Den Entry Research Articles

Extrinsic and intrinsic factors drive the timing of gestation and reproductive success of Scandinavian brown bears

IntroductionClimate change is altering the reproductive phenology of many organisms, but the factors that influence the timing of gestation in ursids are still poorly understood. Higher temperatures in spring are already causing an earlier den exit in some brown bear populations, and a temporal mismatch between hibernation and reproduction could have dramatic consequences for reproductive success. Therefore, understanding the factors that control the timing of these events is important to forecast the consequences of climate change on population growth rates.MethodsIn this study, we used abdominal temperature loggers and GPS collars with acceleration sensors on 23 free-ranging pregnant female brown bears living in two areas in Sweden (61°N and 67°N latitude) to pinpoint hibernation and reproductive events. We investigated how intrinsic and extrinsic factors influence the termination of embryonic diapause and parturition, as well as their impact on reproductive success.ResultsThe termination of embryonic diapause was later in the northern area compared to the southern area and occurred earlier when ambient temperature at den entry was higher in both areas. In the southern area, young adults (i.e., females = 7 years old) had a delayed parturition when bilberry abundance was low the year of mating. Additionally, young adults had a lower reproductive success than adults and their probability to reproduce successfully was dependent on bilberry abundance, whereas adult females were not affected by this parameter.DiscussionAs den exit occurs later in the northern study area, we suggest that a later parturition might ensure that females lactate their cubs in the den for a reasonable amount of time while fasting. Similarly, a later parturition combined with an earlier emergence could allow young adults to spend less time in the den lactating if they could not reach an optimal body condition prior to hibernation. But as a result, their cubs are younger and more vulnerable when they leave the den leading to lower survival rates. Our results suggest that a decreased berry abundance in the fall could impact the reproductive and hibernation phenology of Scandinavian brown bear females and lead to a lower cub survival with potential consequences on the population dynamics.

Seasonality in Biological Rhythms in Scandinavian brown Bears.

Biological rhythms, such as rhythms in activity and body temperature, are usually highly synchronized and entrained by environmental conditions, such as photoperiod. However, how the expression of these rhythms changes during hibernation, when the perception of environmental cues is limited, has not yet been fully understood for all hibernators, especially in the wild. The brown bear (Ursus arctos) in Scandinavia lives in a highly seasonal environment and adapts to harsh winter conditions by exhibiting hibernation, characterized by reduced metabolism and activity. In this study, we aimed to explore the expression of biological rhythms in activity, body temperature and heart rate of free-ranging brown bears over the annual cycle, including active, hibernation and the transition states around den entry and exit. We found that rhythms in physiology and activity are mostly synchronized and entrained by the light-dark cycle during the bears’ active state with predominantly diel and ultradian rhythms for body temperature, activity and heart rate. However, during hibernation, rhythms in body temperature and heart rate were considerably slowed down to infradian rhythms, influenced by the amount of snow in the denning area, whereas rhythms in activity remained diel. Rhythms in the transition states when bears prepared for entering or coming out of hibernation state displayed a combination of infradian and diel rhythms, indicating the preparation of the body for the change in environmental conditions. These results reveal that brown bears adjust their biological rhythms to the seasonal environment they inhabit. Rhythms in physiology and activity show simultaneity during the active state but are partly disconnected from each other during hibernation, when bears are most sheltered from the environment.

Denning in brown bears.

