Yes, the American Bison is classified as a ruminant, which explains its remarkable ability to thrive on the tough grasses of the prairie. This classification points to a sophisticated digestive strategy that enables the animal to extract maximum nutrition from fibrous plant matter. Like other grazing mammals in this group, the bison possesses a unique, multi-chambered stomach system specifically adapted for fermenting and processing high-cellulose diets.
What Defines a Ruminant
A ruminant is an even-toed, hoofed mammal that engages in rumination, or chewing the cud. This process involves partially chewing food, swallowing it, and later regurgitating the material to chew it again. This repeated chewing mechanically reduces the size of the coarse plant material, making it easier for microbes to access nutrients.
The ruminant digestive system is characterized by a single stomach divided into four distinct compartments: the rumen, the reticulum, the omasum, and the abomasum. This complex arrangement facilitates foregut fermentation. This system is an adaptation allowing these animals to subsist on diets high in cellulose.
Mammals cannot produce the enzyme necessary to break down cellulose, so they rely on a symbiotic relationship with a dense population of microorganisms. The digestive tract provides the ideal warm, moist, and anaerobic environment for these microbes to flourish and begin the chemical work of digestion.
The Bison’s Specialized Stomach
The bison’s stomach is a massive organ, occupying nearly 75% of its abdominal cavity, with the four compartments working sequentially to process large amounts of low-quality forage. The first and largest chamber is the rumen, which acts as a vast fermentation vat, holding up to 40 gallons of material in a mature animal. Here, ingested grasses are mixed with saliva and separated into layers, beginning the microbial breakdown.
Connected to the rumen is the reticulum, a smaller compartment with a honeycomb lining that functions primarily as a filter. The reticulum traps heavy objects and helps form the bolus that is regurgitated for rechewing. After the cud is swallowed again, the fine particles pass into the omasum, a chamber with many folds of tissue.
The omasum’s primary role is to absorb excess water and volatile fatty acids. Finally, the digesta enters the abomasum, considered the true stomach because it functions similarly to that of non-ruminant animals. This chamber secretes hydrochloric acid and digestive enzymes to break down the material and digest the microbes themselves as a source of high-quality protein.
How Bison Digest Tough Grasses
The bison’s successful survival on the North American prairie is directly linked to the microbial activity within its rumen. Billions of bacteria, protozoa, and fungi housed in the rumen ferment the cellulose found in tough grasses, chemically transforming it into a usable energy source for the bison.
This fermentation process yields three primary Volatile Fatty Acids (VFAs): acetic acid, propionic acid, and butyric acid. These VFAs are absorbed directly through the rumen wall into the bison’s bloodstream, providing the majority of the animal’s energy requirements.
The long retention time of food, which can take nearly 80 hours, maximizes the amount of fiber that can be broken down. This allows the bison to utilize grasses and sedges more efficiently than some other grazers, especially when forage quality is low. This ability to extract nutrients from tough plants is why bison became the dominant large herbivore on the prairie ecosystem.