Bison move to the lower elevations of the park in the winter to feed. They use their huge heads to shovel the snow and ice out of the way, and feed on dead grasses.If the winter is very harsh they will continue to move to lower elevations which take them out of the park where they are hunted, or rounded up and killed because they have been exposed to a disease called brucellosis. This bacterial disease causes bison and cattle to abort calves, and can be transmitted if an animal eats grass near where another has given birth. Local ranchers are concerned that the bison could spread the disease to their cattle. Ironic since cattle gave the disease to bison in the first place.