ABSTRACT
Grasslands supplied a new nutritional environment that was inhospitable compared with those previously available. Readily available nutrients became progressively diluted with fiber. Fibrous material tends to be bulky, and digestion is slow. Digestion in mammalian herbivores is generally a two-stage process; namely, fermentation and enzymatic digestion. Protein digestion takes place without either mechanical maceration such as chewing the cud or initial microbial attack of cell-wall components. Consequently, protein extraction is less efficient than in ruminants but protein is immediately assimilated as amino acids. Digestion is less efficient than in ruminants of similar size, more food can pass through the gut during the same period of time. Ruminants underwent particularly spectacular adaptative radiation. The omasum is small and functionally unimportant in the smallest ruminants and absent in the ancestral tragulids. In ruminants, most microbes are destroyed and digested in the abomasum where the pH is usually two.
