Amino acid δ 15N values are associated with underweight status in a captive population of muskoxen

Abstract

Measures of protein balance, particularly changes in body mass, are important in understanding the health of wildlife populations but can be difficult or impossible to obtain from free-living animals. One potential biomarker, the δ15N value of bulk tissue, has had mixed associations with body mass. δ15N values of amino acids (AAs) have been shown to be more sensitive measures of trophic position than the bulk δ15N value and may also be more sensitive to changes in body mass. Here we evaluate AA δ15N values as a method to assess underweight status using archived serum samples from a captive population of muskoxen (Ovibos moschatus) that had decreased body mass resulting from an inadvertent dietary trace element deficiency. We predicted that AA δ15N values would be associated with deviations from the expected healthy body mass based on sex and age, and that they would be more sensitive than bulk δ15N values. We evaluated several linear mixed-effect models with δ15N values of bulk serum, AAs and other variables against deviations from expected body mass. In females, the δ15N values of glutamate/glutamine, leucine, aspartate/asparagine and serine, plus pregnancy status, were best associated with the difference from expected mass; in males, the δ15N values of glutamate/glutamine, leucine and proline were best associated with the difference from expected mass. Bulk δ15N values were not associated with the difference from expected mass. δ15N values of AAs have potential as a tool for researchers to evaluate body condition in animals.