INSECTS: FROM THE DIVERSITY TO THE LIMITS - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)


It seems astounding to us that some frogs can survive months being frozen, or that a bird as small as a kinglet can stay warm and survive even one winter night, much less a whole northern winter. But why, really, are we surprised? I think it’s because we compare them to ourselves. We feel uncomfortable when we chill only a degree or so and we can’t imagine how a tiny bird keeps warm in a blizzard. Yet for every kinglet that we find in the winter woods hundreds of thousands of invertebrate animals exponentially tinier than a kinglet survive by doing what for us seems unimaginable. Even when we do know what they accomplish we still tend to withhold respect. Why? It’s because most of them are insects. They are animals so different from us that it’s as if they were from an alien world; we find it difficult to identify with their problems. Yet they face the same problems of cold, freezing, and energy balance that we or a kinglet deal with. They have evolved some of the same, and also different solutions, but with different constraints.

To an entomologist and anyone who aspires to be one, there are no life-forms on earth as diverse, varied, tough, and inventive as the insects. In their teeming millions of species, they own the world. We may not like many of them that compete with us for food, fiber, timber, or that suck our blood and spread our diseases, but we are obliged to acknolwedge their tenacious success, and we may admire many of them for their stunning beauty. Within the animal world they have collectively pushed the limits of things possible, in terms of diversity, beauty, noxiousness, social organization, architecture, powers of flight, sensory capabilities, and ability to survive extremes of climate. And when I contemplate these organisms that are much more ancient than us, and that will long survive us, I wonder about the “secret” of their success and then I am forced to confront how differently a physical scientist and a life scientist sees the world.

The physical scientist tries to understand the world according to mathematical precision by reducing its composition and functioning to a very few “laws of nature.” Such laws, in analogy with our own civil laws, are as if handed down from some higher authority who then enforces them by virtue of that authority because a law is, by definition, general. It applies uniformly. There are no exceptions, or no exceptions are allowed.

Insects’ success is derived from exploiting individual specificity. No one way is best. Insects achieve their success through their diversity, where each individual case is special within the generalizations. Each species is adapted ever more specifically into a specialist niche, catering to specific individual needs. An ever-greater narrowing down to the specific has resulted in miniaturization, and ever-greater diversity. Insects exhibit an exhilaration and a celebration of the exceptions, where anything goes that can. There are few boundaries, because there has been no enforcement or encapsulation by or in laws. That is why they are so successful, and I suspect that it would be difficult to find an entomologist who is also a theist, who believes that there is a force or a power that hands down rules because “he” deems them good. No entomologist could fathom why fleas, mosquitoes, tsetse flies, migratory locusts, and dozens of other insects would have been deliberately created and let loose to cause indiscriminate and unimaginable agony to millions of totally innocent human children and adults over all the ages of humanity.

And it is this indiscriminate capacity to individually do what is best for them that has allowed insects to advance far into the winter world, one species at a time, to explore and exploit the many possibilities.

My fascination with insects started before I was ten years old, thanks to my father, who took me with him on winter adventures to look for hibernating ichneumon wasps. The white grub-like larvae of these insects feed inside (and eventually kill) caterpillars after the wasps inject their eggs through the caterpillar’s skin with their sharp hollow ovipositor (that also serves as the stinger in bees). Papa collected inchneumon wasps as passionately when he was eighty years old as when he was twenty. At that time he had only barely scratched the surface of their astounding abundance and diversity, even in Maine, and many of these exquisitely elegant creatures that keep other insect populations from exploding, still remained to be discovered.

Promethia moth caterpillar and ichneumanid parasite.

The males of these and almost all Hymenoptera, the family of ants, bees, and wasps to which ichneumonids belong, don’t solve overwintering. They simply all die by fall, by which time they are superfluous since the overwintering females are by then inseminated. Only the females seek overwintering sites in places where there is moisture so that they do no desiccate. Temperatures must also be low enough to dampen the metabolic fires and thus conserve the limited stores of energy that they lay up in body fat. As with most insects, however, temperatures cannot be so low as to cause freezing-injury. Typically when unadapted soft tissue thaws out after freezing solid, it turns into a brown mush. A quick routine way to kill summer-active insects (such as specimens for one’s collection, or the pests that are eating one’s collection) is by putting them into the freezer compartment of the refrigerator at about -10°C. This is a very modest temperature, relative to those that some species routinely survive in the wild, in northern winters.

