FILE - In this Dec. 5, 2007 file picture polar bear Knut walks in the enclosure during the celebration of his first birthday in the Zoo Berlin. Scientists in Germany say they have finally figured out what killed Knut the polar bear four years ago. Knut drowned after swelling of his brain caused him to collapse and fall into his enclosure's pool at Berlin Zoo. In an article published Thursday Aug. 27, 2015 by the journal Nature Scientific Reports, researchers say Knut suffered from anti-NMDA receptor encephalitis. (AP Photo/Markus Schreiber,file)