A black bear can lose a maximum of about 30% of its body weight during hibernation. Most of that loss comes from stored fat, which supplies energy during months without eating or drinking.
Before entering a den, an American black bear enters a period of intense feeding called hyperphagia. It may add substantial fat reserves during late summer and autumn, when berries, nuts, insects, and other foods are available. Hibernation usually lasts around three to eight months, depending on climate and location.
The bear’s metabolism falls dramatically, reaching roughly one-quarter of its normal basal rate. Its heart rate can drop from about 40–50 beats per minute to around 8 beats per minute, while body temperature decreases only modestly compared with that of smaller mammalian hibernators.
Black bears also have remarkable adaptations for preserving muscle and bone. They generally do not urinate or defecate in the den, and they recycle nitrogenous waste to help maintain muscle tissue. Reported losses vary by individual and study, but 30% is the accepted upper figure for this quiz.