Blossom bats, like many other pteropodid species, face increased risk of severe heat stress and death due to rising ambient temperatures associated with climate change. Our observational captive study investigates the behavioural thermoregulatory responses to Ta between 17 and 36 degrees C in the southern blossom bat (Syconycteris australis). Under hot conditions, postural adjustment to increase surface area and licking for evaporative cooling were observed, but wing flapping, seen in many larger pteropodid species, was absent. Our study found a significant increase in licking behaviour for temperatures between 33 and 36 degrees C, indicating that onset of severe heat stress commences by Ta of 36 degrees C for S. australis captured in NSW. Field observations in published literature suggest that S. australis likely attempts to avoid hot Ta by appropriate choice of microclimate. In the face of rising temperatures associated with climate change, retaining adequate thermal refuge options within daily flying range of foraging sites is critical to the survival of this and probably other small solitary tree-roosting pteropodids.

Djungarian hamsters (Phodopus sungorus), originating from Asian steppes, change fur color from brown in summer to white in winter when they reduce body mass and size of reproductive organs. The species also enters energy-conserving daily torpor, characterized by a substantial temporal reduction of metabolic rate (MR) and body temperature (Tb). However, spontaneous daily torpor (food ad libitum) is only used by winter-acclimated hamsters. In contrast, short and shallow torpor induced by food restriction may also occur in summer- acclimated hamsters. To better understand the seasonal physiology of torpor, we examined patterns of spontaneous and induced torpor in two groups of hamsters, one was maintained under natural photoperiod in winter (‘winter’ hamsters) the other on a constant long summer photoperiod (‘summer’ hamsters); both were maintained in unheated rooms with an ambient temperature (Ta) of 9 ± 2 °C during the time of measurements in late winter. Largely white winter hamsters (n = 9 of 10), entered spontaneous torpor frequently (51.5

Energy-saving torpor in songbirds is widely believed to be rare, shallow and brief. In contrast, new work establishes that free-ranging white-backed swallows use deeper and longer torpor than thought possible, opening new research opportunities on this highly diverse avian group.

In Australia's arid zone, brush-tailed mulgaras (Dasycercus blythi) occupy burrows throughout the year, use bouts of energy/water saving torpor during the winter reproductive season, but little is known about their thermoregulation outside of winter. We used temperature-telemetry to investigate the daily body temperature (T-b) fluctuations during both winter and summer. In winter, males and females readily used torpor (males = 88.0% +/- 10.6% from 150 days tracked, n = 3; females = 85.4% +/- 5.8% from 151 days tracked, n = 2) and in summer torpor was rare in both sexes (males = 1.6% +/- 1.9%, 163 days tracked, n = 4; females = 3.0% +/- 2.8%, 157 days tracked, n = 4). The minimum winter T-b was 11.7 degrees C (mean T-b = 23.3 degrees C +/- 4.2 degrees C males; 19.6 degrees C +/- 5.1 degrees C females); in summer T-b did not fall below 30 degrees C. Torpor bouts in winter lasted for 7.1 +/- 6.1 h in males and 11.8 +/- 7.8 h in females; in summer torpor bouts were < 120 min in both sexes. Torpor expression differed between sexes during winter, likely due to different selective pressures, with females using deeper and longer torpor bouts than males. In summer, no sexual differences in torpor patterns were observed. Summer torpor use by mulgaras is likely crucial for energy and water conservation in the arid zone, and future conservation work should focus on summer adaptations.

The southern blossom bat (Syconycteris australis) is a small (∼18 g), solitary, tree roosting pteropodid, which forages on nectar, pollen, and small fruits. In Australia, the distribution of S. australis extends along the eastern coast from the tropics to subtropical northern New South Wales (NSW). In NSW, Banksia spp. are an important food source for the species. S. australis can lower metabolic rate (MR) and body temperature (Tb) in a controlled manner to save energy and combat short-term food shortage. While it is known that in S. australis torpor use may vary by season, specifically how ambient temperature (Ta) and food availability influence torpor induction is not understood. Our study therefore aimed to provide the first quantitative examination of the relationships between food availability/restriction, Ta, Tb, and energy expenditure in captive male S. australis. To this end, the relationship between Ta (17-37 °C) and diet ('fed'/'unfed') to subcutaneous temperature (Tsub) and MR was examined using flow-through respirometry (in S. australis Tsub is normally within 0.6 °C of Tb). At Ta < 25 °C, mean resting MR and Tsub were consistently lower in 'unfed' compared with 'fed' S. australis. Below the thermoneutral zone (i.e. below Ta ∼30 °C), the effect of Ta on Tsub depended on food availability, with Tsub decreasing at 0.25 × Ta in 'unfed' bats but at only 0.05 × Ta in 'fed' bats. In contrast, above the thermoneutral zone, Tsub rapidly increased with Ta regardless of feeding treatment (at 0.9 × Ta in 'fed' and 1.0 × Ta in 'unfed' bats). The results demonstrate that under a mild thermal challenge, food availability, rather than Ta, is fundamental in determining whether S. australis reduces MR and Tb. The ability for regulated Tb reductions in response to food restriction is likely crucial for survival of S. australis.

