Each sled dog on Buck's team (besides Spitz) was given the personality of one of the dwarfs from the Disney classic Blanche-Neige et les Sept Nains (1937): Dolly is Bashful, Pike is Grumpy, Joe is Happy, Billy is Sleepy, Dave is Dopey, Dub is Sneezy, and Sol-Leks is Doc.
Unlike the source novel, John Thornton has a back story. They wanted Thornton's experience to be similar to Buck's for this film adaptation to show both man and dog finding their strength after overcoming tragedy.
Buck is half St. Bernard (giving him size) and half Scotch shepherd (a medium-sized herding dog thought to be the ancestor of the Rough and Smooth Collie). He is described in the original novel as looking like a large wolf, rather than his mixed-breed look in the film. Additionally, none of the sled team look the way they do in the book. In the book, they are all Huskies - in the film, only Spitz is a Husky, and he is the wrong colour (black and white, as opposed to all-white as in the book).
The fully CGI model of Buck is a digital scan of Buckley, a real dog that director Chris Sanders' wife, Jessica Steele-Sanders, adopted as a family pet from an Emporia, Kansas animal shelter during production. Buckley is thought to be a cross between a St. Bernard and a Scotch shepherd (a medium-sized herding dog, similar to a rough collie or to the border collie and Australian shepherd descended from it), the same mixed breed as Buck in Jack London's book (though in the book he is described as looking wolf-like, Buck was based on a mixed-breed dog named "Jack," owned by Jack London's friends Marshall Latham Bond and Louis Witford Bond). At the time, Buck's CG design in the movie was based more on a Bernese mountain dog at the request of one of the producers - but when the producers saw Buckley and learned that he was the same sort of mixed breed as the dog in the book (and the real-life dog upon which Buck was based), it was decided that Buck in the film would be a digital scan of Buckley. This is the first time in "The Call of the Wild"'s film adaptations that Buck has been portrayed as a mixed-breed dog rather than a purebred (he has also been portrayed as a full-blooded St. Bernard, a German shepherd, and a Leonberger).
When Buck reaches Skagway for the first time, you can briefly see Charles Chaplin's tramp character (portrayed by actor Colin Woodell, who also plays Mercedes' husband Charles in the film) walking by and gesturing hello to the camera. This is an Easter egg referencing the movie La Ruée vers l'or (1925), supposed to be set in the same area at the same time. The scene in which he appears is immediately after the shot of the man being shaved with an ax. He is in the upper right of the frame deliberately away from the focal point of the action.