When eating with the other dogs, Buck learns that he has to steal in order to have enough to eat. Not only is he a bit bigger than the other dogs and so needs more, but also the others steal his rations at the least opportunity. At first he fights the dogs when they steal from him, but this does him little good because while he's busy fighting the non-combatants finish off what remains of his food. Therefore he has to adapt, and takes his cue from his companions. He sees them stealing from Francois and Perrault and he begins to do the same. He learns that he has to do whatever it takes in order to survive in this harsh environment where food is scarce. This is the survival of the fittest, where fair play means little:
He swiftly lost the fastidiousness which had characterised his old life. (chapter 2)
In other words, he is no longer bothered about such things as stealing. Back in the civilised South, he would never have stolen food, especially from his masters; here, it doesn't matter. All that matters is that he keeps himself strong.