Fasting affects amino acid nitrogen isotope values: a new tool for identifying nitrogen balance of free-ranging mammals

Changes in the nutritional status of free-ranging animals have a strong influence on individual fitness, yet it remains challenging to monitor longitudinally. Nitrogen (delta N-15) and carbon (delta C-13) 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 delta N-15 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 delta N-15 values of 0.6 +/- 0.5 parts per thousand and 1.3-1.8 parts per thousand, respectively, in comparison to segments unaffected by fasting. We also found a large increase (2-6 parts per thousand) in delta N-15 values for most glucogenic amino acids and a simultaneous depletion (2-3 parts per thousand) 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 delta N-15 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 delta N-15 values and can lead to erroneous bulk tissue or amino acid isotope-based reconstructions of foraging habits.