Climate change and population persistence in a hibernating marsupial

Climate change has physiological consequences on organisms, ecosystems and human societies, surpassing the pace of organismal adaptation. Hibernating mammals are particularly vulnerable as winter survival is determined by short-term physiological changes triggered by temperature. In these animals, winter temperatures cannot surpass a certain threshold, above which hibernators arouse from torpor, increasing several fold their energy needs when food is unavailable. Here, we parameterized a numerical model predicting energy consumption in heterothermic species and modelled winter survival at different climate change scenarios. As a model species, we used the arboreal marsupial monito del monte (genus Dromiciops), which is recognized as one of the few South American hibernators. We modelled four climate change scenarios (from optimistic to pessimistic) based on IPCC projections, predicting that northern and coastal populations (Dromiciops bozinovici) will decline because the minimum number of cold days needed to survive the winter will not be attained. These populations are also the most affected by habitat fragmentation and changes in land use. Conversely, Andean and other highland populations, in cooler environments, are predicted to persist and thrive. Given the widespread presence of hibernating mammals around the world, models based on simple physiological parameters, such as this one, are becoming essential for predicting species responses to warming in the short term.

Evolutionary trade-offs in dormancy phenology

Seasonal animal dormancy is widely interpreted as a physiological response for surviving energetic challenges during the harshest times of the year (the physiological constraint hypothesis). However, there are other mutually non-exclusive hypotheses to explain the timing of animal dormancy, that is, entry into and emergence from hibernation (i.e. dormancy phenology). Survival advantages of dormancy that have been proposed are reduced risks of predation and competition (the 'life-history' hypothesis), but comparative tests across animal species are few. Using the phylogenetic comparative method applied to more than 20 hibernating mammalian species, we found support for both hypotheses as explanations for the phenology of dormancy. In accordance with the life-history hypotheses, sex differences in hibernation emergence and immergence were favored by the sex difference in reproductive effort. In addition, physiological constraint may influence the trade-off between survival and reproduction such that low temperatures and precipitation, as well as smaller body mass, influence sex differences in phenology. We also compiled initial evidence that ectotherm dormancy may be (1) less temperature dependent than previously thought and (2) associated with trade-offs consistent with the life-history hypothesis. Thus, dormancy during non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously thought.

Non-Torpid Heterothermy in Mammals: Another Category along the Homeothermy-Hibernation Continuum

Variability in body temperature is now recognized to be widespread among whole-body endotherms with homeothermy being the exception rather than the norm. A wide range of body temperature patterns exists in extant endotherms, spanning from strict homeothermy, to occasional use of torpor, to deep seasonal hibernation with many points in between. What is often lost in discussions of heterothermy in endotherms are the benefits of variations in body temperature outside of torpor. Endotherms that do not use torpor can still obtain extensive energy and water savings from varying levels of flexibility in normothermic body temperature regulation. Flexibility at higher temperatures (heat storage or facultative hyperthermia) can provide significant water savings, while decreases at cooler temperatures, even outside of torpor, can lower the energetic costs of thermoregulation during rest. We discuss the varying uses of the terms heterothermy, thermolability, and torpor to describe differences in the amplitude of body temperature cycles and advocate for a broader use of the term "heterothermy" to include non-torpid variations in body temperature.

Rare and Opportunistic Use of Torpor in Mammals-An Echo from the Past?

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.

The Rate of Cooling during Torpor Entry Drives Torpor Patterns in a Small Marsupial

AbstractTo 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.

Blood transcriptomics mirror regulatory mechanisms during hibernation-a comparative analysis of the Djungarian hamster with other mammalian species

Hibernation enables many species of the mammalian kingdom to overcome periods of harsh environmental conditions. During this physically inactive state metabolic rate and body temperature are drastically downregulated, thereby reducing energy requirements (torpor) also over shorter time periods. Since blood cells reflect the organism´s current condition, it was suggested that transcriptomic alterations in blood cells mirror the torpor-associated physiological state. Transcriptomics on blood cells of torpid and non-torpid Djungarian hamsters and QIAGEN Ingenuity Pathway Analysis (IPA) revealed key target molecules (TMIPA), which were subjected to a comparative literature analysis on transcriptomic alterations during torpor/hibernation in other mammals. Gene expression similarities were identified in 148 TMIPA during torpor nadir among various organs and phylogenetically different mammalian species. Based on TMIPA, IPA network analyses corresponded with described inhibitions of basic cellular mechanisms and immune system-associated processes in torpid mammals. Moreover, protection against damage to the heart, kidney, and liver was deduced from this gene expression pattern in blood cells. This study shows that blood cell transcriptomics can reflect the general physiological state during torpor nadir. Furthermore, the understanding of molecular processes for torpor initiation and organ preservation may have beneficial implications for humans in extremely challenging environments, such as in medical intensive care units and in space.

Modeling heterothermic fitness landscapes in a marsupial hibernator using changes in body composition

Hibernation is an adaptive strategy that allows animals to enter a hypometabolic state, conserving energy and enhancing their fitness by surviving harsh environmental conditions. However, addressing the adaptive value of hibernation, at the individual level and in natural populations, has been challenging. Here, we applied a non-invasive technique, body composition analysis by quantitative magnetic resonance (qMR), to calculate energy savings by hibernation in a population of hibernating marsupials (Dromiciops gliroides). Using outdoor enclosures installed in a temperate rainforest, and measuring qMR periodically, we determined the amount of fat and lean mass consumed during a whole hibernation cycle. With this information, we estimated the daily energy expenditure of hibernation (DEEH) at the individual level and related to previous fat accumulation. Using model selection approaches and phenotypic selection analysis, we calculated linear (directional, β), quadratic (stabilizing or disruptive, γ) and correlational (ρ) coefficients for DEEH and fat accumulation. We found significant, negative directional selection for DEEH (βDEEH = - 0.58 ± 0.09), a positive value for fat accumulation (βFAT = 0.34 ± 0.07), and positive correlational selection between both traits (ρDEEH × FAT = 0.24 ± 0.07). Then, individuals maximizing previous fat accumulation and minimizing DEEH were promoted by selection, which is visualized by a bi-variate selection surface estimated by generalized additive models. At the comparative level, results fall within the isometric allometry known for hibernation metabolic rate in mammals. Thus, by a combination of a non-invasive technique for body composition analysis and semi-natural enclosures, we were characterized the heterothermic fitness landscape in a semi-natural population of hibernators.

