FILE - In this March 23, 2007 file photo, Knut, the polar bear cub, has its first public appearance with his keeper Thomas Doerflein in the Berlin zoo. A Berlin zoo official says world-famous polar bear Knut has died.  Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon March 19, 2011 while alone in his compound. He says the cause is not yet clear. (AP Photo/Herbert Knosowski, File)

Photo by: Herbert Knosowski

FILE - In this March 23, 2007 file photo, Knut, the polar bear cub, has its first public appearance with his keeper Thomas Doerflein in the Berlin zoo. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon March 19, 2011 while alone in his compound. He says the cause is not yet clear. (AP Photo/Herbert Knosowski, File)