Did you ever hear the park rangers talk about the size of the Bison in Yellowstone? According to the last talk I heard, the Bison in Yellowstone are about one third smaller than most other Bison because of the malnutrition that results during the winter months. The grasses they eat all winter have little nutrition and if the herds could migrate south they would as they would be able to find better foraging. Still, the Bison are impressive.