Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period

Declining Arctic sea ice is increasing polar bear land use. Polar bears on land are thought to minimize activity to conserve energy. Here, we measure the daily energy expenditure (DEE), diet, behavior, movement, and body composition changes of 20 different polar bears on land over 19-23 days from August to September (2019-2022) in Manitoba, Canada. Polar bears on land exhibited a 5.2-fold range in DEE and 19-fold range in activity, from hibernation-like DEEs to levels approaching active bears on the sea ice, including three individuals that made energetically demanding swims totaling 54-175 km. Bears consumed berries, vegetation, birds, bones, antlers, seal, and beluga. Beyond compensating for elevated DEE, there was little benefit from terrestrial foraging toward prolonging the predicted time to starvation, as 19 of 20 bears lost mass (0.4-1.7 kg•day-1). Although polar bears on land exhibit remarkable behavioral plasticity, our findings reinforce the risk of starvation, particularly in subadults, with forecasted increases in the onshore period.

Unlock the Endangered Species Act to address GHG emissions

For the first time, ESA evaluations can include impacts on polar bears from greenhouse gas emissions.

A body composition model with multiple storage compartments for polar bears (Ursus maritimus)

Climate warming is rapidly altering Arctic ecosystems. Polar bears (Ursus maritimus) need sea ice as a platform from which to hunt seals, but increased sea-ice loss is lengthening periods when bears are without access to primary hunting habitat. During periods of food scarcity, survival depends on the energy that a bear has stored in body reserves, termed storage energy, making this a key metric in predictive models assessing climate change impacts on polar bears. Here, we developed a body composition model for polar bears that estimates storage energy while accounting for changes in storage tissue composition. We used data of dissected polar bears (n = 31) to link routinely collected field measures of total body mass and straight-line body length to the body composition of individual bears, described in terms of structural mass and two storage compartments, adipose and muscle. We then estimated the masses of metabolizable proteins and lipids within these storage compartments, giving total storage energy. We tested this multi-storage model by using it to predict changes in the lipid stores from an independent dataset of wild polar bears (n = 36) that were recaptured 8-200 days later. Using length and mass measurements, our model successfully predicted direct measurements of lipid changes via isotopic dilutions (root mean squared error of 14.5 kg). Separating storage into two compartments, and allowing the molecular composition of storage to vary, provides new avenues for quantifying energy stores of individuals across their life cycle. The multi-storage body composition model thus provides a basis for further exploring energetic costs of physiological processes that contribute to individual survival and reproductive success. Given bioenergetic models are increasingly used as a tool to predict individual fitness and population dynamics, our approach for estimating individual energy stores could be applicable to a wide range of species.

The use of infrared thermography to noninvasively measure the surface temperature of polar bears during bouts of social play

Infrared thermography or thermal imagery is a noninvasive tool that can be used to measure the temperature of surfaces. Typically, thermal imagery is used for construction or military purposes but is increasingly used as a noninvasive tool in wildlife studies. We investigated the use of thermal imagery to measure surface temperature changes as a proxy for energetic expenditure. We measured the surface temperature of polar bear (Ursus maritimus) eyes, while immobilized, to determine whether the eye is a thermal window that can accurately indicate internal temperature. We found a significant difference (2.68 ± 0.41°C) between the surface temperature of the eye measured with thermal imagery and the internal rectal temperature. Additionally, we measured surface temperature changes in polar bears after bouts of social play as a proxy for energy expenditure. Mean temperature of the eye increased by 1.34 ± 0.43°C after social play, indicating that this activity increased energy expenditure. During the fasting season, polar bears rely on fat stores, and any energy expenditure beyond what is required to travel may be costly to their survival, particularly in years of low resource availability. We conclude that thermal imagery is a useful tool to noninvasively investigate the energetics of social play.