Blossom bats, like many other pteropodid species, face increased risk of severe heat stress and death due to rising ambient temperatures associated with climate change. Our observational captive study investigates the behavioural thermoregulatory responses to Ta between 17-36 ºC in the southern blossom bat, Syconycteris australis . Under hot conditions, postural adjustment to increase surface area and licking for evaporative cooling were observed, but wing flapping, seen in many larger pteropodid species, was absent. Our study found a significant increase in licking behaviour for temperatures between 33-36 °C, indicating that onset of severe heat stress commences by Ta of 36 °C for S. australis captured in NSW. Field observations in published literature suggest that S. australis likely attempts to avoid hot Ta by appropriate choice of microclimate. In the face of rising temperatures associated with climate change, retaining adequate thermal refuge options within daily flying range of foraging sites is critical to the survival of this and probably other small solitary tree-roosting pteropodids.

Torpor is the most effective means of energy conservation available to mammals and birds. Torpor is often viewed as a state of utter inactivity devoid of any behavioral aspects. However, recent work has shown that even torpid individuals do express behaviors. Torpid mammals can move at low body temperatures from a torpor site into the sun to passively rewarm and minimize energy expenditure. Social torpor involves coordinated interaction among individuals, and some species even eat or mate while torpid. A decrease in activity and a corresponding increase in torpor expression can be used to deal with natural disasters such as fires and storms. Behaviors expressed before the torpor season include selection of suitable hibernacula and storage or hoarding of appropriate and sufficient amounts of fuel.

AbstractDuring periods of torpor, hibernators can reduce metabolic rate (MR) and body temperature (Tb) substantially. However, to avoid physiological dysfunction at low temperatures, they defend Tb at a critical minimum, often between ~0°C and 10°C via an increase in MR. Because thermoregulation during torpor requires extra energy, individuals with lower Tb's and thus minimal MR during torpor should be selected in colder climates. Such inter- and intraspecific variations occur in some placental mammals, but for the evolutionary separate marsupials, available information is scarce. Marsupial eastern pygmy possums (Cercartetus nanus; ~22 g body mass), widely distributed along the Australian southeastern coast including subtropical to alpine areas, were used to test the hypothesis that the defended Tb of torpid individuals is related to the climate of their habitat. Possums were captured from five regions, 1,515 km apart, with midwinter (July) minimum environmental temperatures (min Tenv's) ranging from -3.9°C to 6.6°C. Captive possums in deep torpor were slowly cooled with ambient temperature (Ta), while their MR was measured to determine the minimum torpor metabolic rate (TMR), the Ta at which their MR increased for thermoregulation (min Ta), and the corresponding minimum Tb (min Tb). Partial least squares regression analysis revealed that Ta and Tenv were the strongest explanatory variables for the min Tb. The min Tb and Ta were also correlated with latitude but not elevation of the capture sites. However, the best correlations were observed between the min Tenv and the min Tb and Ta for individuals experiencing min Tenv>0°C; these individuals thermoconformed to min Ta's between -0.8°C and 3.7°C, and their min Tb ranged from 0.5°C to 6.0°C and was 0.5°C-2.6°C below the min Tenv at the capture site. In contrast, individuals experiencing a min Tenv of -3.9°C regulated Tb at 0.6°C±0.2°C or 4.5°C above the Tenv. The minimum TMR of all possums did not differ with Ta and thus did not differ among populations and was 2.6% of the basal MR. These data provide new evidence that thermal variables of marsupials are subject to regional intraspecific variation. It suggests that min Tb is a function of the min Tenv but only above 0°C, perhaps because the Tb-Ta differential for torpid possums in the wild, at a min Tenv of -3.9°C, remains small enough to be compensated by a small increase in MR and does not require the physiological capability for a reduction of Tb below 0°C.

