Grizzly Bear

They can be distinguished from black bears by longer, less curved front claws, humped shoulders and a more concave facial profile.

Grizzly bears are generally larger and heavier than other bears. Adult males average 400 to 600 pounds (200 to 300 kilograms) and adult females 250 to 350 pounds (110 to 160 kilograms) in the 48 contiguous states.

Their coloration varies from light brown to nearly black. The coat features longer guard hairs over a dense underfur with tips that are usually silver or golden in color – hence the name grizzly.

Grizzly bears are long-lived mammals, generally living to be around 25 years old, although some wild bears have lived for over 35 years.

Grizzly bears have three life stages: dependent young, subadults and adults. Dependent young are usually less than 2 years old and depend on and are associated with their mother, relying on her for food, protection, and survival. There are two primary sub-categories of dependent young: cubs, defined as cubs born during the most recent denning season and less than one year old, and yearlings.

Grizzly bears have a promiscuous mating system. Mating occurs from May through July, with a peak in mid-June.  Although females mate in spring and early summer, their fertilized embryos do not implant into the uterus for further development until late fall. Fat stores obtained by female grizzly bears at the end of fall are positively correlated with earlier birth dates and faster growth rates of their cubs. Additionally, a body fat threshold may exist below which females may not produce cubs, even when bred. Cubs are born in the den in late January or early February and nurse for 3 to 4 months inside the den and after den emergence, but also increasingly eat foods with their mother once outside the den.  Yearlings den with their mother but do not nurse in the den. Outside of the den, yearlings eat the same foods as their mother, but also occasionally nurse.

Shortly after den emergence, 2-year-old offspring generally leave their mother to become subadults. Subadults are typically not sexually mature enough to breed; however, a small percentage of 3-year-old females do breed and produce cubs as 4-year-olds. Some subadults, generally males, may disperse away from their mother and establish their own home range.

Adult bears are more than 4-years-old and have reached sexual maturity. Some bears may not breed until they are older than 5-years-old, but they have the ability to reproduce once they reach the adult stage. Adults generally live into their mid- to late-20s, although some wild bears have lived over 35 years. Female reproductive senescence starts around age 25 for those long-lived individuals.

Age of first reproduction, which averages 5.8 to 6.3 years, litter size, which averages 2.1 to 2.19 cubs per litter, and inter-birth interval, which is the average number of years between litters, averages 2.78 to 3.4 years, may be related to nutritional state and, or, density dependent effects and varies between ecosystems. Grizzly bears have one of the slowest reproductive rates among terrestrial mammals, resulting primarily from these reproductive factors: late age of first reproduction, small average litter size and the long inter-birth interval. Given these factors, it may take a female grizzly bear 10 or more years to replace herself in a population. The slow reproductive rate should also be understood in the context of having one of the longer life spans of terrestrial mammals. To that end, Ursus arctos in 90th percentile for longevity. With a population being made up of numerous overlapping generations, it is possible for mothers, daughters and granddaughters to be reproductively active at the same time. Grizzly bear females typically cease reproducing some time in their mid-to-late 20s.

Grizzly bears use a variety of habitats. In general, a grizzly bear’s individual habitat needs and daily movements are largely driven by the search for food, water, mates, cover, security or den sites. The available habitat for bears is also influenced by people and their activities. Human activities are the primary factor impacting habitat security and the ability of bears to find and access foods, mates, cover and den sites. Other factors influencing habitat use and function for grizzly bears include overall habitat productivity, which is defined by food distribution, quality and abundance. Overall habitat productivity is also defined by the availability of habitat components, like denning areas and cover types. Additionally, grizzly bear social dynamics, learned behavior and preferences of individual grizzly bears, as well as grizzly bear population density and random variation are important aspects. Water is an important habitat requirement as well; however, we have no information to suggest that water is limiting in the habitat that bears currently occupy, but may have limited distribution in portions of historical range. 

