By BBC - Polar bear and dog story from Churchill
By FirstScience TV - wild polar bears playing with chained up dogs
By Barcroft TV - not a wild bear. Perhaps someday, this will be 'normal' behavior, with wild bears.
By OLN Canada - playing or defending?
Above are some videos that relate to the polar bear and dog story noted in the book, as well as a couple of others of note.
Background photo by W.C. Peace - location noted in videos where dogs and polar bears have played together - Churchill, Manitoba, Canada