to see animals they would never otherwise encounter. But study results published today in the journal Nature indicate that the greater the difference between an animal's natural home range and its confined environment, the worse it will fare in captivity.

Large animals often show signs of boredom in zoos. In a strange example from 1994, Gus--a polar bear living in the Central Park Zoo in New York City--spent much of his time swimming incessant figure eights in his pool. Subsequent changes to his surroundings and more interactive activities seemed to cheer Gus up, but it remained unclear why some animals thrive in captivity whereas others do not. Ros Clubb and Georgia Mason of the University of Oxford collected and analyzed data on 35 captive carnivores in zoos around the world. In addition, they obtained field data related to the creatures' natural habitats, including median home range size, daily travel distance, time spent foraging and reliance on hunting. The researchers found that animals with larger home ranges, such as lions, were more likely to engage in pacing behavior in captivity than were creatures with small home ranges. Typically, a polar bear enclosure is only one millionth the size of its minimum home range.

The study also examined the effects of captivity on reproduction. Clubb and Mason determined that captive carnivores with vast home ranges, such as polar bears, have higher infant mortality rates than do animals with smaller natural dominions--the American mink, for example. The findings suggest a need for fundamental improvements to the caging of naturally wide-ranging carnivores. The authors note: "Our results show, to our knowledge for the first time, that a particular lifestyle in the wild confers vulnerability to welfare problems in captivity."