Hibernation represents an adaptation for coping with unfavorable environmental conditions. For brown bears Ursus arctos, hibernation is a critical period as pronounced temporal reductions in several physiological functions occur.Here, we review the three main aspects of brown bear denning: (1) den chronology, (2) den characteristics, and (3) hibernation physiology in order to identify (a) proximate and ultimate factors of hibernation as well as (b) research gaps and conservation priorities.Den chronology, which varies by sex and reproductive status, depends on environmental factors, such as snow, temperature, food availability, and den altitude. Significant variation in hibernation across latitudes occurs for both den entry and exit.The choice of a den and its surroundings may affect individual fitness, for example, loss of offspring and excessive energy consumption. Den selection is the result of broad‐ and fine‐scale habitat selection, mainly linked to den insulation, remoteness, and availability of food in the surroundings of the den location.Hibernation is a metabolic challenge for the brown bears, in which a series of physiological adaptations in tissues and organs enable survival under nutritional deprivation, maintain high levels of lipids, preserve muscle, and bone and prevent cardiovascular pathologies such as atherosclerosis. It is important to understand: (a) proximate and ultimate factors in denning behavior and the difference between actual drivers of hibernation (i.e., factors to which bears directly respond) and their correlates; (b) how changes in climatic factors might affect the ability of bears to face global climate change and the human‐mediated changes in food availability; (c) hyperphagia (period in which brown bears accumulate fat reserves), predenning and denning periods, including for those populations in which bears do not hibernate every year; and (d) how to approach the study of bear denning merging insights from different perspectives, that is, physiology, ecology, and behavior.

The seasonal sensitivity of brown bear denning phenology in response to climatic variability

BackgroundFor brown bears (Ursus arctos), hibernation is a critical part of the annual life cycle because energy savings during hibernation can be crucial for overwintering, and females give birth to cubs at that time. For hibernation to be a useful strategy, timing is critical. However, environmental conditions vary greatly, which might have a negative effect on the functionality of the evolved biological time-keeping. Here, we used a long-term dataset (69 years) on brown bear denning phenology recorded in 12 Russian protected areas and quantified the phenological responses to variation in temperature and snow depth. Previous studies analyzing the relationship between climate and denning behavior did not consider that the brown bear response to variation in climatic factors might vary through a period preceding den entry and exit. We hypothesized that there is a seasonal sensitivity pattern of bear denning phenology in response to variation in climatic conditions, such that the effect of climatic variability will be pronounced only when it occurs close to den exit and entry dates.ResultsWe found that brown bears are most sensitive to climatic variations around the observed first den exit and last entry dates, such that an increase/decrease in temperature in the periods closer to the first den exit and last entry dates have a greater influence on the denning dates than in other periods.ConclusionsOur study shows that climatic factors are modulating brown bear hibernation phenology and provide a further structuring of this modulation. The sensitivity of brown bears to changes in climatic factors during hibernation might affect their ability to cope with global climate change. Therefore, understanding these processes will be essential for informed management of biodiversity in a changing world.

Factors affecting the success of grizzly bear translocations

ABSTRACTEvaluations of wildlife translocation can be traditional assessments of survival and reproductive success or can be expanded to include valuable but seldom used measures of behavior and physiology in reference to baseline data from a resident population. In Alberta, Canada where grizzly bears (Ursus arctos) are listed as a threatened species, there has been research on resident grizzly bear populations but limited follow‐up of translocated individuals associated with management actions. We determined an outcome for 110 grizzly bear translocation events (77 failed events, 33 successful events) between 1974 and 2014. We used logistic regression to investigate the effects of individual bear characteristics, management strategies, and habitat factors on translocation success. A translocation event was successful if the bear did not require further management action and if the bear survived at least one year without homing. We also compared the home range size, habitat selection, and denning behavior of translocated bears to the resident population over time to assess the long‐term effects of translocation. The odds of translocation success were higher if bears were moved early in the year and decreased by 47% for each unit increase in the level of mortality risk (based on road density, water, and edge features) at the release site. The odds of homing decreased substantially at translocation distances >100 km, but bears translocated outside the Bear Management Area (BMA) of capture had annual home ranges that were 3.25 times larger on average than resident bears. Translocated bears were initially selecting high quality habitat similar to areas used by resident bears, but this behavior appeared to decline after the first year of translocation. Den entry dates, den exit dates, and the denning period of translocated bears did not differ significantly from resident bears. Our findings can aid managers in making more informed decisions when considering translocation as a tool for managing human–bear conflict or supporting grizzly bear conservation efforts. © 2018 The Wildlife Society.

