A polar bear named Tonja gave birth to a cub at the Berlin Zoo on December 1. Polar bear cubs are extremely sensitive, especially during the first 10 days, according to zookeepers. The birth of a new baby is a happy occasion for the zoo after its famous polar bear Knut died in 2011. For now, the newest baby bear is doing well. If the baby survives, visitors can hope to catch their first glimpse in the spring.