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The process of foregut fermentation
Herbivorous mammals face several challenges when feeding: plant matter is tough to digest, relatively low in fat and protein, and the majority of nutrients are located within tough cell walls. Mammals lack the enzymes needed to break down these cell wall compounds, which include cellulose and lignin, and thus can only access a small portion of the nutrients in vegetation by themselves.

Many microorganisms, on the other hand, can synthesize cellulytic enzymes. A symbiotic relationship exists between almost all herbivorous mammals and microbes: mammals provide food and living space for the microbes, and receive the byproducts of fermentation (including fatty acids and sugars) in return. However, microbial fermentation is a slow process, requiring time and space in order to occur. In the majority of herbivorous mammals (including the Perissodactyla) the large intestine is expanded to house microorganisms (a special sac called a caecum is frequently present). The downside to such hind-gut fermentation is that many byproducts of fermentation cannot be absorbed so far down the alimentary canal. Ruminants are able to take advantage of these byproducts because fermentation occurs BEFORE digestion occurs, with microbes being given space in their capacious stomach.

The stomach of all ruminants has four chambers: the first chamber (the rumen) forms a huge fermentation vat, containing billions of microbes (bacteria, protozoans, and fungi) which anaerobically break down the ingested plant material before it is enzymatically digested by the ruminant. The second and third chambers, the reticulum and omasum, act as filters, allowing liquids (including microorganisms) and small, digestible food particles to pass through to the fourth chamber, the true (enzymatic) stomach known as the abomasum. Larger particles are selectively filtered out and retained in the rumen to be broken down further, allowing ruminants to extract maximal nutrition from their diet.

To complement the fermentation process, ruminants periodically regurgitate and rechew their food. Aptly called rumination (or "chewing the cud" ), this mechanical digestion physically breaks down the larger food particles to increase the surface area for microbial action. By chewing their food twice, ruminants avoid having to chew their food thoroughly while foraging (often a vulnerable activity) - proper mastication can be performed while resting, with the head raised and alert for danger.

As in hind-gut fermentation, microbes in the rumen produce sugars and short-chain fatty acids during the digestion of cellulose - these are the principal source of energy for the ruminant. However, the same microbes also synthesize vitamins, detoxify plant poisons, and recycle nitrogenous compounds into proteins - all actions which help the microbes survive. Unfortunately for the microbes, as fluids are selectively filtered out of the fermentation chambers, the microorganisms are also carried to the stomach to be digested: ruminants actually derive the majority of their nutrients from microbes, not directly from plant material! While this is a highly successful strategy, neither foregut fermentation nor rumination are defining characters of this suborder: similar strategies have evolved independently in camels, kangaroos, and colobine primates, among others.

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Literature Cited

Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia.