Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, Savoyenstraße 1, Vienna A-1160, Austria.
Animal. 2020 Mar;14(S1):s124-s132. doi: 10.1017/S1751731119003240.
Ruminants living in seasonal environments face a two-fold challenge during winter. The energetic cost of maintaining a high body temperature is higher at lower ambient temperatures, and this is compounded by poor availability and quality of feed. Wild ruminants acclimatize to this energetic challenge by hypothermia, that is, reduced endogenous heat production and abandoning the maintenance of a high body temperature, particularly in peripheral body parts. Further but lesser contributions to lower energy expenditure during winter are reduced foraging activity; lower heat increment of feeding; and reduced maintenance cost of size-reduced organs. Altogether, metabolic rate, estimated by the continuous measurement of heart rate, during winter is downregulated to more than half of the summer level, as is voluntary food intake, even in animals fed ad libitum. The transformation from the summer into the thrifty winter phenotype is also evident in the physiology of digestion. Microbial protein synthesis is less facilitated by diminished phosphorus secretion into the shrunk rumen during winter. In line with this result, the concentration of ammonia, the end-product of protein digestion in the rumen, peaks in rumen liquid in spring, whereas the molar proportion of acetate, an indicator of fermentation of a diet rich in fiber, peaks in winter. In contrast to reduced stimulation of growth of ruminal microbes during winter, active transport of nutrients across the intestinal epithelium is increased, resulting in more efficient exploitation of the lower amount and quality of ingested winter feed. Nevertheless, the energy balance remains negative during winter. This is compensated by using fat reserves accumulated during summer, which become a major metabolic fuel during winter.
生活在季节性环境中的反刍动物在冬季面临双重挑战。在较低的环境温度下，维持高体温的能量成本更高，而且饲料的可得性和质量也很差，这使问题更加复杂。野生反刍动物通过体温过低来适应这种能量挑战，也就是说，减少内源性产热并放弃维持高体温，尤其是在身体的外周部位。在冬季，减少觅食活动、降低饲料的热增效应以及减少缩小器官的维持成本等因素，进一步降低了能量消耗。总之，通过连续测量心率来估计，冬季的代谢率会下调到夏季水平的一半以上，就像自由采食的动物的自愿食物摄入量一样。这种从夏季到节俭的冬季表型的转变也体现在消化生理学上。冬季，由于磷分泌减少，微生物蛋白质合成的促进作用减弱。与这一结果一致的是，氨的浓度，即瘤胃中蛋白质消化的终产物，在春季达到瘤胃液的峰值，而指示富含纤维的饮食发酵的乙酸摩尔比例在冬季达到峰值。与冬季瘤胃微生物生长刺激减少相反，营养物质通过肠上皮的主动转运增加，从而更有效地利用摄入的冬季饲料的低数量和低质量。然而，冬季的能量平衡仍然是负的。这通过利用夏季积累的脂肪储备来补偿，这些储备在冬季成为主要的代谢燃料。
