Ever since Knut, the Berlin Zoo’s 4-year-old polar bear, died last month, I’ve been wondering: How can I tell if a zoo is taking good care of its animals?

This is a tricky question. Some animal advocates argue that zoos, by definition, are bad for wild animals. Even so, there’s no question that certain zoos treat their animals better than others. Unfortunately, it can be difficult to tell the difference between them.

In the United States, zoos must have a license from the U.S. Department of Agriculture, but this bar is set quite low. The federal regulations that govern animal-exhibition licensing are vague and essentially guarantee only that the animals aren’t facing imminent death. Enclosure sizes, for instance, are basically required to be large enough only for the animals to stand up and turn around. And enforcement of the Animal Welfare Act, the basis for the regulations, is woefully underfunded, with 114 inspectors keeping tabs on 8,371 facilities, including research labs. When exhibitors lose their licenses, it often has more to do with visitor safety than animal welfare.

A far higher standard is accreditation by the Association of Zoos and Aquariums, the group that represents most of the country’s well-known facilities. Approximately 2,700 animal exhibitors have a USDA license, but fewer than 10 percent have earned AZA’s accreditation, which requires that the facility submit reams of paperwork on its enclosures, veterinary care and financial stability. (The AZA Web site includes a list of accredited institutions, among them the National Zoo, but an AZA zoo will also probably display the organization’s emblem somewhere near its entrance.)

To stay accredited, zoos must participate in wildlife conservation and community education. They also must provide the animals with appropriate stimulation to keep their minds and muscles active — by having them follow a keeper’s commands or let them forage for their food, for instance.

(Michael Sloan for The Washignton Post)

Inspectors also try to evaluate animals’ psychological well-being. Repeated purposeless motions — what zoo professionals call “stereotypic behavior” — are a pretty clear sign of a troubled animal. Stressed polar bears have a tendency to pace, while elephants shift their weight back and forth for minutes at a time. Even more alarming is when animals do nothing but lie in a heap at the back of their exhibit. Stress can also manifest itself in physical injuries such as sores and bald patches.

As for enclosures, exhibits should give an animal enough room to break into a run or fly. There should be variation in surface type and elevation, so that the animals can decide where to spend the day. Social animals should be housed in appropriately sized groups, while solitary animals should be kept alone. The habitat should mimic the animal’s natural environs to the extent possible, and there should be enough shade for all the exhibit’s animals to use simultaneously.

These can be helpful rules of thumb, but they’re rife with exceptions. Keepers sometimes have to isolate social animals because they’re sick or can’t get along with others. A creature that doesn’t budge during your visit may just be nocturnal — say, a koala or spectacled bear — or cycling through a natural period of torpor. An enclosure that looks small might actually connect to an extensive habitat out of public view. Zoo officials often ship discontented animals to different facilities, so some troubling behaviors may be a remnant of past trauma rather than present discomfort. And unlike one-day visitors, AZA inspectors get access to an animal’s full social and veterinary history, and they sometimes even measure its corticosteroid levels as an indicator of stress.

Even so, AZA accreditation doesn’t satisfy everyone. Critics point out that the organization’s standards are developed and approved by members of the zoo community. Moreover, they believe the standards are set not by the needs of the animals but by what the top zoos are realistically capable of providing. If no zoo could achieve accreditation, after all, the standards would become rather meaningless. In their view, accreditation is more an assurance of non-awfulness than a mark of excellence.

Elephants provoke some of the sharpest debate. Some critics argue that even the best zoos do not provide appropriate room for an elephant to wander or socialize. The average Asian elephant herd has 20 calves, juveniles and adult females. (The bulls typically roam solo.) Elephants need enough space, the critics argue, to walk seven miles per day. But AZA officials say that’s an overstatement: Just because an animal walks miles and miles on the savanna doesn’t mean that that much exercise is critical to its health.

To support their argument, the anti-zoo advocates often point to a 2008 study in the journal Science, in which researchers found that African elephants in European zoos live far shorter lives than their counterparts in a Kenyan national park: a median of 16.9 years in zoos vs. 56 years in the park. (The researchers cited stress and obesity as likely causes of early death.)

So what’s the bottom line? First, you shouldn’t support an unaccredited zoo unless you have investigated its animal care, a task few people can accomplish on a Saturday morning. Second, even if you’re at a good zoo, keep your eyes and ears open for signs of distress and don’t be afraid to ask the docents about what you’re seeing; better zoos will keep their people informed and capable of providing honest answers. Finally, if you can’t stand the sight of an animal staring longingly through a glass pane, you probably should just stay home.

The Green Lantern is produced by the Web magazine Slate and can be read at www.slate.com . The Lantern thanks Chris Draper of the Born Free Foundation, Steve Feldman of the AZA and Don Moore of the National Zoo for their assistance on this story. Is there an environmental quandary that’s been keeping you up at night? Send it to ask.the.lantern@gmail.com.