Wood frogs can survive winter with ice forming outside their cells.

Ice between cells draws water away from them and creates severe physiological stress. Wood frogs tolerate this controlled freezing rather than remaining fully unfrozen throughout the winter.

The liver supplies large amounts of glucose that help protect tissues. These protective solutes reduce cellular damage while ice occupies spaces outside the cells.

Freezing tolerance has limits. It does not mean a frog survives any temperature, every rate of cooling, or an indefinitely long winter, and it is not a technique that can be reproduced safely in a household freezer.

Wood frogs spend winter close to the ground under leaf litter rather than depending entirely on deep water as a refuge from cold. The National Park Service describes this strategy in northern Alaska, where the species occupies environments inhospitable to most amphibians. The small animal survives a season that seems incompatible with its soft, water-rich body.

As freezing progresses, ice can develop in the abdominal cavity and between the skin and muscles. Circulation and breathing cease in the frozen state. These observations are remarkable precisely because the normal delivery of oxygen and nutrients is interrupted. The frog is not actively moving around beneath a thin icy coating.

For most animals, freezing creates several kinds of injury. Ice inside cells can disrupt their structures, while ice outside cells can draw away water and cause dangerous dehydration. The wood frog's protection must therefore address more than a low thermometer reading. Its tissues must withstand changed water balance and the consequences of greatly reduced metabolic activity.

The liver's release of glucose is a central part of that response. The National Park Service describes the frog as “going sweet”: sugar helps prevent damaging intracellular freezing while extracellular ice is tolerated. This is not a claim that ordinary sugar makes any animal freeze-proof. The protective chemistry works within a specialized physiological response developed by this species.

When conditions improve, the frog thaws and resumes activity. Emerging early allows it to breed in temporary pools formed by spring meltwater, sometimes before larger bodies of water have completely thawed. Those shallow pools can dry later in the season, so early access has ecological value. The winter strategy is connected to the timing and location of reproduction, not merely to surviving cold in isolation.

“Frozen solid” is a vivid phrase, but it can mislead if it implies that all body water becomes ice. The distinction between extracellular ice and protected cellular interiors is the core of the explanation. Similarly, the absence of a detectable heartbeat during dormancy is not equivalent to irreversible death in an animal adapted to tolerate that state. Survival is assessed by recovery of coordinated function after thawing. Researchers studying the process must track actual tissue condition and subsequent recovery rather than relying only on whether the animal looks rigid, feels cold, or appears lifeless at one moment.

Freeze tolerance is a seasonal physiological strategy, not a general ability to survive any freezing treatment. A wood frog prepares for winter in a setting where ice can form around and within parts of its body. The location of that ice matters because a frozen body surface and lethal damage inside cells are not equivalent events. The survival claim depends on preserving the machinery needed for recovery.