The various populations of caribou display a wide range of size. Generally, populations that occur farther south are larger than their northern cousins. Caribou can have shoulder heights of up to 120 cm and total length ranges from 150 to 230 cm. They have short tails. Males are larger than females, sometimes twice as large. The coat of caribou is an excellent, lightweight insulation against the extreme cold temperatures they face. The hairs are hollow and taper sharply which helps trap heat close to the body and also makes them more buoyant. Color varies by subspecies, region, sex, and season from the very dark browns of woodland caribou bulls in summer to nearly white in Greenland (R. t. groenlandicus) and high Arctic caribou. White areas are often present on the belly, neck, and above the hooves. The hooves are large and concave, which support them in snow and soft tundra, conditions that they often face. The broad hooves are also useful when swimming. Caribou make an audible clicking noise while walking, which is produced from tendons rubbing across a bone in the foot. Caribou are the only species of deer in which both sexes have antlers. Mature bulls can carry enormous and complex antlers, whereas cows and young animals generally have smaller and simpler ones. Mature bulls usually shed their antlers shortly after the rut whereas cows can keep theirs until spring.
Caribou are found in North America and Eurasia in a large circle around the north pole, this kind of distribution is called "circumpolar". The woodland subspecies of caribou can be found as far south as 46o north latitude, while other subspecies can be found as far north as 80o north latitude. Caribou were once found as far south as Germany, Great Britain, Poland, and Maine (USA), but over-hunting and habitat destruction have reduced their populations to only a portion of their historic range.
Caribou inhabit arctic tundra and subarctic (boreal) forest regions.
Males compete for access to females during the fall breeding season, or rut. This occurs in October and early November. During this time males may engage in battles that leave them injured and exhausted. Dominant males restrict access to small groups of 5 to 15 females. Males stop feeding during this time and may lose weight rapidly.
In late August and September, prime bulls shed the velvet that surrounds their antlers. Fighting among males (sparring) begins shortly after that, with the rut (breeding season) usually occurring in October. Females can breed as early as 16 months of age but usually begin to breed at 28 months. With good nutrition females give birth to calves each year, but may skip years in areas with low quality forage. A single calf, weighing 3 to 12 kg, is born after about 228 days, in May or June. Twins have been reported, but are rare. Calves are weaned during the first week of July, but also begin to graze on grasses soon after birth. Calves rely mainly on grazing for nutrition after 45 days old.
Newborn calves are precocial, being able to suckle minutes after birth, follow their mother after an hour and are capable of outrunning a human at one day of age. Calves nurse exclusively for their first month, after which they begin to graze. They will continue to nurse occasionally through early fall, when they become independent.
Females generally have longer life spans than males, some over 15 years. Bulls are highly susceptible to predation after the rut, which can leave them injured and/or exhausted. Bulls typically live less than 10 years in the wild. Average life expectancy is 4.5 years.
Caribou are known to travel distances greater than any other terrestrial mammal. The can travel more than 5,000 kilometers in a year, with extensive migrations in spring and fall. They can reach speeds of 80 km/hr. Spring migration leads the caribou off their winter range back to their warm-weather calving grounds (areas where females give birth to the young). Caribou herds are defined by the location of their traditional calving grounds. Caribou are gregarious and the largest groups, which can number in the tens of thousands, are found during the summer months. This grouping behavior is thought to bring about some relief from the huge swarms of harassing mosquitoes, warble flies, and nose bot flies found in the arctic tundra during summer. As cooler weather arrives, groups become smaller but caribou may aggregate again during the rut (breeding season) and fall migration. Most caribou winter in forested areas, where they escape the deep snow and blowing winds of the tundra. Caribou are able to locate forage (grasses, sedges, and lichens) under snow, apparently by their ability to smell it. To reach the forage they use their front paws to dig craters. Dominant caribou frequently take over craters dug by less dominant animals.
Caribou communicate among themselves through vocal, visual, chemical, and tactile cues. They have a keen sense of smell, which allows them to find food buried deep under snow.
Caribou are mainly grazing herbivores. Their diet varies depending on the season. In summer they eat the leaves of willows and birches, mushrooms, cotton grass, sedges, and other ground dwelling kinds of vegetation. In the winter lichens are an important food source, although they continue to eat whatever vegetation is available.
Calves are vulnerable to predation by bears, wolves, and other predators during their first week of life. Healthy adult caribou are less susceptible to predation until old age and illness weakens them. By traveling in herds, caribou increase the number of individuals that can watch for predators.
Through their foraging activities, caribou have a dramatic impact on communities of vegetation throughout their range. They are also important prey species for large predators, such as bears and wolves, especially during the calving season.
There are no negative impacts of caribou.
Caribou have been used extensively for their meat, fur and antlers. Reindeer, the domesticated subspecies of caribou, have been herded throughout their range for thousands of years.
Although Alaska, with its more than 30 herds, has nearly double the number of caribou (1,000,000) than people, caribou in the lower 48 United States are considered endangered. Caribou in Alaska are of the barren-ground subspecies, whereas living (in Washington and Idaho) and extinct (Maine) herds are of the woodland subspecies. The Selkirk Herd, inhabiting Washington, Idaho, and southern British Columbia numbers only around 30 members. They are listed as Endangered under the Endangered Species Act in these regions. Loss of habitat, overhunting, and other factors has contributed to the precarious position of woodland caribou in the United States. Worldwide, the caribou population is estimated to be around 5 million. The largest herds now occur in Alaska, Canada, and Russia. Humans have heavily hunted this species. They have been extinct in most parts of Europe since at least the 1600s. Exploration for oil and minerals in Canada may threaten woodland caribou habitat. High Arctic caribou populations are also thought to be vulnerable.
Despite their status in the wild, domestic herds of reindeer flourish in the Old World, in Canada, in Alaska, and in the lower 48 states including Michigan.
Caribou, and their domestic counterparts - reindeer, have been very important in the cultures of native peoples througout the arctic. Several Siberian, Scandinavian, and American native cultures are built around herding caribou.
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