Hibernation Explained: What It Is and How It Works

Hibernation is a physiological and behavioral adaptation that permits survival during seasonal periods of energy shortage through a combination of pre-hibernal energy storage and hibernal metabolic depression known as torpor (Perspective: rheostasis revisited-hibernation and tanycytes). For students, researchers, life-science professionals, and informed general readers, this article defines hibernation, explains the physiological changes that occur during torpor, and provides examples of true hibernators. The practical utility includes a glossary of hibernation-related terms and a list of species that genuinely hibernate, which supports accurate identification and study of this phenomenon.

What Hibernation Means in Biological Terms

Hibernation is an adaptive strategy some mammals use to survive highly seasonal or unpredictable environments (Transcriptomics in the wild: Hibernation physiology in free-ranging dwarf lemurs). The term describes a state of prolonged metabolic suppression that allows an animal to conserve energy when food is scarce and environmental conditions are harsh. Hibernation is not sleep, and it is not a single uniform state across all species. It involves profound physiological changes including dramatic reductions in metabolic, heart, and respiratory rates and core body temperature (Renal adaptation during hibernation).

Small hibernating mammals repeatedly alternate between the torpid state and the interbout euthermic state over a relatively short timescale of days to weeks for the entire hibernation season (Perspective: rheostasis revisited-hibernation and tanycytes). This pattern is known as torpor-arousal cycling. Hibernation is therefore characterized by extreme shifts in energy homeostasis instead of a single continuous low-energy state.

The distinction between hibernation and other forms of dormancy matters for practical identification. True hibernation involves a controlled drop in body temperature, metabolic rate, and heart rate that the animal actively regulates. This differs from cold torpor, which is a less profound state that some animals enter when temperatures drop. For example, research on the Chinese soft-shelled turtle noted that although the study focused on hibernation, cold torpor may be a more accurate description of the state observed (Induction of Hibernation and Changes in Physiological and Metabolic Indices in Pelodiscus sinensis).

At a Glance: Hibernation Terms and True Hibernators

The table below provides a quick reference for key hibernation terms and examples of species that exhibit true hibernation.

The Physiological Changes During Hibernation

Body Temperature Regulation

Most mammals adapt thermal physiology around 37 degrees Celsius, and large deviations from that range result in organ dysfunction and death (Cold resistance of mammalian hibernators). Hibernators are a prominent exception. They resist long-term exposure to severe low body temperature that is lethal to non-hibernators including humans and mice.

The relationship between body temperature and metabolic suppression is not fixed. Research on the Madagascan fat-tailed dwarf lemur demonstrated that hypometabolism in hibernating animals is not necessarily coupled to a low body temperature (Physiology: hibernation in a tropical primate). This tropical primate relies on a flexible thermal response that depends on the properties of its tree hole. If the hole is poorly insulated, body temperature fluctuates widely and passively follows ambient temperature. If well insulated, body temperature stays fairly constant and the animal undergoes regular spells of arousal.

Heart Rate and Respiratory Changes

Hibernation involves dramatic reductions in heart and respiratory rates (Renal adaptation during hibernation). These reductions occur alongside the drop in metabolic rate and body temperature. The cardiovascular system must adapt to hypoperfusion and hypothermia during hibernation, affecting glomerular filtration and renal plasma flow.

The kidneys show specific adaptations in architecture, vasculature, and the renin-angiotensin system during hibernation (Renal adaptation during hibernation). These adaptations include upregulation of possible protective mechanisms during the extreme conditions endured by hibernating mammals. Understanding these protective mechanisms may provide insights into therapies for organ injury during cold storage and reimplantation during transplantation.

Metabolic Suppression and Fuel Switching

Hibernation requires a fundamental shift in energy metabolism. Chipmunks show a significant change in energy supply pattern from glucose metabolism to fat metabolism during hibernation, along with a compensatory increase in liver volume (The Role of Lipid Metabolic Reprogramming in the Hibernation of Chipmunks). This change in energy metabolism allows chipmunks to cope with cold conditions.

