Ungulates are large mammals which support their weight on the tips of their hoofs when they walk. Yellowstone is filled with ungulates. There are several members of the deer family including elk and moose and the largest ungulate in North America the bison. At one time there was estimated to be over 50 million bison that roamed North America but they were hunted by the early European settlers until they were almost extinct.
The bison herds of Yellowstone are protected from hunters. Grizzly Bears and wolves occasionally take calves, the sick and old. The main thing attacking bison in Yellowstone are fleas, flies, ticks and other pests. In the top photo a dust bath on the prairie helps to remove some of the parasites. If that does not work you might be able to get an occasional hitch hiker to help out. Birds such as cowbirds and European starling, above, often ride around on the back of the bison and pick off insects. This is a symbiotic relationship which provides food for the birds and some relief from the bison.