Bison are the largest land mammal in North America. Males can weigh over 1,800 pounds and females over 1000. At one time there were over 30 million bison roaming the western plains of North America. Then in the 1800's European settlers began to slaughter the bison herds in order to make room for farms, cattle and horse grazing and the railroads. By the early 1900's the bison herds which once numbered over 30 million were down to several hundred.
Yellowstone National Park is the only place in the lower 48 states of the US where a population of wild bison have continued to exist. In 1902 the bison population in the park dropped to a low of 50 animals. At that point the park imported 21 bison from private herds and began to captively raise them at the Buffalo Ranch in the Lamar Valley. By the 1920 the captive raised bison began to mix with the wild herd. In the 1930's bison were transplanted into the Hayden Valley and around the Firehole River. By 1954 the Yellowstone Bison population was close to 1500 animals.
Today around 3,500 bison roam Yellowstone. Typically you find them in open valleys, such as Hayden Valley and Lamar Valley but you can find them almost any where, including crossing bridges as in the pic above. During the winter they usually stay near the thermal areas of the park where the snow is not as deep. They use their large heads, swaying them from side to side, to clear the snow and get at the grasses below. Winter is hardest on pregnant females who carry the calf over the winter. Breeding takes place in July and August and the calves are born in April and May. We usually visit the park late in May so their are plenty of calves frolicking in the fields or snoozing curled up next to mom.