Most animals can’t digest fiber well, and it travels through them without much happening. Rabbits, however, have this nifty way of breaking down fibrous plant material and getting to all the nutrition stored inside.
In the rabbit GI system, food goes into the mouth, down the throat, through the stomach, through the intestines, then through the colon and out the other end, just like it does for most mammals. But…the enzymes present during that process can’t break down fiber well at all. So rabbits divert much of the fiber to the caecum. (diagram credit: Pet Habitat)