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December 19, 2016

Caribou, North America’s wild reindeer, have lives apart from their famous role on Christmas Eve. Reindeer and caribou are large, cold-adapted, herding herbivores related to deer, elk and moose.

To learn more about how these arctic antler-bearers spend the other 364 days of the year, we talked to USGS caribou expert Dr. Layne Adams, who has studied these animals for more than 30 years.

Caribou, North America’s wild reindeer, have lives apart from their famous role on Christmas Eve. Reindeer and caribou (Rangifer tarandus) are large, cold-adapted, herding herbivores related to deer, elk and moose.

To learn more about the biology behind these arctic antler-bearers, we turn to our colleagues at the USGS Alaska Science Center, who conduct a wide variety of earth science and ecological science surveys throughout our northernmost state.

We asked USGS caribou (and large mammal) expert Layne Adams, Ph.D., about the lives of caribou for those other 364 days of the year. Adams has studied caribou and their predators in Denali National Park and elsewhere in Alaska for more than 30 years, helping land managers understand the best ways to manage these important species. Adams, a wildlife biologist at the Alaska Science Center, did an online chat with the Washington Post a few years ago:

USGS scientist Layne Adams places a radiocollar on a sedated large bull caribou in Denali National Park, Alaska
Don't worry, this large bull caribou is only resting! USGS scientist Layne Adams places a radiocollar on a sedated caribou in Denali National Park, Alaska. (USGS photo, public domain.)

Here are other Qs and As about reindeer that Dr. Adams answered:

Mother and young caribou on Yukon-Alaska border
A mother caribou and her offspring, east of Chicken, Alaska (on the Yukon-Alaska border).  (Credit: Jamey Jones, USGS. Public domain.)

Why are reindeer sometimes called caribou and caribou sometimes called reindeer?

“Reindeer” and “caribou” are two common names for the same species (Rangifer tarandus), which occurs throughout the circumpolar North.

“Reindeer” is the common name for Rangifer in Europe and Asia, whereas “caribou” is the North American name.

The name “caribou” is a French derivative of a Native American word that means snow shoveler, which is a reference to the fact that caribou are often pawing through the snow to find food underneath.

What are reindeer or caribou?

Caribou and reindeer are part of the deer family — related to deer, moose and elk. They are the only deer species where males and females both grow antlers. Females and young males have antlers that are similar in size, but males older than 2 or 3 years have much larger antlers.

Caribou and reindeer have been around for over a half-million years; their ancestors lived at the same time as now-extinct woolly mammoths and saber-toothed cats.

Caribou are widely distributed across northern North America ranging from the Canadian High Arctic islands to the mountains and boreal forests of the Canadian southern provinces.  A small, endangered population in northern Idaho and adjacent northeastern Washington are the southernmost group in North America.

The most numerous are those in the large migratory populations that occur from Alaska throughout much of northern Canada.

What are “domestic reindeer”?

Reindeer were domesticated across northern Europe and Asia several thousand years ago and are the basis of herding cultures in those regions.

Domestic reindeer also occur in Alaska and Canada.  A little more than a century ago, 1300 reindeer were imported from Siberia to northwest Alaska in an attempt to establish a herding economy among the Native people in the region. At the time, caribou were scarce along the northwest coast of Alaska.

Reindeer herding expanded widely across the west and north coasts of Alaska, as well as into northern Canada, such that around 600,000 domestic reindeer occurred throughout Alaska by the 1930s. During the Great Depression, the reindeer industry in Alaska collapsed and retracted to the Seward Peninsula of northwest Alaska where it continues today.

While the main goal for domestic reindeer has been to provide meat and hides to local people, reindeer have been trained to pull sleds as a mode of transportation.

Caribou hair on the Alaskan tundra
White caribou hair stands out in contrast from the bright-colored tunda crowded with lichen and fall cranberry leaves.(Public domain.)

What do they eat?

Caribou forage on a variety of plants throughout the year. During winter, lichens are their most frequent food, with shrubs and grass or sedges making up the rest.

