Effects of Earlier Sea Ice Breakup on Survival and Population Size of Polar Bears in Western Hudson Bay

ABSTRACT Some of the most pronounced ecological responses to climatic warming are expected to occur in polar marine regions, where temperature increases have been the greatest and sea ice provides a sensitive mechanism by which climatic conditions affect sympagic (i.e., with ice) species. Population-level effects of climatic change, however, remain difficult to quantify. We used a flexible extension of Cormack-Jolly-Seber capture-recapture models to estimate population size and survival for polar bears (Ursus maritimus), one of the most ice-dependent of Arctic marine mammals. We analyzed data for polar bears captured from 1984 to 2004 along the western coast of Hudson Bay and in the community of Churchill, Manitoba, Canada. The Western Hudson Bay polar bear population declined from 1,194 (95% CI = 1,020-1,368) in 1987 to 935 (95% CI = 794-1,076) in 2004. Total apparent survival of prime-adult polar bears (5–19 yr) was stable for females (0.93; 95% CI = 0.91-0.94) and males (0.90; 95% CI = 0.88-0.91). Survival of juvenile, subadult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival. It may also explain why Churchill, like other communities along the western coast of Hudson Bay, has experienced an increase in human-polar bear interactions in recent years. Earlier sea ice breakup may have resulted in a larger number of nutritionally stressed polar bears, which are encroaching on human habitations in search of supplemental food. Because western Hudson Bay is near the southern limit of the species' range, our findings may foreshadow the demographic responses and management challenges that more northerly polar bear populations will experience if climatic warming in the Arctic continues as projected.     

Spring fasting behavior in a marine apex predator provides an index of ecosystem productivity

The effects of declining Arctic sea ice on local ecosystem productivity are not well understood but have been shown to vary inter‐specifically, spatially, and temporally. Because marine mammals occupy upper trophic levels in Arctic food webs, they may be useful indicators for understanding variation in ecosystem productivity. Polar bears (Ursus maritimus) are apex predators that primarily consume benthic and pelagic‐feeding ice‐associated seals. As such, their productivity integrates sea ice conditions and the ecosystem supporting them. Declining sea ice availability has been linked to negative population effects for polar bears but does not fully explain observed population changes. We examined relationships between spring foraging success of polar bears and sea ice conditions, prey productivity, and general patterns of ecosystem productivity in the Beaufort and Chukchi Seas (CSs). Fasting status (≥7 days) was estimated using serum urea and creatinine levels of 1,448 samples collected from 1,177 adult and subadult bears across three subpopulations. Fasting increased in the Beaufort Sea between 1983–1999 and 2000–2016 and was related to an index of ringed seal body condition. This change was concurrent with declines in body condition of polar bears and observed changes in the diet, condition and/or reproduction of four other vertebrate consumers within the food chain. In contrast, fasting declined in CS polar bears between periods and was less common than in the two Beaufort Sea subpopulations consistent with studies demonstrating higher primary productivity and maintenance or improved body condition in polar bears, ringed seals, and bearded seals despite recent sea ice loss in this region. Consistency between regional and temporal variation in spring polar bear fasting and food web productivity suggests that polar bears may be a useful indicator species. Furthermore, our results suggest that spatial and temporal ecological variation is important in affecting upper trophic‐level productivity in these marine ecosystems.     

Hemoglobin, pH and DPG/chloride shifting

In this study a decreased DPG response by polar bear (Ursus maritimus) hemoglobin was observed, and this response was interpreted as an example of gradual DPG/chloride shifting. This sort of mechanism has been suggested to occur in ruminants and, intuitively, one might guess that for ruminants the DPG/Cl- shifting might have been beneficial and hence selected for at the time of the latest Ice Age. However, suggestion that this is purely a temperature effect in polar bears and ruminants conflicts with the existence, in the hot savanna, of mammals that have Hb modulated by chloride. However, acidosis effects caused by routine periods of food shortage, induced in extreme environments may explain the responses of the hemoglobins of animals adapted to extreme habitats. The chloride effect is bound to specific amino acid substitutions in key positions. In polar bear Hb, they are specific, additional (with respect to human HbA) O2-linked chloride binding sites located between Lys-76 (beta) and Lys-8 (beta). The amino acids operate as an additional H+ binding site for a chloride anion. Additionally, with respect to human adult HbA, the primary structure of polar bear Hb was characterized by two substitutions in beta chains: Pro-5 (A2)--> Gly and Ala-76 (E20)-->Lys. The increased flexibility of the A helix causes the lower DPG effect. We further hypothesize that the resulting widening of the central cavity allows the Lys-82 (beta) terminus to be free and constitute an additional, chloride-binding site.     

