The unexpected death of celebrity polar bear Knut has rocked the Berlin Zoo, which closed for the day as mourners came to leave flowers and messages. "I am deeply shocked," said the zoo's business director. "It's like a friend has left us." Animal rights groups are more angry than shocked. PETA criticized the zoo director and bear keeper for putting Knut in an enclosure with three older female polar bears. They also said raising Knut by hand caused the lovable bear to develop an orientation toward people rather than bears, and led to behavioral problems. Wolfgang Apel, head of the German animal protection association, said Knut's “short and distressful life shows us again that polar bears do not belong in zoos, even if they are called Knut.” Knut's cause of death is still unknown. At four years and three months, he was well below the average life expectancy for polar bears—they typically live to 35.