Endocrine Function in American Black Bears (Ursus americanus)
American Association of Zoo Veterinarians Conference 2010
Stephanie McCain1,2, DVM; Ed Ramsay1, DVM, DACZM; Claudia Kirk1, DVM, PhD, DACVIM
1University of Tennessee, Knoxville, TN, USA; 2Birmingham Zoo, Birmingham, AL, USA


Obesity is one of the most common health problems of captive American black bears (Ursus americanus). In addition to not being able to provide a truly natural diet or the quantity of physical activity wild bears experience, many zoos do not allow their bears to hibernate over winter. Studies examining wild, or recently captured, black bears show reduced thyroid hormone levels during hibernation but comprehensive examination of thyroid hormones or glucose metabolism in captive and wild bears have not been reported.1-3

Wild (n=4–5), captive hibernating (n=5), and captive non-hibernating (n=5) bears were evaluated at times corresponding to their three major physiologic stages: fall (hyperphagic stage), winter (hibernation stage), and summer (normal activity stage). The same bears were not evaluated at each stage. Blood samples were analyzed for T4, T3, free T4, and free T3 concentrations and a combined insulin and glucose tolerance test was performed (a loading dose of 0.5 g/kg dextrose IV and serial blood samples obtained at pre- and 15, 30, 45 and 60 min post-glucose administration; 0.03 IU/kg Humulin R insulin was administered immediately after the 15 min sample was obtained; each sample was analyzed for insulin and glucose concentrations). The glucose curves were similar for all bears within a given stage. All three groups of bears had evidence of insulin resistance during the winter as compared to the summer on glucose curves. Analysis of thyroid hormone concentration varied, and distinct patterns or similarities were not apparent.


Funding for this study was provided by the AAZV Mazuri grant. The authors would also like to thank the following institutions and people for providing animals used in this study: Knoxville Zoo, Memphis Zoo, North Carolina Zoo, Ober Gatlinburg Municipal Black Bear Habitat, Western North Carolina Nature Center, Grandfather Mountain, Great Smoky Mountains National Park, Kelcey Burguess.

Literature Cited

1.  Azizi F, Mannix JE, Howard D, Nelson RA. Effect of winter sleep on pituitary thyroid axis in American black bear. Am J Physiol. 1979;237: E227–E230.

2.  Nelson RA, Wahner HW, Jones JD, Ellefson RD, Zollman PE. Metabolism of bears before, during, and after winter sleep. Am J Physiol. 1973;224:491–496.

3.  Tomasi TE, Hellgren EC, Tucker TJ. Thyroid hormone concentrations in black bears (Ursus americanus): hibernation and pregnancy effects. Gen Comp Endocrinol. 1998;109:192–199.


Speaker Information
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Stephanie McCain, DVM
University of Tennessee
Knoxville, TN, USA

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