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Freeze tolerance

A freeze-tolerant animal makes itself freeze on purpose. A slow freeze starting at −1 °C is survivable; the sudden one that follows deep supercooling is not.

Some animals permit themselves to freeze. Ice fills the spaces between their cells while the cells themselves stay liquid, protected by sugars, and the animal thaws and works again. It is the rarer of the two cold strategies, and it is controlled rather than endured.

The instinct is to treat freezing as something an animal either resists well or resists badly, with the toughest resisting longest. That picture is wrong in an interesting way, because the two things animals actually do are opposites. One group avoids freezing: it clears its gut of anything ice could nucleate on, sometimes produces proteins that stop small crystals growing, and stays liquid at temperatures far below zero — a supercooled state that is stable until it isn’t. The other group tolerates freezing, and does something that sounds like a mistake: it produces proteins whose purpose is to make ice form, as early and as high above the danger zone as possible. The reason is that supercooling has a catastrophic failure mode. A deeply supercooled animal that finally nucleates at −8 °C freezes almost instantly and throughout, including inside its cells, and intracellular ice is fatal. An animal that nucleates deliberately at −1 °C freezes slowly, over many hours, and the ice grows in the spaces between cells where it can be managed. As that external ice takes up water, the cells shrink and their contents concentrate, which is its own hazard — and this is what the cryoprotectants are for. A wood frog floods its tissues with glucose from its liver within minutes of the first ice forming. What results is an animal with ice through two-thirds of its body water, no heartbeat and no breathing, which thaws in spring into a working frog.

Developed coverage · 50% complete · reviewed 2026-09-03

What this page covers

Freeze tolerance occurs in many insects, in a handful of frogs and turtles, in some intertidal molluscs and nematodes, and in a few reptiles. Among vertebrates it is rare and concentrated in animals that overwinter close to the surface, where they cannot escape the frost.

Often confused with: Freeze avoidance, which is the opposite strategy and much commoner; Cold tolerance in general, which usually means functioning while cold rather than while frozen; Cryopreservation, where freezing is fast and deliberate and the organism is not preparing for it

Quick facts

Two strategies, opposite
Keep ice out entirely, or let it in under control
The counter-intuitive part
Tolerant animals make proteins to start ice early, on purpose
Where the ice goes
Between the cells, never inside them — intracellular ice is fatal
What predicts the strategy
Where the animal overwinters, more than how cold the winter gets

Two solutions, and they are opposites

One group spends autumn removing what the other group spends autumn manufacturing.

An animal facing sub-zero temperatures either stops ice forming or permits it. These are opposite solutions: one group removes everything ice could start from, the other makes proteins whose job is to start it.

Established

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

Cold-hardiness strategies in ectotherms divide into freeze avoidance — maintenance of a supercooled liquid state through removal of ice nucleators and expression of thermal-hysteresis proteins — and freeze tolerance, in which extracellular ice formation is initiated at high sub-zero temperatures by ice-nucleating agents and its extent and location controlled.

Who this applies to
Established across insects, amphibians and reptiles, which are the groups where overwintering strategy has been measured directly.
Studied in
Insecta, Amphibia, Reptilia, Arachnida
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Both strategies are directly measurable — supercooling point, thermal hysteresis, ice content — and the two produce opposite laboratory signatures in the same assays.

How far it can be extended

Both strategies have been characterised independently in insects and in ectothermic vertebrates, with the same mechanistic signatures.

Caveats

  • The strategies are not always exclusive within a species: some animals avoid freezing in early winter and tolerate it later, as their preparation changes.
  • Which strategy a species uses tracks where it overwinters more closely than how cold the winter gets.

Still unanswered

  • Why freeze tolerance is common in some groups and absent from others that face identical winters.

