Birds leave on migratory journeys, bears and rodents hide in hibernation. A dominant presence in summer, grizzly bears go underground in September and October and remain there until May. Pregnant females are the first to den and, in the company of their newborn cubs, the last to leave their winter shelters in spring. Amazingly, pregnant bears give birth during this cycle of extended fasting and then nurse their cubs for several months while still in hibernation.
Bears tend to build their dens around the time of the first heavy snowfalls in fall or winter.
Denali's grizzlies have been observed digging dens as early as August, in response to six-inch snowfalls. Grizzlies don't need to defecate, urinate, eat, or drink during their months-long dormancy, and their body temperature and metabolic rates drop only slightly. The hibernation mechanisms of rodents are vastly different from bears. They rouse themselves periodically throughout the winter, bringing body temperature and heart rates back to normal, perhaps to eliminate body wastes. Marmots too go into a deep dormancy that is periodically broken.