Food availability positively affects the survival and somatic maintenance of hibernating garden dormice (Eliomys quercinus).

BACKGROUND: Torpor is an energy saving strategy achieved by substantial reductions of metabolic rate and body temperature that enables animals to survive periods of low resource availability. During hibernation (multiday torpor), the frequency of periodic rewarming-characterised by high levels of oxidative stress-is associated with shortening of telomeres, a marker of somatic maintenance. OBJECTIVES: In this study, we determined the impact of ambient temperature on feeding behaviour and telomere dynamics in hibernating garden dormice (Eliomys quercinus) over winter. This obligate hibernator prepares for hibernation by accumulating fat stores but can also feed during hibernation. METHODOLOGY: Food intake, torpor pattern, changes in telomere length, and body mass change were assessed in animals housed at experimentally controlled temperatures of either 14 °C (i.e., a mild winter) or 3 °C (i.e., a cold winter) over 6 months. RESULTS: When hibernating at 14 °C, dormice experienced 1.7-fold more frequent and 2.4-fold longer inter-bout euthermia, and spent significantly less time torpid, compared to animals hibernating at 3 °C. Higher food intake enabled individuals to compensate for increased energetic costs when hibernating at milder temperatures (14 °C vs. 3 °C), to buffer body mass loss and thus increase winter survival. Interestingly, we observed a significant increase of telomere length over the entire hibernation period, irrespective of temperature treatment. CONCLUSION: We conclude that higher temperatures during winter, if associated with sufficient food availability, can have a positive effect on the individual's energy balance and somatic maintenance. These results suggest that winter food availability might be a crucial determinant for the survival of the garden dormouse in the context of ever-increasing environmental temperatures.

Body mass is associated with hibernation length, body temperature, and heart rate in free-ranging brown bears.

BACKGROUND: Despite centuries of research, debate remains on the scaling of metabolic rate to mass especially for intraspecific cases. The high variation of body mass within brown bears presents a unique opportunity to study the intraspecific effects of body mass on physiological variables. The amplitude of metabolic rate reduction in hibernators is dependent on body mass of the species. Small hibernators have high metabolic rates when euthermic but experience a drastic decrease in body temperature during torpor, which is necessary to reach a very low metabolic rate. Conversely, large hibernators, such as the brown bear (Ursus arctos), show a moderate decrease in temperature during hibernation, thought to be related to the bear's large size. We studied body mass, abdominal body temperature, heart rate, and accelerometer-derived activity from 63 free-ranging brown bears (1-15 years old, 15-233 kg). We tested for relationships between body mass and body temperature, heart rate, and hibernation duration. RESULTS: The smallest individuals maintained lower body temperatures during hibernation, hibernated longer, and ended hibernation later than large bears. Unlike body temperature, winter heart rates were not associated with body mass. In summer, the opposite pattern was found, with smaller individuals having higher body temperature and daytime heart rates. Body mass was associated with body temperature in the winter hypometabolic state, even in a large hibernating mammal. Smaller bears, which are known to have higher thermal conductance, reached lower body temperatures during hibernation. During summer, smaller bears had higher body temperatures and daytime heart rates, a phenomenon not previously documented within a single mammalian species. CONCLUSION: We conclude that the smallest bears hibernated more deeply and longer than large bears, likely from a combined effect of basic thermodynamics, the higher need for energy savings, and a lower cost of warming up a smaller body.

An hourglass mechanism controls torpor bout length in hibernating garden dormice.

Hibernating mammals drastically lower their rate of oxygen consumption and body temperature (Tb) for several weeks, but regularly rewarm and stay euthermic for brief periods (<30 h). It has been hypothesized that these periodic arousals are driven by the development of a metabolic imbalance during torpor; that is, the accumulation or the depletion of metabolites or the accrual of cellular damage that can be eliminated only in the euthermic state. We obtained oxygen consumption (as a proxy of metabolic rate) and Tb at 7 min intervals over entire torpor-arousal cycles in the garden dormouse (Eliomys quercinus). Torpor bout duration was highly dependent on mean oxygen consumption during the torpor bout. Oxygen consumption during torpor, in turn, was elevated by Tb, which fluctuated only slightly in dormice kept at ∼3-8°C. This corresponds to a well-known effect of higher Tb on shortening torpor bout lengths in hibernators. Arousal duration was independent from prior torpor length, but arousal mean oxygen consumption increased with prior torpor Tb. These results, particularly the effect of torpor oxygen consumption on torpor bout length, point to an hourglass mechanism of torpor control, i.e. the correction of a metabolic imbalance during arousal. This conclusion is in line with previous comparative studies providing evidence for significant interspecific inverse relationships between the duration of torpor bouts and metabolism in torpor. Thus, a simple hourglass mechanism is sufficient to explain torpor/arousal cycles, without the need to involve non-temperature-compensated circadian rhythms.

