Hibernation Phase Research Articles

Jeff Koons’ Moon Odyssey: The Return of Post-Minimal Sculpture

The two-year epic of sending a sculpture by Jeff Koons to the Moon on an expedition aboard the Odysseus spaceship is almost over: the module with the world’s first authorized public sculpture on the surface of another extraterrestrial body of the Solar System made a landing but overturned, entering the phase of hibernation. It is not yet clear whether and when it will be possible to directly land the sculpture on the surface of the Earth’s satellite. However, despite the obstacles of its inhospitable field conditions, the project has, in addition to the orbital conquistador mission, two other functional parts that are quite effective: ground collection consisting of approximated elements of the object, which are advertised in media, and digital NFT site-specific sculpture. Expert analysis of the available information allowed us to state another recycling of the sculptural ideas of the 1960s–1970s, known in art history under the definition of “postminimalism” or “systemic art,” which were associated with the “Art in Public Place” programs of the National Council on Arts and the NASA-compatible program “Space City. USA.” Therefore, a much talked-about breakthrough into new horizons of quasi-modern plastic art has not taken place.

Liver transcriptomic and methylomic analyses identify transcriptional mitogen-activated protein kinase regulation in facultative hibernation of Syrian hamster.

Hibernation consists of alternating torpor-arousal phases, during which animals cope with repetitive hypothermia and ischaemia-reperfusion. Due to limited transcriptomic and methylomic information for facultative hibernators, we here conducted RNA and whole-genome bisulfide sequencing in liver of hibernating Syrian hamster (Mesocricetus auratus). Gene ontology analysis was performed on 844 differentially expressed genes and confirmed the shift in metabolic fuel utilization, inhibition of RNA transcription and cell cycle regulation as found in seasonal hibernators. Additionally, we showed a so far unreported suppression of mitogen-activated protein kinase (MAPK) and protein phosphatase 1 pathways during torpor. Notably, hibernating hamsters showed upregulation of MAPK inhibitors (dual-specificity phosphatases and sproutys) and reduced levels of MAPK-induced transcription factors (TFs). Promoter methylation was found to modulate the expression of genes targeted by these TFs. In conclusion, we document gene regulation between hibernation phases, which may aid the identification of pathways and targets to prevent organ damage in transplantation or ischaemia-reperfusion.

See you in spring: overwinter survival is higher than post summer in the Alpine marmot

Animal species living in highly seasonal environments developed different strategies to cope with the periodical drastic change of environmental conditions. Hibernating mammals survive the winter season by reducing their activity and metabolism, and by centring their activities during the favourable season. Thus, the demography of these species depends upon both hibernating and active periods. In this study, we explored the apparent survival of Alpine marmots monitored between 2007 and 2018 in the North-Western Italian Alps. We fit Cormack-Jolly-Seber (CJS) models to quantify changes in the apparent survival and capture probability of marmots, after the hibernation phase and at the end of the summer. Apparent survival after winter almost reached 100% and it was higher than post-summer survival. Moreover, while post-summer apparent survival remained almost stable over lifetime, with a slight increase with age, characterised however by wide confidence intervals, overwinter survival decreased with age, especially after 6 years of age. No temporal trends, nor changes between areas at different elevations, were found. We suggest that these results arise from a combination of climatic conditions, predation pressure and social dynamics, which is a pivotal feature of this species and cannot be ignored when considering population dynamics of Alpine marmots.

The importance of assessing the population structure and biology of psylla species for pest monitoring and management in pear orchards

