Size and Growth Patterns of the Yellowstone Grizzly Bear

Abstract

Listen

Translate article

Weights and/or measurements of 151 grizzly bears (Ursus arctos) captured 261 times were recorded from 1975 to 1985. Males were consistently heavier than females within all age classes beginning at age 2. Mean weight for 65 adult males (5+ years old) was 192 kg and 135 kg for 63 adult females (5 + years old). Mean monthly weights by sex and age class indicated adults lost weight from den emergence through July, generally regaining emergence weight by August. Weaned yearlings lost weight July-September, whereas unweaned yearlings gained weight during the same period. Sexual dimorphism in body measurements within age classes was apparent in cubs and became significant in all body measurements by age 3. Girth was the measurement most closely correlated with weight for both males and females. Adults feeding at garbage dumps weighed more than bears relying on natural food sources. Bears were smaller and weighed less in this study than during the period 1959-70, when major dumps were available as a food source. Mean annual weights of nondump females were highly correlated with annual habitat productivity indices for Yellowstone Park. Correlations between mean adult female weight and cub litter size (r = 0.92) and mean age at 1st cub production (r = ?0.52) were apparent. In general, females with reliable high-energy foods tended to attain larger body sizes, mature at an earlier age, and have larger cub litters than females using relatively low-energy foods. Int. Conf. Bear Res. and Manage. 7:99-107 Differences in size, weight, and growth patterns have been reported for several populations of grizzly bears in North America (Pearson 1975; Reynolds 1976, 1981; Ballard 1980; Glenn 1980; Spraker et al. 1981; Craighead and Mitchell 1982; Nagy et al. 1984). Nutrition has been suspected to be the major factor producing these differences in grizzly bears (Rausch 1963) and black bears (U. americanus) (Rogers et al. 1976, Beecham 1980). The Yellowstone grizzly bear population was intensively studied from 1959 to 1970 by Frank and John Craighead (Craighead et al. 1974). During that period, major dumps were available to grizzly bears and provided a stable seasonal food source. Closure of those dumps in 1970 and 1971 eliminated that food supply for bears within Yellowstone National Park. Dumps serving communities adjacent to the Park were closed in 1982. Eliminating these food sources seriously affected the distribution and dy? namics of the population (Knight and Eberhardt 1985). Effects of food supply changes on the size, weight, and growth patterns of bears from that pop? ulation are reported here.