Demographic risk assessment for a harvested species threatened by climate change: polar bears in the Chukchi Sea

Climate change threatens global biodiversity. Many species vulnerable to climate change are important to humans for nutritional, cultural, and economic reasons. Polar bears Ursus maritimus are threatened by sea-ice loss and represent a subsistence resource for Indigenous people. We applied a novel population modeling-management framework that is based on species life history and accounts for habitat loss to evaluate subsistence harvest for the Chukchi Sea (CS) polar bear subpopulation. Harvest strategies followed a state-dependent approach under which new data were used to update the harvest on a predetermined management interval. We found that a harvest strategy with a starting total harvest rate of 2.7% (˜85 bears/yr at current abundance), a 2:1 male-to-female ratio, and a 10-yr management interval would likely maintain subpopulation abundance above maximum net productivity level for the next 35 yr (approximately three polar bear generations), our primary criterion for sustainability. Plausible bounds on starting total harvest rate were 1.7-3.9%, where the range reflects uncertainty due to sampling variation, environmental variation, model selection, and differing levels of risk tolerance. The risk of undesired demographic outcomes (e.g., overharvest) was positively related to harvest rate, management interval, and projected declines in environmental carrying capacity; and negatively related to precision in population data. Results reflect several lines of evidence that the CS subpopulation has been productive in recent years, although it is uncertain how long this will last as sea-ice loss continues. Our methods provide a template for balancing trade-offs among protection, use, research investment, and other factors. Demographic risk assessment and state-dependent management will become increasingly important for harvested species, like polar bears, that exhibit spatiotemporal variation in their response to climate change.

Yes, they can: polar bears Ursus maritimus successfully hunt Svalbard reindeer Rangifer tarandus platyrhynchus

The polar bear Ursus maritimus is one of the species most endangered by the rapidly declining sea–ice cover in the Arctic, which they use as a platform to hunt fatty, high-energy seals. In recent decades, more polar bears have been forced to remain longer on land, so their access to seals is limited. The importance of terrestrial food to polar bears is disputable, and more data are needed. Terrestrial ungulates could be an attractive substitute prey for them. Svalbard reindeer Rangifer tarandus platyrhynchus are prevalent and their distribution is completely within the range of polar bears. They constitute an attractive potential prey offering a significant energy return. Pre-2000 sources state that polar bears do not attack Svalbard reindeer. This report is the first description and documentation of the complete course of a polar bear hunt for adult reindeer in Hornsund, SW Spitsbergen, and also of the bear’s hunting behaviour and the reindeer’s response. Further, we report several other recent instances of bear–reindeer interactions in Svalbard, suggesting that polar bears now hunt reindeer more frequently than they used to. This increase in hunting is probably linked to the reduced ice cover, with bears spending more time on land, and a growing reindeer population. This study adds to earlier papers on how polar bears in Svalbard have increasingly shifted to a more terrestrial diet, and indicates that they may have an enhanced role as an apex predator in the terrestrial ecosystem.

Long-term increases in pathogen seroprevalence in polar bears (Ursus maritimus) influenced by climate change

The influence of climate change on wildlife disease dynamics is a burgeoning conservation and human health issue, but few long-term studies empirically link climate to pathogen prevalence. Polar bears (Ursus maritimus) are vulnerable to the negative impacts of sea ice loss as a result of accelerated Arctic warming. While studies have associated changes in polar bear body condition, reproductive output, survival, and abundance to reductions in sea ice, no long-term studies have documented the impact of climate change on pathogen exposure. We examined 425 serum samples from 381 adult polar bears, collected in western Hudson Bay (WH), Canada, for antibodies to selected pathogens across three time periods: 1986-1989 (n = 157), 1995-1998 (n = 159) and 2015-2017 (n = 109). We ran serological assays for antibodies to seven pathogens: Toxoplasma gondii, Neospora caninum, Trichinella spp., Francisella tularensis, Bordetella bronchiseptica, canine morbillivirus (CDV) and canine parvovirus (CPV). Seroprevalence of zoonotic parasites (T. gondii, Trichinella spp.) and bacterial pathogens (F. tularensis, B. bronchiseptica) increased significantly between 1986-1989 and 1995-1998, ranging from +6.2% to +20.8%, with T. gondii continuing to increase into 2015-2017 (+25.8% overall). Seroprevalence of viral pathogens (CDV, CPV) and N. caninum did not change with time. Toxoplasma gondii seroprevalence was higher following wetter summers, while seroprevalences of Trichinella spp. and B. bronchiseptica were positively correlated with hotter summers. Seroprevalence of antibodies to F. tularensis increased following years polar bears spent more days on land, and polar bears previously captured in human settlements were more likely to be seropositive for Trichinella spp. As the Arctic has warmed due to climate change, zoonotic pathogen exposure in WH polar bears has increased, driven by numerous altered ecosystem pathways.

