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Dormancy

Hibernation, torpor, aestivation, diapause and cryptobiosis were named by five different fields studying five different animals. Underneath, they are running much the same machinery.

Dormancy is a regulated shutdown: an animal actively holds its metabolism far below normal, for hours or for years, and can restart. It has been named five or six different ways by different fields, and underneath the names much of the machinery is the same.

Hibernation was named by mammalogists, diapause by entomologists, aestivation by people watching lungfish, cryptobiosis by a protozoologist, anaerobiosis by biochemists. Each field found its own animals, coined its own word, and for most of a century they were treated as separate phenomena. Then somebody put the metabolic measurements side by side and found that a ground squirrel in winter, a snail sealed to a rock, a brine shrimp cyst and a diapausing moth pupa were doing recognisably similar things: suppressing protein synthesis and ion pumping in a coordinated way, switching enzymes off by phosphorylation so they could be switched back on, and keeping a small protective subset of genes running while nearly everything else stopped. That convergence is the reason this page exists as a single subject. It is not that the traditional names are wrong — the ecology really does differ, and an insect that commits to diapause in August is doing something categorically unlike a hummingbird dropping into torpor overnight. It is that the differences are in the trigger, the depth and the duration, while the machinery is largely shared. The most important thing to understand about any of these states is that they are active. A hibernating animal is not a stopped animal. Its metabolism falls further than the cold alone can explain, which means something is doing the lowering, and something has to keep running to keep it there.

Developed coverage · 63% complete · reviewed 2026-09-03

What this page covers

Dormant states occur in mammals, birds, reptiles, amphibians, fish, insects, crustaceans, molluscs, nematodes, rotifers, tardigrades, plants and fungi. They arose independently many times, which is why the shared machinery underneath them is interesting.

Often confused with: Sleep, which is a different state with a different brain signature and does not lower metabolism much; Being cold and slow, which is a passive consequence rather than a regulated state; Death, which the older literature genuinely struggled to distinguish it from

Quick facts

What they share
Coordinated suppression of protein synthesis and ion pumping, reversibly
What differs
The trigger, the depth, and the duration — not the underlying biochemistry
Not switched off
Metabolism falls further than the temperature drop can explain
The odd one out
Diapause is a prediction; the others are responses

One phenomenon, five or six names

The names come from the fields that found them, not from the biology.

Hibernation, torpor, aestivation, diapause and cryptobiosis were named separately by different fields, but underneath they run much the same machinery. What actually differs is what starts them and how long they last.

Well supported

Good evidence backs this, though some details remain open.

Metabolic depression across taxonomically and ecologically distinct dormant states converges on a shared set of controls — coordinated suppression of protein synthesis and ion transport, reversible enzyme phosphorylation, and selective maintenance of protective gene expression — while differing in inducing cue, depth and duration.

Who this applies to
Documented across mammals, birds, amphibians, reptiles, fish, insects and several invertebrate phyla.
Studied in
Animalia
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The convergence of controls is well documented in the species where it has been looked for, and those species are few relative to the range of animals that go dormant. The mechanism is better established than its universality.

How far it can be extended

The convergence was established by comparing measured metabolic and molecular data across phyla that acquired these states independently.

Caveats

  • Mechanistic work is concentrated in a handful of tractable species — ground squirrels, wood frogs, land snails, brine shrimp — which were not chosen at random.
  • Shared machinery does not imply shared ancestry: most of these states evolved independently, so the convergence is on solutions rather than on inheritance.

Still unanswered

  • Whether the shared controls reflect a deeply conserved ancient programme or repeated independent recruitment of the same available mechanisms.

Last reviewed 2026-09-03

The evidence (2 studies)
The dormant states, by what starts them and how long they run
StateTypical triggerDurationThe distinguishing feature
TorporCold night, or a bad night’s feedingHoursEntered and exited daily; a response to now
HibernationSeason, with internal timingWeeks to months, in boutsLong bouts broken by costly rewarmings
AestivationHeat and droughtWeeks to yearsThe problem is water and heat rather than cold
DiapauseDaylength, sensed in advanceA season, sometimes longerAnticipatory, and cannot be ended by good conditions
CryptobiosisDrying, freezing, or oxygen lossYears to decadesNo metabolism can be measured at all

Read down the last column and the family resolves. Four of these five are responses to a condition the animal is currently in or about to be in, and they differ mainly in how long they last. The fifth, diapause, is doing something categorically different: it is committing in advance, on a cue that carries no information about the hardship itself except that the hardship is coming.

