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Conversations about Caribou

Caribou Migration
Caribou Biology

Caribou Migration

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

In spring the Porcupine caribou herd migrates hundreds of miles from winter ranges located south of the Brooks Range in Alaska, and from areas in Yukon Territory, to its traditional calving grounds on the Arctic Refuge's coastal plain and foothills (maps of yearly calving locations). In years when there's deep snow on the winter range and along the migration routes, and when the spring snow melt is delayed by cold temperatures, caribou cows are delayed in reaching these preferred calving grounds. When this happens, the calves are born along the migration routes and on calving habitats in Canada. This happened in 1987 and 2000.

Sometimes caribou are not delayed during the spring migration, but when they arrive at the edge of the foothills they find it is still covered by snow because cold temperatures have delayed snow-melt. In this case, the cow caribou give birth in snow free or partially snow free areas to the south, near or in the northern mountain valleys. This happened in 1988. In both 1987 and 1988, nearly all of the herd continued north toward the traditional calving grounds after the young calves were able to travel with their mothers. After calving, the cows and calves are joined by the bulls and yearlings. Almost every year, no matter where calving occurs, the caribou then gather on the Refuge's coastal plain and foothills in immense numbers to feed on the abundant vegetation. The caribou later move to nearby coastal areas to escape from harassing insects.

Variation in snow melt patterns and the timing and location of plant growth on the calving grounds determine where the cows choose to have their calves each year. Although there are some years when Porcupine Caribou herd cows have their calves outside the traditional area, long-term data show that most Porcupine Caribou herd calves are born within the foothills and coastal plain of the Arctic National Wildlife Refuge.

Why do caribou migrate?

Some animals stay in one area their entire life. Others, like caribou, migrate on long journeys. Caribou migrate between summer and winter ranges. Their summer range provides nutritious food that helps the new calves and the other caribou grow healthy and fat before winter. But the summer range is a harsh and windy place during winter, so the caribou move to a winter range where conditions (weather, food, snow cover) are more agreeable.

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 129,000 animals, migrates between summer and winter ranges that are about 400 miles apart. The Central Arctic herd, which contains about 20,000 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 would indicate. They move to and fro over a wide area, adding many miles to their journeys.

Will caribou cross barriers when they migrate?

It is quite common to find situations where caribou are reluctant to cross roads, berms 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, etc.) which can influence caribou reactions to roads 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 build 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.

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 migration paths, the density 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 are displaced from their traditional calving grounds when oil development occurs there.

On the other hand, caribou are less sensitive and more scattered during winter, so fewer animals would be affected if development occurred in a portion of the winter range.

If oil development in a calving ground resulted in a population reduction, the smaller herd would probably occupy a smaller range, and migration routes would change from the longer, traditional routes.

Do caribou migrate the same distance each year?

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.) There are different migration routes leading from the various portions of winter range, so caribou migrate different distances each year.

Do caribou migrate for weather or food?

Certain weather conditions, such as the first severe storm in the fall, stimulate caribou to migrate toward their winter ranges. After the calves are born in the spring, all the caribou in the herd come together on the summer range. 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. When these large herds form, their rate of movement naturally increases.

How many caribou die crossing raging rivers?

We can't tell you how many die while crossing rivers. Many of these places are very remote, and nobody is there to watch what happens to the caribou. 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 without crossing.

Caribou Biology

Do caribou stand around and let wolves eat them? NEW

It sometimes may look like caribou are ignoring predators such as wolves or bears. Unless they are incapacitated, however, they will run from a predator who gets too close to them (actually, they will run from anything that "spooks" them. We've seen a cow repeatedly run away from her calf who happened to get a black plastic bag caught around it's neck.)

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

1) Caribou may come up and investigate something they aren't sure about. One of our biologists unintentionally lured caribou to her just by sitting on a rock eating lunch. They walked right up, wondering what she was. As soon as they figured it out, they were gone. You can also lure them in by acting strangely. Her favorite ploy is to lie on the ground on her back, and to slowly wave one of her legs in the air. Again, they'll come quite close to see what's what, before running away.

2) 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 quite 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). Reindeer are a domesticated variety of caribou that are herded by humans and used for pulling sleds. Most reindeer occur in Scandinavia and Siberia. They generally are smaller and have shorter legs than their wild relatives. In Siberia, caribou are referred to as "wild" reindeer.

