Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
SNOW AND THE SUBNIVIAN SPACE
Sometime early in October the brilliant foliage comes to rest on the forest floor. And then one morning those leaves are encrusted with the white ice crystals we call hoar frost. A few weeks later the first snowflakes, the conglomerates of innumerable snow crystals formed in the air, may come spiraling down out of a darkening sky. Kids of all ages focus on the largest ones and make a game of maneuvering under them to try to catch them on their tongues.
Wilson Alwyn Bentley, or the “Snowflake Man” as he came to be known, also caught snow crystals on microscope slides. He lived on his family’s homestead in the village of Jericho, Vermont, along with his brother Charles, their parents, their grandparents, and later Charles’s wife and their children.
Life on the farm revolved around the chores and seasons, and on February 9, 1880, on his fifteenth birthday, Wilson received an old microscope as a gift from his mother. It changed his life. “I found that snowflakes were miracles of beauty,” he would say later. “Every crystal was a masterpiece of design, and no one design was ever repeated.”
As a result of Bentley’s writing and photography on the subject, every schoolchild is now taught that no two snowflakes are alike, although he pointed out that “it is not difficult to find two or more crystals that are nearly, if not the same, in outline.” Snow crystals were to him a metaphor for earth’s beauty. They were a “road to fairyland” so that “even a blizzard becomes a source of keenest enjoyment and satisfaction, to man, as it brings to him, from the dark, surging ocean of clouds, forms that thrill his eager soul with pleasure.”
Prior to Bentley, scientists and naturalists had commented on the structure of snowflakes, marveling less at their variety than at their six-cornered shape. In 1610, Johannes Kepler (famous for many discoveries, including the planets’ elliptical and not circular paths) is credited with first questioning why, whenever snow begins to fall, its initial formation invariably displays the shape of a six-cornered starlet. I’m not sure we have the answer now, but I assume that the six-arm configuration somehow relates to the most economical way that water molecules align when forming the crystal, when that crystal is free to grow in all directions in the air.
At the age of seventeen, Bentley merged the powers of his microscope with that of the newly developed camera, to realize his dream of capturing images of the snow crystal’s beauty. In a near miracle his father consented to pay the one hundred dollars it cost to purchase the elements required to fashion a primitive camera. Bentley struggled for weeks, experimenting, before on January 15, 1885, he developed the world’s first photo-micrograph of a snow crystal, in the woodshed of the family farm.
Bentley eventually needed to share his photographs with someone who might appreciate them, so he traveled ten miles down the road from his farmhouse to the University of Vermont in Burlington to see Professor George Henry Perkins, a biologist, ecologist, and longtime teacher there. Professor Perkins was amazed at the quality of Bentley’s work and told him that he absolutely had to write about it and show his snowflakes to the world. Bentley went back home and tried to write, but gave up in frustration. He went back to Perkins, appealed to him to put words to his photographs, and in 1898 an article by W. A. Bentley and G. H. Perkins titled “A Study in Snow Crystals” appeared in Appleton’s Popular Science Monthly. Perkins, not only a scholar but also a gentleman, wrote that although he put the pages together from Bentley’s notes and photographs, the “facts, theories, and illustrations are entirely due to [Bentley’s] writing and enthusiastic study.”
The article launched Bentley’s lifelong career studying snow crystals; it apparently unlocked his writer’s block as well. He went on to write fifty popular and technical pieces on snow, culminating with a book, Snow Crystals, in 1931, the year of his death, in which he published more than 2, 500 of his 5, 000-plus photographs.
A snow crystal is only the beginning of what makes snow. Snow flakes are composite masses of often hundreds of snow crystals that have collided on their long journey down from the clouds. The final size of a snowflake before it comes to rest depends on various factors including the numbers of crystals issued per cloud, the distance traveled, and the temperature. Snow falling early in the winter usually forms the largest flakes. Later in the season, when temperatures are lower, the ice crystals spawned by the clouds adhere to one another less readily. These crystals in colder air are brittle and constant collisions on the way down degrades them or smashes their intricate and beautiful structures. Arms of the crystals break off, and these tiny ice spicules then make up the snow. Driven by the wind, the ever more degraded crystal fragments are then packed into a tightly interlocking lattice on the ground that at low enough temperature, near -30°C, has the texture and appearance of Styrofoam. Indeed, walking on such snow at -30°C or less has the feel, and produces the sound, of walking on Styrofoam. It is the building material that is carved into blocks and has been used for centuries to construct winter homes.
