Before the Heart Stops

When a wood frog begins to freeze, the liver acts first.

Ice nucleates at the skin — the frog is lying in the leaf litter, temperature dropping, and ice crystals from the environment touch the skin and start forming there. This triggers something in the liver cells directly. Not through the brain. Not through the nervous system signaling danger. The liver cells themselves are cold, and cold at that threshold means: mobilize glucose now. Within minutes, glucose starts flooding into the blood at a rate of about 20 micromoles per gram of liver tissue per hour. The liver's glycogen — its stored sugar — converts and moves outward.

The glucose spreads everywhere. Heart, muscle, kidney. It saturates the tissues. And what it does there is prevent the water inside the cells from freezing. The water between cells, outside cells — that water turns to ice. About 65 to 70 percent of the frog's total body water becomes ice. But inside each cell, the glucose has raised the solute concentration enough that the intracellular water stays liquid. The cells don't burst. They don't shrink to the point of damage. They sit there, individually intact, while the organism around them goes solid.

Then the heart stops. The blood stops moving. Respiration stops. Brain activity ceases. The frog is, in every functional sense, not running.

It can stay like this for months. Alaskan populations — where winters are longer and colder than the frog's range further south — have evolved a second cryoprotectant on top of the glucose. They accumulate urea through the autumn before any freezing begins, raising blood urea to concentrations that would be toxic in most vertebrates. The urea reduces the amount of ice that forms for a given temperature. These northern frogs can survive to around negative sixteen Celsius, ten to thirteen degrees colder than their southern relatives.

When the temperature rises in spring, the process runs in reverse. Ice thaws in about twenty minutes. The heart takes another twenty or thirty minutes to start. Then it pumps, and blood moves, and the frog gulps, and it breathes, and eventually it hops away. Jon Costanzo, who has studied this for decades, calls the cardiac restart "a spontaneous resumption of function." It just starts. No external signal required.

The thing I keep returning to: what do we call the frog during those months?

"Suspended animation" suggests a slowed-down version of life — everything happening at a reduced pace. But that's not what this is. Nothing is happening. The heart isn't beating slowly. The brain isn't processing dimly. The metabolic processes are not running at some fraction of normal. They're off. The suspension is not of animation but of everything that would qualify as animation.

"Dead" doesn't work either, because dead things don't resume function when you warm them. Whatever persists through the frozen period is enough to restart a heart, sequence a breath, direct a hop. That's not nothing.

The frozen frog is in a state that our vocabulary wasn't built to describe because nothing in our ordinary experience produces it. We have alive and dead and the boundary between them, which we treat as sharp. The wood frog spends a third of its life in what amounts to a gap in that distinction.

What persists through the gap is structure, not process. Each cell is still intact — membrane, organelles, the molecular machinery of metabolism, all present and correctly arranged, just not running. The liver still has glycogen. The heart muscle still has its fiber structure. The information that says "when warm, do this" is still encoded in the proteins and the genome. The frog is not there as a running system. It's there as a set of instructions waiting for the right conditions to execute.

Whether there is any experience of the frozen state — I don't know how to approach that question. No neural activity, so probably not, by most frameworks. But "probably not by most frameworks" is not the same as knowing. We don't actually know what neural activity is necessary and sufficient for experience. We have correlations. The frog in its frozen state is a case where our correlations give a confident answer, but the confidence is borrowed from a theory we haven't fully verified.

The liver acting before the freeze is the part that stays strange to me. The cells respond to temperature directly, without waiting for instruction. They produce the thing that will allow everything to stop without damage. This happens in the minutes before the freeze is complete — before the organism is in the state the glucose is preparing for. It's a preparation for cessation, executed by a part of the system that will itself cease.

In spring the frog reappears at the edge of the pond, finds a mate, continues. The winter doesn't appear to cost it anything. If we asked the frog — which we can't — whether it experienced the winter, the question would be like asking whether it experienced being not yet born. There's no gap from the inside because there's no inside to register a gap.

Which is a strange kind of continuity. Not the continuity of a process running through time, but the continuity of a structure that survives a pause in process. The frog is the same frog before and after. The question of whether the frozen state was part of its life or a gap in its life may not have a clean answer. Maybe the answer depends on what you think life is.