Grizzly bears use a variety of cover types to rest and shelter. Grizzly bears often select bed sites with horizontal and vertical cover, especially at day bed sites suggesting that bed site selection is important for concealment from humans. The interspersion of open areas as feeding sites associated with cover are important, probably because diverse habitat complexes, such as forest interspersed with moist grass-forb meadows, provide both abundant food and cover. Generally, areas with vegetative cover are important to grizzly bears for use as bedding sites. Beds underneath any type of vegetative cover, not necessarily always forest cover, provide bears shade during the hottest parts of the day and a place to sleep at night. 

The six ecosystems occur in mountainous ecoregions and each ecosystem provide the habitat heterogeneity necessary for adequate food, denning and cover resources. Because there are limited opportunities to increase or control these habitat components, the objective for grizzly bear habitat management has been, and continues to be, to reduce or mitigate the risk of human-caused mortality and displacement. An effective habitat management tool for reducing grizzly bear mortality risk on public lands is managing motorized access to ensure bears have secure areas away from humans. 

Unmanaged motorized access: 

• Increases human interaction and potential grizzly bear mortality risk
• Increases displacement from important habitat
• Increases habituation to humans
• Decreases habitat where energetic, meaning food, requirements can be met

Managing motorized access on public lands helps ameliorate these impacts. Other habitat management tools that minimize displacement and reduce grizzly bear mortality risk include regulating livestock allotments and developed sites on public lands. Implementing food storage orders on public lands also reduces mortality risk for both humans and grizzly bears. Requiring users and recreationists in grizzly bear habitat to store their food, garbage and other bear attractants in bear-proof, inaccessible ways reduces encounters and human-grizzly bear conflicts. In addition, encouraging users and recreationists to carry bear spray, and know how to use it, helps reduce the potential for injury to people and bears. 

The 48 contiguous states provide highly diverse landscapes which contain a wide array of habitat types and bear foods across and within the ecosystems. Plant communities vary from grasslands at lower elevations, which are defined as less than 6,230 feet (1,900 meters) to shrub fields that are created by fires, avalanches or timber harvest, to conifer forests at mid-elevations and subalpine and alpine meadows at higher elevations, defined as greater than 7,870 feet (2,400 meters). Grizzly bears are opportunistic omnivores and display great diet plasticity - even within a population. As such, individuals shift their diet according to foods that are most nutritious, for example, available foods that are high in fat, protein, and, or, carbohydrates. An extensive literature review documented more than 260 species of foods that grizzly bears consume in the Greater Yellowstone Ecosystem, which represents four of the five kingdoms of life. The ability to use whatever food resources are available is likely one reason brown bears are the most widely distributed bear species in the world, occupying habitats from deserts to alpine mountains and everything in between. This ability to live in a variety of habitats and eat a wide array of foods makes grizzly bears a generalist species. In contrast, specialist species like mountain lions, eat only a few specific foods or live in only one or two specific habitat types. Morphological adaptations that support a diverse diet include crushing molars and the greatest intestinal length relative to body length of any carnivore.

Grizzly bear diets are highly variable among individuals, seasons and years, and between ecosystems. They opportunistically seek and consume whatever plant and animal foods are available to them. Grizzly bears will consume almost any food available including living or dead mammals or fish, insects, worms, plants, human-related foods and garbage. In areas where animal matter is less available, berries, grasses, roots, bulbs, tubers, seeds and fungi are important in meeting protein and caloric requirements. Grizzly bears often sample new foods so that they have alternative options in years when preferred foods are scarce. In the Greater Yellowstone Ecosystem, it has been noted that, after 10 years of food habits data collection, new feeding strategies continued to appear annually in this population.

In addition, grizzly bears opportunistically prey on livestock, agricultural crops, like grain, corn, garbanzo beans and melons, as well as other human foods. Cattle and sheep depredation rates are generally higher where bear densities are higher, and in later summer months. In the Greater Yellowstone and Northern Continental Divide ecosystems, depredation is generally higher where livestock is more abundant, such as areas with livestock allotments and privately owned ranchland. Livestock grazing is less common in the Cabinet-Yaak and Selkirks ecosystems, and depredation rates are correspondingly lower. Grizzly bears also opportunistically prey on small livestock, such as chickens, llamas, and goats, which primarily occur on private land.

