Hibernation is an adaptive strategy characterized by a drastic suppression of metabolism, activity and body temperature that allows animals to survive during periods with little or no food availability; hibernation is widespread among mammals. Two types of mammalian hibernators exist: obligate hibernators, such as black bears and ground squirrels, hibernate every winter under the regulation of a circannual clock whose molecular mechanisms are largely unknown.“
Source : Decreases in body temperature and body mass constitute pre-hibernation remodelling in the Syrian golden hamster, a facultative mammalian hibernator Royal Society Open Science, Apr. 2016.
Some mammals - notably the Arctic Ground Squirrel - can reach and maintain body temperatures of −2.9 °C for several months. It's not currently known how it's possible for the animals to do this without fatal damage caused by blood freezing and nerve-cell death.
In addition to the mystery surrounding how hibernation is triggered and maintained, there is also no explanation as to how and why hibernating animals often come out of hibernation for short periods and then return.
Obligate hibernators are animals that spontaneously, and annually, enter hibernation regardless of ambient temperature and access to food. Obligate hibernators include many species of ground squirrels, other rodents, mouse lemurs, the European hedgehog and other insectivores, monotremes, marsupials, and even butterflies such as the small tortoiseshell. These undergo what has been traditionally called “hibernation”: the physiological state where the body temperature drops to near ambient (environmental) temperature, and heart and respiration rates slow drastically. The typical winter season for these hibernators is characterized by periods of torpor interrupted by periodic, euthermic arousals, wherein body temperatures and heart rates are restored to euthermic (more typical) levels. The cause and purpose of these arousals is still not clear. The question of why hibernators may experience the periodic arousals (returns to high body temperature) has plagued researchers for decades, and while there is still no clear-cut explanation, there are myriad hypotheses on the topic.”
Source and further reading Wikipedia
Note : It has been suggested that some animals periodically come out of hibernation in order to
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