Caribou are found in Alaska and Canada. Caribou used to live in Maine and the northern Great Lakes states, but they are now extinct in those areas. A small, endangered caribou herd exists in 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.

(A population of 8 reindeer are believed to live at the North Pole. These unusual animals, guided by one with a red nose, are reported to have the capability of flight!)

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 destruction. Woodland caribou do poorly when the forests are "cut up" 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 three very large herds of caribou, the Western Arctic herd in northwest Alaska, the George River herd in northern Quebec, and the Taimyr Peninsula herd in Siberia. Each herd is currently estimated at 500,000 or more individuals. Due to different census techniques and schedules, as well as annual fluctuations in populations, it is not possible to say which of these three herds is currently the largest.

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 following the fall breeding season (young males retain their antlers longer that mature males). Pregnant females shed their antlers 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.

Will caribou trample humans?

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. If you sit on the tundra in the path of the herd, and make no rapid movements to frighten the animals, they will walk around you at quite close range.

What keeps caribou populations in equilibrium?

When factors having negative effects on caribou productivity and survival occur more frequently (more bad years than good years), populations decline. Caribou populations increase when the opposite occurs. If positive and negative effects are balanced, caribou populations remain stable.

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 often 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?

Male caribou live about seven to eight years. Females live slightly longer, to 10 or more 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 (getting smaller over the years), 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, and 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, the calf will not survive.

Do animals have emotions?

Scientists are wary of assuming that animals have emotions, because they can not ask the animals how they are feeling. When a lost baby caribou runs up to its mother and starts nursing, is it happy, or just hungry? We often interpret how humans feel by watching facial expressions, but many animals do not have facial muscles that allow expression. Even if they do, we can't be sure we're seeing a grin, or a grimace, or something totally different.

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). Reindeer are a domesticated variety of caribou that are herded by humans and used for pulling sleds. Most reindeer occur in Scandinavia and Siberia. They generally are smaller and have shorter legs than their wild relatives. In Siberia, caribou are referred to as "wild" reindeer.

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

What do Wildlife Refuges protect caribou from?

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 can walk on bumpy ice without slipping, but if they get onto shiny, smooth ice, their hooves slide out from under them. They sometimes fall down, with all their legs sticking out on the ice around them. 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 129,000 animals and travels over 96,000 square miles of land. This is about 0.75 caribou per square mile. If a similar herd contained only 100 animals, they'd need at least 75 square miles. 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.

How many caribou are at the Arctic National Wildlife Refuge?

There are two caribou herds that use the Arctic Refuge; all 129,000 animals of the Porcupine Caribou herd, and about 10,000 animals (half) of the Central Arctic herd. The caribou live in the Refuge, and in neighboring lands in 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.

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 causes 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 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.

What would 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 can 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.

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Why we archived the ANWR website at

Note: This is the archive of the FWS Arctic National Wildlife Refuge website. In December, 2001 FWS took this website offline, making it unavailable to the public. It includes 90 plus pages of information and many maps. As of 2006 the important information contained in this, the original "unsanitized" version of the FWS website, has yet to return to the internet, so we will continue to maintain it here as a permanent archive to help inform activists and concerned citizens. If you find any broken links, please report them to me at [email protected] and I will attempt to make the repairs. January, 2008 update - A small part of the original information that was present in 2001 has made it back into the current ANWR website. There is also an archive that contains a small amount of the original information, but it is not readily available from the main website.

Click here to visit our homepage. Click here for NRDC's message about ANWR from Robert Redford.

For more information on why this website was "pulled," Check here. And, you can also view the maps of caribou calving areas that the FWS did not want you to see here.

January 29, 2008: Visit Our New ANWR News for Updates

This page should be cited as follows:

U.S. Fish and Wildlife Service. 2001. Potential impacts of proposed oil and gas
       development on the Arctic Refugeďż˝s coastal plain: Historical overview and
       issues of concern. Web page of the Arctic National Wildlife Refuge,
       Fairbanks, Alaska. 17 January 2001.

Archived by Visit us at is an independent firm specializing in the publication of educational and research resources. We created the first U.S. based interactive toxic chemical facility maps on the internet in 1996 and we have been online ever since. Learn more about us and view some of our projects and services.

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