Denning in brown bears

1. 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.
2. 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.
3. 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.
4. 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.
5. 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 effects of primiparity on reproductive performance in the brown bear

We studied the effects of primiparity on litter size, offspring size, and cub loss in brown bears (Ursus arctos) in two study areas (north, south) in Sweden from 1987 to 2006. Sexually selected infanticide (SSI) has been suggested previously as a mortality factor in our study populations. Females in the south became primiparous earlier than females in the north. Primiparous females had significantly smaller litters of cubs than multiparous females. We found no evidence that primiparity was costly in terms of the interlitter interval. Primiparous mothers had a higher probability of cub loss than multiparous mothers. The probability of cub loss was analyzed separately for the pre-mating and the mating season. The probability of cub loss by primiparous females in the pre-mating season increased with both increasing population density and deteriorating food conditions, whereas the probability of cub loss during the mating season decreased with increasing age of primiparity and increased with male turnover (a variable predicting SSI). The temporal patterns of cub loss by primiparous females suggested that the critical times for reproductive success by primiparous females were the pre-mating season (from birth to shortly after leaving the den) and the mating season. Cub loss in these periods was independent and caused by different factors. Cub loss before the mating season seemed to be most influenced by food conditions, whereas that during the mating season appeared to be caused by SSI.     

Phylogeography of mitochondrial DNA variation in brown bears and polar bears

We analyzed 286 nucleotides of the middle portion of the mitochondrial cytochrome b gene of 61 brown bears from three locations in Alaska and 55 polar bears from Arctic Canada and Arctic Siberia to test our earlier observations of paraphyly between polar bears and brown bears as well as to test the extreme uniqueness of mitochondrial DNA types of brown bears on Admiralty, Baranof, and Chichagof (ABC) islands of southeastern Alaska. We also investigated the phylogeography of brown bears of Alaska's Kenai Peninsula in relation to other Alaskan brown bears because the former are being threatened by increased human development. We predicted that: (1) mtDNA paraphyly between brown bears and polar bears would be upheld, (2) the mtDNA uniqueness of brown bears of the ABC islands would be upheld, and (3) brown bears of the Kenai Peninsula would belong to either clade II or clade III of brown bears of our earlier studies of mtDNA. All of our predictions were upheld through the analysis of these additional samples.     

Evidence of non-hibernation in Cantabrian brown bears

Evidence of non-hibernation in brown bears Ursus arctos Linnaeus, 1758 on the Iberian Peninsula has existed since the Middle Ages. We systematically monitored brown bears in the Cantabrian Mountains (Northern Spain) by recording tracks and sightings from 1998 to 2007 to document hibernation behaviour. Our results indicate that females with yearlings and solitary yearlings were more active in winter than bears over two years old. Intensive snow tracking and direct observations of five family groups indicated that they travelled, fed and defecated in winter, which are activities not compatible with the physiological state of hibernation. Also, based on tracking data, the maximum period between two consecutive locations of active family groups in winter was less than that needed by bears to emerge from a state of hibernation (6 days). We conclude that the family groups which we monitored in winter did not hibernate.     

Inversely density-dependent natal dispersal in brown bears Ursus arctos

There is considerable controversy in the literature about the presence of density dependence in dispersal. In this study, we exploit a data series from a long-term study (>18 years) on radio-marked brown bears (Ursus arctos L.) in two study areas in Scandinavia to investigate how individual-based densities influence the probability of natal dispersal and natal dispersal distances. Cumulatively, 32% and 46% of the females and 81% and 92% of the males dispersed before reaching 5 years of age in the northern and southern study area, respectively. Density had a negative effect on both the probability of dispersal and dispersal distances for the dispersing animals, when controlling for study area, sex and age, making this the first study to show that natal dispersal probability and distances are inversely density dependent in a large carnivore. We suggest that female–female competition for space caused females in higher density areas to settle closer to their natal area. For males, however, merging of demes, resulting in decreased relatedness and increased heterozygosity in an expanding population, might be the reason for shorter dispersal distances in males living at higher densities. This has been hypothesised for small mammals. The high proportion of dispersing female brown bears in Scandinavian compared with North American studies might be due to lower densities in Scandinavia and recent population expansion, with unoccupied areas available at the edges of the population. The longer dispersal distances in female Scandinavian brown bears suggest less social constraints on movements than for North American females. The longer dispersal distances by Scandinavian males may be due to increased searching for potential mates in peripheral areas with lower densities of females. These results, in addition to results of other brown bear studies, suggest that brown bears might be more territorial than previously thought, and that density is regulated by social interactions.     

