Grizzly bears are often imagined as fearsome beasts, better known for their brawn than their brain. A brief study of their hibernating habits, however, would suggest a smarter, more strategic creature. Grizzlies go underground into cozy dens and hibernate for up to seven months in a year. During hibernation, they become hyper-efficient; their respiration decreases from six to ten breaths every minute to just one breath every forty-five seconds. They do not eat, drink, urinate, or defecate during hibernation, living off a layer of fat built up during the summer and autumn months.