John P. Whiteman

The subnivium: a deteriorating seasonal refugium

For many terrestrial organisms in the Northern Hemisphere, winter is a period of resource scarcity and energy deficits, survivable only because a seasonal refugium – the “subnivium” – exists beneath the snow. The warmer and more stable conditions within the subnivium are principally driven by snow duration, density, and depth. In temperate regions, the subnivium is important for the overwintering success of plants and animals, yet winter conditions are changing rapidly worldwide. Throughout the Northern Hemisphere, the impacts of climate change are predicted to be most prominent during the winter months, resulting in a shorter snow season and decreased snow depth. These climatic changes will likely modify the defining qualities of the subnivium, resulting in broad-scale shifts in distributions of species that are dependent on these refugia. Resultant changes to the subnivium, however, will be spatially and temporally variable. We believe that ecologists and managers are overlooking this widespread, crucia...

Summer declines in activity and body temperature offer polar bears limited energy savings

Not that unusual after all As polar ice recedes, polar bears are facing a changed habitat with reduced summer foraging opportunities. It has been hypothesized that they might be able to resist summer food shortages by reducing their metabolic needs in a sort of “walking hibernation.” Whiteman et al. monitored energy expenditure in polar bears both on and off the ice and found energy reductions, but that these were more akin to normal mammalian fasting levels. Thus, it appears that polar bears have no energetic protections against reduced summer food supplies and will face increasing starvation threats if summer foraging habitats continue to decline. Science, this issue p. 295 Polar bears show no inherent energetic resistance to summer food shortages. Polar bears (Ursus maritimus) summer on the sea ice or, where it melts, on shore. Although the physiology of “ice” bears in summer is unknown, “shore” bears purportedly minimize energy losses by entering a hibernation-like state when deprived of food. Such a strategy could partially compensate for the loss of on-ice foraging opportunities caused by climate change. However, here we report gradual, moderate declines in activity and body temperature of both shore and ice bears in summer, resembling energy expenditures typical of fasting, nonhibernating mammals. Also, we found that to avoid unsustainable heat loss while swimming, bears employed unusual heterothermy of the body core. Thus, although well adapted to seasonal ice melt, polar bears appear susceptible to deleterious declines in body condition during the lengthening period of summer food deprivation.

Lipid and amino acid composition influence incorporation and discrimination of 13C and 15N in mink

Abstract The incorporation of dietary macronutrients and associated isotopic signatures of carbon (&dgr;13C) and nitrogen (&dgr;15N) into animal tissues is a result of the interaction between growth, nutritional status, and the composition of the diet. In mammalian carnivores incorporation is further complicated by lack of carbohydrates in the diet and allocation of large quantities of dietary macronutrients to fetuses and milk production. In this study, we explored the effects of diet composition, growth, pregnancy, and milk production on isotopic incorporation of 13C and 15N in captive mink (Neovison vison) fed 3 experimental diets (Beef, Fish, and a Mixture of the 2) that differed in lipid and amino acid composition. In nursing kits, growth was the main factor influencing isotopic incorporation rates into muscle. Similarly, in adults, changes in body mass influenced the dynamics of isotopic incorporation in red blood cells, although the rates differed for &dgr;13C and &dgr;15N, as well as among the 3 experimental groups. Effects of allocation of dietary macronutrients to fetuses and milk did not differ from body mass changes, potentially because those macronutrients were assimilated in tissues other than blood cells. Although incorporation of &dgr;13C followed the expected exponential form, &dgr;15N incorporation lagged in the Beef and Mixture diet treatments, and both &dgr;13C and &dgr;15N incorporation rates differed substantially for the Fish-fed mink. These differences in isotopic incorporation can be attributed to the differential oxidation of dietary amino and fatty acids. Thus, we advocate the development of compound-specific isotopic analyses to estimate dietary contributions through the incorporation of essential dietary fatty and amino acids.

DNA-based approach to aging martens (Martes americana and M. caurina)

Abstract Demographic structure is central to understanding the dynamics of animal populations. However, determining the age of free-ranging mammals is difficult, and currently impossible when sampling with noninvasive, genetic-based approaches. We present a method to estimate age class by combining measures of telomere lengths with other biologically meaningful covariates in a Bayesian network. We applied this approach to American and Pacific martens (Martes americana and M. caurina) and compared predicted age with that obtained from counts of cementum annuli. Using telomere length and the covariates sex, species, and estimates of population density obtained from commercial trapping records, we assigned martens to juvenile (<1 year) or adult (≥1 year) classes with 75–88% accuracy. In our analysis for live-captured martens, for which information on body size and whether animals were juveniles or adults would be available, we achieved 90–93% accuracy when assigning individuals to 5 discrete age classes (0–4+ years). This general approach could be extended to other species for noninvasive estimation of age class, or in place of invasive aging methods, and enable demographically based population analyses that have heretofore been impossible.

