So Buck won the fight to the death against Spitz, the stronger dog, while using strategy.. I'd say he's the new lead dog
After many successful message carriages under Perrault and Francois with Buck as the leader of the pack, they were sold
Under The Scot's supervision doing mail runs, the work as a sled dog got tougher than it was previously.
Under Hal and Charles' supervision participating in the gold rush, the pack of sled dogs suffer through abuse and exhaustion due to the inexperienced oblivious overseers
Some of the dogs die
After lots of suffering, the crew stumble upon John's camp where they are warned about ice on the trail, The warning is ignored and a physical altercation breaks out but ultimately ends in John's victory and him keeping Buck.
Due to the crew ignoring the warning, they all die and Buck grows to truly love John