No one knows just how big a single bison herd used to be, but estimates are anywhere from 4-12 million individuals.
Bison are highly social animals, today living in herds of thousands. Adult males, called bulls, are more solitary and join herds during the breeding season. Female bison -- cows -- typically give birth to one calf a year. Bison calves have reddish hair at birth; the dark brown hair doesn't fully emerge until they are about four months old.