Food resources are especially important during the period leading up to hibernation when grizzly bears must consume energetically rich foods to build up fat reserves to survive denning and post-denning periods. Fat stores provide a source of energy and insulate the bear from cold temperatures during hibernation. Also, fat stores obtained by female grizzly bears at the end of fall are positively correlated with earlier birth dates and quicker growth rates of their cubs. Additionally, a body fat threshold may exist below which females may not produce cubs, even when bred; studies have shown that females with less than 20% body fat are unlikely to produce cubs. However, we are unaware of a minimum body fat threshold for survival during the denning period.

Grizzly bears hibernate in winter; hibernation is a life history strategy bears use to cope with seasons of low food abundance. In preparation for hibernation, bears increase their food intake dramatically during a period called hyperphagia. Hyperphagia occurs throughout the two to four months prior to den entry, which runs August through November. During hyperphagia, excess food is converted into fat, and grizzly bears may gain as much as 3.64 pounds a day (1.65 kilograms a day). Grizzly bears must consume foods rich in protein and carbohydrates in order to build up fat reserves to survive denning and post-denning periods. Fat stores are crucial to the hibernating bear as they provide a source of energy and insulate the bear from cold temperatures, and are equally important in providing energy to the bear upon emergence from the den when food is still sparse relative to metabolic requirements. However, we are unaware of a minimum body fat threshold for survival during the denning period and documentation of natural mortality in independent-age bears is low for non-collared individuals.

Grizzly bears in the 48 contiguous states hibernate in dens for four to six months each year, typically entering dens between October and December, with males entering their dens later than females. Females give birth to cubs in the den in late January to early February. On average, males exit dens from early March to late April. Females typically emerge from their dens from mid-March to mid-May, with females with cubs emerging later from mid-April to late-May. 

Grizzly bears typically hibernate alone in dens, except for females with young and subadult siblings who occasionally hibernate together. Grizzly bears usually dig dens on steep slopes where wind and topography cause an accumulation of deep snow and where the snow is unlikely to melt during warm periods. Most dens are located at higher elevations, above 8,000 feet (2,500 meters) in the Greater Yellowstone Ecosystem and 6,400 feet (1,942 meters) in the Northern Continental Divide Ecosystem, and on slopes ranging from 30 to 60 degrees. In the Cabinet-Yaak Ecosystem, the majority of den sites occurred above 5,248 feet (1,600 meters), often on northerly and easterly aspects, though all aspects were used. In the Selkirks Ecosystem, the majority of dens were located above 5,248 feet (1,600 meters), often on easterly aspects, but all aspects were used. The North Cascades Ecosystem contains large areas at high elevations with isolated, steep, snow-packed slopes and many natural caves to serve as potential den sites. Additional areas associated with ridge systems stemming from major volcanic peaks may provide den sites at lower elevations within the North Cascades. In 1991, Davis and Butterfield assessed the northern part of the Bitterroot Ecosystem recovery zone and areas to the immediate north, and concluded that deep snow and mountainous terrain provides adequate denning habitat.

Denning increases survival during periods of food scarcity and inclement weather. During this period, bears do not eat, drink, urinate or defecate. Hibernating grizzly bears exhibit a marked decline in heart and respiration rate, but only a slight drop in body temperature. Due to their relatively constant body temperature in the den, hibernating grizzly bears may be aroused and have been known to exit or relocate dens when disturbed by seismic or mining activity or other human activities. Dens are rarely used twice by an individual, although individuals usually use the same general area from year-to-year. Females display stronger area fidelity than males and generally stay in their dens longer, depending on reproductive status. Females with cubs usually spend a few weeks close to their den upon emergence, unlike solitary bears.