Annual behavioral and biochemical patterns of black bears (Ursus americanus), grizzly bears (Ursus arctos horribilis), and polar bears (Ursus maritimus) were reviewed. We propose that black and grizzly bears show 4 annual physiological stages: Stage I-hibernation, in which lean body mass is preserved and body fat supplies energy; Stage II-walking hibernation, in which the biochemistry of hibernation is integrated with physical activity, but food and water intake are minimal; Stage Illnormal activity, in which patterns are consistent with those of nonhibernating mammals; and Stage IV-hyperphagia, which increases fat reserves for hibernation. For polar bears, using published reports and recently collected data, we propose that all 4 stages are possible and that polar bears appear able to shift between Stages I and II in both summer and winter, which permits successful adaptation to the arctic environment. lnt. Conf. Bear Res. and Manage. 5:284-290 Field and laboratory observations of behavior of black and grizzly bears have indicated that they pass through 4 annual biochemical and physiological stages. These have been designated as Stage I-hibernation, Stage II-walking hibernation, Stage III1-normal activity, and Stage IVhyperphagia (Nelson et al. 1979). The purpose of this paper is to further define the 4 stages and to determine, using the literature and experimental data, whether the polar bear corresponds. PHYSIOLOGICAL STAGES OF BLACK AND GRIZZLY BEARS Stage I-Hibernation Studies by Folk (1974) have shown that black and grizzly bears are hibernators in winter in the true sense, showing physiological patterns similar to those of deep hibernators: distinct decreases in heart rate, metabolic rate, and body temperature. However, hibernation in black and grizzly bears differs from that of deep hibernators in that bears hibernate at a near-normal body temperature, 31-35 C, and their dormancy is continuous from 3 to 7 months. Deep hibernators (bats, insectivores, and rodents) hibernate at near 0 C and undergo periodic arousals (Folk 1974). Although expending about 4,000 kcal per day (calculations based on body fat utilization rates),