Diagram

The dormant states, by trigger and duration

Shared machinery underneath; the trigger and the length are what differ.

Same machinery underneath; the trigger and the length differStateStarted byLastsWhat sets it apartTorporA cold nightHoursEntered and left dailyHibernationThe seasonWeeks to monthsBouts, with costly wakingsAestivationHeat and droughtWeeks to yearsWater, not coldDiapauseDaylength, earlyA season or moreA prediction, not a responseCryptobiosisDrying or freezingYears to decadesNo metabolism measurableDiapause is highlighted because it alone commits before the hardship.
The same explanation in words

A five-row table. Torpor is started by a cold night, lasts hours, and is entered and left daily. Hibernation is started by the season, lasts weeks to months, and runs in bouts broken by costly wakings. Aestivation is started by heat and drought, lasts weeks to years, and is about water rather than cold. Diapause is started by daylength sensed early, lasts a season or more, and is a prediction rather than a response — it is highlighted because it alone commits before the hardship arrives. Cryptobiosis is started by drying or freezing, lasts years to decades, and is the state in which no metabolism can be measured.

A dormant animal is not a stopped animal

Metabolism falls further than the cooling can account for, so something is doing the lowering.

A hibernating animal is not a stopped animal. Its metabolism falls further than the cold alone would explain, which means something is actively holding it down — and something is running the whole time to keep it alive.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Metabolic rate during torpor and hibernation falls below the level predicted by temperature coefficients from the reduction in body temperature alone, indicating active metabolic inhibition supplementary to the passive thermal effect, maintained by regulated biochemical controls.

Who this applies to
Measured across hibernating and daily-torpid mammals and birds.
Studied in
Mammalia, Aves
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Directly measured by respirometry in many species, with the passive thermal contribution calculable and consistently insufficient to account for the observed fall.

How far it can be extended

The excess reduction beyond thermal effects has been measured independently in many mammal species and in birds.

Caveats

  • The relative contribution of active inhibition versus passive thermal effect differs between species and between phases of a torpor bout.
  • In large hibernators such as bears, body temperature falls only a few degrees while metabolic rate falls far more, so the active component dominates.

Still unanswered

  • What sets the floor on how far metabolism can be depressed before the animal cannot restart.

Last reviewed 2026-09-03

The evidence (2 studies)

The arithmetic is what makes this convincing. Chemical reactions slow predictably as temperature falls — roughly two to three times slower for every ten degrees — so from a measured body temperature you can calculate how slow an animal’s metabolism ought to be. In torpid and hibernating animals it is slower than that, sometimes by a large margin. The excess is not explained by the cold, so something else is producing it. That something is a set of regulated biochemical brakes: enzymes switched off by adding a phosphate group, which is a modification that can be undone quickly when the animal needs to restart.

A hibernating bear’s body temperature falls only a few degrees while its metabolic rate falls by roughly three-quarters. Almost none of that reduction can be attributed to being cold.

The bear is the clearest case because the cooling is so small. In an animal that drops to near-freezing, the passive and active contributions are hard to separate by eye. In a bear at 33 °C they are not: the temperature has barely moved and the metabolism has collapsed, and the only available explanation is active suppression.

Diagram

Further down than the cooling explains

Schematic. The shortfall is the evidence that something is doing the lowering.

Metabolism falls further than the cooling can explainMetabolicrateAwake, warmCooling alonewould predictActuallymeasuredthe gap is actively held downSchematic. The shortfall is the evidence that something is doing the lowering.
The same explanation in words

Three bars of decreasing height against a metabolic-rate axis. The first, tallest, is an awake warm animal. The second, much shorter, is what the drop in body temperature alone would predict. The third, shorter still, is what is actually measured in a torpid or hibernating animal. An arrow marks the gap between the second and third bars: that gap cannot be attributed to being cold, so something is actively holding metabolism down. The figure is schematic and carries no numeric axis.

The expensive part is waking up

Hibernation is not continuous, and the interruptions consume most of the energy saved.

A hibernating ground squirrel does not sleep through the winter in one go. It hibernates in bouts of a week or two, then rewarms itself to near-normal temperature for a few hours, then drops back. Those arousals are metabolically enormous — a substantial share of the whole winter’s energy budget is spent on them — and the animal does it anyway, repeatedly, which tells you that whatever the arousals are for is not optional. What that is remains genuinely unsettled. Sleep is one candidate, and the odd finding that hibernating animals appear to arouse in order to sleep is one of the better puzzles in the field. Clearing metabolic waste, restarting immune function and rebalancing ion gradients are others. Nobody has shown which, or whether it is all of them.

