Reduction of metabolic rate and thermoregulation during daily torpor

Torpor energetics are related to the interaction between body mass and climate in bats of the family Vespertilionidae

Torpor is an adaptive strategy allowing heterothermic animals to cope with energy limitations. In birds and mammals, intrinsic and extrinsic factors, such as body mass and ambient temperature, are the main variables influencing torpor use. A theoretical model of the relationship between metabolic rate during torpor and ambient temperature has been proposed. Nevertheless, no empirical attempts have been made to assess the model predictions under different climates. Using open-flow respirometry, we evaluated the ambient temperature at which bats entered torpor and when torpid metabolic rate reached its minimum, the reduction in metabolic rate below basal values, and minimum torpid metabolic rate in 11 bat species of the family Vespertilionidae with different body mass from warm and cold climates. We included data on the minimum torpid metabolic rate of five species we retrieved from the literature. We tested the effects using mixed-effect phylogenetic models. All models showed a significant interaction between body mass and climate. Smaller bats went into torpor and reached minimum torpid metabolic rates at warmer temperatures, showed a higher reduction in the metabolic rate below basal values, and presented lower torpid metabolic rates than larger ones. The slopes of the models were different for bats from different climates. These results are likely explained by differences in body mass and the metabolic rate of bats, which may favor larger bats expressing torpor in colder sites and smaller bats in the warmer ones. Further studies to assess torpor use in bats from different climates are proposed.

Energy and time optimization during exit from torpor in vertebrate endotherms

Torpor is used in small sized birds and mammals as an energy conservation trait. Considerable effort has been put towards elucidating the mechanisms underlying its entry and maintenance, but little attention has been paid regarding the exit. Firstly, we demonstrate that the arousal phase has a stereotyped dynamic: there is a sharp increase in metabolic rate followed by an increase in body temperature and, then, a damped oscillation in body temperature and metabolism. Moreover, the metabolic peak is around two-fold greater than the corresponding euthermic resting metabolic rate. We then hypothesized that either time or energy could be crucial variables to this event and constructed a model from a collection of first principles of physiology, control engineering and thermodynamics. From the model, we show that the stereotyped pattern of the arousal is a solution to save both time and energy. We extended the analysis to the scaling of the use of torpor by endotherms and show that variables related to the control system of body temperature emerge as relevant to the arousal dynamics. In this sense, the stereotyped dynamics of the arousal phase necessitates a certain profile of these variables which is not maintained as body size increases.

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.

Estrogen-sensitive medial preoptic area neurons coordinate torpor in mice

Homeotherms maintain a stable internal body temperature despite changing environments. During energy deficiency, some species can cease to defend their body temperature and enter a hypothermic and hypometabolic state known as torpor. Recent advances have revealed the medial preoptic area (MPA) as a key site for the regulation of torpor in mice. The MPA is estrogen-sensitive and estrogens also have potent effects on both temperature and metabolism. Here, we demonstrate that estrogen-sensitive neurons in the MPA can coordinate hypothermia and hypometabolism in mice. Selectively activating estrogen-sensitive MPA neurons was sufficient to drive a coordinated depression of metabolic rate and body temperature similar to torpor, as measured by body temperature, physical activity, indirect calorimetry, heart rate, and brain activity. Inducing torpor with a prolonged fast revealed larger and more variable calcium transients from estrogen-sensitive MPA neurons during bouts of hypothermia. Finally, whereas selective ablation of estrogen-sensitive MPA neurons demonstrated that these neurons are required for the full expression of fasting-induced torpor in both female and male mice, their effects on thermoregulation and torpor bout initiation exhibit differences across sex. Together, these findings suggest a role for estrogen-sensitive MPA neurons in directing the thermoregulatory and metabolic responses to energy deficiency.

