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First published online July 17, 2009
Journal of Experimental Biology 212, 2313-2323 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.026146
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A body composition model to estimate mammalian energy stores and metabolic rates from body mass and body length, with application to polar bears

Péter K. Molnár1,2,*, Tin Klanjscek3, Andrew E. Derocher2, Martyn E. Obbard4 and Mark A. Lewis1,2

1 Centre for Mathematical Biology, Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, AB, Canada T6G 2G1
2 Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E9
3 Department for Marine and Environmental Research, Ruder Boskovic Institute, POB. 180, Bijenicka 54, HR-10002 Zagreb, Croatia
4 Wildlife Research and Development Section, Ontario Ministry of Natural Resources, DNA Building, Trent University, 2140 East Bank Drive, Peterborough, ON, Canada K9J 7B8

* Author for correspondence (e-mail: pmolnar{at}ualberta.ca)

Accepted 27 April 2009

Many species experience large fluctuations in food availability and depend on energy from fat and protein stores for survival, reproduction and growth. Body condition and, more specifically, energy stores thus constitute key variables in the life history of many species. Several indices exist to quantify body condition but none can provide the amount of stored energy. To estimate energy stores in mammals, we propose a body composition model that differentiates between structure and storage of an animal. We develop and parameterize the model specifically for polar bears (Ursus maritimus Phipps) but all concepts are general and the model could be easily adapted to other mammals. The model provides predictive equations to estimate structural mass, storage mass and storage energy from an appropriately chosen measure of body length and total body mass. The model also provides a means to estimate basal metabolic rates from body length and consecutive measurements of total body mass. Model estimates of body composition, structural mass, storage mass and energy density of 970 polar bears from Hudson Bay were consistent with the life history and physiology of polar bears. Metabolic rate estimates of fasting adult males derived from the body composition model corresponded closely to theoretically expected and experimentally measured metabolic rates. Our method is simple, non-invasive and provides considerably more information on the energetic status of individuals than currently available methods.

Key words: structure, storage, dynamic energy budgets, energy reserve, body fat, lean body mass, isotopic water dilution, bioelectrical impedance analysis, body condition index, nutritional status, Ursus maritimus


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