Weighing in at nearly a tonne (2,200 lb), the American bison (Bison bison) is the largest terrestrial animal in North America and the largest land animal to migrate. Although 60 million bison once roamed North America, they were overhunted to near extinction by the end of the 19th century, and wild populations are now limited to a handful of national parks such as Yellowstone in Wyoming and the Wichita Mountains National Wildlife Refuge in Oklahoma. A seasonal migrant, the bison moves to lower elevations in the winter to seek shelter and food.