An ichneuman wasp, of subfamily ichneumoninae.

We found the ichneumon wasp females at three kinds of overwintering sites. We had the most success finding a variety of species in the decaying wood of old moss-covered tree stumps. Other species turned up only under moss, and still others were revealed when we pried apart sedge or grass hummocks. With hatchet and knives, we sometimes rendered a dozen or more stumps to mulch and found not a single insect. After all this work and anticipation it was exciting to suddenly find even one of these beautifully colored little wasps tucked comatose under moss or in decaying wood. Occasionally we hit a jackpot and found a dozen or more of them in a single stump along with an assortment of spiders, centipedes, millipedes, ground beetles, beetle larvae, and occasionally even an overwintering hornet or bumblebee queen. The insects we found could crawl only very slowly in the cold, but they quickly became lively in the warmth of one’s hand.

Despite our frequent and long-standing searches in the winter woods of Maine, there were many species in my father’s collection that we never found in hibernation. Perhaps they have specialized hibernation sites that we didn’t know about. We found some species in hibernation that we never saw during the summer, so that our winter searches served as a way to enlarge our survey of the diversity of species.

Virtually nothing was known then of the physiological and behavioral mechanisms insects use to survive winter. But thanks to numerous researchers and a huge amount of work in recent decades, we now know that even in any one locality the different insect species exhibit a diversity of winter survival tactics. As previously indicated in the overwintering moth larvae that the kinglets feed on, a few species have amazingly evolved to survive being frozen solid, sometimes at temperatures much lower than those that would kill freezing-tolerant frogs. Some seek special shelter. A tiny number migrate. Many species avoid freezing by physiological mechanisms that lower the freezing point of their tissues. No one strategy is best since each has envolved under a different set of constraints and opportunities. The widest variety of ways of surviving winter is exhibited by the Lepidoptera (moths and butterflies), and I turn to them for a comparative view to explore the range of possibilities. They are also the best studied so far.

Spring azure pupa and adult.

Mourning cloak butterfly.

From northern New England and all the way across Canada, Europe, and Alaska, one of those which stays and overwinters as an adult is the mourning cloak butterfly (Nymphalis antiopa). In the summer the massed crowds of their black-and-red spiny caterpillars commonly feed on willow. Two generations are commonly produced per year, and second-brood caterpillars that I’ve reared in the fall always pupate and produce their adults before winter. The mourning cloaks, like many other butterflies in their family, the Nymphalidae, overwinter only in the adult stage. Mourning cloaks hibernate in hollow trees and these long-lived (in excess of 10 months) butterflies feed on fermenting tree sap such as a sapsucker licks. I never fail to see them in early spring as they bask with open wings in sunshine usually long before all the snow has melted.

Probably my favorite early spring butterfly is the spring azure (Celastrina ladon). Those brilliant blue mites that, like winged jewels, flutter over the just-emerging brown earth after the long winter. Females mate on the first day after emergence, lay their eggs on the second, and rarely live beyond the third. The larvae are tended by ants, and the pupae then spend most of the summer and then the winter in diapause.

In all overwintering insects only one life stage is adapted to survive winter. In the eastern tent caterpillar moth (Malacosomia americanum) that stage is the egg. The moth lays her eggs all in one bunch in a ring around a cherry or apple twig where they are encased in a foam that hardens so that the eggs become solidly attached to the twig. The eggs are exposed to the winter’s lowest air temperatures but what shields them from the cold is glycerol, an antifreeze chemical that used to be commonly poured into car radiators in the fall. In early spring, the glycerol is depleted from the eggs and the larvae hatch. The group of larvae emerging from any one egg mass then spin the silk web in a fork between some branches that constitutes the familiar “tent” from which their name is derived. After completing their growth in early June, the larvae leave their tents and wander about, searching for places to pupate. Their light yellowish cocoons are placed into crevices under bark, and are a common sight on the sides and corners of buildings. By late summer the moths emerge, lay their eggs in time for overwintering, and die. Tent caterpillars, like most caterpillars, are unable to survive the freezing temperatures of winter.

