Bitterroot Mountain Snow
© Sarah Walker
Remains of subnivean tunnels appear when snow melts in the Bitterroot Mountains, long after the small mammals who used them during the winter have returned to summer's sunlight, warm days, and green plants.
While Lewis and Clark wintered in rainy Astoria, snow buried their tracks on K'useyneiskit. When they returned here, even in late June of 1806, Lewis noted, "We found ourselves invelloped in snow from 12 to 15 feet." The same snow that deterred the Corps on both their crossings of K'useyneiskit provides the winter environment for those smaller animals who neither hibernate nor migrate, but remain to deal with whatever winter hands out in Idaho's Bitterroot Mountains.
Deep snow forms an insulating blanket over the summer-warmed earth. Underneath the deep snowpack lies the subnivean (undersnow) world where small mammals—and occasionally birds—seek shelter and find food.
In the secret recesses of this dim environment the snow layer closest to the ground separates into coarse crystals interspersed with openings. This layer is called "depth hoar" and while it signals avalanche danger to backcountry skiers, it provides shelter for smaller animals like shrews, voles, and mice to carry out daily life protected from falling snow, fluctuating temperatures, and winds. Larger species, like weasels, squirrels, and pine martens, excavate networks of runways and burrows throughout the snowpack.
The smallest predators found in the subnivean are shrews1—tiny, energetic mammals with pointy snouts, that forage restlessly along the ground for insects and other small invertebrates. As fewer and fewer insects and spiders remain active during winter, shrews switch to insect eggs and pupae, seeds, or even carrion in the frantic search to feed their tiny bodies (shrews consume more than their body weight every day). Fiercely territorial in summertime, they avoid combat during winter, as a way to conserve energy. Shrews eat constantly and are active round the clock during their short lifespan of one year. They serve as prey for weasels and owls.
The demands of winter may find some species abandoning their solitary lives to practice communal living. In the subnivean world, voles and mice stay warm by huddling together in grassy nests. Voles2 spend time arranging their network of separate nesting, food storage, and toilet areas, perhaps to keep their homes secret from weasels or the other many carnivores and raptors that dine on them. Fortunately, vole mortality is offset by high birth rates, because voles are considered important contributors to forest health.3
Tree squirrels4, arboreal mammals whose noisy chatter is heard throughout the forest, move into tree cavities for winter sleeping and resting. But they too must tunnel into the snowpack to access their winter larders of cached cones and dried mushrooms stored at (or below) ground level. During severe storms, tree squirrels may wait out the weather by temporarily moving into their pantries under the snow. Like voles, squirrels benefit the forest by dispersing fungal spores.
Higher in the subnivean food web are weasels5, predators that are oddly ill-suited to cold: their long slim shape loses heat, and their coats do not become significantly thicker during winter. But weasels are constant hunters whose ability to slip through subnivean runways earns them meals of voles, mice, and squirrels. One of the more macabre ways that weasels create a warm microclimate under the snow is to line their dens with fur plucked from the small mammals that constitute most of their prey. Weasels themselves are preyed upon by owls.
Do predators and prey end up sharing the same travel ways in the subnivean world? Perhaps avoidance is made easier under the snow where the moister air aids scenting. Still, there must be unintentional meetings when winter traffic is limited to tunnels in the snowpack and openings in the depth hoar. Pocket gophers6, who move their summertime underground tunneling activity up to the ground's surface in winter, are able to scurry in reverse, a useful adaptation in such a world! The subterranean tunnels of these expert excavators loosens and aerates soil, a benefit to the plants pocket gophers love to eat.
Even birds, and snowshoe hares, sometimes seek shelter in deep snow. The grouse that Lewis observed7 along K'useyneiskit have a curious winter practice of diving under the snow for short periods to escape what Lewis called "winter with all it's rigors." Imagine Lewis' surprise had he witnessed their explosive exits, a startling event memorable to many backcountry skiers in the Bitterroot Mountains.
Content reviewed by Jay Shepherd, University of Idaho
Louise R. Forrest, Field Guide to Tracking Animals in Snow. Harrisburg PA: Stackpole Books, 1988.
James C. Halfpenny and Roy Ozanne, Winter: An Ecological Handbook. Boulder CO: Johnson Publishing, 1989.
Peter J. Marchand, Life in the Cold. Hanover NH: University Press of New England, 1991. 2nd edition.
Peter J. Marchand, "The Underside of Winter," Natural History, February 1993, pp. 51-56.
Chris Maser, Mammals of the Pacific Northwest. Corvallis OR: Oregon State University Press, 1998.
1. Sorex (SORE-ex), "shrew-mouse."
2. Microtus (my-CROH-tus), "small ears."
3. Voles love to eat truffles, the fruiting bodies of the kinds of fungi whose association with tree roots benefits growth. The voles help make the fungi available to trees by spreading fungal spores around the forest in their fecal pellets.
4. Tamiasciurus (TAM-ee-uh-sy-OR-us), "hoarders with tails that shade them."
5. Mustela (muss-TELL-a), Latin for weasel.
6. Thomomys (tho-MO-miss), "heap mouse" for the piles of excavated soil brought to the surface.
7. Lewis described three grouse species: The blue grouse, Dendragapus obscurus (den-DRAG-uh-pus ob-SCURE-us), "secretive tree-lover"; the spruce grouse, Dendragopus canadensis (den-DRAG-uh-pus can-uh-DEN-siss), "tree-lover of Canada"; and the blue grouse, Bonasa umbellus (bone-AH-sah um-BELL-us), "bison" (refers to the low sounds made by males), and "umbrella" (refers to the shape of its fanned tail).
Supported in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee