JACKSON, Wyo. — According to the Department of the Interior, bison rut season has begun in the Greater Yellowstone Ecosystem (GYE).
Tyler Greenly, professional wildlife guide with Jackson Hole EcoTour Adventures, explained to Buckrail last year that rutting behavior in bison begins as early as late June and starts picking up in late July going into August. Young males have the potential to mate when they’re as young as four or five years old, but typically aren’t mating with females until at least eight years of age.
For bison, as well as other herbivores that are extremely social including wildebeests in Africa and caribou in Canada, bulls mate with multiple females but will do what’s known as “tending.” Greenly said this behavior means one male will attach himself to a female that is coming into heat and stay with her for up to a week. While he’s with her, the male will guard her from all other males while he continuously performs a flehmen response, which looks like a grimace or an exposure of the mouth/teeth, to see if she’s receptive to mating.
According to Greenly, the flehmen response is a way for mammals to communicate with each other. It’s associated with an organ located in the nasal cavity known as Jacobson’s organ, which is located behind the upper palate of bison because bison, like other even-toed ungulates, lack upper incisor teeth.
While all mammals have a Jacobson’s organ, not all use it; in humans, Greenly says the organ is like a tailbone that no longer serves a true function.
Bison males inhale to try and smell a certain pheromone that a female bison releases when she starts ovulating. The males will pull back their lips and stick out their tongues. Greenly says the pheromone is best detected through the female’s urine, so males will follow the female they’re tending and smell every time she pees.
Male bison will also urinate in a dust wallow and roll in their own urine to seem more impressive to the females and more intimidating to other males.
“It’s like bison cologne,” Greenly said.
Greenly points out that females have a lot more choice in bison herds than people might think. If a female doesn’t like the male she’s with, she’ll run away from him.
“Females are a lot lighter and smaller than the males, so they can usually run faster,” Greenly said. “They’ll gallop away, and then all of these other males will see this happening and think this is their chance. You’ll have all these other males also chase her. On tours, we’ve seen up to nine different males chasing a female.”
Because of this social competition and stress, Greenly emphasizes that through September, bull bison are at their most heightened state of aggression. While visitors to Yellowstone and Grand Teton national parks should never approach a bison closer than 25 yards, Greenly said that bison are even less tolerant to human behavior right now and might need to be given more space if they’re showing signs of stress.
According to Greenly, there are three big ways that people can identify if a bison is stressed before it charges or acts aggressively. One is that a bison will lift its tail straight up in the air like a flag. Another sign is that bull bison will bulge their eyes.
“We see this with a lot of male animals under stress,” Greenly said. “You can see the whites of their eyes because they open them super wide, and are looking around quite a bit.”
Lastly, head-tossing or swinging their head around can be a sign of anxiety, particularly with younger bull bison.
Greenly said visitors to the GYE are lucky that bison are still relatively tolerant to humans. He said in Africa, species like hippos and Cape buffalo don’t have that same tolerance to people or their vehicles.
This story runs annually.