The tent caterpillars are not endearing to most people, but the black-and-rust-red banded woolly bear caterpillars come close. They are the larvae of the Isabella tiger moth (Pyrrharctia isabella) that is specialized to overwinter in the caterpillar/larval stage. The Isabella tiger moth (formerly Isis isabella) is a member of the family Arctiidae, in which most species are beautifully colored in striking patterns of bright pink, red, black, yellow, orange, pure white, and blue while in the adult, moth, stage. The Isabella moth, in contrast to most of its group, is plain colored with predominantly dirty-yellow forewings and with a pinkish-yellow hue to the hindwings. W.J. Holland in his classic Moth Book (originally published in 1903) writes: “Both the moth and the larva are common objects, with which every American schoolboy who has lived in the country is familiar; and unhappy is the boy who has not at some time or other in his life made the country his home.”

I’m happy to report from the countryside of Vermont and Maine that the “banded woolly bear” is still familiar to most people. According to local folklore, the width of the central reddish band reflects the severity of the coming winter. However, the caterpillars subtly change color over successive molts through the summer, becoming less black and more reddish as they age, i.e., as winter approaches. When touched, this caterpillar characteristically curls up into a defensive posture with stout bristles sticking out in all directions, much like the European hedgehog. In this species as in other Arctiids, only the caterpillars survive winter.

In the fall of 2001, I picked up three woolly bear caterpillars and I wondered how they, as well as three hatchling snapping turtles that I had retrieved as they were digging out of their nest in sandy soil, might bury themselves for winter. If I let these animals loose I could, of course, never hope to find them in winter. My question was simple: Will they bury themselves in moist soil, hide under the dead leaves, or stay on top of the leaves under the snow? To find out I filled a two-gallon plastic jar with soil, put leaf litter on the soil, and then buried the jar in the woods up to the level of the soil. Within a month frosts had hardened the top layers of the soil, and the jar was buried in snow.

Woolly bear caterpillar in defensive and hibernating posture. Isabella moth at rest.

Winter was not yet over by the end of February, but whatever the caterpillars (and turtles) were going to do to prepare for it, they should have done by then, so I shoveled off the snow and pulled the jar up to examine its contents. I found the three caterpillars almost immediately. They were unfrozen and curled up just under the leaf litter. The turtles were not in sight. Due to the snow insulation, the ground was, as usual, unfrozen, and I dumped the soil from the jar to sift through it. It was difficult to find the turtles. They were caked in mud with their heads and feet retracted into their shells and tails curled alongside. They looked like muddy pebbles. I distinguished them from pebbles at first only because they were compressible with my fingers.

To find out how these hibernators might act when warmed up, I brought them all, caterpillars and turtles, inside the house. After being washed with water at room temperature, the turtles “instantly” extended their limbs and heads, started rubbing their eyes and swimming in a bowl of water. The next day they started feeding on raw meat that I offered them. Their hibernation was over. Maybe they had simply been inactivated by the cold, after perhaps being induced by the cold to bury themselves.

The behavior of the turtles, as such, may not seem unique or surprising, except when seen in terms of the caterpillars’ behavior. The three caterpillars remained on my desk in a jar in moist moss and green grass (one of their food plants) retrieved from under the snow in a field. I had expected them, like the turtles, to immediately start wandering and feeding on the grass when they warmed up. They didn’t. They moved only enough to crawl under the moss and, even while experiencing the warmth of my study, to curl up again in the same hibernation posture that I had found them in. One month later the woolly bears had not moved. They seemed dead. Suddenly, in the last week of March, they all encased themselves in lightly spun cocoons (that incorporate the spiny hairs that they shed). When I tested the frost-hardiness of the pupae by subjecting them to moderately low temperature (-14°C in the freezer compartment of our refrigerator) they froze solid and were dead.

