Black Bear Reproductive Biology, Denning Biology, Habitat Use, and Movements in Northern Wisconsin

Abstract

Few studies have documented reproductive rates, denning biology, habitat use, or movements (including dispersal) of black bears (Ursus americanus) in Wisconsin. This knowledge is required to effectively monitor and manage Wisconsin's bear population, currently estimated at 6095 bears. All known bear dens in north-central Wisconsin were visited during 1981-1988 to document litter size and den characteristics. Sixteen adult female, yearling, and subadult bears were tracked using radio-telemetry during 1986-1988 to define habitat use and movements. Litter sizes and survival were among the highest documented in the literature. Mean litter size was 2.92 at approximately 6-weeks-of-age. Cub survival estimates ranged from 89-94%, resulting in litter sizes of 2.58 at 1-year-of-age. Litter sizes increased with increased age of females (P = 0.039). Mean date of den entry was 28 September for pregnant females, 12 October for females with cubs, 15 October for yearlings, and 21 October for adult males (P = 0.0213). Mean dates of entry were 12 days earlier during years with an apparent lack of food than years with an apparent abundance of natural foods. Date of entry was positively correlated with mean weekly ambient temperatures (r2 = 0.462, P = 0.0002). All bears chose similar den types (P > 0.367) and established dens in similar habitat types (P > 0.183). Volume of nest material was greater for males than for females (P = 0.0433). Den availability in northern Wisconsin was not a limiting factor for bears. The aspen habitat type (Populus spp.) ranked highest in frequency and importance of use by bears for all seasons, years, and sex-age classes. (P < 0.00). Alder, swamp conifer, northern hardwoods, and swamp hardwoods were the next most important habitats used by bears, in descending order. Annual home range size was 18.50 km2 and length was 6.69 km for all bears combined (adult males not sampled). Bears 3-6 years-of-age used the largest home areas 30.46 km2 and 9.31 km in length (P < 0.0233). Females with cubs had smaller annual ranges (13.81 km2) than solitary females (21.18 km2) (P = 0.001 ). Area and length measures showed similar variations between sex, age, and seasonal changes in home areas (P < 0.0388). Average movement rates while foraging ranged between 0.5-1.25 km/hr. Maximum rate of travel was 3.08 km/hr. Moon phase did not affect activity periods (P > 0.2456) but did affect movement rates (P = 0.0001 ). Nighttime movement rate increased with increases in available moonlight and coincided with the timing of moon rise. Bears exhibited daily highs and lows in activity levels. The timing and intensity of activity varied between seasons of the year. Activity was crepuscular during the pre-breeding season, mostly diurnal during the breeding and foraging periods, and became more nocturnal during the fall denning period. Family units broke-up between 7 June and 16 June (n = 3) and did not re-unite. Yearlings (3, of 7) and subadults (2) dispersed between 16 May and 18 June. Average dispersal distance was 36.1 km (n = 4). Physical measurements were taken on 247 bears. Neck girth and chest girth provided reliable estimates of body weights (r2 = 0.902 and r2 = 0.8591, respectively). Body temperatures as low as 21.92° C were recorded, whereas previous researchers felt 32° C was the lowest tolerable body temperature (Hock 1957). The amount of tranquilizer required to sedate bears was correlated with weight (P < 0.005) and analysis of dosages administered in this study indicate that smaller dosages (20-25%) than previously recommended may be adequate.