How did Billie die in call of the wild?

In "The Call of the Wild," Billie, a character and one of Buck's fellow sled dogs, dies from exhaustion and the harsh conditions of their journey. He is worn down by the grueling demands of pulling the sled and ultimately succumbs to the relentless struggles of survival in the brutal environment. His death symbolizes the harsh realities faced by the dogs in the story, highlighting the toll of the unforgiving wilderness.