It had previously been reported that cold-hardened woolly bear caterpillars remain unfrozen, even down to about -30°C, through a combination of supercooling and antifreeze. Low temperatures in the fall were reported to stimulate them to convert their glycogen stores into glycerol and sorbitol, and the amounts of these alcohols (up to 5 percent body weight) reduced the freezing point of their blood to about -10°C, and the rest—the prevention of ice formation of the whole animal down to -30°C—was presumably due to supercooling. Could this really be true for the New England population? I recalled having sometimes found woolly bear caterpillars in outdoor winter woodpiles in Vermont, and although I had no reason to study them, I did have the impression that they were occasionally hard frozen. But I didn’t test if they were dead.

I wanted to put my woolly bear caterpillars to a test, and when I found two of them just out of hibernation (the following spring as I was writing this on Easter weekend) I put them (as previously the pupae) into a film vial and to -14°C in the freezer compartment of our refrigerator. Two hours later they were indeed quick-frozen into blocks of ice. They were solid. I could tap the table with them. When thawed out an hour later, they were alive and well!

This had been a severe test, since the caterpillars had already spontaneously aroused from hibernation and since freezing-survival (as I’ll show later) requires slow freezing. Not believing my senses, I immediately repeated the experiment with the same two caterpillars. The result was the same: Woolly bear caterpillars (like the aforementioned geometrical caterpillars that the kinglets eat) do survive freezing—even multiple freezing—whereas the pupae don’t. No wonder my caterpillars had waited so long to pupate after coming out of hibernation.

Cecropia moth cocoon on red maple twig.

Sawfly cocoon (left) on beaked hazel twig with male catkin bud (right).

Another woolly caterpillar, Gynaephora groenlandica (unrelated to the tiger moth), that lives in the High Arctic, has no chance to escape freezing solid. It is routinely subjected to the very much lower temperatures (to near -70°C) on the tundra where snow cover is thin and the ground is permafrosted. This species is one of the very few moths that has evolved to live within 83 degrees of the North Pole. During the short arctic summer the caterpillars briefly thaw out and feed. Because temperatures are low even then, they spend most of the year frozen solid. They only grow slightly in any one year before again freezing solid and their freezing-thawing cycle is repeated thirteen to fourteen years before they are finally ready to spin a cocoon on an exposed rock to catch direct sunlight for heating. They then molt, first into a pupa and then into adult moths that mate, lay eggs, and die a few days later.

The Gynaephora caterpillars living near the North Pole are surely exotic, and very few people get to see them (I was one of the lucky few who was invited by Jack Duman and Olga Kukal, two colleagues from the University of Notre Dame, to travel north to study them). However, there are also marvels at one’s doorstep. At the farm in Maine, I collected nectar-sipping sphinx moths humming around the milkweeds by the barn. Later, my mothing took me to Los Angeles, where in the lab of George Bartholomew at the University of California, I tried to decipher if and how they regulated their body temperature. A couple of decades later I came full circle and returned east and discovered winter moths for the first time. These winter moths are not seen by the average person—only by those who go out into the woods at night with a bucket and a brush and paint trees with an ambrosiacal concoction of fermenting mashed fruit (apple, banana, or peach will do) spiked liberally with beer or some other alcoholic beverage. Each lepidopterist has his or her own special concoction that works best, which I suspect has as much to do with individual taste as with science.

Adults of the winter moths (Cuculiinae) don’t just survive the winter in torpor. They live as adults in the winter world. There are numerous species of these moths belonging to the Noctuidae, or owlet moths, which is possibly the most species-rich group of Lepidoptera on earth, with thousands of species in temperate and tropical regions. The cuculiinae are distinguishable from other noctuids primarily by their so-called reversed life cycle. Whereas the vast majority of moths overwinter as pupae, the cuculiinids overwinter as adults and they also fly during the winter thaws when temperatures reach near or slightly above the freezing point of water. By mating and then maturing their eggs in the winter and laying them on the just-opening leaf buds in early spring, the adults are less likely to be eaten by bats, and the larvae also encounter less bird predation, since growth to the pupal stage can be finished before their predatory migrants return to reoccupy the northern woodlands.

