American Bison

The American bison is the biggest land animal in North America and the animal most tied to the story of the continent’s grasslands — their shaping, their near-destruction, and the slow work of putting some of it back. In 2016 it was named the national mammal of the United States.

Overview

Bison are animals of open grassland. Before the 19th century, enormous herds moved across the Great Plains from Canada into Mexico, grazing, trampling, wallowing, and fertilising an ecosystem that had evolved around them for millennia. Today wild, free-ranging bison exist in only a few places — Yellowstone, a scattering of national and tribal reserves, and parts of the Canadian north — while the great majority live in fenced conservation herds or on ranches raised for meat.

Physical characteristics

A bull bison stands nearly two metres at the shoulder and can weigh a tonne, front-heavy with a pronounced shoulder hump of muscle, a massive head, a dark shaggy cape and beard, and short curved horns in both sexes. Despite the bulk, bison are agile: they run at 55 km/h, clear 1.8 m fences, and turn on the spot, which makes them dangerous to approach.

The winter coat is dense and insulating enough that snow settles on a bison’s back without melting.

The keystone grazer

Bison do not graze a pasture evenly. They crop it in patches, move on, and return later, creating a shifting patchwork of closely grazed lawn and tall rank grass. That structural variety supports a wider range of prairie plants, insects, and ground-nesting birds than uniform grazing or no grazing at all.

Their wallows — bare hollows where bison roll to shed fur and deter insects — hold rainwater and become seasonal ponds used by amphibians and invertebrates. Their heavy hooves press seeds into the soil, and their dung and, eventually, their carcasses recycle nutrients and feed everything from dung beetles to wolves.

Behaviour and life cycle

Bison live in herds, usually separated by sex outside the breeding season: cow–calf groups and looser bull groups. During the summer rut, bulls bellow, wallow, paw the ground, and fight head-to-head for access to cows, and the sound of a rutting herd carries for kilometres.

A single reddish calf — a “red dog” — is born in spring, can run within hours, and stays with its mother’s group for about a year.

Threats and conservation

The bison is listed as Near Threatened, and the challenges now are less about numbers than about what a wild bison is:

• Genetics. Many herds carry a small amount of domestic cattle DNA from past crossbreeding, and conservationists work to maintain and expand genetically pure herds.
• Room to be wild. Very few places allow bison to roam, migrate, and be shaped by predators and seasons at a natural scale; most are managed like livestock.
• Disease politics. The Yellowstone herd carries brucellosis, and bison that leave the park boundary are often culled to protect cattle ranching interests.

Tribal nations, through the InterTribal Buffalo Council and similar efforts, are returning bison to grasslands across the plains, both as an ecological restoration project and as a cultural one.