In the Rocky Mountains, tree line is around 11,000 to 12,000 feet (3,350 to 3,650 meters) above sea level! Tree line means the elevation where conditions are too harsh for trees to grow, due to low temperatures, high winds, or other environmental factors. For humans, traveling above tree line while hiking, skiing, or mountaineering is limited to brief visits due to exposure to the elements and physically strenuous conditions. Remarkably, some animals are specially adapted to live their entire lives near or above tree line! Here are five amazing animals that you might spot on your next visit above tree line!
Mountain goats are found living in the mountains of Canada and the United States, from Alaska to Colorado. Mountain goats thrive above tree line, foraging in alpine meadows where they eat a variety of alpine grasses, forbs, shrubs, moss, and lichens. Grazing in alpine areas comes with its challenges – a lack of trees in the mountain goat’s preferred foraging habitat leaves these creatures exposed and vulnerable to predators. Consequently, the mountain goat has evolved to be an excellent climber. With large, muscular forequarters and elbow joints that can “lock” as it bounds upwards, the mountain goat is a natural and efficient climber. When it encounters a predator such as a mountain lion or grizzly bear, a mountain goat uses its unique climbing ability to rapidly scale hill slopes and rocky cliffs that are too steep for its predators. These steep parts of the landscape are called escape terrain and are essential for mountain goats to avoid predation above tree line. Their shaggy white coat may also help them camouflage with snow.
Some mountain goats may retreat slightly below tree line in the winter season, but others do not. Regardless, they must face a variety of harsh weather conditions near tree line year-round. Winds over 40 miles per hour (64 kilometers per hour) and snow may occur in the alpine in any month of the year. When snow is shallow, mountain goats may kick through snow with their hooves to uncover plants to eat. When snow is deepest in winter, their capacity for mountain climbing allows them to relocate to the steepest parts of the terrain where snow cannot pile up, or to wind-scoured or melted faces of the mountain. Truly the mountain goat is a creature equipped to survive and thrive in the harshest of alpine conditions!
Rocky Mountain Bighorn Sheep
While sometimes confused with the mountain goat, the Rocky Mountain bighorn sheep is in fact a different animal species in the same family, Bovidae. With a little practice, you can recognize the differences between bighorns and mountain goats. Male bighorns have large, rounded horns, distinct from the smaller, straighter horns of mountain goats. Female bighorns appear more similar to mountain goats, with shorter, less rounded horns. However, all bighorns have distinctly shorter and darker coats, while mountain goats are shaggy and light-colored. The bighorn’s white rump is also useful when identifying them (pictured above).
Like mountain goats, Rocky Mountain bighorn sheep need escape terrain to avoid predation. But unlike mountain goats, bighorns are found across a wide variety of elevations. Bighorns are well suited to a variety of habitats from mountains to canyons and deserts, so long as there is steep terrain. During the summer, some bighorn may travel as high as 14,000 feet (4,300 meters)! Bighorns also have some special adaptations to make them excellent climbers. Their hooves are hardened on the edges with a soft, grippy center, allowing them to dig into snow or dirt but also keep their footing on steep or slippery terrain. Bighorns are widely recognized for another specialized feature: the males’ large, curled horns. Male bighorns duel with one another by “butting heads” during the rutting season, a time in the fall when bighorns select their mates.
In addition to the risks of escaping predators, scaling high elevation terrain, and dueling on steep cliffs, bighorns face a threat from diseases. Domesticated sheep farming occurs in parts of bighorns’ natural range. When in close contact, domesticated sheep may pass diseases to bighorns. Limiting interaction between wild bighorn and domesticated sheep populations is important for protecting bighorn populations from disease.
Among the best adapted alpine animals is a small member of the grouse family, so inconspicuous that you might not spot it while hiking until you are nearly on top of it. The white-tailed ptarmigan is an excellently camouflaged bird species, which sheds its brownish gray feathers each fall to reveal brilliant snow-white winter plumage! These non-migratory birds live in mountains at or near tree line year-round. Ptarmigans are built for life in the snow. Their densely feathered feet act like snowshoes, allowing them to walk on top of the snow in their alpine habitats.
Since there are no trees to roost in, the ptarmigan hunkers down in snowbanks during the coldest parts of the year. Ptarmigans reduce their activity level through the winter to conserve energy and warmth. During the warmer seasons, the ptarmigan can be seen out and about in the alpine, foraging for fruit, seeds, and insects. Ptarmigan may also be seen bathing themselves in permanent alpine snowfields on warm summer days. These birds are so well adapted to cold weather that any temperatures above about 70°F (21°C) are actually harmful to them!
Our next amazing alpine animal, the American pika, is a member of the Lagomorph family, the family which includes rabbits and hares. The pika, evolved for life above tree line, is smaller than its Lagomorph cousins and has relatively smaller ears. The pika lives its entire life higher than most animals ever venture, in the scree and talus slopes, areas of loose rock, above tree line on the sides of mountains. Due to their habitat preference, they are limited to living only where there are large mountains, including the American and Canadian Rocky Mountains, as well as some mountain ranges in Washington, Oregon, and California.
