Berlin ? Knut the superstar polar bear turns 2 today 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 because of a 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. Scientists estimate that there are between 20,000 and 25,000 polar bears in the wild, according to the International Union for Conservation of Nature, and the species is listed as threatened under the U.S. Endangered Species Act. Canada and Russia have listed polar bears as a species of concern, citing shrinking habitats.

“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.

With Knut likely on the way out, the zoo is keeping his birthday low-key. It is planning a cake made of fruit rather than a big party like the one it threw last year for the Vanity Fair “cover bear,” who shared the front of one issue with Leonardo DiCaprio.

There is no shortage of potential suitors who would be happy to offer a suitably roomy new home for Knut and a future mate.

“Of course there are others — it’s the world’s most famous animal,” said Torbjorn Wallin, chief executive of Orsa Gronklitt AB, which runs the Orsa bear park 200 miles northwest of Stockholm. “It’s as if Madonna would move to Stockholm.”