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The Science Behind the Elk Rut in Yellowstone: What Causes This Natural Phenomenon?

You've all probably heard of the rut, but what exactly is it? Why do they do it? How long does it last?

We'll answer all your burning questions and more!

The rut is what scientists call mating season (the word "rut" is derived from the Latin word "rugire" meaning "to roar"). This post specifically discusses the elk rut, but there are many different animals that go through a "rut", including bison, white-tail deer, pronghorn, moose, elephants and even skunks!

Elk rut typically occurs from early-September to mid-October. Although it usually lines up with the changing of the season and cooler temperatures, the rut phase is actually initiated by the length of days. As the days get shorter, the pineal gland releases hormones that increases male testosterone, and makes female elk, called cows, go into estrous (which means they are ready to breed). When a cow goes into estrous, they are only ready and willing to breed for 24 hours, and if the bull elk misses out, he will have to wait another 20 days until the next estrous. A cow elk can have up to four estrous periods during mating season, but is typically impregnated after the first or second.

Once a cow becomes pregnant, it has a 245-day gestation period, and gives birth in early June.

Bull elk are capable of breeding at two years old, although they are in their prime when they are 6-8 years old. Cow elk are capable of becoming pregnant at 18 months, if they weigh at least 430 pounds.

During the rut, bull elk will gather a herd of cows, called a harem. The harem will consist of cows and calves, and is smaller than an elk herd that you can see the rest of the year. Bull elk will frequently bugle to attract females and showcase their presence and fitness. They also bugle to warn off, or accept a challenge from another bull. Typically an elk's bugle starts off as a high-pitched cry and can turn into a deep grunt.

Research shows that they are different types of bugles for different purposes. One type of bugle is meant to attract females to the harem; another bugle is to warn other bulls that he is in the area with his harem; a different bugle is to warn a female when it is wandering too far from the rest of the group, and yet another is to tell other bulls that they are too close to the harem and that there will be trouble if they come any closer.

Turn up the volume and watch the video below to hear three bull elk bugle at each other!

Bulls will obsessively guard their harem, and occasionally another male will come too close, or try to steal the harem. In that case, they will both display their dominance by bugling loudly, marching side to side and thrashing the ground with their antlers. If neither backs down, they will begin to fight, interlocking their antlers to pull the other to the ground. The fights are typically short, but exhausting. And, they are not intended to cause harm, just to show power and strength.

It is a loud and amazing show of dominance, and incredible to witness (from a safe distance, more on that below).

While it may seem like the bull is in charge, it is actually the females that have the power. They will tolerate a male that they find attractive, or run away from one that they don't-- for them, it is all about calf survival, they want the calf that they birth in the spring to have the best chance of survival, so choosing the healthiest and strongest mate is extremely important. She also wants to find security in a harem so she is not harassed by younger bulls who chase down unattached females, this way she can spend more time eating before the winter starts.

How to safely experience the rut? Great question.

Keep your distance, it's as easy as that. Make sure you are 100 yards away from a bull elk and his harem, so make sure you have your binoculars handy!

Each year, there are visitors who are injured because they are too close, and a bull elk (or other animal) chases after them. Being too close puts you and the wildlife in danger, so do your part, and watch them respectfully!

All photos and videos were taken by Matt Hergert. Visit his photography website at


Featured trip: Heart of the Park

Join us on a guided backpacking trip through the “Heart of the Park” and experience a little bit of everything that Yellowstone has to offer!

We'll start at the Grand Canyon of Yellowstone, where we'll see waterfalls and thermal features. Then, move on to experience backcountry lakes, mega fauna, solitude and massive wide-open valleys.

If you're seeking an adventure away from the crowds, or a challenging hike that very few people have the chance to do, this is the one for you!

We can't wait to see you on the trail!

(this is a moose, not an elk)

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