Combined effect of environmental temperature and density-dependent processes on the evolution of seasonal metabolic rate patterns

Species exhibit large diversity in their seasonal metabolic patterns which are traditionally explained by habitat specific seasonal temperature changes and various thermoregulatory adaptations. However, due to seasonal changes in resource abundances, density-dependent, ecological processes can also be important selective forces shaping the evolution of metabolic patterns. In the present theoretical study, the combined effect of environmental temperature and resource availability on the evolutionarily stable metabolic strategies is investigated in a consumer-resource model. Our results suggest that, under a broad range of circumstances, density-dependent mechanisms favor the selection of active metabolic regulation, where the metabolic rate differs from the thermally optimal value. This effect may be temporary, limited only to a brief period at seasonal changes, or permanent depending on the implied energetic cost and the relative timescale of environmental changes as compared to the generation time.

Bats are not squirrels: Revisiting the cost of cooling in hibernating mammals

Many species use stored energy to hibernate through periods of resource limitation. Hibernation, a physiological state characterized by depressed metabolism and body temperature, is critical to winter survival and reproduction, and therefore has been extensively quantified and modeled. Hibernation consists of alternating phases of extended periods of torpor (low body temperature, low metabolic rate), and energetically costly periodic arousals to normal body temperature. Arousals consist of multiple phases: warming, euthermia, and cooling. Warming and euthermic costs are regularly included in energetic models, but although cooling to torpid body temperature is an important phase of the torpor-arousal cycle, it is often overlooked in energetic models. When included, cooling cost is assumed to be 67% of warming cost, an assumption originally derived from a single study that measured cooling cost in ground squirrels. Since this study, the same proportional value has been assumed across a variety of hibernating species. However, no additional values have been derived. We derived a model of cooling cost from first principles and validated the model with empirical energetic measurements. We compared the assumed 67% proportional cooling cost with our model-predicted cooling cost for 53 hibernating mammals. Our results indicate that using 67% of warming cost only adequately represents cooling cost in ground squirrel-sized mammals. In smaller species, this value overestimates cooling cost and in larger species, the value underestimates cooling cost. Our model allows for the generalization of energetic costs for multiple species using species-specific physiological and morphometric parameters, and for predictions over variable environmental conditions.

Turning down the heat: Down-regulation of sarcolipin in a hibernating mammal

Hibernation in mammals is a whole-body phenotype that involves profound reductions in oxygen consumption, metabolic reactions, core body temperature, neural activity and heart rate. An important aspect of mammalian hibernation is the ability to reverse this state of hypothermic torpor by rewarming and subsequent arousal. Brown adipose tissue (BAT) and skeletal muscle shivering have been characterized as the predominant driving forces for thermogenesis during arousal. Conversely, the thermogenic contribution of these organs needs to be minimized as hibernating mammals enter torpor. Because skeletal muscle accounts for approximately 40% of the dry mass of the typical mammalian body, we aim to broaden the spotlight to include the importance of down-regulating skeletal muscle non-shivering thermogenesis during hibernation to allow for whole-body cooling and long-term maintenance of a depressed core body temperature when the animal is in torpor. This minireview will briefly describe the current understanding of thermoregulation in hibernating mammals and present new preliminary data on the importance of skeletal muscle and the micro-peptide sarcolipin as a major thermogenic target.

Exogenous hydrogen sulfide gas does not induce hypothermia in normoxic mice

Hydrogen sulfide (H₂S, 80 ppm) gas in an atmosphere of 17.5% oxygen reportedly induces suspended animation in mice; a state analogous to hibernation that entails hypothermia and hypometabolism. However, exogenous H₂S in combination with 17.5% oxygen is able to induce hypoxia, which in itself is a trigger of hypometabolism/hypothermia. Using non-invasive thermographic imaging, we demonstrated that mice exposed to hypoxia (5% oxygen) reduce their body temperature to ambient temperature. In contrast, animals exposed to 80 ppm H₂S under normoxic conditions did not exhibit a reduction in body temperature compared to normoxic controls. In conclusion, mice induce hypothermia in response to hypoxia but not H₂S gas, which contradicts the reported findings and putative contentions.

Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria (resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Flexibility in thermoregulatory physiology of two dunnarts, Sminthopsis macroura and Sminthopsis ooldea (Marsupialia; Dasyuridae)

Stripe-faced dunnarts (Sminthopsis macroura) and Ooldea dunnarts (S. ooldea) were acclimated for 2 weeks to ambient temperature (T(a)) regimes of 12-22°C, 18-28°C and 25-35°C, and then measured for standard, basal (BMR) and maximum (MMR) metabolic rate using flow-through respirometry. Sminthopsis macroura maintained a stable body temperature under all experimental T(a) and acclimation regimes. Although its BMR was not statistically different between the three acclimation regimes, the lower end of the thermoneutral zone (TNZ) shifted from 30°C under the 18-28°C and 12-22°C acclimation regimes to 35°C under the 25-35°C acclimation regime. MMR increased significantly at the cooler acclimation regimes. EWL increased at T(a)=35°C, compared with lower T(a), in all acclimation regimes, but an increase in evaporative water loss (EWL) at T(a)=10°C observed in cool acclimations did not occur at the 25-35°C regime. In contrast, S. ooldea had variable body temperature between experimental T(a) in all acclimation regimes, but no acclimational shift in TNZ, which was between 30 and 35°C. Neither BMR nor MMR was affected by exposure to the three acclimation regimes. EWL did not change across T(a) or with acclimation regime. Sminthopsis macroura was flexible in many aspects of its thermoregulation (involving energy and water balance) in response to thermal acclimation, presumably allowing it to balance its energy and water requirements over a broad range of climatic conditions. Sminthopsis ooldea seems to have an inflexible energetic and water balance in response to thermal acclimation, but has low nominal expenditure of either resource on thermoregulation because it thermoregulates less precisely than S. macroura. It seems that S. ooldea is adapted to a more narrow, stable climate.

Comparative thermoregulatory physiology of two dunnarts, Sminthopsis macroura and Sminthopsis ooldea (Marsupialia : Dasyuridae)

Metabolic rate and evaporative water loss (EWL) were measured to quantify the thermoregulatory patterns of two dasyurids, the stripe-faced dunnart (Sminthopsis macroura) and the Ooldea dunnart (S. ooldea) during acute exposure to Ta between 10 and 35°C. S. macroura maintained consistent Tb across the Ta range, whereas S. ooldea was more thermolabile. The metabolic rate of both species decreased from Ta = 10°C to BMR at Ta = 30°C. Mass-adjusted BMR at Ta = 30°C was the same for the two species, but there was no common regression of metabolic rate below the thermoneutral zone (TNZ). There was no significant difference between the species in allometrically corrected EWL at Ta = 30°C. Total EWL increased significantly at Ta = 10 and 35°C compared with the TNZ for S. macroura, but was consistent across the Ta range for S. ooldea. At any Ta below the TNZ, S. macroura required more energy per gram of body mass than S. ooldea, and had a higher EWL at the lower critical Ta. By being thermolabile S. ooldea reduced its energetic requirements and water loss at low Ta. The more constant thermoregulatory strategy of S. macroura may allow it to exploit a broad climatic envelope, albeit at the cost of higher energetic and water requirements. Since S. ooldea does not expend as much energy and water on thermoregulation this may be a response to the very low productivity, ‘hyperarid’ conditions of its central Australian distribution.

Phylogenetic differences of mammalian basal metabolic rate are not explained by mitochondrial basal proton leak

Metabolic rates of mammals presumably increased during the evolution of endothermy, but molecular and cellular mechanisms underlying basal metabolic rate (BMR) are still not understood. It has been established that mitochondrial basal proton leak contributes significantly to BMR. Comparative studies among a diversity of eutherian mammals showed that BMR correlates with body mass and proton leak. Here, we studied BMR and mitochondrial basal proton leak in liver of various marsupial species. Surprisingly, we found that the mitochondrial proton leak was greater in marsupials than in eutherians, although marsupials have lower BMRs. To verify our finding, we kept similar-sized individuals of a marsupial opossum (Monodelphis domestica) and a eutherian rodent (Mesocricetus auratus) species under identical conditions, and directly compared BMR and basal proton leak. We confirmed an approximately 40 per cent lower mass specific BMR in the opossum although its proton leak was significantly higher (approx. 60%). We demonstrate that the increase in BMR during eutherian evolution is not based on a general increase in the mitochondrial proton leak, although there is a similar allometric relationship of proton leak and BMR within mammalian groups. The difference in proton leak between endothermic groups may assist in elucidating distinct metabolic and habitat requirements that have evolved during mammalian divergence.

