Ruminant Digestion

Ruminant digestion in Bos taurus
    Like other vertebrates, ruminant Artiodactyla (including cattle, deer, and their relatives) are unable to digest plant material directly, because they lack enzymes to break down cellulose in the cell walls. Digestion in ruminants occurs sequentially in a four-chambered stomach. Plant material is initially taken into the Rumen, where it is processed mechanically and exposed to bacteria than can break down cellulose (foregut fermentation). The Reticulum allows the animal to regurgitate & reprocess particulate matter ("chew its cud"). More finely-divided food is then passed to the Omasum, for further mechanical processing. The mass is finally passed to the true stomach, the Abomassum, where the digestive enzyme lysozyme breaks down the bacteria so as to release nutrients. Use of plant material is thus indirect, with primary processing by the bacterial flora maintained in the stomach.

    The Perissodactyla (including horses, rhinoceroses, and tapirs) have evolved a less efficient form of ruminant digestion. Bacterial fermentation occurs primarily in the intestine (hindgut fermentation), such that extraction of nutrients from plant material is less complete. [Compare horse droppings with 'cow flops': the former contains more or less intact plant material that may be scavenged by birds, whereas the latter is essentially amorphous].

    Although all mammals have lysozyme, the enzymatic properties of ruminant lysozyme have evolved to be especially efficient. In a superb example of convergent evolution, some leaf-eating monkeys have evolved a lysozyme with similar enzymatic properties, due to selection on independent mutations to produce identical amino acids at key active sites.