The near constancy of hibernation mass-specific metabolic rate regardless of body size: Insights from ontogeny and comparative physiology

I employ the comparative method to improve understanding of the metabolic rates expressed by mammalian hibernators when hibernating at body temperatures ≤ 10°C (deep hypothermia). The weight of evidence is that the mass-specific metabolic rate of adult hibernators in hibernation (HibMR) is constant or nearly constant regardless of body size. I compare to HibMR the metabolic rates of neonatal mice of two species when the neonates are in deep hypothermia. The neonates have mass-specific metabolic rates statistically identical to the HibMR of adult hibernators, suggesting that mammals may express a relatively fixed minimum mass-specific metabolic rate during deep hypothermia, regardless of the cause of hypothermia (hibernation or neonatal cooling). I also compare the relationship between metabolic rate and body size in polar species of teleost fish with the metabolism-size relationship in hibernating mammals. The mass-specific metabolic rates of resting polar fish at polar temperatures – although similar in order of magnitude to mass-specific HibMR in hibernating mammals – exhibit unambiguous allometry in relation to body size, in contrast to the lack or near-lack of allometry in the mammals, suggesting that comparative studies of the two groups might help reveal the mechanisms underlying a switch between allometry and lack of allometry.