(c) Jack Ballard
Among humans who inhabit the mountains, there exist two types of people. One group consists of recreational property owners who stream into the highlands in early summer, unlocking homes and cabins in anticipation of passing cool, aspen-shaded weekends away from the heat and bustle of the city. These folks delight in the season when wildflowers color the landscape and berries drip from the prickly stems of wild raspberries. Come the autumnal equinox, though, the summer dwellers are as industrious as the beavers at work in a nearby pond gnawing softwoods for a winter food supply. Windows are shuttered, pipes drained and a fresh coat of sealer applied to the back deck. When the key turns in the lock after the final winterizing tasks are completed and the SUV points its shiny snout down the driveway for the last time, the fair weather residents won’t pass another night in the mountains until spring.
In contrast are the stalwarts for whom tire chains, snowplows and mercury in the bottom of the bulb are as predictable in the yearly cycle as the tang of evergreen pollen and ripening rose hips. Ranchers in isolated mountain valleys and residents of ski towns exit the same front doors in January as June, having learned to take the hardships of winter in stride, perhaps even reveling in the severe beauty of driving snow and hoarfrost.
Like humans, many animals that share summer quarters in the alpine zone part company autumn. Some migrate to the lowlands to escape the hardship of deep snow and intense cold, while others remain in the high country, capitalizing on habitat niches that allow them to secure shelter and nourishment sufficient to weather the winter months in health. From a biological standpoint, the presence of living creatures at or near timberline in temperatures and conditions that would tax the survival skills of the most well-prepared human is something of a mystery. Why don’t these animals descend to more hospitable elevations? What challenges do they face for survival and what adaptations make it possible for them to hang tough in their thin air strongholds? Are there any biological advantages associated with wintering at high elevations?
Among large ungulates common to the Rocky Mountains, populations within several species routinely winter at elevations much loftier than those commonly associated with their kind. While most bighorn sheep herds descend from the mountains to more temperate lowland areas, some actually remain on remarkably high ridges. At such elevations, annual snowfall may easily exceed ten feet, an accumulation more than three times the height of the average bighorn.
Do the stately mountain sheep burrow around beneath the snow like a pika? Not hardly. On the exposed ridges at timberline, most of the snow drifts into the timber or sheltered pockets on the leeward side of the mountains due to the seemingly endless action of the howling alpine wind. As a result, broad expanses of forage are left uncovered by snow. Here the bighorns graze on cured grasses completely exposed by the scouring wind, or paw through a layer of snow much thinner than that which settles in the timber. Bedding areas on south-facing slopes provide exposure to the sun for warmth during the day, and at night, temperatures are often milder on the high ridges than in the mountain valleys where the coldest air tends to settle.
The winter solitude of high elevation habitats may also hold another advantage for bighorns. Predators capable of taking an adult sheep, most notably wolves and mountain lions, seek out areas with high densities of their favored prey. In the western states, elk and deer are the species most regularly targeted by these persistent cullers of ungulate herds. As the majority of mountain dwelling deer and elk migrate to valleys and foothills for the winter, wolves and lions follow, leaving the bighorns up on the ridges at lower risk of predation. Fewer encounters with predators requiring flight also allow bighorns to conserve energy reserves to combat the more persistent threats of lingering cold and snow.
Although elk and deer typically migrate to lower winter ranges, one notable exception occurs in each of these species. While the vast majority of females and young males congregate in the lowlands, older males often winter up on ridges and mountainsides. Several factors contribute to this atypical pattern of winter behavior in mature bucks and bulls. First of all, the sheer mass of older males makes it possible for them to survive winter conditions that would prove fatal to smaller animals of their kind. For decades biologists have understood that body mass greatly enhances an animal’s ability to withstand cold and the scarcity of foodstuff associated with winter. Accordingly, mule deer of the far north are larger on average than those of the south.
In seasons of severity, winter mortality follows a predictable pattern among hoofed mammals. Small bodied species such as antelope are the first to succumb, followed by deer. Elk and bison, with their larger body mass, are at acute risk for starvation only during the most prolonged and harsh winters.
By the same token, the animals within a species holding the greatest body mass are most capable of surviving an extreme winter. Among mule deer, a mature buck may weigh twice as much as a similar aged doe or a young male. A similar situation exists among elk. As a result, the middle-aged men and graybeards of the species can successfully remain at higher elevation than the women and kids. As with bighorn sheep, the smaller herds and isolation associated with these loftier winter habitats also gives bucks and bulls a measure of insulation from predation not afforded to the masses in the valleys.
In addition to enhancing an animal’s ability to withstand severe weather, the larger body mass of older males among species such as elk and bison give them another advantage over their counterparts. Extra height and muscle also allows them to move about in deep snow that would be a serious impediment to smaller members of their kind. Research reported by the Wildlife Management Institute indicates that when snow reaches a depth of around two feet, the mobility of elk calves is impaired. Mature cows can handle about another foot of accumulation, but isolated herds of large bulls have been known to winter in snow depths exceeding five feet.
