Bison often rub their shoulders and rumps against boulders and tree trunks, and they enjoy taking mud and dust baths. This behaviour scratches off fly larvae and other parasites that live on their hides.
Mature bulls move in separate groups from the cows. In the mating season, in late summer, the males join the females. They fight for the females by ramming each other head-on. After mating, a bull will guard his mate for several days to prevent other rival males from mating with her.