The species of pear sucker, also called pear psylla or pear psyllid, which are destructive insects belonging to the genus Cacopsylla (Hemiptera: Psyllidae) cause substantial damage to pear tree plantations. Two consecutive years of research were conducted on the population structure and biological cycle of psylla in a small, elderly, and unmaintained pear orchard in northern Romania. Of the two identified species (C. pyri L. and C. pyricola Forster), C. pyri dominated the psylla population with a percentage of 77.8-80.1%. Adults of both species emerge from hibernation in the first part of March and produce three generations per year. First-generation adults emerge in the first decade of June, the second generation in the last decade of July, and the third generation in the first decade of September as they enter the hibernation phase. Larvae of the first generation appear in the second half of April, the second generation at the end of June - the beginning of July, and the third generation in the second decade of August. There have been reports of up to 11 adults and 27 larvae per leaf, 9 larvae per petiole, and 14 larvae per fruit. The number of adults captured on yellow sticky traps exposed on the southern side of the tree crown was significantly higher compared to the northern side. Following the intense attack, the well-known cv. ‘Williams’ was heavily affected, pear trees were badly defoliated, blackened, and aged prematurely. The climatic conditions of the two years did not influence the phenology of the pests, but the importance of monitoring psyllids is widely argued, considering that it remains the key to integrated protection programs in pear orchards.

Hypothalamic orexinergic neuron changes during the hibernation of the Syrian hamster

Hibernation in small mammals is a highly regulated process with periods of torpor involving drops in body temperature and metabolic rate, as well as a general decrease in neural activity, all of which proceed alongside complex brain adaptive changes that appear to protect the brain from extreme hypoxia and low temperatures. All these changes are rapidly reversed, with no apparent brain damage occurring, during the short periods of arousal, interspersed during torpor—characterized by transitory and partial rewarming and activity, including sleep activation, and feeding in some species. The orexins are neuropeptides synthesized in hypothalamic neurons that project to multiple brain regions and are known to participate in the regulation of a variety of processes including feeding behavior, the sleep-wake cycle, and autonomic functions such as brown adipose tissue thermogenesis. Using multiple immunohistochemical techniques and quantitative analysis, we have characterized the orexinergic system in the brain of the Syrian hamster—a facultative hibernator. Our results revealed that orexinergic neurons in this species consisted of a neuronal population restricted to the lateral hypothalamic area, whereas orexinergic fibers distribute throughout the rostrocaudal extent of the brain, particularly innervating catecholaminergic and serotonergic neuronal populations. We characterized the changes of orexinergic cells in the different phases of hibernation based on the intensity of immunostaining for the neuronal activity marker C-Fos and orexin A (OXA). During torpor, we found an increase in C-Fos immunostaining intensity in orexinergic neurons, accompanied by a decrease in OXA immunostaining. These changes were accompanied by a volume reduction and a fragmentation of the Golgi apparatus (GA) as well as a decrease in the colocalization of OXA and the GA marker GM-130. Importantly, during arousal, C-Fos and OXA expression in orexinergic neurons was highest and the structural appearance and the volume of the GA along with the colocalization of OXA/GM-130 reverted to euthermic levels. We discuss the involvement of orexinergic cells in the regulation of mammalian hibernation and, in particular, the possibility that the high activation of orexinergic cells during the arousal stage guides the rewarming as well as the feeding and sleep behaviors characteristic of this phase.

The future of insolvency law in a post‐pandemic world

AbstractThe COVID‐19 crisis has encouraged many countries to amend their insolvency laws. In most cases, these amendments took place temporarily, especially during the hibernation phase of the pandemic. In other countries, however, the pandemic has led to permanent changes in their insolvency legislation. More importantly, the COVID‐19 crisis has accelerated the insolvency reforms that were already on the political agenda of many countries, and it has encouraged other jurisdictions to reassess the desirability of their insolvency and restructuring frameworks. This article analyses the current trends, reforms and policy discussions that are expected to reshape the future of insolvency law in a post‐pandemic world.

Variations in oxidative stress and antioxidant defense level during different phases of hibernation in common Asian toad, Duttaphrynus melanostictus.