The hibernation signature in dwarf lemurs is characterized by suppression of lipid biosynthesis, pyruvate metabolism, and mitochondrial-associated functions, with accumulation of transcripts encoding ribosomal components and iron-storage proteins (Transcriptomics in the wild: Hibernation physiology in free-ranging dwarf lemurs). The data support a key role for pyruvate dehydrogenase kinase isoenzyme 4 in regulating the shift in fuel economy during periods of severe food deprivation. This pattern holds true across representative hibernating species from disparate mammalian groups, suggesting that the genetic underpinnings of hibernation may be ancestral to mammals.

Skeletal Muscle Adaptations

Skeletal muscle undergoes significant changes during hibernation. Research on small hibernators including the thirteen-lined ground squirrel and garden dormouse, and larger hibernators including brown bears and American black bears, showed preservation of myosin structure in bears during hibernation (Remodeling of skeletal muscle myosin metabolic states in hibernating mammals). In the small hibernators, changes in myosin metabolic states during torpor led to higher levels of energy expenditure in type II fast-twitch muscle fibers at ambient lab temperatures of 20 degrees Celsius.

When experiments were repeated at 8 degrees Celsius, near the body temperature of torpid animals, myosin ATP consumption in type II muscle fibers was reduced by 77 to 107 percent during torpor compared to active periods (Remodeling of skeletal muscle myosin metabolic states in hibernating mammals). Myh2 hyper-phosphorylation during torpor in the thirteen-lined ground squirrel was predicted to stabilize the myosin molecule, potentially mitigating myosin-associated increases in skeletal muscle energy expenditure during torpor in response to cold exposure.

Gastrointestinal and Liver Remodeling

The transition from summer active to winter hibernation seasons comes with significant remodeling at cellular, organ, and whole organism levels (How the gut and liver hibernate). In the thirteen-lined ground squirrel, hibernation alters intestinal epithelial, immune, and cell survival pathways in ways that point to a protective phenotype in the face of prolonged fasting and major fluctuations in nutrient and oxygen delivery during torpor-arousal cycles.

Prolonged fasting associated with hibernation alters lipid metabolism and systemic cholesterol dynamics, with both the gut and liver participating in these changes (How the gut and liver hibernate). Fasting also affects the gut microbiota, altering the abundance, composition, and diversity of gut microbes and impacting the metabolites they produce. Interventional studies have demonstrated that the hibernation phenotype confers resistance to experimental ischemia-reperfusion injury in both gut and liver.

How Hibernation Is Controlled

Brain Regulation and the Rheostasis Concept

Hibernation can be viewed as rheostasis, a term referring to a change in a regulated homeostatic level or set point (Perspective: rheostasis revisited-hibernation and tanycytes). This applies both over the annual timescale of the seasonal hibernation cycle and over the much shorter torpor-arousal cycle. The brain sites through which these homeostatic shifts are controlled have not been fully identified.

A specialized glial cell type lining the third ventricle of the mediobasal hypothalamus, known as MBH tanycytes, is of particular interest (Perspective: rheostasis revisited-hibernation and tanycytes). These cells have a privileged anatomical position contacting the periphery and the hypothalamic control centers of the brain. They have documented sensing and signaling functions within the hypothalamus, making them a strong candidate cell type for the control of energy homeostasis. The proposal is that MBH tanycytes could act as a rheostat, shifting their sensitivity to metabolic feedback over the annual timescale and the torpor-arousal cycle.

Genetic and Epigenetic Control

Extreme metabolic adaptations can elucidate genetic programs that govern mammalian metabolism (Genomic convergence in hibernating mammals elucidates the genetics of metabolic regulation in the hypothalamus). Research using convergent evolutionary changes in hibernating lineages has defined conserved cis-regulatory elements and metabolic programs. Multi-omics approaches have pinpointed cis-regulatory elements, hub genes, regulatory programs, and cell types underlying lineage divergence.

Hibernators accumulated loss-of-function effects for cis-regulatory elements regulating hypothalamic responses, and the refeeding period after fasting served as a key phase for molecular processes with convergent evolutionary changes (Genomic convergence in hibernating mammals elucidates the genetics of metabolic regulation in the hypothalamus). This work provides a genetic framework for harnessing hibernator adaptations to understand human metabolic control.