MicroRNAs facilitate skeletal muscle maintenance and metabolic suppression in hibernating brown bears.

Hibernating brown bears, Ursus arctos, undergo extended periods of inactivity and yet these large hibernators are resilient to muscle disuse atrophy. Physiological characteristics associated with atrophy resistance in bear muscle have been examined (e.g., muscle mechanics, neural activity) but roles for molecular signaling/regulatory mechanisms in the resistance to muscle wasting in bears still require investigation. Using quantitative reverse transcription PCR (RT-qPCR), the present study characterized the responses of 36 microRNAs linked with development, metabolism, and regeneration of skeletal muscle, in the vastus lateralis of brown bears comparing winter hibernating and summer active animals. Relative levels of mRNA of selected genes (mef2a, pax7, id2, prkaa1, and mstn) implicated upstream and downstream of the microRNAs were examined. Results indicated that hibernation elicited a myogenic microRNA, or "myomiR", response via MEF2A-mediated signaling. Upregulation of MEF2A-controlled miR-1 and miR-206 and respective downregulation of pax7 and id2 mRNA are suggestive of responses that promote skeletal muscle maintenance. Increased levels of metabolic microRNAs, such as miR-27, miR-29, and miR-33, may facilitate metabolic suppression during hibernation via mechanisms that decrease glucose uptake and fatty acid oxidation. This study identified myomiR-mediated mechanisms for the promotion of muscle regeneration, suppression of ubiquitin ligases, and resistance to muscle atrophy during hibernation mediated by observed increases in miR-206, miR-221, miR-31, miR-23a, and miR-29b. This was further supported by the downregulation of myomiRs associated with a muscle injury and inflammation (miR-199a and miR-223) during hibernation. The present study provides evidence of myomiR-mediated signaling pathways that are activated during hibernation to maintain skeletal muscle functionality in brown bears.

Always a price to pay: hibernation at low temperatures comes with a trade-off between energy savings and telomere damage.

We experimentally tested the costs of deep torpor at low temperatures by comparing telomere dynamics in two species of rodents hibernating at either 3 or 14°C. Our data show that hibernators kept at the warmer temperature had higher arousal frequencies, but maintained longer telomeres than individuals hibernating at the colder temperature. We suggest that the high-energy demand of frequent arousals is counteracted by a lower temperature differential between torpid and euthermic body temperature and that telomere length is restored during arousals when the body temperature is returned to normothermic values. Taken together, our study shows that hibernation at low body temperatures comes with costs on a cellular level and that hibernators need to actively counterbalance the shortening of telomeres.

Seasonal changes in eicosanoid metabolism in the brown bear.

Polyunsaturated fatty acids (PUFAs) exert several important functions across organ systems. During winter, hibernators divert PUFAs from oxidation, retaining them in their tissues and membranes, to ensure proper body functions at low body temperature. PUFAs are also precursors of eicosanoids with pro- and anti-inflammatory properties. This study investigated seasonal changes in eicosanoid metabolism of free-ranging brown bears (Ursus arctos). By using a lipidomic approach, we assessed (1) levels of specific omega-3 and omega-6 fatty acids involved in the eicosanoid cascade and (2) concentrations of eicosanoids in skeletal muscle and blood plasma of winter hibernating and summer active bears. We observed significant seasonal changes in the specific omega-3 and omega-6 precursors. We also found significant seasonal alterations of eicosanoid levels in both tissues. Concentrations of pro-inflammatory eicosanoids, such as thromboxane B2, 5-hydroxyeicosatetraenoic acid (HETE), and 15-HETE and 18-HETE, were significantly lower in muscle and/or plasma of hibernating bears compared to summer-active animals. Further, plasma and muscle levels of 5,6-epoxyeicosatrienoic acid (EET), as well as muscle concentration of 8,9-EET, tended to be lower in bears during winter hibernation vs. summer. We also found lower plasma levels of anti-inflammatory eicosanoids, such as 15dPGJ2 and PGE3, in bears during winter hibernation. Despite of the limited changes in omega-3 and omega-6 precursors, plasma and muscle concentrations of the products of all pathways decreased significantly, or remained unchanged, independent of their pro- or anti-inflammatory properties. These findings suggest that hibernation in bears is associated with a depressed state of the eicosanoid cascade.

