Grizzly bear (Ursus arctos) COSEWIC assessment and update update status report: chapter 5

5. General biology

5.1 General

The essential biology of the grizzly bear has been thoroughly described, and excellent reviews can be found in LeFranc et al. (1987), Pasitschniak-Arts (1993), Craighead et al. (1995), Pasitschniak-Arts and Messier (2000), and Schwartz et al. (in press). The following sections address recent advances in the knowledge of life-history characteristics that are pertinent to the species’ status.

5.2 Reproduction

Age at primiparity, litter size, and interbirth interval for grizzly bears are variable, and appear to be influenced by habitat quality (Hilderbrand et al. 1999; Ferguson and McLoughlin 2000). Typically, females produce their first litters at 5-7 years of age, and have litters of 1-3 cubs about every 3 years (Schwartz et al. in press). Successful first breeding has been documented for females as young as 3.5 years (Aune et al. 1994; Wielgus and Bunnell 1994) and as old as 9.5 years (Case and Buckland 1998). Some reproductive parameters for grizzly bears in and near Canada are presented in Table 3.

5.3 Survival

Survival rates in grizzly bear populations have been estimated in several Canadian locations (Table 4). Direct comparisons among studies is confounded by the variety of means used to estimate rates (Schwartz et al. in press), but general trends are apparent. Adult female survival is typically high (>0.90). Adult male survival tends to be lower, particularly in hunted populations, due to legal protection of females with young, in concert with hunter preferences for larger bears. Subadult survival is variable, but is typically relatively low for males. In most populations, survival of cubs is lowest of all age classes, but increases for yearlings.

5.4 Physiology

The most notable element of grizzly bear physiology are those features related to denning. Although some grizzly bears in some areas do not den every year (Van Daele et al. 1990; Murphy et al. 1998), lack of food and harsh weather compels most bears to hibernate during winter. Duration of denning depends on the class of bear; pregnant females generally enter dens first and emerge last, and adult males usually spend the shortest time denned. The duration of den occupancy is also related to latitude, with bears at higher latitudes generally entering dens earlier and remaining denned longer (Schwartz et al. In Press). Grizzly bears in Banff National Park spent, on average, about 4.5 months each year in dens (Vroom et al. 1980). In the central Canadian Arctic, average duration of den occupancy was 185 days (6.2 months) for males and 199 days (6.6 months) for females (McLoughlin et al. 2002b). On the Tuktoyaktuk Peninsula, bears were estimated to occupy dens for 6-7 months per year (Nagy et al. 1983a).

 

Table 3. Estimated reproductive parameters of grizzly bears in and adjacent to Canada. Rates were estimated using various methods and comparisons must be made cautiously.
 Location Age (yrs)Footnote a at first litter Litter sizeFootnote b Interbirth interval (yrs):  Reference
Mean (n) Range Mean (n) Range Mean (n) RangeFootnote c
Flathead River, British Columbia (BC) 6.0 (5) 5 - 8 2.3 (31) 1 - 3 2.7 (9) 1 - 4 McLellan 1989c
N. Continental Divide, Montana (MT) 5.7 (10) 4 - 7 2.1 (56) 1 - 4 2.7 (16) 2 - 4 Aune et al. 1994
Tuktoyaktuk Peninsula, Northwest Territories (NT) 5.9 (10) 5 - 8 2.3 (18) 1 - 3 3.3 (8) 3 - 4 Nagy et al. 1983a
Kananaskis, Alberta (AB) 5.5 (3) 3 - 6 1.4 (5) -- 3.0 (3) -- Wielgus and Bunnell 1994
Selkirk Mountains, US / BC 7.3 (5) 6 - 7 2.2 (10) 2 - 3 3.0 (6) 2 - 4 Wielgus et al. 1994
Khutzeymateen Valley, BC -- -- 2.4 (8) 1 - 3 -- -- MacHutchon et al. 1993
Swan Mountains, MT 5.7 (3) 4 - 8 1.6 (17) 1 - 2 3.0 (6) 2 - 4 Mace and Waller 1998
Mackenzie Mountains, NT -- 8 - ? 1.8 (6) -- 3.8 (5) -- Miller et al. 1982
Kluane National Park, Yukon Territory (YT) 6.7 (7) 6 - 8 1.7 (11) -- -- 3 - 4 Pearson 1975
Richardson Mountains, NT -- 5 - 6 2 (?) -- -- -- Nagy and Branigan 1998
Kugluktuk, Nunavut (NU) 8.7 (6) 7 - 10 2.3 (19) 1 - 4 2.6 (8) 1 - 4 Case and Buckland 1998
Anderson-Horton Rivers, NT 10.8 (12) 6 - ? 2.3 (37) 1 - 3 4.3 (15) 3 - 5 Clarkson and Liepins 1993
Brock-Hornady Rivers, NT -- 5 - 6 1.5 (?) -- -- -- Nagy and Branigan 1999
Berland River, AB -- 6 - ? 1.8 (5) 1 - 3 -- -- Nagy et al. 1988
Northern Yukon, YT -- 6 - 8 2.0 (6) 1 - 3 -- 3 - 5 Nagy et al. 1983b
Eastern Slopes, AB 6.7 (8) 6 - 12 1.9 (24) 1 - 3 3.8 (24)Footnote d 2 - 7+ Garshelis et al. 2001

