Sometimes the agitated animals will run for many kilometres, stopping to rest only when exhausted or when high winds temporarily disperse the insects. Running from insects places great energy demands on the caribou, and may retard their rate of growth by temporarily reducing their foraging. In July the herds start to move en masse and feed on flowers, grasses, and leaves of shrubs. The mating season the rut occurs in late October and early November. By late September the herds fat and in good condition are arriving in pre-rutting areas.
Bulls spar a great deal and sometimes fight for possession of cows.
The migration routes have always been so well established that, in past years, Native hunters lay in wait for caribou at places where they would cross lakes or rivers. Occasionally, however, the caribou did change their migration routes, and hunters and their families located near the traditional migration path faced starvation. The wolf is a natural predator of the caribou. Wolf packs follow the migrating herds from summer to winter range and back. Caribou are relatively free from this predator only during calving time, when the breeding wolves are raising their young in areas distant from the calving ground.