Future sea ice conditions in Western Hudson Bay and consequences for polar bears in the 21st century

The primary habitat of polar bears is sea ice, but in Western Hudson Bay (WH), the seasonal ice cycle forces polar bears ashore each summer. Survival of bears on land in WH is correlated with breakup and the ice‐free season length, and studies suggest that exceeding thresholds in these variables will lead to large declines in the WH population. To estimate when anthropogenic warming may have progressed sufficiently to threaten the persistence of polar bears in WH, we predict changes in the ice cycle and the sea ice concentration (SIC) in spring (the primary feeding period of polar bears) with a high‐resolution sea ice‐ocean model and warming forced with 21st century IPCC greenhouse gas (GHG) emission scenarios: B1 (low), A1B (medium), and A2 (high). We define critical years for polar bears based on proposed thresholds in breakup and ice‐free season and we assess when ice‐cycle conditions cross these thresholds. In the three scenarios, critical years occur more commonly after 2050. From 2001 to 2050, 2 critical years occur under B1 and A2, and 4 under A1B; from 2051 to 2100, 8 critical years occur under B1, 35 under A1B and 41 under A2. Spring SIC in WH is high (>90%) in all three scenarios between 2001 and 2050, but declines rapidly after 2050 in A1B and A2. From 2090 to 2100, the mean spring SIC is 84 (±7)% in B1, 56 (±26)% in A1B and 20 (±13)% in A2. Our predictions suggest that the habitat of polar bears in WH will deteriorate in the 21st century. Ice predictions in A1B and A2 suggest that the polar bear population may struggle to persist after ca. 2050. Predictions under B1 suggest that reducing GHG emissions could allow polar bears to persist in WH throughout the 21st century.     

Dietary habits of polar bears in Foxe Basin,Canada: possible evidence of a trophic regime shift mediated by a new top predator

Polar bear (Ursus maritimus) subpopulations in several areas with seasonal sea ice regimes have shown declines in body condition, reproductive rates, or abundance as a result of declining sea ice habitat. In the Foxe Basin region of Nunavut, Canada, the size of the polar bear subpopulation has remained largely stable over the past 20 years, despite concurrent declines in sea ice habitat. We used fatty acid analysis to examine polar bear feeding habits in Foxe Basin and thus potentially identify ecological factors contributing to population stability. Adipose tissue samples were collected from 103 polar bears harvested during 2010–2012. Polar bear diet composition varied spatially within the region with ringed seal (Pusa hispida) comprising the primary prey in northern and southern Foxe Basin, whereas polar bears in Hudson Strait consumed equal proportions of ringed seal and harp seal (Pagophilus groenlandicus). Walrus (Odobenus rosmarus) consumption was highest in northern Foxe Basin, a trend driven by the ability of adult male bears to capture large‐bodied prey. Importantly, bowhead whale (Balaena mysticetus) contributed to polar bear diets in all areas and all age and sex classes. Bowhead carcasses resulting from killer whale (Orcinus orca) predation and subsistence harvest potentially provide an important supplementary food source for polar bears during the ice‐free period. Our results suggest that the increasing abundance of killer whales and bowhead whales in the region could be indirectly contributing to improved polar bear foraging success despite declining sea ice habitat. However, this indirect interaction between top predators may be temporary if continued sea ice declines eventually severely limit on‐ice feeding opportunities for polar bears.     

