How do animals hibernate and survive without eating for months?

Before hibernation, animals like bears and groundhogs enter a hyperphagia phase where they eat enormous amounts to build up fat reserves. During hibernation, their metabolism drops dramatically, sometimes to just 2-5% of normal rates. Body temperature falls, heart rate slows from dozens of beats per minute to just a few, and breathing becomes very shallow. The body shifts to burning stored fat as its primary fuel source instead of carbohydrates. True hibernators like ground squirrels can drop their body temperature near freezing, while bears experience a lighter torpor with less dramatic temperature drops. Muscle and organ tissue is largely preserved through biochemical adaptations that prevent the protein breakdown you would expect from such prolonged fasting. Water is recycled internally from metabolic processes, reducing the need to drink.