The diversity of rodents in Asian deserts is high. Nevertheless, little is known about their use of daily torpor and hibernation, which are employed by many small mammals worldwide for energy and water conservation to permit survival under adverse environmental conditions. We quantified for the first time, using temperature transponders and data loggers, long-term torpor expression and patterns in sympatric desert hamsters Phodopus roborovskii, striped hamsters Cricetulus barabensis and 3-toed jerboas Dipus sagitta under controlled conditions. Animals were live-trapped in Inner Mongolia in August and held in captivity under short photoperiods and low ambient temperatures (T(a)s) for about 6 months. Both hamster species (similar to half of individuals) expressed spontaneous (food available) daily torpor. Daily torpor in desert hamsters was less frequent and shallower than that in striped hamsters, which also had longer torpor bouts during torpor at T-a 15.8 +/- 0.4 degrees C. Only one individual jerboa entered hibernation spontaneously at T-a 6.2 +/- 0.5 degrees C, but all hibernated after food deprivation. The 2 hamster species only slightly changed their body mass during the acclimation, whereas jerboas greatly increased their body mass by 27.9% during the first 2 months of acclimation probably as a preparation for the hibernating season. Our data show that short photoperiod and moderately low T-a induces spontaneous daily torpor in the 2 hamster species, suggesting that it is used regularly in the wild. Hibernation in Jerboas occurred at T-a 6.2 +/- 0.5 degrees C especially when food was withheld suggesting limited food availability is the proximate trigger of their hibernation.

Torpor is a highly effective response to counter various ecological and physiological bottlenecks in endotherms. In this study, we examined interrelations between thermoregulatory responses and key environmental variables in free-living squirrel gliders (Petaurus norfolcensis) in a habitat with drastic climatic and ecological changes across seasons. To this end, we measured body temperature (Tb) and heart rate (fH) simultaneously throughout the year using implanted data loggers. Squirrel gliders in our study experienced fluctuations in ambient temperature (Ta) between −4.0°C and 44.1°C and expressed torpor at different times during the year. In contrast to our expectations, torpor seemed to be employed flexibly, on demand, and most frequently in spring rather than during the coldest and/or hottest periods. Torpor bouts lasted, on average, about 5 h, and Tb during torpor dropped as low as 17.9°C. The fH during torpor decreased below 50 bpm, which is about one-third of the basal level. The ability to record fH alongside Tb enabled us to also report periods of low fH during thermoconforming hyperthermia at Ta’s above 35°C that likely occurred to conserve energy and water. Our findings double the body size of Australian gliders for which data on torpor are available and advance our ecological understanding of the dynamics of torpor expression in wild mammals and of how animals cope with varying conditions. Moreover, they highlight that the flexibility of physiology and thermoregulatory responses are clearly more complex than previously thought.

To maximize energy savings, entry into torpor should involve a fast reduction of metabolic rate and body temperature (Tb); that is, animals should thermoconform. However, animals often defend against the decrease in Tb via a temporary increase in thermoregulatory heat production, slowing the cooling process. We investigated how thermoregulating or thermoconforming during torpor entry affects temporal and thermoenergetic aspects in relation to body mass and age in juvenile and adult fat-tailed dunnarts (Sminthopsis crassicaudata; Marsupialia: Dasyuridae). During torpor entry, juvenile thermoconformers cooled twice as fast as and used less energy during cooling than juvenile thermoregulators. While both juvenile and adult thermoconformers had a lower minimum Tb, a lower torpor metabolic rate, and longer torpor bouts than thermoregulators, these differences were more pronounced in the juveniles. Rewarming from torpor took approximately twice as long for juvenile thermoconformers, and the costs of rewarming were greater. To determine the difference in average daily metabolic rate between thermoconformers and thermoregulators independent of body mass, we compared juveniles of a similar size (∼13 g) and similarly sized adults (∼17 g). The average daily metabolic rate was 7% (juveniles) and 17% (adults) less in thermoconformers than in thermoregulators, even though thermoconformers were active for longer. Our data suggest that thermoconforming during torpor entry provides an energetic advantage for both juvenile and adult dunnarts and may aid growth for juveniles. While thermoregulation during torpor entry is more costly, it still saves energy, and the higher Tb permits greater alertness and mobility and reduces the energetic cost of endogenous rewarming.