Aestivation in mammals and birds

Aestivation, which in the context of this paper refers to avian and mammalian torpor in summer/at high ambient temperatures (T (a)), does not appear to differ functionally from other forms of torpor, and to a large extent reflects the higher body temperatures (T (b)) caused by high T (a). However, from an ecological point of view, aestivation results in different challenges and requirements than does torpor use in winter, because heat can cause reduced food and water availability in many regions, but without the access to low T (a) for a substantial reduction of T (b). Aestivation is used by a diversity of adult mammals and birds both in the field and laboratory, as well as by growing young to reduce thermoregulatory energy expenditure. Torpor occurs at high T (a) including the thermo-neutral zone and even under these conditions the reduction in energy expenditure and water requirements or water loss is substantial. Although data from the laboratory and, especially, from the field are limited, they show that torpor at high T (a) is an effective survival strategy and suggest that it is employed by many mammals and birds in a diversity of habitats.

The key to winter survival: daily torpor in a small arid-zone marsupial

Mammalian hibernation, which lasts on average for about 6 months, can reduce energy expenditure by >90% in comparison to active individuals. In contrast, the widely held view is that daily torpor reduces energy expenditure usually by about 30%, is employed for a few hours every few days, and often occurs only under acute energetic stress. This interpretation is largely based on laboratory studies, whereas knowledge on daily torpor in the field is scant. We used temperature telemetry to quantify thermal biology and activity patterns of a small arid-zone marsupial, the stripe-faced dunnart Sminthopsis macroura (16.9 g), in the wild and to test the hypothesis that daily torpor is a crucial survival strategy of this species in winter. All individuals entered torpor daily with the exception of a single male that remained normothermic for a single day (torpor on 212 of 213 observation days, 99.5%). Torpor was employed at air temperatures (T (a)) ranging from approximately -1 degrees C to 36 degrees C. Dunnarts usually entered torpor during the night and aroused at midday with the daily increase of T (a). Torpor was on average about twice as long (mean 11.0 +/- 4.7 h, n = 8) than in captivity. Animals employed sun basking during rewarming, reduced foraging time significantly, and occasionally omitted activity for several days in sequence. Consequently, we estimate that daily torpor in this species can reduce daily energy expenditure by up to 90%. Our study shows that for wild stripe-faced dunnarts daily torpor is an essential mechanism for overcoming energetic challenges during winter and that torpor data obtained in the laboratory can substantially underestimate the ecological significance of daily torpor in the wild.

Chronic food shortage and seasonal modulations of daily torpor and locomotor activity in the grey mouse lemur (Microcebus murinus)

The extent to which seasonal plasticity in torpor displayed by one of the smallest Malagasy primates (Microcebus murinus) will help survival in the context of ongoing global change-induced chronic food shortage, is unknown. Body temperature (Tb) and locomotor activity were measured by telemetry in short- (SD, winter-acclimated) and long-days (LD, summer-acclimated) males (n = 24) during an experimental 35-day calorie restriction of 40 or 80%. Under SD exposure, regardless of calorie restriction intensity, mouse lemurs immediately increased torpor depth and duration by 4.6-fold, and showed greater phase-advanced entry into torpor (2.4-fold). Tb adjustments were efficient under 40% calorie restriction to maintain body mass, whereas they did not prevent a 0.71 +/- 0.11 g/day mass loss during 80% calorie restriction. The 40% food-deprived LD animals combined an early shallow deepening of torpor (1 degrees C) and a late 18% decrease in locomotor activity, resulting in a moderate 6% mass loss. After 15 days of 80% calorie restriction, LD animals exhibited a SD phenotype by increasing their torpor duration and phase-advancing the entry of torpor (16 min/day). Those adjustments had no impact on mass loss (0.93 +/- 0.07 g/day) as locomotor activity increased four-fold. Daily torpor allows M. murinus to face moderate food shortage whatever the photoperiod but poorly mitigates energy imbalance during severe food deprivation, especially under LD exposure. Although the behavioral thermoregulation role warrants further investigation in energy savings, M. murinus survival would be impaired during long-term food shortage in summer.

