# Animals in the Snow > Why do caribou crowd into some patches of forest and avoid others? In this 1960 classic, William Pruitt shows that the answer lies in the snow itself: its hardness, density and depth. He takes readers beneath the snow too, into a dark, quiet world where voles dig air shafts and red squirrels shelter from the deepest cold. Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/1960-jr-animals-snow/ ## Citation and access - **Authors:** William O. Pruitt Jr. - **Year published:** 1960 - **Journal or source:** Scientific American - **Full citation:** Pruitt, W. O., Jr. 1960. Animals in the snow. Scientific American 202(1): 60–68. - **DOI:** 10.1038/scientificamerican0160-60 - **Access status:** Publisher link only - **Where to find it:** https://doi.org/10.1038/scientificamerican0160-60 ## Tags - **Species:** Mammals > American red squirrel; Mammals > Caribou; Mammals > Moose; Mammals > Red-backed vole; Mammals > Snowshoe hare - **Region:** North America > United States - **Behaviour and ecology:** Behaviour; Feeding; Habitat selection; Migration; Movement; Predation - **Methods and technology:** Aerial surveys; Indigenous knowledge - **Data type:** New field data; Review - **Observer qualification:** Not needed - **Discipline:** Behaviour and ecology; Indigenous knowledge - **Year published:** 1960 ## Detailed keywords - **Species:** Studied or described: caribou (North American caribou), moose, varying hare (Lepus americanus), red squirrel (Tamiasciurus hudsonicus), red-backed vole, shrews, voles and lemmings, including the varying or “hoofed” lemming. Also mentioned: fox, wolf, lynx, elk, owls (said to catch voles at their snow shafts), migratory insect-eating and ground-feeding birds. Given as examples of animals that cannot cope with snow: pronghorn antelope, wild turkey and opossum. Extinct mammals discussed: mammoth, mastodon, an antelope, certain musk oxen, woolly rhinoceros and the “Irish elk”. The paper gives scientific names only for the varying hare and red squirrel. - **Location and habitat:** Mainly the taiga (subarctic spruce forest) of interior Alaska, United States, with tundra and frozen lakes for comparison. The opening photograph shows caribou on a Canadian lake. Snow words come from the Kobuk valley (Alaska), Dindye (Fort Yukon, Alaska) and Chipewyan (northern Alberta, Canada) languages. Work by Russian scientists in the taiga of the Union of Soviet Socialist Republics (U.S.S.R.) is also discussed. Snow covers more than half of the land in the Northern Hemisphere at some time of year. - **Topics:** snow ecology; snow cover structure (depth hoar, wind crust, wind slab, sun crust, snow shadow); hardness and density of snow layers; winter distribution of caribou; digging through snow for food; migration as a response to snow; anatomical adaptations (long legs, large feet); packed trails, “yards” and runways; life under the snow (the subnivean environment); snow as insulation; under-snow temperature, light, sound and humidity; carbon dioxide build-up and vole “ventilator shafts”; owl predation at shaft openings; red squirrels moving into tunnels in deep cold; Formozov’s classification of animals by snow adaptation (chionophobes, chioneuphores, chionophiles); snow and Pleistocene extinctions; Indigenous snow vocabulary and technology (komatik sled, toboggan, snowshoes, snow houses). - **Statistics or models:** None - **Data type:** New field data from the author’s own work in interior Alaska (aerial surveys of wintering caribou combined with snow measurements on the spot; soil, moss and air temperatures; live trapping of voles under the snow), with under-snow temperatures recorded through the winters of 1954–1955 and 1955–1956. Other dates of fieldwork are not given. Also a popular review of work by A. N. Formozov and N. V. Bashenina. A general-audience magazine article with no reference list. Possible inconsistency: the text gives about 60 grams per square centimetre as the critical hardness that keeps caribou out, but the snow profile figure for a lake where caribou concentrated shows a bottom layer of hardness 100. The pages carry a 1959 copyright line, though the issue is January 1960. ## Synopsis ### 1. What they did William Pruitt argues that snow, which most people only want to clear away, is one of the most important parts of the environment for northern animals. Drawing on his own work in the taiga, the spruce forest of interior Alaska, he explains how snow changes after it falls and how animals live with it. He used several kinds of evidence: - Snow measurements. He measured the hardness, density and depth of each snow layer in cross sections of the snow cover, in forests, on lakes and on the tundra. - Aerial surveys of caribou. He surveyed wintering bands of caribou from the air and compared where they were with the snow measured on the spot. - Temperatures. He recorded temperatures in the air, at the moss surface and in the soil under the snow, including through two winters (1954 to 1955 and 1955 to 1956) at one Alaskan station. - Live traps under the snow. He sampled small mammals to see where red-backed voles were active. He also drew on the snow words and know-how of northern Indigenous peoples (the paper calls them Eskimos and Indians), whom he calls “the wisest instructors” on snow, and on the work of the Russian naturalists A. N. Formozov and N. V. Bashenina. ### 