Little is known about the use of heterothermy by wild bats during summer, especially for tree-roosting species. Because thermal conditions within tree roosts can fluctuate widely with ambient temperature, which affects thermoregulatory energy expenditure during diurnal roosting, we measured skin temperatures of free-ranging male 'Nyctophilus geoffroyi' (8 g) to quantify the relation between summer torpor use and roost thermal conditions. Bats roosted under bark on the northern (sunny) side of trees and entered torpor every day, usually near sunrise. Bats exhibited two bouts of torpor on most days: the first occurred in the morning, was terminated by partially passive rewarming, and was followed by a period of normothermy during the warmest part of the day; a second torpor bout occurred in the late afternoon, with arousal near sunset. On the warmest days, bats had only a single, short morning bout. On the coolest days, bats remained torpid throughout the day, and one 2-d bout was observed. Thus, presumably owing to their poorly insulated roosts and the high energetic cost of normothermy at temperatures below 300C, the extent and timing of heterothermy was closely related to the cycle of diurnal temperatures. Our study indicates that torpor use is important for energy maintenance during summer diurnal roosting of 'N. geoffroyi' and likely of other small, tree-roosting bats.

Small insectivorous bats commonly use torpor while day-roosting, even in summer. However, reproductive female bats are believed to benefit from avoiding torpor because a constant, elevated body temperature maximises the rate of offspring growth, which could increase offspring survival. We used temperature-sensitive radio-transmitters to locate roosts and document the thermal biology of pregnant and lactating females of Nyctophilus geoffroyi (9 g) and N. gouldi (11 g) at a woodland in a cool temperate climate. Unlike males, reproductive female Nyctophilus spp. roosted as small groups (<25) within insulated tree cavities. Roost switching occurred every 3.7 ± 1.5 (N. geoffroyi) or 1.7 ± 0.8 days (N. gouldi), and radio-tagged individuals roosted together and apart on different days. Skin temperature during roosting was most often between 32 and 36°C, and torpor was used infrequently. Male Nyctophilus have been shown in previous studies to use torpor daily during summer. These contrasting torpor patterns likely reflect the warmed cavities occupied by maternity colonies and the thermally unstable shallow crevices occupied by individual males. Our results support the hypothesis that availability of thermally suitable roosts will influence thermoregulatory patterns of reproductive females and hence the growth rates and survival of their offspring. Thus, it is important to conserve woodland habitat with trees in a range of decay stages to provide opportunities for selection and movement among roost trees by reproductive female bats.

Marsupials, unlike placental mammals, are believed to be unable to increase heat production and thermal performance after cold-acclimation. It has been suggested that this may be because marsupials lack functional brown fat, a thermogenic tissue, which proliferates during cold-acclimation in many placentals. However, arid zone marsupials have to cope with unpredictable, short-term and occasionally extreme changes in environmental conditions, and thus they benefit from an appropriate physiological response. We therefore investigated whether a sequential two to four week acclimation in 'Sminthopsis macroura' (body mass approx. 25 g) to both cold (16°C) and warm (26°C) ambient temperatures affects the thermal physiology of the species. Cold-acclimated 'S. macroura' were able to significantly increase maximum heat production (by 27%) and could maintain a constant body temperature at significantly lower effective ambient temperatures (about 9°C lower) than when warm-acclimated. Moreover, metabolic rates during torpor were increased following cold-acclimation in comparison to warm-acclimation. Our study shows that, despite the lack of functional brown fat, short-term acclimation can have significant effects on thermoenergetics of marsupials. It is likely that the rapid response in 'S. macroura' reflects an adaptation to the unpredictability of the climate in their habitat.

Food availability, ambient temperatures (Ta), and prevailing weather conditions have long been presumed to influence torpor use. To a large extent, this is based on measurements in the laboratory of animals placed on restricted diets and kept at low Ta. Information on the determinants of torpor employment in the field is limited. We assessed winter torpor by insectivorous, free-ranging Australian owlet-nightjars ('Aegotheles cristatus'; 22 birds, 834 bird-days over six winters). Birds in three habitats were investigated to test whether torpor use is affected by annual Ta, rainfall, and arthropod abundance. Owlet-nightjars entered daily torpor regularly at all sites. Torpor frequency, depth and bout duration were greatest during two periods with lower arthropod abundance, providing rare evidence of the link between food availability and torpor patterns of wild birds. Temporal organization of torpor was similar among sites, and nocturnal torpor was more frequent than previously reported. Our findings quantitatively demonstrate that reduced food resources affect torpor usage independently from Ta, and support the view that food availability is a primary ecological determinant of torpor use in the wild.

Fires have occurred throughout history, including those associated with the meteoroid impact at the Cretaceous-Palaeogene (K-Pg) boundary that eliminated many vertebrate species. To evaluate the recent hypothesis that the survival of the K-Pg fires by ancestral mammals was dependent on their ability to use energy-conserving torpor,we studied body temperature fluctuations and activity of an egg-laying mammal, the echidna ('Tachyglossus aculeatus'), often considered to be a 'living fossil', before, during and after a prescribed burn. All but one study animal survived the fire in the prescribed burn area and echidnas remained inactive during the day(s) following the fire and substantially reduced body temperature during bouts of torpor. For weeks after the fire, all individuals remained in their original territories and compensated for changes in their habitat with a decrease in mean body temperature and activity. Our data suggest that heterothermy enables mammals to outlast the conditions during and after a fire by reducing energy expenditure, permitting periods of extended inactivity. Therefore, torpor facilitates survival in a fire-scorched landscape and consequently may have been of functional significance for mammalian survival at the K-Pg boundary.

