Sex steroid and prolactin profiles in male American black bears (Ursus americanus) during denning

Serum sex steroid and prolactin profiles were examined in the male American black bear, Ursus americanus during denning. Sera collected in December an d the following March from 8 denning male black bears in Minnesota, U.S.A. were assayed for testosterone, estradiol-17 beta and prolactin. Eight bears were confirmed to be the denning mode based on a serum urea to creatinine ratio less than 10. Serum testosterone concentrations tended to increase fr om December to the subsequent March whereas serum estradiol-17 beta concent rations tended to decrease during this period. There were few changes in se rum prolactin concentrations between December and March. These findings sug gest that spermatogenesis and testicular steroidogenesis initiated during d enning may be influenced by changes in serum sex steroid concentrations in the American black bear.