Germany is in mourning after the life of the world's only rock-star animal, Knut the polar bear, ended tragically with his death at Berlin Zoo at the tender age of four. Knut grew up at the zoo after he was shunned by his circus bear mother as a cub. The cute young bear became an overnight media sensation and a poster boy for the battle against global warming. But animal rights groups alleged that fame was too much for Knut. They even suggested he was suffering from depression or 'bipolar' disorder. A necropsy will be performed to determine the exact cause of Knut's death, while German officials are considering a formal funeral for their former furry friend.