Physiological consequences of Arctic sea ice loss on large marine carnivores: unique responses by polar bears and narwhals

Rapid environmental changes in the Arctic are threatening the survival of marine species that rely on the predictable presence of the sea ice. Two Arctic marine mammal specialists, the polar bear (Ursus maritimus) and narwhal (Monodon monoceros), appear especially vulnerable to the speed and capriciousness of sea ice deterioration as a consequence of their unique hunting behaviors and diet, as well as their physiological adaptations for slow-aerobic exercise. These intrinsic characteristics limit the ability of these species to respond to extrinsic threats associated with environmental change and increased industrial activity in a warming Arctic. In assessing how sea ice loss may differentially affect polar bears that hunt on the ice surface and narwhals that hunt at extreme depths below, we found that major ice loss translated into elevated locomotor costs that range from 3- to 4-fold greater than expected for both species. For polar bears this instigates an energy imbalance from the combined effects of reduced caloric intake and increased energy expenditure. For narwhals, high locomotor costs during diving increase the risk of ice entrapment due to the unreliability of breathing holes. These species-specific physiological constraints and extreme reliance on the polar sea ice conspire to make these two marine mammal specialists sentinels of climate change within the Arctic marine ecosystem that may foreshadow rapid changes to the marine ecosystem.

Influence of sea ice dynamics on population energetics of Western Hudson Bay polar bears

The Arctic marine ecosystem has experienced extensive changes in sea ice dynamics, with significant effects on ice-dependent species such as polar bears (Ursus maritimus). We used annual estimates of the numbers of bears onshore in the core summering area, age/sex structure and body condition data to estimate population energy density and storage energy in Western Hudson Bay polar bears from 1985 to 2018. We examined intra-population variation in energetic patterns, temporal energetic trends and the relationship between population energetics and sea ice conditions. Energy metrics for most demographic classes declined over time in relation to earlier sea ice breakup, most significantly for solitary adult females and subadult males, suggesting their greater vulnerability to nutritional stress than other age/sex classes. Temporal declines in population energy metrics were related to earlier breakup and longer lagged open-water periods, suggesting multi-year effects of sea ice decline. The length of the open-water period ranged from 102 to 166 days and increased significantly by 9.9 days/decade over the study period. Total population energy density and storage energy were significantly lower when sea ice breakup occurred earlier and the lagged open-water period was longer. At the earliest breakup and a lagged open-water period of 180 days, population energy density was predicted to be 33% lower than our minimum estimated energy density and population storage energy was predicted to be 40% lower than the minimum estimated storage energy. Consequently, over the study, the total population energy density declined by 53% (mean: 3668 ± 386 MJ kg-1/decade) and total population storage energy declined by 56% (mean: 435900 ± 46770 MJ/decade). This study provides insights into ecological mechanisms linking population responses to sea ice decline and highlights the significance of maintaining long-term research programs.

