It is my opinion that the title of Upton Sinclair's novel The Jungle was accurately named. Sinclair shows how men will resort to animalistic tendencies to stay alive. How men who have won the struggle regard the ones losing the same way a lion will see a herd of gazelles.
The first reason is that during the late 1800s/early 1900s the poor working man in Chicago had to fight a battle against the elements and starvation such as an animal. Much like the animals of the world, the unheroic hero of the story has to work, fight, steal or kill to fill his stomach. If he is unable to do this he will die and be allowed to die such as in the animal kingdom. If he is injured and cannot provide for himself he will be thrown aside to freeze or starve.