Bison

Overview

The American bison is a large ungulate characterized by its distinctive shoulder hump, massive head with curved horns, and shaggy brown coat. Adult males (bulls) can weigh up to 2,000 pounds, while females (cows) are somewhat smaller. Bison were once the most numerous large mammal on the continent, with populations estimated between 30-60 million before European colonization. Historically, bison ranged from Alaska to Mexico and from Nevada to the Appalachian Mountains, playing a crucial role in grassland ecosystems through grazing patterns that influenced plant diversity and nutrient cycling. Their near-extinction by the late 1800s, when fewer than 1,000 individuals remained, represents one of the most dramatic wildlife collapses in history, driven by commercial hunting, habitat loss, and deliberate extermination policies. Today, bison exist in fragmented populations across North America, primarily in protected areas, tribal lands, and private ranches. Conservation efforts have brought the species back from the brink, though most populations are small and geographically isolated. Bison are ecologically extinct across most of their former range, meaning they no longer fulfill their historic role in shaping prairie ecosystems. Bison are highly social animals that form herds with complex hierarchies. They are well-adapted to harsh climates and can use their massive heads to sweep snow aside to reach vegetation in winter. Two subspecies are recognized: the plains bison (B. b. bison) and the wood bison (B. b. athabascae), with the latter being larger and darker.

How to identify

Adult bison are unmistakable: massive bovids with a pronounced shoulder hump, large head carried low, short curved horns in both sexes, and dark brown woolly fur on the head, neck, and forequarters. Body length reaches 7-11.5 feet, shoulder height 5-6.5 feet, and weight ranges from 800-2,000 pounds. Males are significantly larger than females. The coat is lighter brown on hindquarters and legs. Calves are reddish-brown. Both sexes have horns that curve upward and inward. Bison have a distinctive beard under the chin and mane on the shoulders. Vocalizations include grunts, snorts, and bellows, particularly during rutting season.

Habitat

Historically occupied grasslands, prairies, river valleys, and open woodlands. Plains bison prefer mixed-grass and shortgrass prairies, while wood bison inhabit boreal forests and meadows in northern regions. Bison require areas with adequate grazing, water sources, and space to accommodate their migratory and wallowing behaviors. They utilize diverse vegetation types and create habitat heterogeneity through selective grazing and trampling.

Distribution

Historically ranged across most of North America from Alaska and northern Canada south to Mexico, and from the Rocky Mountains east to the Appalachians. Current distribution is highly fragmented, with populations in Yellowstone National Park, Wind Cave National Park, Grand Teton National Park, tribal lands (particularly in Montana, South Dakota, and Oklahoma), Canadian national parks (Wood Buffalo, Elk Island, Grasslands, Prince Albert), and numerous state parks and private ranches across the Great Plains.

Diet

Herbivorous grazers feeding primarily on grasses and sedges. Bison consume a variety of grass species including blue grama, buffalo grass, wheatgrass, and needle-and-thread grass. During winter or drought, they supplement their diet with forbs, woody vegetation, and lichens. Bison are ruminants with a four-chambered stomach that allows them to digest fibrous plant material. They spend 9-11 hours per day grazing and require large quantities of forage, consuming approximately 1.6% of their body weight daily.

Behavior

Highly social, forming herds that historically numbered in the thousands, though modern herds are typically smaller. Female-led groups include cows, calves, and young bulls, while mature bulls form bachelor groups or remain solitary outside breeding season. Bison engage in wallowing behavior, rolling in dust or mud to thermoregulate and deter insects. They undertake seasonal movements in search of forage and have strong swimming abilities. Bison are generally docile but can be unpredictable and dangerous, particularly during rutting season (July-September) or when protecting calves. They can run at speeds up to 35-40 mph despite their size.

Reproduction & life cycle

Breeding season (rut) occurs July through September, peaking in late July and August. Bulls compete for mating access through displays, vocalizations, and occasional combat. Females reach sexual maturity at 2-3 years, males at 3 years but typically don't successfully breed until 6 years. Gestation lasts approximately 285 days (9-9.5 months), with most calves born in April-May. Single calves are typical, twins rare. Calves weigh 30-70 pounds at birth and can stand within 30 minutes. They are weaned at 7-12 months but may remain with their mothers for up to 3 years. Females typically give birth every year or every other year.

When to look

Year-round residents; calving occurs primarily April-May; rutting season July-September; most visible in open grasslands during spring through fall; winter viewing possible but animals may be less active and concentrated near thermal areas or sheltered valleys

Cultural significance

Bison hold profound spiritual, cultural, and economic significance for many Indigenous peoples of North America, who depended on them for food, clothing, shelter, tools, and ceremonial purposes for millennia. The deliberate destruction of bison herds in the 19th century was used as a strategy to subjugate Native populations. Today, bison restoration efforts are central to tribal sovereignty and cultural revitalization initiatives. Bison appears on U.S. currency, state flags, and became the national mammal of the United States in 2016. The species symbolizes conservation success, American wilderness, and the frontier era. Bison meat is commercially produced and considered a lean, healthy alternative to beef.

Similar species

• European bison (Bison bonasus)
• Domestic cattle (Bos taurus)
• Muskox (Ovibos moschatus)

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