  • Why do hibernators periodically rewarm at such enormous cost?

    Why it matters: Periodic arousals consume a large share of the energy hibernation saves. Whatever they accomplish must be something that cannot be done in the depressed state and cannot be postponed — which would identify the real physiological limit on how long metabolism can stay suppressed.

    What would settle it: Experimentally preventing or extending arousals while measuring what accumulates or fails, though the intervention is difficult to do without confounding the animal.

  • What sets the floor on metabolic depression?

    Why it matters: Different species stop at very different depths, and cryptobiotic animals go far below anything a mammal reaches. Knowing what makes restarting impossible below some level would explain why the depths differ so much.

The states in detail

Each state, and what makes it its own thing

The research behind this page

12 studies, newest first. Each one has a page explaining what it found and what it could not show.

2015Biological Reviews

Daily torpor and hibernation in birds and mammals

The two states separate statistically rather than grading continuously: hibernators reach lower body temperatures and much longer bouts, and the distribution of bout lengths is bimodal.

2007Annual Review of Physiology

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

Anoxia-tolerant vertebrates suppress metabolism dramatically, reduce the electrical activity of the brain in a controlled way rather than losing it, and manage the resulting acid — in turtles by dissolving buffer from their own shells.

2006Journal of Insect Physiology

Eco-physiological phases of insect diapause

Much apparent disagreement in the diapause literature comes from authors using the same word for different phases; separating the phases resolves conflicting reports about what ends diapause.

2004Biological Reviews

Metabolic rate depression in animals: transcriptional and translational controls

Depression is actively controlled rather than passive: the animal shuts most of its gene expression down while keeping a specific protective subset running, and the state is held by modifications that can be reversed quickly.

2004Annual Review of Physiology

Metabolic rate and body temperature reduction during hibernation and daily torpor

Hibernation and daily torpor differ quantitatively in every measured dimension: bout length, minimum body temperature and the depth of metabolic suppression, which in deep hibernators falls to a few per cent of basal rate.

2004Journal of Experimental Biology

Hypoxic survival strategies in two fishes: extreme anoxia tolerance in the North European crucian carp and natural hypoxic preconditioning in a coral-reef shark

Crucian carp survive months of complete anoxia in ice-covered ponds by converting lactate to ethanol and excreting it across the gills, avoiding the acid build-up that kills other fish.

2002Annual Review of Entomology

Regulation of Diapause

Diapause is a programmed developmental arrest initiated in advance of adverse conditions, not a response to them.

2001Comparative Biochemistry and Physiology Part B

Cryptobiosis — a peculiar state of biological organization

In the deepest cryptobiotic states no metabolism can be measured by available methods, and the organism’s survival depends on the physical preservation of its structures rather than on any ongoing repair.

1999Biological Reviews

Metabolic depression in animals: physiological perspectives and biochemical generalizations

Depressed states across very distant phyla converge on a small set of mechanisms — coordinated suppression of protein synthesis and ion pumping, reversible phosphorylation of enzymes, and a general shutdown rather than a targeted one — despite having been named and studied independently.

1992Annual Review of Physiology

Anhydrobiosis

Organisms that survive drying accumulate protective sugars that substitute for water at membrane and protein surfaces and hold the cell contents in a glass, preventing the structural collapse that otherwise occurs as water leaves.

1991Neuroscience Letters

Warming up for sleep? Ground squirrels sleep during arousals from hibernation

The electroencephalogram during arousals showed the signature of slow-wave sleep, and the amount of it was greatest after the longest torpor bouts — the pattern expected if sleep pressure had accumulated during torpor rather than being discharged by it.

1959Proceedings of the Royal Society of London B

The problem of anabiosis or latent life: history and current concept

There exists a genuine state in which no metabolic activity can be measured and the organism nonetheless revives, and it is distinct from a merely very low metabolic rate.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 63% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 9 claims and answers 2 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • Aestivation is described in the table and does not yet have a page of its own.
  • Anaerobiosis — dormancy driven by oxygen loss rather than temperature or drying — is only mentioned.
  • Plant and seed dormancy share a good deal of this logic and are not covered here.