What animal does Buck harass and attack over the longest period of time?

In "The Call of the Wild," Buck engages in a prolonged and intense interaction with a moose. This encounter showcases Buck's growing strength, instincts, and assertiveness as he adapts to his wild surroundings. Throughout the story, Buck's development includes honing his hunting skills, and the moose represents a significant challenge due to its size and strength.

While Buck does encounter other animals, such as wolves and deer, his relentless pursuit and eventual successful harassing of the moose stand out as a key moment that demonstrates his connection to the wild and his evolution from a domestic dog into a creature of the wilderness. This particular struggle is vital in illustrating the themes of survival and adaptation central to Buck's character arc.