Caribou are a species of the northern landscapes, known for undertaking some of the longest terrestrial migrations on Earth. These mammals, also referred to as reindeer in Eurasia, navigate vast distances across diverse terrains. Understanding their migration patterns offers insights into their biology and the ecosystems they inhabit.
Major Caribou Migration Pathways
Caribou migrations span extensive geographical regions, primarily across the Arctic and sub-Arctic areas of North America and Eurasia. In North America, barren-ground caribou herds are known for their movements. The Porcupine Caribou Herd, for instance, travels over 1,500 miles (2,400 km) annually between Alaska, Yukon, and the Northwest Territories, making it the longest land migration of any mammal. Its calving grounds are on the Arctic coastal plain of the Arctic National Wildlife Refuge.
The Western Arctic Herd, another barren-ground population in Alaska, undertakes migrations across northwestern Alaska. The Central Arctic Herd in northern Alaska also migrates over large distances. These barren-ground caribou herds move from their wintering grounds, often in boreal forests, to northern calving grounds on the open tundra.
Woodland caribou, found in Canada and the northern U.S., exhibit more localized movements. While some are sedentary, certain herds undertake seasonal movements, often involving altitudinal migrations within forested or mountainous regions. For example, some mountain caribou herds may cover 140-240 kilometers during spring migration. In Scandinavia and Russia, reindeer populations follow seasonal migration patterns, traveling between summer and winter pastures for food. The Taimyr herd in Russia, one of the largest globally, migrates to winter taiga pastures in Evenkia and the Putoran Mountains.
Driving Forces Behind Migration
Caribou migration is driven by environmental factors that influence their survival. A primary factor is the seasonal availability of food. In winter, caribou rely on lichens, which they access by digging through snow. In spring and summer, they seek fresh vegetation like grasses, sedges, and shrubs, necessitating movement to areas where these food sources are abundant.
Another driver is the avoidance of predators, particularly during the calving season. Pregnant caribou migrate to remote calving grounds, typically on open tundra, which provides visibility to spot predators like wolves and bears.
Seeking relief from insect harassment also compels caribou to move. During warmer months, caribou move to cooler, windier areas, such as high mountains or coastal regions, to escape biting insects like mosquitoes and warble flies.
Annual Migration Cycle
The annual migration cycle of caribou is a progression tied to their life stages and seasonal changes in their environment. In spring, around March or April, pregnant cows lead the northward migration towards traditional calving grounds. These grounds are open tundra landscapes that offer reduced predator presence and early forage.
After calving in early to mid-June, female caribou and their calves form large aggregations. This behavior helps protect them from predators and provides relief from intense insect activity.
Throughout the summer, caribou continue grazing, often moving to higher elevations or coastal areas to access diverse forage and escape insects. As summer transitions into fall, by August and September, insect disturbance decreases, and caribou disperse into smaller groups, feeding heavily to gain weight before the rutting season.
The fall migration begins as caribou move towards their wintering grounds, often in forested areas that offer shelter and access to lichens. This southward journey is influenced by colder temperatures and the first snowfalls. The rut, or breeding season, occurs in October, impacting the movements of bulls as they compete for mates. By November, many herds are settled in their winter ranges, where they remain until the cycle begins again in spring.
Conserving Migration Corridors
Maintaining intact migration corridors is important for caribou survival. These pathways allow caribou to access essential seasonal habitats, find food, avoid predators, and adapt to environmental changes. However, these corridors face threats that can disrupt the caribou’s ability to complete their journeys.
Habitat fragmentation is a concern, often caused by human activities such as industrial development, roads, pipelines, and logging. These disturbances can create barriers, making it harder for caribou to follow their traditional routes and potentially forcing them into less suitable habitats.
Climate change also poses a threat, altering snow patterns, vegetation availability, and the safety of ice crossings, which caribou rely on during their migrations. Warmer winters can lead to less predictable snow and ice conditions, affecting movement and access to food. Increased human disturbance, including noise and activity, adds to the challenges faced by migrating herds.
Conservation efforts focus on protecting these routes through measures like establishing protected areas, implementing careful land use planning, and fostering international cooperation to manage transboundary herds. These efforts are vital for the species’ future.