Caribou are well-known for their extensive seasonal movements. This migratory behavior is a fundamental survival strategy, allowing them to access necessary resources and avoid environmental challenges. Their large-scale movements are among the longest terrestrial migrations on Earth, highlighting their adaptation to Arctic and sub-Arctic environments.
Seasonal Movements
Caribou migrations follow a predictable annual cycle, involving distinct spring and fall journeys. The spring migration, usually beginning in April, sees pregnant female caribou leading the northward trek towards specific calving grounds. This movement is often synchronized across herds. After calving, caribou aggregate into groups during the summer months, providing relief from biting insects.
The fall migration generally commences from August and can continue through November, as herds move southward to their winter ranges. This movement is largely influenced by decreasing temperatures and the first significant snowfalls. While the timing is generally consistent, variations occur based on specific herds and prevailing environmental conditions each year.
Driving Forces Behind Migration
Caribou migrations are driven by a combination of biological needs and environmental pressures. A primary motivator is the search for abundant and high-quality forage. Caribou shift their locations to access new plant growth on the tundra during summer and to find lichens, their main winter food source, in more sheltered boreal forests. This constant movement prevents overgrazing in any single area.
Another significant factor is the avoidance of predators, particularly during the vulnerable calving period. By migrating to remote, often open, calving grounds, pregnant females enhance the survival chances of their newborn calves from predators like grizzly bears, wolves, and golden eagles. Caribou also migrate to escape harsh weather conditions, such as deep snow and extreme cold, and to mitigate harassment from biting insects during warmer months.
Journeys Across Landscapes
Caribou undertake journeys across diverse landscapes, covering vast distances between their seasonal habitats. Some herds travel up to 2,000 miles in a single year, with round-trip migrations commonly reaching 1,350 kilometers (approximately 838 miles).
During the summer and calving seasons, caribou primarily inhabit the open Arctic tundra. As winter approaches, many herds move into the boreal forest, also known as taiga, which offers sheltered conditions and access to winter forage. Caribou utilize traditional migration corridors used for millennia. However, these routes can shift in response to changing environmental conditions or resource availability.
Influences on Migration Patterns
External factors can significantly influence the timing, routes, and overall success of caribou migrations. Environmental variables, such as snow depth, ice conditions, and temperature fluctuations, play a role in guiding their movements. For instance, warmer fall temperatures can delay the onset of snow, which caribou often use as a cue to begin their southward migration. Changes in vegetation, including the spread of shrubs on the tundra or increased wildfires, can also alter food availability and impact migratory pathways.
Human activities also present challenges to caribou migration. Industrial developments, such as oil and gas extraction, mining operations, and the construction of roads and pipelines, can fragment habitats and disrupt traditional routes. Caribou have been observed to avoid areas with increased human activity, sometimes by considerable distances.
The cumulative effect of these disturbances can significantly impact caribou populations and their ability to complete their annual journeys. Climate change further complicates migration patterns, leading to thinner ice on water crossings, more frequent rain-on-snow events that make foraging difficult, and increased insect harassment during warmer summers.