The diet of polar bears (Ursus maritimus) from Svalbard, Norway, inferred from scat analysis

Polar bears (Ursus maritimus) are heavily dependent on marine prey, in particular ice-associated seals, which they hunt on landfast sea ice or free-floating pack ice. Dramatic current (and predicted) losses of sea ice habitat make it increasingly important to gain more knowledge of the relative use by bears of all types of prey from the marine food web as well as from terrestrial sources. This study uses frequency of occurrence of food items in 119 polar bear scats sampled on the sea ice as well as on shore in coastal areas in the Svalbard Archipelago, mainly in spring, between 2003 and 2010 to explore the diet of bears in the region. Ringed seals (Pusa hispida) occurred in 62.2 % (CI 52.8–70.9 %) of the scat samples examined. Various terrestrial plants (32.8 %, CI 24.4–42.0 %) and marine algae (21.8 %, CI 14.8–30.4 %) also occurred frequently in the scats; the significance of this high occurrence of plants and algae is not clear. Bearded seals (Erignathus barbatus) and various bird species constituted only minor components of the diet, while Svalbard reindeer (Rangifer tarandus platyrhynchus) occurred in 9.2 % (CI 4.7–15.9 %) of the scats, indicating that this species may play a more important role than previously reported. The novel combination of genetic analyses of material in the fecal samples along with detailed exploration of the physical–structural properties of prey hairs and plant parts provided a much fuller picture of the diet of polar bears than would have been possible from observational studies of polar bear predation behavior alone. This approach may provide an important tool for monitoring the responses of polar bears to ongoing ecosystem changes that will result from continued warming in the Arctic.     

Effects of inedible, manipulable objects on captive bears

Bears in captivity often show abnormal behaviors such as stereotypies and periods of excessive inactivity. Most of the research aimed at reducing abnormal behaviors in bears has focused on feeding enrichment. However, the effects of feeding enrichment only seem to last as long as it takes a bear to consume the food. This study investigated the effects of inedible, manipulable objects on bears. Two polar bears (Ursus maritimus) received plastic floats. A sloth bear (Melursus ursinus) and a spectacled bear (Treniarctos ornatus) received plastic balls. During a 7-week period, polar bears nearly doubled their activity in the presence of toy floats, and the spectacled bear halved its pacing in the presence of toys. The sloth bear did not respond to the objects. These findings indicate that access to manipulable nonfood objects such as plastic toys can result in prolonged engagement in appetitive behaviors by polar bears.     

Assessing stress in Western Hudson Bay polar bears using hair cortisol concentration as a biomarker

The development of novel biomarkers to help assess whether polar bear (Ursus maritimus) health is impacted by long-term physiological stress associated with climate change represents an emerging area of research. Reductions in sea ice cover and food availability are potentially stressful, and chronic stress can have deleterious effects that may impair individual and population level health. Cortisol is the principal effector hormone of the stress response and has previously been linked to aspects of polar bear life history (e.g., reproduction and growth) known to be negatively influenced by environmental change. Understanding stress is important for polar bears at the southern limit of their range, such as those in Western Hudson Bay (WH), where rapidly changing sea ice phenology threatens population viability. We examined the relationships between age, reproductive status, and body condition (fatness) and hair cortisol concentration (HCC) in 729 polar bear hair samples collected in WH from 2004–2013. Overall, there was a negative relationship between fatness and HCC, suggesting that bears in poorer body condition experienced higher levels of stress. However, when reproductive status was included in our analysis, this relationship only held for male and lone female bears. Females with dependent offspring had consistently low fatness and elevated HCC, likely because of the high cost of maternal care. We also found a positive correlation between HCC and age for: (1) bears in poor body condition, possibly due to nutritional stress compounding effects of aging; and (2) male bears, potentially due to stress and injury associated with intrasexual mate competition. These findings support the use of HCC as a biomarker for polar bear health. Furthermore, we have established a HCC benchmark against which future population-level effects of climate change in WH polar bears can be compared.     

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.     

Site fidelity of Svalbard polar bears revealed by mark-recapture positions

Quantifying the degree of site fidelity in polar bears’ (Ursus maritimus) use of denning and mating areas in spring is of considerable interest for both basic and applied purposes. We analyzed 276 spring-to-spring movements (displacements) of 178 polar bears obtained from mark-recapture in the period 1987–2010 in Svalbard, Norway. Male and female subadults and adults showed site fidelity (only subadult females non-significantly) when their movements were compared to a scenario of random movements between all capture locations. The median observed displacement was 47.6 km (bootstrapped 95 % CI: 38.6–57.8 km), significantly smaller than the median potential displacement for random movements of 206.3 km (bootstrapped 95 % CI: 187.3–219.6 km). Subadult females tended to have the longest displacements, followed by adult males and adult females. However, large individual variation both in displacement distances and recapture frequency tended to blur age and sex differences. Analysis restricted to one movement for each bear showed that the difference between adult males and adult females was small and non-significant. This indicates that findings based on telemetry, which is almost always restricted to females, may be relatively representative of the whole adult Barents Sea population in the spring season.     

