Northern ungulates undergo pregnancy in winter when food supplies are low. Consequently, females employ a capital breeding strategy that relies upon maternal body stores of energy and protein for fetal growth. We studied captive female reindeer (Rangifer tarandus tarandus; n = 6; 118 kg), caribou (Rangifer tarandus granti; n = 6; 97 kg), and muskoxen (Ovibos moschatus; n = 8; 205 kg) in late winter (February to April) to track body protein use in late pregnancy when feeding is often interrupted by snow storms and when wild animals begin movements to spring and summer ranges where they give birth. We used high and low protein rations (14–16 and 8% protein) as complete diets for Rangifer and high protein ration as a supplement (500 g/day) to hay (7.3% protein; δ15N = 1.1‰) for Ovibos. Animals were fasted for 2 days to evaluate isotopic responses to an acute deficit of energy and protein on each diet in a sequence from δ15N 1.6‰, to 6.9‰ to 3.2‰ over 58 days. Dietary shifts in δ15N were reflected in plant fibers in the feces (fecal fiber) but not in blood cells (Rangifer 6.5 ± 0.3‰; Ovibos 5.2 ± 0.1‰). Serum proteins were higher in δ15N than blood cells whereas serum amino acids were lower in δ15N than blood cells and more responsive to changes in dietary δ15N indicated by fecal fiber. Fasting did not affect δ15N of serum proteins or serum amino acids. Values for δ15N in urea were strongly affected by both shifts in diet and by fasting, which indicated that excretory urea N was derived from cellular proteins and isotopically heavy proteins released from organs into the serum. Inter-organ exchanges of transport proteins may minimize oxidation of stored amino acids and conserve body protein stores for fetal growth and milk production in Rangifer and Ovibos. A capital breeding strategy in these long-lived, iteroparous herbivores relies upon routing of body proteins to simultaneously sustain maternal function and maternal investment through common metabolic pathways that conserve lean body mass for survival.

中文翻译：

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冬天来了：保护雌性驯鹿、驯鹿和麝牛的身体蛋白质

当食物供应不足时，北方有蹄类动物会在冬天怀孕。因此，雌性采用一种资本繁殖策略，依靠母体身体储存的能量和蛋白质来促进胎儿生长。我们在晚冬（二月）研究了圈养雌性驯鹿（Rangifer tarandus tarandus；n = 6；118 kg）、驯鹿（Rangifer tarandus granti；n = 6；97 kg）和麝香牛（Ovibos moschatus；n = 8；205 kg）到 4 月）以跟踪怀孕后期的身体蛋白质使用情况，此时喂食经常被暴风雪中断，以及当野生动物开始迁移到它们分娩的春季和夏季范围时。我们使用高蛋白和低蛋白日粮（14-16% 和 8% 蛋白质）作为 Rangifer 的完整日粮，高蛋白日粮作为 Ovibos 干草（7.3% 蛋白质；δ15N = 1.1‰）的补充（500 克/天）。动物禁食 2 天，以评估对每种饮食中能量和蛋白质急性缺乏的同位素反应，按顺序从 δ15N 1.6‰ 到 6.9‰ 到 3.2‰ 超过 58 天。δ15N 的膳食变化反映在粪便中的植物纤维（粪便纤维）中，但未反映在血细胞中（Rangifer 6.5 ± 0.3‰；Ovibos 5.2 ± 0.1‰）。δ15N 中的血清蛋白高于血细胞，而 δ15N 中的血清氨基酸低于血细胞，并且对粪便纤维指示的膳食 δ15N 变化更敏感。禁食不影响血清蛋白或血清氨基酸的δ15N。尿素中 δ15N 的值受到饮食变化和禁食的强烈影响，这表明排泄尿素 N 来自细胞蛋白质和从器官释放到血清中的同位素重蛋白质。在 Rangifer 和 Ovibos 中，转运蛋白的器官间交换可以最大限度地减少储存氨基酸的氧化，并为胎儿生长和产奶保留体内蛋白质储存。在这些长寿、迭代生育的食草动物中，一种资本育种策略依赖于身体蛋白质的路由，以通过共同的代谢途径同时维持母体功能和母体投资，从而保持瘦体重以维持生存。