BERLIN - Knut the superstar polar bear turned 2 on Friday looking nothing like the button-eyed ball of white fluff who captured hearts around the world.

The star of the Berlin Zoo is a fully grown bear with yellowish fur who, at 440 pounds, has grown too big for his enclosure. Worried fans are lobbying for him to stay, but zoo officials say he will have to move if they do not build a new enclosure, which appears virtually impossible due to lack of space.

Knut lives in a small section of the current enclosure, home to Knut's parents, Tosca and Lars, and two older females. Bearkeeper Heiner Kloes said Knut, who will reach sexual maturity around the age of 6, urgently needs enough space for both him and a fertile mate.

The zoo's two eligible female bears will be too old to have cubs by the time Knut is ready to reproduce.

"The survival of the species is more important than any individual," Kloes said.

"I won't hang on to Knut if it means keeping him with an old lady," Kloes said, noting the zoo is already filled with enclosures for other animals.

The zoo has credited Knut with a 27 percent increase in visitors in 2007 and profits of nearly $8.6 million from products with Knut's image, including stuffed animals, T-shirts, mugs and DVDs.

Doris Webb, 65, has followed Knut since he was first presented to the world by the keeper who raised him by hand after he was rejected by his mother. Since April, Webb has gathered more than 21,000 signatures in support of keeping Knut in Berlin. "We want to show how important it is for Berlin, for the people here - and for Knut himself," she said.

Web site Unibet is placing odds on the zoo likeliest to get the bear, with Zoom Erlebniswelt in Germany the top contender.