Frequently Asked Questions about Caribou

This information is based on research listed in the partial bibliography of scientific research pertaining to the Refuge. Some specific references are noted within the text.

Caribou Migration

Caribou Biology


Caribou Migration

Why are some Porcupine Caribou herd calves born in Canada or the mountains?

In spring the Porcupine caribou herd migrates hundreds of miles from winter ranges located south of the Brooks Range in Alaska and the central Yukon Territory, to its traditional calving grounds on the Arctic coastal plain, which is an area of relatively flat tundra located between the Brooks Range mountains and the Beaufort Sea. Some calving also occurs in the foothills along the southern edge of the coastal plain. The entire calving range of the herd includes areas in both northern Yukon and northeastern Alaska. Within this area, in any given year calving may occur mostly in Alaska, mostly in Canada, or split across the border. Where caribou choose to calve each year seems to be strongly influenced by weather, and especially snow conditions during the migration. During “average” years, snowmelt begins shortly before caribou start migrating, and the animals are able to travel to the coast of northern Yukon Territory, then turn west and continue on to their preferred calving range in the Arctic Refuge*. However, in years when spring snow melt is delayed by cold temperatures or spring storms, caribou migration may be delayed or slowed and caribou may be unable to migrate as far before calving begins. When this happens, the calves are born along the migration routes and in northwestern Canada. The size of the Porcupine herd’s calving range allows caribou to select whatever part of the area is best for calving in response to weather conditions each year.

After calving, the cows and calves are joined by the bulls and yearlings, who generally migrate a week or so later than the cows. Almost every year, no matter where calving occurs, the caribou then gather on the Refuge's coastal plain and foothills to feed on the abundant vegetation that appears soon after the snow melts. In late June and early July, mosquitos emerge and caribou move to windy coastal areas or up into the mountains to escape from harassing insects.

caribou calf map 490 x 236

[*Griffith, B., D. C. Douglas, N. E. Walsh, D. D. Young, T. R. McCabe, D. E. Russell, R. G. White, R. D. Cameron, and K. R. Whitten. 2002. The Porcupine caribou herd. Pages 8-37 in D. C. Douglas, P. E. Reynolds, and E. B. Rhode, editors. Arctic Refuge coastal plain terrestrial wildlife research summaries. U.S. Geological Survey, Biological Resources Division, Biological Science Report USGS/BRD/BSR-2002-0001.] 


Why do caribou migrate?

Some animals stay in one area their entire life. Others, like caribou, migrate on long journeys between summer and winter ranges. This behavior helps large caribou herds spread their grazing over large areas so that the food supply is not used up. Migration to remote calving grounds may also help caribou avoid predators when their calves are very small. The newly-sprouting tundra vegetation (such as Cottongrass, or Eriophorum sedge) on the coastal plain is especially nutritious during the few weeks immediately after calving occurs. Later in summer, better food and relief from insects are found further inland. In winter, the best food consists of the thick layer of lichens (also called “reindeer moss”) found in the boreal forest south of the Brooks Range. Long-distance migrations allow caribou to access whatever type of food is most abundant and nutritious during each season.


What happens to the young when caribou migrate?

Soon after birth, the caribou calf and its mother develop a strong bond. They try to stay close to each other, and they can recognize each other by smell and by the sounds they make. This is important because the caribou calves are fast runners within hours of their birth. When caribou migrate, the calves run with their mothers. If they become separated, the mother searches for many hours to find her calf.


How far do caribou migrate?

Caribou herds migrate different distances. Large herds are more apt to migrate long distances, while smaller herds often migrate shorter distances. For example, the Porcupine caribou herd, which contains about 218,000 animals, migrates between summer and winter ranges that are about 400 miles apart. The Central Arctic herd, which contains about 30,000 animals, migrates between summer and winter ranges that are about 120 miles apart.

Biologists have discovered, by using satellites to track caribou, that the herds actually travel much farther than the straight-line distance between summer and winter ranges would indicate. They move to and fro over a wide area, adding many miles to their journeys. Porcupine Caribou herd animals, for example, have been observed to travel over 3000 miles per year.


Will caribou cross barriers when they migrate?

It is quite common to find situations where caribou are reluctant to cross roads, berms, pipelines and other related obstacles. Being terrestrial migrators, caribou must deal with what ever is placed on the land by human development (birds are able to fly over most human structures and continue their migratory habits). Researchers have learned* there are many factors (traffic levels, time of year, degree of visual obstruction, reproductive status, etc.) which can influence caribou reactions to roads and other potential obstacles, and thus their chances of crossing successfully. Caribou need to move freely over vast areas to forage, avoid predators, escape from harassing insects, and reach favorable summer and winter ranges.

