The tundra is a biome characterized by permanently frozen subsoil, scientifically known as "permafrost." The Arctic tundra's annual average temperature is below 41 degrees Fahrenheit, with annual precipitation of less than 10 inches -- essentially a frozen desert; in the winter these tundras, in the extreme Northern hemisphere, receives almost no sunlight. Tundra conditions also exist in Alpine conditions, on very high mountains. Animals living in these harsh conditions must possess specific characteristics to survive.
Due to the cold, dry climate of the tundra, tundra animals must have natural protection against the elements, which can take many forms. Some animals have special adaptations, like the Arctic Fox; this animal has very small ears, short stature, and a rounded body, to minimize surface exposure to the elements. The Musk Ox is another animal specifically suited to the tundra; it has two layers of fir, a short and a long layer, which traps air. This air, in turn, warms with the ox's body heat, turning the frigid air into natural insulation. Most of the larger tundra animals are characterized by a thick mass due to heavy layers of insulating fat.
Hibernation is common among tundra animals -- particularly in the Arctic tundra, where the winters days provide little or no sunshine. Animals like the brown bear consume extreme amounts of food in the summer, then sleep all winter, fueled and warmed by the insulating fat they amassed. Along with this hibernation cycle comes a very specific mating cycle; animals who hibernate during cold tundra winters quickly breed and raise their young during the short summer months.
Many animals that live in the tundra and don't hibernate migrate to warmer climates during the tundra's frigid winters. This is particularly true in the Alpine tundras, where animals can easily travel down the mountain to more moderate climates.
The specific requirements of tundra life also limit the number of species found there, causing populations to be extremely interrelated. This means fluctuations in population in one group significantly influence others. For example, a diminished lemming population would lead to a subsequent decrease in Snowy Owls, birds who rely primarily on lemmings as a food source.
Many tundra inhabitants are just what you would expect to find in Arctic climes: caribou and reindeer, polar bears, and arctic foxes and hares. Mountain goats, elk and sheep are common in alpine tundras. Insects also reside in the tundra, as well as fish, but due to the extremely cold temperatures, reptiles and amphibians are almost unheard of. Birds who appear there -- like grouses in the alpine tundras, falcons, ravens and more in the arctic tundras -- are almost invariably migratory.