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Title: Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal

Ecologists increasingly determine the δ15N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ15N values in consumers. Thus, trophic‐source δ15N offsets (i.e., Δ15NT‐S) reflect trophic position in a food web. However, even minor variations in δ15Nsource AAvalues may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDFT‐S). Diet digestibility and protein content can influence the δ15N values of bulk animal tissue, but the effects of these factors on AA Δ15NT‐Sand TDFT‐Sin mammals are unknown.


We fed captive mice (Mus musculus) either (A) a low‐fat, high‐fiber diet with low, intermediate, or high protein; or (B) a high‐fat, low‐fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for bulk tissue δ15N using elemental analyzer‐isotope ratio mass spectrometry (EA‐IRMS), and were also hydrolyzed into free AA that were analyzed for δ15N using gas chromatography‐combustion‐IRMS.


As dietary protein increased, Δ15NConsumer‐Dietslightly declined for bulk muscle tissue in both experiments; increased for AA in the low‐fat, high‐fiber diet (A); and remained the same or decreased for AA in the high‐fat, low‐fiber diet (B). The effects of dietary protein on Δ15NT‐Sand on TDFT‐Svaried by AA but were consistent between variables.


Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ15NConsumer‐Dietvalues. However, the similar responses of Δ15NT‐Sand of TDFT‐Sto diet variation suggest that if diet samples are available, Δ15NT‐Saccurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ15NT‐Svalues. The trophic‐source offset of Pro‐Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu‐Phe).

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Author(s) / Creator(s):
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Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Rapid Communications in Mass Spectrometry
Medium: X
Sponsoring Org:
National Science Foundation
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