Structures such as highways may deflect caribou movements, and reduce their chances for survival. A single road within a caribou herd's range usually is not as serious as a system of many roads. In some instances, roads and pipelines can be constructed in ways that reduce problems for caribou. For example, a ramp may be built to direct caribou over a road, and a pipeline may have buried sections for caribou to pass over. These modifications can help, but do not always work.

[*Fancy, S. G., L. F. Pank, K. R. Whitten, and W. L. Reglin. 1989. Seasonal movements of caribou in arctic Alaska as determined by satellite. Canadian Journal of Zoology 67:644-650.]


Will oil development change migration patterns?

The effect that oil development may have on caribou migrations depends on many things, such as the location of the development in relation to critical habitats and migration paths, the density and design of the buildings, pipes, and roads, as well as the time of year that caribou are in the development area. For example, caribou are most sensitive at calving time, and studies have shown that caribou may be displaced from their traditional calving grounds when oil development occurs there. Other factors, such as weather, may also influence how much development may affect caribou. Caribou may be able to tolerate higher levels of disturbance when the weather is good and caribou are in good shape, but they may be more vulnerable to disturbance when they are also stressed by bad weather or a scarce food supply.


Do caribou migrate the same distance each year?

Based on several years' observation of satellite-collared animals of the Porcupine Caribou herd, caribou do not migrate the same distance each year. This is because they often use different portions of their winter range from year to year. By using this strategy, caribou are able to eat winter food over a wide area, which reduces the possibility of eating up all the available food in any one area. Although parts of the migration routes are used almost every year, there are variations in the specific routes that are taken, depending on weather, snow conditions, and where caribou spent the winter. As a result, caribou migrate different distances each year.


Do caribou migrate for weather or food?

We don’t know for certain why caribou migrate or what triggers migration to begin. Migration probably helps caribou access the best type of food available during each season, and also helps the herd avoid predation (because predators are generally less mobile, and thus only have access to the caribou for part of each year). Some weather conditions, such as the first severe storm in the fall, seem to stimulate caribou to begin migrating toward their winter ranges and spring migration is probably stimulated by the pregnant cows feeling the instinct to go to their preferred calving grounds. During summer, caribou movements are driven by changes in the food value of the growing vegetation and the caribou’s desire to avoid insect harassment. For large herds such as the Porcupine Caribou herd in the Arctic National Wildlife Refuge, the caribou must keep moving so they don't eat all the available food.

PCH in 1002 490 x 333



How many caribou die crossing rivers?

We can't tell you how many die while crossing rivers, although this undoubtedly does happen. We do know, however, that caribou have been encountering rivers for many thousands of years, and yet the herds survive.

Caribou have many strategies that help them cross rivers. They (including the calves) are excellent swimmers, and their hollow winter hair helps them float high in the water. Caribou do not always plunge blindly into rivers: sometimes they scout out safer crossing areas or wait for better crossing conditions. We've observed migrating pregnant females decide not to cross a raging river, but instead, give birth before crossing.


Caribou Biology


What is a “herd” of caribou? Is this the same as a population?

The meaning of the term “herd” is not quite the same as “population”. To biologists, a population is a group of animals that commonly interbreed among themselves, but do not interbreed with animals from neighboring populations. In the case of caribou, breeding between neighboring herds does occur, but is usually much less common than breeding between caribou within a herd. Biologists and wildlife managers define a “herd” of caribou as a group of animals that occupy the same seasonal ranges and follow the same migratory patterns, while maintaining some degree of separation from other groups of caribou in the same region. The most important thing used to define a “herd” of caribou in Alaska is the location of the calving ground. Each herd maintains a unique calving range, separated from those of other herds.


Why are they called the Porcupine Herd?

Most caribou herds are named after the area where they are found, or a prominent geographic feature of their range. The Porcupine Herd is named for the Porcupine River, which flows through the middle of the herd’s range.


Do caribou stand around and let wolves eat them?

It sometimes may look like caribou are ignoring predators such as wolves or bears. Unless they are incapacitated, however, caribou will run from a predator who gets too close to them.

There are three characteristics of caribou that may make them appear to show less than the expected amount of fear toward a predator:

1) Caribou may come up and investigate something they aren't sure about.

2) Caribou are very fast runners, and can outrun most predators over short distances, thus, caribou may appear unafraid of a predator that is not actively chasing them.

