Yellowstone National Park is home to one of North America’s largest and most genetically pure herds of free-roaming bison. These massive herbivores are an iconic symbol of the American West, and each year they engage in a reproductive period known as the rut. This annual event is a captivating display of natural behavior.
Rut Season Timeline
The bison rut in Yellowstone National Park occurs from late July through September. Activity peaks during early to mid-August, when the park transforms into a dynamic showcase of primal behavior. This breeding season is influenced by environmental signals and the bison’s biological rhythms. The timing ensures that calves are born in the spring, when conditions are optimal for their survival.
During this period, bull bison can lose nearly 200 pounds or 12% of their body mass. This weight loss occurs because the bulls dedicate their time and energy to breeding activities rather than grazing. The rut’s duration and intensity can vary based on factors like weather patterns and the overall health of the herd.
Behaviors During the Rut
During the rut, male bison, known as bulls, become more aggressive and restless, driven by a surge of testosterone. They engage in various behaviors to establish dominance and attract females, or cows. Bulls bellow, producing deep vocalizations that can be heard across the landscape, displaying their presence and vigor.
Wallowing is a behavior where bulls roll in dirt depressions, often urinating in them to cover themselves in pheromones. This action creates a distinctive “bison cologne,” signaling their identity and dominance to other bison. Sparring, or head-butting, is also common as males compete for mating rights, with these impacts sometimes sounding like thunder. When a bull finds a receptive cow, he will stay close, performing courtship rituals such as licking, nuzzling, and a low rumble known as “tending” to signal his intentions.
Observing the Rut Safely
For visitors hoping to observe the bison rut, areas within Yellowstone National Park include Lamar Valley and Hayden Valley. Bison are visible from pullouts in these locations, especially during the cooler parts of the day such as early morning and late afternoon. However, rutting behavior can be observed throughout the day.
It is important to maintain a safe distance from all wildlife, particularly during this period. The National Park Service recommends staying at least 25 yards (23 meters) away from bison and elk. For more dangerous animals like bears and wolves, a distance of at least 100 yards (91 meters) is advised. Bison are unpredictable and can run up to three times faster than humans, making close encounters dangerous.
If a bison exhibits signs of agitation, such as head bobbing, pawing the ground, bellowing, snorting, or a raised tail, it indicates you are too close. Visitors should use binoculars or telephoto lenses for viewing to avoid disturbing the animals. Remaining in your vehicle can also provide a protective barrier while observing these creatures. Approaching, feeding, or attempting to interact with bison is prohibited and can lead to dangerous situations for both people and wildlife.