Caribou, also known as reindeer in some regions, are mammals that inhabit the northern regions of Europe, North America, Asia, and Greenland. They are well-adapted to survive in the harsh Arctic and subarctic environments, and their diet plays a crucial role in their survival. As herbivores, caribou have a varied diet that changes with the seasons, allowing them to thrive in different habitats.
Seasonal Diet Variations
Caribou exhibit a remarkable ability to adapt their diet to the changing seasons and availability of food resources. This dietary flexibility is essential for their survival in the Arctic and subarctic regions, where food availability can fluctuate dramatically throughout the year.
Summer Diet: A Focus on Growth
During the summer months, when the tundra comes alive with vegetation, caribou primarily feed on grasses, the leaves of willows, sedges, flowering tundra plants, and mushrooms. These food sources are rich in nutrients and provide the energy needed for growth, reproduction, and building up fat reserves for the winter. The Porcupine caribou herd, for example, has been found to consume diamond-leaf willow, netleaf willow, dwarf birch, Arctic sweet coltsfoot, and tussock cottongrass during the summer.
Winter Diet: A Focus on Survival
As winter approaches and snow blankets the landscape, caribou shift their diet to lichens, dried sedges (grasslike plants), and small shrubs (like blueberry). Lichens, often referred to as "reindeer moss," become a staple food source during the winter months, sometimes making up 80 to 90 percent of their diet. Caribou use their large, hollowed-out hooves to dig through the snow in search of these food sources.
The Importance of Lichens
Lichens are a unique composite organism formed by the symbiotic relationship between a fungus and an alga. They are abundant in Arctic and subarctic environments and provide caribou with essential carbohydrates for energy. Caribou are well-adapted to digesting lichens, thanks to their specialized digestive system.
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Nutritional Value of Forage
Researchers analyze the nutrient content of plant and lichen samples collected from caribou summer ranges to understand the nutritional value of their forage. This analysis involves quantifying the energy, protein, and mineral content of the samples. The nutritional value varies across different areas and throughout the growing season. This information is used to model forage conditions and predict where caribou can find the most nutritious food, especially during critical calving and post-calving seasons.
The Impact of Climate Change on Caribou Diet
Climate change is significantly impacting caribou habitat and food availability. Warmer temperatures, altered precipitation patterns, and changes in vegetation composition are all affecting the caribou's diet.
Insect Harassment
Changes in climate cause earlier and longer Arctic growing seasons, which leads to increased insect activity. Responding to insect harassment reduces the time caribou spend foraging, while simultaneously increasing the energy they expend running or seeking refuge in cooler, windier areas on water, gravel, or snowfields. These energy expenditures can result in a subsequent decline in caribou survival and reproduction.
Changes in Vegetation
Vegetation is changing in some areas, with less lichen and more shrubs, which are harder to digest and less energy-rich. Precipitation patterns are also changing, and warm winter temperatures followed by freezing rain can create a frozen ice crust that locks the animals’ vegetation under ice.
Caribou Digestive System
Caribou, like other ruminants, have a four-chambered stomach that allows them to efficiently digest plant material. The four chambers of the stomach are the rumen, reticulum, omasum, and abomasum.
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The Rumen and Reticulum
When caribou first ingest food, it travels to the rumen and reticulum, the first two chambers of their stomach. These chambers contain a diverse community of microorganisms, including bacteria, protozoa, and fungi, which break down cellulose, a complex carbohydrate found in plant cell walls. The rumen and reticulum also serve as a fermentation vat, where food is mixed and churned.
Regurgitation and Re-Chewing
After spending some time in the rumen and reticulum, the caribou regurgitates the food back into its mouth and chews it again. This process, known as rumination, further breaks down the plant material and increases the surface area for microbial digestion.
The Omasum and Abomasum
After being re-chewed, the food is swallowed again and passes into the omasum, the third chamber of the stomach. The omasum absorbs water and other nutrients from the food. Finally, the food enters the abomasum, the fourth and final chamber of the stomach. The abomasum is similar to the stomach of monogastric animals, such as humans, and secretes acids and enzymes that further digest the food.
Caribou Herds and Migration
Caribou are nomadic animals that are constantly moving in search of food. They often migrate long distances between their summer and winter ranges. Some herds travel more than 600 miles (965 kilometers) along well-traveled routes. The Porcupine caribou herd completes the longest overland migration of any terrestrial mammal on earth, traveling roughly 450 miles every spring to reach the coastal plain.
Caribou as a Food Source for Humans
Caribou are hunted by Indigenous northern people throughout much of their range. They are an important source of food and other resources for these communities. Alaska hunters shoot about 22,000 caribou each year for food.
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Threats to Caribou Populations
Caribou populations are facing numerous threats, including habitat loss, climate change, predation, and human disturbance.
Habitat Loss
Habitat loss and fragmentation due to human activities, such as logging, mining, and road construction, are major threats to caribou populations.
Predation
Wolves, grizzly bears, and golden eagles are major predators of caribou, especially newborn calves.
Human Disturbance
Disturbance from human activities, such as aircraft and snowmobiles, can disrupt caribou behavior and reduce their ability to forage and reproduce.
Conservation Efforts
Various conservation efforts are underway to protect caribou populations and their habitat. These efforts include habitat protection, predator management, and hunting regulations. It is crucial to understand how caribou respond to these changes for identifying critical habitat areas and predicting future population trends. By using a combination of video collars, field sampling, and laboratory analyses, researchers are examining caribou responses to changing habitat conditions and gleaning information that could help manage and conserve the herd.