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Bear Ecology



The brown or grizzly bear (Ursus arctos) is one of the largest bears in the world, averaging 400-600 pounds for males and 250-350 pounds for females.  Male bears can stand up to 8 feet tall.  However, they vary in size from region to region, depending on the richness of food sources.  They live up to 35 years in the wild, and are excellent swimmers. Some have even been known to climb trees.  Amazingly, they can run up to 30-40 mph.


Their color varies from black to blond.  During the western frontier days, the brown bear was dubbed the “grizzly” due to its frosted or “grizzled” coat.  The grizzly bear is distinguished from the black bear by its humped shoulders, more upturned snouts and longer claws. 




Grizzly bears arrived in North America about 30-50,000 years ago.  Until 100-300 years ago, the grizzly occupied most of the temperate, boreal and arctic regions of the West.  Today, the largest populations of grizzlies remain in Alaska and Canada. In Eurasia, Russia supports by far the largest population of brown bears, although small populations hang on in Austria, Finland, France, Italy, Norway, Spain, Sweden, Syria, India, Japan and other countries.  They are absent from the hottest and driest deserts – except for the Gobi in Asia.  About, 1,800 grizzlies remain in the lower-48 states, mainly in the Yellowstone and Glacier/Waterton ecosystems of Idaho, Montana, Wyoming, and adjacent lands in Alberta and British Columbia.




The density and home ranges of grizzlies vary by orders of magnitude (50-2,500 square kilometer ranges), primarily depending on the availability of energy-rich foods.  Grizzlies are highly dependent upon learned the learned locations of food and other resources within their home ranges, and are less successful than other carnivores, such as wolves, in colonizing new ranges.




Grizzlies are highly intelligent and opportunistic omnivores.  Although grizzlies are typically solitary, except when mothers are raising young, they will often gather at concentrations of rich food such as a bison carcass or streams full of spawning salmon.


Until the last few decades, four key foods have been critical to grizzly bear survival in the Greater Yellowstone Ecosystem (GYE) of Idaho, Montana and Wyoming. These include: 1. bison, elk, and other large ungulates; 2. whitebark pine seeds; 3. Yellowstone cutthroat trout; and 4. army cutworm moths.

Due to drought, climate change and the illegal introduction of nonnative Lake trout into Yellowstone Lake, cutthroat trout have been decimated and no longer provide reliable late spring and early summer food for bears. There is slim chance that trout will ever recover. Similarly, whitebark pine has been largely eliminated by the nonnative pathogen White Pine Blister Rust, and climate-driven outbreak of mountain pine beetle. Because of climate warming, whitebark pine is not expected to recover. The federal government considers whitebark pine to be endangered. 


Bears also feed on berries, underground stems and roots, grubs, rodents, and numerous plants – but none of these foods match the caloric value of “the big four.” In the wake of losing two of the four mainstay foods (trout and pine seeds), grizzly bears are turning increasingly to eating meat, especially elk and livestock – and, as a result dying at greater numbers because of conflicts with big game hunters and livestock operators. 




Grizzlies have the lowest reproductive rate of all land-dwelling carnivores.  Bears typically first reproduce at four to seven years old, and have two cubs per litter every third year, on average.  The breeding season occurs between May and July.  Remarkably, implantation of the embryo is delayed to allow the mother to fatten up and weigh the chances (at least at a physiological level) of successfully rearing cubs. The actual gestation period is quite short, and the cubs, when born, are the smallest relative to the size of their mom (a tiny 12-16 ounces) of any placental mammal. 


Cubs can spend two to three years under the care of their mother.  After weaning, female adolescents typically occupy ranges in or near the range of their mother, while males can disperse 30 miles or more.  This conservative female behavior makes connecting grizzly bear populations a very long-term proposition.




In the fall, bears excavate dens, which shortly after are typically well covered in snow.  They occupy their dens up to six months, typically beginning in November.  Large reserves of body fat, accumulated during the previous summer, are critical to surviving the period of winter sleep.  During this time, grizzlies do not eat, urinate, defecate or lose any bone mass.




In 1975 the grizzly bear was listed as threatened in the lower - 48 US states under the Endangered Species Act (ESA).  Elsewhere in the world, small isolated populations are also considered to be endangered.  Even in Alberta, Canada, grizzlies are considered to be threatened, raising questions about the future of adjacent US grizzly populations. For twenty out of forty years of ESA protections, the federal government and the states have been trying to remove ESA-mandated safeguards for grizzly bears—a process called “delisting.” Delisting is currently being actively pursued in the Glacier and Yellowstone ecosystems. A final rule to delist Yellowstone grizzlies was issued in July of 2017. 



The primary threat to grizzlies has been, and continues to be, deaths caused by humans either directly or indirectly through intrusion into and destruction of habitat. 


Between 1800 and 1890, grizzly populations in the lower-48 states declined drastically in the face of a westward onslaught of Europeans pursuing furs, minerals, ranchlands, and farms.  Bears were also targeted in predator control programs beginning in 1914. 


Despite 40 years of protection, grizzly bear populations in the Yellowstone and Glacier ecosystems have increased only very slowly, with growth rates less than what has been often claimed by the government. In the Cabinet Yaak, Selkirks, and North Cascades populations – which some call “the walking dead” – virtually no growth has occurred. Low reproductive rates, human hostility and climate change threaten progress towards recovery.


In recent years, grizzly bear mortality in the Rocky Mountain West has risen because of management “control” actions, human defense of life and property, and poaching and malicious killing.  Much of this has more to do with peoples’ attitudes rather than unresolvable conflicts. In contrast to Eurasia, North American grizzlies survive only in areas where there are comparatively few armed people and a relatively small chance of encountering them, largely because people on this continent are often intolerant. 


Grizzly Bears are intelligent, inquisitive and generally peaceful animals that, contrary to popular belief, rarely attack humans. Unfortunately, these bears come into the world with formidable strikes against them, including particularly low reproductive rates, small litter sizes, long periods (two to three years) of being raised by their mothers – and, sadly, a bad reputation.

Bear Basics
Yellowstone grizzly bear photo by Roger Hayden

Image © Roger Hayden - all rights reserved

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