Why do some animals hibernate through the entire winter?

Some animals hibernate through winter primarily because food becomes scarce and the energy cost of staying active would exceed what they could obtain from foraging. Rather than burning through fat reserves trying to survive in harsh conditions, they enter a state of dramatically reduced metabolism, dropping their body temperature, heart rate, and breathing to conserve energy. This strategy works especially well for animals like bears, groundhogs, and bats that can build up sufficient fat stores in autumn. Hibernation is essentially a survival trade-off: the animal sacrifices months of activity in exchange for making it to spring when food and warmth return. Animals in colder climates with longer winters tend to hibernate longer because the period of food scarcity is extended.