The reindeer, known as caribou in North America, is an iconic herbivore of the Arctic and sub-Arctic regions. This species thrives in the tundra, an environment characterized by permafrost, extremely low temperatures, and a short growing season. Surviving in this challenging landscape requires a highly specialized, seasonal diet. This strategy allows the reindeer to navigate the stark contrast between the resource-poor winter and the brief, resource-rich summer.
Winter Survival: The Dominance of Lichens and Mosses
During the long, dark winter, when the tundra is locked under snow and ice, the reindeer’s diet shifts dramatically. The primary food source becomes lichens, often mistakenly called “reindeer moss,” which can constitute up to 90% of their winter intake. The most important of these are terrestrial species like Cladonia rangiferina and Cladonia stellaris.
These lichens provide a high concentration of soluble carbohydrates, such as lichenin, which are a dense source of energy necessary for maintaining body temperature and basic metabolic functions in the cold. To access this buried food, reindeer employ a specialized behavior called “cratering,” where they use their hooves to paw through the snow cover. A keen sense of smell aids them in locating the lichens beneath layers of snow.
Despite being energy-rich, lichens are low in crude protein, typically containing only 1.7% to 3.3% crude protein by dry matter. This nutritional imbalance means that reindeer generally lose body mass throughout the winter, relying on the fat reserves accumulated during the summer. They possess a physiological adaptation that allows them to recycle urea, a nitrogen-containing waste product, to synthesize microbial protein in the gut, maximizing the utility of the limited nitrogen available.
The reliance on lichens presents a challenge because these organisms are extremely slow-growing, often increasing by only a few millimeters per year. Heavy grazing can easily deplete a lichen pasture, requiring the herd to undertake long migrations to find new feeding grounds. The winter diet prioritizes simple energy over building or maintaining muscle mass.
Summer Abundance: Grazing on Tundra Vegetation
The arrival of the brief Arctic summer initiates a period of intense feeding and rapid nutritional recovery. The reindeer’s diet transforms from a limited, carbohydrate-heavy one to a diverse, protein-rich buffet of fresh tundra vegetation. This shift is necessary to replenish depleted energy stores and prepare for the next winter cycle.
Reindeer graze selectively on a variety of plants, including grasses, sedges, and the young, tender leaves of shrubs like dwarf birch and willow. These new growths are packed with essential nutrients, with the summer diet containing up to 15% crude protein, a significant increase over the winter months. The high protein and energy content allows the animals to rapidly gain body weight and build the thick layer of fat required for survival.
This period meets the high nutritional demands of reproduction, lactation for females, and antler growth for both sexes. The summer diet provides the necessary vitamins, minerals, and nitrogen to support these energy-intensive biological processes. The reindeer must capitalize on the short window of plant availability, often spending most of the daylight hours grazing intensely.
The rapid digestive turnover of these soft, green plants contrasts with the slow digestion of lichens, facilitating the swift assimilation of nutrients. By late summer, the focus shifts to maximizing fat reserves before the vegetation begins to senesce and freeze. This seasonal gorging ensures the animal has the stored energy to endure the low-intake, high-effort foraging required in the subsequent winter.
Nutritional Supplements and Mineral Needs
Beyond the primary seasonal diets of lichens and vascular plants, reindeer actively seek supplementary items to fulfill nutritional gaps. Fungi, particularly mushrooms, become an important food source in the late summer and autumn. These mushrooms are sought after because they offer a concentrated source of protein and fat just before the onset of winter.
Reindeer also occasionally consume non-plant matter, especially when their diet is lacking in specific trace elements. Documented supplemental foods can include bird eggs, small rodents like lemmings, and even fish. This behavior provides a rare but significant injection of animal protein and fat not otherwise available in the tundra ecosystem.
A consistent challenge in the tundra is obtaining sufficient minerals, as many plants and lichens are mineral-poor. Reindeer compensate for this by engaging in geophagy, the consumption of mineral-rich soil or clay, which provides sodium, calcium, and other trace elements. They will also chew on shed antlers or bones, a direct method of supplementing calcium and phosphorus necessary for skeletal maintenance and antler growth.
The need for specific minerals, such as sodium, is sometimes met by selectively eating mineral-accumulating plants like aquatic herbs or horsetails. These supplementary dietary behaviors demonstrate the reindeer’s complex foraging intelligence and its adaptation to meet seasonal nutritional deficiencies in a harsh environment.