Influence of Food Availability on Matrilineal Site Fidelity of Female Asian Black Bears

Abstract. To describe the maintenance process of matrilineal site fidelity in large, solitary mammals, we investigated both the site fidelity over time and the effect of kinship on the space use at a fine (home range) scale under different levels of food availability. We collected location data for female Asian black bears, Ursus thibetanus, using GPS collars and identified microsatellite DNA markers from bears in the Ashio-Nikko Mountains, Japan. We investigated autumn food resources by assessing hard mast productivity. We found high site fidelity over the years from the denning to summer season, and there was spatial proximity among related females (i.e., mother—daughter, grandmother—granddaughter, and sister—sister pairs) from spring to summer and through mid-autumn in moderate/good hard mast years. In autumns of poor hard mast years, although females used sites far from their primary home ranges, they returned to sites near their primary range before den entry. In habitats, where good denning habitat ...

Identifying shifts in maternity den phenology and habitat characteristics of polar bears (Ursus maritimus) in Baffin Bay and Kane Basin

The phenology and habitat selection of polar bear (Ursus maritimus) maternity dens may shift over time in response to changing environmental conditions. We compared maternity den phenology and habitat characteristics using satellite telemetry data from adult female polar bears from the Baffin Bay (BB) (n = 16 dens; 2009–2015) and Kane Basin (KB) subpopulations (n = 3 dens; 2012–2015) to previously published maternity den data from 1991 to 1997 (BB n = 8 dens; KB n = 3 dens). BB maternity denning duration decreased from a mean of 194.1 days (SD = 21.0 days, n = 8) in the 1990s, to a mean of 167.1 days (SD = 27.6 days, n = 16; p = 0.017) in the 2000s. Delayed den entry accounted for shorter denning durations (1990s entry date $$\bar{x}$$ = 7 September; 2000s entry date $$\bar{x}$$ = 5 October; p = 0.018). For dens habitat characteristics of which could be measured, BB maternity dens in the 2000s occurred at higher elevations ( $$\bar{x}$$ = 707.0 m, SD = 284.9 m, n = 15; p = 0.003) and greater slopes ( $$\bar{x}$$ = 23.1°, SD = 7.4°; p = 0.003) than the 1990s (elevation $$\bar{x}$$ ± SD = 351.3 ± 194.5 m, n = 8; slope $$\bar{x}$$ ± SD = 11.9 ± 6.4°). Aspect also significantly differed between the 1990s ( $$\bar{x}$$ = 51.3°) and 2000s BB maternity dens ( $$\bar{x}$$ = 199.7°; Watson’s U 2 p = 0.042). KB dens were not statistically compared due to low sample size (n = 3 dens in both periods). Shifts in sea ice phenology and snow availability may explain the observed changes.

Temporal effects of hunting on foraging behavior of an apex predator: Do bears forego foraging when risk is high?

Avoiding predators most often entails a food cost. For the Scandinavian brown bear (Ursus arctos), the hunting season coincides with the period of hyperphagia. Hunting mortality risk is not uniformly distributed throughout the day, but peaks in the early morning hours. As bears must increase mass for winter survival, they should be sensitive to temporal allocation of antipredator responses to periods of highest risk. We expected bears to reduce foraging activity at the expense of food intake in the morning hours when risk was high, but not in the afternoon, when risk was low. We used fine-scale GPS-derived activity patterns during the 2 weeks before and after the onset of the annual bear hunting season. At locations of probable foraging, we assessed abundance and sugar content, of bilberry (Vaccinium myrtillus), the most important autumn food resource for bears in this area. Bears decreased their foraging activity in the morning hours of the hunting season. Likewise, they foraged less efficiently and on poorer quality berries in the morning. Neither of our foraging measures were affected by hunting in the afternoon foraging bout, indicating that bears did not allocate antipredator behavior to times of comparably lower risk. Bears effectively responded to variation in risk on the scale of hours. This entailed a measurable foraging cost. The additive effect of reduced foraging activity, reduced forage intake, and lower quality food may result in poorer body condition upon den entry and may ultimately reduce reproductive success.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-016-3729-8) contains supplementary material, which is available to authorized users.

