Mechanism

Causation

For most mammals hibernation consists of cycles of torpor and slight arousal. Although changes in environmental temperature are likely the longterm cause of hibernation, external temperature is not an immediate cause of torpor in endothermic animals, due to their ability to keep a constant body temperature through homeostatic thermoregulation.

The Role of the Mitochondria in Metabolic Suppression

At the onset of hibernation and subsequent states of torpor, reductions in metabolic rate begin before the body temperatures decrease. This suggests that metabolic suppression is an active mechanism of torpor, rather than a result of thermal effects (Staples, 2014). Oxidative phosphorylation is a process along the metabolic pathway that allows cells to oxidize nutrients, which are later used to reform ATP. This process regularly takes place within the mitochondria. Liver mitochondrial metabolism is down regulated during hibernation and daily torpor, primarily by a suppression of substrate oxidation. This suppression contributes to secondary energy savings (Staples and Brown, 2008).

Methylation of Promoter Regions

There has been an observed enhanced methylation of promoter regions as a means of turning off transcription for certain genes during hibernation. Hibernation responsive genes, however, have hypomethylation of promoter regions which permits increased transcription during hibernation. (Tessier and Storey, 2014)

The Role of Circadian Rhythms and the Suprachiasmatic Nucleus in Hibernation

Circadian rhythms are shown to be a factor in inducing and regulating hibernation (Ruby 1996). The suprachiasmatic nucleus (SCN) is a small region in the hypothalamus responsible for controlling circadian rhythms. In a study conducted with ground squirrels, intact control females were compared to a group whose hypothalamic SCN was completely ablated (SCNx). They concluded that the SCN played an integral role in controlling the duration of hibernation and the temporal structure of individual periods of torpor.