It’s the beginning of winter and for the next six months temperatures will plummet to twenty degrees below zero. For warmth, buffalo depend on hot springs, thermal geysers and their own natural protective defensives. Their thick winter coats and extra layers of fat insulate them form the cold, while their large powerful heads act as snow plows. Their humps serve as counterweights to their swinging heads, pushing the deep snow out of the way. Their highly developed sense of smell tells them where to dig; they can detect the scent of grass even under a meter of snow.
In this weather vulnerable calves must stay close to the herd for protection. The bison walk slowly through the deep snow in a single line. The lead animal blazes a path which makes it easy for the herd to follow, especially the younger ones. If an animal strays from the herd, it’s almost certainly in trouble. They are stranded in deep snow without a packed down trail to follow, struggling to survive. The herd is now powerless to help him. The herd must continue on, searching for better grazing. As the herd moves on, once navigable rivers become hidden obstacles. If one buffalo falls into an icy river, it is stuck. Once again, the herd can do nothing, and the animal is left there to die alone.
During a severe winter, the bison often migrate to warmer elevations outside the park, where they are not protected. Some of these animals have been exposed to a disease called brucellosis, there has been no evidence of wild bison transmitting this disease to cattle, but nonetheless the state of Montana is not willing to take this risk, and any animals that stray outside the park can be destroyed. During the terrible winter of 1996 over 1,000 bison were shot and killed, nearly 1/3 of the Yellowstone herd. Winters in Yellowstone are long and difficult, but for the survivors spring means renewal, the beginning of a new cycle.