Season of rut on the range
There’s a pattern to male bison behavior when they are interested in the females. “Bulls begin to court females through a variety of behaviors such as sniffing female genital areas and face-to-face lip curls. Wallowing behaviors also increase, where males will roll violently on the ground to display aggression. This wallowing behavior can cause so much dust to rise that the herds can disappear behind clouds of dust! A bull’s tail also indicates mating status and behavior. A tail held high in a “question mark” fashion indicates a threat or challenge. This question mark signal is commonly seen as bulls lead a female away for mating.
Once a bull finds a receptive female, he will form a tending bond to keep other bulls ways from her. These bonds can last from a few minutes to a few days, depending on when the female will accept copulation. During these tending bonds, the bull demonstrates intolerance for all other group members through a variety of bellowing, wallowing, and threat displays. — National Park Service Bison Bellows: Peak Rutting Season https://www.nps.gov/articles/bison-bellows-7-27-16.htm
When I came up on this herd (staying in my car as required by this park) I was able to identify many behaviors including: sniffing, wallowing, herding, scenting the air through an open mouth and lots of posturing and grunting.
Here are a few of the pictures from this day/activity