Meteorological influence on the occurrence of gastric dilatation-volvulus in military working dogs in Texas

Gastric dilatation-volvulus (GDV) is a life-threatening condition in dogs and other species in which the stomach dilates and rotates on itself. The etiology of the disease is multi-factorial, but explicit precipitating causes are unknown. This study sought to determine if there was a significant association between changes in hourly-measured temperature and/or atmospheric pressure and the occurrence of GDV in the population of high-risk working dogs in Texas. The odds of a day being a GDV day, given certain temperature and atmospheric pressure conditions for that day or the day before, was estimated using logistic regression models. There were 57 days in which GDV(s) occurred, representing 2.60% of the days in the 6-year study period. The months of November, December, and January collectively accounted for almost half (47%) of all cases. Disease risk was negatively associated with daily maximum temperature. An increased risk of GDV was weakly associated with the occurrence of large hourly drops in temperature that day and of higher minimum barometric pressure that day and the day before GDV occurrence, but extreme changes were not predictive of the disease.     

Reply to response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay by Stirling et al. (2008)

We address the three main issues raised by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity]: (1) evidence of the role of climate warming in affecting the western Hudson Bay polar bear population, (2) responses to suggested importance of human–polar bear interactions, and (3) limitations on polar bear adaptation to projected climate change. We assert that our original paper did not provide any “alternative explanations [that] are largely unsupported by the data” or misrepresent the original claims by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, I., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climate change. Arctic 52, 294–306], Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176], and other peer-approved papers authored by Stirling and colleagues. In sharp contrast, we show that the conclusion of Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] – suggesting warming temperatures (and other related climatic changes) are the predominant determinant of polar bear population status, not only in western Hudson (WH) Bay but also for populations elsewhere in the Arctic – is unsupportable by the current scientific evidence.The commentary by Stirling et al. [Stirling, I., Derocher, A.E., Gough, W.A., Rode, K., in press. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecol. Complexity] is an example of uni-dimensional, or reductionist thinking, which is not useful when assessing effects of climate change on complex ecosystems. Polar bears of WH are exposed to a multitude of environmental perturbations including human interference and factors (e.g., unknown seal population size, possible competition with polar bears from other populations) such that isolation of any single variable as the certain root cause (i.e., climate change in the form of warming spring air temperatures), without recognizing confounding interactions, is imprudent, unjustified and of questionable scientific utility. Dyck et al. [Dyck, M.G., Soon, W., Baydack, R.K., Legates, D.R., Baliunas, S., Ball, T.F., Hancock, L.O., 2007. Polar bears of western Hudson Bay and climate change: Are warming spring air temperatures the “ultimate” survival control factor? Ecol. Complexity, 4, 73–84. doi:10.1016/j.ecocom.2007.03.002] agree that some polar bear populations may be negatively impacted by future environmental changes; but an oversimplification of the complex ecosystem interactions (of which humans are a part) may not be beneficial in studying external effects on polar bears. Science evolves through questioning and proposing hypotheses that can be critically tested, in the absence of which, as Krebs and Borteaux [Krebs, C.J., Berteaux, D., 2006. Problems and pitfalls in relating climate variability to population dynamics. Clim. Res. 32, 143–149] observe, “we will be little more than storytellers.”     

What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay

Under current climate trends, spring ice breakup in Hudson Bay is advancing rapidly, leaving polar bears (Ursus maritimus) less time to hunt seals during the spring when they accumulate the majority of their annual fat reserves. For this reason, foods that polar bears consume during the ice‐free season may become increasingly important in alleviating nutritional stress from lost seal hunting opportunities. Defining how the terrestrial diet might have changed since the onset of rapid climate change is an important step in understanding how polar bears may be reacting to climate change. We characterized the current terrestrial diet of polar bears in western Hudson Bay by evaluating the contents of passively sampled scat and comparing it to a similar study conducted 40 years ago. While the two terrestrial diets broadly overlap, polar bears currently appear to be exploiting increasingly abundant resources such as caribou (Rangifer tarandus) and snow geese (Chen caerulescens caerulescens) and newly available resources such as eggs. This opportunistic shift is similar to the diet mixing strategy common among other Arctic predators and bear species. We discuss whether the observed diet shift is solely a response to a nutritional stress or is an expression of plastic foraging behavior.     