 

Table 4. Estimated annual survival rates of grizzly bears in and adjacent to Canada. Rates were estimated using various methods and comparisons must be made cautiously.
Study Location Hunted? Adult Subadult Yearling Cub Reference
Male Female Male Female
1 Flathead River, BCFootnote a.1 Y 0.92 0.94 0.92 0.94 0.88 0.82 McLellan 1989b
2 Flathead River, BCFootnote a.1 Y -- 0.95 -- 0.93 0.94 0.87 Hovey and McLellan 1996
3 North Fork Flathead, BC / MTFootnote a.1 Y 0.89 0.96 0.78 0.94 -- -- McLellan et al. 1999
4 Kananaskis, AB Y 0.70 0.93 0.89 0.89-0.93 --Footnote b.1 0.78 Wielgus and Bunnell 1994
5 Blackfeet-Waterton, MT / AB Y 0.63 0.92 0.80 0.86 -- -- McLellan et al. 1999
6 Mountain Parks, AB / BC N 0.89 0.91 0.74 0.95 -- -- McLellan et al. 1999
7 South Fork Flathead, MT N 0.89 0.89 0.78 0.87 -- -- McLellan et al. 1999
8 Selkirk-Yaak, US / BCFootnote a.1 N 0.84 0.95 0.81 0.93 -- -- McLellan et al. 1999
9 Selkirk Mountains, US / BCFootnote a.1 N 0.81 0.96 0.90 0.78 --Footnote b.1 0.84 Wielgus et al. 1994
10 Swan Mountains, MT N -- 0.90 -- 0.83 0.91 0.79 Mace and Waller 1998
11 Interior Mountains, US / CANFootnote c.1 Y/N 0.88 0.93 0.80 0.92 -- -- McLellan et al. 1999
12 Eastern Slopes, AB N -- 0.95-0.96 -- 0.89-0.95 0.88 0.78 Garshelis et al. 2001


The physiology of hibernating bears is complex and interesting, and is reviewed in Hellgren (1998). Essential elements of bear hibernation include the maintenance of survival metabolic costs through catabolism of stored fat and protein, and the lack of urination or defecation for very long periods. For pregnant females, which give birth during the denning period, costs of latter-stage gestation and lactation must also be met in the absence of foraging. Weight loss in hibernating wild bears over the denning period has ranged from 16 to 37% (Hellgren 1998). In Alaska, adult females lost an average of 73 kg (32%) of body mass over winter (Hilderbrand et al. 2000). Most (56%) of this mass loss was fat. Females emerging from dens with cubs or yearlings were lighter than solitary females, and had less fat and lower lean body mass, indicating the relative costs of hibernation, gestation, and lactation. Total body fat during early summer dropped to as low as 6.3% of body mass in grizzly bears in the central Canadian Arctic, and climbed as high as 33.6% in autumn (Gau 1998).

