Zoo elephants swaying back and forth, polar bears swimming in endless circuits and manic monkeys grooming themselves to baldness.
Such disturbed, trance-like behavior in some zoo animals and the deaths of four elephants in the past year at two U.S. zoos have sparked animal rights protests and renewed a larger debate over the purpose of zoos.
Defenders say zoos serve important purposes, including offering access to researchers, providing money and expertise for habitat preservation elsewhere and as repositories of genetic material for fast-vanishing species.
Critics say captivity is both physically and mentally stressful.
"We might see within our lifetimes a great reduction or extinction of these animals," as their natural habitats are squeezed by the crush of human populations, said Bill Foster, president of the American Zoo and Aquarium Association. "Extinction is not acceptable."
Zoos originally gave city dwellers the chance to marvel at the world's fauna and later promoted habitat preservation, but those purposes have been eclipsed, critics say.
"In the old days, when you didn't have television, children would see animals for the first time at the zoo and it had an educational component," said Tufts University animal behaviorist Nicholas Dodman.
"Now the zoos claim they're preserving the disappearing species, preserving embryos and genetic material. But you don't need to do that in a zoo. There's still a lot of entertainment to zoos," he said.
Elephants are often chosen in surveys as the most popular zoo animals and a newborn calf draws many visitors. But seeing animals behaving oddly in zoos is more disturbing than educational, said a spokesman for People for the Ethical Treatment of Animals (PETA).
Oxford University researchers contended 40 percent of zoo elephants display so-called stereotypical behavior, which their 2002 report defined as repetitive movements that lack purpose.
The report said studies have shown zoo elephants tend to die younger, are more prone to aggression and are less capable of breeding compared with the hundreds of thousands of elephants left in the wild.
Moreover, critics say many zoo elephants, though hardy, spend too much time cramped indoors, get little exercise and become susceptible to infections and arthritis from walking on concrete floors.
Following the deaths of two of three African elephants housed at Chicago's Lincoln Park Zoo in the past four months, animal rights activists said their demise was hastened by the stress brought on by their 2003 move from balmy San Diego.
Zoo curators denied climate was to blame and concluded that Tatima, 35, died from a rare lung infection and Peaches, at 55 the oldest of some 300 elephants in U.S. captivity, suffered from organ failure.
When two elephants in San Francisco's zoo died within weeks of each other last year, the resulting outcry prompted the zoo to opt to send its remaining elephants to a California sanctuary against the wishes of the American Zoo and Aquarium Association.
Detroit's zoo director, who decided his zoo lacked the space or resources to keep elephants, also had a fight with the association about sending his elephants to a Tennessee sanctuary. The association relented only when one elephant showed signs of herpes.
Detroit's zoo was the eighth North American zoo to stop exhibiting elephants since 1991, according to PETA.
"For the modern-day zoo to have elephants does nothing for the preservation or conservation of the species. And it does nothing for the welfare of the elephant," said Carol Buckley, who created a Tennessee sanctuary that now cares for a dozen cast-off zoo and circus elephants.
Foster of the zoo association countered that many northern zoos have successful elephant programs with plans to expand.
Calves born in captivity have higher mortality rates and survivors often have to be isolated for a time from their inexperienced mothers, who may trample them.
Based on the Oxford University report that found 40 percent of zoo elephants engage in stereotypical behavior, the report's sponsor,
Britain's Royal Society for the Prevention of Cruelty to Animals, urged European zoos to stop importing and breeding elephants and to phase out exhibits.
Dodman said he frequently observes stereotypical behavior among zoo animals: polar bears rocking in place or swimming in endless circuits, parrots grooming themselves until they bleed, gorillas regurgitating and re-ingesting meals, and big cats pacing the same routes in trance-like patterns.
Most zoos embrace efforts to enrich the animals' lives by varying feeding rituals and providing toys, with some success; an Alaskan zoo is even building its elephant a treadmill. But elephants and other animals that range widely in the wild are less easily distracted, critics say.
Some zoos give animals behaving stereotypically the same antidepressant drugs found to ease compulsive behaviors in people, Dodman said.
The key is providing more space and companionship for elephants, which often travel in large herds and forage for hours, Buckley said.