Why bears hibernate? Redefining the scaling energetics of hibernation

Hibernation is a natural state of suspended animation that many mammals experience and has been interpreted as an adaptive strategy for saving energy. However, the actual amount of savings that hibernation represents, and particularly its dependence on body mass (the "scaling") has not been calculated properly. Here we estimated the scaling of daily energy expenditure of hibernation (DEEH), covering a range of five orders of magnitude in mass. We found that DEEH scales isometrically with mass, which means that a gram of hibernating bat has a similar metabolism to that of a gram of bear, 20,000 times larger. Given that the metabolic rate of active animals scales allometrically, the point where these scaling curves intersect with DEEH represents the mass where energy savings by hibernation are zero. For BMR, these zero savings are attained for a relatively small bear (~100 kg). Calculated on a per-cell basis, the cellular metabolic power of hibernation was estimated to be 1.3x10-12 ± 2.6x10-13 W/cell, which is lower than the minimum metabolism of isolated mammalian cells. This supports the idea of the existence of a minimum metabolism that permits cells to survive under a combination of cold and hypoxia.

冬眠（Hibernation）是诸多哺乳动物所经历的自然假死状态，此前学界普遍将其视为一种节约能量的适应性策略。然而，冬眠实际所能节约的能量总量，尤其是其与体重的缩放关系（scaling），此前尚未得到准确测算。本研究对冬眠每日能量消耗（Daily Energy Expenditure of Hibernation, DEEH）的缩放关系进行了估算，涵盖了跨度达5个数量级的体重区间。研究发现，冬眠每日能量消耗与体重呈等比例缩放关系，这意味着每克冬眠蝙蝠的代谢水平，与体重为其2万倍的每克冬眠熊的代谢水平相当。鉴于活跃动物的代谢率呈异速缩放关系，两类缩放曲线与冬眠每日能量消耗的交点，即为冬眠节能效应为零的体重阈值。就基础代谢率（Basal Metabolic Rate, BMR）而言，节能效应为零的体重阈值对应约100千克的小型熊类。若以单个细胞为单位计算，冬眠状态下的细胞代谢功率约为1.3×10^-12 ± 2.6×10^-13 瓦/细胞，这一数值低于离体哺乳动物细胞的最低代谢水平。这一结果佐证了“存在最低代谢阈值，可使细胞在低温与低氧的联合环境下存活”这一假说。