It was, by the standards of Knut the superstar bear, a modest birthday party. Five thousand fans and twenty international television crews turned up in front of his pen in Berlin Zoo, as Knut's faithful keeper served up a cake made of rice, fruit and lettuce to celebrate the first birthday of the world's most famous polar bear.

As Knut ripped into the vegetarian confection, impatiently splintering a wooden candle, it became clear that he would rather have preferred a succulent mammal, perhaps even one of the hundreds of children chanting “Happy birthday little bear”.

There is nothing little or cuddly any more about Knut. His white fur is turning brown from dirt and he weighs 112kilograms (246 lbs), almost as much as his patron, Sigmar Gabriel, the German environment minister. His keeper, Thomas Dörflein has been ordered — after months of public sparring with the bear, of mothering, of lullabies and of swimming lessons — to keep his distance. One swipe from Knut would end 44-year-old's career swiftly.

Mr Dörflein said: “My birthday wish for Knut is that he is happily transferred to another zoo — in a big compound with a female partner. He has to learn to separate from me.”

A transfer is likely from next May and several zoos, in Germany and across Europe, are already in contention.

Knut's unusual biography has made him into a cash machine, with about 400,000 more people visiting the zoo over the past year. A children's book has been written about him, seven chart-topping Knut records have been cut, the celebrity photographer Annie Leibovitz has snapped him for Vanity Fair and he has been paraded for stars such as Tom Cruise.

Somehow the Knut story — rejected by his mother at birth, left to die, recovered and nurtured by Mr Dörflein — has mutated into a parable of a planet threatened by climate change. Fans were offered slices of a huge iced pastry cake yesterday and then encouraged to view a film in the zoo restaurant about the disappearing species of the Arctic. First prize in the bear-birthday lottery was a trip to Knut's natural habitat.

“I don't want to win a trip to the Arctic, that would just help destroy the climate even more,” grumbled 12-year-old Lucia Bertorelli, one of the many ecologically conscious visitors to visit the zoo yesterday (schools were given the day off to see Knut). “I would rather win a Knut bike.”

Knut is, in fact, a thoroughly German bear. His mother, Tosca, was an east German circus bear, his father Lars, a long-term resident of German zoos. Lars has mated with three other polar bears in the past year and keepers believe that at least one may be about to give birth. “We have prepared two incubation chambers, heating them round the clock at 37C (98.6F) in case we have another pre-Christmas birth,” Mr Dörflein said.

Mr Dörflein bottle-fed Knut all through last Christmas, massaged baby oil into his skin and strummed Elvis Presley songs to send him to sleep. He enjoyed an extraordinarily intimate relationship with the rejected cub, allowing him to teeth on his Wellington boots. The human intervention stirred suspicion among animal rights activists, who argued that it was against the law to bring up animals in an unnatural way.

A furious international row erupted when one activist seemed to suggest that Knut should have been left to die when his mother refused to feed him.

“He remains very cheeky,” Mr Dörflein said. “The other day he charged into the feeding station knocking over two buckets of dead worms and dead mice. I had to spend the rest of the day sorting out the mice from the worms.”

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