Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor

Abstract

Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (Tb) for MR reduction, perhaps with the exception of very small species and at high torpor Tb, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to Tb effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of Tb are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and Tb reduction at torpor onset. Over the ambient temperature (Ta) range where torpid heterotherms are thermo-conforming, the Tb-Ta differential is more or less constant despite a decline of MR with Ta; however, in thermo-regulating torpid individuals, the Tb-Ta differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower Tb. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Molecular Detection of the Causative Agent of White-nose Syndrome on Rafinesque's Big-eared Bats (Corynorhinus rafinesquii) and Two Species of Migratory Bats in the Southeastern USA

“…In healthy individuals, torpor bouts typically last 12-15 d (Reeder et al 2012;Brownlee-Bouboulis and Reeder 2013), and bouts are separated by brief arousal periods that serve various physiologic purposes (Geiser 2004;Moore et al 2011;Jonasson and Willis 2012). Extensive torpor bouts may facilitate the growth of P. destructans on bats, ultimately causing lesions that disrupt torpor resulting in the premature depletion of energy reserves (Warnecke et al 2012).…”

Elevated expression of protein biosynthesis genes in liver and muscle of hibernating black bears (Ursus americanus)

“…During hibernation, heart and respiration rates, blood flow, and oxygen consumption decrease dramatically to Ͻ10% of normal or basal rates (2,4). These physiological changes do not represent a loss of homeostasis but instead are precisely controlled and spontaneously reversible, and they allow individual animals to survive highly seasonal or unpredictable environments where food availability becomes lacking (7,12). The molecular and genetic basis of hibernation in small mammals has only recently begun to be described, and little is known about its evolutionary history.…”

Energy conservation in hibernating endotherms: Why “suboptimal” temperatures are optimal

“…Changing thermal conductance can represent several important aspects of hibernation energetics. For example, thermal conductance scales allometrically, with larger species having lower thermal conductance (Geiser, 2004), and huddling during hibernation lowers effective thermal conductance (Boyles et al, 2008). We modeled thermal conductance at three levels (0.007, 0.07, and 0.7 ml O 2 /(g h • C)), which span the range of values found in hibernating mammals (Geiser, 2004).…”

“…Because the primary benefit of facultative heterothermy is thought to be energy conservation, most studies have focused on quantifying the relationship between ambient temperature (T a ) and torpid metabolic rate (TMR) (e.g., Geiser, 2004;Willis et al, 2005;Dunbar and Tomasi, 2006). During torpor and hibernation, endotherms defend a reduced body temperature (T b ) setpoint that may be as much as 40 • C below normothermic levels, with fundamentally different T a -TMR relationships above and below the T a where TMR is minimized (T min ).…”

“…During torpor and hibernation, endotherms defend a reduced body temperature (T b ) setpoint that may be as much as 40 • C below normothermic levels, with fundamentally different T a -TMR relationships above and below the T a where TMR is minimized (T min ). At T a > T min , TMR decreases with T a , typically with Q 10 values of 2-3 (Geiser, 2004). At T a < T min , TMR increases linearly with decreasing T a as animals increase metabolic thermogenesis in order to defend their T b and avoid freezing if T a falls below freezing (Buck and Barnes, 2000).…”