Life on the trail is incredibly hard for dogs and humans alike. For one thing, it's absolutely freezing cold, and even the dogs' thick fur can't completely protect them from the biting wind and driving snow. Pulling a heavy sled is hard enough at the best of times, but in the depths of winter it's especially punishing, even for a skilled, hard-working animal like Buck.
Aside from the weather, the dogs have to endure attacks by wolves and wild dogs. These animals are ferocious; even worse, they're fearless. When a pack of wild dogs descends upon the team, Buck is forced to fight for his very life. To make matters worse, he also has to fend off his vicious rival Spitz, who takes advantage of all the chaos and confusion to settle scores with his old enemy.
Disease also takes its toll. After being bitten by a wild dog, it's hinted that Dolly, one of the trail dogs, contracts rabies and goes completely insane. Whatever the precise cause of her insanity, she chases and attacks Buck before being shot dead by Francois. For the dogs of the trail, it really is a case of survival of the fittest, the Darwinian message at the core of the story.