The equivalent during the summer months is aestivation. Often associated with low temperatures, the function of hibernation is to conserve energy when enough 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 in fat deposits. In many small species, food caching replaces eating and becoming fat.
Dormouse |
Some species of mammals hibernate while gestating young,
which are born either while the mother hibernates or shortly afterwards. For
example, female polar bears go into hibernation during the cold winter months
in order to give birth to their offspring. The pregnant mothers significantly
increase their body mass prior to hibernation, and this increase is further
reflected in the weight of the offspring. The fat accumulation enables them to
provide a sufficiently warm and nurturing environment for their newborns.
During hibernation, they subsequently lose 15–27% of 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.
Primates
Fat-tailed dwarf lemur |
While hibernation has long been studied in rodents,
namely ground squirrels, no primate or tropical mammal was known to hibernate
until the discovery of hibernation in the fat-tailed dwarf lemur of Madagascar,
which hibernates in tree holes for seven months of the year. Malagasy winter
temperatures sometimes rise to over 30 °C (86 °F), so hibernation is not
exclusively an adaptation to low ambient temperatures. The hibernation of this
lemur is strongly dependent on the thermal behaviour of its tree hole: if the
hole is poorly insulated, the lemur's body temperature fluctuates widely,
passively following the ambient temperature; if well insulated, the body
temperature stays fairly constant and the animal undergoes regular spells of
arousal. Dausmann found that hypometabolism in hibernating animals is not
necessarily coupled with low body temperature.
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