Methanogens are strict anaerobes that release methane as a waste product of cellular metabolism. Many live in mud at the bottom of lakes and swamps because it lacks oxygen, and some (enteric bacteria) live in the intestinal tracts of animals. A large number of methanogens colonize the intestinal tracts of ruminary grazing animals, where they aid in the digestion of cellulose. The methanogens living in the human digestive tract can cause intestinal gas.