If a bestial human is reborn in the animal or inferior realms for behaving like an animal, wouldn't have mean that every animal that behaves like an animal is also sent down there?

Perhaps the distinction is that the animal lacks the ability to think for itself, thus eventually earns a human existence simply by comprehending the animal one. The human in contrast is given a greater ability to think for them self, and thus has higher responsibilities and descend or ascend depending on whether or not they take advantage of their own intelligence and will power?