Working Hard to Survive the Winter

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.