Arnold Walter, Ruf Thomas, Kuntz Regina
Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, Savoyenstrasse 1, 1160 Vienna, Austria.
J Exp Biol. 2006 Nov;209(Pt 22):4566-73. doi: 10.1242/jeb.02536.
Many large mammals show pronounced seasonal fluctuations of metabolic rate (MR). It has been argued, based on studies in ruminants, that this variation merely results from different levels of locomotor activity (LA), and heat increment of feeding (HI). However, a recent study in red deer (Cervus elaphus) identified a previously unknown mechanism in ungulates--nocturnal hypometabolism--that contributed significantly to reduced energy expenditure, mainly during late winter. The relative contribution of these different mechanisms to seasonal adjustments of MR is still unknown, however. Therefore, in the study presented here we quantified for the first time the independent contribution of thermoregulation, LA and HI to heart rate (f(H)) as a measure of MR in a free-roaming large ungulate, the Przewalski horse or Takhi (Equus ferus przewalskii Poljakow). f(H) varied periodically throughout the year with a twofold increase from a mean of 44 beats min(-1) during December and January to a spring peak of 89 beats min(-1) at the beginning of May. LA increased from 23% per day during December and January to a mean level of 53% per day during May, and declined again thereafter. Daily mean subcutaneous body temperature (T(s)) declined continuously during winter and reached a nadir at the beginning of April (annual range was 5.8 degrees C), well after the annual low of air temperature and LA. Lower T(s) during winter contributed considerably to the reduction in f(H). In addition to thermoregulation, f(H) was affected by reproduction, LA, HI and unexplained seasonal variation, presumably reflecting to some degree changes in organ mass. The observed phase relations of seasonal changes indicate that energy expenditure was not a consequence of energy uptake but is under endogenous control, preparing the organism well in advance of seasonal energetic demands.
许多大型哺乳动物的代谢率（MR）呈现出明显的季节性波动。基于对反刍动物的研究，有人认为这种变化仅仅是由不同水平的运动活动（LA）以及摄食热增量（HI）导致的。然而，最近一项针对马鹿（Cervus elaphus）的研究发现了有蹄类动物中一种此前未知的机制——夜间低代谢，这一机制对能量消耗的减少有显著贡献，主要发生在冬末。然而，这些不同机制对MR季节性调节的相对贡献仍然未知。因此，在本文所呈现的研究中，我们首次量化了体温调节、LA和HI对心率（f(H)）的独立贡献，以此作为自由放养的大型有蹄类动物普氏野马或蒙古野马（Equus ferus przewalskii Poljakow）MR的衡量指标。f(H)全年呈周期性变化，从12月和1月的平均44次/分钟增加了两倍，到5月初达到春季峰值89次/分钟。LA从12月和1月的每天23%增加到5月的每天平均53%，此后又再次下降。冬季每日平均皮下体温（T(s)）持续下降，并在4月初达到最低点（年变化范围为5.8摄氏度），这一最低点出现在气温和LA的年度低点之后很久。冬季较低的T(s)对f(H)的降低有很大贡献。除了体温调节外，f(H)还受到繁殖、LA、HI以及无法解释的季节性变化的影响，这些变化可能在一定程度上反映了器官质量的变化。观察到的季节性变化的相位关系表明，能量消耗不是能量摄取的结果，而是受内源性控制，在季节性能量需求之前就使生物体做好了充分准备。
