Hibernation the summer months is aestivation. Often associated with low temperatures, the function of hibernation is to conserve energy when sufficient food is unavailable. To achieve this energy saving, an endothermic animal decreases its metabolic rate and thereby its body temperature. Hibernation may last days, weeks, or months depending on the species, ambient temperature, time of year, and the individual's body condition. Before entering hibernation, animals need to store enough energy to last through the duration of their dormant period, possibly as long as the entire winter. Larger species become hyperphagic, eating a large amount of food and storing the energy
Hibernation their pre-hibernation weight by using their stored fats for energy. True hibernation is restricted to endotherms; ectotherms by definition cannot hibernate because they cannot actively down-regulate their body temperature or their metabolic rate. Still, many ectothermic animals undergo periods of dormancy which are sometimes confused with hibernation. Some reptile species are said to brumate, but possible similarities between brumation and hibernation are not firmly established. Many insects, such as the wasp "Polistes exclamans", exhibit periods of dormancy which have often been referred to as hibernation, despite their ectothermy. Obligate hibernators are animals that spontaneously, and annually, enter hibernation regardless of