What is the significance of animal representation of human behavior of experience in literature such as Franz Kafka's books and the Animal Farm?
is using this animal allegorical approach common or does it depend on the writer's individuality
The animal figurations in Kafka and Orwell are quite different in terms of the purpose behind this allegorization. In fact, it is more an exploitation of bestial allegory in Animal Farm, where animals are an allegorical detour for avoiding any direct involvement with the political issue at hand. It speaks volumes of animal courage and their right to resist. There is corruption and betrayal even in their world, but there is also class-struggle, revolt, emancipation and heroic sacrifice in it. Orwell also implies a paradox of heroism that a novel like The Red Badge of Courage also underscores, that courage/heroism, ironically enough, a matter more of passion and animal-like instinct rather than human rationality.
In Kafka, however, the physicality of the metamorphosis is important. His animal is not man represented as animal but man turned into, or better still reduced to a state of radical unreason. In stories like Metamorphosis, there is a vision of an absurd world where signification is proliferated out of existence and the radical replacement of rationality of man with the instinctive irrational animal self is a post-Darwinian insight into an absurd link between the man and the world.
Using such allegories seems to be fairly common in literature, as in a book Alley Cat that I read when very young and in the books you cite. However, the usage does tend to depend entirely on the author's individual opinions, views, and goals for their works. What aspects are presented also vary based on the message that the author is trying to convey. Is (s)he trying to show humans as animals, anthropomorphizing animals, or using the animals as a way to give a 3rd party view of a common human condition? To really answer those questions, you have to know the work well. Animal Farm, for example, both depicts the animalistic tendencies of human nature, as well as giving a vital 3rd party view of what was, at the time, a major interpersonal injustice between the wealthy and the working class. Much of it's allegory is still relevant today, but it is no longer as inflammatory a piece as it was when first written and released.
Great question. Best brain workout I've had all week!