ANNUAL LIPID CYCLES IN HIBERNATORS: Integration of Physiology and Behavior

Abstract

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.

Mitochondrial Dysfunction, Bioenergetic Impairment, and Metabolic Down-Regulation in Sepsis

“…For many animals, this hypometabolic state is a protective adaptation to harsh environmental conditions and is a regulated seasonal response (45,46). Hibernation has also been described at the cellular level in response to low oxygen tension as with hypoxic and ischemic insults (41,47,48).…”

Subtropical mouse-tailed bats use geothermally heated caves for winter hibernation

“…This increase in body fat is achieved by switching diet preference during summer (mid-July) from coleopterans and heteropterans to mainly fat-rich winged carpenter ants [11]. This fat-rich food contains a high fraction of saturated and mono-unsaturated fatty acids (SFA and MUFA, respectively), but almost no polyunsaturated fatty acids (PUFA) [12] which are considered important for hibernation success at low temperatures [13][14][15][16][17]. As a result, mouse-tailed bat pre-winter body fat is lacking in PUFAs and is extremely saturated [11,12], which is a-typical for hibernators.…”

Late-born intermittently fasted juvenile garden dormice use torpor to grow and fatten prior to hibernation: consequences for ageing processes

“…Most hibernators do not eat during the winter but rely entirely on body fat stores [3]. Therefore, gaining sufficient fat in autumn to last throughout hibernation is an important process and is essential for the individual's winter survival.…”