Abstract. Bears consuming wild fruits for fall energy accumulation are constrained by several factors, including intake rate, the physiological capacity of the gastrointestinal tract, and the metabolic efficiency of gain in body mass. We measured these relationships through foraging and feeding trials using captive and wild black bears (Ursus americanus) and grizzly bears (Ursus arctos). Four fruit types covering a range of sizes and clustering were offered to captive bears to determine the effect of density, size, and presentation on intake rate. Intake rate (in grams per minute) and bite rates (in bites per minute) increased curvilinearly with increasing fruit density in singly spaced fruits. Maximum intakes ranged from 30 g/min for 0.5-g berries to >200 g/min for 4.2-g fruits. The highest bite rates were obtained during the initial encounter with each patch as bears consumed all visually apparent fruits on the surface. Bite rates quickly dropped by 15-20% as foraging continued within the patch. Maximum bite rates were not depressed until initial fruit density fell to <50 berries/M3. Maximum daily fresh fruit intake for the captive bears averaged 34 ? 6% (mean ? 1 SD) of body mass. The dry-matter digestibility of wild fruits, particularly preferred species, was as high as 72%. While large captive bears could gain body mass very rapidly when given fruit ad libitum, foraging efficiencies increasingly constrained growth rates of wild bears >100 kg. We concluded that large bears, such as grizzlies, must depend on plants that permit large bite sizes or high bite rates through fruit clustering and bush configuration that reduce leaf-to-fruit ratios.

Abstract. Bears consuming wild fruits for fall energy accumulation are constrained by several factors, including intake rate, the physiological capacity of the gastrointestinal tract, and the metabolic efficiency of gain in body mass. We measured these relationships through foraging and feeding trials using captive and wild black bears (Ursus americanus) and grizzly bears (Ursus arctos). Four fruit types covering a range of sizes and clustering were offered to captive bears to determine the effect of density, size, and presentation on intake rate. Intake rate (in grams per minute) and bite rates (in bites per minute) increased curvilinearly with increasing fruit density in singly spaced fruits. Maximum intakes ranged from 30 g/min for 0.5-g berries to >200 g/min for 4.2-g fruits. The highest bite rates were obtained during the initial encounter with each patch as bears consumed all visually apparent fruits on the surface. Bite rates quickly dropped by 15-20% as foraging continued within the patch. Maximum bite rates were not depressed until initial fruit density fell to <50 berries/M3. Maximum daily fresh fruit intake for the captive bears averaged 34 ? 6% (mean ? 1 SD) of body mass. The dry-matter digestibility of wild fruits, particularly preferred species, was as high as 72%. While large captive bears could gain body mass very rapidly when given fruit ad libitum, foraging efficiencies increasingly constrained growth rates of wild bears >100 kg. We concluded that large bears, such as grizzlies, must depend on plants that permit large bite sizes or high bite rates through fruit clustering and bush configuration that reduce leaf-to-fruit ratios.

Knowledge of density-dependent responses in grizzly bears is largely circumstantial yet critical to managing populations near carrying capacity. We studied grizzly bear (Ursus arctos) body condition and demographics in Denali National Park, Alaska, USA, 1991USA, -1998, and compared our results with data from a nearby heavily hunted population to better understand the characteristics of a naturally regulated bear population. We captured grizzly bears just after den emergence in spring (May) and just before den entrance in fall (Sep). We measured body fat using bioelectrical impedance analysis, estimated bear ages, and fitted adult, subadult, and 2-year-old dependent bears with telemetry-collars. We used capture-mark-resight techniques to estimate density. We estimated grizzly bear density at 37 bears/1,000 km 2 ; density of adult and subadult bears was 27 bears/1,000 km 2 . Adult females that produced cubs had 40% more fat and 10% more lean mass pre-denning than non-reproductive females but had similar lean mass and similar but very low body fat (8%) post-denning. Adult females that produced cubs lost more fat and lean mass over winter than nonreproductive adult females. The physiological demand on adult females producing cubs was 29 kg of total body mass split evenly between fat mass and lean mass. Females that did not lose cubs during the cubs' first summer averaged more body fat the previous fall than those that lost partial or entire litters. Reproductive rate (0.34) and average litter size (2.1) were high and comparable to increasing populations (l > 0). Age of dependent young at family breakup (2.9 yr) and age of first reproduction (7.5 yr) were high and comparable to declining populations (l < 0). Cub (0.34), cub litter (0.43), and yearling survival rates (0.46) were low. Subadult and adult survival rates were high (0.94-1.00) for both sexes. The female age structure was bimodal, suggesting episodic recruitment. The percentage of subadult males was low (3% of population and 10% of male segment). We conclude that the grizzly bear population in Denali National Park was near or at carrying capacity relative to available nutritional resources, adult female access to nutritional resources determined whether cubs were born and the likelihood of their survival, and high dependent bear mortality was the primary factor limiting recruitment and regulating the population. Ó

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