Last reviewed 2026-09-03

The evidence (3 studies)
Freeze avoidance and freeze tolerance, side by side
AspectFreeze avoidanceFreeze tolerance
The goalStay liquid below zeroFreeze safely
Ice nucleatorsEliminated — gut emptied, particles clearedManufactured, to start ice early
Antifreeze proteinsUsed to stop crystals growingUsed to stop crystals enlarging during thaw
Failure modeSudden and total, if a crystal ever startsGradual — too fast, too cold, or too much ice
Typical usersMany insects; the Arctic ground squirrel among mammalsWood frogs, painted turtle hatchlings, many insects

The row worth pausing on is the fourth. Avoidance works perfectly until it doesn’t, and then it fails all at once: a supercooled animal that nucleates freezes throughout in moments, and the ice goes everywhere including inside cells. Tolerance has no equivalent cliff, because the thing that would trigger the cliff has already happened, under supervision. That difference in failure mode, rather than any difference in how much cold each can take, is the real distinction between the strategies.

Freezing on purpose, early and slowly

The ice-nucleating proteins are the part that sounds like a mistake and is not.

Freeze-tolerant animals make themselves freeze on purpose, at the highest sub-zero temperature they can manage. A slow freeze starting at −1 °C is survivable; a sudden one at −8 °C is not.

Well supported

Good evidence backs this, though some details remain open.

Freeze-tolerant ectotherms express ice-nucleating proteins that initiate extracellular ice formation at high sub-zero temperatures, preventing the deep supercooling that would otherwise culminate in rapid, extensive and intracellular ice formation.

Who this applies to
Demonstrated in freeze-tolerant insects and in freeze-tolerant amphibians and reptiles.
Studied in
Insecta, Amphibia, Reptilia
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The physiology is measured and the logic is clear: nucleation temperature is directly observable and correlates with survival. The molecular identity of the nucleators is established in only a handful of species.

How far it can be extended

Ice-nucleating activity has been measured in freeze-tolerant species across several groups, though the identity of the nucleator is known in relatively few.

Caveats

  • Not every freeze-tolerant animal makes its own nucleators; some rely on ice contacting the skin from outside, which does the same job.
  • The nucleators are not identified in most species where the physiology has been measured.

Still unanswered

  • How ice is kept out of cells while filling the spaces between them, over hours, without the cells collapsing.

Last reviewed 2026-09-03

The evidence (3 studies)

Pure water does not freeze at 0 °C. It freezes when something gives it a surface to organise around, and without such a surface it will happily stay liquid well below zero until, eventually, a crystal forms spontaneously and the whole volume goes at once. That "at once" is the problem. An animal that has supercooled to −8 °C contains a great deal of stored freezing, and when it comes it comes fast enough that ice forms inside cells as well as between them. Ice inside a cell tears it apart. So the freeze-tolerant strategy is to never get there: nucleate at −1 °C, where the amount of ice that forms immediately is small and the rest grows over hours, entirely in the spaces between cells.

  1. Ice-nucleating proteins in the extracellular fluid start crystals at about −1 °C, before the animal can supercool.
  2. Ice grows slowly in the spaces between cells. Because water is leaving the solution as ice, the fluid around the cells becomes more concentrated.
  3. That concentration draws water out of the cells osmotically, so the cells shrink and their own contents concentrate — which is what keeps them from freezing internally.
  4. Cryoprotectants, mostly glucose or glycerol, flood the cells and stop them shrinking too far or their proteins denaturing at the resulting concentrations.
  5. The animal reaches equilibrium with perhaps two-thirds of its water as ice, and stays there.

Antifreeze proteins turn up in freeze-tolerant animals too, doing a different job: stopping small ice crystals merging into large damaging ones during the repeated thaws of a real winter.

Based on Antifreeze proteins do not work like the antifreeze in a car. They bind to tiny ice crystals and stop them growing, which leaves a strange signature: the freezing point no longer matches the melting point.

Diagram

Where ice starts decides whether the animal survives

Schematic. Avoidance works perfectly until it fails completely.

