Habitat
Across the vast range, caribou habitats include arctic tundra (including tussock tundra and sedge meadow), subarctic taiga, mature coniferous forest, forested peatlands, semi-open and open bogs, rocky ridges with jack pine, and riparian zones (Banfield 1974, COSEWIC 2002, Feldhamer et al. 2003, Hummel and Ray 2008). Some migratory herds in Alaska, Yukon, and Northwest Territories move seasonally between boreal forest (used in winter) and tundra (summer habitat). In summer, woodland caribou frequent open or semi-open habitats such as alpine tundra, upper subalpine, peatlands, islands, and shorelines where nutritious plants such as forbs and sedges are available (COSEWIC 2002). Large, low productivity, lichen-rich mature and old forests and forested peatlands are critical in winter (COSEWIC 2002); such areas provide critical food resources and are important spatial refuges from wolf predation (wolves tend to select more productive habitats in which deer, moose, or elk are more numerous) (Alberta Sustainable Resource Development and Alberta Conservation Association 2010). In winter in northeastern Alberta, woodland caribou selected forested fen peatland complexes; feeding activity was concentrated in forested raised bog islands, which may have been related to increased lichen biomass in these habitats (Bradshaw et al. 1995). "Mountain" caribou of southeastern British Columbia depend on older coniferous forests with high canopy closure, especially in late winter (Apps et al. 2001).
In northern British Columbia, pregnant females seek high south slopes in mountains as calving sites (Bergerud et al. 1984). In the Porcupine Herd of northeastern Alaska and northwestern Yukon, females give birth on patches of bare ground within snowfields (Eastland et al. 1989); cows select areas north of the foothills (snow conditions permitting), thereby reducing exposure of calves to predators.
Ecology
Caribou are generally gregarious; in tundra, they usually are bands of 10-50 or loose herds of several hundred or more than 1,000 individuals. Males and females may segregate seasonally.
Caribou populations often incur high rates of calf mortality resulting from predation, inclement weather, or malnutrition (Bergerud et al. 1984, Bergerud and Ballard 1988).
White-tailed deer carry and disperse into the environment meningeal worms that usually are fatal to moose and caribou but are clinically benign in deer; hence, white-tailed deer, through worm-mediated impacts, may exclude moose and caribou from otherwise suitable areas (see Schmitz and Nudds 1994).
Reproduction
Breeding occurs in late September and October. Cows bear usually 1 young in May or June. Newborn calves are precocious and soon able to walk. Calves and most yearlings commonly are not pregnant (Cichowski et al. 2004). Adult females sometimes skip reproduction for a year, in response to nutritional stress (Cameron 1994). In northeastern Alaska and adjacent Canada, 80% of adult females (age 3 years or older) gave birth each year (Fancy et al. 1994).