Variation in Reproductive Biology of Male Black Bears

Abstract

We compared the reproductive biology of male black bears in two study areas at different latitudes (36 and 47°N) and with dissimilar population characteristics (hunted versus unhunted) to test two null hypotheses stemming from previous studies: 1) no regional variation exists in male reproductive rhythms; 2) social factors do not contribute to variation. Both hypotheses were rejected. Serum–testosterone profiles differed between the two areas, with a sharper peak associated with the May–July breeding season among bears in Minnesota. Bears in Virginia and North Carolina had a longer period of elevated serum testosterone, in apparent accordance with more prolonged breeding periods reported for southerly populations of black bear. In both areas, serum-testosterone concentrations and size of testicles decreased from the breeding months to autumn. By December, when all bears in Minnesota were denned, testicles had ascended into the body cavity, and serum testosterone reached a nadir. Serum testosterone increased during late denning (February–March), and testicles of some bears became scrotal, supporting previous suggestions that the male reproductive cycle is triggered by photoperiod. However, we also observed age–specific differences in secretion of testosterone between study sites that appeared to be related to the effects of differing social structures.