The American Bison (Bison bison) is the largest land mammal in Yellowstone National Park, and its grazing habits are fundamental to the park’s grasslands. These large herbivores shape the landscape, a role ecologists call ecosystem engineering. Understanding their diet is central to appreciating how they maintain their health and influence the biodiversity of the Greater Yellowstone Ecosystem. Their specialized digestive system enables them to thrive on this diet, which drives their seasonal movements.
The Primary Forage Base
The bulk of the Yellowstone bison’s diet during the growing season (late spring through early fall) consists of graminoids. These fibrous plants include various grasses and grass-like sedges, which can constitute over 90% of the animal’s total food volume. Sedges, especially those found in wet bottomlands, are highly utilized forage items.
Bison prefer to graze in open meadows and river valleys where these plants are abundant, optimizing their nutrient intake. They are bulk grazers, consuming large quantities of available forage rather than selecting small, specific parts of plants. This non-selective pattern supports the regrowth of high-quality, young shoots.
While grasses and sedges are the staple, bison also consume forbs (broad-leafed flowering plants) and some woody plants as a supplementary part of their summer diet. This variety helps them meet nutritional demands by varying protein and energy intake. Heavy grazing stimulates the nitrogen cycle, causing plants to regrow with a higher crude protein content.
Seasonal Changes in Diet
The availability and quality of vegetation decline significantly in winter, forcing bison to adjust their diet and feeding behavior. As snow accumulates, herds migrate to lower-elevation winter ranges, such as the northern range or thermal basins, where snow depth is less restrictive. They still face the challenge of accessing forage buried under deep snowpack.
Bison possess powerful neck muscles and a massive shoulder hump, allowing them to use their heads in a unique snow-plowing motion. This action clears snow away from the ground, enabling them to reach dormant grasses and sedges underneath. They may clear snow up to four feet deep to uncover vegetation.
The grasses accessed in winter have low nutritional value because the plant tissues have senesced. To supplement this poor forage, bison occasionally browse on the twigs, bark, and leaves of woody vegetation, such as willows and cottonwoods. Survival relies on their ability to efficiently extract every possible nutrient from this tough, low-quality food source.
Specialized Digestive System
The bison’s ability to thrive on a diet dominated by fibrous plants is due to its classification as a ruminant with a multi-chambered stomach. This system is adapted for foregut fermentation, a process that breaks down cellulose, the structural component of plant cell walls. The four-chambered stomach begins with the rumen, the largest compartment, which acts as a fermentation vat.
The rumen hosts a dense population of specialized microorganisms, including bacteria and protozoa, that produce cellulose-digesting enzymes. After initial swallowing, food is stored in the rumen, then periodically regurgitated for thorough re-chewing, known as “chewing the cud.” This mechanical action further breaks down the fibrous material, making it accessible to the microbes.
Once processed and fermented, the plant material passes through the remaining stomach chambers. The final chamber, the abomasum or “true stomach,” uses strong acids and digestive enzymes to break down the material, including the microbes themselves. This physiological mechanism allows the bison to obtain protein and energy indirectly from the microbes, enabling survival on coarse, high-fiber vegetation.