Mitochondrial polymorphism m.3017C>T of SHLP6 relates to heterothermy

Heterothermic thermoregulation requires intricate regulation of metabolic rate and activation of pro-survival factors. Eliciting these responses and coordinating the necessary energy shifts likely involves retrograde signalling by mitochondrial-derived peptides (MDPs). Members of the group were suggested before to play a role in heterothermic physiology, a key component of hibernation and daily torpor. Here we studied the mitochondrial single-nucleotide polymorphism (SNP) m.3017C>T that resides in the evolutionarily conserved gene MT-SHLP6. The substitution occurring in several mammalian orders causes truncation of SHLP6 peptide size from twenty to nine amino acids. Public mass spectrometric (MS) data of human SHLP6 indicated a canonical size of 20 amino acids, but not the use of alternative translation initiation codons that would expand the peptide. The shorter isoform of SHLP6 was found in heterothermic rodents at higher frequency compared to homeothermic rodents (p < 0.001). In heterothermic mammals it was associated with lower minimal body temperature (T b, p < 0.001). In the thirteen-lined ground squirrel, brown adipose tissue-a key organ required for hibernation, showed dynamic changes of the steady-state transcript level of mt-Shlp6. The level was significantly higher before hibernation and during interbout arousal and lower during torpor and after hibernation. Our finding argues to further explore the mode of action of SHLP6 size isoforms with respect to mammalian thermoregulation and possibly mitochondrial retrograde signalling.

Saving energy via short and shallow torpor bouts

Maintaining a high and stable body temperature as observed in most endothermic mammals and birds is energetically costly and many heterothermic species reduce their metabolic demands during energetic bottlenecks through the use of torpor. With the increasing number of heterotherms revealed in a diversity of habitats, it becomes apparent that triggers and patterns of torpor use are more variable than previously thought. Here, we report the previously overlooked use of, shallow rest-time torpor (body temperature >30 °C) in African lesser bushbabies, Galago moholi. Body core temperature of three adult male bushbabies recorded over five months showed a clear bimodal distribution with an average active modal temperature of 39.2 °C and a resting modal body temperature of 36.7 °C. Shallow torpor was observed in two out of three males (n = 29 torpor bouts) between June and August (austral winter), with body temperatures dropping to an overall minimum of 30.7 °C and calculated energy savings of up to 10%. We suggest that shallow torpor may be an ecologically important, yet mostly overlooked energy-saving strategy employed by heterothermic mammals. Our data emphasise that torpor threshold temperatures need to be used with care if we aim to fully understand the level of physiological plasticity displayed by heterothermic species.

Long-term survival, temperature, and torpor patterns

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_as 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_as 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.

Pronounced differences in heart rate and metabolism distinguish daily torpor and short-term hibernation in two bat species

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 T_b (16 ± 0.5 °C). The data show that the degree of heterothermy defined using T_b 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 T_b reduction, which has clear implications for the modelling of their energy expenditure.

High level of daily heterothermy in desert gerbils

Daily heterothermy is a strategy employed by endothermic birds and mammals to reduce their energetic costs by lowering their metabolic rate. We recorded nocturnal and diurnal rectal temperatures in three Moroccan Gerbillus rodent species to determine the level of heterothermy. A decrease in body temperature from night to day was observed by an average (±SD) of 8.7 (±4.2) in G. gerbillus, 11.1 (±3.0) in G. amoenus, and 7.7 (±3.3)°C in G. sp.1, the first records of heterothermy in the three species. The findings support a prediction that daily heterothermy is found in mammals from arid and semi-arid regions, contributing to further knowledge of thermoregulation in desert rodents.

Transcriptome landscapes that signify Botrylloides leachi (Ascidiacea) torpor states

Harsh environments enforce the expression of behavioural, morphological, physiological, and reproductive rejoinders, including torpor. Here we study the morphological, cellular, and molecular alterations in torpor architype in the colonial urochordate Botrylloides aff. leachii by employing whole organism Transmission electron (TEM) and light microscope observations, RNA sequencing, real-time polymerase chain reaction (qPCR) quantification of selected genes, and immunolocalization of WNT, SMAD and SOX2 gene expressions. On the morphological level, torpor starts with gradual regression of all zooids and buds which leaves the colony surviving as condensed vasculature remnants that may be 'aroused' to regenerate fully functional colonies upon changes in the environment. Simultaneously, we observed altered distributions of hemolymph cell types. Phagocytes doubled in number, while the number of morula cells declined by half. In addition, two new circulating cell types were observed, multi-nucleated and bacteria-bearing cells. RNA sequencing technology revealed marked differences in gene expression between different organism compartments and states: active zooids and ampullae, and between mid-torpor and naive colonies, or naive and torpid colonies. Gene Ontology term enrichment analyses further showed disparate biological processes. In torpid colonies, we observed overall 233 up regulated genes. These genes included NR4A2, EGR1, MUC5AC, HMCN2 and. Also, 27 transcription factors were upregulated in torpid colonies including ELK1, HDAC3, RBMX, MAZ, STAT1, STAT4 and STAT6. Interestingly, genes involved in developmental processes such as SPIRE1, RHOA, SOX11, WNT5A and SNX18 were also upregulated in torpid colonies. We further validated the dysregulation of 22 genes during torpor by utilizing qPCR. Immunohistochemistry of representative genes from three signaling pathways revealed high expression of these genes in circulated cells along torpor. WNT agonist administration resulted in early arousal from torpor in 80% of the torpid colonies while in active colonies WNT agonist triggered the torpor state. Abovementioned results thus connote unique transcriptome landscapes associated with Botrylloides leachii torpor.

