Mercedes the polar bear is subject of a heated debate

For the past 21 years, shes has been a main attraction at Edinburgh Zoo but, while children run freely from one enclosure to another, she paces up and down or swims around in circles.

To animal welfare charities the behaviour of the great "sea bear" is a clear indication that years of confinement in an enclosure less than one millionth of the size of the area she would roam in the wild have taken a toll on her sanity.

There are estimated to be between 22,000 to 27,000 polar bears living in 20 populations across eight Arctic countries but, although the WWF claims the wild population is stable, there are fears that numbers could fall becauseof hunting and global warming.

Although Edinburgh Zoo agreed in 1993 not to replace Mercedes when she dies, officials have said that if the species numbers continue to decrease they will build an improved enclosure to support a captive breeding programme

Yesterday afternoon, Edinburgh Zoo's Committee on Animal Welfare and Ethics met to discuss the future of polar bears at the zoo amid protests against any change to current policy.

One group pointed out it was particularly unfair to keep the great beasts, which can grow up to 10ft tall, in captivity. "Polar bears are about the most unsuitable animals to be kept in zoos," said Ross Minett, director of the Edinburgh-based Advocates for Animals.

As natural hunters, polar bears, which can weigh up to 1,700lb, usually survive on a diet of seals but will also eat walrus, beluga whales, reindeer and birds.

Mercedes, who was captured in Canada while raiding bins for food, arrived at Edinburgh when she was three years old and has been alone since her partner, Barney, died in 1996. Their only cub, Minty, was used as the model for the bear which appears on packets of Fox's Glacier Mints. He died from intestinal problems last year at Antwerp Zoo.

It is estimated only 250 polar bears survive in zoos worldwide and, with the evident low reproduction rate and high infant mortality, opponents fear more polar bears will be removed from the wild to accommodate a breeding programme.

Although polar bears have been known to live to almost 40 in captivity, as opposed to an average expectancy of just 18 years in the wild, their quality of life behind bars tends to take a toll on their physical and mental health.

Mr Minett said: "Anybody who has been to the zoo can see the pitiful sight of Mercedes going crazy in her pathetic small enclosure."

WWF has also joined the chorus of protest against breeding endangered species in zoos. "Our preference is 'in-situ' conservation with species in their natural environment," said a spokesman.

No one from the zoo was available for comment.