Arctic National Wildlife Refuge

Caribou in the Arctic Refuge

Caribou are the most numerous large mammals in the Arctic Refuge. Two herds occur there: the Porcupine Herd (named afer the Porcupine River) and the Central Arctic Herd. The Arctic National Wildlife Refuge is within the main range of the Porcupine Herd, which numbers approximately 152,000 animals, and on the periphery of the range of the smaller Central Arctic Herd with 23,400 animals.

The following discussion focuses on the Porcupine Herd, but basic features of the ecology and annual cycle of events are similar for both groups. During the year, caribou go through seven distinct phases of activities, some involving long migrations.

Spring Migration

The spring migration begins in early March as caribou gradually drift off toward the northern limits of their wintering areas. The Porcupine Herd follows three major routes to the North Slope from primarily wintering areas in Alaska and the Yukon Territory: the Richardson, Barn, and British Mountains; the Old Crow route, which crosses the Porcupine River near the settlement of Old Crow and continues northward through the Old Crow Flats, over the British Mountains and through the Firth Valley; and the Arctic Village/South Brooks Range route which crosses the East Fork of the Chandler River, the Sheenjek, and upper Coleen rivers and follows the Firth River into Canada where it joins the Old Crow route.

The caribou segregate themselves into groups which migrate at different times. Pregnant females along with some yearlings and barren cows are the first to migrate; followed by bulls and the remaining juveniles. In mid-to-late May the pregnant arrive on the North Slope, while the others follow a few weeks later.


Calving takes place during the last week in May and the first two weeks in June in the foothills and Coastal Plain between the Hulahula and Babbage rivers, and area that is generally snow free by early June. Caribou are not distributed evenlyacross the area; instead, they gather in more limited locations which vary from year to year.

There is a high degree of synchrony in the timing of the births, and this synchrony is probably an adaptation to reduce predation by 'swamping' the predators-primarily grizzlies, wolves, and occasionally golden eagles. That is, for a brief period, calves are superabundant, far more numerous than the predators can kill. This allows most of the calves to develop to the state where they are able to escape.

The calves are able to stand and nurse within an hour or two after birth, and within 24 hours they can follow their mothers and even run for short distances. The calves' precociousness is an obvious advantage where cover is sparse and predators common. Many cows with calves assemble to form small nursery bands, and these groups move slowly through the calving grounds, where the cows graze, favoring new-growth cottongrass shoots.

Post-calving Aggregation

As the mosquitoes emerge in late June and early July, the caribou gather into enormous post-calving aggregations, sometimes numbering in the tens of thousands. For example, in 1987, over 93,000 caribou assembled in one group south of Camden Bay. The caribou seek areas where breezes and cooler temperatures reduce the harassment by mosquitoes, and when there is no wind, the caribou move continually. Cold winds offer relief from the mosquitoes and permit the caribou to rest and feed freely.

Caribou rest as the insect harressment begins (late June)

By mid-to-late July, most Porcupine Caribou have moved off the Coastal Plain and into the foothills and mountains. Although some of the Porcupine Caribou occasionally remain on the North Slope for the winter, they usually travel south and east to Canada. When they do stay on the North Slope, the Porcupine Caribou usually move westward from the 1002 area and mingle with caribou from the Central Arctic Herd.


As the mosquitoes decline, the caribou disperse, only to be plagued by two other insect pests-the warble fly and the nose-bot fly. The warble fly, which looks like a small yellow and black bumblebee, lays its eggs in the fur of the legs or abdomen of the caribou. The larvae soon hatch, burrow under the skin, and travel to the back. Here they encapsulate and cut a breathing hole in the skin. Caribou commonly carry over one hundred larvae. It is not until May and June of the following year that the larvae cut exit holes, crawl out and drop to the ground to develop into mature flies.

The nose-bot bears live larvae, which it deposits in the nostrils of the caribou. The bot larvae move through the nasal passages and settle down at the entrance to the throat. By spring the larvae have grown so much that they may form a mass large enough to actually interfere with breathing.

The reaction of the caribou to these flies is different from the reaction to mosquitoes. The warble and nose-bot flies are strong fliers and the caribou cannot avoid them simply by seeking breezy places or moving into the wind. Instead the caribou stand, heads held low, alert for the approach of the flies. During July and early August, caribou can be seen violently shaking their heads, stamping their feet, and racing wildly over the tundra, for no apparent reason; they are seeking to evade warble or bot flies. The fly season is followed by a month's respite where the caribou can feed unmolested.

Fall Migration

The fall migration may begin any time from late August to mid-October as the caribou start to move generally southward. This migration will carry the caribou one hundred to three hundred miles south into the area south of the Brooks Range, in the vicinity of Arctic Village, Alaska, and into the southern Richardson and Ogilvie mountains in the Yukon Territory. The caribou continue to lay on fat as they move south; the males will need energy reserves for the rut and all will need it during the winter. At this time, the bulls are shedding the velvet from their antlers and rubbing them against trees and shrubs. The bulls are also becoming increasingly aggressive, engaging in brief sparring matches.


The rut lasts for a two-week period in midOctober. During this time the bulls fast, relying on their reserves of body fat. This brief breeding period helps to explain why the calving season is similarly brief. Even during the rut, the animals continue on to their winter ranges.


Winter is a difficult period for the caribou. Food plants are often covered by snow, so that the caribou have to dig to gain access; nor are the food plants in the winter as nutritious or as easily digested as are those available in summer. Over the winter the animals usually lose weight.

Central Arctic Herd

The other caribou in ANWR, the Central Arctic Herd, follow the same basic annual pattern as the Porcupine Herd, except that migrations are much shorter. Caribou from the Central Arctic Herd move between the arctic coast and the Brooks Range mountains, with most animals remaining north of the continental divide all year. Central Arctic Caribou use the northwestern part of the Coastal Plain during summer, and in most years several hundred to a thousand spend the winter near the Sadlerochit Mountains.

Caribou Populations

Both the Porcupine and Central Arctic Herds are biologically healthy. After a long period of stability at around 100,000 animals, the Porcupine Herd began to grow steadily during the late 1970s and 1980s and reached 180,000 animals by 1989. The herd then decreased during a series of severe winters and was down to 160,000 in 1992. In 1994, the Porcupine Herd numbered 152,000, but the caribou were in excellent physiological condition and overwinter calf survival had improved to levels comparable to the 1980s.

The Central Arctic Herd also increased during the 1970s and 1980s from 6,000 in 1978 to 23,400 in 1982. Rapid growth stopped in the late 1980s, however, and the herd now appears stable. Relatively low calf production and survival in recent years may result from severe winter weather which has also depleted moose and Dali sheep populations in the central arctic area. It is also possible that the Central Arctic Herd is approaching range carrying capacity.

Subsistence Uses

The caribou in the two herds which utilized portions of ANWR during their migration are an important subsistence food source for Inupiat Eskimos and Athabascan Indians who live in communities near the migratory routes of the caribou herds.