Conserved noncoding cis-elements associated with hibernation modulate metabolic and behavioral adaptations in mice (Conserved Noncoding Cis-Elements Associated with Hibernation Modulate Metabolic and Behavioral Adaptations in Mice). Genomic analyses revealed topologically associated domains enriched for convergent changes in hibernators, including the Fat Mass and Obesity locus. Deletions of individual cis-elements in mice differentially altered Fto, Irx3, and Irx5 expression, reshaping downstream gene expression programs and affecting metabolism, torpor, obesogenesis, and foraging in distinct ways.

Epigenetic Aging and Longevity

Species that hibernate generally live longer than would be expected based solely on their body size (Hibernation slows epigenetic ageing in yellow-bellied marmots). The hibernation-ageing hypothesis proposes that aging is suspended during hibernation. Testing in yellow-bellied marmots showed that epigenetic age increased during the active season and essentially stalled during the hibernation period.

The results are consistent with the hibernation-ageing hypothesis and may explain the enhanced longevity in hibernators (Hibernation slows epigenetic ageing in yellow-bellied marmots). This has implications for understanding the relationship between metabolic rate and longevity across mammalian species.

Examples of True Hibernators

Mammalian Hibernators

The thirteen-lined ground squirrel is a well-studied hibernator that undergoes significant remodeling of the gastrointestinal tract, liver, and other organs during hibernation (How the gut and liver hibernate). Research on this species has provided much of the current understanding of hibernation physiology.

The yellow-bellied marmot spends 7 to 8 months per year hibernating (Hibernation slows epigenetic ageing in yellow-bellied marmots). This species has been used to study the relationship between hibernation and epigenetic aging.

Brown bears and American black bears are larger hibernators that preserve myosin structure in skeletal muscle during hibernation (Remodeling of skeletal muscle myosin metabolic states in hibernating mammals). The physiology of hibernation in bears has been studied for decades, with research focusing on the metabolic and physiological adaptations that allow bears to remain inactive for extended periods (Physiology of hibernation in bears).

Primate Hibernators

The Madagascan fat-tailed dwarf lemur hibernates in tree holes for seven months of the year, even though winter temperatures rise to over 30 degrees Celsius (Physiology: hibernation in a tropical primate). This tropical primate relies on a flexible thermal response that depends on the properties of its tree hole.

Crossley's dwarf lemurs from the high-altitude forest of Tsinjoarivo in central-eastern Madagascar also hibernate (First direct evidence of hibernation in an eastern dwarf lemur species (Cheirogaleus crossleyi) from the high-altitude forest of Tsinjoarivo, central-eastern Madagascar). Research on this species has provided insights into the transcriptomics of hibernation in a natural population of primate hibernators (Transcriptomics in the wild: Hibernation physiology in free-ranging dwarf lemurs).

Non-Mammalian Hibernation

Hibernation-like states occur in non-mammalian species as well. The Chinese soft-shelled turtle is a commonly cultivated turtle species with a habit of hibernation (Induction of Hibernation and Changes in Physiological and Metabolic Indices in Pelodiscus sinensis). Research on this species has examined changes in histone expression and methylation during hibernation induction.

The brown frog undergoes a prolonged period of hibernation lasting from several months to half a year (Transcriptome Analysis Reveals Cross-Tissue Metabolic Pathway Changes in Female Rana dybowskii during Emergence from Hibernation). This species is extensively cultured in northeast China, and research has examined the molecular mechanisms underlying emergence from hibernation.

Some insects also exhibit hibernation-like states. In the Western Palaearctic, many species of Darwin wasps exhibit a form of diapause known as free-living adult diapause, similar to hibernation in certain beetle, bumblebee, and butterfly species (First comprehensive catalogue of hibernating Darwin wasps in the Western Palaearctic). A comprehensive review confirmed free-living adult diapause in 340 of 439 species reviewed.

