Annual Testosterone Rhythm in the Black Bear (Ursus americanus)1

Abstract

Thirty-one blood samples were obtained from wild male black bears at different times of the calendar year. The samples were assayed for serum testosterone and the results were correlated with testicular size, time of year and nutritional status. A significant annual testosterone rhythm was found with high values during March through mid-July (mean 153 ng per 100 ml) and low values from mid-July through November (mean 47 ng per 100 ml, P<0.001). Testicular length was also greater during March through mid-July (mean 6.9 cm) than mid-July through November (mean 5.8 cm, P<0.005). Five animals were studied in the spring while still in their dens after having spent the winter isolated and not feeding. Despite a mean 19 percent loss in body weight and a winter of social isolation, serum testosterone was found to be elevated in these animals (mean 161 ng per 100 ml). This suggests that the annual testosterone cycle of this species is independent of nutritional and social factors.