Jack London included the detail because "it is a part of the exposition that introduces Sol-leks' characteristics."
Explanation:
The detail is an exposition that introduces Sol-leks characteristics.
Exposition means showing or narrating what happens at the start of the story. Generally, it talked about the background of a milieu, or a character, which in this scenario is Sol-leks.
The details illustrate what kind of dog Sol-leks is, what are his likes and dislikes. Because these characteristics or details will play a crucial role as the story goes.
As the story goes, the position of Buck and Sol-leks will be reversed. Sol-leks being afraid of Buck because of a brute strength he displayed later on.
