What purpose does the dog serve in "To Build a Fire"? How do London's descriptions of the dog reveal its purpose?

Without the dog, the unnamed protagonist would be all alone. That would force the author Jack London to remain inside the man's mind from beginning to end. This could become tedious. The dog offers a diversion. Although the dog cannot talk, it is almost like a conversation between two characters because they have two different points of view. Jack London was very good at interpreting what was going on in dogs' minds, as he did...

Without the dog, the unnamed protagonist would be all alone. That would force the author Jack London to remain inside the man's mind from beginning to end. This could become tedious. The dog offers a diversion. Although the dog cannot talk, it is almost like a conversation between two characters because they have two different points of view. Jack London was very good at interpreting what was going on in dogs' minds, as he did in in his novel The Call of the Wild, although he might have been taking some "poetic license" and did not really know what a dog or any other animal might be "thinking." Here is a good example of how Jack London makes the shift away from the man's consciousness into that of the dog:

The animal was depressed by the tremendous cold. It knew that it was no time for traveling. Its instinct told it a truer tale than was told to the man by the man's judgment. In reality, it was not merely colder than fifty below zero; it was colder than sixty below, than seventy below. It was seventy-five below zero. Since the freezing-point is thirty-two above zero, it meant that one hundred and seven degrees of frost obtained. The dog did not know anything about thermometers. Possibly in its brain there was no sharp consciousness of a condition of very cold such as was in the man's brain. But the brute had its instinct. It experienced a vague but menacing apprehension that subdued it and made it slink along at the man's heels, and that made it question eagerly every unwonted movement of the man as if expecting him to go into camp or to seek shelter somewhere and build a fire. The dog had learned fire, and it wanted fire, or else to burrow under the snow and cuddle its warmth away from the air.

Oddly enough, we learn a great deal from the dog even though it is an animal and can neither think nor speak. It relies on its instinct, but it has a long lineage going back into the prehistoric past. Its ancestors were wolves. Those who survived in this merciless environment passed on the instincts that had enabled them to do so. Ironically, the dog knows a great deal more than the man but is unable to tell him anything. The man would not listen anyway. The dog's other purpose is to show the contrast between instinct and judgment. Every time we find ourselves in the dog's point of view, we invariably feel that the dog is in most respects not only wiser but better than the brutal man. In the end, the man freezes to death in the snow while the dog trots away looking for shelter and warmth. We feel that the dog is lucky not only to have survived, but to have gotten away from such a cruel and selfish master.