When does buck pull a 1000 pounds call of the wild?

In Jack London's "The Call of the Wild," Buck pulls a 1,000-pound sled during a pivotal moment in the story when he is challenged by the team to prove his strength and leadership. This feat occurs after Buck has adapted to the harsh realities of life in the Yukon and has become a strong, resourceful dog. His ability to accomplish this task signifies his transformation and acceptance of his primal instincts.