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What Causes the Bison Rut in Yellowstone National Park?

It's that time of year, friends: the bison rut in Yellowstone! August is the peak of the bison rut, and evidence can be seen and heard in Lamar Valley and Hayden Valley, where herds increase in size from about 200 members to 1,000 during July and August.

Go back and read our post The Rut for info about the elk rut, because this post will be all about 2,000 pounds of sexual aggression— the bison.

What exactly is the rut?

Bison rut in Yellowstone is characterized by the season in which female bison (known as cows) go into heat and are receptive to breeding, called estrus. Cows go into estrus for approximately 9 to 24 hours, every 21 days. This season lasts from June to September, with the peak being in August.

What marks the peak of bison rut?

Male bison will join mixed-sex groups to compete for mating partners... key word there, "partners". A male bison (known as bulls) will mate with many cows, but the cow will only mate with one bull. To prove that they are a fit partner, the bull will increase their wallowing behaviors, begin courting cows, bellow loudly and even clash their horns against competition. It is quite a sight and sound!

Once a bull finds a receptive cow, he will form a tending bond, a temporary pairing forced on the cow by the bull. When the bond is formed, the bull will be within feet of the cow at all times, demonstrating an intolerance for all other members of the herd by bellowing, wallowing and threat displays. The bond will last for a few hours or a few days, depending on when the cow enters estrus. Then, the bull will move on to another receptive cow.

Bison have a olfactory gland that allows them to detect if a cow is in estrus by the smell of her urine, which is an important adaptation because if she doesn't become pregnant in the 9-24 hours that she is in estrus, her next opportunity will be in three weeks.

How to bulls establish dominance?

Ultimately the cow will choose who she mates with, and will typically select a larger and more mature bull. In order to prove dominance, bulls will wallow on dry ground, in a dust bath. During the rut, they will urinate while wallowing, spreading their scent to inform others of their identity, age and social status within the herd.

Another way to establish dominance is by making distinct noises, known as bellows. During the rut, these bellows are made to announce their presence and establish dominance. "Scientific studies indicate that the acoustic qualities such as the frequency, length, and volume of the bellows demonstrate which males are more dominant, and therefore, a better mate to females."

If these nonverbal forms of communication aren't enough, a physical altercation can occur. When one bull won't back down, it can lead to a violent challenge of two 2,000 pound animals ramming their giant heads into each other (I have a headache just thinking about that).

What happens after the rut?

Well, first of all, Lamar Valley becomes a much quieter and less exciting place. Just kidding...kinda.

After courtship, mature males will leave the herd and spend the rest of the year alone or in small groups. In Yellowstone, herd sizes decrease throughout the fall and winter, reaching their smallest numbers in March and April, when the average herd is about 20 individuals.

Cows are pregnant for 285 days, and give birth, called calving season in April and May. Bison calves are born with reddish-tan fur, giving them the common nickname "red dog". They can stand and nurse within 10-30 minutes of being born, and can graze on grass and drink water within one week; and it isn't much longer that they can interact with other calves and keep up on long distance walks with the herd. Cows are great mothers and are very protective of their young, about 80% of bison calves survive to weaning age.

bison calf with herd in yellowstone

For more information about bison, check out the following links:


Featured trip: Lamar to Grand Canyon

This guided backpacking trip covers some of the best scenery Yellowstone has to offer. You'll hike through wide-open meadows scattered with big mammals, up and down on rolling hills and over Mist Pass to Pelican Valley.

Then, we'll head north to alpine lakes, old-growth forests and a diverse ecosystem. And finally, end the trip at the Grand Canyon of the Yellowstone.

This trip is for experienced backpackers who want to see a little bit of everything that Yellowstone has to offer!!


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