Biologists, hikers and others are reporting that grizzly bears are emerging from hibernation across the greater Yellowstone area, and experts urge caution for those venturing into grizzly country. Bears begin looking for food soon after they emerge from their dens. They are attracted to elk and bison that have died during the winter. Carcasses are an important food source that can cause bears to sometimes react aggressively if they are surprised while feeding on them.