Carbon isotopes in exhaled breath track metabolic substrates in brown bears (Ursus arctos)

Abstract Dietary carbon is oxidized and exhaled as CO2, thus &dgr;13Cbreath values can provide information on diet and substrate use for energy. However, physiological phenomena such as fat deposition and fasting can alter values of &dgr;13Cbreath such that interpretation of source contributions may be unclear. Consequently, before application to free-ranging animals, inferences about feeding and nutritional states based on &dgr;13Cbreath should be validated with controlled experiments using captive individuals. Here, we report &dgr;13Cbreath values for 4 captive brown bears (Ursus arctos) under different conditions: the bears were 1st given a diet containing carbohydrate, lipid, and protein; they were then switched to a carbohydrate-free diet consisting of salmon and fish oil; and finally they were placed on a fast leading to winter hibernation. Following the switch to the carbohydrate-free diet, values of &dgr;13Cbreath and &dgr;13Cplasma suggested that although oxidation included a substantial portion of dietary proteins, dietary lipids were preferentially utilized. After the onset of fasting, &dgr;13Cbreath values did not change, possibly because of selective mobilization of endogenous fatty acids accreted by bears while consuming the experimental diet. Examination of our data suggests that because CO2 production and exhalation are influenced differentially by fasting state and diet composition, additional measurements such as respiratory exchange ratio should be used to aid interpretation of carbon isotope analysis of breath.

Increased Arctic sea ice drift alters adult female polar bear movements and energetics

Abstract Recent reductions in thickness and extent have increased drift rates of Arctic sea ice. Increased ice drift could significantly affect the movements and the energy balance of polar bears (Ursus maritimus) which forage, nearly exclusively, on this substrate. We used radio‐tracking and ice drift data to quantify the influence of increased drift on bear movements, and we modeled the consequences for energy demands of adult females in the Beaufort and Chukchi seas during two periods with different sea ice characteristics. Westward and northward drift of the sea ice used by polar bears in both regions increased between 1987–1998 and 1999–2013. To remain within their home ranges, polar bears responded to the higher westward ice drift with greater eastward movements, while their movements north in the spring and south in fall were frequently aided by ice motion. To compensate for more rapid westward ice drift in recent years, polar bears covered greater daily distances either by increasing their time spent active (7.6%–9.6%) or by increasing their travel speed (8.5%–8.9%). This increased their calculated annual energy expenditure by 1.8%–3.6% (depending on region and reproductive status), a cost that could be met by capturing an additional 1–3 seals/year. Polar bears selected similar habitats in both periods, indicating that faster drift did not alter habitat preferences. Compounding reduced foraging opportunities that result from habitat loss; changes in ice drift, and associated activity increases, likely exacerbate the physiological stress experienced by polar bears in a warming Arctic.

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

Polar bears experience skeletal muscle atrophy during winter whether they hibernate or not. Bears spending summer on sea ice in the Arctic basin undergo a second period of atrophy caused by fasting, whereas bears with shore access to adequate, alternative food do not. Summer atrophy may influence fitness.

Heightened immune system function in polar bears using terrestrial habitats

Climate change is altering the distribution of some wildlife species while warming temperatures are facilitating the northward expansion of pathogens, potentially increasing disease risk. Melting of Arctic sea ice is increasingly causing polar bears (Ursus maritimus) of the southern Beaufort Sea (SBS) to spend summer on land, where they may encounter novel pathogens. Here, we tested whether SBS polar bears on shore during summer exhibited greater immune system activity than bears remaining on the sea ice. In addition, we tested whether the type of immune response correlated with body condition, because adaptive responses (slowly developing defenses against specific pathogens) often require less energy than innate responses (rapid defenses not based on pathogen identity). After accounting for body condition, we found that polar bears on shore exhibited higher total white blood cell counts, neutrophils, and monocytes than bears on the ice, suggesting more infections. Lymphocytes, eosinophils, basophils, and globulins did not differ. C-reactive protein, an indicator of inflammation, also did not differ between habitats. Body condition was associated with variables indicative of both innate and adaptive immunity, suggesting that neither response was uniquely limited by energy resources. Our data indicate that as more polar bears spend longer periods of time on shore, they may experience more infections. We encourage continued health monitoring of this species and studies of the long-term fitness consequences from disease.

Characterization of blood lipoproteins and validation of cholesterol and triacylglycerol assays for free-ranging polar bears (Ursus maritimus)

Blood triacylglycerol (TG) and lipoproteins are important variables for evaluating nutritional status of wildlife, but measurements are often expensive and difficult. Performance of a small, portable blood analyzer intended for human medical diagnostics was evaluated in measuring these variables in plasma and serum from free-ranging polar bears (Ursus maritimus), which are experiencing nutritional stress related to sea ice loss. The analyzer accurately tracked changes in concentration of total cholesterol (Ctotal), cholesterol associated with high-density lipoprotein (CHDL), and TG during a validation protocol of diluting samples and spiking them with exogenous cholesterol and glycerol. Values of Ctotal and TG agreed well with values obtained by other methods (ultracentrifugation followed by colorimetric assays); agreement was variable for values of cholesterol associated with specific lipoproteins. Similar to a study of captive polar bears, ultracentrifugation methods revealed greater TG in very low-density lipoproteins than in low-density lipoprotein, which is unusual and merits additional study.