Siberian hamsters nonresponding to short photoperiod use fasting-induced torpor

Nonresponding Siberian hamsters Phodopus sungorus do not develop the winter phenotype with white fur, low body mass (mb) and spontaneous torpor use in response to short photoperiod. However, their thermoregulatory response to fasting remains unknown. We measured body temperature and mb of 12 nonresponders acclimated to short photoperiod and then to cold, and fasted four times for 24h. Four individuals used torpor and in total we recorded 19 torpor bouts, which were shallow, short, and occurred at night. Moreover fasting increased the heterothermy index in all hamsters. Low mb was not a prerequisite for torpor use and mb loss correlated with neither heterothermy index nor torpor use. This is the first evidence that individuals which do not develop the winter phenotype, can use torpor or increase body temperature variability to face unpredictable, adverse environmental conditions. Despite the lack of seasonal changes, thermoregulatory adjustments may increase winter survival probability of nonresponders.

Assigning metabolic rate measurements to torpor and euthermy in heterothermic endotherms: "torpor", a new package for R

Torpor is a state of controlled reduction of metabolic rate (M) in endotherms. Assigning measurements of M to torpor or euthermy can be challenging, especially when the difference between euthermic M and torpid M is small, in species defending a high minimal body temperature in torpor, in thermolabile species, and slightly below the thermoneutral zone (TNZ). Here, we propose a novel method for distinguishing torpor from euthermy. We use the variation in M measured during euthermic rest and torpor at varying ambient temperatures (T_a) to objectively estimate the lower critical temperature (T_lc) of the TNZ and to assign measurements to torpor, euthermic rest or rest within TNZ. In addition, this method allows the prediction of M during euthermic rest and torpor at varying T_a, including resting M within the TNZ. The present method has shown highly satisfactory results using 28 published sets of metabolic data obtained by respirometry on 26 species of mammals. Ultimately, this novel method aims to facilitate analysis of respirometry data in heterothermic endotherms. Finally, the development of the associated R-package (torpor) will enable widespread use of the method amongst biologists.

Summary: The presented method and its associated R-package (torpor) enable the assignment of metabolic rate measurements to torpor or euthermy, ultimately improving the standardization of respirometry analyses in heterotherms.

Body Mass and Tail Girth Predict Hibernation Expression in Captive Dwarf Lemurs

AbstractHibernation, a metabolic strategy, allows individuals to reduce energetic demands in times of energetic deficits. Hibernation is pervasive in nature, occurring in all major mammalian lineages and geographical regions; however, its expression is variable across species, populations, and individuals, suggesting that trade-offs are at play. Whereas hibernation reduces energy expenditure, energetically expensive arousals may impose physiological burdens. The torpor optimization hypothesis posits that hibernation should be expressed according to energy availability. The greater the energy surplus, the lower the hibernation output. The thrifty female hypothesis, a variation of the torpor optimization hypothesis, states that females should conserve more energy because of their more substantial reproductive costs. Contrarily, if hibernation's benefits offset its costs, hibernation may be maximized rather than optimized (e.g., hibernators with greater fat reserves could afford to hibernate longer). We assessed torpor expression in captive dwarf lemurs, primates that are obligate, seasonal, and tropical hibernators. Across 4.5 mo in winter, we subjected eight individuals at the Duke Lemur Center to conditions conducive to hibernation, recorded estimates of skin temperature hourly (a proxy for torpor), and determined body mass and tail fat reserves bimonthly. Across and between consecutive weigh-ins, heavier dwarf lemurs spent less time in torpor and lost more body mass. At equivalent body mass, females spent more time torpid and better conserved energy than did males. Although preliminary, our results support the torpor optimization and thrifty female hypotheses, suggesting that individuals optimize rather than maximize torpor according to body mass. These patterns are consistent with hibernation phenology in Madagascar, where dwarf lemurs hibernate longer in more seasonal habitats.

Disparate roost sites drive intraspecific physiological variation in a Malagasy bat

Many species are widely distributed and individual populations can experience vastly different environmental conditions over seasonal and geographic scales. With such a broad ecological reality, datasets with limited spatial and temporal resolution may not accurately represent a species and could lead to poorly informed management decisions. Because physiological flexibility can help species tolerate environmental variation, we studied the physiological responses of two separate populations of Macronycteris commersoni, a bat widespread across Madagascar, in contrasting seasons. The populations roost under the following dissimilar conditions: either a hot, well-buffered cave or within open foliage, unprotected from the local weather. We found that flexible torpor patterns, used in response to prevailing ambient temperature and relative humidity, were central to keeping energy budgets balanced in both populations. While bats’ metabolic rate during torpor and rest did not differ between roosts, adjusting torpor frequency, duration and timing helped bats maintain body condition. Interestingly, the exposed forest roost induced extensive use of torpor, which exceeded the torpor frequency of overwintering bats that stayed in the cave for months and consequently minimised daytime resting energy expenditure in the forest. Our current understanding of intraspecific physiological variation is limited and physiological traits are often considered to be fixed. The results of our study therefore highlight the need for examining species at broad environmental scales to avoid underestimating a species’ full capacity for withstanding environmental variation, especially in the face of ongoing, disruptive human interference in natural habitats.