ABSTRACTTo assess redox status during hibernation with metabolic depression, oxidative stress parameters and antioxidant defense were assessed during different phases of hibernation including active period, hibernation, arousal, and post-arousal period, in the liver and brain tissues of Duttaphrynus melanostictus. We hypothesized low levels of oxidative stress and antioxidant defense during the hibernation period in comparison to the summer active period, due to hypometabolism and their subsequent increase during the arousal period following an increase in body temperature and metabolism. Contrary to our hypothesis, increased oxidative stress with significantly higher lipid peroxidation, protein carbonylation, oxidized glutathione (GSSG): glutathione (GSH) ratio, and elevated antioxidants defense consisting of higher catalase activity and high ascorbic acid content to control oxidative stress were found during hibernation. However, GSH and uric acid levels were found low with super oxide dismutase (SOD) activities at a steady level during hibernation. Supporting our hypothesis, increased oxidative stress with high lipid peroxidation and GSSG:GSH ratio were found during arousal from hibernation owing to increased oxygen consumption and rewarming. Augmented catalase and SOD activities and nonenzymatic antioxidants (GSH, ascorbic acid, and uric acid) level were found to counteract oxidative stress during arousal periods as it was expected. A steady level of protein carbonylation, indicating no oxidative damage during arousal from hibernation due to elevated antioxidant defense, shows the significance of hibernation to overcome food and water scarcity and cold climatic condition. Decrease in antioxidants levels accompanying coming down of lipid peroxidation, protein carbonylation, and GSSG:GSH ratio to their lower levels during the post-arousal period showing normalcy in redox status as it was during active period indicates controllability of oxidative stress in hibernating toads.

Reversible thrombocytopenia during hibernation originates from storage and release of platelets in liver sinusoids

Immobility is a risk factor for thrombosis due to low blood flow, which may result in activation of the coagulation system, recruitment of platelets and clot formation. Nevertheless, hibernating animals—who endure lengthy periods of immobility—do not show signs of thrombosis throughout or after hibernation. One of the adaptations of hemostasis in hibernators consists of a rapidly reversible reduction of the number of circulating platelets during torpor, i.e., the hibernation phase with reduction of metabolic rate, low blood flow and immobility. It is unknown whether these platelet dynamics in hibernating hamsters originate from storage and release, as suggested for ground squirrel, or from breakdown and de novo synthesis. A reduction in detaching forces due to low blood flow can induce reversible adhesion of platelets to the vessel wall, which is called margination. Here, we hypothesized that storage-and-release by margination to the vessel wall induces reversible thrombocytopenia in torpor. Therefore, we transfused labeled platelets in hibernating Syrian hamster (Mesocricetus auratus) and platelets were analyzed using flow cytometry and electron microscopy. The half-life of labeled platelets was extended from 20 to 30 h in hibernating animals compared to non-hibernating control hamsters. More than 90% of labeled platelets were cleared from the circulation during torpor, followed by emergence during arousal which supports storage-and-release to govern thrombocytopenia in torpor. Furthermore, the low number of immature platelets, plasma level of interleukin-1α and normal numbers of megakaryocytes in bone marrow make platelet synthesis or megakaryocyte rupture via interleukin-1α unlikely to account for the recovery of platelet counts upon arousal. Finally, using large-scale electron microscopy we revealed platelets to accumulate in liver sinusoids, but not in spleen or lung, during torpor. These results thus demonstrate that platelet dynamics in hibernation are caused by storage and release of platelets, most likely by margination to the vessel wall in liver sinusoids. Translating the molecular mechanisms that govern platelet retention in the liver, may be of major relevance for hemostatic management in (accidental) hypothermia and for the development of novel anti-thrombotic strategies.

Влияние повышения температуры зимних месяцев на сроки основных фенологических фаз жизнедеятельности Spermophilus pygmaeus Pallas, 1778 на территории Прикаспийской низменности (Астраханская область)

This article provides comparison analysis between 1950–1959’s and 2000–2009’s periods for duration and schedule dates of awakening, spreading and hibernation phases for Spermophilus pygmaeus populations located in the Caspian Depression (by example of Astrakhan Oblast). We learned that within a period of maximum temperature rise in February 2000–2009’s durations of S.pygmaeus’ populations of every phonological phase have shortened and been rescheduled with earlier dates (in comparison to 1950–1959’s). The study also shows that these changes caused its population reduction on the Caspian Depression’s territory.