Although roost choice in bats has been studied previously, little is known about how opposing roost colours affect the expression of torpor quantitatively. We quantified roost selection and thermoregulation in a captive Australian insectivorous bat, 'Nyctophilus gouldi' (n=12) in winter when roosting in black and white coloured boxes using temperature-telemetry. We quantified how roost choice influences torpor expression when food was provided ad libitum or restricted in bats housed together in an outdoor aviary exposed to natural fluctuations of ambient temperature. Black box temperatures averaged 5.1 °C (maximum 7.5 °C) warmer than white boxes at their maximum daytime temperature. Bats fed ad libitum chose black boxes on most nights (92.9%) and on 100% of nights when food-restricted. All bats used torpor on all study days. However, bats fed ad libitum and roosting in black boxes used shorter torpor and spent more time normothermic/active at night than food-restricted bats and bats roosting in white boxes. Bats roosting in black boxes also rewarmed passively more often and to a higher skin temperature than those in white boxes. Our study suggests that 'N. gouldi' fed ad libitum select warmer roosts in order to passively rewarm to a higher skin temperature and thus save energy required for active midday rewarming as well as to maintain a normothermic body temperature for longer periods at night. This study shows that colour should be considered when deploying bat boxes; black boxes are preferable for those bats that use passive rewarming, even in winter when food availability is reduced.

Although storms provide an extreme environmental challenge to organisms and are predicted to increase in frequency and intensity due to climate change, there are no quantitative observations on the behaviour and physiology of animals during natural disasters. We provide the first data on activity and thermal biology of a free-ranging, arboreal mammal during a storm with heavy rain and category 1 cyclone wind speeds. We studied a population of sugar gliders ('Petaurus breviceps'), a species vulnerable to bad weather due to their small body size and mode of locomotion, in a subtropical habitat during spring when storms are common. Although torpor is generally rare in this species, sugar gliders remained inactive or reduced foraging times during the storm and further minimized energy demands by entering deep torpor. All animals survived the storm and reverted to normal foraging activity during the following night(s). It thus appears that heterothermic mammals have a crucial adaptive advantage over homeothermic species as they can outlast challenging weather events, such as storms and floods, by reducing metabolism and thus energetic needs.

The recent observation that torpor plays a key role in post-fire survival has been mainly attributed to the reduced food resources after fires. However, some of these adjustments can be facilitated or amplified by environmental changes associated with fires, such as the presence of a charcoal-ash substrate. In a previous experiment on a small terrestrial mammal the presence of charcoal and ash linked to food restriction intensified torpor use. However, whether fire cues also act as a trigger of torpor use when food is available and whether they affect other species including arboreal mammals remains elusive. To evaluate whether smoke, charcoal and ash can act as proximate triggers for an impending period of food shortage requiring torpor for mammals, we conducted an experiment on captive sugar gliders (Petaurus breviceps), a small, arboreal marsupial, housed in outside aviaries under different food regimes and natural ambient conditions. When food was available, fire simulation via exposure to smoke and charcoal-ash substrate caused a significant earlier start of activity and a significant decrease in resting body temperature. In contrast, only when food was withheld, did smoke and charcoal-ash exposure significantly enhance torpor depth and duration. Thus, our study not only provides evidence that fire simulation does affect arboreal and terrestrial species similarly, but also suggests that smoke and ash were presumably selected as cues for torpor induction because they indicate an impending lack of food.

Many mammals save energy during food shortage or harsh weather using controlled reductions in body temperature and metabolism called torpor. However, torpor slows offspring growth, and reproductive individuals are thought to avoid using it because of reduced fitness resulting from delayed offspring development. We tested this hypothesis by investigating torpor during reproduction in hoary bats ('Lasiurus cinereus', 'Vespertilionidae') in southern Canada. We recorded deep, prolonged torpor bouts, which meet the definition for hibernation, by pregnant females. Prolonged torpor occurred during spring storms. When conditions improved females aroused and gave birth within several days. Our observations imply a fitness advantage of torpor in addition to energy conservation because reduced foetal growth rate could delay parturition until conditions are more favourable for lactation and neonatal survival.

The physiological signal for torpor initiation appears to be related to fuel availability. Studies on metabolic fuel inhibition in placental heterotherms show that glucose deprivation via the inhibitor 2-deoxy-D-glucose (2DG) initiates a torpor-like state, whereas fatty acid deprivation via mercaptoacetate (MA) does not. As previous studies using inhibitors were limited to quantifying body temperature in placentals, we investigated whether inhibition of glucose or fatty acids for cellular oxidation induces torpor in the marsupial hibernator 'Cercartetus nanus', and how the response of metabolic rate is related to body temperature. Glucoprivation initiated a torpor-like state in 'C. nanus', but animals had much higher minimum body temperatures and metabolic rates than those of torpid food-deprived animals and arousal rates were slower. Moreover, 2DG-treated animals were thermoregulating at ambient temperatures of 20 and 12°C, whereas food-deprived torpid animals were thermo-conforming. We suggest that glucoprivation reduces the hypothalamic body temperature set point, but only by about 8°C rather than the approximately 28°C during natural torpor. Reduced fatty acid availability via MA also induced a torpor-like state in some 'C. nanus', with physiological variables that did not differ from those of torpid food-deprived animals. We conclude that reduced glucose availability forms only part of the physiological trigger for torpor initiation in 'C. nanus'. Reduced fatty acid availability, unlike for placental heterotherms, may be an important cue for torpor initiation in 'C. nanus', perhaps because marsupials lack functional brown adipose tissue