Heterothermy in a Small Passerine: Eastern Yellow Robins Use Nocturnal Torpor in Winter

Torpor is a controlled reduction of metabolism and body temperature, and its appropriate use allows small birds to adapt to and survive challenging conditions. However, despite its great energy conservation potential, torpor use by passerine birds is understudied although they are small and comprise over half of extant bird species. Here, we first determined whether a free-living, small ∼20 g Australian passerine, the eastern yellow robin (Eopsaltria australis), expresses torpor by measuring skin temperature (Ts) as a proxy for body temperature. Second, we tested if skin temperature fluctuated in relation to ambient temperature (Ta). We found that the Ts of eastern yellow robins fluctuated during winter by 9.1 ± 3.9°C on average (average minimum Ts 30.1 ± 2.3°C), providing the first evidence of torpor expression in this species. Daily minimum Ts decreased with Ta, reducing the estimated metabolic rate by as much as 32%. We hope that our results will encourage further studies to expand our knowledge on the use of torpor in wild passerines. The implications of such studies are important because species with highly flexible energy requirements may have an advantage over strict homeotherms during the current increasing frequency of extreme and unpredictable weather events, driven by changing climate.

A Mesocosm Experiment in Ecological Physiology: The Modulation of Energy Budget in a Hibernating Marsupial under Chronic Caloric Restriction

AbstractDuring the past 60 years, mammalian hibernation (i.e., seasonal torpor) has been interpreted as a physiological adaptation for energy economy. However, direct field comparisons of energy expenditure and torpor use in hibernating and active free-ranging animals are scarce. Here, we followed the complete hibernation cycle of a fat-storing hibernator, the marsupial Dromiciops gliroides, in its natural habitat. Using replicated mesocosms, we experimentally manipulated energy availability and measured torpor use, hibernacula use, and social clustering throughout the entire hibernation season. Also, we measured energy flow using daily food intake, daily energy expenditure (DEE), and basal metabolic rate (BMR) in winter. We hypothesized that when facing chronic caloric restriction (CCR), a hibernator should maximize torpor frequency to compensate for the energetic deficit, compared with individuals fed ad lib. (controls). However, being torpid at low temperatures could increase other burdens (e.g., cost of rewarming, freezing risks). Our results revealed that CCR animals, compared with control animals, did not promote heat conservation strategies (i.e., clustering and hibernacula use). Instead, they gradually increased torpor frequency and reduced DEE and, as a consequence, recovered weight at the end of the season. Also, CCR animals consumed food at a rate of 50.8 kJ d^-1, whereas control animals consumed food at a rate of 98.4 kJ d^-1. Similarly, the DEE of CCR animals in winter was 47.3±5.64 kJ d^-1, which was significantly lower than control animals (DEE=88.0±5.84 kJ d^-1). However, BMR and lean mass of CCR and control animals did not vary significantly, suggesting that animals maintained full metabolic capacities. This study shows that the use of torpor can be modulated depending on energy supply, thus optimizing energy budgeting. This plasticity in the use of heterothermy as an energy-saving strategy would explain the occurrence of this marsupial in a broad latitudinal and altitudinal range. Overall, this study suggests that hibernation is a powerful strategy to modulate energy expenditure in mammals from temperate regions.

Body Temperature Frequency Distributions: A Tool for Assessing Thermal Performance in Endotherms?

There is increasing recognition that rather than being fully homeothermic, most endotherms display some degree of flexibility in body temperature. However, the degree to which this occurs varies widely from the relatively strict homeothermy in species, such as humans to the dramatic seasonal hibernation seen in Holarctic ground squirrels, to many points in between. To date, attempts to analyse this variability within the framework generated by the study of thermal performance curves have been lacking. We tested if frequency distribution histograms of continuous body temperature measurements could provide a useful analogue to a thermal performance curve in endotherms. We provide examples from mammals displaying a range of thermoregulatory phenotypes, break down continuous core body temperature traces into various components (active and rest phase modes, spreads and skew) and compare these components to hypothetical performance curves. We did not find analogous patterns to ectotherm thermal performance curves, in either full datasets or by breaking body temperature values into more biologically relevant components. Most species had either bimodal or right-skewed (or both) distributions for both active and rest phase body temperatures, indicating a greater capacity for mammals to tolerate body temperatures elevated above the optimal temperatures than commonly assumed. We suggest that while core body temperature distributions may prove useful in generating optimal body temperatures for thermal performance studies and in various ecological applications, they may not be a good means of assessing the shape and breath of thermal performance in endotherms. We also urge researchers to move beyond only using mean body temperatures and to embrace the full variability in both active and resting temperatures in endotherms.

Interspecific variation in evaporative water loss and temperature response, but not metabolic rate, among hibernating bats

Hibernation is widespread among mammals in a variety of environmental contexts. However, few experimental studies consider interspecific comparisons, which may provide insight into general patterns of hibernation strategies. We studied 13 species of free-living bats, including populations spread over thousands of kilometers and diverse habitats. We measured torpid metabolic rate (TMR) and evaporative water loss (two key parameters for understanding hibernation energetics) across a range of temperatures. There was no difference in minimum TMR among species (i.e., all species achieved similarly low torpid metabolic rate) but the temperature associated with minimum TMR varied among species. The minimum defended temperature (temperature below which TMR increased) varied from 8 °C to < 2 °C among species. Conversely, evaporative water loss varied among species, with species clustered in two groups representing high and low evaporative water loss. Notably, species that have suffered population declines due to white-nose syndrome fall in the high evaporative water loss group and less affected species in the low evaporative water loss group. Documenting general patterns of physiological diversity, and associated ecological implications, contributes to broader understanding of biodiversity, and may help predict which species are at greater risk of environmental and anthropogenic stressors.