Drivers of hibernation: linking food and weather to denning behaviour of grizzly bears

Climate-induced changes in the phenology of hibernation for bear species could result in altered energy budgets, reduced cub survival and fitness and increased human-bear conflicts. Using 11 years of data, we determined the amount of variation in den entry and den exit dates that could be attributed to sex and reproductive status, weather and berry availability for 15 male and 58 female grizzly bears (Ursus arctos). We estimated berry availability during autumn using a probability surface of berry productivity within the home range of 13 individuals over 3 years. Sex and reproductive status explained 22 and 14 % of the variation in den entry and den exit dates, respectively. Weather did not influence the timing of den entry but berry availability in autumn explained 39 % of the variation observed in den entry, and high berry availability was associated with late den entry. Elevation and spring temperatures, and elevation and winter precipitation, respectively, explained 26 and 21 % of the variation observed in den exit dates. Increasing spring average monthly maximum temperature by 4 °C resulted in bears emerging from dens 10 days earlier and an increase of 1.25 m in snow precipitation delayed den exit by 1 week. We demonstrate that although the phenology of hibernation for grizzly bears depends on sex and reproductive status, den entry appears to be driven by food availability, while den exit is more linked to weather. Extended growing seasons and mild meteorological conditions should result in shorter denning periods for grizzly bears. Climate change is altering the phenology of spring green-up and the onset of winter, disrupting the seasonal behaviours of species. Climate change can act as an additional strain on threatened populations, especially during energetically demanding periods such as hibernation. We quantified the importance of intrinsic and extrinsic factors including food availability and weather in the hibernation behaviour of grizzly bears. High berry availability was associated with late den entry, while low winter precipitation and high spring temperature resulted in early den exit. We conclude that den entry is more driven by food availability while den exit is more linked to weather. This dichotomy in factors affecting den entry and den exit has implications for the long-term conservation of grizzly bear populations because extended growing seasons and mild meteorological conditions expected under future climate conditions should result in shorter denning periods.

Drivers of hibernation in the brown bear.

BackgroundHibernation has been a key area of research for several decades, essentially in small mammals in the laboratory, yet we know very little about what triggers or ends it in the wild. Do climatic factors, an internal biological clock, or physiological processes dominate? Using state-of-the-art tracking and monitoring technology on fourteen free-ranging brown bears over three winters, we recorded movement, heart rate (HR), heart rate variability (HRV), body temperature (Tb), physical activity, ambient temperature (TA), and snow depth to identify the drivers of the start and end of hibernation. We used behavioral change point analyses to estimate the start and end of hibernation and convergent cross mapping to identify the causal interactions between the ecological and physiological variables over time.ResultsTo our knowledge, we have built the first chronology of both ecological and physiological events from before the start to the end of hibernation in the field. Activity, HR, and Tb started to drop slowly several weeks before den entry. Bears entered the den when snow arrived and when ambient temperature reached 0 °C. HRV, taken as a proxy of sympathetic nervous system activity, dropped dramatically once the bear entered the den. This indirectly suggests that denning is tightly coupled to metabolic suppression. During arousal, the unexpected early rise in Tb (two months before den exit) was driven by TA, but was independent of HRV. The difference between Tb and TA decreased gradually suggesting that bears were not thermoconforming. HRV increased only three weeks before exit, indicating that late activation of the sympathetic nervous system likely finalized restoration of euthermic metabolism. Interestingly, it was not until TA reached the presumed lower critical temperature, likely indicating that the bears were seeking thermoneutrality, that they exited the den.ConclusionsWe conclude that brown bear hibernation was initiated primarily by environmental cues, but terminated by physiological cues.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0140-6) contains supplementary material, which is available to authorized users.

