In the first chapter of the novel, Buck enjoys his carefree life on Judge Miller's estate, which is located in the sunny Santa Clara Valley. London goes on to write that Buck is a St. Bernard/Scotch shepherd mix breed and weighs roughly around one hundred and forty pounds. Buck's father was a massive St. Bernard named Elmo, who had been Judge Miller's inseparable companion, and his mother was a small Scotch Shepherd named Shep.
At one hundred and forty pounds, Buck is still considered a large dog, whose defining features are his strength, tenacity, and courage. As the novel progresses, Buck is transported to the Northland, where he becomes a sled dog alongside huskies. Buck is significantly larger than the huskies on his dog team and is typically twice their size. Despite his large size, Buck keeps pace with the other dogs and eventually earns the position of lead dog.