Few sights are more symbolic of the American frontier then the bison. The American bison is the largest terrestrial animal in North America. They are good swimmers and can cross rivers over half a mile (1 km) wide. They are nomadic grazers and travel in herds. The bulls leave the herds of females at two or three years of age, and join a male herd, which are generally smaller than female herds. Mature bulls rarely travel alone. Towards the end of the summer, for the reproductive season mixed sex herds are formed. During the rut, which lasts from June through August, the bulls will fight for dominance within the now commingled herds with the winners being rewarded with reproductive rights.
American bison are known for living in the Great Plains. The bison were nearly hunted to extinction in the latter half of the 19th century when an estimated 50 million bison were slaughtered for their hides and meat with only a few hundred surviving at the turn of the century. Now numbering close to 400K, most bison are now raised as domesticated livestock. The Yellowstone herds, numbering between 4-5K are one of the few wild herds still extant.
On a personal note, bison are one of my favorite subjects on my annual visits to Yellowstone. Yes, I like more stellar species such as grizzlies, foxes, wolves, etc., but I find the bison to be both interesting and photogenic and can spend hours observing a herd waiting for a behavior of interest to occur. They seldom disappoint me.