File photo of polar bear cub Knut playing in the Berlin zoo in an undated picture, released on March 2, 2007. Knut, the polar bear who became a global celebrity as a cute cub, died in Berlin zoo March 19, 2011 aged just four. Knut, the first polar bear born at the zoo for more than 30 years, was rejected by his mother and fed instead by his keeper Thomas Doerflein. Thousands of visitors came to watch keeper and cub playing together, and Knut's fame soon spread around the world. The cause of death was unknown and he will undergo an autopsy. [Xinhua/Reuters Photo]