Ecophysiology of omega Fatty acids: a lid for every jar.

Omega fatty acids affect various physiological functions, such as locomotion, cardiac function, and thermogenesis. We highlight evidence from animal models that points to pathways by which specific omega fatty acids exert differential effects. We suggest that optimizing the omega fatty acid composition of tissues involves trade-offs between costs and benefits of specific fatty acids.

Late-born intermittently fasted juvenile garden dormice use torpor to grow and fatten prior to hibernation: consequences for ageing processes.

Torpor is thought to slow age-related processes and to sustain growth and fattening of young individuals. Energy allocation into these processes represents a challenge for juveniles, especially for those born late in the season. We tested the hypothesis that late-born juvenile garden dormice (Eliomys quercinus) fed ad libitum ('AL', n = 9) or intermittently fasted ('IF', n = 9) use short torpor bouts to enhance growth and fat accumulation to survive winter. IF juveniles displayed more frequent and longer torpor bouts, compared with AL individuals before hibernation. Torpor frequency correlated negatively with energy expenditure and water turnover. Hence, IF juveniles gained mass at the same rate, reached similar pre-hibernation fattening and displayed identical hibernating patterns and mass losses as AL animals. We found no group differences in relative telomere length (RTL), an indicator of ageing, during the period of highest summer mass gain, despite greater torpor use by IF juveniles. Percentage change in RTL was negatively associated with mean and total euthermic durations among all individuals during hibernation. We conclude that torpor use promotes fattening in late-born juvenile dormice prior to hibernation. Furthermore, we provided the first evidence for a functional link between time spent in euthermy and ageing processes over winter.

Evolutionary trade-offs in dormancy phenology.

Seasonal animal dormancy is widely interpreted as a physiological response for surviving energetic challenges during the harshest times of the year (the physiological constraint hypothesis). However, there are other mutually non-exclusive hypotheses to explain the timing of animal dormancy, that is, entry into and emergence from hibernation (i.e. dormancy phenology). Survival advantages of dormancy that have been proposed are reduced risks of predation and competition (the 'life-history' hypothesis), but comparative tests across animal species are few. Using the phylogenetic comparative method applied to more than 20 hibernating mammalian species, we found support for both hypotheses as explanations for the phenology of dormancy. In accordance with the life-history hypotheses, sex differences in hibernation emergence and immergence were favored by the sex difference in reproductive effort. In addition, physiological constraint may influence the trade-off between survival and reproduction such that low temperatures and precipitation, as well as smaller body mass, influence sex differences in phenology. We also compiled initial evidence that ectotherm dormancy may be (1) less temperature dependent than previously thought and (2) associated with trade-offs consistent with the life-history hypothesis. Thus, dormancy during non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously thought.

Comparative transcriptomics of the garden dormouse hypothalamus during hibernation.

Torpor or heterothermy is an energy-saving mechanism used by endotherms to overcome harsh environmental conditions. During winter, the garden dormouse (Eliomys quercinus) hibernates with multiday torpor bouts and body temperatures of a few degrees Celsius, interrupted by brief euthermic phases. This study investigates gene expression within the hypothalamus, the key brain area controlling energy balance, adding information on differential gene expression potentially relevant to orchestrate torpor. A de novo assembled transcriptome of the hypothalamus was generated from garden dormice hibernating under constant darkness without food and water at 5 °C. Samples were collected during early torpor, late torpor, and interbout arousal. During early torpor, 765 genes were differentially expressed as compared with interbout arousal. Twenty-seven pathways were over-represented, including pathways related to hemostasis, extracellular matrix organization, and signaling of small molecules. Only 82 genes were found to be differentially expressed between early and late torpor, and no pathways were over-represented. During late torpor, 924 genes were differentially expressed relative to interbout arousal. Despite the high number of differentially expressed genes, only 10 pathways were over-represented. Of these, eight were also observed to be over-represented when comparing early torpor and interbout arousal. Our results are largely consistent with previous findings in other heterotherms. The addition of a transcriptome of a novel species may help to identify species-specific and overarching torpor mechanisms through future species comparisons.

