Above photo from National Park Service utilized respectfully in accordance with Fair Use.
They primarily eat ground and tree lichens. It takes 80 to 150 years for a forest to grow enough lichens for caribou.
Caribou are the only deer species in which both sexes grow antlers. Every year they shed their antlers and the next year they grow new ones!
Historically, woodland caribou inhabited the forests of the Northern United States from Maine to Washington State, but have been reduced to one small herd in the Selkirk Mountains of northern Idaho, eastern Washington and southern British Columbia. This last U.S. herd is reduced to approximately 40 members that tend to stay mostly in the Canadian part of its range.
Worldwide, mountain caribou are found only in northern Washington and Idaho and British Columbia. The northern ecotype of woodland caribou have a broader distribution in Canada.
Caribou are well-known for their ability to use tree growing (arboreal) lichens as a major food source. As a result they are most often associated with mature coniferous forests that provide substantial quantities of tree lichens.
Mating Season: Early to mid-October
Gestation: October to early June
Number of offspring: 1 calf 
: Defenders of Wildlife, “Basic Facts About Caribou”: www.defenders.org/woodland-caribou/basic-facts
: Conservation Northwest, “Mountain caribou Rangifer tarandus caribou”: www.conservationnw.org/what-we-do/wildlife-habitat/woodland-caribou
: Defenders of Wildlife, “Threats (to Caribou)”: www.defenders.org/woodland-caribou/threats