Quantitative Support for a Subjective Fatness Index for Immobilized Polar Bears

Authors

  • IAN STIRLING,

    Corresponding author
    1. Canadian Wildlife Service, 5320 122 Street, Edmonton, AB T6H 3S5, Canada, and Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada
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  • GREGORY W. THIEMANN,

    1. Department of Biology, Dalhousie University, Halifax, NS B3H 4J1, Canada
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    • Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E, Canada

  • EVAN RICHARDSON

    1. Canadian Wildlife Service, 5320 122 Street, Edmonton, AB T6H 3S5, Canada
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ian.stirling@ec.gc.ca

Abstract

ABSTRACT  Adequate stores of body fat are essential for survival and reproduction of polar bears (Ursus maritimus). However, polar bear body fat levels can be difficult to quantify in the field. For >30 years, biologists have subjectively estimated relative fatness of immobilized polar bears by assigning individuals a rating from 1 to 5, with 1 being leanest and 5 most obese. Although previous studies suggested this fatness index (FI) rating accurately reflects large-scale differences in body condition, its relationship to more quantitative measures of condition has not been explored. We compared the FI rating of individual polar bears in western Hudson Bay and the Beaufort Sea to 2 quantitative measures of body condition: the Quetelet Index (ratio of mass to length2) and the relative lipid content of adipose tissue. We found a significant relationship between FI rating and both Quetelet Index values and adipose lipid content. Our data demonstrate that the FI rating accurately reflects overall body condition, regardless of polar bear age, sex, or nutritional phase. We suggest that continued field use of the FI rating could provide valuable information on ecological effects of large-scale environmental change on polar bear populations.

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