Preparation for a long fast includes hyperphagia, particularly of high-caloric foods such as berries and carcasses. This compulsion to generate fat stores adequate to minimize muscle catabolism during the denning period drives foraging and directs much grizzly bear behaviour during late summer and autumn. For example, grizzly bears in central coastal British Columbia roamed widely during berry season, using 10 berry species in divergent habitats (Hamilton and Bunnell 1987), and fall migrations to salmon streams have been widely reported for coastal bear populations (LeFranc et al. 1987).

5.5 Movements/dispersal

5.5.1 Home range

As with most species, home range size in grizzly bears is negatively correlated with general habitat quality. Bears with access to predictably abundant, high-quality foods and long growing seasons, such as in temperate coastal areas, tend to have small home ranges. For example, home ranges on Admiralty Island, Alaska, averaged 115 km2 for males and 24 km2 for females (Schoen et al. 1986). Bears living in dryer and colder interior or northern environments typically require much larger home ranges. The largest reported grizzly bear home ranges are from the central Canadian Arctic; they are up to 2 orders of magnitude larger than coastal Alaskan home ranges (Table 5). Home ranges are typically several times larger for male bears than for females, presumably due to male breeding activity and perhaps influenced by the increased energetic demands of larger body size (Gau 1998; McLoughlin et al. 1999).

Local climate affects grizzly bear home range size by influencing primary productivity, and thereby food availability and accessibility (McLoughlin and Ferguson 2000). Most grizzly bear home range sizes in Canada lie between the extremes cited above, as do the applicable climatic conditions. Typical grizzly bear home ranges, however, are large irrespective of location (Table 5).

 

Table 5. Estimated density and adult home range sizes (100% minimum convex polygon) for grizzly bear populations inCanada. Densities are based on radio telemetry studies, except where noted. Grizzly bear zone refers to descriptions by Banci (1991); see Table 12.
Study area Grizzly: bear zone DensityFootnote a.2:
(bears/1000 km2)
Home range size (km2) Source
Males Females
Northern Yukon, YT 1 26 - 30 645 210 Nagy et al. 1983b
Richardson Mountains, NT 1 19     Nagy and Branigan 1998
Anderson-Horton Rivers, NT 1 8.2 - 9.1 3433 1182 Clarkson and Liepins 1994
Brock-Hornady Rivers, NT 1 6     Nagy and Branigan 1998
Tuktoyaktuk Peninsula, NT 1 4 1154 670 Nagy et al. 1983a
Ivvavik National Park, YT 1   435 144 MacHutchon 1996
Central Arctic, NT / NU 1,2 3.5 8171 2434 Penner 1998; McLoughlin et al. 1999
Mackenzie Mountains, NT 4 12   265 Miller et al. 1982
Kluane National Park, YT 5 37 287 86 Pearson 1975
Prophet River, BCFootnote e 5,6 14.5 - 16.9     Boulanger and McLellan 2001Footnote b.2
Prophet River, BCFootnote e 5 29     Poole et al. 2001Footnote b.2
Prophet River, BCFootnote e 6 10     Poole et al. 2001Footnote b.2
Swan Hills, AB 6 7.4 - 9.6 244 113 Nagy and Russell 1978
South Wapiti, AB 6 7.4     cited in Nagy and Gunson 1990
Berland River, AB 6 4.6 1918 252 Nagy et al. 1988; Nagy and Gunson 1990
Yellowhead, AB 6,10 14.9 1733 668 Boulanger 2001Footnote b.2; Stenhouse and Munro 2001
Hart Ranges, BC 7 49 77 47 Mowat et al. 2001Footnote b.2; Ciarniello et al. 2001
Khutzeymateen Valley, BC 8 43 - 90 125 52 MacHutchon et al. 1993
Upper Fraser Basin, BC 9 12 1697 326 Mowat et al. 2001Footnote b.2; Ciarniello et al. 2001
Central Selkirk Mountains, BC 10 26.6     Mowat and Strobeck 2000Footnote b.2
Selkirk Mountains, BC 10 14.1     Wielgus et al. 1994
West Slopes, BC 10   318 89 Woods et al. 1997
Jasper National Park, AB 10 9.8 - 11.7 948Footnote c.2 331Footnote c.2 Russell et al. 1979
Flathead River, BC 12 57 - 80 668 253Footnote d.1 McLellan 1989a; B.N. McLellan, pers. commun.
Kananaskis, ABFootnote e 12 16.2     Wielgus and Bunnell 1994
Kananaskis, ABFootnote e 12 12.2 - 14.5 1183 179 Carr 1989
Crowsnest, AB 12 15     Mowat and Strobeck 2000Footnote b.2
Central Rockies, AB & BC 12 9.8 - 16 1560 305 Gibeau et al. 1996; Gibeau and Herrero 1997


