Annual Lipid Cycles in Hibernators: Integration of Physiology and Behavior

• Published in: Annual review of nutrition 2005-01, Vol.25 (1), p.469-497
• Main Author: Dark, J
• Format: Article
• Language: English
• Subjects: Adipose tissue; Adipose Tissue - metabolism; Adipose Tissue, Brown; Animals; Biological and medical sciences; Body temperature; brown adipose tissue; Chronobiology; Eating; energy balance; Energy conservation; Energy Intake; Female; Females; Food availability; food intake; Fundamental and applied biological sciences. Psychology; heat production; hibernation; Hibernation - physiology; Hormones; Hypothermia; Leptin - blood; Lipid Metabolism; Lipids; lipogenesis; Male; Mammals; nutrient partitioning; Periodicity; Polyunsaturated fatty acids; Reproduction; Sciuridae; Seasons; Spring; thermoregulation; Thermoregulation. Hibernation. Estivation. Ecophysiology and environmental effects; ultradian rhythm; Vertebrates: anatomy and physiology, studies on body, several organs or systems; white adipose tissue
• ISSN: 0199-9885; 1545-4312
• DOI: 10.1146/annurev.nutr.25.050304.092514

Mammalian hibernation is a temporary suspension of euthermia allowing endotherms to undergo reversible hypothermia and generate a marked savings in energy expenditure. In most fat-storing hibernator species, seasonal changes in food intake, triacylglycerol deposition, metabolism, and reproductive development are controlled by a circannual clock. In ground-dwelling sciurid rodents (ground squirrels and marmots), for example, energy intake increases during a summer body mass gain phase, and toward the end of this phase metabolic rate also begins to decrease, resulting in a profound increase in lipid deposition as fat. Increased activity of lipogenic hormones and enzymes correspond with this increase. The hibernation mass loss phase begins after the body mass peak in the fall and ends in spring. During this phase, stored lipids are slowly utilized in a programmed manner by undergoing deep torpor or hibernation during which the hypothalamic setpoint for body temperature is typically reduced to just above 0 degrees C. Throughout the hibernation season, bouts of deep torpor are punctuated by periodic arousals in which brown adipose tissue thermogenesis plays a critical role. Lipid oxidation nearly exclusively fuels deep torpor and most of the rewarming process. The fatty acid composition of stored lipids can affect the depth and duration of deep torpor, and saturated fatty acids may be preferentially used during hibernation, whereas polyunsaturated fatty acids may be preferentially retained. Female and underweight male hibernators terminate hibernation in spring when aboveground food becomes available; in contrast, heavier males with sufficient lipid reserves spontaneously terminate hibernation several weeks before females and independent of food availability. Mating occurs shortly after emergence from hibernation, and the lipid cycle begins again with the completion of reproduction. Lipid deposition and mobilization, temperature regulation, reproduction, and circannual timing are intimately interdependent. The unique manner in which they are controlled during the annual cycle, especially lipid reserves, makes hibernators valuable and promising models for research into the mechanisms underlying these processes in all mammals.