What do bears and other hibernators rely on for energy during their winter sleep?

Bears and other hibernators rely on stored body fat for energy during their winter sleep. Before hibernation, many animals eat heavily and build substantial fat reserves, a preparation called hyperphagia. During the dormant season, their metabolism, breathing, and heart rate slow, allowing those reserves to last for weeks or months.

This strategy is especially important for bears, which may spend an entire winter in a den without eating. Their bodies use fat not only as fuel but also as a source of metabolic water. Bears can lose a significant portion of their pre-hibernation body mass while preserving much of their muscle through specialized protein recycling and other physiological adaptations.

A common mix-up is “stored food.” Some small hibernators, such as chipmunks, cache seeds or nuts and wake periodically to eat them. Bears generally do not stockpile meals in their dens; they depend primarily on fat accumulated before winter. Hibernation is therefore an energy-budgeting system, not simply a long nap.