Mercedes steals from the fish sacks and feeds the dogs more than their rations when no one is watching; this action proves detrimental because it creates a food shortage later on.
Mercedes, the sister of Hal, the cruel man who obtains Buck is a soft, pretty woman accustomed to being pampered. Because of this condition, she brings too much on their trip, weighing down the sled; in addition, she insists upon riding in one of the sleds, adding another one hundred and twenty pounds to the pulling load. Obviously, she does not understand the merciless conditions of the Yukon, and what must be done for survival because "the heavy load they [the dogs] dragged sapped their strength severely."
Then, because of Mercedes's overfeeding, the food rations are depleted and Hal must underfeed the dogs as there is nowhere for him to obtain more food. Besides this condition, the dogs were made to go farther, and the men did not no how to adjust to other conditions. "Not only did they not know how to work dogs, but they did not know how to work themselves." These instances of incompetence point to the naturalistic themes of Jack London in which man is conditioned and controlled by environment, instinct, or chance.