Ecology and physiology of a black bear (Ursus americanus) population in the Great Dismal Swamp and reproduction physiology in the captive female black bear

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Virginia Polytechnic Institute and State University


Ecology and physiology of black bears in Great Dismal Swamp (GDS) National Wildlife Refuge and surrounding area, a forested wetland in eastern Virginia and northeastern North Carolina, were studied from April 1984 to March 1987. A total of 101 bears (71M, 30F) were captured 120 times. Males dominated the capture sample (P< 0.001), but age did not differ between sexes (P= 0.74). Mean (±SE) age was 4.2±0.3 years. Litter size (=2.1; N=12), modal age at primiparity (4 years), and interbirth interval (slightly greater than 2 years) were indicative of good-quality habitat. Estimated annual adult survival rates were 0.84 for females and 0.58 for males. Causes of mortality included legal (outside the Refuge) and illegal harvest, vehicle collisions, depredation permit kills, research, and predation. Population density for the study area was estimatcd by 6 techniques at 0.47-0.68 bears/km², corresponding to 262-377 bears for the 555 km² study area. Demographic data suggested a stable and productive population.

Three major levels of diet quality were observed in terms of crude fiber, fat, and protein. Spring diets were high in protein but moderate in crude fiber, while fall diets were low in crude protein and high in ether extract. Condition indices and several blood characteristics (e.g. total protein, albumin, HCT, hemoglobin, and RBC) were at peaks in spring and late fall and at a low during summer. Serum creatinine concentrations also varied seasonally (P<0.001), with a peak during denning and high levels in spring and late fall, perhaps resulting from transition from and to hibernation. A urea/creatinine (U/C) ratio ≤10 was not a good indicator of the hibernating state, as 39 of 120 (32.5%) trapped, active bears had U/C ratios ≤10. Creatinine and total protein were the best indicators of the hibernating state. Albumin, HCT, hemoglobin, and RBC were the best indicators of condition during active stages, as indicated by significant (P< 0.1) correlations of condition indices and blood variables. Nine blood variables varied with age (P< 0.1). Multivariate analysis of variance and discriminant function analysis using blood variables failed to reject the hypothesis that bears cycled through 4 metabolic stages throughout the year. Results showed that metabolic shifts were tied to concomitant seasonal changes in diet quality, diet composition, and body condition, but also may have an endogenous component.

Habitat and range use dynamics were described by radiotracking of 24 female and 22 male bears. Median annual range size estimates were 79.1 km² for males (N=10), 33.1 km² for subadult h females (N=7), and 21.4 km² for adult females (N=11). Preferred (P< 0.05) habitats on an annual basis were pocosins and mesic areas for females and gum-cypress and maple-coniferous stands for males. Seasonally, pocosin, gum-cypress, mesic, and disturbed areas were important for females. Bear distribution analysis indicated that roads were preferred (P< 0.05) during all seasons except early fall, when bears made fall excursions to feeding areas far from Swamp roads and close to the study area boundary. Range overlap was extensive for both sexes, although it appeared that females maintained exclusive ranges during spring and early summer.

Denning ecology was described by monitoring 35 bears (26F, 9M). Five bears (2M, 3F) remained active throughout the winter. Den types included 14 elaborate ground nests, 11 excavated ground cavities, 2 ground-level tree cavities, 1 above-ground-level tree cavity, and l den in a stump. Females with cubs denned earlier, (P< 0.02) emerged later (P< 0.001), and denned longer (P< 0.001; 119 ±4 days vs. 78 ±4 days) than all other bear groups. Dry den sites did not appear to be limited.

Present population management (protection from hunting and no public vehicular access) should be continued in the Refuge. The small effective population size (N=66) in GDS indicated the need for study of dispersal and genetics in the GDS and other southeastern wetland populations to determine the degree of isolation and extent of genetic variability. Maintenance and enhancement of pocosins, mature gum, oak, and disturbed habitats would benefit black bears in southeastern wetlands by providing a wide variety of natural foods throughout the year. Large den trees may not be necessary for successful denning and reproduction in certain southeastern wetlands because bears can use dense cover and microelevational factors to overwinter. Black bear conservation strategies in the Southeast are a critical need due to increasing habitat fragmentation.

Six adult female black bears were maintained in captivity in Virginia from August 1987 to April 1988. Serum samples, as well as data on body weight and rectal temperatures, were collected from each bear at approximately 10-day intervals from 25 September to 30 March. Four of the six bears hibernated, not feeding for periods of 56 to 121 days (=94 d). Rectal temperature declined in both active and hibernating bears during winter, but to a greater extent (P= 0.013) in hibernators. Average weight loss during hibernation represented 27.9% of peak body weight. Mean serum urea/creatinine (U/C) ratios were similar between physiological groups during the prehibernation phase. However, U/C ratios differed (P< 0.025) after the onset of hibernation. Concentrations of total serum protein, serum urea nitrogen, and serum creatinine were similarly affected by significant time-group interactions (P< 0.01). Alkaline phosphatase, phosphorus, sodium, and chloride changed significantly (P< 0.05) during the course of the experiment, without regard to physiological group. U/C ratio was a good indicator of the hibernating state, but the sensitivity of serum urea levels to diet suggests careful use of U/C ratio as a field index. Serum progesterone (P) concentrations slowly increased from 2 to 5 ng/ml during October and November, then increased 2-2.5 fold 58 ±5 days before parturition in 2 bears that produced cubs. After the implantation peak, P declined, reaching undetectable levels 1-2 days postpartum. Similar P profiles were observed in 3 of 4 bears that did not produce any observed cubs. P also was assayed in 38 active wild black bears to relate to reproductive status in the den. Changes in serum estradiol-17ß concentrations during gestation also were profiled. The occurrence of pseudopregnancy or early embryonic mortality in bears with elevated serum progesterone concentrations is discussed.