Changes in the nutritional status of free-ranging animals have a strong influence on individual fitness, yet it remains challenging to monitor longitudinally. Nitrogen (δ15N) and carbon (δ13C) isotope values measured chronologically along the length of metabolically inert keratinous tissues can be used as a nutritional biomarker to retrospectively reconstruct the foraging ecology and eco-physiology of consumers. We quantitatively describe the physiological effects of fasting on amino acid metabolism using sequentially measured bulk tissue and amino acid δ15N values along the length of whiskers sampled from free-ranging juvenile, subadults, adult female, and male southern elephant seals (SES; Mirounga leonina) on Marion Island in the Southern Ocean. For both juveniles and adult females, whisker segments representing fasting had significantly higher bulk tissue δ15N values of 0.6 ± 0.5‰ and 1.3-1.8‰, respectively, in comparison to segments unaffected by fasting. We also found a large increase (2-6‰) in δ15N values for most glucogenic amino acids and a simultaneous depletion (2-3‰) of alanine in segments reflecting fasting, which enabled us to accurately predict (74%) the nutritional status of our model species. We hypothesize that the glucose-alanine cycle is the mechanism driving the observed depletion of alanine δ15N values during fasting. We demonstrated that keratinaceous tissues can be used as a longitudinal nutritional biomarker to detect changes in the nitrogen balance of an individual. Moreover, it is evident that physiological factors have an important influence on tissue δ15N values and can lead to erroneous bulk tissue or amino acid isotope-based reconstructions of foraging habits.

中文翻译：

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空腹影响氨基酸氮同位素值：一种用于识别自由放养哺乳动物氮平衡的新工具。

自由放养动物的营养状况变化对个体适应性有很大影响，但纵向监测仍然具有挑战性。沿代谢惰性角质组织的长度按时间顺序测得的氮（δ15N）和碳（δ13C）同位素值可用作营养生物标志物，以追溯地重建消费者的觅食生态和生态生理。我们定量地描述了禁食对氨基酸代谢的生理影响，方法是使用顺序测量的大块组织和沿着自由放养的幼年，亚成年，成年雌性和雄性南象海豹（SES; Mirounga leonina）采样的胡须长度的氨基酸δ15N值在南大洋的马里恩岛上。对于少年和成年女性，与不受禁食影响的节段相比，代表禁食的晶须节段的大体组织δ15N值分别显着更高，分别为0.6±0.5‰和1.3-1.8‰。我们还发现，大多数糖原氨基酸的δ15N值大幅增加（2-6‰），而反映禁食的区段中丙氨酸同时消耗（2-3‰），这使我们能够准确预测（74％）营养状况我们的模型物种。我们假设葡萄糖-丙氨酸循环是在禁食期间驱动观察到的丙氨酸δ15N值消耗的机制。我们证明角质组织可以用作纵向营养生物标志物，以检测个体的氮平衡变化。此外，