Torpor is an incredibly efficient energy-saving strategy that many endothermic birds and mammals use to save energy, by lowering their metabolic rates, heart rates, and typically body temperatures. Over the last few decades, the study of daily torpor-in which torpor is used for less than 24 hours per bout-has advanced rapidly. The papers in this issue cover the ecological and evolutionary drivers of torpor, as well as some of the mechanisms governing torpor use. We identified broad focus areas that need special attention: clearly defining the various parameters that indicate torpor use and identifying the genetic and neurological mechanisms regulating torpor. Recent studies on daily torpor and heterothermy, including the ones in this issue, have furthered the field immensely. We look forward to a period of immense growth in this field.

Synopsis Torpor was traditionally seen as a winter survival mechanism employed by animals living in cold and highly seasonal habitats. Although we now know that torpor is also used by tropical and subtropical species, and in response to a variety of triggers, torpor is still largely viewed as a highly controlled, seasonal mechanism shown by Northern hemisphere species. To scrutinize this view, we report data from a macroanalysis in which we characterized the type and seasonality of torpor use from mammal species currently known to use torpor. Our findings suggest that predictable, seasonal torpor patterns reported for Northern temperate and polar species are highly derived forms of torpor expression, whereas the more opportunistic and variable forms of torpor that we see in tropical and subtropical species are likely closer to the patterns expressed by ancestral mammals. Our data emphasize that the torpor patterns observed in the tropics and subtropics should be considered the norm and not the exception.

Torpor, a controlled reduction in metabolism and body temperature, reduces energy expenditure substantially. However, torpor expression in wild passerines is currently understudied. We show that skin temperature (Ts) of resting White-throated Treecreepers (N = 4) fell by 5 °C on average in both summer and winter, independent of ambient temperature, but we could not confirm torpor use (Ts reduction > 5 °C). It is possible that roosting in tree hollows provides sufficient insulation to minimise energy loss, or torpor is used only during extreme conditions. Further studies are needed to characterise the physiological flexibilities of species and, therefore, their capability to cope with changing environmental conditions.

Mammalian and avian torpor is highly effective in reducing energy expenditure. However, the extent of energy savings achieved and thus long-term survival appear to differ between species capable of multiday hibernation and species restricted to daily heterothermy, which could, however, be due to thermal effects. We tested how long-term survival on stored body fat (i.e. time to lean body mass), crucial for overcoming adverse periods, is related to the pattern of torpor expressed under different ambient temperatures (T a : 7 °C typical of hibernation, 15 and 22 °C typical of daily torpor) in the small marsupial hibernator the pygmy-possum ( Cercartetus nanus ). Possums expressed torpor at all T a s and survived without food for 310 days on average at T a 7 °C, 195 days at T a 15 °C, and 127 days at T a 22 °C. At T a 7 and 15 °C, torpor bout duration (TBD) increased from < 1–3 to ~ 5–16 days over 2 months, whereas at T a 22 °C, TBD remained at < 1 to ~ 2 days. At all T a s daily energy use was substantially lower and TBD and survival times of possums much longer (3–12 months) than in daily heterotherms (~ 10 days). Such pronounced differences in torpor patterns and survival times even under similar thermal conditions provide strong support for the concept that torpor in hibernators and daily heterotherms are physiologically distinct and have evolved for different ecological purposes.

Torpor, and its differential expression, is essential to the survival of many mammals and birds. Physiological characteristics of torpor appear to vary between those species that express strict daily heterothermy and those capable of multiday hibernation, but comparisons are complicated by the temperature-dependence of variables. Previous reviews have compared these different torpor strategies by measuring the depth and duration of torpor in multiple species. However, direct comparison of multiple physiological parameters under similar thermal conditions are lacking. Here, we quantified three physiological variables; body temperature, metabolic rate (MR) and heart rate (HR) of two small heterothermic bats (daily heterotherm Syconycteris australis, and hibernator Nyctophilus gouldi) under comparable thermal conditions and torpor bout durations. When normothermic and resting both MR and HR were similar for the two species. However, during torpor the minimum HR was more than fivefold higher, and minimum MR was 6.5-fold higher for the daily heterotherm than for the hibernator at the same subcutaneous Tb (16 ± 0.5 °C). The data show that the degree of heterothermy defined using Tb is not necessarily a precise proxy for physiological capacity during torpor in these bats and is unlikely to reveal accurate energy budgets. Our study provides evidence supporting a distinction between daily torpor in a daily heterotherm and short term torpor in a hibernator, at least within the Chiroptera with regard to these physiological variables. This exists even when individuals display the same degree of Tb reduction, which has clear implications for the modelling of their energy expenditure.