Ancient DNA reveals differences in behaviour and sociality between brown bears and extinct cave bears

Ancient DNA studies have revolutionized the study of extinct species and populations, providing insights on phylogeny, phylogeography, admixture and demographic history. However, inferences on behaviour and sociality have been far less frequent. Here, we investigate the complete mitochondrial genomes of extinct Late Pleistocene cave bears and middle Holocene brown bears that each inhabited multiple geographically proximate caves in northern Spain. In cave bears, we find that, although most caves were occupied simultaneously, each cave almost exclusively contains a unique lineage of closely related haplotypes. This remarkable pattern suggests extreme fidelity to their birth site in cave bears, best described as homing behaviour, and that cave bears formed stable maternal social groups at least for hibernation. In contrast, brown bears do not show any strong association of mitochondrial lineage and cave, suggesting that these two closely related species differed in aspects of their behaviour and sociality. This difference is likely to have contributed to cave bear extinction, which occurred at a time in which competition for caves between bears and humans was likely intense and the ability to rapidly colonize new hibernation sites would have been crucial for the survival of a species so dependent on caves for hibernation as cave bears. Our study demonstrates the potential of ancient DNA to uncover patterns of behaviour and sociality in ancient species and populations, even those that went extinct many tens of thousands of years ago.     

Social play in wild brown bears of varying age-sex class

Social play behaviour is a well-described phenomenon, almost ubiquitous among mammals. Despite its prevalence, social play takes several forms and may vary in function across species. For solitary species, the function of play outside of the family group remains unclear. Here, we describe the motor patterns of play among non-littermate wild brown bears Ursus arctos of different age-sex class. Play was documented during a time of abundant food availability in three different scenarios: play among non-littermate subadults, play among non-littermate cubs, and play among a ‘group’ of bears of different age and sex class. We used a previously described behavioural ethogram to recognise play. Play followed typical motor patterns and postures expressed by bears during play-fighting: relaxed face, puckered-lip, ears partially flattened to crescent, wrestling, jaw gaping, play-biting, paw-swatting, and lunging. No vocalisations were conducted during play bouts. Older bears displayed ‘self-handicapping’ and ‘role-reversal’ in the play postures they selected when playing with younger bears, suggesting that tactics vary according to age class and dominance ranking. Playing likely allows for the evaluation of conspecifics in a non-aggressive way during times of reduced competition and could also relieve stress in complex social situations.     

Genomic evidence of geographically widespread effect of gene flow from polar bears into brown bears

Polar bears are an arctic, marine adapted species that is closely related to brown bears. Genome analyses have shown that polar bears are distinct and genetically homogeneous in comparison to brown bears. However, these analyses have also revealed a remarkable episode of polar bear gene flow into the population of brown bears that colonized the Admiralty, Baranof and Chichagof islands (ABC islands) of Alaska. Here, we present an analysis of data from a large panel of polar bear and brown bear genomes that includes brown bears from the ABC islands, the Alaskan mainland and Europe. Our results provide clear evidence that gene flow between the two species had a geographically wide impact, with polar bear DNA found within the genomes of brown bears living both on the ABC islands and in the Alaskan mainland. Intriguingly, while brown bear genomes contain up to 8.8% polar bear ancestry, polar bear genomes appear to be devoid of brown bear ancestry, suggesting the presence of a barrier to gene flow in that direction.     

Genetics and conservation of European brown bears Ursus arctos

• 1 We review the genetics research that has been conducted on the European brown bear Ursus arctos, one of the genetically best‐studied mammalian species.
• 2 The first genetics studies on European brown bears were on phylogeography, as a basis for proposed population augmentations. Two major mitochondrial DNA lineages, western and eastern, and two clades within the western lineage were found. This led to a hypothesis that brown bears had contracted to southern refugia during the last glacial maximum. More recent results suggest that gene flow among brown bears blurred this structure and they survived north of these putative refugia. Thus, today's structure might be a result of population fragmentation caused by humans.
• 3 The nuclear diversity of European brown bears is similar in range to that in North American bears: low levels occur in the small populations and high levels in the large populations.
• 4 Many non‐invasive genetic methods, developed during research on brown bears, have been used for individual identification, censusing populations, monitoring migration and gene flow, and testing methods that are easier to use in endangered populations and over large areas.
• 5 Genetics has been used to study many behavioural and population ecological questions that have relevance for the conservation and management of brown bears.
• 6 The European brown bear has served, and will continue to serve, as a model for the development of methods, analyses and hypotheses in conservation genetics.