Biomechanical consequences of rapid evolution in the polar bear lineage

The polar bear is the only living ursid with a fully carnivorous diet. Despite a number of well-documented craniodental adaptations for a diet of seal flesh and blubber, molecular and paleontological data indicate that this morphologically distinct species evolved less than a million years ago from the omnivorous brown bear. To better understand the evolution of this dietary specialization, we used phylogenetic tests to estimate the rate of morphological specialization in polar bears. We then used finite element analysis (FEA) to compare the limits of feeding performance in the polar bear skull to that of the phylogenetically and geographically close brown bear. Results indicate that extremely rapid evolution of semi-aquatic adaptations and dietary specialization in the polar bear lineage produced a cranial morphology that is weaker than that of brown bears and less suited to processing tough omnivorous or herbivorous diets. Our results suggest that continuation of current climate trends could affect polar bears by not only eliminating their primary food source, but also through competition with northward advancing, generalized brown populations for resources that they are ill-equipped to utilize.     

Annual changes in serum sex steroids in male and female black (Ursus americanus) and polar (Ursus maritimus) bears

The adaptation of black and polar bears to their environments is proportional to the severity of climate and food restriction. Both black and polar bears mate during the spring, despite differences in their recent metabolic state. Reproductive activity in black bears follows 4 mo of torpor, whereas reproduction in polar bears occurs prior to torpor. The goals of this study were to measure the annual changes in serum sex steroids in male and female black and polar bears, and to determine if changes in serum levels of these steroids were associated with metabolic condition or photoperiod. Serum testosterone (T) concentrations were elevated during spring in black and polar bears. Moreover, this increase in serum T in polar bears during spring was correlated with age and testis size. Serum progesterone (P4) concentrations increased in pregnant polar bears in fall coincident with the time of expected implantation. No increases in serum P4 were observed in nonpregnant black and polar bears. Serum estradiol (E2) was elevated in nonpregnant and pregnant polar bears 2 mo prior to the time of expected implantation. We found that serum sex steroids measured in black and polar bears change independent of torpor. Therefore, our results suggest that photoperiod may be a more important regulator of serum steroid levels and reproduction than metabolic condition.     

Field Metabolic Rate and PCB Adipose Tissue Deposition Efficiency in East Greenland Polar Bears Derived from Contaminant Monitoring Data

Climate change will increasingly affect the natural habitat and diet of polar bears (Ursus maritimus). Understanding the energetic needs of polar bears is therefore important. We developed a theoretical method for estimating polar bear food consumption based on using the highly recalcitrant polychlorinated biphenyl (PCB) congener, 2,2′,4,4′,55-hexaCB (CB153) in bear adipose tissue as an indicator of food intake. By comparing the CB153 tissue concentrations in wild polar bears with estimates from a purposely designed individual-based model, we identified the possible combinations of field metabolic rates (FMR) and CB153 deposition efficiencies in East Greenland polar bears. Our simulations indicate that if 30% of the CB153 consumed by polar bear individuals were deposited into their adipose tissue, the corresponding FMR would be only two times the basal metabolic rate. In contrast, if the modelled CB153 deposition efficiency were 10%, adult polar bears would require six times more energy than that needed to cover basal metabolism. This is considerably higher than what has been assumed for polar bears in previous studies though it is similar to FMRs found in other marine mammals. An implication of this result is that even relatively small reductions in future feeding opportunities could impact the survival of East Greenland polar bears.     