Lichens are a combination of fungus and algae that grow together. On alpine and arctic tundra ranges, caribou primarily feed on terrestrial lichens, sometimes called reindeer moss, that occur within the low-growing grasses and shrubs that make up the tundra vegetation.

In southern or boreal forest ranges, where caribou winter in deep snow, arboreal lichens that grow on trees are the predominant caribou forage.

During summer, caribou shift to eating a wide variety of green plants including grasses or sedges, growing shrubs, and a variety of small forbs or flowering plants. In some regions, mushrooms abundant in late summer are an important food for caribou.


A bull caribou grazes in autumn at the Lake Clark National Park and Preserve in Alaska. (Public domain.)

What do caribou do in the fall and winter?

In the fall, some caribou herds start migrating — when they migrate is dictated by cues in changing day length in combination with the onset of snowfall as the long winter begins. Fall is also the breeding season when mature bulls compete with each other for opportunities to breed with females as the females become receptive.

In winter, Arctic caribou generally migrate south into the northern fringe of the boreal forest or onto tundra winter ranges where terrestrial lichens are abundant. Some larger caribou herds migrate long distances, 300-400 miles, between their winter ranges and their calving and summer ranges.

Smaller mountain populations migrate out of the higher mountains onto the tundra and forest ranges adjacent to their mountainous summer ranges, while small boreal forest populations are generally sedentary throughout the year.

Once on their winter range, caribou remain there throughout the winter, from about early October to late April.

How do they thrive in such cold temperatures?

Caribou are well adapted to living in cold regions and thrive in areas where winter temperatures can reach 70 or 80 degrees below zero. These animals have a very dense haircoat, made up of wooly underfur and hollow guardhair, over their entire body (except the very tip of their nose) that provides superior insulation. They also have relatively large, wide hooves for walking and digging through snow.

What do caribou do in the summer?

After the females calve, caribou generally gather together in large groups to help them better avoid predators and to escape incredibly bothersome mosquitoes and parasitic flies.

The different herds of caribou stay together in the high mountains and along the Alaskan seacoasts where the winds and cooler temperatures help protect them from summer heat and those pesky insects.

After the number of insects decline in late July, the caribou herds scatter into smaller groups. This is an important time for caribou — they use the time before winter arrives to feed as much as possible on remaining green grasses and sedges, willow leaves, and even mushrooms to regain their body weight.

USGS biologist Gretchen Roffler weighs a newborn caribou calf in Denali National Park, Alaska
USGS biologist Gretchen Roffler weighs a newborn caribou calf in Denali National Park, Alaska. (Public domain.)

How big are calves?

We’ve weighed quite a few newborn calves in Denali and on average they weigh about 17 pounds. Calves are born in May and early June throughout Alaska, with most calves being born in any herd within about a 10-day period.

Caribou cows produce one calf each year and generally begin producing calves when they are 2 to 4 years old depending on the nutritional status of a given population.

In small herds, such as the Denali Caribou Herd, calves are subject to intense predation primarily by wolves and grizzly bears — fewer than half survive beyond 2 weeks of age.

In the large, migratory populations, early calf survival is markedly higher because the huge number of calves born over a brief interval can greatly swamp the ability of local predators to kill them.

How big are adult caribou?

In Denali National Park, where I currently study caribou, mature adult males average about 500 pounds but can weigh more than 600 pounds. Adult females are about half as big, averaging about 240 pounds (225- to 320-pound range).

In the large, migratory herds, caribou are smaller with adult males and females averaging about 400 pounds for males and 200 pounds for females.

How many herds are in Alaska?

Simulation domain and winter ranges of the Central Arctic and Porcupine caribou herds, Alaska and Yukon.(Public domain.)

There are 31 caribou herds recognized in Alaska, with 7 large migratory populations numbering 30,000 to 206,000 animals. These herds currently total just under 600,000 animals and account for about 97 percent of the caribou in the State. The remaining 24 herds are much smaller ranging from about 30 to 3000 animals each.

