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Everything you need to know about grizzly bears in Yellowstone

Yellowstone is home to two species of bears: the black bear and the grizzly bear. Although the grizzly bear population in Yellowstone has been closely monitored since the 1970s, there has been very little continuous research conducted on black bears. The last black bear study was in the 1960s when bears were still eating from garbage dumps and roadside feedings (see our blog post Bears in Yellowstone for more info).

Black bears have a large range across North America, and can be found in Canada, Alaska, the Rocky Mountains, the upper Midwest, the Appalachian Mountains, parts of the southern United States, Texas and down into Mexico.

Grizzly bears are a subspecies of brown bear that once roamed in large regions of prairies and mountains of the American West. But now, the Greater Yellowstone Ecosystem (GYE) and northwest Montana are the only regions south of Canada that have large grizzly bear populations.

In 1975, the grizzly bear was listed as a threatened species. Since being listed, the GYE experienced an increase of population from 136 grizzlies in 1975 to a peak of 757 in 2014. As of 2021, there are an estimated 728 grizzly bears, with approximately 150 having home ranges wholly or partially in the park.

Pictured: Sow grizzly and cub in Yellowstone's Lamar Valley.

Grizzly bears are considered generalist omnivores, meaning their diet changes depending on what is most nutritious and available. Although they can be found in forested areas, they are more likely to be in open meadows and valleys; they use their long claws and muscular shoulders to dig up plants and rodents.

Pictured: Female grizzly feeding on bison carcass in late May.

When they emerge from hibernation, in March to May, their diet consists of winter-killed elk, bison and other ungulates (hoofed mammals), and elk calves killed by predation.

In the summer, they eat grasses, dandelions, thistle and ants, in addition to whitebark pine nuts, which are a high-energy food rich in fats, carbohydrates, and protein and most importantly, army cutworm moths.

Each year, hundreds of thousands of army cutworm moths travel to Yellowstone to feed on the sweet nectar of alpine wildflowers. They feed at night and spend the day resting in the cool shadow

of boulders on steep mountain slopes. Both grizzly bears and black bears take advantage the rich food source and spend hours flipping over the boulders to scoop up the moths underneath. A grizzly can eat more than 40,000 moths in one day. And, usually solitary animals and hunters, bears will tolerate eating in the same area as other bears since there is plentiful food.

Pictured: Grizzly bear with a dandelion hanging out it's mouth, Glacier Bay National Park and Preserve, southeast Alaska.

The period before hibernation is called "hyperphagia", and bears may gain up to three pounds each day to prepare for the winter. Scientists believe that bears evolved to hibernate as a result of seasonal food shortages. Bears hibernate during the cold winter months in most parts of the world; the length of denning depends on the bears location. In Mexico, hibernation could last a few days or weeks, and in Canada or Alaska, it could last six months.

Grizzly bears will occasionally use a den year after year in Yellowstone, especially if it is located in a rock shelter or at the base of a large tree. Excavating a den typically takes 3-7 days and the bear can move up to 2,000 pounds of material. Each den consists of an entrance, tunnel and chamber. To prevent heat loss, the entrance and tunnel are just large enough for the bear to squeeze through, and the chamber is insulated with bedding materials such as spruce duff or whatever is available at the site.

Unlike many other animals that hibernate (some of which freeze for the entire winter!), the body temperature of a hibernating grizzly bear in Yellowstone remains within 12 degrees of their normal body temperature. This allows them to react quickly to danger since they don't have to wait for their body to warm up. Because they stay so warm through the winter, they are able to reduce their metabolic rate to 50-60%, decrease their respirations to one every 45 seconds (as opposed to a normal respiratory rate of 6-10 breaths per minute), and drop their heart rate to 8-19 beats per minute (normal heart rate is 40-50 beats per minute).

Occasionally, bears will leave their den in the middle of the winter, but they typically do not eat, drink, defecate or urinate during hibernation. The urea that is produced from fat metabolism is broken down and rebuilt as protein that allows the bear to maintain muscle mass. So, while bears lose 15-30% of their body weight during hibernation, they increase lean body mass.

Pictured: Fresh grizzly print in Yellowstone.

Pregnant females tend to hibernate earlier and longer than those who are not pregnant. Because implantation of the fertilized egg is delayed, the embryo doesn't start to grow until late November or December, about one month after the mother has denned.

Mating season for bears is in May or June. The fertilized eggs develops into a blastocyst, and then stops developing and remains unattached. It seems like the bears' body has evolved to delay the implantation so the female can conserve energy in the months leading up to hibernation. Once she enters the den, the egg implants and cubs are born a few months later in January. Cubs are born blind and hairless and weigh 8 to 12 ounces. They do not hibernate; they sleep next to their mother, nurse and grow rapidly (at ten weeks, cubs weigh 10-20 pounds). Grizzly cubs will spend 2.5 to 3.5 years with their mother before they are chased away by her or a prospective suitor so she can mate again. Subadult females frequently establish as home range within the vicinity of their mother, but males will go further.

Pictured: Mother grizzly with two hefty cubs in Yellowstone in the fall.

As mentioned above, in 1975, the grizzly bear was placed on the threatened species list. The Fish and Wildlife Service noted that four populations of grizzly bears had been reduced to 2% of their former range, and decreased in population to 800-1000 total bears.

The Endangered Species Act of 1973, made the Fish and Wildlife Service (FWS) responsible for the listing and delisting of any species (animal, tree, coral, etc.), protecting them from any future harm, and creating a plan for restoration. The federal agency works with the National Park Service, Native American tribes, states, and the public to ensure peaceful existence between grizzly bears and humans.

In 2005, FWS proposed delisting the grizzly bear from the threatened species list, and in July of 2017, it was finally removed from the list. Although it seems like a negative thing, it is actually a positive move since the grizzly bear had such a great recovery and increased in population. There are many reasons that a species can be delisted — according to FWS: "we assess the population and its recovery achievements; we assess the existing threats; and, we seek advice from species experts in and outside of the Service. To assess the existing threats, the Service must determine that the species is no longer threatened or endangered based on five factors."

However, in September 2018, a federal judge restored protection of the grizzly bear within the entire GYE, which stretches far beyond the boundaries of the park. The protection and preservation of the grizzly bear will not change within Yellowstone and Grand Teton National Parks where the Park Service will still maintain their long-term monitoring system, and hunting will still be prohibited inside the National Parks.


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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1 comentário

Great write up!! Would love to see the bear flipping rocks for yummm moths

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