Knut was born at Berlin Zoo. His mother was Tosca, a 20-year-old polar bear. She was a circus performer from East Germany, born in Canada, and her 13-year-old mate, Lars, was from a park in Munich. Tosca gave birth to Knut and his unnamed brother on 5 December 2006. However, she left her cubs on a rock in the polar bear enclosure and did not take care of them. The zookeepers rescued the cubs with a fishing net, but Knut's brother died from sickness four days later. Knut was only the size of a guinea pig, and had to spend the first 44 days of his life in an incubator before zookeeper Thomas Dörflein began raising the cub.