Hibernation and Circadian Rhythms of Body Temperature in Free-Living Arctic Ground Squirrels

Abstract

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.

CSF in Alzheimer's Disease

“…There are at present only three conditions in addition to AD in which elevated CSF P-tau has been reported: (i) in term and preterm newborns, possibly reflecting physiological tau phosphorylation in brain development [87], (ii) in herpes encephalitis [88], and (iii) in superficial CNS siderosis [89,90]. Clearly, these conditions are no important differential diagnoses to AD, but they may shed light on mechanisms behind CSF P-tau increase (as may the data on tau phosphorylation in hibernating squirrels [91] and hamsters [92]). …”

Persistence, Entrainment, and Function of Circadian Rhythms in Polar Vertebrates

“…In freeliving arctic ground squirrels, body temperature rhythms are abolished during hibernation, and the preemergent euthermic intervals of reproductive males in spring, but daily body temperature rhythms are reestablished coincident with exposure to light and emergence from the hibernacula, and these rhythms persist under the midnight sun (FIGURE 2; Refs. 133,134). Interestingly, body temperature rhythms of male arctic ground squirrels free run (Ͼ24 h) in individuals that are sequestered below ground but euthermic before initiating torpor in autumn (133) strongly suggest that the body temperature rhythm is endogenously driven and not simply a result of masking.…”

Animal activity around the clock with no overt circadian rhythms: patterns, mechanisms and adaptive value

“…The adaptive significance of post-hibernation arrhythmicity in ground squirrels that remain in constant conditions may be maximizing rates of fat deposition in spring in recovery from winter periods of low food availability [20]. The lack of circadian timing in behaviour in winter and summer has been also interpreted as a cost reduction in the absence of circadian predation pressure or need for synchronized social activity.…”