Grizzly bear (Ursus arctos horribilis)
Taxonomy:
View taxonomy in ITISListing Status: Threatened and Experimental Population, Non-Essential
General Information
Grizzly bears (Ursus arctos horribilis) are generally larger and heavier than other bears. Adult males average 200 to 300 kilograms (kg) (400 to 600 pounds (lb)) and adult females 110 to 160 kg (250 to 350 lb) in the lower-48 States. Their coloration varies from light brown to nearly black. They can be distinguished from black bears by longer, less curved front claws, humped shoulders, and a facial profile that appears concace. 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 into their mid- to late-20s, although some wild bears have lived for over 35 years. Grizzly bears are a member of the brown bear species (U. arctos) that occurs in North America, Europe, and Asia. The subspecies U. a. horribilis is limited to North America and historically existed throughout much of the western half of the lower-48 States, central Mexico, western Canada, and most of Alaska. Prior to 1800, an estimated 50,000 grizzly bears were distributed in one large contiguous area throughout all or portions of 18 the westernmost States. Grizzly bears were probably most common in the Rocky Mountains, along the Upper Missouri River, and in California. Grizzly bears were less common or did not occur in large expanses of the North American deserts and Great Plains ecoregions. With the arrival of Europeans to North America, grizzly bears were seen as a threat to livestock and human safety and, therefore, an impediment to westward expansion and settlement. In the 1800s, in concert with European settlement of the American West and government-funded bounty programs aimed at eradication, grizzly bears were shot, poisoned, and trapped wherever they were found. The resulting declines in range and population were dramatic with rapid extinction of populations from Mexico and most of the central and southwestern United States and California. Grizzly bears were reduced to close to 2 percent of their former range in the lower-48 States by the 1930s, with a corresponding decrease in population. By 1975, grizzly bear populations in the lower-48 States had been reduced in number to 700800 and restricted largely to the confines of National Parks and Wilderness areas in Washington, Idaho, Montana, and Wyoming. The 1993 Recovery Plan identified six ecosystems, with recovery zones at the core of each, to further recovery efforts. Each recovery zone represents an area large enough and of sufficient habitat quality to support a recovered grizzly bear population. The Recovery Plan recognized that grizzly bears will move and reside permanently in areas outside the recovery zones and that movement between recovery areas would be necessary for isolated populations to increase and sustain themselves at recovery levels. The recovery zones identified are: (1) the Greater Yellowstone (GYE) in northwestern Wyoming, eastern Idaho, and southwestern Montana; (2) the Northern Continental Divide (NCDE) of north-central Montana; (3) the North Cascades area of north-central Washington; (4) the Selkirks (SE) area of northern Idaho, northeast Washington, and southeast B.C.; (5) the Cabinet-Yaak (CYE) area of northwestern Montana and northern Idaho; and (6) the Bitterroot (BE) in the Bitterroot Mountains of central Idaho and western Montana. Currently, there are at least 1,913 individuals in the lower-48 States (737 in the GYE demographic monitoring area, 1,068 in the NCDE, 5560 in the CYE, and a minimum of 53 in the U.S. portion of the SE, although some bears have home ranges that cross the international border). In addition, grizzly bears have been verified in areas outside of current distributions; however, there are likely few resident grizzly bears in the lower-48 States outside of the GYE, NCDE, CYE, and SE. There are currently no known populations within the North Cascades and BE.The species historical range included Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming. See below for information about where the species is known or believed to occur.
Population detail
The following populations are being monitored: Grizzly bear
» Range Information
Current Range
Current range maps are only shown within the jurisdictional boundaries of the United States of America. The species may also occur outside this region.
U.S.A., conterminous (lower 48) States, except where listed as an experimental population
Listing status: Threatened
- States/US Territories in which this population is known to or is believed to occur: Idaho, Montana, Washington, Wyoming
- US Counties in which this population is known to or is believed to occur: View All
- USFWS Refuges in which this population is known to occur: Benton Lake Wetland Management District, Little Pend Oreille National Wildlife Refuge, Lost Trail National Wildlife Refuge ...Show All Refuges
North Cascades Ecosystem Non-Essential Experimental Population
Listing status: Experimental Population, Non-Essential
- States/US Territories in which this population is known to or is believed to occur:
- US Counties in which this population is known to or is believed to occur: View All
- USFWS Refuges in which this population is known to occur:
» Candidate Information
No Candidate information available for this species.
No Candidate Assessments available for this species.
No Uplisting Documents currently available for this species.
» Federal Register Documents
» Species Status Assessments (SSAs)
» Recovery
- Species with Recovery Documents Data Explorer
- Recovery Priority Number: 9C
No Delisting Documents currently available for this species.
