Bison. Bison are nomadic grazers and travel in herds. The bulls leave the herds of females at 2-3 years of age, and join a male herd which is generally smaller than the female herds. Mature bulls rarely travel alone. Towards the end of the summer, for the reproductive season, the sexes necessarily commingle. American bison are known for living in the Great Plains. Both species were hunted close to extinction during the 19-20th centuries, but have since rebounded.