Hibernation and daily torpor in Australian mammals

Abstract

Torpor is the most effective means for energy conservation available to mammals and is characterized by substantial reductions in body temperature (Tb) and metabolic rate (MR). Most Australian terrestrial mammals are small with high mass-specific energy requirements and, although it is widely believed that torpor is not needed in a 'warm' country like Australia, a large number of species are heterothermic (i.e. capable of changing Tb, in contrast to homeothermic mammals with a constant high Tb). These heterothermic species (estimated >43% of terrestrial Australian native mammals) employ periods of daily torpor or prolonged multi-day torpor (hibernation) to conserve energy. Daily torpor is used by dasyurids (e.g. dunnarts, antechinus, quolls), myrmecobiids (numbat), tarsipedids (honey-possum), petaurid possums (e.g. sugar glider), rodents (but only known in the introduced house mouse), and small megabats (blossom-bats). During daily torpor, Tb is reduced from ~35°C during the active or normothermic phase to ~10-25°C during torpor, and the torpor MR (TMR) is ~30% of the basal metabolic rate (BMR). Daily torpor is often, but not exclusively, used during the rest phase and, between bouts of torpor, animals usually forage and feed. Recent evidence shows that free-ranging arid zone dasyurids may employ daily torpor on every day in winter and that torpor may last twice as long as in captivity (often up to around 20 hours), which will reduce energy expenditure and thereby food requirements by up to 80%. Hibernation or prolonged torpor has been observed in the Monotremes (echidna), marsupials (pygmy-possums and feathertail glider) and insectivorous bats (e.g. long-eared bats). During prolonged torpor, which often, but not exclusively, is expressed in winter, Tb is usually reduced to a minimum of ~0-10°C, and torpor bouts may last for several days or weeks, but in all species periodic arousals with brief normothermic periods (hours) between bouts of torpor have been observed. The TMR during hibernation is extremely low and can be as little as 1-5% of the BMR; daily energy expenditure can be reduced to only 3% of that in active individuals permitting survival on stored body fat for months without the need to feed. Daily and prolonged torpor in many Australian mammals appear to be opportunistic and not only important for survival of adverse seasonal conditions, but apparently also for dealing with unpredictable events such as droughts and perhaps fires and floods. As torpor substantially reduces energy requirements its use will in turn reduce the need for foraging and consequently exposure to predators. Predator avoidance by employing torpor and minimising foraging may be one of the reasons why none of the known heterothermic Australian species has gone extinct. In contrast many of the similar-sized perceived homeothermic species, such as rodents and bandicoots, have suffered high rates of extinction possibly because they must forage long and frequently to meet large energetic demands and thus are more vulnerable to predation by introduced foxes and cats.