Male bison sparring in Yellowstone National Park, Wyoming. The rutting season (mating season) takes place from July to September, and this was taken in October, so the bulls had pretty much established their dominance but still had some extra testosterone to burn off. During the mating season is when big bulls rejoin the herds, and it's when they are most dangerous. Bison are unpredictable, and will change from appearing lazy to charging at 35 mph in the blink of an eye, often without warning or reason. The tree I was hiding behind while photographing this herd did little to help me feel safe, and these bulls eventually got too close for comfort and I had to move further away from the action.