Denning Chronology of Female Black Bears: Effects of Food, Weather, and Reproduction

Abstract

We studied factors affecting the chronology of denning by 104 female black bears ( Ursus americanus ) from three areas in Maine from 1982 to 1988. At two areas in northern Maine, entry into dens followed an alternate-year pattern with most bears denning in mid to late October when beechnuts ( Fagus grandifolia ) were scarce, and in mid to late November the next year when nuts were abundant. This pattern may have occurred for ≥11 years and has not been reported previously for a black bear population. From 1986 to 1988, patterns of habitat use reflected another behavioral response to beechnut crops; bears used hardwood forests more during autumns when nuts were abundant. Annual variation in den entry was not strongly associated with weather patterns during autumn. Where beech trees were less common in central Maine, den entry and habitat use varied less among years. Pregnant females tended to den earlier than other females. Den emergence was similar among reproductive classes, generally occurring during April within 7 days of the final snow melt. Denning periods for individuals ranged from 134 to 197 days. We concluded that black bears generally remain active in autumn until a negative energy balance occurs, but we hypothesized that pregnant females may den after they store adequate fat reserves for reproduction.