     

Noninvasive genetic studies of brown bears using power poles

One difficulty in the conservation of endangered wildlife is the lack of reliable information on its status. This lack of knowledge can often be attributed to financial and logistic constraints as well as the lack of trained personnel to collect data. We test a simple method to study bears in the southern Balkans by inspecting power poles, which are used by bears for marking and rubbing purposes. We created a network of barbed-wire fitted poles for the collection of hair samples, evenly distributed throughout six study areas. During 87 sampling sessions in the main study area, we collected 191 samples and identified six microsatellite loci that were variable enough for individual bear identification. The most and best-quality hair samples were collected during the mating period, and DNA was most successfully extracted from samples remaining <4 weeks in the field. In the six study areas, we identified 47 bears. An advantage of using power poles for hair sampling is their availability and accessibility; no bait is required, and the network can be easily set up. A drawback may be an unequal capture probability of sex and age classes of bears. Despite this limitation, using power poles proved to be a simple and cheap method for the noninvasive genetic study of bears that did not require any prior knowledge on habitat use and activity patterns. The method is suitable for large-scale surveys to estimate distribution and relative densities of bears and could also be applied for studying other species.     

Age estimation based on blood DNA methylation levels in brown bears

Age is an essential trait for understanding the ecology and management of wildlife. A conventional method of estimating age in wild animals is counting annuli formed in the cementum of teeth. This method has been used in bears despite some disadvantages, such as high invasiveness and the requirement for experienced observers. In this study, we established a novel age estimation method based on DNA methylation levels using blood collected from 49 brown bears of known ages living in both captivity and the wild. We performed bisulfite pyrosequencing and obtained methylation levels at 39 cytosine-phosphate-guanine (CpG) sites adjacent to 12 genes. The methylation levels of CpGs adjacent to four genes showed a significant correlation with age. The best model was based on DNA methylation levels at just four CpG sites adjacent to a single gene, SLC12A5, and it had high accuracy with a mean absolute error of 1.3 years and median absolute error of 1.0 year after leave-one-out cross-validation. This model represents the first epigenetic method of age estimation in brown bears, which provides benefits over tooth-based methods, including high accuracy, less invasiveness, and a simple procedure. Our model has the potential for application to other bear species, which will greatly improve ecological research, conservation, and management.     

Yew intoxication in brown bears; a novel approach to diagnosis

Tissues from two juvenile brown bears (Ursus arctos), suspected to have died from yew (Taxus baccata) toxicosis, were chemically examined using thin layer chromatography and high performance liquid chromatography. Extraction and analysis were conducted on heart and liver tissue samples as well as stomach content from the two bears and also on fresh material from an authenticated yew tree which was used as a standard for comparison. The taxine complex comprised of taxine B, 1-deoxytaxine B, isotaxine B, 2-acetyltaxin B, 1-deoxyisotaxine B, 2-acetylisotaxine B, and cinnamates were detected in all extracted samples. taxine B and its isomer isotaxine B are the main toxic constituents in yew. The heart, the target organ of taxine B, is arrested when reaching the lethal tissue concentration of taxine complex. In the present cases, the concentration in the hearts was found to be 37 and 17 μg taxine complex per gram tissue, respectively. As no further absorption into the heart occurs following cardiac arrest, the concentration determined is the actual lethal tissue concentration. Yew ingestion was confirmed by microscopic examination of stomach contents of both bears. Histopathological findings of contraction band necrosis in heart specimens were also consistent with yew intoxication.     

Minimal seasonal alterations in the skeletal muscle of captive brown bears

Abstract Previous studies on wild black bears (Ursus americanus) have shown that skeletal muscle morphology, composition, and overall force-generating capacity do not differ drastically between seasons despite prolonged inactivity during hibernation. However, the amount and characteristics of the seasonal variations were not consistent in these studies. The goals of this study were to compare the amount of muscle atrophy in captive brown bears (Ursus arctos) with that observed in wild black bears and measure seasonal differences in twitch characteristics. Samples from the biceps femoris muscle were collected during the summer and winter. Protein concentration, fiber-type composition, and fiber cross-sectional area were measured along with twitch characteristics. The protein concentration of the winter samples was 8.2% lower than that of the summer samples; fiber cross-sectional area and the relative proportion of fast and slow fibers remained unchanged between seasons. Myosin heavy chain isoforms I, IIa, and IIx were identified by immunoblotting and electrophoresis, and the proportions did not change between seasons. The half-rise time in the twitch contractions increased in winter relative to summer samples, which is unexpected under disuse conditions. These results agreed with a study that showed minimal skeletal muscle atrophy between seasons in wild black bears.     

Patterns of animal–enrichment interaction in captive brown bears

We studied the behavioral responses of three brown bears (Ursus arctos) to different types of enrichment devices to assess the predominant pattern of animal–enrichment interaction (PAI) to each type of enrichment. We assessed the bears' responses to feeding, sensory, and occupational enrichments over nine sessions. Using Pearson's correlation (r) and the coefficient of variation (CV)—we describe four models of PAIs: habituation, increasing, continuous, and fluctuating. The habituation model (r < 0 and p < 0.05; CV (%) > 0) consists of a loss of interest in the device over time and it occurred with the occupational device. The increasing model (r > 0 and p < 0.05; CV (%) > 0) consists of a sustained interest in the device over time and it was not observed for any device of this study. The continuous model (0 > r > 0 and p > 0.05; CV (%) < 100) consists of a consistent and unchanging interaction over time and it occurred with the feeding enrichments used in this study. The fluctuating PAI (0 > r > 0 and p > 0.05; CV (%) ≥ 100) consists of wide oscillations in the degree of interaction over time and we observed this pattern with sensory devices. Review of previous literature supports these classes of PAI, and suggests similar relationships between PAIs and the type of enrichment as we observed in this study.     