3) Caribou do not want to spend any more energy than they have to. They know what's a safe distance from a predator, and they can tell by watching how a wolf or bear is behaving whether the predator is a threat. So it is very possible to see a bear or wolf pass through a herd of caribou. While the predator ambles along, the caribou do not run away, but continue to feed or walk. When the predator begins running toward a group or an individual animal, then those caribou run away.

When caribou are not alarmed, they walk slowly, extending the head forward and downward. When alarmed, caribou perform a special behavior to warn other caribou of danger. They'll do this if a predator gets too close, but isn't about to catch them (or after they figure out that you're a person sitting on a rock). An alarmed caribou will trot with the head held high and parallel to the ground, and the short, normally floppy tail held up in the air. They gallop very quickly when being chased closely by a predator.


Where are Caribou found?

Caribou and reindeer are the same species (Rangifer tarandus). Caribou are native to North America, while reindeer are found in Scandinavia and northern Asia, where some have been domesticated.  Domestic reindeer are herded by humans and used for food and for pulling sleds. They generally are smaller and have shorter legs than caribou. 

Caribou are found throughout Alaska and northern Canada, and along the west coast of Greenland. Caribou used to live in Maine and the northern Great Lakes states, but they are no longer found in those areas. A small, endangered woodland caribou herd exists along the Canadian border with northern Idaho and northwest Montana.

An unusual situation exists at South Georgia, an island near Antarctica, where reindeer from Norway were introduced in the early 1900's. Because of the opposite seasons in the southern hemisphere, these animals had to change the timing of breeding and calving by a half year.


Why are the caribou in Idaho and Montana endangered?

The woodland caribou in that region live in old growth forests. They do not migrate very far between their summer and winter ranges (less than 40 miles, in some cases), but they are very sensitive to human disturbance and to forest loss. Woodland caribou do poorly when forests are fragmented into small areas by roads or by logging operations. As human activities increase, woodland caribou, and other animals sensitive to disturbance, have disappeared. Only a few areas in the United States still support woodland caribou, and the animals are endangered in these areas.


What is the largest herd of caribou?

There are currently four very large herds of caribou and wild reindeer: the Porcupine Herd, the Western Arctic herd in northwest Alaska, the Qamanirjuag Herd in Nunavut, Canada, and the Taimyr Peninsula herd in northern Siberia. Each herd is currently estimated at over 200,000 individuals. Due to different census techniques and schedules, as well as annual fluctuations in populations, it is not possible to say for certain which of these three herds is currently the largest. Several caribou herds in northern and northeastern Canada that used to be quite large have recently declined to very low numbers, causing concern about the future of these herds.


What is the size of caribou?

Adult caribou range in size from 3 to 4 feet tall. Their size and weight varies by sex and region. For example, caribou are fairly small in northern Alaska. Males average about 275 to 375 pounds, females about 200 pounds. In southern Alaska, caribou are considerably larger -- males average 400 to 600 pounds and females average 200 to 300 pounds.


How long do caribou keep their antlers?

Caribou are the only deer in which both sexes have antlers. Males shed their antlers in late fall, just after the breeding season (young males retain their antlers longer that mature males). Pregnant females keep their antlers all winter and shed them soon after the calves are born in the spring. Non-pregnant females shed their antlers during the winter.


How does caribou meat compare with beef?

Caribou do not store much of their fat in muscle tissue, so their meat is leaner than beef which often is "marbled" with fat. Caribou meat is considered more healthy than beef, and is quite tasty.


Can you get close to a herd of caribou?

Caribou commonly gather in large herds about three weeks after the calves are born. At this time the great herds increase their rate of movement, and caribou tend to be less wary when they are in very large groups. Caribou will generally not let you approach them, but if you sit quietly in the path of a group that is moving in your direction, you might be fortunate enough to see them up close as they pass by.

coast caribou 490w 70h



What keeps caribou populations in equilibrium?

Caribou populations are rarely stable for very long. Under natural conditions, the size of a particular herd will tend to increase slowly for 10-20 years, then decline, sometimes rapidly, before again beginning to increase. A herd is considered to be at “equilibrium” if the increases match the declines over long time spans (perhaps 50 years or more). Many things can influence whether a herd is increasing or declining. When factors having negative effects on caribou births and deaths occur more frequently (more bad years than good years), populations decline. Caribou populations increase when the opposite occurs.

Usually a combination of factors cause caribou numbers to change. Harsh weather can reduce plant growth, which causes poor caribou nutrition, and reduced survival. Some years, insect harassment interferes with caribou foraging, which also decreases survival. If it rains during the winter, ice can prevent caribou from getting their food. They may starve when this happens.