Survivable hypothermia or torpor in a wild-living rat: rare insights broaden our understanding of endothermic physiology

Maintaining a high and stable body temperature as observed in endothermic mammals and birds is energetically costly. Thus, it is not surprising that we discover more and more heterothermic species that can reduce their energetic needs during energetic bottlenecks through the use of torpor. However, not all heterothermic animals use torpor on a regular basis. Torpor may also be important to an individual’s probability of survival, and hence fitness, when used infrequently. We here report the observation of a single, ~ 5.5 h long hypothermic bout with a decrease in body temperature by 12 °C in the native Australian bush rat (Rattus fuscipes). Our data suggest that bush rats are able to rewarm from a body temperature of 24 °C, albeit with a rewarming rate lower than that expected on the basis of their body mass. Heterothermy, i.e. the ability to withstand and overcome periods of reduced body temperature, is assumed to be an evolutionarily ancestral (plesiomorphic) trait. We thus argue that such rare hypothermic events in species that otherwise appear to be strictly homeothermic could be heterothermic rudiments, i.e. a less derived form of torpor with limited capacity for rewarming. Importantly, observations of rare and extreme thermoregulatory responses by wild animals are more likely to be discovered with long-term data sets and may not only provide valuable insight about the physiological capability of a population, but can also help us to understand the constraints and evolutionary pathways of different phenologies.

Camera-trap data do not indicate scaling of diel activity and cathemerality with body mass in an East African mammal assemblage

Diel activity patterns of animal species reflect constraints imposed by morphological, physiological, and behavioral trade-offs, but these trade-offs are rarely quantified for multispecies assemblages. Based on a systematic year-long camera-trap study in the species-rich mammal assemblage of Lake Manyara National Park (Tanzania), we estimated activity levels (hours active per day) and circadian rhythms of 17 herbivore and 11 faunivore species to determine the effects of body mass and trophic level on activity levels and cathemerality (the degree to which species are active throughout the day and night). Using generalized least squares and phylogenetic generalized least squares analyses, we found no support for the hypothesis that trophic level is positively associated with activity levels. We found no support for activity levels to scale positively with body mass in herbivores or to differ between ruminants and nonruminants; in faunivores, we also did not detect relationships between body mass and activity levels. Cathemerality was positively associated with activity levels but did not scale significantly with body mass. Overall, our findings caution against trophic level or body mass-associated generalized conclusions with regard to diel activity patterns.

Heterothermy as the Norm, Homeothermy as the Exception: Variable Torpor Patterns in the South American Marsupial Monito del Monte (Dromiciops gliroides)

Hibernation (i.e., multiday torpor) is considered an adaptive strategy of mammals to face seasonal environmental challenges such as food, cold, and/or water shortage. It has been considered functionally different from daily torpor, a physiological strategy to cope with unpredictable environments. However, recent studies have shown large variability in patterns of hibernation and daily torpor ("heterothermic responses"), especially in species from tropical and subtropical regions. The arboreal marsupial "monito del monte" (Dromiciops gliroides) is the last living representative of the order Microbiotheria and is known to express both short torpor episodes and also multiday torpor depending on environmental conditions. However, only limited laboratory experiments have documented these patterns in D. gliroides. Here, we combined laboratory and field experiments to characterize the heterothermic responses in this marsupial at extreme temperatures. We used intraperitoneal data loggers and simultaneous measurement of ambient and body temperatures (T A and T B, respectively) for analyzing variations in the thermal differential, in active and torpid animals. We also explored how this differential was affected by environmental variables (T A, natural photoperiod changes, food availability, and body mass changes), using mixed-effects generalized linear models. Our results suggest that: (1) individuals express short bouts of torpor, independently of T A and even during the reproductive period; (2) seasonal torpor also occurs in D. gliroides, with a maximum bout duration of 5 days and a mean defended T B of 3.6 ± 0.9°C (one individual controlled T B at 0.09°C, at sub-freezing T A); (3) the best model explaining torpor occurrence (Akaike information criteria weight = 0.59) discarded all predictor variables except for photoperiod and a photoperiod by food interaction. Altogether, these results confirm that this marsupial expresses a dynamic form of torpor that progresses from short torpor to hibernation as daylength shortens. These data add to a growing body of evidence characterizing tropical and sub-tropical heterothermy as a form of opportunistic torpor, expressed as daily or seasonal torpor depending on environmental conditions.

Be cool to be far: Exploiting hibernation for space exploration

In mammals, torpor/hibernation is a state that is characterized by an active reduction in metabolic rate followed by a progressive decrease in body temperature. Torpor was successfully mimicked in non-hibernators by inhibiting the activity of neurons within the brainstem region of the Raphe Pallidus, or by activating the adenosine A1 receptors in the brain. This state, called synthetic torpor, may be exploited for many medical applications, and for space exploration, providing many benefits for biological adaptation to the space environment, among which an enhanced protection from cosmic rays. As regards the use of synthetic torpor in space, to fully evaluate the degree of physiological advantage provided by this state, it is strongly advisable to move from Earth-based experiments to 'in the field' tests, possibly on board the International Space Station.

On the modulation and maintenance of hibernation in captive dwarf lemurs

In nature, photoperiod signals environmental seasonality and is a strong selective "zeitgeber" that synchronizes biological rhythms. For animals facing seasonal environmental challenges and energetic bottlenecks, daily torpor and hibernation are two metabolic strategies that can save energy. In the wild, the dwarf lemurs of Madagascar are obligate hibernators, hibernating between 3 and 7 months a year. In captivity, however, dwarf lemurs generally express torpor for periods far shorter than the hibernation season in Madagascar. We investigated whether fat-tailed dwarf lemurs (Cheirogaleus medius) housed at the Duke Lemur Center (DLC) could hibernate, by subjecting 8 individuals to husbandry conditions more in accord with those in Madagascar, including alternating photoperiods, low ambient temperatures, and food restriction. All dwarf lemurs displayed daily and multiday torpor bouts, including bouts lasting ~ 11 days. Ambient temperature was the greatest predictor of torpor bout duration, and food ingestion and night length also played a role. Unlike their wild counterparts, who rarely leave their hibernacula and do not feed during hibernation, DLC dwarf lemurs sporadically moved and ate. While demonstrating that captive dwarf lemurs are physiologically capable of hibernation, we argue that facilitating their hibernation serves both husbandry and research goals: first, it enables lemurs to express the biphasic phenotypes (fattening and fat depletion) that are characteristic of their wild conspecifics; second, by "renaturalizing" dwarf lemurs in captivity, they will emerge a better model for understanding both metabolic extremes in primates generally and metabolic disorders in humans specifically.