Like sphinx moths and other noctuids, these winter moths have robustly built thick bodies powering short wings that require a high wing-beat frequency to support flight. In order to generate sufficient power for rapid wingbeats they must warm up their musculature to over 30°C. They do that by shivering. A shivering moth extends its antennae, raises its wings, and then you see wing vibrations as the upstroke and downstroke wing muscles are activated nearly simultaneously. A covering of thick insulating scales on the thorax acts, like fur or feathers, to approximately half their rate of heat loss. Further heat retention from the working thoracic flight muscles is enhanced by countercurrent mechanisms of the blood circulatory system that reduced heat loss into and from the abdomen. These moths are unique in their willingness and ability to start shivering when their muscle temperature is as low as 0°C. (Most others require 15° to 20°C higher temperatures.) Once shivering begins and the muscles begin to warm up, then shivering proceeds more vigorously, to produce even more heat until the suitably high muscle temperatures needed for flight are attained.

THE LEPIDOPTERA DEMONSTRATE diversity of adaptation to winter within a single group. However, a discussion of frost-hardiness must necessarily include a specific and very famous maggot, that of the goldenrod fly (Eurosta solidaginis). Eurosta is as necessary to an understanding of insect frost-tolerance as the fruit fly Drosophila is to genetics.

As implied by the name, the fly’s life cycle is inextricably bound to the goldenrod’s. The adult fly injects an egg into a young and rapidly growing goldenrod stem in the spring or early summer. Chemicals that are either injected with the egg or produced by the young larva then subvert the plant stem’s normal growth, causing it to produce a thick tumorlike growth, called a gall. The gall has soft tissue on the inside, and is enclosed within a hard woody exterior. From within it the larva taps the plant’s resources and uses them for its own growth.

Goldenrod gall, showing cross section with gall fly larva (center) and two galls on one stalk, both excavated by downy woodpecker (right).

In the goldenrod fly, as in the previously discussed Arctiid moths, only the larval stage is physiologically specialized to overwinter, and the insect’s life cycle of one generation per year is adjusted to bring the larval stage to winter. After passing through the winter the larvae pupate and then emerge as adult flies in time to parasitize tender goldenrod shoots. By late summer the goldenrod, the gall, and the larva have stopped growing and the larva then chews an escape tunnel from the center of the gall all the way to, but not through, the outermost edge. Retreating back into the center of the tough woody gall, it spends the winter there, in hibernation. That the larva makes the escape hatch when it does is essential, because the adult gall fly does not have chewing mouth-parts and it would otherwise remain entombed within the gall. Having prepared both an overwintering site and a means for the fly emerging in the spring to escape from it, the larva next prepares physiologically for meeting the winter cold. Northern populations of the fly have different responses than the southern.

Northern Eurosta larvae prepare for winter by producing both glycerol (an alcohol) and sorbitol (a sugar) in response to lowered temperature in the fall. The lower the temperature, the more glycerol and sorbitol they produce and the lower the freezing point of their blood. But their frost-hardiness doesn’t end there. It involves far more complex and sometimes counterintuitive mechanisms, all acting in concert, that not all readers will likely want to follow. This includes, for example, the paradox that the northern larvae produce and release a protein into their blood that promotes freezing. In effect, the protein mimics ice crystals by providing nucleation sites for ice crystal formation. It thereby prevents the animal from achieving supercooling. By preventing supercooling, the protein causes the larvae to freeze earlier, already at higher temperatures than they would otherwise.

How can the apparent promotion of freezing aid in winter survival, even as the animals produce compounds with antifreeze properties? The answer is complex, and elegant. It relates to the fact that verylow temperatures may be encountered in the northern larvae populations, and antifreeze alone would then be insufficient to guarantee absence of ice formation. With no guarantee of avoiding freezing, the animals have then found a way to survive it. Their antifreeze glycerol serves a dual function. It lowers the freezing point, thus reducing the probability of freezing, but when or if freezing does occur, then the glycerol acts to reduce the damage caused by ice crystals. Indeed, glycerol is found in both freeze-intolerant and freeze-tolerant species, having a different function in each.