To use or not to use torpor? Activity and body temperature as predictors

When food is limited and/or environmental conditions are unfavourable, many mammals reduce activity and use torpor to save energy. Nevertheless, reliable predictors for torpor occurrence, especially in the wild, are currently not available. Interrelations between torpor use and other energy conserving strategies are also poorly understood. We tested the hypothesis that reductions in normothermic body temperature (T(b)) and the period of activity before torpor events could be used as predictors for torpor occurrence in sugar gliders, Petaurus breviceps (body mass, approximately 125 g), known to display daily torpor in the wild. Occurrence of torpor was preceded by significant (approximately 10-25%) reductions of the duration of the activity phase. Moreover, the normothermic resting T(b) fell by an average of 1.2 degrees C over 3 days before a torpor event, relative to individuals that did not display torpor. Our new findings suggest that before entering torpor, sugar gliders, which appear to use torpor as an emergency measure rather than a routine energy saving strategy, systematically reduce activity times and normothermic resting T(b)s to lower energy expenditure and perhaps to avoid employing torpor. Thus, reduced activity and normothermic T(b) may provide a predictive tool for the occurrence of daily torpor in the wild.

Development of thermoregulation and torpor in a marsupial: energetic and evolutionary implications

Altricial mammals and birds become endothermic at about half the size of adults and presumably would benefit energetically from entering torpor at that time. Because little is known about torpor during development in endotherms, we investigated whether after the establishment of endothermic thermoregulation (i.e. the ability to maintain a high body temperature during cold exposure), Sminthopsis macroura, a small (approximately 25 g) insectivorous marsupial, is capable of entering torpor and whether torpor patterns change with growth. Endothermic thermoregulation was established when the nest young reached a body mass of approximately 10 g, and they were capable of entering torpor early during development at approximately 10-12 g, lending some support to the view that torpor is a phylogenetically old mammalian trait. Torpor bout length shortened significantly and the minimum metabolic rate during torpor increased as juveniles approached adult size, and consequently total daily energy expenditure increased steeply with age. Relationships between total daily energy expenditure and body mass during development of S. macroura (slope approximately 1.3) differed substantially from the relationship between basal metabolism and body mass in adult endotherms (slope approximately 0.75) suggesting that the energy expenditure-size relationship during the development differs substantially from that in adults under thermo-neutral conditions. Our study shows that while torpor can substantially reduce energy expenditure during development of endotherms and hence is likely important for survival during energy bottlenecks, it also may enhance somatic growth when food is limited. We therefore hypothesize that torpor during the development in endotherms is far more widespread than is currently appreciated.

Exogenously administered leptin leads to weight loss and increased physical activity in the marsupial Sminthopsis crassicaudata

The adipose tissue derived cytokine leptin, modifies energy balance via effects on both food intake and energy expenditure. It is not clear, however, whether the component of energy expenditure accounted for by voluntary (nonexercise) physical activity is increased in response to leptin. The aim of this study was to investigate the effect of exogenously administered leptin on physical activity in the marsupial Sminthopsis crassicaudata. Body weight, tail width and food intake, were measured daily and physical activity was measured hourly in normal lean S. crassicaudata (n=8) with ad libitum access to standard laboratory diet. After 5 days baseline the animals were divided into two equal groups (n=4), and either human recombinant leptin (2.5 mg/kg) or placebo was administered twice daily intraperitoneally. Approximately 81% of the total daily activity during the baseline period occurred during the nocturnal phase. After 9 days of leptin administration, there were significant decreases in body weight (P<0.001) and fat content (P<0.01), which were not accompanied by a decrease in total energy intake. Overall daily physical activity increased (P=0.028); this effect was confined to the dark phase (P=0.033). We conclude that in lean S. crassicaudata the exogenous administration of human recombinant leptin results in a decrease in adiposity which occurs in the absence of a measurable effect on food intake and is associated with an increase in non-exercise physical activity at least over the duration of this study.

