Caribou, a large species of deer-like animals native to northern climates, hold the world record for the longest overland migration. Each year, 3 million caribou make seasonal journeys across the Arctic tundra. They literally seek greener pastures, always traveling to find fresh grazing grounds. The distance traveled varies by herd, with larger herds traveling farthest. Some cover more than 2,000 miles each yearIn 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. 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, 2000 and 2001.
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 1987, 1988, 2000 and 2001, 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 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 123,000 animals, migrates between summer and winter ranges that are about 400 miles apart. The Central Arctic herd, which contains about 27,000 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.(arctic.fws.gov)