The Future of Insolvency Law in a Post-Pandemic World

The COVID-19 crisis has encouraged many countries to amend their insolvency laws. In most cases, these amendments took place temporarily – especially during the hibernation phase of the pandemic. In other countries, however, the pandemic has led to permanent changes in the insolvency legislation. More importantly, the COVID-19 crisis has accelerated the insolvency reforms already existing in the political agenda of many countries, and it has encouraged other jurisdictions to reassess the desirability of their insolvency and restructuring frameworks. This article analyzes the current trends, reforms and policy discussions that are expected to reshape the future of insolvency law in a post-pandemic world.

Development of circularity indicators based on the in-use occupation of materials

Circularity can help to prevent depletion of the earth’s limited material resources, which are vital for human’s modern society. Circular economy strategies of slowing and closing loops of resources have the ultimate goal of keeping materials useful (i.e. in-use) while avoiding losses (dissipation) and hibernation of materials; therefore, appropriate indicators that can measure these aspects are necessary. We propose a measurement of the circularity of materials by quantifying their in-use occupation, that is, the maintenance of materials in a useful state in products for as long as possible, avoiding dissipation or hibernation. Specifically, two indicators were developed: in-use occupation ratio (UOR) and final retention in society (FRS). These indicators were applied in two case studies (materials in a laptop and wood products) with three scenarios each (linear, reuse, and recycling). The reuse scenarios generally presented a higher UOR (41–48% for laptop materials and 53% for wood) compared to recycling scenarios (29–45% for laptop materials and 52% for wood). Only two scenarios of wood products resulted in retaining materials for the next generation (FRS > 0%). We argue that the differentiation between supply, in-use, and hibernation phases is essential for a circular economy. The occupation of materials is shown in charts to facilitate understanding by non-experts and can provide a scientific basis for policies supporting technologies or products with increased in-use occupation and retention of materials.

An investigation of hibernating members from the Culex pipiens complex (Diptera, Culicidae) in subterranean habitats of central Germany

The Culex pipiens complex encompasses five species and subspecies of the genus Culex. Over time, a multitude of morphologically indistinguishable species has been assigned to this complex with several species being classified as important vectors for different diseases. Some species of this complex hibernate in subterranean habitats, and it has been proven that viruses can survive this phase of hibernation. However, studies focusing on the environmental requirements, ecology and spatial and temporal distribution patterns of mosquitos in underground habitats are sparse. Here, we investigate the main environmental factors and dependencies of Culex, considering the number of individuals and survival probabilities in underground habitats during the winter months. Methods. Since the State of Hesse, Germany harbors about 3500 to 4000 subterranean shelters ample availability of subterranean habitats there provides a good opportunity to conduct detailed investigations of the Culex pipiens complex. In this study, we identified a sample of 727 specimens of overwintering females within the Culex pipiens complex from 52 different underground sites collected over a period of 23 years using qPCR. A complete data set of samplings of hibernating mosquitos from 698 subterranean habitats in Central Germany over the same period was available to study the spatial and temporal patterns and the effect of temperature and precipitation conditions on these hibernating populations using a generalized linear model (GLM). Results. Our qPCR-results show, similar to aboveground studies of mosquitos, that Culex pipiens pipiens and Culex torrentium occur sympatrically. On the other hand, Culex pipiens molestus occurred very rarely. The GLM revealed no shifts in species composition over time, but different preferences for subterranean hibernacula, chemical effects on overwintering populations as well as effects of annual and seasonal mean temperature and precipitation during the active phase from March to November. Cx. p. pipiens and Cx. torrentium are the most common species within Hessian caves and other underground habitats during winter. They co-occur with different frequency without any patterns in species composition. Weather conditions influence the number of overwintering mosquitos during the activity phase. Depending on cave parameters, the number of mosquitos decreases during the winter months.

The Dynamics of Body Temperature of the Eastern European Hedgehog (Erinaceus roumanicus) during Winter Hibernation

The dynamics of body temperature in the Eastern European hedgehog during hibernation is described. The average body temperature in autumn before hibernation is maintained at 34.3 ± 0.2°C. During hibernation, the value averages 6.3 ± 0.4°C. A decrease in temperature down to –1.3°С is revealed for hedgehogs for the first time. Periodic normothermic phases each lasting less than a day were detected, as is typical of other hibernators. An active phase of hibernation (warming up and normothermia) takes up 16.5 ± 1.5% of the total time of hibernation. Changes in body temperature in the passive phase (cooling and hibernation) correlate with the ambient temperature. A reliable negative correlation between the duration of bouts and the body temperature is shown.