Effects of hypoxia on the respiratory and metabolic responses to progressive cooling in newborn rodents that range in heterothermic expression

NEW FINDINGS

What is the central question of this study? Adult homeotherms and heterotherms differ in cold and hypoxia tolerance and in how they match O₂ supply and demand in response to these stressors. It has never been ascertained whether these differences reflect different developmental trajectories or whether they are already present at birth. What is the main finding and its importance? When exposed to cold and hypoxia, newborn rodents differed in how they matched O₂ supply and demand, with responses reflecting the degree of heterothermic expression and tolerance. Our findings indicate that elements of the adult phenotype are already present at birth.

ABSTRACT

There are physiological differences in how adult rodents regulate O₂ supply and O₂ demand when exposed to hypoxia in the cold. We examined whether these differences reflect divergent developmental trajectories of homeotherms and heterotherms or whether the differences are already present at birth. We exposed newborn rodents (0-4 days old) that ranged in heterothermic expression [a homeotherm, the rat (Rattus norvegicus); two facultative heterotherms, the mouse (Mus musculus) and the hamster (Mesocricetus auratus); and an obligate heterotherm, the ground squirrel (Ictidomys tridecemlineatus)] to either normoxia (21% O₂ ) or hypoxia (7% O₂ ) and measured their metabolic, thermoregulatory and ventilatory responses while progressively reducing the ambient temperature from 33 to 15°C. All newborns reduced their body temperature, O₂ consumption rate and ventilation during progressive cooling, both in normoxia and in hypoxia. When progressively cooled in hypoxia, however, the homeothermic rats exhibited the greatest thermogenic response, depressed their O₂ consumption rate the least and increased ventilation the most. In contrast, the obligate heterotherm, the ground squirrel, did not mount a thermogenic response, exhibited the greatest reduction in O₂ consumption rate and increased O₂ uptake not by increasing ventilation like the rat, but by extracting ≤80% of the O₂ from each breath. Facultative heterotherms (mice and hamsters) exhibited responses in between these two extreme phenotypes. We conclude that even as newborns, homeotherms and heterotherms diverge in how they match O₂ supply and O₂ demand when progressively cooled in hypoxia, with responses reflecting the degree of heterothermic expression, in addition to reported hypoxia and cold tolerance.

How dryland mammals will respond to climate change: the effects of body size, heat load and a lack of food and water

Mammals in drylands are facing not only increasing heat loads but also reduced water and food availability as a result of climate change. Insufficient water results in suppression of evaporative cooling and therefore increases in body core temperature on hot days, while lack of food reduces the capacity to maintain body core temperature on cold nights. Both food and water shortage will narrow the prescriptive zone, the ambient temperature range over which body core temperature is held relatively constant, which will lead to increased risk of physiological malfunction and death. Behavioural modifications, such as shifting activity between night and day or seeking thermally buffered microclimates, may allow individuals to remain within the prescriptive zone, but can incur costs, such as reduced foraging or increased competition or predation, with consequences for fitness. Body size will play a major role in predicting response patterns, but identifying all the factors that will contribute to how well dryland mammals facing water and food shortage will cope with increasing heat loads requires a better understanding of the sensitivities and responses of mammals exposed to the direct and indirect effects of climate change.

Body Protein Sparing in Hibernators: A Source for Biomedical Innovation

Proteins are not only the major structural components of living cells but also ensure essential physiological functions within the organism. Any change in protein abundance and/or structure is at risk for the proper body functioning and/or survival of organisms. Death following starvation is attributed to a loss of about half of total body proteins, and body protein loss induced by muscle disuse is responsible for major metabolic disorders in immobilized patients, and sedentary or elderly people. Basic knowledge of the molecular and cellular mechanisms that control proteostasis is continuously growing. Yet, finding and developing efficient treatments to limit body/muscle protein loss in humans remain a medical challenge, physical exercise and nutritional programs managing to only partially compensate for it. This is notably a major challenge for the treatment of obesity, where therapies should promote fat loss while preserving body proteins. In this context, hibernating species preserve their lean body mass, including muscles, despite total physical inactivity and low energy consumption during torpor, a state of drastic reduction in metabolic rate associated with a more or less pronounced hypothermia. The present review introduces metabolic, physiological, and behavioral adaptations, e.g., energetics, body temperature, and nutrition, of the torpor or hibernation phenotype from small to large mammals. Hibernating strategies could be linked to allometry aspects, the need for periodic rewarming from torpor, and/or the ability of animals to fast for more or less time, thus determining the capacity of individuals to save proteins. Both fat- and food-storing hibernators rely mostly on their body fat reserves during the torpid state, while minimizing body protein utilization. A number of them may also replenish lost proteins during arousals by consuming food. The review takes stock of the physiological, molecular, and cellular mechanisms that promote body protein and muscle sparing during the inactive state of hibernation. Finally, the review outlines how the detailed understanding of these mechanisms at play in various hibernators is expected to provide innovative solutions to fight human muscle atrophy, to better help the management of obese patients, or to improve the ex vivo preservation of organs.

