Late-summer visitors to Yellowstone National Park get to witness the annual bison rut. The rut—a term used for bison mating season—lasts from June to September, with most of the activity occurring between July and August.
August is typically the peak of the bison rut, and evidence of courting is concentrated in Lamar Valley and Hayden Valley, where herds can increase in size from about 200 members to 1,000.
As males join the herds, they court and mate with multiple females; though females will only mate with one bull each year. Visitors might see mature males with their mouth open, tongue out and upper lips pulled back. This is how they sniff pheromones and determine whether a female is ready to breed, according to park officials.
Bulls will also increase wallowing behaviors, bellow loudly and even clash their horns against competition. The winners of these bouts are more likely to find receptive females. Cows tend to prefer larger and more mature bulls as mates.
While this is an exciting time to see bison in action, the rut can also pose threats to visitors. With testosterone raging among the bulls of the herd, the bison rut means bull are on high alert and can get aggravated easily. Park officials always encourage visitors to stay at least 25 yards away from bison, but advise additional caution during the rut.
“Bison are unpredictable and can run three times faster than humans,” warn park officials. “Use extra caution and give them additional space during this time.”
After courtship, mature males will once again leave the herd and spend the rest of the year alone or in small groups. In March and April, herd sizes decrease to their smallest numbers, averaging about 20 bison.
Cows are pregnant for 285 days, and give birth in April and May, bringing new life and excitement to the park once again. Bison make great mothers and about 80% of calves survive to weaning age and become a part of the herd.
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