Bison and other  animals migrate to lower elevations in search of forage.Yellowstone national park is not a zoo it has no fences keeping wildlife in the park.The animals migrate outside the park boundaries,about half of bison test positive exposure to brucellosis.Brucellosis is a disease that can cause susceptible domestic cattle to abort their first calf.Yellowstone bison migrate into Montana,their exposure to brucellosis concerns the cattle industry in that state.