Reindeer, known as caribou in North America, are an arctic and sub-arctic species of deer inhabiting the tundra and boreal forests across the Northern Hemisphere. These animals survive in challenging environments where extreme seasonal changes dictate the availability and quality of food. Their diet is highly specialized, shifting dramatically between the brief summer and the long, resource-scarce winter. This adaptation allows them to thrive where other large herbivores cannot, relying on specialized foraging and a unique digestive system to extract sustenance from low-quality forage.
The Foundation of the Diet: Lichens and Mosses
The reindeer’s survival during the long winter months is uniquely tied to lichens, particularly species colloquially known as “reindeer moss” (Cladonia rangiferina). Lichens are a symbiotic organism composed of a fungus and an alga or cyanobacterium, and they represent a reliable carbohydrate source when green vegetation is absent. These organisms can make up over 50% of the reindeer’s winter diet and are one of the few food items available under a deep blanket of snow.
While lichens are low in protein, they are rich in easily digestible carbohydrates, making them an efficient energy source for maintaining body heat. To access this food, reindeer use their hooves and muzzles to “crater” through snow, sometimes digging up to a meter deep. This foraging process is guided by their acute sense of smell, which allows them to locate the lichens beneath the snowpack. The availability of this primary food source is paramount, as a thick layer of ice or hard-crusted snow can block access to the lichens, leading to starvation.
Seasonal Shifts in Foraging
The summer season brings an abrupt and complete change in the reindeer’s diet, shifting from carbohydrate-heavy lichens to protein-rich green vegetation. This period is when they must replenish body reserves and support the growth of calves, requiring a substantial increase in nutrient intake. The diet expands to include a wide variety of plants, such as sedges (Carex species), grasses, willow leaves, and the inflorescence of cotton grass.
The consumption of fresh, new plant growth is highly selective, as the reindeer focus on the parts with the highest concentration of protein and energy. During late summer and autumn, fungi, or mushrooms, become an important seasonal food item, offering a dense source of protein and minerals. This switch to a high-quality, high-protein summer diet is necessary to accumulate the fat reserves required to endure the subsequent winter.
Digestive Adaptations
Reindeer are classified as ruminants, possessing a four-chambered stomach that allows them to process the complex plant matter they consume. This digestive process relies on symbiotic microorganisms, including bacteria and fungi, housed in the largest stomach chamber, the rumen. These microbes ferment the ingested plant material, breaking down tough components like cellulose in grasses and the complex carbohydrates in lichens.
The reindeer is considered an intermediate ruminant, meaning its digestive strategy is flexible and can adapt to seasonal changes in forage quality. During the winter, the microbial community in the rumen efficiently breaks down the high-carbohydrate content of lichens. This efficiency is necessary to extract maximum energy from the low-quality winter forage, enabling the animal to survive on stored fat reserves.