Existing comment: American Bison
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.
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