Practical Assessment of Hibernation States

Observational Indicators

Identifying whether an animal is truly hibernating requires observation of multiple physiological parameters. The key indicators include reduced body temperature, reduced heart rate, reduced respiratory rate, and reduced metabolic rate (Renal adaptation during hibernation). These changes occur together and are actively regulated by the animal.

For researchers and life-science professionals, the distinction between hibernation and other states matters. Cold torpor involves less profound metabolic suppression and may not involve the same regulatory mechanisms as true hibernation (Induction of Hibernation and Changes in Physiological and Metabolic Indices in Pelodiscus sinensis). The duration and depth of metabolic suppression are key distinguishing features.

Measurement Approaches

Measuring hibernation state requires appropriate equipment and protocols. Body temperature can be measured using implanted data loggers or external sensors. Heart rate and respiratory rate require physiological monitoring equipment. Metabolic rate can be assessed through oxygen consumption measurements.

For field studies, capture-mark-recapture techniques can track the same animals over extended periods (Transcriptomics in the wild: Hibernation physiology in free-ranging dwarf lemurs). This approach allows researchers to compare gene expression profiles during distinct physiological states in the same individuals.

Records and Documentation

Maintaining accurate records is essential for hibernation research. Key data to record include species, individual identification, date and time of observations, ambient temperature, body temperature, heart rate, respiratory rate, and behavioral state. For long-term studies, these records allow researchers to track patterns across seasons and years.

The value of field-based data cannot be overstated. Research on Darwin wasps highlighted the importance of field-based data and cautioned against relying solely on collection dates to study diapause (First comprehensive catalogue of hibernating Darwin wasps in the Western Palaearctic). Collection dates alone may not accurately reflect hibernation status.

Common Misconceptions and Failure Patterns

Confusing Hibernation with Sleep

Hibernation is frequently confused with sleep, but they are fundamentally different states. Hibernation involves profound metabolic suppression that is actively regulated by the animal. Sleep is a normal physiological state that occurs regularly and does not involve the same degree of metabolic depression.

Confusing Hibernation with Cold Torpor

Cold torpor is a less profound state of reduced activity and metabolism that some animals enter in response to cold temperatures. Unlike true hibernation, cold torpor may not involve the same regulatory mechanisms or the same degree of metabolic suppression (Induction of Hibernation and Changes in Physiological and Metabolic Indices in Pelodiscus sinensis). The distinction matters for both research and practical management.

Assuming Hibernation Is Continuous

Hibernation is not a continuous state. Small hibernating mammals repeatedly alternate between the torpid state and the interbout euthermic state over a relatively short timescale of days to weeks (Perspective: rheostasis revisited-hibernation and tanycytes). This torpor-arousal cycling occurs throughout the hibernation season.

Assuming Body Temperature Always Drops

Hypometabolism in hibernating animals is not necessarily coupled to a low body temperature (Physiology: hibernation in a tropical primate). The Madagascan fat-tailed dwarf lemur demonstrates that hibernation can occur at relatively high ambient temperatures, with body temperature fluctuations depending on the insulation properties of the hibernaculum.

Welfare and Safety Considerations

Ethical Treatment of Hibernating Animals

Researchers and animal care professionals must consider the welfare of hibernating animals. Disturbing hibernating animals can have significant physiological consequences. The torpor-arousal cycle involves significant energy expenditure during arousal periods, and repeated disturbances can deplete energy reserves.

Cold Resistance and Cellular Protection

Hibernators resist long-term exposure to severe low body temperature that is lethal to non-hibernators (Cold resistance of mammalian hibernators). This cold resistance is supported at least in part by intrinsic cellular properties. Primary or immortalized cells from several hibernator species can survive longer than those from non-hibernators when cultured at cold temperatures.

Recent studies have suggested that cold-induced cell death fulfills the hallmarks of ferroptosis, a type of necrotic cell death that accompanies extensive lipid peroxidation by iron-ion-mediated reactions (Cold resistance of mammalian hibernators). Understanding these pathways may inform approaches to organ preservation and transplantation.