Where ice starts decides whether the animal survives it0 °C−1−8Tolerator: ice made to start hereSlow, outside the cells, survivableAvoider: stays liquid, supercoolingif it nucleates hereit freezes at once, inside cellsOne group manufactures ice nucleators. The other spends autumn removing them.Schematic. Avoidance works perfectly until it fails completely.
The same explanation in words

A temperature axis running from 0 °C down to about −8 °C, with two traces. The freeze-tolerator’s trace levels off at about −1 °C, where its own ice-nucleating proteins start ice deliberately: the freeze is slow, confined to the spaces between cells, and survivable. The freeze-avoider’s trace continues downward, staying liquid and supercooled, until it nucleates around −8 °C — at which point it freezes almost instantly and throughout, including inside cells, which is fatal. One group manufactures ice nucleators; the other spends the autumn removing them.

What the tolerance is bounded by

Three limits, and the preparation that has to happen first.

  • Rate. Cool a freeze-tolerant animal too quickly and it dies, because the ice outruns the osmotic redistribution that protects the cells.
  • Extent. Beyond some fraction of body water frozen — around two-thirds in wood frogs — the cells have lost too much water to recover.
  • Temperature. Colder means more of the water is ice, so the temperature limit is really the extent limit measured a different way.
  • Preparation. The tolerance is seasonal. A summer wood frog frozen without its autumn preparation dies; the capacity has to be built.

The animals and the molecules

The research behind this page

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

2014Journal of Experimental Biology

Wood frog adaptations to overwintering in Alaska: new limits to freezing tolerance

Alaskan wood frogs survived far lower temperatures and longer freezing than laboratory work had established, accumulating much higher glucose concentrations, and the repeated partial thaws of autumn appeared to drive that accumulation.

2013Journal of Experimental Biology

Avoidance and tolerance of freezing in ectothermic vertebrates

Avoidance and tolerance are alternative solutions with different failure modes, and the strategy a species uses tracks where it overwinters rather than how cold it gets.

2001Annual Review of Physiology

Antifreeze and ice nucleator proteins in terrestrial arthropods

Antifreeze proteins act by binding to the surface of small ice crystals and preventing their growth, producing a gap between the freezing and melting points; ice-nucleating proteins do the reverse.

1998Annual Review of Physiology

Biological ice nucleation and ice distribution in cold-hardy ectothermic animals

Freeze-tolerant animals often produce ice-nucleating proteins that trigger freezing at high sub-zero temperatures in extracellular spaces, while freeze-avoiding animals do the opposite — clearing their guts and removing nucleators to stay liquid as far below zero as possible.

1988Physiological Reviews

Freeze tolerance in animals

Freeze-tolerant animals permit ice in extracellular spaces while keeping cells themselves unfrozen, accumulate low-molecular-weight cryoprotectants such as glucose and glycerol, and frequently use proteins that deliberately start ice formation in safe locations at high sub-zero temperatures.

1987Canadian Journal of Zoology

Freeze tolerance and the dynamics of ice formation in wood frogs

Around two-thirds of the frog’s body water froze, in extracellular spaces, over a period of many hours, and the animals recovered on thawing.

1971Science

Glycoproteins as biological antifreeze agents in Antarctic fishes

The glycoproteins lowered the freezing point without lowering the melting point by the same amount — a gap known as thermal hysteresis — showing that they act on ice crystal growth rather than by ordinary colligative depression.

1969Science

Freezing resistance in some Antarctic fishes

The fish remained unfrozen at temperatures below the freezing point predicted from their dissolved salts, indicating an additional and previously unidentified antifreeze substance in the blood.

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 50% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 4 claims and answers 4 mapped search questions.

  • 5 high-priority search intent(s) not yet covered
  • 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
  • Insect cold-hardiness is the largest literature here and is drawn on rather than covered in its own right.
  • How ice is excluded from cells over many hours is stated as the mechanism and is not fully explained by the sources.
  • Freeze tolerance in intertidal molluscs, which may be the most extreme case of all, is only mentioned.