Rabbits have three special features of their digestive system that makes feeding the right diet important. Firstly, their teeth grow continuously—an evolutionary adaptation to a coarse or rough fibrous diet found in the wild. In order to keep their teeth from overgrowing, rabbits chew their food in a circular direction—front to back and side to side. This brings all their teeth into wear and keeps them at the right length. This chewing motion is therefore essential for good dental health but only occurs if they spend long periods of time eating long fibrous material like fresh grass and grass hay (this should comprise about 80% of the overall diet). Eating an excessive amount of short-length foodstuffs (such as grain, chaff, and pellets) encourages them to chew up and down like people. The teeth then don't come into wear and dental disease develops quickly. Secondly, fiber is needed to keep the intestinal tract moving (peristalsis). Low fiber diets can slow, or even stop, this movement. Bloat quickly develops, which is often fatal. Thirdly, rabbits are termed "hindgut fermenters," meaning that they have a large caecum (a little like our appendix but much, much bigger). Fiber is digested in the caecum (fermentation), releasing nutrients essential to their health. Without high levels of fiber in their diet, this process stops, and the rabbit weakens and dies. According to the passage, where do rabbits' essential nutrients come from?