Freezing-avoidance by supercooling is of course also potentially adaptive. But only at consistently modest low temperatures. At even occasionally very low environmental temperatures, when freezing is highly possible, if not inevitable, then supercooling is dangerous because it could cause instant freezing and sure death. To understand why, we need to keep two things in mind. First, supercooled larvae, if seeded with an ice crystal, would freeze nearly instantly (the exact speed, in seconds, would depend on the amount of supercooling). Second, although some insects can indeed survive being frozen, they can do so only if that freezing proceeds slowly. Any insect—or any animal—that freezes instantly, as when supercooled, then also dies instantly.

Rate of freezing is important for cell survival because of a compartmentalization of ice crystals that relates ultimately to dehydration. During slow freezing the fluid surrounding the cells freezes first, because it is more dilute than the fluid within the cells. As ice crystals form extracellularly, they use up water and leave pockets of fluid of higher concentration. These pockets then act to osmotically withdraw water from the inside of the cell. In effect, gradual freezing results in the extracellular ice formations with a concomitant dehydration of the cell contents, so that no ice crystals are formed within the cell—ice crystals that would otherwise tear the cell organelles. Or, if ice crystals do form inside the cell, they are small and less damaging. Alternatively, the water may solidify into glass form—a type of liquid that is hard and in which the remaining water molecules are unavailable to form ice crystals perhaps because they closely adhere to the molecules of the cell structure. In contrast, during rapid freezing there is not sufficient time for the osmotic exchange of water (between intra-and extracellular) compartments, and so without prior dehydration the cell contents freeze, resulting in large, jagged ice crystals that shear and tear the cell organelles and membranes.

Let us return now to why northern Eurosta populations avoid supercooling by having a protein in the blood that promotes ice formation. They must thus have freezing-tolerance. Southern populations lack the protein that prevents supercooling and they can and most likely do supercool. Not having been selected for freezing-tolerance, they’ve perfected instead the mechanisms to prevent freezing rather than those that would help them survive it. The combination of antifreeze and supercooling is reliably sufficient to preclude ice formation in their environment. Rate of ice formation is no longer an issue when ice formation is unlikely, so no precautions for ensuring slow freezing (to promote cell dehydration freezing survival) are necessary.

A convincing demonstration that dehydration secondarily confers freezing tolerance can be found in the larvae of the African desert fly (Polypedilum vanderplanki), which periodically dry up in the temporary desert pools within which they live. The larvae are adapted to survive losing 92 percent of their body water, and such desiccated larvae are essentially immortal and can survive immersion in liquid helium (to -269°C), within 4°C or potentially at absolute zero, or 0°K, the lowest temperature in the universe. When rehydrated by dropping them into water they become “instant insects.” They then again have narrow temperature tolerances, surviving only from 10° to 42°C. Are they alive before they are wetted? I think not. What they are is potentially alive, and I base that supposition on some oblique illumination from recent research in geology and microbiology.

About 250 million years ago—that’s about 190 million years before the dinosaurs went extinct—there was an inland sea in North America. The ocean eventually evaporated where there is now the New Mexico desert. It left salt deposits a half mile below the now sunbaked ground. Tiny pockets of ancient seawater became trapped in these salt deposits and from them microbiologists have reported isolating, and then growing (actively metabolizing and reproducing), novel Bacillus-like bacteria. Additionally, they found other microbes whose DNA does not match DNA of known organisms, so that these novel forms are thought to be the first, or among the first, ancient microorganisms that inhabited the planet. Bacteria had previously been isolated from guts of dead insects entrapped in amber for 125 million years, and the microbiologists reported that these bacteria could also be grown in culture, making it possible that living 125-million-year-old organisms had been found.

