I think that the first answer is probably the right one, but there is at least one other possible answer.
Another thing that saves Buck's life is the lesson that he got when Curly was killed by the pack of dogs early in the story. From that episode, he learned that you must at all costs not fall down in a fight or else the other dogs will all jump on you and kill you.
This saves Buck's life, you can say, when he fights with Spitz. He knows that he has to do everything he can to avoid being knocked down. This was a very vicious fight, and without that lesson he might not have won.
Buck's best human master, John Thornton, saves him just minutes before the rest of the dog team perishes in Call of the Wild. The inexperienced group who had purchased the team decided to push onward despite the fact that the dogs were weak and starving. When Hal began to whip them again, Buck and the other dogs refused to move. Hal continued with the whip and then the club, and John Thornton could take no more. He "sprang upon the man who wielded the club."
"If you strike that dog again, I'll kill you," he at last managed to say in a choking voice.
When Hal pulled a knife, "Thornton rapped Hal's knuckles with the axe handle, knocking the knife to the ground." Since Buck was "too near dead to be of further use," Hal allowed him to be left behind. Within minutes, Hal's party and the rest of the dog team plunged through thin ice and disappeared "into a yawning hole." When Thornton showed his sympathy toward Buck, the grateful dog licked his hand. A new friendship had begun.