» Critical Habitat
To learn more about critical habitat please see https://ecos.fws.gov/crithab
» Conservation Plans
» Petitions
» Biological Opinions
To see all FWS Issued Biological Opinions please visit the BO Report.
» Life History
Habitat Requirements
Grizzly bears use a variety of habitats. In general, a grizzly bears 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 (e.g., food distribution, quality, and abundance), the availability of habitat components (e.g., denning areas, cover types), grizzly bear social dynamics, learned behavior and preferences of individual grizzly bears, grizzly bear population density, and random variation. Water is an important habitat requirement; 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: (1) increases human interaction and potential grizzly bear mortality risk; (2) increases displacement from important habitat; (3) increases habituation to humans; and (4) decreases habitat where energetic (i.e., 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 so that they are inaccessible to bears 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.
Food Habits
The lower-48 States provides highly diverse landscapes containing a wide array of habitat types and bear foods across and within the ecosystems. Plant communities vary from grasslands at lower elevations (less than 1,900 m (6,230 ft)) to shrub fields created by fires, avalanches, or timber harvest, to conifer forests at mid-elevations and subalpine and alpine meadows at higher elevations (greater than 2,400 m (7,870 ft)). Grizzly bears are opportunistic omnivores and display great diet plasticityeven within a population, shifting their diet according to foods that are most nutritious (i.e., high in fat, protein, and/or carbohydrates) and available. An extensive literature review documented over 260 species of foods consumed by grizzly bears in the GYE, representing 4 of the 5 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 (e.g., 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 GYE, 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 (e.g., grain, corn, garbanzo beans, melons), and other human foods. Cattle and sheep depredation rates are generally higher where bear densities are higher and in later summer months. In the GYE and NCDE, 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 CYE and SE, 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. As discussed in Other Life History below, 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 percent body fat are unlikely to produce cubs. However, we are unaware of a minimum body fat threshold for survival during the denning period.
Movement / Home Range
Adult grizzly bears are normally solitary except when breeding or when females have dependent young, but they are not territorial and home ranges of adult bears frequently overlap. Home range size is affected by resource availability, sex, age, and reproductive status. Generally, females with cubs-of-the-year or yearlings have the smallest home range sizes. The large home ranges of grizzly bears, particularly males, enhance maintenance of genetic diversity in the population by enabling males to mate with numerous females. Young, female grizzly bears usually establish home ranges within or overlapping their mothers. This pattern of home range establishment can make dispersal of females across landscapes a slow process. Radio-telemetry and genetic data suggest females typically establish home ranges an average of 9.8 to 14.3 km (6.1 to 8.9 mi) away from the center of their mothers home range, whereas males generally disperse farther, averaging 29.9 to 42.0 km (18.6 to 26.0 mi) away from the center of their mothers home range. Maximum male dispersal distances of 67176 km (42109 mi) have been documented in the GYE and NCDE. Studies also indicate that females can and do disperse long distances up to 8090 km (5056 mi), typically on the periphery of expanding populations. Although the frequency of long-distance dispersal by females is much lower than males, it can contribute to range expansion and demographic connectivity between populations. Home range sizes vary among the ecosystems because of population densities and habitat productivity as well as methodology. In the lower-48 States, observed average annual adult home ranges vary from 130 km2 (50 mi2) to 358 km2 (138 mi2) and average annual adult home range vary from 475 km2 (183 mi2) to 2,162 km2 (835 mi2).
Reproductive Strategy
Grizzly bears have three life stages: dependent young, subadults, and adults. Dependent young are usually less than two 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: (1) cubs, defined as cubs born during the most recent denning season and less than one year old; and (2) 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, two-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 (see Movement / Home Range below for further details). Adult bears are more than four years old and have reached sexual maturity. Some bears may not breed until they are older than five years old, but they have the ability to reproduce once they reach the adult stage. Female reproductive senescence starts around age 25 for those long-lived individuals. Age of first reproduction (averages 5.86.3 years), litter size (averages 2.12.19 cubs per litter), and inter-birth interval (the average number of years between litters; averages 2.783.4 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 the above 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 (Ursus arctos in 90th percentile for longevity). A population is 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.
Other
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 2 to 4 months prior to den entry (i.e., August through November). During hyperphagia, excess food is converted into fat, and grizzly bears may gain as much as 1.65 kg/day (3.64 lb/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 lower-48 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 2,500 m (>8,000 ft) in the GYE and 1,942 m (6,400 ft) in the NCDE and on slopes ranging from 30 to 60 degrees. In the CYE, the majority of den sites occurred above 1,600 m (5,248 ft), often on northerly and easterly aspects, though all aspects were used. In the SE, the majority of dens were located above 1,600 m (5,248 ft), often on easterly aspects, but all aspects were used. The North Cascades 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. Davis and Butterfield (1991) assessed the northern part of the BE 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.
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