ABSTRACT
Circadian rhythms of rodents are dependent on a pacemaker localized in the suprachiasmatic nucleus (SCN) of the hypothalamus. Generation or expression of circadian rhythms during deep hibernation appears to be mediated by neural pathways that include the SCN. Daily torpor in short daylengths was eliminated in Siberian hamsters with SCN lesions, possibly because of endocrine sequelae of the neural insult. Torpor was restored by food restriction, presumably because it reversed some of the hormonal changes induced by the lesions. Successful hibernators delay entry into hibernation until they have stored sufficient white adipose tissue or cached enough food to fuel energy expenditures during the winter months. The oscillators that generate annual rhythms of hibernation, body mass, and reproduction have so far eluded identification; they presumably are localized in the central nervous system. The SCN has been implicated in circannual organization of body mass rhythms of squirrels housed for several years at ambient temperature of 23°C.
