Turning down the Fires of Life: Metabolic Regulation of Hibernation and Estivation

The ability to suppress basal metabolic rate and enter a hypometabolic or dormant state is a lifesaver for many animals. When food is unavailable or environmental conditions too stressful to maintain normal life, many species reduce their energy consumption and, thereby, extend the time that fixed reserves of endogenous metabolic fuel stores can sustain life. Metabolic suppression can come in varying intensities, ranging from a relatively shallow 20-30 % reduction in metabolic rate for a few hours during nightly torpor in small birds and mammals to a >95% reduction in metabolic rate for many weeks during seasonal hibernation in mammals or diapause in insects through to a virtually ametabolic state that allows desiccated seeds, spores and cysts of many plant and animal species to remain viable for many years (Storey and Storey, 1990; Hand and Hardewig, 1996). The present review focuses on two forms of aerobic hypometabolism in vertebrates mammalian hibernation and amphibian estivation (Geiser, 1988; Wang, 1989; Land and Bernier, 1995) and describes some of our recent advances in understanding the metabolic regulation and changes in gene expression that underlie the phenomena.