Buck follows the wounded bull moose until he is able to catch it and take it down.
Buck follows the wounded bull moose for days. There is a tribe of band of twenty moose. Buck sees that “among them was a great bull” and wants to take the bull down. Buck fights him until he wounds him, but then he gets away. Buck follows.
From then on, night and day, Buck never left his prey, never gave it a moment's rest, never permitted it to browse the leaves of trees or the shoots of young birch and willow. (Ch. 7)
Buck is able to catch the bull moose. "Three hundredweight more than half a ton he weighed" and he had come to end of his life. Buck was persistent, and this shows that he could take down even large and strong prey. It is part of his final transition from domesticated to wild beast.