According to About.com, caribou, or reindeer as they are commonly called, are considered to be a grazing herbivore, and their diet mainly consists of the leaves of willow and birch trees, grasses, sedges and lichen. Reindeer moss, in particular, is a common dietary staple.
The caribou's diet changes according to the season. Lichen is the caribou's primary winter food source. Research suggests caribou occasionally eat fish, such as arctic char, small rodents, including lemmings, and bird eggs during spring when sustenance is not always abundantly available. During one research project, a specific group of reindeer was observed eating mushrooms with great enthusiasm during the late summer.
Caribou have a four-chambered stomach, which makes them a ruminant. Besides some gastropods, they are the only animals to have been found with the enzyme lichenase, which is used to break down lichenin to glucose. Because of this, caribou are the only large mammal capable of metabolizing lichen.
Caribou have hooves that change according to the season. During winter, their hooves become denser, which allows them to dig through the snow to access lichen. Their hooves become softer and more sponge-like during the summer, which allows them to walk comfortably on the thawed tundra ground.