Remodelling of Skeletal Muscle Myosin Metabolic States in Hibernating Mammals.

Hibernation is a period of metabolic suppression utilized by many small and large mammal species to survive during winter periods. As the underlying cellular and molecular mechanisms remain incompletely understood, our study aimed to determine whether skeletal muscle myosin and its metabolic efficiency undergo alterations during hibernation to optimize energy utilization. We isolated muscle fibers from small hibernators, Ictidomys tridecemlineatus and Eliomys quercinus and larger hibernators, Ursus arctos and Ursus americanus. We then conducted loaded Mant-ATP chase experiments alongside X-ray diffraction to measure resting myosin dynamics and its ATP demand. In parallel, we performed multiple proteomics analyses. Our results showed a preservation of myosin structure in U. arctos and U. americanus during hibernation, whilst in I. tridecemlineatus and E. quercinus, changes in myosin metabolic states during torpor unexpectedly led to higher levels in energy expenditure of type II, fast-twitch muscle fibers at ambient lab temperatures (20°C). Upon repeating loaded Mant-ATP chase experiments at 8°C (near the body temperature of torpid animals), we found that myosin ATP consumption in type II muscle fibers was reduced by 77-107% during torpor compared to active periods. Additionally, we observed Myh2 hyper-phosphorylation during torpor in I. tridecemilineatus, which was predicted to stabilize the myosin molecule. This may act as a potential molecular mechanism mitigating myosin-associated increases in skeletal muscle energy expenditure during periods of torpor in response to cold exposure. Altogether, we demonstrate that resting myosin is altered in hibernating mammals, contributing to significant changes to the ATP consumption of skeletal muscle. Additionally, we observe that it is further altered in response to cold exposure and highlight myosin as a potentially contributor to skeletal muscle non-shivering thermogenesis.

Remodeling of skeletal muscle myosin metabolic states in hibernating mammals.

Hibernation is a period of metabolic suppression utilized by many small and large mammal species to survive during winter periods. As the underlying cellular and molecular mechanisms remain incompletely understood, our study aimed to determine whether skeletal muscle myosin and its metabolic efficiency undergo alterations during hibernation to optimize energy utilization. We isolated muscle fibers from small hibernators, Ictidomys tridecemlineatus and Eliomys quercinus and larger hibernators, Ursus arctos and Ursus americanus. We then conducted loaded Mant-ATP chase experiments alongside X-ray diffraction to measure resting myosin dynamics and its ATP demand. In parallel, we performed multiple proteomics analyses. Our results showed a preservation of myosin structure in U. arctos and U. americanus during hibernation, whilst in I. tridecemlineatus and E. quercinus, changes in myosin metabolic states during torpor unexpectedly led to higher levels in energy expenditure of type II, fast-twitch muscle fibers at ambient lab temperatures (20 °C). Upon repeating loaded Mant-ATP chase experiments at 8 °C (near the body temperature of torpid animals), we found that myosin ATP consumption in type II muscle fibers was reduced by 77-107% during torpor compared to active periods. Additionally, we observed Myh2 hyper-phosphorylation during torpor in I. tridecemilineatus, which was predicted to stabilize the myosin molecule. This may act as a potential molecular mechanism mitigating myosin-associated increases in skeletal muscle energy expenditure during periods of torpor in response to cold exposure. Altogether, we demonstrate that resting myosin is altered in hibernating mammals, contributing to significant changes to the ATP consumption of skeletal muscle. Additionally, we observe that it is further altered in response to cold exposure and highlight myosin as a potentially contributor to skeletal muscle non-shivering thermogenesis.