Metabolic rate and body temperature reduction during hibernation and daily torpor

Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (Tb) for MR reduction, perhaps with the exception of very small species and at high torpor Tb, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to Tb effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of Tb are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and Tb reduction at torpor onset. Over the ambient temperature (Ta) range where torpid heterotherms are thermo-conforming, the Tb-Ta differential is more or less constant despite a decline of MR with Ta; however, in thermo-regulating torpid individuals, the Tb-Ta differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower Tb. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Effects of temperature acclimation on maximum heat production, thermal tolerance, and torpor in a marsupial

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 would 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 degrees C) and warm (26 degrees 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 degrees 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.

Radiant heat affects thermoregulation and energy expenditure during rewarming from torpor

The high expenditure of energy required for endogenous rewarming is one of the widely perceived disadvantages of torpor. However, recent evidence demonstrates that passive rewarming either by the increase of ambient temperature or by basking in the sun appears to be common in heterothermic birds and mammals. As it is presently unknown how radiant heat affects energy expenditure during rewarming from torpor and little is known about how it affects normothermic thermoregulation, we quantified the effects of radiant heat on body temperature and metabolic rate of the small (body mass 25 g) marsupial Sminthopsis macroura in the laboratory. Normothermic resting individuals exposed to radiant heat were able to maintain metabolic rates near basal levels (at 0.91 ml O(2) g(-1) h(-1)) and a constant body temperature down to an ambient temperature of 12 degrees C. In contrast, metabolic rates of individuals without access to radiant heat were 4.5-times higher at an ambient temperature of 12 degrees C and body temperature fell with ambient temperature. During radiant heat-assisted passive rewarming from torpor, animals did not employ shivering but appeared to maximise uptake of radiant heat. Their metabolic rate increased only 3.2-times with a 15- degrees C rise of body temperature (Q(10)=2.2), as predicted by Q(10) effects. In contrast, during active rewarming shivering was intensive and metabolic rates showed an 11.6-times increase. Although body temperature showed a similar absolute change between the beginning and the end of the rewarming process, the overall energetic cost during active rewarming was 6.3-times greater than that during passive, radiant heat-assisted rewarming. Our study demonstrates that energetic models assuming active rewarming from torpor at low ambient temperatures can substantially over-estimate energetic costs. The low energy expenditure during passive arousal provides an alternative explanation as to why daily torpor is common in sunny regions and suggests that the prevalence of torpor in low latitudes may have been under-estimated in the past.

Molecular identification of uncoupling proteins (UCP2 and UCP3) and absence of UCP1 in the marsupial Tasmanian bettong, Bettongia gaimardi

This study has identified the expression of uncoupling proteins in a marsupial using molecular techniques. The Tasmanian bettong, Bettongia gaimardi, increases non-shivering thermogenesis (NST) in response to cold exposure and norepinephrine, although previous studies have been unable to demonstrate the presence of brown adipose tissue or uncoupling protein 1 (UCP1). This study used molecular techniques to confirm the absence of UCP1 as well as ascertain if this species expresses UCP2 and/or UCP3. Tissue samples from four B. gaimardi were taken prior to and post-cold exposure at 4-5 degrees C for 2 weeks. The tissues were then examined for UCP1, UCP2 and UCP3 expression using Western blotting. UCP2 and UCP3 were amplified through RT-PCR and subsequently sequenced to confirm molecular identity. Our work confirms that B. gaimardi does not express UCP1 and that this species expresses both uncoupling proteins 2 and 3. The sequencing of the amplified B. gaimardi UCP2 and UCP3 cDNAs have revealed a 74% homology with rat UCP2 cDNA, and 65% homology with rat UCP3 cDNA. Although this work has not yet characterised the functional properties of these proteins in the marsupial, it does suggest a possible mechanism to explain the existence of NST in B. gaimardi.

