Playful, mischievous and much-beloved, Mac was just two years old when he became the latest Asian elephant to succumb to the herpes virus at the Houston Zoo last month.

For animal welfare advocates, every early death is another piece of evidence that these 8,000-pound (3,625-kilogram) proboscideans don't belong behind bars, where they can become obese, diseased and stressed out. A new study published today in Science provides the strongest evidence to date that zoo life is harmful to an elephant's health.

Researchers led by Ros Clubb, an animal behavior expert at the Royal Society for the Prevention of Cruelty to Animals (RSPCA) in West Sussex, England, compiled data on over 4,500 African and Asian elephants over 45 years in European zoos and compared their life spans with the median life expectancy of elephants in preserves in their home countries. (The median, rather than the average, is used to reduce the effects of extreme values at either end of the age spectrum that would skew the results.)

African elephants, they say, can expect to live 36 years in Kenya's sprawling Amboseli National Park, more than double the 17-year life span of zoo elephants. It is not unusual for Asian elephants like Mac to make it to the ripe old age of 42 after having toiled in timber camps in Burma, but they can expect to live just 19 years in a zoo, according to the new study.

Overall, infant mortality in Asian elephants is as much as three times higher in zoos than in native protected areas. The new results show that captive-born Asian elephants fare worse than wild-born elephants in zoos, suggesting that problems arise in gestation and early infancy. The scientists say these endangered animals are compromised further by breeding programs that transfer animals between zoos.

Consequently, zoos are far from their long-promised goal of producing self-sustaining captive populations—and they clearly do not send animals back into the wild to bolster diminishing populations there. "It's hard for a female to produce lots of babies if she comes in at 10 and is dead by 25," says senior study author Georgia Mason, an animal behaviorist at the University of Guelph in Ontario.

The RSPCA has long called for phasing out zoo elephants by ending importation and breeding programs. Mason, however, says she believes that importation could resume once zoos solve their alleged problems, although she notes her co-authors may disagree.

Despite the findings, zoo scientists deny their elephants fare poorly. "The problem is the topic is very emotionally driven," says conservation biologist Peter Leimgruber, an expert on wild Asian elephants at the Smithsonian National Zoological Park in Washington, D.C. "You can play numbers different ways just to make your point."

Population biologist Robert Wiese, director of collections at the San Diego Zoo and Wild Animal Park, who has criticized earlier RSPCA work on elephant life expectancy in zoos, said he had seen the findings but was reticent to comment on them. "I can't say without really having the data sets they have," he says.

Wiese says that the study, at best, is a reflection of the way zoos once were run and does not take into account animal-friendly changes that have been made, such as leaving offspring with their mothers longer periods of time. "Most of the things zoo critics bring up about elephants were discovered by zoo people," he says. "Right now, we can assess how animals were cared for in the 1960s and 1970s. It's going to be a long period before we can tell how we're doing today."

The seven Asian elephants at the San Diego Zoo are all in their late 40s and 50s—well past their life expectancy of about 42. He says seven of the zoo's eight adult African elephants were imported from the Mkhaya Game Reserve and Hlane Royal National Park in Swaziland in 2003, where burgeoning elephant populations were set to be culled. They are all about 17 years old today. An additional African female elephant, Tembo, who previously appeared in the TV series Born Free, has been at the zoo for 20 years and is now probably 36 years old, according to a zoo spokesperson.

Wiese argues that zoos are important to provide visitors who may not otherwise have the opportunity to see wild animals up close and personal. By doing so, he contends, they make a personal connection with, say, elephants, encouraging them to donate time and money to conservation projects in their home countries designed to protect them.

And life in the wild is not particularly idyllic compared with captivity: Of 1,089 elephants in the Amboseli sample, 142 were killed by ritual spearing, gunshot wounds, or other mishaps. Besides, Wiese notes, survivorship is not the only gauge of an animal's quality of life, a point even Mason concedes.

"It's not the case that life span is a clear-cut indicator," she says. "Maybe being overweight is absolutely wonderful. You're just living your life like Homer Simpson."