Elevated plasma concentrations of haptoglobin in European brown bears during hibernation

Haptoglobin (Hp), a hemoglobin-binding protein, is known as an acute phase protein and increases during the acute phase of inflammation in most mammals. We reported previously in brown bears that the mean Hp concentrations were higher in blood samples obtained in winter than those in spring. To examine a possible relation of the seasonal variations of Hp to hibernation, in the present study, we measured the plasma concentrations of Hp as well as some other acute phase proteins (alpha(2)-macroglobulin, alpha(1)-antitrypsin, C-reactive protein) in 6 European brown bears (Ursus arctos), from which blood samples were obtained at 5-6 different months of year including February, the time of hibernation. The Hp concentrations showed clear seasonal variations, being highest in February. The alpha(2)-macroglobulin concentrations also showed a similar but much smaller rise in February, but those of alpha(1)-antitrypsin and C-reactive protein did not show any seasonal variations. Our results suggest that the seasonal variation of plasma Hp concentration in brown bears is associated with a hibernation-specific mechanism more than that of acute phase response.     

Factors influencing lifespan dependency on agricultural crops by brown bears

Landscape and Ecological Engineering - Investigating factors underlying human-wildlife conflicts in agricultural landscapes is important for both preventing crop damage and wildlife conservation....     

Size-selective and sex-selective predation by brown bears on sockeye salmon

Breeding activity increases the vulnerability of many animals to predation, and such predation can affect the subset of animals successfully reproducing. To study the ways in which predation might affect the evolution of Pacific salmon, we measured the intensity and selectivity of predation by bears (primarily brown bears, Ursus arctos) on mature sockeye salmon (Oncorhynchus nerka) breeding in a series of small, spring-fed ponds and creeks near Pedro Bay, Alaska, from 1994 to 1998. Bears killed male salmon more often than females; males constituted 60% of the kills but only 35% of the salmon that died of senescence. The bears also killed fish that were larger, on average, than those dying of senescence (males: 462 vs 452 mm; females: 453 vs 443 mm). The level of predation varied greatly, from 4% (females) and 10% (males) in 1994 to 100% of both sexes in 1996 and 1997. The rate of predation also varied among habitats, being lower in larger ponds than in smaller, shallower ponds and the very small interconnecting creeks. Despite the intense and size-selective predation, the salmon in safer habitats (large ponds) were not larger than those in riskier habitats, and salmon densities were only slightly higher in the safer areas. Compared to a nearby population that experiences no bear predation (Woody Island), the male sockeye salmon from the Pedro Pond system had shallower bodies (i.e., less exposure in shallow water) for a given length, consistent with the hypothesis that selective predation can affect the extent of sexual dimorphism among populations. However, the average length at age for both males and females was greater in the Pedro Pond fish, indicating that selective factors besides predation affect length. Overall, the results indicate that bears can be an agent of natural selection within (and perhaps between) sockeye salmon populations, and predation can greatly affect reproductive success among individuals and years for the population as a whole. Received: 6 April 1999 / Accepted: 1 June 1999     

Population-level resource selection by sympatric brown and American black bears in Alaska

Distribution theory predicts that for two species living in sympatry, the subordinate species would be constrained from using the most suitable resources (e.g., habitat), resulting in its use of less suitable habitat and spatial segregation between species. We used negative binomial generalized linear mixed models with fixed effects to estimate seasonal population-level resource selection at two spatial resolutions for female brown bears (Ursus arctos) and female American black bears (U. americanus) in southcentral Alaska during May–September 2000. Black bears selected areas occupied by brown bears during spring which may be related to spatially restricted (i.e., restricted to low elevations) but dispersed or patchy availability of food. In contrast, black bears avoided areas occupied by brown bears during summer. Brown bears selected areas near salmon streams during summer, presumably to access spawning salmon. Use of areas with high berry production by black bears during summer appeared in response to avoidance of areas containing brown bears. Berries likely provided black bears a less nutritious, but adequate food source. We suggest that during summer, black bears were displaced by brown bears, which supports distribution theory in that black bears appeared to be partially constrained from areas containing salmon, resulting in their use of areas containing less nutritious forage. Spatial segregation of brown and American black bears apparently occurs when high-quality resources are spatially restricted and alternate resources are available to the subordinate species. This and previous work suggest that individual interactions between species can result in seasonal population-level responses.