The reindeer, or caribou, is a large ungulate species uniquely adapted to survive the extreme, resource-scarce environments of the Arctic and subarctic tundra. These animals undertake some of the longest terrestrial migrations on the planet, moving between seasonal feeding grounds across Northern Europe, Siberia, and North America. While commonly classified as herbivores that feed exclusively on plant matter, this classification does not tell the complete story of their nutritional strategy in an environment defined by scarcity.
The Foundation of Reindeer Diet: Herbivory
Reindeer are anatomically and physiologically classified as ruminant herbivores, meaning their digestive system is optimized for breaking down high-fiber plant material. During the short but productive summer months, their diet is rich and varied, consisting of grasses, sedges, and the leaves of willow and birch trees. This summer grazing allows them to build up the necessary fat reserves to endure the long, desolate winter.
As the seasons change, their feeding habits shift dramatically to accommodate deep snow cover and limited vegetation. In winter, the primary food source becomes lichens, often called “reindeer moss,” which they locate and excavate using their specialized hooves. Lichens are highly digestible and rich in carbohydrates, providing the energy needed to maintain body temperature, but this diet severely lacks other nutrients.
The Drive for Minerals and Protein
Survival in the Arctic often involves nutrient deficiencies, particularly during the winter and early spring when forage quality is lowest. A diet dominated by lichen contains extremely low levels of crude protein, often dropping below 5% of the total dry matter intake. This low protein content is insufficient to support the high metabolic demands of growth, pregnancy, and lactation.
The need for essential minerals, such as calcium and sodium, also becomes challenging in the winter. Calcium is a high-demand nutrient because reindeer grow and shed antlers annually, a process requiring substantial mineral investment. Furthermore, pregnant and lactating females require a significant influx of calcium and phosphorus to support fetal development and milk production. Since winter vegetation is poor in these necessary inorganic compounds, the reindeer are compelled to seek out alternative sources.
Documented Consumption of Non-Plant Matter
Driven by the need for protein, calcium, and iron, reindeer occasionally exhibit behavior that deviates from strict herbivory. One of the most documented examples is osteophagia, the gnawing on shed antlers, bones, and even caribou carcasses left by predators. This behavior is a direct response to mineral scarcity, allowing the animals to supplement their diet with concentrated sources of calcium and phosphorus.
Beyond bone consumption, there are documented instances of reindeer consuming animal matter opportunistically. They have been observed eating bird eggs from ground nests, and in rare cases, actively preying on small rodents, such as lemmings or shrews.
This consumption of non-plant matter, though not a routine part of their diet, is a functional adaptation that helps them bridge the nutritional gaps created by their extreme environment. This confirms that while reindeer are predominantly herbivores, they are highly opportunistic feeders capable of exploiting animal protein and minerals when necessary.