London often uses imagery to intensify the reader's imagination of Buck's experience in the Yukon. In Chapter 2, he uses imagery to describe the changes Buck's body undergoes as he becomes less domesticated:
Sight and scent became remarkably keen, while his hearing developed such acuteness that in his sleep he heard the faintest sound and knew whether it heralded peace or peril. He learned to bite the ice out with his teeth when it collected between his toes; and when he was thirsty and there was a thick scum of ice over the water hole, he would break it by rearing and striking it with stiff fore legs. His most conspicuous trait was an ability to scent the wind and forecast it a night in advance.
London describes how Buck's senses are becoming heightened, but he does not simply state that this is the case. Rather, he uses imagery to invite the reader to engage their own senses by imagining what Buck is experiencing. The reader gets to imagine listening for "the faintest sound" while asleep and smelling the weather in the air. The reader also gets to imagine the feeling of ice between their toes, the stiff joints cold can bring on, and the intense thirst that hard work can cause. London even invites the reader to imagine what it must be like to bite and punch the ice that covers everything in the Yukon, including Buck's body.
All of this imagery helps the reader sympathize with Buck and consider what it must be like to be ripped away from home and thrust into a harsh environment. London uses imagery to urge readers to sympathize not just with Buck's discomfort, but also with the more general discomfort of workers whose jobs force them to labor in practically unlivable conditions.
In Chapter 5, London uses an extended metaphor and vivid imagery to describe Buck and the other starving dogs:
As it was with Buck, so was it with his mates. They were perambulating skeletons. There were seven all together, including him. In their very great misery they had become insensible to the bite of the lash or the bruise of the club. The pain of the beating was dull and distant, just as the things their eyes saw and their ears heard seemed dull and distant. They were not half living, or quarter living. They were simply so many bags of bones in which sparks of life fluttered faintly.
London calls the dogs "perambulating skeletons," or walking skeletons. The metaphor helps the reader visualize how thin the dogs are becoming as Hal, Charles, and Mercedes starve them. It is easy to imagine the contour of their ribs showing through their skin: rather than strong, muscular dogs, they look like mere "bags of bones." Beyond this visual imagery, the metaphor also creates the sense that the dogs are eerie, undead creatures who have been hollowed out. Hal has just taken an ax to Billee and killed him for falling down while harnessed in the traces. It is almost as if the dogs left alive perished most of the way with Billee: although their bodies keep "perambulating," they bear up under the "bite of the lash" and "bruise of the club" without even "sensing" them. The "sparks of life" remain, barely lit, inside their bodies.
Hal, Charles, and Mercedes are clearly running an unsustainable operation. The dogs will not make it much farther before the sparks of life go out. And yet their bodies continue working past the time when their spirits are broken, so Hal, Charles, and Mercedes temporarily benefit from driving the dogs this hard. London was a socialist, and over the course of his life he developed great contempt for a labor system in which wealthy business-owners exploited their workers and kept them subjugated. The way the humans transform the dogs into "perambulating skeletons" parallels the labor system London was so eager to critique. Workers had very few rights at the beginning of the 20th century. Labor organizers began agitating for their rights by asking not only for their jobs not to kill them physically but also for their jobs not to snuff out their spirits. For example, several years after London published The Call of the Wild, women employed as textile workers in Lawrence, Massachusetts, went on strike and used "bread and roses" as their slogan. The slogan referred to a poem and song about how people deserve "bread" to sustain their bodies and also metaphorical "roses" to sustain their hearts. In other words, the textile workers refused to be treated as walking skeletons. London describes the dogs' physical and spiritual starvation to demonstrate that exploitative labor practices only work so long and are bad for everyone in the end.