The Torpid State: Recent Advances in Metabolic Adaptations and Protective Mechanisms†

Torpor and hibernation are powerful strategies enabling animals to survive periods of low resource availability. The state of torpor results from an active and drastic reduction of an individual's metabolic rate (MR) associated with a relatively pronounced decrease in body temperature. To date, several forms of torpor have been described in all three mammalian subclasses, i.e., monotremes, marsupials, and placentals, as well as in a few avian orders. This review highlights some of the characteristics, from the whole organism down to cellular and molecular aspects, associated with the torpor phenotype. The first part of this review focuses on the specific metabolic adaptations of torpor, as it is used by many species from temperate zones. This notably includes the endocrine changes involved in fat- and food-storing hibernating species, explaining biomedical implications of MR depression. We further compare adaptive mechanisms occurring in opportunistic vs. seasonal heterotherms, such as tropical and sub-tropical species. Such comparisons bring new insights into the metabolic origins of hibernation among tropical species, including resistance mechanisms to oxidative stress. The second section of this review emphasizes the mechanisms enabling heterotherms to protect their key organs against potential threats, such as reactive oxygen species, associated with the torpid state. We notably address the mechanisms of cellular rehabilitation and protection during torpor and hibernation, with an emphasis on the brain, a central organ requiring protection during torpor and recovery. Also, a special focus is given to the role of an ubiquitous and readily-diffusing molecule, hydrogen sulfide (H2S), in protecting against ischemia-reperfusion damage in various organs over the torpor-arousal cycle and during the torpid state. We conclude that (i) the flexibility of torpor use as an adaptive strategy enables different heterothermic species to substantially suppress their energy needs during periods of severely reduced food availability, (ii) the torpor phenotype implies marked metabolic adaptations from the whole organism down to cellular and molecular levels, and (iii) the torpid state is associated with highly efficient rehabilitation and protective mechanisms ensuring the continuity of proper bodily functions. Comparison of mechanisms in monotremes and marsupials is warranted for understanding the origin and evolution of mammalian torpor.

Tropical bats counter heat by combining torpor with adaptive hyperthermia

Many tropical mammals are vulnerable to heat because their water budget limits the use of evaporative cooling for heat compensation. Further increasing temperatures and aridity might consequently exceed their thermoregulatory capacities. Here, we describe two novel modes of torpor, a response usually associated with cold or resource bottlenecks, as efficient mechanisms to counter heat. We conducted a field study on the Malagasy bat Macronycteris commersoni resting in foliage during the hot season, unprotected from environmental extremes. On warm days, the bats alternated between remarkably short micro-torpor bouts and normal resting metabolism within a few minutes. On hot days, the bats extended their torpor bouts over the hottest time of the day while tolerating body temperatures up to 42.9°C. Adaptive hyperthermia combined with lowered metabolic heat production from torpor allows higher heat storage from the environment, negates the need for evaporative cooling and thus increases heat tolerance. However, it is a high-risk response as the torpid bats cannot defend body temperature if ambient temperature increases above a critical/lethal threshold. Torpor coupled with hyperthermia and micro-torpor bouts broaden our understanding of the basic principles of thermal physiology and demonstrate how mammals can perform near their upper thermal limits in an increasingly warmer world.

Elevated ambient temperature accelerates aspects of torpor phenology in an obligate hibernator

The thirteen-lined ground squirrel (Ictidomys tridecemlineatus) is assumed to be an obligate hibernator - commencing and terminating hibernation on a circannual rhythm, regardless of environmental conditions - but, until now, this assumption had never been fully tested. We housed three groups of captive-born ground squirrels from Aug. 2017 to Aug. 2018 under constant photoperiod (12 h L:12 h D) at 5, 16 or 25 °C, and monitored hibernation using body temperature loggers. At 5 and 16 °C all animals hibernated from autumn to spring with no differences in date of first/last torpor or duration of interbout euthermic periods (IBE), but torpor bout duration was 25% shorter at 16 °C. One of 4 animals housed at 25 °C did not hibernate. For the other three 25 °C animals, the first torpor date did not differ from the other groups, but the last torpor bout (5 Feb.) occurred almost 8 weeks earlier. These animals aroused from torpor more frequently and IBE lasted significantly longer, so the total time spent torpid was less than 50% of the other groups. Unlike the 5 or 16 °C animals, 25 °C animals re-entered torpor in late spring 2018. Taken together these data suggest that this species is an obligate hibernator, but that high ambient temperatures can accelerate the endogenous circannual hibernation rhythm.

Are southern African solitary mole-rats homeothermic or heterothermic under natural field conditions?

Abandoning of a stable body temperature (T_b), a phenomenon known as heterothermy, is an adaptation to cope mainly with a lack of food and water, especially in species inhabiting daily or seasonally variable environments. There is increasing evidence that African mammals avoid adverse conditions by heterothermy and eventually by entering torpor. Members of subterranean rodent family, the African mole-rats (Bathyergidae), are suitable candidates to study both phenomena, because of the diversity of their strategies in respect of maintaining stable T_b ranging from homeothermic species to a mammal with the most labile T_b, the naked mole-rat. Currently, there are field data on daily and seasonal T_b in one social species only and such information are lacking for any solitary mole-rat. In our study, we recorded yearly T_b in two solitary bathyergids, the Cape mole-rat Georychus capensis and the Cape dune mole-rat Bathyergus suillus from South Africa using intraperitoneally implanted dataloggers. Since this region is characterised by changing ecological characteristics, we expected either decreases of T_b within 24 h indicating daily torpor and/or longer-term decreases of T_b, which would indicate multiday torpor. Although we found seasonally phase shifted low amplitude daily T_b cycles, we did not find any remarkable and regular daily and/or seasonal T_b deviations, likely showing an absence of torpor in both species. Due to absence of this energy saving mechanism, we may speculate that both species could be vulnerable to ongoing global climatic change.

