The bison, also called buffalo, has long symbolized the wild power of the American West and the Great Plains. Prehistoric bison entered North America from Siberia more than 350,000 years ago, and as the species evolved, modern herds once ranged from the eastern slopes of the Rocky Mountains to the western slopes of the Appalachian Mountains. Their numbers are estimated to have peaked between 50 and 60 million animals in the mid-1800s. Bison are North America's largest land mammal: a full-grown bull stands more than six feet tall at the shoulder and can weigh more than 2000 pounds, while a cow may stand up to five feet tall and weigh around 1000 pounds. Despite their size, they are quick and agile, able to sprint upwards of 40 mph and turn sharply because of their small hind quarters. In winter, bison use their heads as snowplows to uncover grasses and small shrubs, and their heavy fur insulates nearly their entire bodies against harsh cold. Although commonly called buffalo, they are distinct from the buffalo species of South Africa and Southeast Asia, and the name buffalo for bison arose in the mid-1800s from the French term les boeufs. Most calves are born between mid-March and early June with reddish fur that later turns dark brown, and within a year they develop horns, a shoulder hump, larger size, and may gain more than 350 pounds. Calves can stand within minutes of birth, run slowly within three hours, and begin grazing on grass after about one week, though most nourishment comes from their mother's milk until they are yearlings. Bison also roll in dirt wallows that help rid their fur of biting insects. During the rutting season from July to mid-September, bulls compete for breeding rights by bellowing and butting heads, then tend cows until they are ready to mate; bulls can mate by age three, cows by age two, and a bull may lose up to 300 pounds during this season.