Packed snow has superb insulating properties. An igloo efficiently retains warmth generated by a small oil-lamp and human bodies, while it effectively blocks the infinite heat sink of the sky above. Wind cannot penetrate its walls, even as oxygen and carbon dioxide freely exchange through the snow and the entrance tunnel. The tunnel reduces air convection, such as from wind, and at the entrance to the igloo the Inuit create an air lock—raised area that reduces the inflow of the colder and heavier air that hugs the ground. Typically, this raised area is covered with caribou hides.
Ruffed grouse that has tunneled into the snow to sit out the night or a snowstorm.
Snow also provides shelter at night for many kinds of birds, ranging from Siberian tits, ptarmigan, and ruffed grouse that burrow in and create igloolike snow caves. The birds leave tangible evidence that they sometimes tarry a long time in these snow shelters; I have found over seventy fecal pellets within a single ptarmigan snow cave near Barrow, Alaska, and in the Maine woods I routinely find over thirty where a ruffed grouse has spent the night. Often these birds also stay the day in their shelter, because on snowy, cold days I have flushed them out from underfoot under the snow even at noon.
From all appearance, many northern birds are excited by snow, especially that of the first snowstorm of the year. Both ravens and ptarmigan then become visibly animated, rolling, sliding, and bathing in the fluff when it is not yet packed. Owls, crows, finches, tits, and kinglets also bathe in snow (Thaler 1982).
Snow has been such a constant feature of their environment that many northern animals have become well adapted to it and now depend on it. Perhaps none depends on snow more than the snowshoe hare. The size of this hare’s tracks are all out of proportion to the animal’s size. As a result of its low foot-loading, the hare can walk, hop, and run very near the top of the fluffiest snow. As a consequence, the more that snow accumulates throughout the winter, the more easily the hare can reach its food, the fresh twigs of small trees and brush. Thus, the twigs feed the hares, who are in turn reincarnated into fox, bobcat, lynx, fisher, weasel, great horned owl, goshawk, and red-tailed hawk. Yet despite the hares’ rapid recycling into other lives, their populations persist because of tricks of individual survival coupled with a legendary reproductive potential. (A female hare may have four litters per year, with up to eight young per litter. The young are furred when born and almost ready to run and soon ready to reproduce.)
Regardless of how fast the hares reproduce, it would not take long for the predators to deplete them if it were not for their camouflage. The snowshoe, also called varying hare, changes from brown to a coat of pure white fur in winter. However, the more effective the camouflage is for one season, the less effective for another, and a hare’s trick to survival requires getting the timing just right. It is hard to be exact in when it is best to become white or brown, because the coat change requires a month or more, and a snowstorm can transform a landscape in minutes. Proximally the hare’s timing is determined by day length, but ultimately it must be dictated by the average time that the ground is snow-covered; the timing of the genetically fixed color change of locally adapted hares necessarily reflects the historic patterns of when there was snow cover because off-color hares are the first to be eaten and have their meat be reconverted into the next life, that of predators.
In the woods in western Maine the hares are almost all white by the end of November, the most usual time that there is continuous snow cover. However, in some years when the first snowfall is late, the hares show up for the whole duration of that lateness as if they had been marked in hunter’s fluorescent orange. Within a few minutes after it snows, however, they become practically invisible. I doubt that a hare knows whether or not it is invisible because the totally white hares I’ve seen on brown background made no apparent effort to hide. Nevertheless, they could still have some change in behavior that compensates for their inability to accurately time the molt with snowfall events. For example, I routinely see almost no hares’ tracks in the woods of western Maine after early snows in late October and early November, although their tracks are common in the same year and at the same sites in December. I had first suspected that the hares might migrate, until I happened to walk one November up on the ridges of Mount Bald near my camp. A hare could get up there in minutes. Here in the spruces near the top, I suddenly found many hare tracks, making me wonder if the molted animals move up into the hills where the snow comes earlier, and then later come down after the first snow falls in at the swamps, their preferred habitat.
In March the hares’ white fluffy winter fur begins to drop out and is again replaced with the summer brown. Golden-crowned kinglets take advantage of this fortuitous timing of the hares’ coat change to gather the cast fur for insulating their nests.