Wolf populations in caribou winter ranges can increase in response to higher levels of other prey such as moose. When caribou return to the winter range they are preyed on more heavily by the increased number of wolves. On the other hand, when arctic foxes reach a high in their population cycle, they sometimes spread rabies to neighboring wolves. This results in reduced wolf predation on caribou.


How long do caribou live?

Most male caribou live about seven to eight years. Females often live longer, to 10-15 years. These are very general numbers. Every animal faces its own set of situations that lead to a shorter or longer life. If a caribou lives in a herd that is declining, it probably will have a shorter life than a caribou in a healthy or expanding herd. Also, many caribou die within the first year after they are born, so never reach adult age.


Are orphan caribou calves "adopted"?

Orphan caribou calves are not adopted by other caribou mothers. If the mother dies, or the calf becomes permanently separated from its mother before the end of its first summer of life, the calf will probably not survive.


What are caribous' natural predators?

Several species are known to prey on caribou. Wolves prey on caribou throughout the year, but most frequently in the winter. Bears prey on caribou during spring, summer and fall. Golden eagles take young calves during the early summer, and lynx are able to kill calves in the fall when caribou migrate into forested areas. When snow is deep, wolverines are sometimes able to kill caribou. Humans have hunted caribou for many thousands of years.


Do female caribou pick males with large antlers to breed with?

The female doesn't actually pick males with large antlers, but the females do often end up breeding with males that have large antlers. This is because the mature males (those with the largest antlers) work hard to keep younger males (with smaller antlers) away from the females during breeding time.

The males with the largest antlers are in the best health, and they have been good at finding food all their lives (so their bodies can grow these large antlers). When these animals do most of the breeding, their genes are passed on to new generations, and this ensures that the herd remains healthy.


Are caribou and elk related?

Both caribou and elk are hoofed mammals of the deer family. Caribou (males weigh about 500 pounds) are generally smaller than elk (males weigh about 700 pounds). Caribou often occur in large herds which migrate over long distances. Elk generally occur in smaller herds, and migrate over relatively short distances. They usually migrate between summer ranges at higher elevations, and winter ranges in mountain valleys.


Are caribou and reindeer related?

Caribou and reindeer are the same species (Rangifer tarandus). Caribou are native to North America, whereas reindeer are found in Scandinavia and northern Asia. Some reindeer have been domesticated by humans for hundreds of years. These are used for food and for pulling sleds. Reindeer are smaller and have shorter legs than caribou.

A number of reindeer have been imported to Alaska, primarily to the Seward Peninsula. These herds are owned by Alaska Natives. 


How do Wildlife Refuges protect caribou?

In the U.S., the National Wildlife Refuge System preserves a national network of lands and waters for the conservation and management of fish, wildlife and plants for the benefit of present and future generations. There are currently over 500 different wildlife refuges. Refuge lands are legally protected from activities and developments which are harmful to wildlife or their habitat. Human activities which are compatible with refuge purposes are allowed on wildlife refuges. In Alaska, the Arctic National Wildlife Refuge protects the primary calving grounds and some of the wintering areas of the Porcupine caribou herd, one of the major caribou herds in North America.


How can caribou travel in the cold and on ice?

Caribou are well equipped to survive in cold, snowy places. In winter, their hair is about three inches long. This winter hair is hollow inside, to trap air and keep warmth near their bodies. This hollow hair also helps the caribou to cross rivers and lakes after spring thaws, because it acts like a life jacket full of air, and helps them to float.

Caribou have four hoofed "toes" on each foot. They usually walk on the two larger ones, like a cow does. When they are in snow, however, these four "toes" spread out wide to act like snow-shoes, which help the caribou walk on deep snow.

Caribou are well adapted to cold, dry conditions that were typical in the Arctic region since the last ice age. When the snow is cold and dry, winter winds will often blow it away, exposing the ground and allowing caribou to walk around and graze more easily. Warmer, wetter winters bring deeper, denser snow and ice that makes it difficult for caribou to dig down to the lichens they eat during winter.

Caribou can walk on bumpy ice without slipping, but if they get onto shiny, smooth ice, their hooves slide out from under them and they may fall. Sometimes they even break their legs this way.


How big does a refuge need to be for 100 caribou?

The answer depends on whether the caribou are wild, or if they are captive animals being cared for by people.