Translational Research Implications

Hibernation research has potential applications for human health. Hibernation can serve as a model for understanding metabolic diseases, providing insights into reversible insulin resistance and energy homeostasis (Comparative physiology and biomimetics in metabolic and environmental health). Insights from comparative physiology could help identify novel therapeutic targets for obesity, type 2 diabetes, and cardiovascular disease.

Mammalian hibernation offers a natural, reversible model of systemic cellular quiescence mediated by dynamic epigenetic reprogramming (Mimicking mammalian hibernation to lock cancer cells in safe quiescence). Studying and pharmacologically mimicking the epigenetic landscape of hibernators may provide a novel class of agents to lock disseminated tumor cells into safe dormancy.

Hibernation-like neuroprotection has been explored in the context of stroke by attenuating brain metabolic dysfunction (Hibernation-like neuroprotection in stroke by attenuating brain metabolic dysfunction). The protective mechanisms that allow hibernators to survive extreme conditions may inform therapeutic approaches for organ injury.

Professional Escalation Criteria

When to Consult Specialists

Researchers and practitioners should consult specialists when they encounter situations that exceed their expertise. This includes cases where an animal's hibernation pattern appears abnormal, where hibernation is occurring outside the expected seasonal window, or where an animal fails to arouse from torpor at the expected time.

When to Seek Veterinary Care

Animals that show signs of distress during hibernation or that fail to maintain normal torpor-arousal cycling may require veterinary assessment. Signs of concern include abnormal body temperature patterns, failure to arouse, weight loss beyond expected levels, or signs of injury or illness.

When to Escalate Research Findings

Researchers who observe unexpected hibernation patterns or who generate findings that contradict established understanding should escalate their observations to appropriate research communities. The study of hibernation continues to evolve, and new findings may refine current understanding.

Limitations of Current Knowledge

Incomplete Understanding of Control Mechanisms

The brain sites through which homeostatic shifts are controlled during hibernation have not been fully identified (Perspective: rheostasis revisited-hibernation and tanycytes). While MBH tanycytes are a promising candidate cell type, further research is needed to confirm their role.

Species-Specific Variation

Hibernation physiology varies significantly across species. Findings from one species may not directly apply to another. For example, the myosin metabolic changes observed in small hibernators differed from those observed in bears (Remodeling of skeletal muscle myosin metabolic states in hibernating mammals).

Environmental Threats

Accelerating anthropogenic environmental change threatens even the most resilient animal species (Comparative physiology and biomimetics in metabolic and environmental health). Climate change may affect the timing and duration of hibernation seasons, with potential consequences for hibernating species. A holistic approach to conservation and environmental protection is needed to preserve these species and the lessons they offer.

Frequently Asked Questions

What is the difference between hibernation and sleep?

Hibernation is a state of profound metabolic suppression that allows animals to conserve energy during seasonal periods of energy shortage. Sleep is a normal physiological state that occurs regularly and does not involve the same degree of metabolic depression. During hibernation, body temperature, heart rate, and respiratory rate drop dramatically, and the animal enters a torpid state that can last for days or weeks.

Do all animals that hibernate experience the same physiological changes?

No. Hibernation physiology varies significantly across species. Small hibernators such as ground squirrels and marmots show different patterns than larger hibernators such as bears. Some hibernators maintain relatively stable body temperatures while others allow body temperature to fluctuate with ambient conditions. The Madagascan fat-tailed dwarf lemur demonstrates that hypometabolism is not necessarily coupled to low body temperature.

How long do animals stay in hibernation?

The duration of hibernation varies by species and environmental conditions. Yellow-bellied marmots spend 7 to 8 months per year hibernating. The Madagascan fat-tailed dwarf lemur hibernates for seven months of the year. Brown frogs may hibernate for several months to half a year. Small hibernating mammals alternate between torpor and arousal states throughout the hibernation season.

What is torpor-arousal cycling?

Torpor-arousal cycling is the pattern in which small hibernating mammals repeatedly alternate between the torpid state and the interbout euthermic state over a relatively short timescale of days to weeks (Perspective: rheostasis revisited-hibernation and tanycytes). During arousal periods, body temperature and metabolism return to normal levels before the animal enters torpor again.

Why do hibernators live longer than expected based on body size?

