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Fatty acid profiles of feeding and fasting bears: estimating calibration coefficients, the timeframe of diet estimates, and selective mobilization during hibernation


Accurate information on diet composition is central to understanding and conserving carnivore populations. Quantitative fatty acid signature analysis (QFASA) has emerged as a powerful tool for estimating the diets of predators, but ambiguities remain about the timeframe of QFASA estimates and the need to account for species-specific patterns of metabolism. We conducted a series of feeding experiments with four juvenile male brown bears (Ursus arctos) to (1) track the timing of changes in adipose tissue composition and QFASA diet estimates in response to a change in diet and (2) quantify the relationship between consumer and diet FA composition (i.e., determine “calibration coefficients”). Bears were fed three compositionally distinct diets for 90–120 days each. Two marine-based diets were intended to approximate the lipid content and composition of the wild diet of polar bears (U. maritimus). Bear adipose tissue composition changed quickly in the direction of the diet and showed evidence of stabilization after 60 days. During hibernation, FA profiles were initially stable but diet estimates after 10 weeks were sensitive to calibration coefficients. Calibration coefficients derived from the marine-based diets were broadly similar to each other and to published values from marine-fed mink (Mustela vison), which have been used as a model for free-ranging polar bears. For growing bears on a high-fat diet, the temporal window for QFASA estimates was 30–90 days. Although our results reinforce the importance of accurate calibration, the similarities across taxa and diets suggest it may be feasible to develop a generalized QFASA approach for mammalian carnivores.

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Availability of data and materials

The datasets generated during the current study are available from the corresponding author on reasonable request.

Code availability (software application or custom code)

All analyses were conducted in R, version 4.0.0.


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Financial support was provided by Natural Sciences and Engineering Research Council (NSERC) Canada, U.S. Fish and Wildlife Service Marine Mammals Management program, US Geological Survey, Interagency Grizzly Bear Committee, FRI Research Grizzly Bear Program, USDA National Institute of Food and Agriculture (Hatch project WNP 00226), Raili Korkka Brown Bear Endowment, Nutritional Ecology Endowment, and Bear Research and Conservation Endowment at Washington State University.

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GWT, KDR, and CTR conceived the work; GWT analyzed the data and wrote the initial manuscript; JAE and CTR conducted the experiments; SMB analyzed the samples. All authors contributed to writing and editing the final version.

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Correspondence to Gregory W. Thiemann.

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Communicated by P. Withers.

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Thiemann, G.W., Rode, K.D., Erlenbach, J.A. et al. Fatty acid profiles of feeding and fasting bears: estimating calibration coefficients, the timeframe of diet estimates, and selective mobilization during hibernation. J Comp Physiol B 192, 379–395 (2022).

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