Unlike many cold-climate mammals, the pika does not hibernate. During the warm season, the pika must collect and store foods such as grasses and forbs to eat during the colder parts of the year. The pika, which has a naturally low body temperature, survives the harsh alpine winter by living in tunnels in scree and talus, insulated from the extreme cold by the overlying layers of snow. Wildlife and ecosystem scientists consider the pika to be an indicator species – a species whose health (or lack thereof) signals the overall health of an ecosystem. Due to the pika’s need for insulating snowpack in its winter habitat, the pika is sensitive to changes in the amount of snow in alpine areas. Mountain snowpack, the amount of snow that accumulates on mountain tops in the winter, is decreasing due to climate change. Certain recent studies have thrown into question just how sensitive the pika is to change and suggest that this species’ ability to adapt to an altered climate is more complex than initially expected. Scientists are continuing to monitor pika populations very closely.
When traveling above tree line, you will often hear pika before you see them. Their distinctive, high-pitched “cheep” can be heard in many scree and talus fields above tree line. This call serves as a vocal defense of territory or an alarm to warn neighboring pikas of danger.
While pikas are our “alpine rabbit”, marmots are our “alpine squirrel”. As the largest member of the squirrel family (Sciuridae), the marmot is often described as looking like a mountain woodchuck. Unlike the pika, the marmot can tolerate living both above and below tree line. Also different from the pika, the marmot survives the long, cold alpine winter by hibernating: lowering its activity level to decrease its metabolism. Due to the length of the cold season at high elevations, a marmot will spend close to half of its life hibernating!
To survive such a long hibernation, it is important for marmots to build up plenty of fat reserves to last through the winter. During the summer months, marmots can be seen foraging in high elevation meadows and tundra. Marmots are omnivores, who will eat a variety of plant and animal foods depending on what is available: grasses, forbs, seeds, insects, and bird eggs are all on the marmot menu. When they are not busy eating, marmots have active social lives. They tend to live in groups of kin during the summers and will even hibernate in small social or familial groups during winters. Many hikers will describe marmots as curious creatures; when traveling on popular alpine trails, hikers often easily spot marmots peeking out from behind rocks at summits and overlooks. As cute and curious as marmots are, it is important to never feed or touch a marmot. Marmots are wild animals and feeding them will change their natural behaviors, threatening their ability to survive. Marmots also may carry fleas and ticks, which can be vectors of human diseases like bubonic plague and Rocky Mountain spotted fever. It is best to give marmots their space and admire them from a distance.
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References and Further Reading
- All About Birds. (2019). White-tailed ptarmigan. Available: https://www.allaboutbirds.org/guide/White-tailed_Ptarmigan/overview
- B.C. Conservation Data Centre. (2010). Species Summary: Marmota flaviventris. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/
- Beecham, J. J., Collins, C. P., & Reynolds, T. D. (2007). Rocky mountain bighorn sheep (Ovis canadensis): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region.
- Blumstein, D. T., Im, S., Nicodemus, A., & Zugmeyer, C. (2004). Yellow-bellied marmots (Marmota flaviventris) hibernate socially. Journal of Mammalogy, 85(1), 25-29.
- Denver Zoo. (n.d.). Rocky Mountain Bighorn Sheep. Available: https://denverzoo.org/animals/rocky-mountain-bighorn-sheep/
- Innes, R. J. (2011). Oreamnos americanus. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/animals/mammal/oram/all.html
- Lewinson, R. T., & Stefanyshyn, D. J. (2016). A descriptive analysis of the climbing mechanics of a mountain goat (Oreamnos americanus). Zoology, 119(6), 541-546.
- Merten, D., Gallegos, M. (2017). Yellow-Bellied Marmot. Colorado Parks and Wildlife. Available: https://cpw.state.co.us/learn/Lists/Wildlife%20Species/DispForm.aspx?ID=62
- Millar, C. I., Delany, D. L., Hersey, K. A., Jeffress, M. R., Smith, A. T., Van Gunst, K. J., & Westfall, R. D. (2018). Distribution, climatic relationships, and status of American pikas (Ochotona princeps) in the Great Basin, USA. Arctic, Antarctic, and Alpine Research, 50(1), e1436296.
- National Bighorn Sheep Center. (n.d.). Scientific Classification of American Wild Sheep. Available: https://bighorn.org/about-bighorns/
- National Park Service. (n.d.). White-tailed ptarmigan. Available: https://www.nps.gov/romo/learn/nature/ptarmigan.htm
- National Wildlife Federation. (n.d.). American Pika. Available: https://www.nwf.org/Educational-Resources/Wildlife-Guide/Mammals/American-Pika
- Smith, A. T., & Millar, C. I. (2018). American pika (Ochotona princeps) population survival in winters with low or no snowpack. Western North American Naturalist, 78(2), 126-132.
- U.S. National Park Service. (2018). Marmot. Available: https://www.nps.gov/romo/learn/nature/marmot.htm