Wild caribou need areas that are large enough for them to find food and shelter, and avoid predators during every month of the year. If summer food is far from safe winter areas, they need a very large area. For example, the Porcupine caribou herd now has about 218,000 animals and travels over about 96,000 square miles of land. Captive animals do not need room to run away from predators, and they don't have to find their own food. If people keep caribou pens clean, the animals can live in quite small areas, just as a horse can.


Do mosquitoes play a role in caribou behavior?

Mosquitoes do play an important role in caribou behavior. Mosquitoes appear in early summer, just as the caribou are shedding their long winter hair. The insects can easily draw blood from the caribou at this time, and seriously torment the animals. The problem is worst when the weather is warm, winds are calm, and the caribou are in damp tundra areas where the mosquitoes breed. Caribou try to avoid mosquitoes by a variety of strategies, depending on where they live: they run; move to higher areas that may be windy and dry; move to snow or ice patches that are too cool for the insects to be active; move out into large lakes or shallow salt water; and/or bunch up into very dense groups.

The running, blood loss, and inability to spend time eating cause caribou to lose weight during a time of year when they need to be getting fat for the coming winter. Mosquitoes are therefore a major influence in the lives of caribou.


Are calving grounds essential for caribou survival?

Yes. Each spring, pregnant female caribou begin long migrations towards their traditional calving grounds. Their instinct to reach these areas is very strong, and enables them to travel through deep snow and storms, and to cross rivers flooding with icebergs to reach the calving grounds at just the right time. Soon after they arrive on the calving grounds, the calves are born. Studies have shown that predators are less abundant on the calving grounds, so the young calves are safer at a time when they are too weak to escape from wolves and bears. The preferred calving grounds also have an abundance of highly nutritious new plant growth* which enables the mother caribou to produce rich milk for their calves. This is very important as it allows the calves to grow rapidly so that they can escape from predators and harassing insects, and keep up with the herd as it migrates to the winter range. In summary, it is the special conditions of the calving grounds which improve the survival of calves and ultimately the entire herd.

[*Griffith, B., D. C. Douglas, N. E. Walsh, D. D. Young, T. R. McCabe, D. E. Russell, R. G. White, R. D. Cameron, and K. R. Whitten. 2002. The Porcupine caribou herd. Pages 8-37 in D. C. Douglas, P. E. Reynolds, and E. B. Rhode, editors. Arctic Refuge coastal plain terrestrial wildlife research summaries. U.S. Geological Survey, Biological Resources Division, Biological Science Report USGS/BRD/BSR-2002-0001.]


What might happen if development occurred on a caribou calving area?

This is a very interesting question, and one of much concern. Caribou calving grounds are special areas which are essential to survival of the young, and ultimately the health of the entire herd.

Pregnant caribou, and females with young calves, are especially sensitive to disturbances such as the presence of humans, vehicles and sounds*. This heightened sensitivity enables females to avoid predators, which improves the chances of their young surviving. Studies show that caribou move away from disturbances during the calving season. This could prevent caribou from using valuable areas of a calving ground, and result in increased mortality of young by predators. It can also prevent mothers from getting the most nutritious food, which in turn can lead to poor nutrition for them and their nursing calves. Displacement of caribou from preferred habitats can result in crowded conditions in low-quality areas, making it even harder for caribou to get proper nutrition. If normal growth and nutrition are reduced on the calving grounds, caribou will enter the winter without the fat reserves they need for survival, and females may not be able to produce calves the following spring.

These events contribute to reduced productivity and increased mortality, which ultimately results in decline of the caribou population.

[*Cameron, R. D., W. T. Smith, R.G. White, and B. Griffith, B. 2002. The Central Arctic Caribou Herd. Pages 38-45 in D.C. Douglas, P.E. Reynolds, and E.B. Rhode, editors. Arctic Refuge coastal plain terrestrial wildlife research summaries. U.S. Geological Survey, Biological Resources Division, Biological Science Report USGS/BRD/BSR-2002-0001.]


How many caribou are at the Arctic National Wildlife Refuge?

There are two caribou herds that use the Arctic Refuge; all 218,000 animals of the Porcupine Caribou herd, and about 20,000 animals (60%) of the Central Arctic herd. The caribou live in the Refuge, and in neighboring lands in the U.S. and Canada. The Arctic Refuge is about 200 miles north to south, and about 200 miles east to west (it's almost the same size as South Carolina). This remote area remains pretty much as it has been since glaciers covered North America. Because it is so wild, half of the Refuge has been designated as a Wilderness Area,the largest in all the National Wildlife Refuges.

National Wildlife Refuges belong to all Americans, and their purposes are to protect wildlife and habitats, and provide opportunities for people to enjoy these areas now and in the future.