The Realm of Animals
The world of animals is that of instinctive gratification of the biological drives of hunger and sexuality. All endeavor is directed to the satisfaction of physical desires and self-preservation. This world is of the ignorant refusal or inability to see beyond the natural needs of the body.
A human can be reborn as a certain animal if his character and habits resemble it. "Human-animals" also exist in our world - a couch potato who only eats, sleeps and watches television all day might gradually resemble a pig and become a "good for nothing" being physically lazy and mindless.
The "Human-animal" is the one who willfully refuses to look at the meaning and purpose of life. His bodily needs for nourishment, sleep and sex might be healthy and he gets satisfaction and enjoyment from them.
But their fulfillment becomes an end to itself. For him, life has no other significance. Though he is ignorant in that he fails to see any higher noble destiny, he is not necessarily stupid in the practical sense, but he has no practical ideals and there is nothing beyond himself which he lives for.
He lives without vision or culture when he can choose to cultivate his mind. Such people are dominated by mental state of ignorance. One who dies tuned to this mindstate is likely to be reborn as an animal.