The stress of Arctic warming on polar bears

Arctic ecosystems are changing rapidly in response to climate warming. While Arctic mammals are highly evolved to these extreme environments, particularly with respect to their stress axis, some species may have limited capacity to adapt to this change. We examined changes in key components of the stress axis (cortisol and its carrier protein-corticosteroid binding globulin [CBG]) in polar bears (Ursus maritimus) from western Hudson Bay (N = 300) over a 33 year period (1983-2015) during which time the ice-free period was increasing. Changing sea ice phenology limits spring hunting opportunities and extends the period of onshore fasting. We assessed the response of polar bears to a standardized stressor (helicopter pursuit, darting, and immobilization) during their onshore fasting period (late summer-autumn) and quantified the serum levels of the maximum corticosteroid binding capacity (MCBC) of CBG, the serum protein that binds cortisol strongly, and free cortisol (FC). We quantified bear condition (age, sex, female with cubs or not, fat condition), sea ice (breakup in spring-summer, 1 year lagged freeze-up in autumn), and duration of fasting until sample collection as well as cumulative impacts of the latter environmental traits from the previous year. Data were separated into "good" years (1983-1990) when conditions were thought to be optimal and "poor" years (1991-2015) when sea ice conditions deteriorated and fasting on land was extended. MCBC explained 39.4% of the variation in the good years, but only 28.1% in the poor ones, using both biological and environmental variables. MCBC levels decreased with age. Changes in FC were complex, but more poorly explained. Counterintuitively, MCBC levels increased with increased time onshore, 1 year lag effects, and in poor ice years. We conclude that MCBC is a biomarker of stress in polar bears and that the changes we document are a consequence of climate warming.

Interrelated ecological impacts of climate change on an apex predator

Climate change has broad ecological implications for species that rely on sensitive habitats. For some top predators, loss of habitat is expected to lead to cascading behavioral, nutritional, and reproductive changes that ultimately accelerate population declines. In the case of the polar bear (Ursus maritimus), declining Arctic sea ice reduces access to prey and lengthens seasonal fasting periods. We used a novel combination of physical capture, biopsy darting, and visual aerial observation data to project reproductive performance for polar bears by linking sea ice loss to changes in habitat use, body condition (i.e., fatness), and cub production. Satellite telemetry data from 43 (1991-1997) and 38 (2009-2015) adult female polar bears in the Baffin Bay subpopulation showed that bears now spend an additional 30 d on land (90 d in total) in the 2000s compared to the 1990s, a change closely correlated with changes in spring sea ice breakup and fall sea ice formation. Body condition declined for all sex, age, and reproductive classes and was positively correlated with sea ice availability in the current and previous year. Furthermore, cub litter size was positively correlated with maternal condition and spring breakup date (i.e., later breakup leading to larger litters), and negatively correlated with the duration of the ice-free period (i.e., longer ice-free periods leading to smaller litters). Based on these relationships, we projected reproductive performance three polar bear generations into the future (approximately 35 yr). Results indicate that two-cub litters, previously the norm, could largely disappear from Baffin Bay as sea ice loss continues. Our findings demonstrate how concurrent analysis of multiple data types collected over long periods from polar bears can provide a mechanistic understanding of the ecological implications of climate change. This information is needed for long-term conservation planning, which includes quantitative harvest risk assessments that incorporate estimated or assumed trends in future environmental carrying capacity.

Fatty acid-based diet estimates suggest ringed seal remain the main prey of southern Beaufort Sea polar bears despite recent use of onshore food resources

Polar bears (Ursus maritimus) from the southern Beaufort Sea (SB) subpopulation have traditionally fed predominantly upon ice-seals; however, as the proportion of the subpopulation using onshore habitat has recently increased, foraging on land-based resources, including remains of subsistence-harvested bowhead whales (Balaena mysticetus) and colonial nesting seabirds has been observed. Adipose tissue samples were collected from this subpopulation during the springs of 2013-2016 and analyzed for fatty acid signatures. Diet estimates were generated for the proportional consumption of ringed seal (Pusa hispida), bearded seal (Erignathus barbatus), and beluga whale (Delphinapterus leucas), relative to onshore foods, including bowhead whale remains and seabird, as represented by black guillemot (Cepphus grylle mandtii) nestlings and eggs. Quantitative fatty acid signature analysis (QFASA) estimated that the ice-obligate prey, ringed seal, remained the predominant prey species of SB polar bears (46.4 ± 1.8%), with much lower consumption of bearded seal (19.6 ± 2.0%), seabird (17.0 ± 1.2%), bowhead whale (15.0 ± 1.4%), and hardly any beluga whale (2.0 ± 0.5%). Adult and subadult females appeared to depend more on the traditional ringed seal prey than adult and subadult males. Diet estimates of SB polar bears showed significant interannual variability for all prey (F 12, 456 = 3.17, p < .001). Longer-term estimates suggested that both types of onshore prey, bowhead whale remains and seabird, have represented a moderate proportion of the food resources used by SB polar bears since at least the start of the 21st Century.