Many animals use hibernation as a tactic to survive harsh winters. During this dormant, inactive state, animals reduce or limit body processes, such as heart rate and body temperature, to minimise their energy use. To conserve energy during hibernation, animals can use different approaches. For example, garden dormice undergo periodic states of extremely low core temperatures (down to 4­8^oC); whereas Eurasian brown bears see milder temperature drops (down to 23­25^oC). An important organ that changes during hibernation is skeletal muscle. Skeletal muscle typically uses large amounts of energy, making up around 50% of body mass. To survive, hibernating animals must change how their skeletal muscle uses energy. Traditionally, active myosin ­ a protein found in muscles that helps muscles to contract ­ was thought to be responsible for most of the energy use by skeletal muscle. But, more recently, resting myosin has also been found to use energy when muscles are relaxed. Lewis et al. studied myosin and skeletal muscle energy use changes during hibernation and whether they could impact the metabolism of hibernating animals. Lewis et al. assessed myosin changes in muscle samples from squirrels, dormice and bears during hibernation and during activity. Experiments showed changes in resting myosin in squirrels and dormice (whose temperature drops to 4­8^oC during hibernation) but not in bears. Further analysis revealed that cooling samples from non-hibernating muscle to 4­8^oC increased energy use in resting myosin, thereby generating heat. However, no increase in energy use was found after cooling hibernating muscle samples to 4­8^oC. This suggest that resting myosin generates heat at cool temperatures ­ a mechanism that is switched off in hibernating animals to allow them to cool their body temperature. These findings reveal key insights into how animals conserve energy during hibernation. In addition, the results show that myosin regulates energy use in skeletal muscles, which indicates myosin may be a potential drug target in metabolic diseases, such as obesity.

Seasonal variation in telomerase activity and telomere dynamics in a hibernating rodent, the garden dormouse (Eliomys quercinus).

Telomere dynamics in hibernating species are known to reflect seasonal changes in somatic maintenance. Throughout hibernation, the periodic states of rewarming, known as inter-bout euthermia or arousals, are associated with high metabolic costs including shortening of telomeres. In the active season, if high energetic resources are available, telomere length can be restored in preparation for the upcoming winter. The mechanism for telomere elongation has not been clearly demonstrated, although the action of the ribonucleoprotein complex, telomerase, has been implicated in many species. Here we tested for levels of telomerase activity in the garden dormouse (Eliomys quercinus) at different seasonal time points throughout the year and across ages from liver tissues of male juveniles to adults. We found that telomerase is active at high levels across seasons (during torpor and inter-bout euthermia, plus in the active season) but that there was a substantial decrease in activity in the month prior to hibernation. Telomerase levels were consistent across age groups and were independent of feeding regime and time of birth (early or late born). The changes in activity levels that we detected were broadly associated with changes in telomere lengths measured in the same tissues. We hypothesise that i) telomerase is the mechanism used by garden dormice for maintenance of telomeres and that ii) activity is kept at high levels throughout the year until pre-hibernation when resources are diverted to increasing fat reserves for overwintering. We found no evidence for a decrease in telomerase activity with age or a final increase in telomere length which has been detected in other hibernating rodents.

Elevated plasma phospholipid n-3 docosapentaenoic acid concentrations during hibernation.

Factors for initiating hibernation are unknown, but the condition shares some metabolic similarities with consciousness/sleep, which has been associated with n-3 fatty acids in humans. We investigated plasma phospholipid fatty acid profiles during hibernation and summer in free-ranging brown bears (Ursus arctos) and in captive garden dormice (Eliomys quercinus) contrasting in their hibernation patterns. The dormice received three different dietary fatty acid concentrations of linoleic acid (LA) (19%, 36% and 53%), with correspondingly decreased alpha-linolenic acid (ALA) (32%, 17% and 1.4%). Saturated and monounsaturated fatty acids showed small differences between summer and hibernation in both species. The dormice diet influenced n-6 fatty acids and eicosapentaenoic acid (EPA) concentrations in plasma phospholipids. Consistent differences between summer and hibernation in bears and dormice were decreased ALA and EPA and marked increase of n-3 docosapentaenoic acid and a minor increase of docosahexaenoic acid in parallel with several hundred percent increase of the activity index of elongase ELOVL2 transforming C20-22 fatty acids. The highest LA supply was unexpectantly associated with the highest transformation of the n-3 fatty acids. Similar fatty acid patterns in two contrasting hibernating species indicates a link to the hibernation phenotype and requires further studies in relation to consciousness and metabolism.

Temperature Effects on DNA Damage during Hibernation.

