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Validating the use of bulk tissue stable isotope and amino acid δ15N values measured in molted hair and epidermis of elephant seals to assess temporal foraging niche specialization

Nico Lübcker, Seth D. Newsome, Marthán N. Bester, P. J. N. de Bruyn

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ABSTRACT: The degree of dietary generalization versus specialization exhibited by populations can impact their ability to respond to changing environmental conditions. Naturally shed hair and epidermis are easily collected and may provide a suitable substrate for stable isotope analysis to assess trophic niche specialization. Whether fasting influences the isotopic composition of molted tissues has not been assessed. Here, we gauged the validity of using carbon (δ13C) and nitrogen (δ15N) isotope analysis of molted hair and the outermost epidermis as proxies to study the foraging ecology of southern elephant seals (Mirounga leonina) at Marion Island, southern Indian Ocean. Similarity in both bulk tissue (δ13C and δ15N) and amino acid (δ15N) isotope values of molted tissues and whiskers known to reflect fasting confirmed that physiological changes associated with fasting influenced hair and epidermis isotope values. We recommend caution when using the isotopic composition of molted hair and epidermis to make ecological inferences regarding diet and/or movement. Despite these limitations, molted hair can be used to detect extreme foraging behaviors, which we show using an extensive dataset of hair sampled from marked individuals (n = 331) that hauled out on Marion Island during the 2012–2013 annual pelage molt. Other factors, such as ontogenetic shifts in diet and potential environmental (baseline) isotopic variation, also influence hair δ13C and δ15N values, which complicates the use of isotopic data from molted tissues to assess the temporal consistency of an individuals’ isotopic niche.