Integrating Mortality Risk and the Adaptiveness of Hibernation

Low mortality rate is often associated with slow life history, and so far, has mainly been assessed through examinations of specific adaptations and lifestyles that limit mortality risk. However, the organization of activity time budgets also needs to be considered, since some activities and the time afforded for performing them may expose animals to higher mortality risks such as increased predation and/or increased metabolic stress. We examined the extent of activity time budgets contribution to explaining variation in life history traits in mammals. We specifically focused on hibernating species because of their marked seasonal cycle of activity/inactivity associated with very different mortality risks. Hibernation is considered a seasonal adaptation to prolonged periods of food shortage and cold. This inactivity period may also reduce both extrinsic and intrinsic mortality risks, by decreasing exposure to predators and drastically reducing metabolic rate. In turn, reduction in mortality may explain why hibernators have slower life history traits than non-hibernators of the same size. Using phylogenetically controlled models, we tested the hypothesis that longevity was positively correlated with the hibernation season duration (the time spent between immergence and emergence from the hibernaculum or den) across 82 different mammalian species. We found that longevity increased significantly with hibernation season duration, an effect that was particularly strong in small hibernators (<1.5 kg) especially for bats. These results confirm that hibernation not only allows mammals to survive periods of energy scarcity, but further suggest that activity time budgets may be selected to reduce mortality risks according to life history pace.

Ambient Temperature Cycles Affect Daily Torpor and Hibernation Patterns in Malagasy Tenrecs

Hibernation and daily torpor (heterothermy) allow endotherms to cope with demanding environmental conditions. The depth and duration of torpor bouts vary considerably between tropical and temperate climates, and tropical hibernators manage to cope with a wider spectrum of ambient temperature (T_a) regimes during heterothermy. As cycles in T_a can have profound effects on activity and torpor patterns as well as energy expenditure, we examined how these characteristics are affected by daily fluctuating versus constant T_a in a tropical hibernator, the lesser hedgehog tenrec (Echinops telfairi). Throughout the study, regardless of season, the tenrecs became torpid every day. In summer, E. telfairi used daily fluctuations in T_a to passively rewarm from daily torpor, which led to synchrony in the activity phases and torpor bouts between individuals and generally decreased energy expenditure. In contrast, animals housed at constant T_a showed considerable variation in timing and they had to invest more energy through endogenous heat production. During the hibernation season (winter) E. telfairi hibernated for several months in constant, as well as in fluctuating T_a and, as in summer, under fluctuating T_a arousals were much more uniform and showed less variation in timing compared to constant temperature regimes. The timing of torpor is not only important for its effective use, but synchronization of activity patterns could also be essential for social interactions, and successful foraging bouts. Our results highlight that T_a cycles can be an effective zeitgeber for activity and thermoregulatory rhythms throughout the year and that consideration should be given to the choice of temperature regime when studying heterothermy under laboratory conditions.

Seasonal Expression of Avian and Mammalian Daily Torpor and Hibernation: Not a Simple Summer-Winter Affair†

Daily torpor and hibernation (multiday torpor) are the most efficient means for energy conservation in endothermic birds and mammals and are used by many small species to deal with a number of challenges. These include seasonal adverse environmental conditions and low food/water availability, periods of high energetic demands, but also reduced foraging options because of high predation pressure. Because such challenges differ among regions, habitats and food consumed by animals, the seasonal expression of torpor also varies, but the seasonality of torpor is often not as clear-cut as is commonly assumed and differs between hibernators and daily heterotherms expressing daily torpor exclusively. Hibernation is found in mammals from all three subclasses from the arctic to the tropics, but is known for only one bird. Several hibernators can hibernate for an entire year or express torpor throughout the year (8% of species) and more hibernate from late summer to spring (14%). The most typical hibernation season is the cold season from fall to spring (48%), whereas hibernation is rarely restricted to winter (6%). In hibernators, torpor expression changes significantly with season, with strong seasonality mainly found in the sciurid and cricetid rodents, but seasonality is less pronounced in the marsupials, bats and dormice. Daily torpor is diverse in both mammals and birds, typically is not as seasonal as hibernation and torpor expression does not change significantly with season. Torpor in spring/summer has several selective advantages including: energy and water conservation, facilitation of reproduction or growth during development with limited resources, or minimisation of foraging and thus exposure to predators. When torpor is expressed in spring/summer it is usually not as deep and long as in winter, because of higher ambient temperatures, but also due to seasonal functional plasticity. Unlike many other species, subtropical nectarivorous blossom-bats and desert spiny mice use more frequent and pronounced torpor in summer than in winter, which is related to seasonal availability of nectar or water. Thus, seasonal use of torpor is complex and differs among species and habitats.

Hibernation and daily torpor in Australian and New Zealand bats: does the climate zone matter?

We aim to summarise what is known about torpor use and patterns in Australian and New Zealand (ANZ) bats from temperate, tropical/subtropical and arid/semiarid regions and to identify whether and how they differ. ANZ bats comprise ~90 species from 10 families. Members of at least nine of these are known to use torpor, but detailed knowledge is currently restricted to the pteropodids, molossids, mystacinids, and vespertilionids. In temperate areas, several species can hibernate (use a sequence of multiday torpor bouts) in trees or caves mostly during winter and continue to use short bouts of torpor for the rest of the year, including while reproducing. Subtropical vespertilionids also use multiday torpor in winter and brief bouts of torpor in summer, which permit a reduction in foraging, probably in part to avoid predators. Like temperate-zone vespertilionids they show little or no seasonal change in thermal energetics during torpor, and observed changes in torpor patterns in the wild appear largely due to temperature effects. In contrast, subtropical blossom-bats (pteropodids) exhibit more pronounced daily torpor in summer than winter related to nectar availability, and this involves a seasonal change in physiology. Even in tropical areas, vespertilionids express short bouts of torpor lasting ~5 h in winter; summer data are not available. In the arid zone, molossids and vespertilionids use torpor throughout the year, including during desert heat waves. Given the same thermal conditions, torpor bouts in desert bats are longer in summer than in winter, probably to minimise water loss. Thus, torpor in ANZ bats is used by members of all or most families over the entire region, its regional and seasonal expression is often not pronounced or as expected, and it plays a key role in energy and water balance and other crucial biological functions that enhance long-term survival by individuals.