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.

In The Call of the Wild by Jack London, Buck was a St. Bernard/Scotch shepherd mix. His father was a very large St. Bernard named Elmo, and his mother was the shepherd. Buck was not as big as his father but still weighed about 140 pounds, which is large for any dog. He started out his life as a family pet, living with a judge and his family, until Manuel the gambling gardener took him and sold him, which was, of course, how Buck ended up in the Arctic amidst the gold rush. His days in the sunny Santa Clara valley were over. Buck's breed was very helpful to him because he had the St. Bernard's thick furry coat and the shepherd's intelligence. Though he may not have chosen such a life for himself, he was able to adapt pretty quickly and become a leader among dogs and wolves and even men.