Overall, Alaska’s caribou population was relatively low in the mid 1970’s, numbering around 250,000 statewide.  Caribou numbers increased to about 950,000 by the mid-1990s, as a couple of the large herds grew to historic high numbers.  Since then, caribou numbers have declined to around 620,000 today. Such wide fluctuations in caribou numbers over the time scale of decades are not unusual.

Can you talk a little more about predators — what eats caribou?

In general, the primary predators of caribou in Alaska are grizzly bears and wolves. Grizzly bears are very effective at killing young caribou calves less than a couple weeks old, although they also kill older caribou on occasion. Wolves are important predators of both young calves and older caribou. Other predators on caribou include black bears, golden eagles, wolverine, and coyotes.

Humans are also important predators of caribou. Caribou are a mainstay of local subsistence in Bush Alaska, and a sought-after quarry for other Alaskan residents, as well as sport hunters from all over the world. On average, people harvest about 22,000 caribou a year in Alaska.

Predation affects the number of caribou, particularly in the smaller, more sedentary populations. The large, migratory herds are able to reduce the negative effects of predation to some degree just due to their sheer numbers; the tradeoff is that they are more likely to be affected by the nutritional limitations of their ranges compounded by competition with their herd mates.

Caribou are more vulnerable in deep snow

Layne with caribou.(USGS photo, public domain.)

A main goal of my research has been to understand the interrelationships of caribou and wolves in Denali National Park. For caribou, an important factor that affects how many are killed by wolves is the amount of snow during winter.

In years with less snow, caribou have large expanses of wind-blown, snow-free land to seek their food, and they commonly make it through the winter in good shape. They can also more easily evade wolves because they can run unimpeded across the bare, frozen tundra. During such times, wolves are primarily able to kill those caribou that are old, injured, not in good shape, or just plain unlucky. We’ve found that when it is harder for wolves to catch caribou, the wolf packs tend to be smaller.

But the balance shifts in favor of wolves when there is a lot of snow.

Caribou then have a harder time finding enough to eat because they have to dig through deep or crusted snow or must seek food on high mountain ridges where there is little snow, but also little food.

The caribou also have a harder time escaping from wolves in deep snow. In fact, wolves will sometimes chase caribou into areas with deep snow where the caribou are very vulnerable, even if they are in good shape.

In those years, wolf packs tend to be bigger and some packs produce more pups. In contrast, our research shows that after severe winters, not only is a cow less likely to breed, but calves that are born are lighter, grow more slowly, and are more likely to be killed by predators in the weeks after they are born.

Is climate change affecting caribou?

We know, from our studies, that weather may be the most important factor affecting the yearly cycles of large hoofed mammals (such as caribou, moose and muskox) and their predators.

However, the longer-term effects of climate change are much more complex. Unlike polar bears, which are highly dependent on sea ice that is declining due to warming temperatures, caribou are likely influenced by a wide variety of factors that will be affected by a warming climate, and some effects will be positive and some negative.

For example, with a warming climate, we expect the growing season to be longer and provide caribou with green, nutritious forage earlier and for a longer period of time for a positive effect.  Our recent studies on Alaska’s Arctic Coastal Plain have been geared toward understanding how a warming climate is affecting the plants that caribou eat during summer; this information will help managers forecast how future habitat condition might affect the well-being of these large herbivores.

However, we have also done research that indicates that with increasing temperatures we can expect more fires on boreal forest winter ranges for caribou that will likely result in reduced availability of lichen, their primary winter forage, which tends to not grow back for about 70 to 80 years after a fire.

The overall effect of a warming climate on caribou will be dependent on how these and many other climate-related effects interact and that is very difficult to predict. Further, responses to climate change are likely to differ among the various caribou populations across North America.

What does some of your research focus on?

Currently, I am continuing long-term studies of the population dynamics of the Denali Caribou Herd and overseeing research on the summer habitat selection of caribou relative to forage quality and weather on Alaska’s Arctic Coastal Plain.

Read More:

Resilience of Caribou to Climatic Shifts in the Arctic

USGS Alaska Science Center Large Mammal Ecology webpages

Battling Flies and Fog in Search of Reindeer Poop

The Changing Arctic Initiative (including caribou)

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