The hares’ winter survival depends not only on ability to hide, but also to run when needed. Unlike many animals in winter they can and do stay lean, accumulating essentially no body fat, because food is almost always within reach and food energy need not be stored. Being lightweight and big-footed gives them an advantage for moving quickly on the snow. But sinking in even a little bit slows a runner down. Yet there is a limit to how big the feet can be before they hinder rather than enhance running speed, and snowshoe hares are probably already close to as light-and big-footed as they can get. They have, however, another behavioral trait that is apparent at a glance (the patterns of their tracks in winter woods): hares follow others’ tracks and thus pack down the snow, making well-trodden highways. Hares traveling along these paths then clip the twigs along the way, and knowing the paths well, get the jump on any predator giving chase.
The snow can be an enemy, too. Small animals in the subnivian zone, that area in or under the snow, can at times be sealed in when the upper snow surface melts in the sun and then freezes at night into a solid crust. Grouse sometimes get trapped under the crust and become prey to foxes. Shrews that emerge onto the crust and do not quickly find a hole back down to safety, may be taken by a predator or simply freeze to death.
Polar bears have nothing to fear from the crust. They dig their dens into drifts of snow where they have their cubs, suckling them in warmth and safety, and hibernate for the six months of the arctic winter. My Winter Ecology students, like polar bears and Athabaskan hunters, also make temporary shelters in mounds of snow.
Every winter I take ten to thirteen students with me to my camp in the Maine woods, where we live in my homemade log cabin (two-story) that is without electricity but with a woodstove. We get water from snow we melt, or from a well at some distance. We bake our own bread and have been known to fry our own voles. We take long unstructured walks through the woods in the first week. In the second two weeks everyone settles on an independent research project that I guide. The hard part comes when everyone gets back to campus during the spring semester and analyzes their results and writes their scientific reports.
Building snow shelters is not one of the official projects. But we occasionally make them nevertheless. We start by heaping up a great pile of snow. A few hours after the snow has been heaped up, ice crystals interlock and combine to make a solid mass that can then be excavated to produce a snug and warm cave for overnighting.
Near the top of any snowpack, the snow gets denser as the crystals bond together. Meanwhile, close to the ground, where it is warmer than at the surface, water vapor from disintegrating snow crystals migrates upward and recondenses and freezes onto the upper snow pack crystals. In time, the growth of the upper ice gains at the expense of the lower, and a latticework of ice pillars and columns and extensive air spaces at ground level create the subnivian space that is, in a sense, a continuous snow cave inhabited by mice, voles, and shrews.
Within this space, temperatures are physically “regulated” within a degree or two of the freezing point of water, all winter long. Several factors are involved. First, as already mentioned, snow affords remarkable insulation and, even at -50°C, heat rising from the earth generally keeps the temperatures near the ground close to 0°C. When both ice and water are balanced at near 0°C in the subnivian zone, the temperature is stabilized since whenever heat is lost through the snowpack to cool this space slightly below 0°C, there is then a water-to-ice-crystal conversion, which releases heat. Similarly, whenever ice turns to water, the process requires or uses up heat. Thus, as long as both ice and water exist side by side, they constitute a thermostat keeping temperatures constant. Only the amounts of ice and water vary, depending on the amount of heat loss or input.
In New England, the subnivian zone is the home of voles (a type of short-tailed mouse): principally the meadow vole (Microtus pennsylvanicus) and the red-backed vole (Clethrionomys gapperi) as well as the masked shrew (Sorex cinerius), smoky shrew (Sorex fumeus), pygmy shrew (Microsorex hoyi), and short-tailed shrew (Blarina brevicauda). Every spring, right after the snow melts, or just as the last inch or two is melting, I see the labyrinth of the Microtus tunnels fully exposed on the surface of the ground. Also fully exposed lie the grass nests of these rodents, many of which will soon be occupied by bumblebee queens starting new colonies.
Mice in the subnivian zone, eating bark.
The spring of 2001 provided an especially impressive demonstration of the importance of the subnivian world to meadow voles. Record amounts of snow had fallen in March in Vermont, and the voles appeared to be having a population explosion. Like lemmings, their close relatives, meadow voles have an awesome reproductive potential. One well-fed captive vole produced seventeen litters in one year, averaging five babies each after twenty-one-day gestation periods. The young females, in turn, can produce their own litters in one month. At such reproductive potential, it would not take long for them to carpet the earth. Luckily, such horrors of exponential growth are seldom realized. Instead, the voles’ role in the economy of nature is, like that of hares, to convert vegetation into the protein-rich dietary staple of many predators that rely on them in winter, principally foxes, weasels, fishers, coyotes, and bobcats. The summer shift includes hawks and snakes.
