Repeated migration informs amino acid nitrogen isotope incorporation in the African elephant Loxodonta africana

ABSTRACT

Accurate interpretation of an organism’s isotopic composition relies heavily on the assumption of steady state, which is often violated owing to limited appreciation for isotope incorporation. We present amino acid (AA) nitrogen isotope (δ15N) records from the tail hair of an African elephant (Loxodonta africana) that frequently migrated between the Samburu National Wildlife Reserve and Mount Kenya—these regions have drastically different baseline δ15N values of ~ 10‰. We used this baseline isotopic variation to estimate 15N incorporation for 13 AAs. We observe that incorporation in most AAs is best described by a two-pool reaction progress variable. Amino acids closely connected with metabolic nitrogen cycling that have higher rates of trans- and deamination, often termed ‘trophic AAs’, exhibited higher contributions from a short pool (41–75%) with faster incorporation (T50 = 5–37 days). Conversely, AAs associated with lower rates of trans- and deamination, often termed ‘source AAs’, exhibited higher contributions from a long pool (50–64%) with slower incorporation (T50 > 365 days). Calculation of relative trophic position using glutamic acid and phenylalanine revealed high variabslitty across the time series (TP = 0.3–3.2), suggesting a decoupling of isotopic steady state between AAs as the individual moved among ecosystems with inherently different δ15N baselines. Failure to consider that incorporation varies across AAs associated with different degrees of nitrogen mobilization has broad implications for trophic position estimates using AA δ15N values and could lead to erroneous interpretation across ecological systems.

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