Regulation of metabolism during hibernation in brown bears (Ursus arctos): Involvement of cortisol, PGC-1α and AMPK in adipose tissue and skeletal muscle

The purpose of this study was to investigate changes in expression of known cellular regulators of metabolism during hyperphagia (Sept) and hibernation (Jan) in skeletal muscle and adipose tissue of brown bears and determine whether signaling molecules and transcription factors known to respond to changes in cellular energy state are involved in the regulation of these metabolic adaptations. During hibernation, serum levels of cortisol, glycerol, and triglycerides were elevated, and protein expression and activation of AMPK in skeletal muscle and adipose tissue were reduced. mRNA expression of the co-activator PGC-1α was reduced in all tissues in hibernation whereas mRNA expression of the transcription factor PPAR-α was reduced in the vastus lateralis muscle and adipose tissue only. During hibernation, gene expression of ATGL and CD36 was not altered; however, HSL gene expression was reduced in adipose tissue. During hibernation gene expression of the lipogenic enzyme DGAT in all tissues and the expression of the FA oxidative enzyme LCAD in the vastus lateralis muscle were reduced. Gene and protein expression of the glucose transporter GLUT4 was decreased in adipose tissue in hibernation. Our data suggest that high cortisol levels are a key adaptation during hibernation and link cortisol to a reduced activation of the AMPK/PGC-1α/PPAR-α axis in the regulation of metabolism in skeletal muscle and adipose tissue. Moreover, our results indicate that during this phase of hibernation at a time when metabolic rate is significantly reduced metabolic adaptations in peripheral tissues seek to limit the detrimental effects of unduly large energy dissipation.

Hibernation induces changes in the metacerebral neurons of Cornu aspersum: distribution and co-localization of cytoskeletal and calcium-binding proteins

Pulmonate gastropods provide unique opportunities to examine physiological and biochemical adaptation strategies when cellular metabolic activity is reduced. In this study, cytochemical changes in metacerebral neurons of the cerebral ganglia were investigated in the garden snail Cornu aspersum during the hibernation phase. The immunocytochemical expression of three cytoskeletal markers: microtubule-associate protein 2-like (MAP-2-li), phosphorylated form of tau-like (P-Tau-li) and heavy subunit of neurofilaments-like (NF-H-li), and of two calcium-binding proteins: calmodulin-like (CaM-li) and parvalbumin-like (PV-li) was compared in active and hibernated snails. The immunopositivity for all the markers increased during hibernation versus activity in metacerebral neurons, with the notable exception of PV-li, which remained highly expressed during the whole annual cycle. Strongly positive aggregates of MAP-2-li and P-Tau-li were detected in the somata of hibernated snail neurons. P-Tau-li aggregates co-localized with CaM-li-labelled masses during hibernation. In addition, increased labelling of NF-H-li epitopes was associated with enhancement of CaM immunopositivity. These changes may reflect neural plasticity mechanisms mainly mediated by microtubule-associated proteins and CaM. Moreover, neuroprotective strategies may allow neurons to endure the prolonged hypometabolic conditions, taking into account that many of the functions controlled by the metacerebrum, such as feeding and movement, are suspended during hibernation. In this context, the molluscan ganglia model offers an easy opportunity to understand the molecular mechanisms behind these life cycle changes in cell physiology and to investigate possible cytological similarities among distantly related animals that adapt to the same environmental challenges through hibernation.

