If human beings are traditionally distinguished from animals by their capacity for thought, language, and social feeling, how do we categorize Gregor, who seems to exhibit all of these human capacities but resides in the body of an insect? The Metamorphosis shows Gregor questioning his own humanity as he grows more accustomed to the life of a bug. But it also casts doubt on the humanity of the other characters by showing how they too mimic animal behavior. (Note: we're sticking to a traditional way of looking at animals as not having consciousnesses or minds because we're looking at a work of literature from the early twentieth century. Whether animals have consciousness is a question that biologists and philosophers are still hammering out today.)
As the story progresses, Gregor maintains his human intelligence and feeling, despite the pressing needs of his animal body.
Gregor's behavior as an insect brings out how the other characters behave in an animalistic way: their similarities demonstrate that human beings have an animal side that they cannot ignore.