Abiotic and biotic factors affecting the denning behaviors in Asiatic black bears Ursus thibetanus

Abstract For bears, numerous associations between biotic and abiotic factors have been reported to correlate with the timing of den entry and emergence; however, an analysis showing which factors influence the timing of den entry and emergence has not been performed enough. In this study, a generalized linear mixed model was generated using 66 entry dates for 26 females and 40 entry dates for 26 males, and 56 emergence dates for 26 females and 25 emergence dates for 18 males between 1999 and 2012. Regarding factors for den entry, the average temperature in November and mast production of Mongolian oaks were significant for both males and females. For the date of den emergence, the average temperature in March affected strongly. For males, good mast production of Mongolian oaks in the previous year was found to be associated with early den emergence. For females, the presence of newborns had a significant influence on their den entry and emergence. This study indicated that denning behavior appears to be regulated by several abiotic and biotic factors and regulation factors are sex specific.

Den Entry Behavior in Scandinavian Brown Bears: Implications for Preventing Human Injuries.

Encounters between Scandinavian brown bears (Ursus arctos) and humans that result in human injuries and fatalities typically coincide with den entry in October and November, and commonly occur near a den. Our aim was to determine when bears arrive at their dens, identify potential predictors of this event, document behavior and activity associated with this period, and attempt to explain the increased risk of bear-caused human injuries in this period. We analyzed global positioning system (GPS) location and activity data from brown bears in south-central Sweden, using generalized linear mixed models, statistical process control, and activity analyses. Bears arrived at their den sites between 6 October and 1 December. Timing varied by reproductive category, bear age, and year. Half of all bears significantly reduced their activity before arriving at the den area: on average 2,169 m away from the den and 1.8 days before arrival. The other half reduced their activity after arriving at the den area. The latter bears took longer time to reach hibernation activity levels, but we did not find a difference in the start date of hibernation between the 2 groups. Bears also appeared to be sensitive to disturbance in this period, with higher den abandonment rates than later in winter, particularly for males and for bears that had not visited their den sites previously. Den entry occurred from October to December, with high variability and poor predictability of its timing. Therefore, restricting hunting or other recreation activities to reduce risk of injury by bears and disturbing bears probably would be both impractical and ineffective. Our findings can be used to educate hunters about bear behavior at this time of year. Many people associate dens with an increased risk of a bear responding aggressively to disturbance to defend its den, but our results indicate that other behavioral, and possibly physiological, changes in this period also may be involved. © 2014 The Authors. The Journal of Wildlife Management published by The Wildlife Society.

Factors affecting date of implantation, parturition, and den entry estimated from activity and body temperature in free-ranging brown bears.

Knowledge of factors influencing the timing of reproduction is important for animal conservation and management. Brown bears (Ursus arctos) are able to vary the birth date of their cubs in response to their fat stores, but little information is available about the timing of implantation and parturition in free-ranging brown bears. Body temperature and activity of pregnant brown bears is higher during the gestation period than during the rest of hibernation and drops at parturition. We compared mean daily body temperature and activity levels of pregnant and nonpregnant females during preimplantation, gestation, and lactation. Additionally we tested whether age, litter size, primiparity, environmental conditions, and the start of hibernation influence the timing of parturition. The mean date of implantation was 1 December (SD = 12), the mean date of parturition was 26 January (SD = 12), and the mean duration of the gestation period was 56 days (SD = 2). The body temperature of pregnant females was higher during the gestation and lactation periods than that of nonpregnant bears. The body temperature of pregnant females decreased during the gestation period. Activity recordings were also used to determine the date of parturition. The parturition dates calculated with activity and body temperature data did not differ significantly and were the same in 50% of the females. Older females started hibernation earlier. The start of hibernation was earlier during years with favorable environmental conditions. Dates of parturition were later during years with good environmental conditions which was unexpected. We suggest that free-ranging pregnant brown bears in areas with high levels of human activities at the beginning of the denning period, as in our study area, might prioritize investing energy in early denning than in early parturition during years with favorable environmental conditions, as a strategy to prevent disturbances caused by human.