Modifications of the axon initial segment during the hibernation of the Syrian hamster

Mammalian hibernation is a natural process in which the brain undergoes profound adaptive changes that appear to protect the brain from extreme hypoxia and hypothermia. In addition to a virtual cessation of neural and metabolic activity, these changes include a decrease in adult neurogenesis; the retraction of neuronal dendritic trees; changes in dendritic spines and synaptic connections; fragmentation of the Golgi apparatus; and the phosphorylation of the microtubule-associated protein tau. Furthermore, alterations of microglial cells also occur in torpor. Importantly, all of these changes are rapidly and fully reversed when the animals arouse from torpor state, with no apparent brain damage occurring. Thus, hibernating animals are excellent natural models to study different aspects of brain plasticity. The axon initial segment (AIS) is critical for the initiation of action potentials in neurons and is an efficient site for the regulation of neural activity. This specialized structure-characterized by the expression of different types of ion channels and adhesion, scaffolding and cytoskeleton proteins-is subjected to morpho-functional plastic changes upon variations in neural activity or in pathological conditions. Here, we used immunocytochemistry and 3D confocal microscopy reconstruction techniques to measure the possible morphological differences in the AIS of neocortical (layers II-III and V) and hippocampal (CA1) neurons during the hibernation of the Syrian hamster. Our results indicate that the general integrity of the AIS is resistant to the ischemia/hypoxia conditions that are characteristic of the torpor phase of hibernation. In addition, the length of the AIS significantly increased in all the regions studied-by about 16-20% in torpor animals compared to controls, suggesting the existence of compensatory mechanisms in response to a decrease in neuronal activity during the torpor phase of hibernation. Furthermore, in double-labeling experiment, we found that the AIS in layer V of torpid animals was longer in neurons expressing phospho-tau than in those not labeled for phospho-tau. This suggests that AIS plastic changes were more marked in phospho-tau accumulating neurons. Overall, the results further emphasize that mammalian hibernation is a good physiological model to study AIS plasticity mechanisms in non-pathological conditions.

Seasonal and Sexual Variation in Metabolism, Thermoregulation, and Hormones in the Big Brown Bat (Eptesicus fuscus)

In response to seasonal variation in energy availability and thermal environment, physiological and endocrine mechanisms have evolved in temperate zone animals. Seasonal changes in hormone activity affect metabolism, body temperature, and reproductive activity. We examined the seasonal regulatory role of hormones on basal metabolic rate (BMR) and regulatory nonshivering thermogenesis (RNST) in 98 female and 17 male adult Eptesicus fuscus (big brown bat). We measured BMR, RNST, and plasma levels of thyroid hormone (T3), leptin, and cortisol in bats captured in maternity colonies in eastern Massachusetts from May to August (from arousal from the hibernation phase to the prehibernation phase). We hypothesized that all three hormones are seasonally primarily metabolic hormones and secondarily thermogenic hormones. In males, only BMR significantly changed seasonally. In females, all five variables significantly changed seasonally. The seasonal pattern of plasma leptin and cortisol levels correlated with the seasonal pattern of BMR, with an initial increase followed by a decrease, suggesting that leptin and cortisol are primarily metabolic hormones. The seasonal pattern of plasma T3 levels generally paralleled the basic seasonal pattern of RNST, with both increasing at the second half of the season, suggesting that T3 is primarily a thermogenic hormone. The observed decrease in plasma leptin levels may be necessary to allow for the observed seasonal decrease in BMR, with the similar cortisol pattern important for leptin regulation. While T3 is needed to maintain BMR, it may play a more critical role in the seasonal regulation of RNST than of BMR.

Brain inflammatory cytokines and microglia morphology changes throughout hibernation phases in Syrian hamster