Although habitat quality determines minimum home range size required to meet energetic needs, actual home range size used by grizzly bears may be influenced by population density. Nagy and Haroldson (1990) concluded that reduced bear density resulting from man-caused mortalities suppressed competition for resources, including space, and permitted use of larger areas.


5.5.2 Movements

Male grizzly bears generally have higher rates of movement than do females (LeFranc et al. 1987). In the central Canadian Arctic, male grizzly bears move faster than females in all seasons (McLoughlin et al. 1999). Movement rates of males were highest in spring, when energetic demands are high and males seek mates, and generally declined through autumn. Female movement rates peaked during summer when, in this area, food availability was considered low.

In some mountainous areas, an annual pattern of altitudinal migrations is typical in response to seasonal changes in vegetation phenology and the availability of other foods (LeFranc et al. 1987). For example, bears may emerge from relatively high-elevation dens and descend to valley bottoms to seek ungulate carcasses and early-emergent plants. As snow melt proceeds upslope, bears ascend to follow the emergence of fresh vegetation.


5.5.3 Dispersal

Subadult male grizzly bears usually disperse upon independence, whereas subadult females are commonly philopatric (LeFranc et al. 1987; Blanchard and Knight 1991). Dispersal distances for young grizzly bears are short compared with some other large carnivores. Mean dispersal distance for 4 subadult males in Yellowstone National Park was 70 km (Blanchard and Knight 1991). In southeastern British Columbia, male and female dispersals averaged 29.9 km and 9.8 km, respectively (McLellan and Hovey 2001b). The longest dispersals from maternal home ranges in this study were 67 km for a male and 20 km for a female. However, the species is large and mobile, and capable of long movements. One radio-marked subadult male grizzly bear in northeastern BC was shot 340 km from his maternal home range (P.I. Ross, unpubl. data). In the central Arctic, 1 subadult male moved 471 km in less than 1 month (R. Gau, pers. commun.).

Dispersal in grizzly bears is a gradual process, taking 1-4 years (McLellan and Hovey 2001b). Because of this, grizzly bears must be able to live in dispersal corridors, rather than simply disperse through them.

5.6 Nutrition and interspecific interactions

Like most bear species, grizzlies share the basic digestive anatomy and physiology of other members of Carnivora, but consume relatively large volumes of vegetation. The degree of herbivory varies among and within grizzly bear populations, but in most, a variety of plants are highly important foods, and consequently there is a strong influence of season on diet. Conversely, bears in some areas are highly carnivorous, and in some cases, predatory. Based on stable isotope signatures, the contribution of vegetation to diets of adult female grizzly bears ranged from 19% in coastal Alaska to 98% in Kluane National Park (Hilderbrand et al. 1999a). Grizzly bears are probably best described as opportunistically omnivorous (Schwartz et al. In press).

Grizzly bear food habits are widely variable among regions. Many food-habits studies have been reported, and thorough reviews are provided in LeFranc et al. (1987), Pasitschniak-Arts (1993), Pasitschniak-Arts and Messier (2000), and Schwartz et al. (In press). Following are highlights of several recent Canadian studies.

In central coastal British Columbia, 65 distinct food items, including 49 plants, were identified (MacHutchon et al. 1993). In spring, sedges were the most commonly eaten food. Several forb species dominated the summer diet and persisted into the fall. From early August to mid-October, salmon (Oncorhynchus spp.) were the major food item. Bears were also observed feeding on mammals and a variety of intertidal invertebrates.

