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Ice nucleation across the skin of wood frogs (Lithobates sylvaticus) rapidly induces endogenous production of glucose, a cryoprotectant necessary for freeze tolerance. In laboratory studies of freeze tolerance, wood frogs are cooled slowly, often at -0.05°C h-1, to facilitate high cryoprotectant production and survival. Under natural conditions in Alaska, however, wood frogs accumulate maximal tissue glucose concentrations while cooling at much faster rates, -0.35° to -1.6°C h-1, and in addition undergo multiple successive freeze-thaw cycles before remaining frozen for the winter. We examined whether simulating these ecologically relevant cooling rates and repeated freeze-thaw events in captive wood frogs results in the high glucose concentrations found in naturally frozen wood frogs. We found that over successive freezing and thawing events, glucose concentrations increased stepwise in all measured tissues. Short thawing periods did not result in a statistically significant decline of glucose concentrations. Wood frogs that experienced three freeze-thaw events had fresh weight glucose concentrations that approached values found in tissues of wood frogs frozen in natural conditions. Laboratory wood frogs survive frozen for 2 mo, while wood frogs frozen under natural conditions survive frozen for up to 7 mo at temperatures below -18°C. We hypothesize that repeated freeze-thaw cycles with rapid cooling and warming rates allow for greater survival in Alaskan wood frogs through enhanced cryoprotectant production. [ABSTRACT FROM AUTHOR]

Prolonged disuse (e.g., physical inactivity) typically results in increased bone porosity, decreased mineral density, and decreased bone strength, leading to increased fracture risk in many mammals. However, bears, marmots, and two species of ground squirrels have been shown to preserve macrostructural bone properties and bone strength during long seasons of hibernation while they remain mostly inactive. Some small hibernators (e.g., 13-lined ground squirrels) show micro-structural bone loss (i.e., osteocytic osteolysis) during hibernation, which is not seen in larger hibernators (e.g., bears and marmots). Arctic ground squirrels (Urocitellus parryii) are intermediate in size between 13-lined ground squirrels and marmots and are perhaps the most extreme rodent hibernator, hibernating for up to 8 mo annually with body temperatures below freezing. The goal of this study was to quantify the effects of hibernation and inactivity on cortical and trabecular bone properties in arctic ground squirrels. Cortical bone geometrical properties (i.e., thickness, cross-sectional area, and moment of inertia) at the midshaft of the femur were not different in animals sampled over the hibernation and active seasons. Femoral ultimate stress tended to be lower in hibernators than in summer animals, but toughness was not affected by hibernation. The area of osteocyte lacunae was not different between active and hibernating animals. There was an increase in osteocytic lacunar porosity in the hibernation group due to increased lacunar density. Trabecular bone volume fraction in the proximal tibia was unexpectedly greater in the hibernation group than in the active group. This study shows that, similar to other hibernators, arctic ground squirrels are able to preserve many bone properties during hibernation despite being physically inactive for up to 8 mo. [ABSTRACT FROM AUTHOR]

Hibernating mammals become sequestered and cease foraging during prolonged seasonal periods of reduced or unpredictable food availability and instead rely on cached food and/or endogenous reserves of fat and protein accumulated during the previous active season. The gain in weight is due to increased food consumption, but it also has been hypothesized that hi-bernators maximize rates of fattening by decreasing costs of maintenance before weight gain, reflected in reduced resting metabolic rate (RMR). We recorded repeated measures of total body, lean, and fat mass in individual adult male and female arctic ground squirrels across their active season and found that squirrels increased body mass by 42% (males) and 62% (females). This gain was achieved through a 17% increase in lean mass and a 7-8-fold increase in fat mass; however, mass gain was not linear and patterns differed between sexes. Contrary to our hypothesis, decreases in RMR were not associated with rapid mass gain. We found RMR of males increased (whole-animal RMR or lean-mass-specific RMR) or remained constant (mass-specific RMR) for most of the active season and decreased only after the majority of mass had been gained. In females, although RMR (whole-animal, mass-specific, and lean-mass RMR) generally decreased across the active season, the greatest decrease occurred late in the active season after the majority of mass had been gained. In conclusion, arctic ground squirrels do not trade off metabolism to facilitate rates of weight gain before hibernation, but they do use energy sparing strategies before hibernation that help maintain peak mass. [ABSTRACT FROM AUTHOR]

In mammals, the circadianmaster clock generates daily rhythms of body temperature (Tb) that act to entrain rhythms in peripheral circadian oscillators. The persistence and function of circadian rhythms during mammalian hibernation is contentious, and the factors that contribute to the reestablishment of rhythms after hibernation are unclear. We collected regular measures of core Tb (every 34 min) and ambient light conditions (every 30 s) before, during, and following hibernation in free-living male arctic ground squirrels. Free-running circadian Tb rhythms at euthermic levels of Tb persisted for up to 10 d in constant darkness after animals became sequestered in their hibernacula in fall. During steady state torpor, Tb was constant and arrhythmic for up to 13 d (within the 0.19°C resolution of loggers). In spring, males ended heterothermy but remained in their burrows at euthermic levels of Tb for 22-26 d; patterns of Tb were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running Tb rhythm (τ = 22.1 h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal Tb rhythms were immediately reestablished co-incident with emergence to the surface and the resumption of surface activity. Our results support the hypothesis that clock function is inhibited during hibernation and reactivated by exposure to light, although resumption of extended surface activity does not appear to be necessary to reinitiate Tb cycles. [ABSTRACT FROM AUTHOR]

Laboratory experiments have demonstrated that the amount of polyunsaturated fatty acids (PUFAs) in the diet before hibernation influences patterns of mammalian torpor. The hibernation ability of ground squirrels is greatest (longest torpor bouts, greatest number of animals entering torpor) when the PUFA content of their fall diets is 33-74 mg/g, under laboratory conditions. The extent to which natural fall diets both (a) vary in PUFA content and (b) influence the torpor patterns of free-ranging populations of hibernating mammals is unknown, however. We conducted a 3-yr study on the diet PUFA contents and subsequent hibernation patterns of free-ranging arctic ground squirrels (Spermophilus parryii) in the Brooks Range of Alaska. We found that the PUFA contents of fall diets varied more than threefold among individuals. Our study also revealed that arctic ground squirrels that consumed a moderate-PUFA (33-74 mg/g) diet had (a) longer torpor bouts, (b) fewer arousals from torpor, (c) shorter arousal periods, (d) more days in torpor, and (e) greater probability of persisting in the population than those that consumed a high-PUFA (>74 mg/g) diet during the fall. No animals were demonstrated to have consumed a diet representing low-PUFA (<33 mg/g) values. Our study is therefore the first to demonstrate that estimated dietary PUFA levels of a free-ranging hibernator influence subsequent torpor patterns. [ABSTRACT FROM AUTHOR]