In some areas all of the young sugar maple trees, box elder, and white ash trees were debarked right up to the snow line, but never above it. The tree damage caused by voles in winter is well known by orchardists, who would lose their young fruit trees every winter if they did not in the fall cover every one of their young trees in an artificial barklike commercial plastic stripping up to the level the winter snows are expected to reach. I learned this the hard way when I planted apple trees in the field by my cabin; by spring, every single one was stripped of bark for a foot above the ground, in what had been the subnivian zone in the winter. Older trees, once they have developed a thick layer of their own bark, are protected. The cambium, or inner live layer of bark of trees, is a favorite food of many herbivores, and the thick outer dead layer is essential armor. Like most armor, its usefulness is only apparent in time of need. For trees, the greatest need for thick bark is in the winter when more easy-to-eat foliage is not available.
Due to the protection of the snowpack and the cozy subnivian zone under it, voles are able to get a jump on spring and reproduce sometimes two to three months before the snowpack has melted. Wild spring flowers of many kinds also get an early start in the relative warmth of the subnivian zone under the snow. Some, like the snowdrops in our gardens, grow in March under the snow and send their flowers directly through the snow.
Peter Marchand, a winter ecologist who has extensively studied snow cover at the Center for Northern Studies in Vermont and elsewhere, has wondered how organisms that are buried under snow get their cue to start growing or breeding. How do they know, as they appear to, that the snowpack is about to melt off? Do they sense the sunlight? To investigate this problem, Marchand and his students studied the light-transmitting properties of the snowpack, finding that as the snow became increasingly more compact, it extinguished more and more light. But only up to a point. To their surprise, they found that when they mimicked the melting and refreezing that occurs in the spring when snow density increased, the snowpack became almost icelike in consistency. Then, despite or because of its greater density, it transmitted more light. Marchand speculates that this snow-penetrating light is sensed by the voles and stimulates them to start reproducing, thereby giving them their legendary reproductive potential. Alternatively, the plants detect the light first, and by growing, produce chemicals that then give the animals that eat them an indirect cue that then stimulates their reproductive activity.
For some animals in the winter woods, the subnivian zone is never totally separate from the subterranean zone. If it were fully separate, then few small mammals would survive the winter, because in some years subzero temperatures occur a month or two before there is an appreciable layer of snow. During these times, the shrews and voles inhabit the space under the leaf mold, or they live in rotting stumps that are riddled with cavities. At times they also burrow into the soil, living beneath the frost line. Still other animals, such as the molelike short-tailed shrew and the star-nosed mole and its cousin, the hairy-tailed mole, stay there permanently. The presence of the subnivian zone merely raises the frost level and allows them to be closer to the surface of the ground, where there are likely more insect prey.
While the snowpack is a haven for many small animals, it provides a severe challenge to the larger ones that hunt them for a living. Some predators would be unable to live in the north in winter if it were not for their specialized ways of hunting the subnivian prey. These winter-active hunters include the aforementioned weasels, foxes, and coyotes, and the great gray owl.
Foxes and coyotes locate mice by sound and pounce on them by crashing with their front paws through the snow. Their bouncing collapses the rodents’ tunnels, temporarily trapping the intended victim. Great gray owls (Strix nebulosa) also have acute hearing and can detect a meadow vole’s movements under snow from thirty meters away. Drawing near one, they plunge from twenty-five feet in the air, and with their balled-up feet can punch through crust thick enough for a person to walk on. They then catch the mice that are temporarily detained by the collapsed snow by repeatedly clenching their toes, sifting through the snow with their long talons. Great grays are among the largest of all owls, being nearly three feet tall, but a large part of their bulk is composed of thick layers of insulating feathers. They are not as powerful as great horned and snowy owls that specialize on hares. In calm weather the grays hunt both at night and during the day, and temperatures as low as -43°C do not cause them to leave their northern haunts. They do leave regularly, though, when their prey is depleted due to disease or overhunting. Rodent population crashes in one area do not preclude population explosions in another, and so the owls wander widely. So I too wander, a hunter of winter marvels.