Bison are incredibly well equipped to survive the harsh winters of Yellowstone National Park and the Great Plains. Their heavy coats protect them from temperatures as low as -25 degrees F. Their bodies are so well insulated against the cold that snow and freezing rain that land on them don't melt. This is why bison are often seen with faces totally covered with snow or frozen moisture as they exhale. Bison use their massive heads to sweep aside deep snow as they search for the grass below.
Their adaptations notwithstanding, about nine of every 100 bison will die during the winter. Most of these were ill, injured, old or in poor condition at the start of winter. Young bison facing their first winter have a higher mortality rate, with between 20 and 40 of every 100 not surviving. Major causes of death include accident, lack of food and predation.
A bull bison's face is coated with snow after using its massive head to sweep away deep snow as it searches for grass in Yellowstone National Park.
Bison coats insulate them so well that snow that falls on the coat doesn't melt, so small is the amount of heat that escapes from the body. Bison can survive temperatures as low as -30 F with little or no discomfort. The dead grass beneath the snow is brown and has little nutritional value.
