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16.5B: The Rumen and Ruminant Animals

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  • Ruminant animals (such as deer and cows) digest food in a four-chambered stomach with the help of special bacteria, protozoa, and fungi.


    Identify how ruminant animals host symbiotic bacteria


    Key Points

    • Ruminant animals use a special four-chambered stomach with a unique microbial flora to digest tough cellulose found in the plants in their diets. Most vertebrates cannot make cellulase, the enzyme that breaks down cellulose, but microbes in the rumen produce it for them.
    • Ruminants chew and ingest plant matter and then swallow it. The plant matter is separated into liquids and solids in the rumen, and liquids drain into the reticulum. Solids in the rumen are then regurgitated into the mouth to be chewed and further broken down.
    • Liquids pass from the reticulum into the omasum, where sugars, fatty acids, and other nutrients are absorbed into the blood stream.
    • After the omasum, food passes into the abomasum, which is much like the stomach in non-ruminant (monogastric) animals, and from there moves into the small intestine, where it is digested.

    Key Terms

    • Rumen: The first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed.
    • Abomasum: The fourth and final stomach compartment in ruminants. It secretes rennin – the artificial form of which is called rennet, and is used in cheese creation.
    • Omasum: The third compartment of the stomach in ruminants. Though its functions have not been well-studied, it appears to primarily aid in the absorption of water, magnesium, and the volatile fatty acids produced.

    A Ruminant’s Multi-chambered Stomach

    Ruminants are mammals that digest plant based food by processing it in a series of chambers in their stomachs. There are about 150 species of ruminants, including both domestic and wild species. Ruminating mammals include cattle, goats, sheep, giraffes, bison, moose, elk, yaks, water buffalo, deer, camels, alpacas, llamas, and antelope.

    Ruminants differ from non-ruminants (called monogastrics) because they have a four-chambered stomach. The four compartments are called the rumen, the reticulum, the omasum, and the abomasum. The rumen and the reticulum are connected and work in concert and are therefore sometimes called the “reticulorumen”.


    The digestive tract of a ruminant: The ruminant digestive tract has four compartments, the rumen, the reticulum, the omasum, and abomasum.

    The Ruminant Digestive Process

    • Ruminants chew plant matter to mix it with saliva and swallow. The food then enters the first two stomach chambers, the reticulum and rumen (or reticulorumen).
    • The reticulum and rumen work together to separate solids and liquids. Contractions push solid food particles back up into the rumen, while liquids are drained into the reticulum. Specialized microbe species live in the rumen and help ruminants break down cellulose.
    • Solids are formed into a bolus, called “cud,” in the rumen and the solid cud is regurgitated back up to the mouth where it is chewed a second time, and returned to the reticulorumen to repeat the process.
    • Liquid digesta in the reticulum is passed into the omasum where nutrients and water are absorbed into the blood stream.
    • After this, digesta is passed into the abomasum, which is similar to the stomach of other animals. After the abomasum, digesta moves through the large and small intestines.

    Ruminants are of interest to microbiologists because they have unique species of bacteria, yeasts, protozoa, and fungi in their rumens. The plant matter consumed by ruminants is high in cellulose, but vertebrates cannot produce cellulase which is the enzyme required to break down cellulose. Thus ruminants depend on the symbiotic microbes in their guts to break down cellulose for digestion. There is no oxygen in the rumen, so bacteria in the rumen are typically anaerobes or facultative anaerobes.