Bison are massive land mammals, encompassing both the American and European species. They are strictly classified as herbivores, naturally adapted to a plant-based diet anchored in their physiology and feeding behaviors. This article explores the details of the bison’s herbivorous diet, the specialized digestive system that enables it, and the rare, non-standard behaviors that sometimes lead to the consumption of non-plant matter.
The Primary Herbivorous Diet
Bison are classified as grazers, meaning their diet consists overwhelmingly of grasses and grass-like plants known as graminoids. In North America’s Great Plains, primary food sources include species such as blue gramma, sand dropseed, and little bluestem. Studies show that grasses can comprise up to 90% or more of their total food intake.
The sheer volume of this plant matter is substantial, and a bison’s grazing habits are a major factor in maintaining the prairie ecosystem. They selectively graze on grasses, which reduces competition and allows other plant species, like forbs, to flourish. This grazing pattern creates a mosaic of vegetation across the landscape.
The bison diet shows a distinct seasonal variation driven by the changing nutritional quality of forage. During the spring and summer, they consume grasses when protein content is highest. In the late summer and fall, bison increase their intake of forbs—broadleaf, non-woody flowering plants—and shrubs to supplement their diet. Woody browse is also utilized, particularly during winter months when primary grasses are covered by snow or have significantly reduced nutritional value.
The Ruminant Digestive System
The biological necessity for the bison’s plant-only diet is rooted in its highly specialized digestive system, which defines it as a ruminant. Like cattle and deer, the bison possesses a stomach divided into four distinct compartments: the rumen, reticulum, omasum, and abomasum. This complex structure is engineered to process the tough, fibrous material found in plants.
The rumen is the largest compartment, acting as a massive fermentation vat where the initial stage of digestion occurs. It houses a diverse population of specialized microbes, including bacteria and protozoa. These microbes produce enzymes capable of breaking down cellulose, the complex carbohydrate that forms the structural walls of plant cells and which non-ruminants cannot digest.
After the plant material is partially broken down, it moves to the reticulum, where coarser particles are sorted and sent back to the mouth to be re-chewed. This process, called rumination or “chewing the cud,” may occupy the bison for 7.5 to 9 hours a day. This repeated chewing and re-swallowing further reduces the particle size of the forage.
The re-chewed cud then passes through the omasum, which absorbs water and fine particles. It finally enters the abomasum, often called the “true stomach,” which uses strong acids and digestive enzymes. The abomasum breaks down the microbes and remaining food material, allowing the bison to absorb nutrients. This slow, microbe-dependent process is optimized for extracting maximum energy from low-quality, high-fiber vegetation.
Are There Any Exceptions to the Rule?
The answer to whether bison eat meat is that they are herbivores, and their anatomy prevents them from being effective predators or regular carnivores. However, like many other large herbivores, bison have been observed engaging in rare behaviors that involve consuming small amounts of animal matter. These isolated instances are driven by nutritional deficiencies, not a carnivorous instinct.
The most common non-plant consumption is known as osteophagia, the practice of chewing or consuming bones. Bison, like cattle, may seek out dried bones or shed antlers to supplement their diet with phosphorus and calcium, minerals that can be scarce in the prairie grasses they consume. This behavior is a form of mineral supplementation, not a predatory act.
A bison’s grazing style, which involves sweeping large mouthfuls of grass, also results in the accidental ingestion of animal matter. They may unintentionally consume insects, small rodents, or bird nestlings hidden within the dense vegetation. While they may occasionally investigate or lick carrion, their digestive system is not adapted to process meat regularly, and such acts are not a meaningful part of their energy intake.