AbstractDuring multiday torpor, deep-hibernating mammals maintain a hypometabolic state where heart rate and ventilation are reduced to 2%-4% of euthermic rates. It is hypothesized that this ischemia-like condition may cause DNA damage through reactive oxygen species production. The reason for intermittent rewarming (arousal) during hibernation might be to repair the accumulated DNA damage. Because increasing ambient temperatures (Ta's) shortens torpor bout duration, we hypothesize that hibernating at higher Ta's will result in a faster accumulation of genomic DNA damage. To test this, we kept 39 male and female garden dormice at a Ta of either 5°C or 10°C and obtained tissue at 1, 4, and 8 d in torpor to assess DNA damage and recruitment of DNA repair markers in splenocytes. DNA damage in splenocytes measured by comet assay was significantly higher in almost all torpor groups than in summer euthermic groups. Damage accumulates in the first days of torpor at Ta=5°C (between days 1 and 4) but not at Ta=10°C. At the higher Ta, DNA damage is high at 24 h in torpor, indicating either a faster buildup of DNA damage at higher Ta's or an incomplete repair during arousals in dormice. At 5°C, recruitment of the DNA repair protein 53BP1 paralleled the increase in DNA damage over time during torpor. In contrast, after 1 d in torpor at 10°C, DNA damage levels were high, but 53BP1 was not recruited to the nuclear DNA yet. The data suggest a potential mismatch in the DNA damage/repair dynamics during torpor at higher Ta's.

The cost of research: Lasting effects of capture, surgery and muscle biopsy on brown bear (Ursus arctos) movement and physiology.

Animal models are a key component of translational medicine, helping transfer scientific findings into practical applications for human health. A fundamental principle of research ethics involves weighing the benefits of the research to society against the burden imposed on the animals used for scientific purposes. The utilisation of wild animals for research requires evaluation of the effects of capture and invasive sampling. Determining the severity and duration of these interventions on the animal's physiology and behaviour allows for refining study methodology and for excluding or accounting for biased data. In this study, 39 Scandinavian brown bears (Ursus arctos) captured either while hibernating in winter or via helicopter in summer and that underwent surgery as part of a human health project had their movement, body temperature and timing of onset of hibernation compared with those of 14 control bears that had not been captured during the same period. Bears captured in winter and summer showed decreased movement from den exit until late summer, compared to those in the control group. Bears captured in summer showed reduced movement and body temperature for at least, respectively, 14 and 3 days, with an 11% decrease in hourly distance, compared to pre-capture levels, but did not differ in the timing of hibernation onset. We reveal that brown bear behaviour and physiology can be altered in response to capture and surgery for days to months, post-capture. This has broad implications for the conclusions of wildlife studies that rely upon invasive sampling.

Hypothesis and Theory: A Two-Process Model of Torpor-Arousal Regulation in Hibernators.

Hibernating mammals drastically lower their metabolic rate (MR) and body temperature (Tb) for up to several weeks, but regularly rewarm and stay euthermic for brief periods. It has been hypothesized that the necessity for rewarming is due to the accumulation or depletion of metabolites, or the accrual of cellular damage that can be eliminated only in the euthermic state. Recent evidence for significant inverse relationships between the duration of torpor bouts (TBD) and MR in torpor strongly supports this hypothesis. We developed a new mathematical model that simulates hibernation patterns. The model involves an hourglass process H (Hibernation) representing the depletion/accumulation of a crucial enzyme/metabolite, and a threshold process Hthr. Arousal, modelled as a logistic process, is initiated once the exponentially declining process H reaches Hthr. We show that this model can predict several phenomena observed in hibernating mammals, namely the linear relationship between TMR and TBD, effects of ambient temperature on TBD, the modulation of torpor depth and duration within the hibernation season, (if process Hthr undergoes seasonal changes). The model does not need but allows for circadian cycles in the threshold T, which lead to arousals occurring predominantly at certain circadian phases, another phenomenon that has been observed in certain hibernators. It does not however, require circadian rhythms in Tb or MR during torpor. We argue that a two-process regulation of torpor-arousal cycles has several adaptive advantages, such as an easy adjustment of TBD to environmental conditions as well as to energy reserves and, for species that continue to forage, entrainment to the light-dark cycle.

Sticking Together: Energetic Consequences of Huddling Behavior in Hibernating Juvenile Garden Dormice.

