The American bison (Bison bison), also commonly known as the American buffalo, is a North American species of bison that once roamed the grasslands of North America in massive herds. A bison has a shaggy, long, dark brown winter coat, and a lighter weight, lighter brown summer coat. The male bison are slightly larger than the female and, in some cases, can be considerably heavier. Adult male bison stand around six feet tall at the shoulder and weigh up to approximately 2000 pounds. The heads and forequarters are massive, and both sexes have short, curved horns that can grow up to 2 feet long, which they use in fighting for status within the herd and for defense. Fort Niobrara refuge is home to a herd of approximately 350 bison.