Temperature effects on energy metabolism: a dynamic system analysis

Q(10) factors are widely used as indicators of the magnitude of temperature-induced changes in physico-chemical and physiological rates. However, there is a long-standing debate concerning the extent to which Q(10) values can be used to derive conclusions about energy metabolism regulatory control. The main point of this disagreement is whether or not it is fair to use concepts derived from molecular theory in the integrative physiological responses of living organisms. We address this debate using a dynamic systems theory, and analyse the behaviour of a model at the organismal level. It is shown that typical Q(10) values cannot be used unambiguously to deduce metabolic rate regulatory control. Analytical constraints emerge due to the more formal and precise equation used to compute Q(10), derived from a reference system composed from the metabolic rate and the Q(10). Such an equation has more than one unknown variable and thus is unsolvable. This problem disappears only if the Q(10) is assumed to be a known parameter. Therefore, it is concluded that typical Q(10) calculations are inappropriate for addressing questions about the regulatory control of a metabolism unless the Q(10) values are considered to be true parameters whose values are known beforehand. We offer mathematical tools to analyse the regulatory control of a metabolism for those who are willing to accept such an assumption.

Thermoregulation and the energetic significance of clustering behavior in the white-backed mousebird (Colius colius)

Thermoregulation and the energetic significance of clustering behavior were assessed in the white-backed mousebird Colius colius. Basal metabolic rate was 40% below the predicted allometric values. Rest-phase body temperature (T(b)) was highly labile and as low as 26 degrees C. Rest-phase T(b) was not regulated with respect to a constant set point temperature, as occurs typically in endotherms. Rather, we observed periods of linear decreases in rest-phase T(b) at a rate dependent on ambient temperature (T(a)) and the number of individuals in a cluster. The apparent inability of individual mousebirds to maintain rest-phase homeothermy suggests that clustering behavior is obligatory in the defense of a rest-phase set point T(b). The low rest-phase body temperatures exhibited by single C. colius hence appear to represent a normothermic state rather than typical avian facultative hypothermia. The birds were able to make significant energy savings by means of clustering behavior. These energy savings were dependent on T(a) and the number of birds in the cluster. At a T(a) of 15 degrees C, the mean energy expenditure of each bird in a cluster of six was 50% of that of a single bird. The metabolic traits of C. colius are likely be adaptive in the arid habitats that this species inhabits.

Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator

Arctic ground squirrels (Spermophilus parryii) overwinter in hibernaculum conditions that are substantially below freezing. During torpor, captive arctic ground squirrels displayed ambient temperature (T(a))-dependent patterns of core body temperature (T(b)), metabolic rate (TMR), and metabolic fuel use, as determined by respiratory quotient (RQ). At T(a) 0 to -16 degrees C, T(b) remained relatively constant, and TMR rose proportionally with the expanding gradient between T(b) and T(a), increasing >15-fold from a minimum of 0.0115 +/- 0.0012 ml O(2). g(-1). h(-1). At T(a) 0-20 degrees C, T(b) increased with T(a); however, TMR did not change significantly from T(b) 0 to 12 degrees C, indicating temperature-independent inhibition of metabolic rate. The overall change in TMR from T(b) 4 to 20 degrees equates to a Q(10) of 2.4, but within this range of T(b), Q(10) changed from 1.0 to 14.1. During steady-state torpor at T(a) 4 and 8 degrees C, RQ averaged 0.70 +/- 0.013, indicating exclusive lipid catabolism. At T(a) -16 and 20 degrees C, RQ increased significantly to >0.85, consistent with recruitment of nonlipid fuels. RQ was negatively correlated with maximum torpor bout length. For T(a) values <0 degrees C, this relationship supports the hypothesis that availability of nonlipid metabolic fuels limits torpor duration in hibernating mammals; for T(a) values >0 degrees C, hypotheses linked to body temperature are supported. Because anterior body temperatures differ from core, overall, the duration torpor can be extended in hibernating mammals may be dependent on brain temperature.

The temporal organization of daily torpor and hibernation: circadian and circannual rhythms

Mammals and birds have evolved the ability to maintain a high and constant body temperature Tb over a wide range of ambient temperatures Ta using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of Tb and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when Tb is reduced substantially and may even fall below 0 degrees C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and Ta. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low Tb.