AbstractHibernation, or multiday torpor, allows individuals to save energy via substantial reductions of metabolism and body temperature but is regularly interrupted by euthermic phases called arousals. Social thermoregulation, or "huddling," can act in synergy with torpor in reducing individuals' energy and heat losses. In the wild, the garden dormouse (Eliomys quercinus) combines both strategies, which are crucial for winter survival of juveniles with limited prehibernation body fat reserves. We investigated via thermographic and temperature measurements (i) the energetic impact of huddling during an arousal from deep torpor, (ii) the dynamics of huddling behavior during hibernation, and (iii) its consequences during the entire winter in juvenile garden dormice. Thermographic images revealed a significant effect of huddling on torpor energetics, as it reduced heat exchange and mass loss by two-thirds in huddling versus single individuals during arousal. Our investigation of the dynamics of huddling further revealed a "random-like mechanistic" behavior during winter hibernation, as arousals from torpor were not always initiated by the same individuals. Animals took turns in initiating rewarming within a group, and the individual with highest body temperature during arousal entered into torpor later than the others within the huddle. The animals share both costs and benefits of huddling during arousals, without any energetic benefit of huddling over the entire winter on an individual level. We conclude that the dynamics of social thermoregulation during hibernation seems to counterbalance its benefit of reducing energetic costs associated against the energy-demanding process of rewarming from torpor.

Seasonality in Biological Rhythms in Scandinavian brown Bears.

Biological rhythms, such as rhythms in activity and body temperature, are usually highly synchronized and entrained by environmental conditions, such as photoperiod. However, how the expression of these rhythms changes during hibernation, when the perception of environmental cues is limited, has not yet been fully understood for all hibernators, especially in the wild. The brown bear (Ursus arctos) in Scandinavia lives in a highly seasonal environment and adapts to harsh winter conditions by exhibiting hibernation, characterized by reduced metabolism and activity. In this study, we aimed to explore the expression of biological rhythms in activity, body temperature and heart rate of free-ranging brown bears over the annual cycle, including active, hibernation and the transition states around den entry and exit. We found that rhythms in physiology and activity are mostly synchronized and entrained by the light-dark cycle during the bears' active state with predominantly diel and ultradian rhythms for body temperature, activity and heart rate. However, during hibernation, rhythms in body temperature and heart rate were considerably slowed down to infradian rhythms, influenced by the amount of snow in the denning area, whereas rhythms in activity remained diel. Rhythms in the transition states when bears prepared for entering or coming out of hibernation state displayed a combination of infradian and diel rhythms, indicating the preparation of the body for the change in environmental conditions. These results reveal that brown bears adjust their biological rhythms to the seasonal environment they inhabit. Rhythms in physiology and activity show simultaneity during the active state but are partly disconnected from each other during hibernation, when bears are most sheltered from the environment.

Dynamic Function and Composition Shift in Circulating Innate Immune Cells in Hibernating Garden Dormice.

Hibernation is characterized by successive torpor bouts during which metabolic rate is down-regulated to 2-4% of euthermic levels along with core body temperatures (T b ) ranging between 0 and 10°C. One characteristic of the torpid state, which is periodically interrupted by a few hours of euthermic phases or arousals during hibernation, resides in an overall impairment of the immune system. The most striking change during torpor is the reduction of circulating white blood cells up to 90%, while their numbers rise to near summer euthermic level upon rewarming. However, potential changes in responsiveness and function of neutrophil granulocytes, accounting for the primary cellular innate immune defense, are unknown. Here we present the first data on shifts in oxidative burst capacity, i.e., the ability to produce reactive oxygen species (ROS), of neutrophils during hibernation. Using a chemiluminescence assay, we measured real-time ROS production in whole blood of hibernating garden dormice (Eliomys quercinus) in early or late torpor, and upon arousals. Accounting for changes in neutrophil numbers along the torpor-arousal cycle, we found significant differences, between torpid and euthermic states, in the neutrophil oxidative burst capacity (NOC), with shallow cell responses during torpor and a highly significant increase by up to 30-fold during arousals. Further, we observed a significant reduction of NOC from aroused animals with euthermic T b of 36.95 ± 0.37°C, when tested at 6°C, whereas no change occurred in NOC from torpid individuals reaching constant T b of 4.67 ± 0.42°C, when measured at 35°C. This dynamic indicates that the reduction in NOC during torpor may be temperature-compensated. These results linked to the understanding of immune function during the torpor-arousal cycle might have clinical relevance in the context of therapeutic hypothermia and reperfusion injury.