Grizzly bears in the Flathead drainage of southeastern British Columbia occur at a density at least twice as high as any other reported interior population (Table 5). McLellan and Hovey (1995) suggested that this high density was a result of the high quantity and diversity of bear foods in the Flathead area. Typical of many mountainous interior study areas (Hamer and Herrero 1987; LeFranc et al. 1987; Hamer et al. 1991), grizzlies in the Flathead fed largely on roots (especially Hedysarum spp.) and ungulates in early spring and again in late fall (McLellan and Hovey 1995). Feeding on whitebark pine (Pinus albicaulus) seeds was rarely observed, although this is an important food item in adjacent Glacier National Park, USA, and other regions where the 2 species overlap (Mattson et al. 2001). A variety of forb species, along with grasses and horsetails (Equisetum sp.), dominated the summer diet, and during late summer berries comprised up to 96% of scat volume. The presence of all known major interior grizzly bear foods, and the abundance of both huckleberry (Vaccinium spp.) and buffaloberry (Shepherdia canadensis) fruit, were considered particularly important in defining the high quality of habitat in the Flathead area.

In Ivvavik National Park, Yukon, grizzly bear seasonal food habits generally paralleled those of southern interior bears (MacHutchon 1996). Hedysarum roots, overwintered berries, and horsetails were important spring foods. Horsetails remained important during summer, but forbs were also heavily used. During fall, berries became important as they ripened, and roots returned to the diet. Grizzlies in Ivvavik Park hunted for ground squirrels (Spermophilus parryii) during summer and fall, and for caribou (Rangifer tarandus) during the brief mid-summer period they were available, but most (96-98%) foraging time was spent on vegetation.

Gau (1998) and Gau et al.(2002) studied food habits of barren-ground grizzly bears in the central Arctic. Caribou was the most prevalent food item, especially in spring, mid-summer, and autumn. In early summer, when caribou were essentially absent, horsetails, sedges (Carex spp.) and Arctic cotton grass (Eriophorum spp.) dominated the diet. During late summer berries became most important, and were judged to be critical for deposition of fat reserves sufficient for denning.

The occurrence of meat in the diet of grizzly bears influences several physical and life history characteristics. Population density, female body mass, and mean litter size were positively correlated with dietary meat content (Hilderbrand et al. 1999a). In most areas, pre-hibernatory mass gain is largely dependent upon the consumption of massive volumes of berries during late summer. However, energetic maintenance costs were lowest, and rate of mass gain was highest, when dietary protein content was about 20-35%, indicating that even when berries were abundant, a mixed diet was most efficient for bears (Rode and Robbins 2000).

The availability of meat to grizzly bears is widely variable across study areas, and is generally seasonal. However, where and when meat is available, grizzly bears indicate a strong preference for it. In coastal Alaska, adult females ate an average of 8.5 kg/day of meat in spring, primarily moose carrion and calves (Hilderbrand et al. 1999b). During summer and fall, they consumed 10.8 kg/day of salmon, and meat contributed 80.4% (59.6% salmon and 20.7% terrestrial) of the fall diet.

Scavenged ungulate carcasses have long been recognized as an important food item, particularly in spring, of virtually all grizzly populations. However, the role of predation in grizzly bear nutrition, and in ungulate population dynamics, has more recently become clear. In southcentral Alaska, grizzly bears killed 44% of moose calves and accounted for 73% of calf mortality (Ballard et al. 1991); they also killed older moose including adult cows. Grizzlies were the primary cause of adult moose mortality in southwestern Yukon (Larsen et al. 1989), and have been identified as important moose predators in other areas as well (e.g., Gasaway et al. 1988; Mattson 1997; Bertram and Vivion 2002). Some classes of bear may be more successful predators than others. In east central Alaska, each adult male grizzly bear killed 3.3-3.9 adult moose annually, whereas each lone adult female killed 0.6-0.8 adult moose per year (Boertje et al. 1988). In that area, grizzlies killed 4 times more animal biomass than they scavenged.

