As the proud owner of pygmy goats, I cannot help marveling at how goats and other herbivores can digest grass and other plants, and “make a living” from it. I can't digest grass, and it seems to me that it would be a useful thing to be able to do. Why can't I or any other human digest grass? How do herbivores do it?
It turns out that vertebrates in general do not have the enzymes to digest cellulose, a key sugar in plant cell walls that is useless to animals, into glucose, a sugar supplying energy to the body. Thus, it would seem that vertebrates cannot gain much metabolic benefit from eating plants. This explains why humans can't cannot gain benefit from eating grass, but it does not explain why herbivores can break down cellulose in grass. After all, they do not naturally produce enzymes to break it down!
Yet, if you look closely into the stomach and intestines of herbivores, one will find that there are indeed the enzymes necessary to break down cellulose. Where do these come from?
Scientists have discovered that living in the guts and intestines of herbivores there are in fact a whole host of micro-organisms such as bacteria and protozoans, which make enzymes to break down cellulose. These organisms live in symbiotic relationships with their host organisms; the digestive tract provides a good living environment and food to these micro-organisms, and in turn the micro-organisms break down cellulose, some of it for their own consumption, and some of it for the host organism. Thus herbivores can't break down grass themselves, but enlist help from other organisms. How cool is that?