The estimated 4,550 bison that roam Yellowstone National Park comprise the nation’s largest bison population on public land. Like their ancient ancestors, Yellowstone’s bison exhibit wild behavior especially during the breeding season known as the rut when they congregate to compete for mates.
During the rut bison mature males display their dominance with vocalizations called bellowing which has been compared to the sound of revving up an old Chevy truck, wallowing where they violently thrash on the ground stirring up dust clouds, and engaging in fights with other bulls. During the rut bison bulls are on high alert and can be aggravated easily.
Yellowstone advises tourists maintain 25 yards from bison throughout the year and recommend increasing that distance during this volatile stage of their life cycle. Clearly these visitors were in violation of the 25 yard rule and quickly discovered that it is in place for their own safety.
One clear indication that this bison was about to charge was the posture of its tail. If a bison’s tail is hanging down it is likely not agitated. As the tail rises so does its level of aggression. A fully raised tail signals a bison highly stressed is associated with aggression, sexual behavior, or perceived danger.
Yellowstone Bison Safety Guidelines:
Bison have injured more people in Yellowstone than any other animal. Bison are unpredictable and can run three times faster than humans. Always stay at least 25 yards (23 m) away from bison.
- Give bison space when they are near a campsite, trail, boardwalk, parking lot, or in a developed area. If need be, turn around and go the other way to avoid interacting with a wild animal in close proximity. 
- Approaching bison threatens them, and they may respond by bluff charging, head bobbing, pawing, bellowing, or snorting. These are warning signs that you are too close and that a charge is imminent.
- Do not stand your ground. Immediately walk or run away from the animal. Spray bear spray as you are moving away if the animal follows you.
