What role does the dog play in the story and what does it come to symbolize?

The dog is an interesting character in this story. The dog survives the attack on him but is no longer able to bark. The animal seems attached to Boyd and follows them as they set out. Over the coming weeks, as the boys travel around Mexico, the dog will travel with them. At one point, the boys wake to some danger and Billy seems aggravated with the dog, but Boyd points out that the dog couldn't have warned them with a bark because of the animal's injury. The animal seems to become a symbol for what the boys have lost in their lives.