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.

冬眠是许多哺乳动物所经历的自然代谢暂停状态，被认为是一种用以节约能量的适应性策略。然而，冬眠实际带来的能量节约量，尤其是其与体质量的依赖关系（即“尺度缩放效应”）此前并未得到准确测算。本研究针对覆盖五个数量级的体质量范围，估算了冬眠每日能量消耗（Daily Energy Expenditure of Hibernation, DEEH）的尺度缩放关系。研究发现，DEEH与体质量呈等尺度缩放，这意味着每克冬眠蝙蝠的代谢速率，与体重为其20000倍的冬眠熊的每克组织代谢速率相近。鉴于活跃动物的代谢速率呈异速尺度缩放，上述两类尺度曲线与DEEH尺度关系的交点，即为冬眠能量节约量为零的体质量阈值。对于基础代谢率（Basal Metabolic Rate, BMR）而言，零节能的体质量点对应相对较大的熊（约100千克）。以单位细胞计算，冬眠状态下的细胞代谢功率经估算为1.3×10^-12 ± 2.6×10^-13 瓦/细胞，这一数值低于分离哺乳动物细胞的最低代谢速率。该结果支持了“存在可使细胞在低温与缺氧联合环境下存活的最低代谢阈值”这一学术观点。