Fat Reserves and Reproduction in Black Bears

The American black bear Ursus americanus currently has a stable population throughout most of its range in the wild, and therefore provides an ideal non-endangered model for the study of reproduction in endangered bear species.

American black bears are known to undergo dramatic seasonal changes in body weight and adiposity. The protein hormone leptin, produced in adipose cells, is important for the regulation of appetite, adiposity, and energy in mammals. Therefore, CRES researchers hypothesized that leptin plays an important role in the seasonal fluctuations in these physiological parameters in the black bear. Adiposity is a crucial factor in the regulation of reproduction in most mammals. Leptin is believed to be associated with reproductive function, and circulating levels of leptin are known to correlate with sex steroids and other hormones involved in reproduction. By characterizing the seasonal profiles of leptin and adiposity, researchers hope to better understand the relationship between the seasonal rhythms of adiposity and reproduction in bears.

Drs. Thomas Spady and Barbara Durrant of the Reproductive Physiology Division of the San Diego Zoo's CRES, with colleagues Dr. Hank Harlow of the University of Wyoming and Dr. George Butterstein of Union College, examined the relationships between serum leptin, body weight, and body fat in semi-free ranging male black bears in western South Dakota. The bears were studied at four time points corresponding with the pre-denning (October), overwintering (January), den emergence (March), and peak breeding season (June).

At all time points serum leptin levels were strongly correlated with the percent of body fat in a positive, linear manner, both when examined within individual bears or when all data sets were pooled. These data indicate the seasonal changes in leptin levels mimic the seasonal changes in adiposity, and implicate leptin as a candidate mechanism by which adiposity regulates reproduction in bears. A better understanding of adiposity and reproduction in bears will allow researchers to optimize captive breeding success of endangered bears through improved nutritional management.

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