Despite centuries of research, debate remains on the scaling of metabolic rate to mass especially for intraspecific cases. The high variation of body mass within brown bears presents a unique opportunity to study the intraspecific effects of body mass on physiological variables. The amplitude of metabolic rate reduction in hibernators is dependent on body mass of the species. Small hibernators have high metabolic rates when euthermic but experience a drastic decrease in body temperature during torpor, which is necessary to reach a very low metabolic rate. Conversely, large hibernators, such as the brown bear (Ursus arctos), show a moderate decrease in temperature during hibernation, thought to be related to the bear’s large size. We studied body mass, abdominal body temperature, heart rate, and accelerometer-derived activity from 63 free-ranging brown bears (1–15 years old, 15–233 kg). We tested for relationships between body mass and body temperature, heart rate, and hibernation duration. The smallest individuals maintained lower body temperatures during hibernation, hibernated longer, and ended hibernation later than large bears. Unlike body temperature, winter heart rates were not associated with body mass. In summer, the opposite pattern was found, with smaller individuals having higher body temperature and daytime heart rates. Body mass was associated with body temperature in the winter hypometabolic state, even in a large hibernating mammal. Smaller bears, which are known to have higher thermal conductance, reached lower body temperatures during hibernation. During summer, smaller bears had higher body temperatures and daytime heart rates, a phenomenon not previously documented within a single mammalian species. We conclude that the smallest bears hibernated more deeply and longer than large bears, likely from a combined effect of basic thermodynamics, the higher need for energy savings, and a lower cost of warming up a smaller body.

中文翻译：

---

自由放养的棕熊的体重与冬眠长度、体温和心率有关

尽管经过几个世纪的研究，关于代谢率与质量的比例仍然存在争议，特别是对于种内情况。棕熊体内体重的高度变化为研究体重对生理变量的种内影响提供了独特的机会。冬眠者代谢率降低的幅度取决于物种的体重。小型冬眠动物在恒温时具有较高的代谢率，但在冬眠期间体温会急剧下降，这是达到非常低的代谢率所必需的。相反，大型冬眠动物，例如棕熊（Ursus arctos），在冬眠期间温度会适度下降，这被认为与熊的体型较大有关。我们研究了 63 只自由放养的棕熊（1-15 岁，15-233 公斤）的体重、腹部体温、心率和加速度计衍生的活动。我们测试了体重与体温、心率和冬眠持续时间之间的关系。最小的个体在冬眠期间保持较低的体温，冬眠时间更长，并且比大型熊更晚结束冬眠。与体温不同，冬季心率与体重无关。在夏季，发现了相反的模式，较小的个体具有较高的体温和白天心率。在冬季代谢低下状态下，体重与体温相关，即使是大型冬眠哺乳动物也是如此。较小的熊具有较高的导热性，在冬眠期间达到较低的体温。在夏季，体型较小的熊的体温和白天心率较高，这种现象以前在单一哺乳动物物种中没有记录到。我们得出的结论是，最小的熊比大型熊冬眠得更深、时间更长，这可能是由于基本热力学、对节能的更高需求以及较小身体的预热成本较低的综合影响。