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We report a novel method of investigating foraging in marine mammals based on detecting biomarkers of strictly dietary origin in blood. Arsenobetaine (AsB), the arsenic analogue of the osmolyte glycine betaine, and trimethylamine N-oxide (TMAO), an osmolyte used by marine fish and invertebrates, were measured in plasma of lactating Weddell seals Leptonychotes weddellii during the postpartum fast and at different stages of lactation. Plasma dietary biomarker concentra- tions were low in early lactation (<14 d postpartum (d p.p.); AsB: 5.5 ± 2 ppb As, TMAO: 66 ± 20 µmol l-1) and increased 10-fold in late lactation (≥27 d p.p.; AsB: 57 ± 17 ppb As, TMAO: 685 ± 199 µmol l-1). In lactating females (n = 6) monitored longitudinally, plasma TMAO remained low for the first 3 wk p.p. Increases in AsB concentrations observed in late lactation were closely correlated (Pearson correlation r = 0.96, p < 0.01, n = 6) with increases in plasma insulin-like growth factor 1 (IGF1), a growth factor known to reflect nutritional status in other mammals. Two seals outfitted with time-depth recorders began regular deep diving at different times p.p. and showed corresponding differences in biomarker concentrations. Our results indicate that (1) most lactating Weddell seals at the study site (70% of 20 seals studied) feed after 3 to 4 wk p.p., (2) individual lactating Weddell seals may forage as early as 9 d p.p., and (3) some Weddell seals may consume little or no food throughout lactation. Dietary biomarkers provide a simple method for distinguishing feeding and fasting in free- living marine mammals.

Detection of food intake in a marine mammal using marine osmolytes and their analogues as dietary biomarkers