Winter bison
Winter bison Bison in the winter © TNC/Lane Ketterman

Stories in North Dakota

North Dakota

Bison seem to thrive in the winter.

North Dakotans know that winter can be a harsh time. Its cold temperatures can cause wildlife to migrate, escaping to warmer climes, or hibernate, literally sleeping through the season.

But bison seem to thrive in the cold. They’re animals built for life on the prairie year-round. Visitors to parks and preserves with bison herds enjoy watching bulls, cows and calves mingling together during the summer months. Come late fall, the bulls wander off to form “bachelor bands” and the cows, calves and non-breeding bulls stay together in small herds. These dispersed, separate groups make for fewer animals looking for food in the same place once winter snows make forage hard to find. It’s a good survival strategy.

Bison also need to eat less in the winter. Their metabolism actually slows, allowing them to survive on less forage and on forage that is of poorer quality than summer’s green growing plants. Bison keep food in their rumen (the first chamber of their stomach) longer than domestic cattle, allowing the microbes in their gut to more fully break-down low-quality forage that can be mostly cellulose. A bison cow in winter may eat 40% less than an Angus cow. Ranchers raising bison for market appreciate this savings. Bison, in fact, are so efficient in their forage use that they do not need to increase their food intact or burn fat reserves unless winter weather brings sustained cold temperatures (at least 20 or more degrees below zero).

Bison are good at finding forage under the snow. Using powerful neck muscles, they swing their massive heads to push snow out of their way. Deep snow is less of an obstacle than hard, crusted snow that would require bison to use their much smaller hooves to break or paw snow. Bison have been seen grazing successfully in snow four feet deep in Yellowstone. They seek out bunch grasses with leaves close together that provide compact sources of nutrition.

Bison also are good at staying warm. Their summer coat turns dark brown in the fall and develops a dense undercoat of woolly hair that provides insulation during cold weather. Guard hairs provide added protection and are longest on the front half of a bison’s body, covering the head, neck, forelimbs and tall hump above the shoulders. The winter coat on a bull bison can be as much as six inches thick over the hump. During a storm, bison turn and face into the wind, causing these long hairs to lie down against the body, maximizing their insulating ability. Their winter coat insulates so well that a storm’s snow simply accumulates on the animals, unable to be melted by their body heat.

Eric Rosenquist, manager of the Conservancy’s Cross Ranch in North Dakota where bison roam on the mixed-grass prairie of the Missouri River hills, is impressed by these snow-encrusted animals. “They are resilient. They can withstand both the heat of summer and the freezing cold of winter. They even seem comfortable in the two feet of snow and 40-below temperatures of the ranch in January.”

Visit Cross Ranch or Theodore Roosevelt National Park to watch bison during the winter or throughout the year.