What is hibernation?  Why do animals hibernate?  We learn about: the adaptation of hibernation, classes of animals that hibernate, estivation (dormancy to conserve energy in hot periods or when habitats are disrupted), preparing for hibernation, hyperphagia, hibernacula (den) preparations, triggers for hibernation, circannual rhythms, physiological changes during hibernation, torpor (modified hibernation), interesting hibernation facts. Detailed graphics, exciting video and multiple-choice reviews reinforce important concepts.