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Reptiles and amphibiansspecies

Wood frog

Lithobates sylvaticus

A frozen wood frog has no heartbeat, no breathing and ice through two-thirds of its body. In spring it thaws out and goes to breed.

The wood frog spends winter frozen. Ice fills roughly two-thirds of its body water, its heart stops, it does not breathe, and in spring it thaws and goes to breed. The ice is outside its cells and it is flooded with glucose from its own liver.

What makes the wood frog the standard example is not that it is the toughest animal in the cold — several insects survive far lower temperatures — but that it is a vertebrate doing something a vertebrate should not be able to do, in a form clear enough to measure. A frozen wood frog has no heartbeat, no breathing, no measurable brain activity and no circulation. By any bedside test it is dead. It is not, and the reason is that the freezing has been managed rather than suffered. When ice first touches its skin, the frog’s liver dumps an enormous quantity of glucose into its blood within minutes — concentrations that would be a medical emergency in a mammal — and that glucose enters the cells and protects them as the water leaves. Ice forms only in the spaces between cells and in body cavities, growing slowly over many hours. The cells themselves never freeze; they dehydrate, which is survivable, and the glucose keeps the dehydration from going too far. Two things are worth keeping in view. The tolerance is prepared, not permanent: a wood frog frozen in July without the autumn’s preparation dies. And the limits found in the laboratory turned out to understate what the animal can do — Alaskan frogs followed through natural winters in their own overwintering sites survived colder and longer than the protocols had suggested, apparently because the repeated freeze–thaw cycles of a real autumn are themselves part of the preparation.

Developed coverage · 40% complete · reviewed 2026-09-03

What this page covers

One species, with a range extending further north than any other North American amphibian — across Canada and into Alaska, above the Arctic Circle. Populations differ substantially in how much freezing they survive.

Often confused with: Other freeze-tolerant frogs, such as the spring peeper and grey treefrog, which tolerate less; Hatchling painted turtles, which are freeze-tolerant by a partly different route

Quick facts

How much freezes
About two-thirds of its body water, all of it outside the cells
What it uses
Glucose, released from the liver within minutes of the first ice
While frozen
No heartbeat, no breathing, for weeks at a time
Not year-round
A summer frog frozen without the autumn’s preparation dies

What happens when a wood frog freezes

It starts with ice touching the skin, and the liver responding within minutes.

A frozen wood frog has ice through roughly two-thirds of its body water, no heartbeat and no breathing, for weeks — and thaws into a working frog. The ice is outside its cells, and it is flooded with glucose.

Established

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

Lithobates sylvaticus tolerates extracellular freezing of approximately 65 per cent of body water, with cardiac and respiratory arrest, sustained for weeks. Survival depends on rapid hepatic glucose mobilisation as a cryoprotectant, controlled slow nucleation, and exclusion of ice from the intracellular compartment.

Who this applies to
The wood frog specifically. Freeze tolerance occurs in a few other frogs and turtles, at different limits.
Studied in
Lithobates sylvaticus
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Directly measured in the laboratory and confirmed in animals overwintering naturally, with the ice fraction quantified rather than estimated.

Caveats

  • The tolerance is prepared, not permanent: a summer wood frog frozen without the autumn’s preparation dies.
  • Limits differ substantially between populations — Alaskan frogs survive far more than southern ones.
  • Survival falls sharply with faster cooling and with deeper freezing, so the capacity is bounded on both.

Still unanswered

  • What restarts the heart on thawing, and in what order the organs resume — a sequence that is still not fully described.
  • What the long-term cost of a winter’s freezing is to an individual frog’s later survival and breeding.

Last reviewed 2026-09-03

The evidence (3 studies)
  1. Ice in the leaf litter touches the frog’s skin and seeds freezing in the fluid just beneath it — the frog does not supercool.
  2. Within minutes, the liver converts stored glycogen to glucose and releases it in quantities that would be a medical emergency in a mammal.
  3. Glucose distributes through the blood into the cells, where it protects proteins and membranes as water leaves.
  4. Ice grows slowly over many hours in the spaces between cells and in the body cavity. The cells shrink; they do not freeze.
  5. Heartbeat slows and stops. Breathing stops. Ice reaches roughly two-thirds of body water and the frog holds there.
  6. On thawing, the ice melts from the inside out. The heart restarts before the animal can move, and normal function returns over hours.

The detail that surprises people is the order of the thaw. It happens from the inside out rather than the outside in, so the heart and brain are working again while the extremities are still frozen. Nobody has fully described what restarts the heart, which is one of the better open questions in the field and one with obvious interest beyond frogs.

Diagram

A winter, in six steps

Prepared, not permanent.