The hibernation-ageing hypothesis proposes that aging is suspended during hibernation (Hibernation slows epigenetic ageing in yellow-bellied marmots). Research on yellow-bellied marmots showed that epigenetic age increased during the active season and essentially stalled during the hibernation period. This may explain the enhanced longevity observed in hibernators.

Can hibernation occur in tropical environments?

Yes. The Madagascan fat-tailed dwarf lemur hibernates in tree holes for seven months of the year even though winter temperatures rise to over 30 degrees Celsius (Physiology: hibernation in a tropical primate). This tropical primate relies on a flexible thermal response that depends on the properties of its tree hole.

What is the difference between hibernation and diapause?

Hibernation is a state of metabolic suppression used by some mammals to survive seasonal energy shortages. Diapause is a broader term used to describe a state of arrested development or activity in various organisms. In the Western Palaearctic, many species of Darwin wasps exhibit a form of diapause known as free-living adult diapause, similar to hibernation in certain beetle, bumblebee, and butterfly species.

How is hibernation research relevant to human health?

Hibernation research has potential applications for human health. Hibernation can serve as a model for understanding metabolic diseases, providing insights into reversible insulin resistance and energy homeostasis (Comparative physiology and biomimetics in metabolic and environmental health). Understanding the mechanisms of protection against organ injury during hibernation may provide insights into potential therapies for organ injury during cold storage and reimplantation during transplantation (Renal adaptation during hibernation).

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Hibernation: neural aspects.. Annual review of physiology, 1979.
• Physiology: hibernation in a tropical primate.. Nature, 2004.
• Transcriptomics in the wild: Hibernation physiology in free-ranging dwarf lemurs.. Molecular ecology, 2018.
• How the gut and liver hibernate.. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 2021.
• Renal adaptation during hibernation.. American journal of physiology. Renal physiology, 2013.
• Genomic convergence in hibernating mammals elucidates the genetics of metabolic regulation in the hypothalamus.. Science (New York, N.Y.), 2025.
• Perspective: rheostasis revisited-hibernation and tanycytes.. Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 2025.
• Hibernation slows epigenetic ageing in yellow-bellied marmots.. Nature ecology & evolution, 2022.
• Mimicking mammalian hibernation to lock cancer cells in safe quiescence.. 2026.
• Cold resistance of mammalian hibernators ∼ a matter of ferroptosis?. 2024.
• First comprehensive catalogue of hibernating Darwin wasps in the Western Palaearctic (Hymenoptera, Ichneumonidae).. 2025.
• Remodeling of skeletal muscle myosin metabolic states in hibernating mammals.. 2024.
• Comparative physiology and biomimetics in metabolic and environmental health: what can we learn from extreme animal phenotypes?. 2026.
• Induction of Hibernation and Changes in Physiological and Metabolic Indices in Pelodiscus sinensis. Biology, 2023.
• Transcriptome Analysis Reveals Cross-Tissue Metabolic Pathway Changes in Female Rana dybowskii during Emergence from Hibernation. Fishes, 2023.
• Conserved Noncoding Cis-Elements Associated with Hibernation Modulate Metabolic and Behavioral Adaptations in Mice. Science, 2025.
• The Role of Lipid Metabolic Reprogramming in the Hibernation of Chipmunks. Animals, 2025.
• Conserved Noncoding Cis-Elements Associated with Hibernation Modulate Metabolic and Behavioral Adaptations in Mice. bioRxiv, 2024.
• Physiology of hibernation in bears. Ursus, 1998.
• Proteomics approaches shed new light on hibernation physiology. Journal of Comparative Physiology B Biochemical Systemic and Environmental Physiology, 2015.
• Humoral induction of mammalian hibernation. Comparative Biochemistry and Physiology Part A Physiology, 1979.
• First direct evidence of hibernation in an eastern dwarf lemur species (Cheirogaleus crossleyi) from the high-altitude forest of Tsinjoarivo, central-eastern Madagascar. Naturwissenschaften, 2010.
• Hibernation-like neuroprotection in stroke by attenuating brain metabolic dysfunction. Progress in Neurobiology, 2017.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.