File photo of polar bear cub Knut during his first presentation in Berlin zoo, March 23, 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. Knut, the celebrity polar bear who died at the weekend, may have succumbed to a brain disorder, Berlin zoo said on Tuesday."The initial results show significant abnormalities in the brain which could be regarded as the reason for the polar bear's sudden death," the zoo said in a statement. [Photo/Agencies]