Bull Bison With Tongue Out – in the Lamar Valley of Yellowstone National Park. The bison mating season or rut, starts in early August and continues late into the month or early into September. This bull was wandering through a small herd of cows and calves of the year looking to perpetuate the species. As is the case with most large mammals during the mating season, bull bison become very aggressive and agitated during the rut. We had stopped the car in a parking area to watch the bulls sort of herd the cows around and fairly quickly found ourselves surrounded by another group that had come up from behind us. I was able to catch the bull bison tasting the air as he looked for a cow who was ready to mate.