Cher has joined a campaign to free the world's "saddest creature" from a zoo in Argentina. The pop superstar was horrified by online images of lonely polar bear Arturo, showing signs of depression following the death of his partner, and took to Twitter in a bid to urge Argentine president, Cristina Fernandez, to order zoo bosses in Mendoza to let him live out his days at a sanctuary in Canada. She wrote, "Don't cry for him, Argentina? No tears of Mrs. Cristina Fernandez de Kirchner for tortured polar bear Arturo. Your hands are stained with his blood when he dies." Activists have been campaigning for the 28-year-old bear's move since late 2013, when a heatwave endangered the life of the only polar bear in Argentina. The director of the zoo insists Arturo will not be moved despite a Change.org petition signed by 500,000 people, encouraging him to change his mind.