Environmental Relationships and the Denning Period of Black Bears in Tennessee

The denning period of black bears (Ursus americanus) in the Great Smoky Mountains National Park was determined through the use of motion-sensitive transmitter collars. Most (14, 83%) instrumented bears entered dens between 25 December and 7 January. Adult females entered dens first (X̄ = 31 December), subadults of both sexes entered dens next (X̄ = 7 January), and adult males entered dens last (X̄ = 12 January). Emergence generally occurred between 25 March and 7 April, with the denning period averaging 90 days. Females with cubs vacated dens later (P < 0.002) and denned longer (P < 0.011) than other adult females. Den entry and strong fidelity to dens by all instrumented bears indicated that the intensity of dormancy did not differ from that in northern regions; however, duration of dormancy was considerably shorter. Cumulative effects of increased precipitation and lower maximum and higher minimum temperatures, which correspond to passage of a low pressure weather front, provided a proximate stimulus to enter dens. Food supply also appeared to affect denning in a proximal manner because bears denned earlier in years with fair to poor mast yields than in years with excellent mast yields (P < 0.0004). Emergence dates were less strongly correlated with environmental factors. Ultimate synchronization of denning behavior with the environment is best explained by a circannual (endogenous) rhythm; this rhythm is easily shortened or lengthened allowing flexibility depending on environmental variation and the ecology of a species. Such a rhythm encompasses the observed variation in environmental factors affecting the denning period of bears over their broad geographic range and diverse ecological conditions.