Beneath the ice, beneath the mud at the bottom of a chilled winter pond, a heart beats one time. Snow drifts across ice to cover frozen soil at pond's edge. Wind whistles through bare branches of dormant hardwood trees. Eleven minutes later the heart beats again.

The heart of a hibernating Painted Turtle may beat as slowly as once every ten to eleven minutes. However, this is a flurry of activity compared to its respiratory rate, which is zero. Painted Turtles can hold their breath for up to five months during winter.

Admittedly, a Painted Turtle is barely alive during the depths of winter. Yet, in order to keep its heart beating and maintain its extremely low metabolism, this animal must have energy.

Energy is obtained from stored carbohydrates. However, with no oxygen available to "burn" carbohydrates efficiently, the turtle switches to an inefficient oxygen-free process, which provides some energy but also produces an unwanted acidic byproduct.

Fortunately, Painted Turtles carry with them a ready reserve of calcium (in their shells) to partially neutralize any acidic accumulation. And so the turtles spend the winter months, never breathing, but doing next to nothing and thereby accumulating few toxic metabolic byproducts. At least the chilled water beneath the frozen pond surface protects them from freezing.

Freezing would be fatal to most animals, but not the Wood Frog. It regularly freezes solid during winter and thaws out in early spring to hop away to woodland ponds and puddles to mate. Other common amphibians, including Spring Peepers, Gray Tree Frogs, and Upland Chorus Frogs also may freeze and thaw several times during the course of a winter. When in the frozen state, the skin pigments of the Gray Tree Frog actually turn blue!

Why don't these animals die when they are frozen? Survival techniques are complex, but in all cases the animals control the manner in which ice forms within their bodies so that cells of their vital organs are not destroyed and remain functional. Although nearly two thirds of their body water may exist as ice, the ice is located in non-vital areas such as the bladder, stomach cavity, beneath the skin, between organs and muscle fibers, and between cells. Ice does not form within the living cells.

Frozen frogs maintain barely detectable nerve activity. They have no heartbeat. They do not breathe. Their blood does not circulate. Yet their vital organs do not deteriorate, even after being frozen solid for several weeks. When these animal icicles thaw out they simply hop away as if nothing unusual had occurred. How do they protect their living cells?

When ice begins to form on a Wood Frog's skin, a hormonal or nervous response stimulates the release of massive amounts of ordinary blood sugar, or glucose, from the frog's liver. This is similar to the adrenaline initiated release of blood sugar associated with the "fight or flight" instinct found in humans and many animals.

However, in ice-nipped Wood Frogs, the amount of blood sugar released is enormous, enough to pack all vital organs in glucose within a few hours. The highly concentrated sugar solution resists freezing and creates a favorable osmotic balance to prevent desiccation or disruption of living tissues.

Organs of frozen frogs survive packed in syrup, surrounded by ice, and essentially undamaged. There is also evidence that the presence of glucose slows the metabolism of living cells, thereby preventing the buildup of toxic waste products within the icy amphibians.

The blood of frozen Wood Frogs may have a glucose level of nearly 5000 milligrams per 100 milliliters. (Humans average 50 to 100 milligrams, diabetics 300 to 400). The extremely high sugar level in Wood Frog blood apparently does no harm because the frozen animal's metabolic rate is extremely low and the sugar is quickly removed from the bloodstream by the liver when thawing occurs. Some of the sugar even fuels the amorous amphibian's trek to the breeding pond.

Frozen frogs are not the only animal ice cubes to survive winter cold snaps in the solid state. Scientists have recently discovered that Garter Snakes, normally the last snakes to retire in autumn and the first to emerge in spring, are able to tolerate a night or two of being frozen if caught away from their hibernation chambers. Adult Box Turtles have some ability to survive freezing and newly hatched Painted Turtles freeze solid and thaw out many times while remaining in their nests during their first winter.

Animals deal with winter cold in a variety of marvelous ways. Some highly mobile creatures, such as birds and Monarch Butterflies, simply fly to warmer climates. Many insects overwinter as eggs or grubs, the adults having perished with the first frost or before. A few adult insects, like the Mourning Cloak Butterfly, survive the winter cold by seeking shelter and perhaps adding biotic antifreeze molecules to their body fluids.

Deer Mice and White Footed Mice, normally solitary creatures, may den communally during winter, huddling together for warmth. Foxes grow dense winter coats which enable them to remain active and without discomfort on all but the coldest of winter days. Chipmunks and Ground Hogs sleep soundly in their snug winter dens, maintaining body temperatures just above the freezing point.

Overwintering birds also may grow a "winter coat" of feathers to help insulate them from the cold. Goldfinch feather weight is about 50% higher in winter than summer. Even with feathers fluffed, however, birds as large as crows can maintain their body temperature by insulation alone only if the air temperature is within 20 degrees of their body temperature. At colder temperatures, they shiver to stay warm.

Chickadees may allow their body temperature to drop as much as 20 degrees during winter, meaning that with feathers fully fluffed they begin shivering when air temperature dips below about 60 degrees. Chickadees, like other overwintering birds, shiver more or less continuously throughout winter except when they are flying.

Humans, too, have adapted to winter cold. Like Chipmunks, we construct shelters to shield us from the cold. Like Foxes, we rely on winter coats to protect us when venturing outdoors. We shiver like Chickadees when our body temperature dips too low and we frequently follow the Monarch Butterfly to tropical winter refuges.

We have not yet learned to freeze and thaw our vital organs like the Wood Frog does, but consider the medical implications if we someday learn this lesson. Perhaps we will trade in diseased or worn out organs for newly thawed replacements, and stroll away as nonchalantly as a Wood Frog hopping to a spring mating pond.