Population substructure and space use of Foxe Basin polar bears

Climate change has been identified as a major driver of habitat change, particularly for sea ice-dependent species such as the polar bear (Ursus maritimus). Population structure and space use of polar bears have been challenging to quantify because of their circumpolar distribution and tendency to range over large areas. Knowledge of movement patterns, home range, and habitat is needed for conservation and management. This is the first study to examine the spatial ecology of polar bears in the Foxe Basin management unit of Nunavut, Canada. Foxe Basin is in the mid-Arctic, part of the seasonal sea ice ecoregion and it is being negatively affected by climate change. Our objectives were to examine intrapopulation spatial structure, to determine movement patterns, and to consider how polar bear movements may respond to changing sea ice habitat conditions. Hierarchical and fuzzy cluster analyses were used to assess intrapopulation spatial structure of geographic position system satellite-collared female polar bears. Seasonal and annual movement metrics (home range, movement rates, time on ice) and home-range fidelity (static and dynamic overlap) were compared to examine the influence of regional sea ice on movements. The polar bears were distributed in three spatial clusters, and there were differences in the movement metrics between clusters that may reflect sea ice habitat conditions. Within the clusters, bears moved independently of each other. Annual and seasonal home-range fidelity was observed, and the bears used two movement patterns: on-ice range residency and annual migration. We predict that home-range fidelity may decline as the spatial and temporal predictability of sea ice changes. These new findings also provide baseline information for managing and monitoring this polar bear population.     

The dietary habits of the Brown bear (Ursus arctos) in western Greece

Knowledge of the dietary habits of an endangered species is essential to its management and conservation. The brown bear (Ursus arctos) exists in two isolated population in Greece, but only initial management actions have been taken to improve the habitat of this species. To improve our understanding of the ecology and habitat requirements of this species, we examined the dietary habits of the brown bear in the Panagia-Grevena region, of Macedonia, Greece. In total, 360 scats of brown bear were collected between 2002 and 2004 during spring, summer and autumn months. Scats were analyzed by their frequency of occurrence, volumetric and dry weights, and their importance values. Microhistological analysis was applied to estimate the proportion of wild and cultivated plants in the diet. The most important type of food in the spring was green vegetation, while, in the summer, fruits of Pyrus sp., Morus sp., Prunus sp. and Rubus sp. were important food items. In autumn, hard masts, mainly oak (Quercus sp.), were essential foods for the brown bear. The frequency of vertebrates in the diet was higher in the summer and autumn while that of invertebrates was higher in the spring. In summary, the brown bear is an omnivorous species that lives in Greece and adapts its diet according to food availability and human activities in its habitat. For this reason, human activities in the study area must take the needs and requirement of brown bears into consideration.     

Ancestral Polymorphisms and Sex-Biased Migration Shaped the Demographic History of Brown Bears and Polar Bears

Recent studies have reported discordant gene trees in the evolution of brown bears and polar bears. Genealogical histories are different among independent nuclear loci and between biparentally inherited autosomal DNA (aDNA) and matrilineal mitochondrial DNA (mtDNA). Based on multi-locus genomic sequences from aDNA and mtDNA, we inferred the population demography of brown and polar bears and found that brown bears have 6 times (aDNA) or more than 14 times (mtDNA) larger population sizes than polar bears and that polar bear lineage is derived from within brown bear diversity. In brown bears, the effective population size ratio of mtDNA to aDNA was at least 0.62, which deviated from the expected value of 0.25, suggesting matriarchal population due to female philopatry and male-biased migration. These results emphasize that ancestral polymorphisms and sex-biased migration may have contributed to conflicting branching patterns in brown and polar bears across aDNA genes and mtDNA.     

Morbillivirus ecology in polar bears (Ursus maritimus)

Polar bear (Ursus maritimus) morbillivirus infection was initially reported by Follmann and co-workers in 1996, based upon serologic results using canine distemper virus (CDV). The impetus for the evaluation of polar bear populations for morbillivirus infections was prompted by epidemics of canine distemper-like disease in seal populations in the north Atlantic regions of Greenland, Europe, and Russia. Since marine morbilliviruses have been further characterized into three major species, phocine distemper virus (PDV), dolphin morbillivirus (DMV) and porpoise morbillivirus (PMV), it was of value to determine the origin of the polar bear infection. One hundred serum samples were selected from a group of sera collected from regions of Alaska and Russia and tested by differential serum neutralization assay against the three major marine morbilliviruses and CDV, to determine the predominant virus infecting the polar bear. Polar bears had higher serum antibody titers to CDV than they did to PDV, DMV, and PMV. These data suggest that polar bears are being infected with a morbillivirus of terrestrial origin. Furthermore, based on the high serum antibody prevalence in the population, the virus may be indigenous to the polar bear and not necessarily the result of interspecies transmission from other arctic mammals susceptible to CDV and/or marine morbilliviruses. Accepted: 20 December 1999