Threonine nitrogen isotopes reveal hidden physiological dimensions of mammalian ecology

Stable nitrogen isotopes of amino acids are widely used to reconstruct trophic position. Most applications rely on only two amino acids despite routinely measuring many others. Here, comparative amino acid {delta}15N values from 88 mammal species reveal that threonine records a physiological dimension beyond trophic position. Adding threonine to the canonical glutamate-phenylalanine framework reveals ecological differentiation obscured by broad dietary categories and opposite isotopic relationships between herbivores and secondary consumers. A mechanistic model links this variation to preferential intestinal utilization of threonine for mucin synthesis and predicts experimentally testable patterns of isotope partitioning. These findings show that amino acid {delta}15N values encode complementary dimensions of organismal ecology, expanding amino acid isotope analysis beyond trophic reconstruction to reveal physiological and ecological variation associated with dietary specialization.