Other important grizzly prey include caribou (Adams et al. 1995; Gau 1998), elk (Cervus elaphus: Hamer and Herrero 1991; Mattson 1997), and a variety of small mammals (especially ground squirrels [Spermophilus sp.] and marmots [Marmota sp.]). Muskoxen (Ovibos moschatus: Gunn and Miller 1982; Case and Stevenson 1991), mule deer (Odocoileus hemionus: Mattson 1997), mountain goats (Oreamnos americanus: Festa-Bianchet et al. 1994), bison (Bison bison: Mattson 1997), and black bears (Ursus americanus: Boertje et al. 1988; Ross et al. 1988) can also be occasional prey for grizzly bears. In the Canadian Arctic, grizzly predation on ringed seals (Phoca hispida) has been documented or inferred from sign (Clarkson and Liepins 1989; M.K. Taylor, pers. commun.; P.I. Ross, unpubl. data). Where available, army cutworm moths (Euxoa auxiliaris), ants, and earthworms may be important seasonal grizzly bear prey (Mattson et al. 1991; Mattson 2001; Mattson et al. in press).

Grizzly bears influence other species in ways aside from just eating them. Wolves (Canis lupus) and grizzly bears compete for live prey and for carcasses, and usurp kills from each other. However, Servheen and Knight (1993) reviewed grizzly bear/wolf interactions and found no evidence of effects on survival or reproduction of either species. The grizzly’s relationship with obligate predators is more one-sided; bears (grizzly and black) visited 24% of cougar (Puma concolor) kills in Yellowstone and Glacier National Parks, and displaced cougars from 10% of carcasses (Murphy et al. 1998). Bears gained up to 113%, and cougars lost up to 26%, of their respective daily energy requirements from these encounters. Bear predation and incomplete consumption of carcasses (especially salmon) provides food for a variety of scavengers.

Grizzly bear digging for bulbs of glacier lily (Erythronium grandiflorum) enhances soil nutrients on those sites, encouraging regrowth and productivity of glacier lilies and other plants and structuring plant communities (Tardiff and Stanford 1998). Consumption of berries and other fruits leads to seed dispersal for those plants (Willson 1993). Grizzly bears also distribute nutrients from salmon carcasses into terrestrial systems. Of the total nitrogen in spruce foliage within 500 m of streams, 15.5-17.8% was derived from salmon, and 83-84% of that was contributed by bears through their urine or feces (Hilderbrand et al. 1999c).

Grizzly bears also interact directly with humans. There are millions of bear-human interactions in North America each year, nearly all with a peaceful, positive outcome. However, during 1990-1999, grizzly bears killed 18 people in North America (S. Herrero, pers. commun.). Over a 30-year period in Alberta and BC, there were about 4 times as many serious injuries as fatalities, suggesting an annual average 9.0 serious and fatal grizzly attacks on humans in North America during the 1990s. The rate of increase in bear-inflicted injuries is higher than the population growth rate in BC but not in Alberta (S. Herrero, pers. commun.).

5.7 Behaviour/adaptability

Individual grizzly bears are clearly capable of learning behaviours and in that sense they are highly adaptable. Examples include innumerable anecdotes about individual bear responses to particular stimuli or situations. Many of these examples are related to conflict situations with humans. Bears which receive anthropogenic food rewards in response to particular behaviours will quickly become food conditioned (McCullough 1982). Habituation, by contrast, is the loss of fear of humans as a result of a lack of negative reinforcement. Both processes contribute to negative bear-human interactions.

Aversive conditioning programs have been tried and implemented in many places to take advantage of bears’ ability to modify their behaviours (e.g., review in LeFranc et al.; Schirokauer and Boyd 1998). One particularly promising approach involves the use of trained Karelian bear dogs to change behaviour of habituated bears (Hunt 2000). However, because most bear behaviour is strongly influenced by their overwhelming nutritional requirements, aversive conditioning is challenging.

Grizzly bear young remain with their mothers typically for 2-4 years (Schwartz et al. In press). This long period of dependence is presumably related to the complexity of behaviours that young bears must learn from their mothers in order to survive on their own.

Adaptability on the species level, however, is much lower, primarily because of their low reproductive rate. Population-wide changes in general behavioural patterns take many generations, and are always mitigated by the tenuousness of bears’ nutritional status. Grizzly bears will engage in risky behaviours for food, and this means that individuals and populations will always be vulnerable to potential conflicts with humans.

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