A winter, in six steps1Ice touches the skin and seeds freezing at about −1 °C2The liver floods the blood with glucose within minutes3Ice grows between the cells, over many hours4Cells shrink as water leaves; glucose protects them5Heart and breathing stop; about two-thirds is ice6Thawing runs inside-out — the heart restarts firstPrepared, not permanent: a summer frog frozen without the autumn dies.
The same explanation in words

Six numbered steps. One: ice in the leaf litter touches the frog’s skin and seeds freezing at about −1 °C, so the frog never supercools. Two: the liver converts stored glycogen and floods the blood with glucose within minutes. Three: ice grows in the spaces between cells over many hours. Four: cells shrink as water leaves them osmotically, with glucose protecting their contents. Five: heartbeat and breathing stop, with roughly two-thirds of body water as ice. Six: on thawing the ice melts from the inside out, so the heart restarts before the animal can move. A summer frog frozen without the autumn’s preparation dies.

Controlled, not endured

Almost everything about the freeze is managed, including when it starts.

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)

A wood frog overwinters in shallow leaf litter, sometimes only a few centimetres down, where it cannot escape the frost the way a burrowing animal can. Given that it is going to freeze, the useful thing is to freeze early and slowly rather than late and suddenly — and it manages this partly by not resisting at all. Ice contacting its permeable skin nucleates the fluid underneath immediately, at around −1 °C, which forecloses the deep supercooling that would end in a catastrophic freeze at −6 or −8 °C.

Wood frogs overwintering in Alaska survived far more freezing than laboratory work had established, accumulating much higher glucose concentrations — apparently because the repeated freeze–thaw cycles of a real autumn are part of the preparation.

Based on A frozen wood frog has ice through roughly two-thirds of its body water, no heartbeat and no breathing, for weeks — and thaws into a working frog. The ice is outside its cells, and it is flooded with glucose.

That result is worth generalising carefully. It is not that laboratory work was wrong; it is that a tidy protocol removed something the animal depends on. An experiment that freezes a frog once, smoothly, is not reproducing a winter — and in this case the messiness of the real thing was doing useful work.

How we know

Letting the winter do the experiment

Laboratory work established how much freezing a wood frog survives. Do frogs in an actual Alaskan winter match it?

Wood frogs were followed through natural winters in their own overwintering sites in Alaska, with body temperature recorded continuously, the sequence of freeze–thaw cycles they experienced logged, and cryoprotectant concentrations measured.

What happened

The Alaskan frogs survived considerably lower temperatures and longer freezing than the laboratory protocols had established, and accumulated much higher glucose concentrations. The repeated freeze–thaw cycles of the natural autumn appeared to drive that accumulation.

What it shows

That a tidy laboratory protocol had removed something the animal depends on. A single smooth freeze is not a winter, and the messiness of the real sequence — freezing and partially thawing repeatedly through autumn — is part of how the frog prepares.

What it does not show

Because the freeze–thaw cycling was observed rather than manipulated, the link between it and the higher glucose is inferred rather than demonstrated. This is also one population at the northern edge of the species range; southern wood frogs tolerate substantially less, so the numbers are not a species constant.

The controls — what makes this evidence rather than a story
  • Laboratory-derived limits from earlier controlled work serve as the comparison.
  • Continuous temperature logging rather than spot measurement, so brief thaws are not missed.
  • Measurements taken across the whole overwintering period rather than at a single point.

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

The limits, and what is not known

Prepared, bounded, and variable between populations.

  • Preparation is required. The autumn build-up of glycogen and the freeze–thaw cycling both matter, and a frog without them dies at temperatures it would otherwise survive.
  • Rate matters. Cooled too fast, the osmotic redistribution that protects the cells cannot keep up.
  • Extent matters. Past roughly two-thirds of body water frozen, recovery fails.
  • Populations differ. Alaskan wood frogs tolerate substantially more than southern ones, which is a real biological difference and not just a difference in how they were tested.

  • What restarts the heart?

    Why it matters: A heart that has been stopped and frozen for weeks resumes on thawing without external intervention. What triggers the first beat, and in what order the organs come back, is not fully described — and the question has obvious interest beyond frogs.

    What would settle it: Continuous physiological monitoring through the thaw, which is technically difficult in a small animal that is partly still ice.

  • What does a winter of freezing cost a frog afterwards?

    Why it matters: Survival is scored in spring. Whether a frozen winter shortens life or reduces breeding success in the following years has not been followed, and it determines whether freeze tolerance is cheap or merely survivable.

Related

The research behind this page

5 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.

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.

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 40% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 2 claims and answers 6 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
  • Breeding biology and ecology beyond overwintering are outside this page’s scope.
  • Other freeze-tolerant amphibians and hatchling turtles are mentioned only as comparisons.
  • The order in which organs resume function on thawing is not described in the sources consulted.