Denning Ecology of Florida Black Bears in North-Central Florida

We studied the denning chronology, den type, and den-site characteristics of Ursus americanus floridanus (Florida Black Bear) in Ocala National Forest (ONF) and the adjacent residential area of Lynne, FL. We monitored 35 radio-collared females for 62 den years from 1999 through 2003. Den entry dates did not differ between parturient females (females that gave birth to cubs during the winter) and nonparturient (solitary females or females with yearlings) (P = 0.139). Females with cubs exited dens later (P < 0.001), and denned longer (mean =113 ± 3.3 days) than females without cubs (mean = 54 ± 6.0 days; P < 0.001). Among females with cubs, primiparous females entered dens on average 28 days later than multiparous females (P = 0.003); however, exit date and duration of denning did not differ between the two groups. Female bears denned in ground nests most frequently (n = 45), followed by excavated dens (n = 7); one female used a tree den. Compositional analysis revealed that denning habitat selection occurred in ONF, with sand pine as the preferred denning habitat, followed by swamp and pine flatwoods habitats. Denning habitat selection was not evident in Lynne, although the majority of females denned in swamp habitats. Parturient females often denned in ecotones with dense vegetation, due perhaps to the fact that such ecotones offer better protection to the female and her cubs from potential predators and weather elements. Habitat management activities should be limited during peak denning of parturient females, from late December to mid-April, particularly in Sand Pine - xeric oak and pine flatwood - swamp ecotones.

Space and Habitat Use by Black Bears in the Elwha Valley Prior to Dam Removal

Dam removal and subsequent restoration of salmon to the Elwha River is expected to cause a shift in nutrient dynamics within the watershed. To document how this influx of nutrients and energy may affect black bear (Ursus americanus) ecology, we used radio-telemetry to record movements of 11 male and two female black bears in the Elwha Valley from 2002–06. Our objective was to collect baseline data on bear movements prior to dam removal. We calculated annual home ranges, described seasonal timing of den entry and emergence, and described seasonal patterns of distribution and habitat use. Adaptive kernel home ranges were larger for males (mean = 151.1 km2, SE = 21.4) than females (mean = 38.8 km2, SE = 13.0). Males ranged widely and frequently left the watershed during late summer. Further, they exhibited predictable and synchronous patterns of elevation change throughout each year. Bears entered their winter dens between 8 October and 15 December and emerged from dens between 10 March and 9 May. Male bears used low-elevation conifer and hardwood forests along the Elwha floodplain during spring, mid- to high-elevation forests and meadows during early summer, high-elevation forests, meadows and shrubs during late summer, and mid-elevation forests, shrubs and meadows during fall. Data acquired during this study provide important baseline information for comparison after dam removal, when bears may alter their late summer and fall movement and denning patterns to take advantage of energy-rich spawning salmon.

Denning Chronology and Design of Effective Bear Management Units

Abstract: Reports on the effectiveness of using late fall hunting seasons to reduce the proportion of female black bears (Ursus americanus) in the harvest are limited, and the geographic scale over which the technique functions as intended has not been examined. During 1992‐2000, we radio‐equipped black bears in New Mexico, USA, obtained estimates of 175 den entry and 137 den emergence dates, and used New Mexico Department of Game and Fish harvest data (1985‐2000) to test for differences in proportion of females in the harvest relative to denning chronology. Bears in northern New Mexico entered dens earlier and emerged later than bears in southern New Mexico (P ≤ 0.001). In northern New Mexico bears displayed the typical pattern of earlier entry and later emergence by reproductive females, proportion of females in the harvest varied over time as expected, and late fall seasons were effective (P &lt; 0.10). In contrast, denning chronology did not differ by sexin southern New Mexico, proportion of females in the harvest did not change over time, and late fall seasons were not effective (P ≤ 0.18). Manipulation of hunting season dates to influence female mortality can be an effective tool, however our study provides an example of an area where denning chronology did not differ by sex and late seasons were not effective. We also observed regional differences in timing of entrance and emergence, which suggest that scale of application may be key. In management jurisdictions that encompass ecologically distinct areas, cover a wide range of latitudes, or are mountainous, successful use of the technique may depend on knowledge of denning chronology at multiple locations and appropriate designation of hunting unit boundaries, season dates, and data analysis units.