Insectivorous bats are particularly susceptible to heat loss due to their relatively large surface area to volume ratio. Therefore, to maintain a high normothermic body temperature, bats require large amounts of energy for thermoregulation. This can be energetically challenging for small bats during cold periods as heat loss is augmented and insect prey is reduced. To conserve energy many bats enter a state of torpor characterized by a controlled reduction of metabolism and body temperature in combination with selecting roosts based upon thermal properties. Our study aimed to quantify torpor patterns and roost preferences of free-ranging little forest bats (Vespadelus vulturnus) during winter to identify physiological and behavioral mechanisms used by this species for survival of the cold season. All bats captured were male (body mass 4.9 ± 0.7 g, n = 6) and used torpor on every day monitored, with bouts lasting up to 187.58 h (mean = 35.5 ± 36.7 h, n = 6, total number of samples [N] = 61). Torpor bout duration was significantly correlated with daily minimum and maximum ambient temperature, mean skin temperature, insect mass, and body mass of individuals and the multiday torpor bouts recorded in the cold qualify as hibernation. The lowest skin temperature recorded was 5.2°C, which corresponded to the lowest ambient temperature measurement of -5.8°C. Most bats chose tall, large, live Eucalyptus trees for roosting and to leave their roost for foraging on warmer days. Many individuals often switched roosts (every 3-5 days) and movements increased as spring approached (every 1-2 days). Our data suggest that V. vulturnus are capable of using the environmental temperature to gauge potential foraging opportunities and as a cue to reenter torpor when conditions are unsuitable. Importantly, frequent use of torpor and appropriate roost selection form key roles in the winter survival of these tiny bats.

Hibernating mammals drastically lower their metabolic rate (MR) and body temperature (Tb) for up to several weeks, but regularly rewarm and stay euthermic for brief periods. It has been hypothesized that the necessity for rewarming is due to the accumulation or depletion of metabolites, or the accrual of cellular damage that can be eliminated only in the euthermic state. Recent evidence for significant inverse relationships between the duration of torpor bouts (TBD) and MR in torpor strongly supports this hypothesis. We developed a new mathematical model that simulates hibernation patterns. The model involves an hourglass process H (Hibernation) representing the depletion/accumulation of a crucial enzyme/metabolite, and a threshold process Hthr. Arousal, modelled as a logistic process, is initiated once the exponentially declining process H reaches Hthr. We show that this model can predict several phenomena observed in hibernating mammals, namely the linear relationship between TMR and TBD, effects of ambient temperature on TBD, the modulation of torpor depth and duration within the hibernation season, (if process Hthr undergoes seasonal changes). The model does not need but allows for circadian cycles in the threshold T, which lead to arousals occurring predominantly at certain circadian phases, another phenomenon that has been observed in certain hibernators. It does not however, require circadian rhythms in Tb or MR during torpor. We argue that a two-process regulation of torpor-arousal cycles has several adaptive advantages, such as an easy adjustment of TBD to environmental conditions as well as to energy reserves and, for species that continue to forage, entrainment to the light-dark cycle.

We show here that evaporative water loss (EWL) is constant over a wide range of ambient relative humidity for two species of small, mesic habitat dasyurid marsupials (Antechinus agilis and Antechinus swainsonii) below thermoneutrality (20°C) and within thermoneutrality (30°C). This independence of EWL from the water vapor pressure deficit between the animal and its environment indicates that EWL is physiologically controlled by both species. The magnitude of this control of EWL was similar to that of two other small marsupials from more arid habitats, which combined with the observation that there were no effects of relative humidity on body temperature or metabolic rate, suggests that control of EWL is a consequence of precise thermoregulation to maintain heat balance rather than a water-conserving strategy at low relative humidities. The antechinus appear to manipulate cutaneous EWL rather than respiratory EWL to control their total EWL by modifying their cutaneous resistance and/or skin temperature. We propose that there is a continuum between enhanced thermoregulatory EWL at high ambient temperature and so-called insensible EWL at and below thermoneutrality.