Size and growth patterns of the Yellowstone grizzly bear

Abstract

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 captive 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-Septmber, 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 weighted 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 an 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.