2. What they found How snow changes - New taiga snow is mostly air: it may weigh only 5% as much as the same volume of water. - Heat and moisture rise from the warmer soil, and the bottom layer turns into coarse grains called depth hoar (“pukak”), sometimes leaving an empty space at ground level. - Thaws, rain, wind and spring sun create hard crusts and dense layers. On the tundra, wind can pack the whole snow cover hard enough to carry a fox, wolf, caribou or person. - Spruce branches catch much of the falling snow (“qali”), leaving a bowl-shaped dip around each tree trunk (“qamaniq”). Caribou - Most caribou leave the tundra’s hard snow in autumn and move inside the tree line. - They gather where snow is soft, light and thin, so they can dig down to their food. These areas are bounded “as if by a fence” by snow that is hard, dense or thick. - The critical hardness seems to be about 60 grams per square centimetre, the critical density about 16% of that of water or more, and the critical depth about two feet. Through the winter the herds move as these “fences” of unsuitable snow shift. - They also rest and chew their cud on lakes where the snow is similar. Moose and varying hares - Moose wade through snow on their long legs, but when snow reaches about three feet they pack it into trails or “yards”. - Varying hares travel on top of the snow on their oversized feet, but where the snow is too soft they too pack regular trails and runways. Life under the snow - Only the larger mammals (hare, fox, wolf, lynx and moose) can stay above the snow in the cold. Shrews, voles and lemmings are too small to keep warm there and live under the snow. - Small mammals go under the snow in autumn once it is about six inches deep. The time between the first hard frosts and that depth is probably their hardest time. - Under the snow the forest floor seldom drops below 20°F, even when the air is 50 or 60 degrees below zero. At one spot under two and a half feet of snow, he found no temperature change for nine days while the air swung between 24°F above and 28°F below zero. - Under the tree snow shadows the ground is colder: with the air at 25 below, the soil an inch down was 2.5 below zero at a tree’s base but 9.5 above just beyond the dip. Live trapping showed red-backed voles avoid these cold spots. - Under the snow it is dark (one foot of snow lets through 8% of the light, two feet only 1%), silent, calm and humid, with temperatures mostly between 15 and 25°F above zero. - Bashenina found that carbon dioxide can build up under taiga snow, and voles then dig “ventilator shafts” to the surface, where owls catch them. Formozov believes this habit lets some owls survive the subarctic winter. - Red squirrels move from the trees into tunnels under the snow when it gets colder than about 25 to 30 degrees below zero, and may not be seen for weeks. Snow and other birds and mammals - Most birds and some large mammals migrate in winter not because of the cold but because they cannot reach food under the snow. Migratory birds are mostly insect-eaters and ground-feeders; among mammals, elk and caribou are the most notable migrants. ### 3. What they concluded - Snow is a major part of the environment of northern animals, not just a hindrance. - Formozov’s classification sorts animals by how they cope with snow: chionophobes cannot adjust to it (pronghorn antelope, wild turkey, opossum); chioneuphores can survive in snowy regions (shrew, fox, vole, moose, elk); chionophiles have special adaptations and live only where there is snow (varying hare, caribou, the “hoofed” lemming). Pruitt says this classification should underlie all wildlife management in snowy regions. - Without the snow cover, the taiga would lose most of its mammals. - He suggests that snowy conditions at the start of the Pleistocene may help explain why mammals such as the mammoth, mastodon and woolly rhinoceros disappeared: they may have been animals of cold places with little snow, trapped by advancing deep snow. - Indigenous snow words describe snow conditions more usefully for animal studies than official weather terms. ### 6. Other technologies, skills or innovations - Measuring snow hardness by the weight each square centimetre can bear, and density relative to water, layer by layer. - Combining aerial surveys of animals with snow measurements on the ground. - Using Indigenous snow vocabulary (a glossary of Kobuk valley, Dindye and Chipewyan words) as precise scientific terms. ### 7. Implications for conservation (from the paper) - Formozov’s classification of animals by their snow adaptations “should also underlie all wildlife management programs in snowy regions”. ### 8. Implications for tracking (Original Wisdom's observations) _These are Original Wisdom's observations, not the authors'._ - Snow conditions decide where many northern animals travel and leave tracks and signs: caribou gather where they can dig, moose and hares pack trails, yards and runways when snow gets deep or soft, and voles open shafts to the surface. - Under-snow life means some animals, such as voles and, in deep cold, red squirrels, may leave few tracks on the surface for long periods, which matters when reading or counting snow tracks. ## Cite this summary Lawrence, K. 2026. Summary of: Animals in the Snow (Jr., 1960). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/1960-jr-animals-snow/