What then, explains the flittering, darting and bounding presence of some the West’s tiniest creatures near timberline in the dead of winter? Swooshing along a backcountry ski trail at 7,500 feet, the nasal yank-yank of a nuthatch announces its presence long before I spot it tiptoeing headfirst down the scaly trunk of a fire-killed pine. Further down the trail, a trio of feathered sprites dart happily about in the beryl branches of fir tree. Jaunty black caps and soft gray jackets betray the identity of the chickadees, but should one harbor a doubt, the unmistakable five note melody of the tiny birds quickly lays all speculation to rest.
What are these downy tidbits doing here? With body mass measured in ounces — and precious few of those — shouldn’t these little folks be passing the winter at a suburban bird feeder instead of the taiga?
Given the opportunity, chickadees and nuthatches are enthusiastic patrons of human handouts, especially in the year’s cold season. However, they’re also possessed of several adaptations that allow them to tough it out in the high country. Birds of both species are known to store or “cache” food items such as nuts and seeds which they return to consume in periods of scarcity. These perky avian midgets also have specialized adaptations for feeding which make them very efficient foragers. Chickadees have unique muscles in their tiny legs which allow them to feed in an upside down position and hang for relatively long periods of time to the ends of tiny branches. Nuthatches cling to the trunk of a tree facing downward as easily as upward. These adaptations allow both species to glean nourishment from locations that would be too difficult to reach by many other birds. Additionally, chickadees are capable of reducing their body temperature during extremely cold periods to conserve energy.
A variety of other birds remain in the high country throughout the winter as well. Gray jays are one such species, raucous robbers of any food supply which share the chickadee’s proclivity for caching booty for a winter larder. Blue grouse also winter up on the mountains, commonly passing the coldest months in evergreens right at timberline. However, unlike jays who spend the entire year at high elevations, grouse actually migrate upward with the first snows of autumn. Seed and berry eaters during the summer, in winter grouse subsist exclusively on the needles and buds of evergreens, preferring fir trees to all others.
Unlike most other creatures for whom a diet of pine needles would prove toxic, blue grouse have special digestive adaptations which allow them to obtain nutrition from the needles of evergreens without harm. However, this dietary peculiarity still doesn’t explain exactly why grouse ascend to harsher climates with the coming of winter. After all, there are evergreens at lower elevations in which the grouse might as easily roost and forage. Research indicates that grouse prefer older trees for their winter feeding. Perhaps a higher percentage of older trees are found at higher elevations. Whatever the reason, blue grouse are peculiar in their autumnal migration which takes them upslope rather than down.
Along with birds, a number of small mammals hang tough at upper elevations throughout the winter. Red squirrels are one such animal. An extremely active creature that requires a high energy diet and typically weighs less than a half a pound, at first glance the red squirrel would seem biologically unsuited to withstand even a mild Rocky Mountain winter. However, these aggressive little loud-mouths have two adaptive strategies which allow them to pass the lean season in relative comfort. First of all, red squirrels create huge caches of seed-laden pine cones in late summer and fall. Sometimes three feet deep and containing thousands of cones, squirrels feed on these throughout the winter. Also, while the furry fireballs occasionally scurry about the trees on warm days, they spend most of the cold months in expansive runways under the snow.
Other tiny chewers of roots and seeds pass the winter under snowdrifts as well. Voles, mice and pika utilize such a survival strategy. However, while a deep, white blanket insulates them from the cold, it doesn’t completely protect them from predators. Pine martens and ermine, two members of the weasel family, also pass the winters at alpine elevations. Both are active hunters, constantly on the prowl to fuel their high metabolism with prey.
Ermine (the name given to short-tailed weasels in their white, winter pelage), burn energy at astonishingly high rates. With a resting metabolism over ten percent higher than other comparable sized mammals and a very small stomach, ermine must eat every few hours to survive the winter. However, the lean, skinny body which limits digestive capacity also gives weasels a shape perfectly adapted to hunting. They can swiftly race through the underground or under-snow burrows of rodents. Having made a kill above or below the soil, ermine use a survival tactic similar to that of their prey. They cache uneaten portions in a handy location for later consumption.
As evidenced in the habits of all creatures that winter at high elevations, access to sufficient nutrition is essential for survival. Ermine require rodents, who in turn need meadows and open areas for their own survival. Red squirrels and grouse must have healthy stands of conifers. Wintering bull elk and bighorns are bound to areas that produce an excess of summer forage so that some remains for winter.
While few humans venture into the highlands when the snow lies heavy on the drooping branches of stoic evergreens, all have a stake in maintaining these habitats for the welfare of wildlife. Natural forest succession and freedom from frequent disturbance are essential for the furred and feathered souls who make a year-round home in the high country. For countless generations, they’ve endured the rigors of the alpine winter. As long as we do our part, they’ll be hanging tough for centuries to come.