Polyphemus moth pupae from the field in winter have three-way protection:
✵ tough shells (cocoons) that are impenetrable by most birds
✵ a camouflage wrapping of dead leaves
✵ biochemical protection to prevent death by freezing

So fantastic are these results that they should be and are viewed with skepticism in the scientific community. However, I think they are fantastic, not because of magical processes new to science that we don’t yet understand, but because of the enormous time spans concerned. (I’m personally skeptical, but will give my verdict shortly.) That is, if the bacteria had lain dormant for one, ten, or a hundred years, nobody would have blinked an eye. Even complex, multicellular animals can revive after being dried for a century: Six-legged primitive insectlike arthropods called tardigrades have been known to walk off after they had been inadvertently preserved in dried moss specimens in museums. What is remarkable is not that these organisms survive any specific number of years while in an apparently lifeless state. What is remarkable, to me, is that they can survive in that state for even one minute.

If even multicellular animals, such as insects with organs, muscles, glands, and nervous, reproductive, digestive, and excretory systems, can survive having all of those systems, each with its thousands of cells, being stopped and potentially disrupted, then how much easier it must be to stop and restart the nevertheless highly orchestrated chain of complex biochemical reactions in a single cell, or an even much simpler system, such as a virus? Viruses are not primitive organisms. They are inordinately elegant life-forms that are functionally reduced to a bare minimum required for growth and reproduction in their specific environment, the interior of cells. Their liveness is a complex series of chemical reactions that is within our grasp of understanding. It is at least theoretically possible for us to synthesize them from chemicals “off the shelf.” A bacterium is admittedly a much more complex combination of molecules than a virus, but the same principles apply. If a fly or a tardigrade can survive being dried for a century (and presumably much longer), then a bacterium could be potentially immortal.

A DNA molecule could remain forever; if it is preserved in a saline solution and is not degraded, then time is irrelevant to it. Only the ambient conditions are vital. Given constant conditions, there is no scientific surprise if it survives or has a “shelf life” of 10 years, 10 million years, or 100 million years. I’m emotionally shocked and surprised that bacteria may have been preserved for 250 million years, but intellectually I’m less surprised that, if indeed preserved, they could still metabolize, grow, and divide when recovered. However, I do not subscribe to the idea that they were alive all this time. They were dead as a rock. And so is an insect that freezes in winter. It is not matter that defines life. Process, such as energy flow, does.

Research on insects has opened the amazing possibility, only broached in folklore, science fiction, and most recently even business, of freezing ourselves solid and later reviving. After being frozen the body is “dead” by most definitions (no movement, heartbeat, circulation, respiration, neurological activity) and there should then be no limit to the length of time a body can be preserved. Thankfully, no humans have been immortalized in ice. But freezing of human embryos has been in practice since 1984; some human embroys have been in storage for eight years before being implanted. There is no obvious physiological reason why they could not be stored for more than a century (see Time magazine, March 2, 1998), but I could think of lots of moral ones.

There is a whimsical story of the townsfolk in a village in the Northeast Kingdom of Vermont—an isolated backwoods area known for its cold winters—where the residents of one little village were said to avoid the awful winters by downing a few stiff drinks in the fall and then freezing themselves solid and then unthawing to resume an active life at an appropriate time in the spring. The movie Icemansimilarly featured a man who had been frozen in ice some ten thousand years ago, who was subsequently thawed and brought back to life. Some people are willing to believe that these stories are in the realm of possibility, which is why a company (TransTime Inc. of San Leandro, California) provides “commercial cryonics and cryonic suspension services”; they will freeze anyone in liquid nitrogen and keep them frozen, presumably for the next millennium or beyond, for a “minimum of $150,000,” at 2002 prices. Walt Disney is said to have bought such cold treatment rather than opting for cremation, despite the no-guarantee (of survival) clause. Currently, some “patients” have been “maintained” for twenty-three years. As far as I know, however, nobody has yet submitted to the treatment while they were still in the prime of health, which is when all the animals do it.

The potential implications of the knowledge of freezing-tolerance do not seem to be lost on the agencies that fund research. One researcher with whom I talked had at one time worked on frost-tolerance in insects. He told me that after switching to work on vertebrate animals, “they practically threw the money at me.” I would probably have taken such money as well, if offered. But I confess to unease. The promise to make us immortal and the specter of frozen bodies in vats horrifies me. The unintended implications alone are all too obvious to need reiteration here. It is pure research, that which has no practical implication whatsoever, that enlivens the human spirit the most.