Polar bears experience skeletal muscle atrophy in response to food deprivation and reduced activity in winter and summer

When reducing activity and using stored energy during seasonal food shortages, animals risk degradation of skeletal muscles, although some species avoid or minimize the resulting atrophy while experiencing these conditions during hibernation. Polar bears may be food deprived and relatively inactive during winter (when pregnant females hibernate and hunting success declines for other demographic groups) as well as summer (when sea ice retreats from key foraging habitats). We investigated muscle atrophy in samples of biceps femoris collected from free-ranging polar bears in the Southern Beaufort Sea (SBS) throughout their annual cycle. Atrophy was most pronounced in April-May as a result of food deprivation during the previous winter, with muscles exhibiting reduced protein concentration, increased water content, and lower creatine kinase mRNA. These animals increased feeding and activity in spring (when seal prey becomes more available), initiating a period of muscle recovery. During the following ice melt of late summer, ~30% of SBS bears abandon retreating sea ice for land; in August, these 'shore' bears exhibited no muscle atrophy, indicating that they had fully recovered from winter food deprivation. These individuals subsequently scavenged whale carcasses deposited by humans and by October, had retained good muscle condition. In contrast, ~70% of SBS bears follow the ice north in late summer, into deep water with less prey. These 'ice' bears fast; by October, they exhibited muscle protein loss and rapid changes in myosin heavy-chain isoforms in response to reduced activity. These findings indicate that, unlike other bears during winter hibernation, polar bears without food in summer cannot mitigate atrophy. Consequently, prolonged summer fasting resulting from climate change-induced ice loss creates a risk of greater muscle atrophy and reduced abilities to travel and hunt.

Windscapes and olfactory foraging in a large carnivore

The theoretical optimal olfactory search strategy is to move cross-wind. Empirical evidence supporting wind-associated directionality among carnivores, however, is sparse. We examined satellite-linked telemetry movement data of adult female polar bears (Ursus maritimus) from Hudson Bay, Canada, in relation to modelled winds, in an effort to understand olfactory search for prey. In our results, the predicted cross-wind movement occurred most frequently at night during winter, the time when most hunting occurs, while downwind movement dominated during fast winds, which impede olfaction. Migration during sea ice freeze-up and break-up was also correlated with wind. A lack of orientation during summer, a period with few food resources, likely reflected reduced cross-wind search. Our findings represent the first quantitative description of anemotaxis, orientation to wind, for cross-wind search in a large carnivore. The methods are widely applicable to olfactory predators and their prey. We suggest windscapes be included as a habitat feature in habitat selection models for olfactory animals when evaluating what is considered available habitat.

Costs of locomotion in polar bears: when do the costs outweigh the benefits of chasing down terrestrial prey?

Trade-offs between locomotory costs and foraging gains are key elements in determining constraints on predator-prey interactions. One intriguing example involves polar bears pursuing snow geese on land. As climate change forces polar bears to spend more time ashore, they may need to expend more energy to obtain land-based food. Given that polar bears are inefficient at terrestrial locomotion, any extra energy expended to pursue prey could negatively impact survival. However, polar bears have been regularly observed engaging in long pursuits of geese and other land animals, and the energetic worth of such behaviour has been repeatedly questioned. We use data-driven energetic models to examine how energy expenditures vary across polar bear mass and speed. For the first time, we show that polar bears in the 125-235 kg size range can profitably pursue geese, especially at slower speeds. We caution, however, that heat build-up may be the ultimate limiting factor in terrestrial chases, especially for larger bears, and this limit would be reached more quickly with warmer environmental temperatures.