Hibernators tolerate low metabolism, reduced cerebral blood flow and hypothermia during torpor without noticeable neuronal or synaptic dysfunction upon arousal. Previous studies found extensive changes in brain during torpor, including synaptic rearrangements, documented both morphologically and molecularly. As such adaptations may represent organ damage, we anticipated an inflammatory response in brain during specific hibernation phases. In this study, signs of inflammation in the brain were investigated in the Syrian hamster hippocampus (Mesocricetus Auratus) both during hibernation (torpor and arousal phases) and in summer and winter euthermic animals.mRNA expression of the pro-inflammatory cytokines TNF-α, IL-6 and IL-1β was quantified by RT-qPCR. Morphological changes of microglia were studied by immunohistochemistry staining for IBA-1. Activation of microglia based on retraction and thickening of the dendritic branches and an increase in cell body size was quantified by calculation of cell body size to total cell size ratio.Expression of pro-inflammatory cytokines was upregulated early in arousal (90 min), and normalized after 8 h of arousal. Substantial loss of microglia ramification was found throughout torpor and early arousal together with a 2-fold increase in the cell body size to total cell size ratio. Notably, microglia changes were fully reversed in late arousal (8 h) to euthermic levels.These results demonstrate an upregulation of inflammatory cytokines and signs of microglia activation during hibernation, which completely resolves by late arousal. Activation of this response may serve to prevent or offset brain damage resulting from the substantial physiological changes accompanying torpor and their rapid change during early arousal.

Reversible temperature-dependent differences in brown adipose tissue respiration during torpor in a mammalian hibernator

Although seasonal modifications of brown adipose tissue (BAT) in hibernators are well documented, we know little about functional regulation of BAT in different phases of hibernation. In the 13-lined ground squirrel, liver mitochondrial respiration is suppressed by up to 70% during torpor. This suppression is reversed during arousal and interbout euthermia (IBE), and corresponds with patterns of maximal activities of electron transport system (ETS) enzymes. Uncoupling of BAT mitochondria is controlled by free fatty acid release stimulated by sympathetic activation of adipocytes, so we hypothesized that further regulation at the level of the ETS would be of little advantage. As predicted, maximal ETS enzyme activities of isolated BAT mitochondria did not differ between torpor and IBE. In contrast to this pattern, respiration rates of mitochondria isolated from torpid individuals were suppressed by ~60% compared with rates from IBE individuals when measured at 37°C. At 10°C, however, mitochondrial respiration rates tended to be greater in torpor than IBE. As a result, the temperature sensitivity (Q10) of mitochondrial respiration was significantly lower in torpor (~1.4) than IBE (~2.4), perhaps facilitating energy savings during entrance into torpor and thermogenesis at low body temperatures. Despite the observed differences in isolated mitochondria, norepinephrine-stimulated respiration rates of isolated BAT adipocytes did not differ between torpor and IBE, perhaps because the adipocyte isolation requires lengthy incubation at 37°C, potentially reversing any changes that occur in torpor. Such changes may include remodeling of BAT mitochondrial membrane phospholipids, which could change in situ enzyme activities and temperature sensitivities.

The awakening of a classical nova from hibernation

Cataclysmic variable stars-novae, dwarf novae, and nova-likes-are close binary systems consisting of a white dwarf star (the primary) that is accreting matter from a low-mass companion star (the secondary). From time to time such systems undergo large-amplitude brightenings. The most spectacular eruptions, with a ten-thousandfold increase in brightness, occur in classical novae and are caused by a thermonuclear runaway on the surface of the white dwarf. Such eruptions are thought to recur on timescales of ten thousand to a million years. In between, the system's properties depend primarily on the mass-transfer rate: if it is lower than a billionth of a solar mass per year, the accretion becomes unstable and the matter is dumped onto the white dwarf during quasi-periodic dwarf nova outbursts. The hibernation hypothesis predicts that nova eruptions strongly affect the mass-transfer rate in the binary, keeping it high for centuries after the event. Subsequently, the mass-transfer rate should significantly decrease for a thousand to a million years, starting the hibernation phase. After that the nova awakes again-with accretion returning to the pre-eruption level and leading to a new nova explosion. The hibernation model predicts cyclical evolution of cataclysmic variables through phases of high and low mass-transfer. The theory gained some support from the discovery of ancient nova shells around the dwarf novae Z Camelopardalis and AT Cancri, but direct evidence for considerable mass-transfer changes prior, during and after nova eruptions has not hitherto been found. Here we report long-term observations of the classical nova V1213 Cen (Nova Centauri 2009) covering its pre- and post-eruption phases and precisely documenting its evolution. Within the six years before the explosion, the system revealed dwarf nova outbursts indicative of a low mass-transfer rate. The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion.