Quote 73
It was a hard trip, with the mail behind them, and the heavy work wore them down. They were short of weight and in poor condition when they made Dawson, and should have had a ten days' or a week's rest at least. But in two days' time they dropped down the Yukon bank from the Barracks, loaded with letters for the outside. The dogs were tired, the drivers grumbling, and to make matters worse, it snowed every day. This meant a soft trail, greater friction on the runners, and heavier pulling for the dogs (4.26)
Buck and his team endure suffering together. From here a sense of camaraderie arises.
Quote 74
They were all terribly footsore. No spring or rebound was left in them. Their feet fell heavily on the trail, jarring their bodies and doubling the fatigue of a day's travel. There was nothing the matter with them except that they were dead tired. It was not the dead-tiredness that comes through brief and excessive effort, from which recovery is a matter of hours; but it was the dead-tiredness that comes through the slow and prolonged strength drainage of months of toil. There was no power of recuperation left, no reserve strength to call upon. It had been all used, the last least bit of it. Every muscle, every fibre, every cell, was tired, dead tired. (5.2)
Buck’s fatigue becomes not just a physical suffering, but emotional and mental as well. He and the other dogs are unable to travel as they are "tired" in many senses of the word.
Quote 75
All day Buck brooded by the pool or roamed restlessly about the camp. Death, as a cessation of movement, as a passing out and away from the lives of the living, he knew, and he knew John Thornton was dead. It left a great void in him, somewhat akin to hunger, but a void which ached and ached, and which food could not fill (7.41)
While Buck’s hunger is initially physical, is becomes emotional as a result of his love for Thornton.