| Popis: |
Hibernation in mammals in response to cold and food shortage in winter is characterized by a profound reduction in metabolism (to 1% or less of normal) and body temperature (to near 0 °C) which may last from a few days to a few weeks. Unlike hibernation in the ectotherms (e.g. frogs and snakes), hibernating mammals are capable of exiting from the depressed metabolic state at any time, rewarming to normal body temperature using exclusively, endogenously produced heat by shivering and non-shivering thermogenesis. Extensive comparative studies on the evolutionary, physiological, biochemical, and neuroendocrinologic aspects have firmly established that mammalian hibernation represents an advanced form of thermoregulation rather than a reversion to primitive poikilothermy, and that it is polyphyletic in origin, acquired independently by members of at least six mammalian orders (see Hudson 1973 and Lyman et al. 1982, for reviews). Because of this latter aspect, divergence in regulatory mechanisms for hibernation is common amongst different species; for instance, Syrian hamsters (Mesocricetus auratus) and chipmunks store food whereas ground squirrels and marmots store fat. This leads to differences in at least two aspects in energy metabolism: (1) the endocrine strategy for food digestion and assimilation vs chronic fasting; and (2) the reliance on carbohydrates as the energy source for hibernation and the biochemical emphasis on glycogenolysis vs gluconeogenesis. Thus, one must be cautious when extrapolating mechanistic findings from one species to another because of the heterogeneity in niche specialization and the species-specific solutions (behavioral and physiological) in achieving the depressed metabolic state exemplified by hibernation. |