Black Bear, &lt;em&gt;Ursus americanus&lt;/em&gt;, Ecology on the Northeast Coast of Labrador

Twenty-three Black Bears (Ursus americanus) were captured, 20 were measured, marked and/or radio collared, in northeastern Labrador, between 1996 and 1997. Bears used sea ice for travel, coastal islands for denning, hunted adult Caribou (Rangifer tarandus), and were the possible cause of Moose (Alces alces) calf mortality. Body sizes were small, median weight of adult females was 48 kg, and the sex ratio for captured subjects was 1:1. Four of six radio-collared females gave birth during the winter of 1997, female reproductive histories suggest delayed sexual maturity. Den entry occurred between October and December 1996; spring emergence occurred between April and May 1997, with estimated denning period ranging from 148-222 days. Visual observations of habitat use by radio collared subjects (n = 10) were not tested statistically but suggest that barren areas are used nearly as much as forest. Location data from three GPS collars deployed on three adult females were analysed using Chi-square goodness-of-fit test with Bonferroni correction; two females appeared to prefer forest habitats (p &lt; 0.05).

Black Bear, &lt;em&gt;Ursus americanus&lt;/em&gt;, Denning Chronology and Den Site Selection in the Northeastern Cascades of Washington

I studied Black Bear denning chronology and den site selection during 1995-1998 in the northeastern Cascade Mountains of Washington. Male Black Bears entered dens between 22 October and 19 November and emerged between 4 April and 7 May. Females entered dens somewhat sooner, between 15 October and 19 November, and emerged later than males, 4 April and 22 May. These den entry and emergence dates were similar to those reported at similar lattitudes in northwestern Montana. Roads had an important influence on den site selection by Black Bears in this study area. Bears selected dens in areas with no open roads &gt;200 ha in size, &gt;500 m from the nearest open road, and 1500-2000 m in elevation. The combination of information on denning chronology and site characterstics should allow managers to coordinate human activities to reduce the potential for disturbance to denning Black Bears.

Population Characteristics and Winter Ecology of Black Bears in Coahuila, Mexico

Biology, status, and distribution of black bears (Ursus americanus) in Mexico are poorly understood. We studied a population of black bears in the Serranias del Burro, Coahuila, Mexico, from 1991 to 1994 to address the hypothesis that the recolonization source for bears in Texas is the Sierra del Carmen range in northern Mexico. We also described aspects of winter ecology in this southerly population. We captured 42 bears (27 M, 15 F) and equipped 28 with radiotransmitters. All pregnant females (n = 13) and 2 of 5 females with yearlings denned, whereas all other bears, including 10 males, remained active during winter. Mean (± SE) den entry and exit dates for pregnant females (n = 5) were 25 December (± 5 days) and 22 April (± 2 days). Mean age at primiparity was 4.5 ± 0.6 years (n = 4), interbirth interval was 2.0 ± 0.0 years (n = 3), and litter size was 2.75 ± 0.25 cubs/female (n = 12). Estimated adult female and cub annual survival rates were 0.94 ± 0.05 and 0.81 ± 0.10, respectively, based on radiotelemetry and observation of family groups. Estimates of subadult male survival ranged from 0.88 ± 0.08 to 1.00 ± 0.00. We estimated density to be 0.35 bears/km 2 using a modification of the Lincoln-Petersen estimator that employs animal-equivalents. Because of high reproductive rates and lack of anthropogenic mortality, our data were consistent with the hypothesis that the area can serve as a source of bears emigrating to western Texas. Future bear research in this region should focus on determination of the source of emigrating bears, and identification and conservation of corridors linking Mexican and Texan populations of black bears.

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

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.