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In mammals, the circadian master clock generates daily rhythms of body temperature (T(b)) 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 T(b) (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 T(b) rhythms at euthermic levels of T(b) persisted for up to 10 d in constant darkness after animals became sequestered in their hibernacula in fall. During steady state torpor, T(b) 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 T(b) for 22-26 d; patterns of T(b) were arrhythmic for the first 10 d of euthermia. One of four squirrels exhibited a significant free-running T(b) rhythm (τ = 22.1 h) before emergence; this squirrel had been briefly exposed to low-amplitude light before emergence. In all animals, diurnal T(b) rhythms were immediately reestablished coincident 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 T(b) cycles.

Precise measures of phenology are critical to understanding how animals organize their annual cycles and how individuals and populations respond to climate-induced changes in physical and ecological stressors. We show that patterns of core body temperature (T (b)) can be used to precisely determine the timing of key seasonal events including hibernation, mating and parturition, and immergence and emergence from the hibernacula in free-living arctic ground squirrels (Urocitellus parryii). Using temperature loggers that recorded T (b) every 20 min for up to 18 months, we monitored core T (b) from three females that subsequently gave birth in captivity and from 66 female and 57 male ground squirrels free-living in the northern foothills of the Brooks Range Alaska. In addition, dates of emergence from hibernation were visually confirmed for four free-living male squirrels. Average T (b) in captive females decreased by 0.5-1.0°C during gestation and abruptly increased by 1-1.5°C on the day of parturition. In free-living females, similar shifts in T (b) were observed in 78% (n = 9) of yearlings and 94% (n = 31) of adults; females without the shift are assumed not to have given birth. Three of four ground squirrels for which dates of emergence from hibernation were visually confirmed did not exhibit obvious diurnal rhythms in T (b) until they first emerged onto the surface when T (b) patterns became diurnal. In free-living males undergoing reproductive maturation, this pre-emergence euthermic interval averaged 20.4 days (n = 56). T (b)-loggers represent a cost-effective and logistically feasible method to precisely investigate the phenology of reproduction and hibernation in ground squirrels.

Black bears hibernate for 5 to 7 months a year and, during this time, do not eat, drink, urinate, or defecate. We measured metabolic rate and body temperature in hibernating black bears and found that they suppress metabolism to 25% of basal rates while regulating body temperature from 30° to 36°C, in multiday cycles. Heart rates were reduced from 55 to as few as 9 beats per minute, with profound sinus arrhythmia. After returning to normal body temperature and emerging from dens, bears maintained a reduced metabolic rate for up to 3 weeks. The pronounced reduction and delayed recovery of metabolic rate in hibernating bears suggest that the majority of metabolic suppression during hibernation is independent of lowered body temperature.

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Circadian systems are principally entrained to 24 h light--dark cycles, but this cue is seasonally absent in polar environments. Although some resident polar vertebrates have weak circadian clocks and are seasonally arrhythmic, the arctic ground squirrel (AGS) maintains daily rhythms of physiology and behavior throughout the summer, which includes 6 weeks of constant daylight. Here, we show that persistent daily rhythms in AGS are maintained through a circadian system that readily entrains to the polar day yet remains insensitive to entrainment by rapid light--dark transitions, which AGS generate naturally as a consequence of their semi-fossorial behavior. Additionally, AGS do not show 'jet lag', the slow realignment of circadian rhythms induced by the inertia of an intrinsically stable master circadian clock in the suprachiasmatic nucleus (SCN). We suggest this is due to the low expression of arginine vasopressin in the SCN of AGS, as vasopressin is associated with inter-neuronal coupling and robust rhythmicity. [ABSTRACT FROM AUTHOR]

The use of animal-borne instruments (ABIs), including biologgers and biotransmitters, has played an integral role in advancing our understanding of adjustments made by animals in their physiology and behavior across their annual and daily cycles and in response to weather and environmental change. Here, we review our research employing body temperature (Tb), light, and acceleration biologgers to measure patterns of physiology and behavior of a free-living, semi-fossorial hibernator, the arctic ground squirrel ( Urocitellus parryii ). We have used these devices to address a variety of physiological, ecological, and evolutionary questions within the fields of hibernation physiology, phenology, behavioral ecology, and chronobiology. We have also combined biologging with other approaches, such as endocrinology and tracking the thermal environment, to provide insights into the physiological mechanisms that underlie fundamental questions in biology including physiological performance trade-offs, timing and functional energetics. Finally, we explore the practical and methodological considerations that need to be addressed in biologging studies of free-living vertebrates and discuss future technological advancements that will increase the power and potential of biologging as a tool for assessing physiological function in dynamic and changing environments. [ABSTRACT FROM AUTHOR]

Integrating physiological and behavioral observations into ecological field studies of animals can provide novel insights into relationships among animal behavior, physiology, and ecology. We describe and evaluate a new technique for simultaneously collecting body temperature (Tb) and burrow use data for semi-fossorial mammals by combining light-sensitive radiotransmitters and implanted temperaturesensitive dataloggers. We used this approach to collect core Tb and activity data for 9 free-ranging arctic ground squirrels (Spermophilus parryii) in northern Alaska, USA, at approximately 5-minute intervals for 30-90 days each to address questions related to thermoregulation, energetics, foraging, sociality, and timing of activity in natural environments. [ABSTRACT FROM AUTHOR]

We investigated influences of the thermal environment on patterns of body temperature (Tb), activity, and use of burrows during the active season in a population of free-living arctic ground squirrels (Spermophilus parryii). Arctic ground squirrels normally exhibited a daily 5°C range in Tb, and had higher Tb when above ground than when in burrows (P < 0.0001). This difference decreased as standard operative temperature (Tes; an index of environmental heat load) increased. Ground squirrels entered burrows more frequently on warmer compared to average or cooler days and when Tb exceeded 39°C. On cool days with heavy precipitation, ground squirrels remained almost exclusively below ground, and peaks in Tb were associated with brief aboveground forays. Time on the surface was maximal (about 80% between 0500 and 2200 h) at Tes = 17-33°C and decreased proportionately with decreasing Tes from 17°C to -2°C. Forty-six percent of variation in timing of presence above ground could be explained by a series of thermal and nonthermal variables related to environmental heat transfer. This suggests that diurnal activity patterns in this arctic environment with 24-h daylight result from a strategy that minimizes thermoregulatory costs. [ABSTRACT FROM AUTHOR]