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

Abstract

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 concentrations were low in early lactation ( ). 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 freeliving marine mammals.

Measurement of marine osmolytes in mammalian serum by liquid chromatography–tandem mass spectrometry

“…This assay can also be applied to measure betaine and other analytes such as proline betaine, trigonelline, and choline in foods [37]. Thus, this assay will have a wide range of potential applications in addition to our objectives to determine the feeding behavior of Weddell seals [5].…”

“…Glycine betaine is metabolized in mammals by betaine-homocysteine methyltransferase (BHMT), and the increase following feeding is short-lived [26] even though it is not excreted in the urine [27]; DMSP especially is rapidly metabolized by BHMT and would be expected to have a short biological half-life [19,28]. AsB enters tissues as a glycine betaine analog and is not a substrate for BHMT [19] and has a long half-life [5]. Homarine is not known to be metabolized by mammals, and there currently are no data on its clearance by mammals.…”

“…Elevated levels of TMAO and AsB have been identified in the blood plasma of the Antarctic Weddell seal (Leptonychotes weddellii) to indicate feeding or fasting [5]. TMAO, AsB, DMSP, and homarine are not synthesized endogenously by mammals.…”

“…1). The osmolyte analog, arsenobetaine (AsB), is also accumulated in tissues by the transport systems that transport glycine betaine but is not metabolized; therefore, it too is a potential biomarker [5].…”

“…TMAO and AsB were previously measured separately using headspace gas 0003-2697/$ -see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ab.2011.09.013 chromatography (GC) and graphite furnace-atomic absorption spectrometry (GF-AAS) [5]. The aim of our study was to develop a method that could be used to identify and quantify all of the osmolytes of interest in a single sample using one analytical system.…”