Regulation of body temperature and energy requirements of hibernating alpine marmots (Marmota marmota)

Body temperature and metabolic rate were recorded continuously in two groups of marmots either exposed to seasonally decreasing ambient temperature (15 to 0 degrees C) over the entire hibernation season or to short-duration temperature changes during midwinter. Hibernation bouts were characterized by an initial 95% reduction of metabolic rate facilitating the drop in body temperature and by rhythmic fluctuations during continued hibernation. During midwinter, we observed a constant minimal metabolic rate of 13.6 ml O(2) x kg(-1) x h(-1) between 5 and 15 degrees C ambient temperature, although body temperature increased from 7.8 to 17.6 degrees C, and a proportional increase of metabolic rate below 5 degrees C ambient temperature. This apparent lack of a Q(10) effect shows that energy expenditure is actively downregulated and controlled at a minimum level despite changes in body temperature. However, thermal conductance stayed minimal (7.65 +/- 1.95 ml O(2) x kg(-1) x h(-1) x degrees C(-1)) at all temperatures, thus slowing down cooling velocity when entering hibernation. Basal metabolic rate of summer-active marmots was double that of winter-fasting marmots (370 vs. 190 ml O(2) x kg(-1) x h(-1)). In summary, we provide strong evidence that hibernation is not only a voluntary but a well-regulated strategy to counter food shortage and increased energy demands during winter.

Suppression of protein synthesis in brain during hibernation involves inhibition of protein initiation and elongation

Protein synthesis (PS) has been considered essential to sustain mammalian life, yet was found to be virtually arrested for weeks in brain and other organs of the hibernating ground squirrel, Spermophilus tridecemlineatus. PS, in vivo, was below the limit of autoradiographic detection in brain sections and, in brain extracts, was determined to be 0.04% of the average rate from active squirrels. Further, it was reduced 3-fold in cell-free extracts from hibernating brain at 37 degreesC, eliminating hypothermia as the only cause for protein synthesis inhibition (active, 0.47 +/- 0.08 pmol/mg protein per min; hibernator, 0.16 +/- 0.05 pmol/mg protein per min, P < 0.001). PS suppression involved blocks of initiation and elongation, and its onset coincided with the early transition phase into hibernation. An increased monosome peak with moderate ribosomal disaggregation in polysome profiles and the greatly increased phosphorylation of eIF2alpha are both consistent with an initiation block in hibernators. The elongation block was demonstrated by a 3-fold increase in ribosomal mean transit times in cell-free extracts from hibernators (active, 2.4 +/- 0.7 min; hibernator, 7.1 +/- 1.4 min, P < 0.001). No abnormalities of ribosomal function or mRNA levels were detected. These findings implicate suppression of PS as a component of the regulated shutdown of cellular function that permits hibernating ground squirrels to tolerate "trickle" blood flow and reduced substrate and oxygen availability. Further study of the factors that control these phenomena may lead to identification of the molecular mechanisms that regulate this state.

Leptin increases energy expenditure of a marsupial by inhibition of daily torpor

Leptin plays an important role in regulating body fat stores of placental mammals, but the contribution of changes in energy expenditure to this adjustment remains controversial. We were interested in how recombinant murine leptin would affect metabolic rate (MR) and body temperature (Tb) of a marsupial mammal (Sminthopsis macroura, 25 g) known to display daily torpor but lacking thermogenetically active brown adipose tissue. In a group of eight animals deprived of food for 1 day at 18 degreesC, leptin treatment halved the duration of torpor bouts (time at Tb < 30 degreesC) and raised the average daily minimum Tb by 4.5 degreesC and minimum MR by 2.2-fold. Leptin treatment thus increased daily energy expenditure by 9%, although MR and Tb during the activity phase were not raised. Body mass was also not affected. These findings in a marsupial suggest that the adjustment of thermoregulatory energy expenditure during the rest phase in accordance with energy availability is a phylogenetically old function of leptin.

Evolution of daily torpor and hibernation in birds and mammals: importance of body size

1. The evolution of hibernation and daily torpor in mammals and birds remains a controversial subject. The original view was that use of torpor reflects a primitive thermoregulation, as it occurs in ancestral groups of mammals. 2. This view is no longer widely supported. However, the interpretation of a polyphyletic derivation of torpor also has been challenged because of the astonishing similarity of torpor patterns among various orders and even the two classes. 3. A recent argument is that mutations required for torpor and hibernation are unlikely to occur simultaneously and that torpor must be plesiomorphic (ancestral), although it is not functionally primitive. Homeothermy is interpreted as a loss of the ability to enter torpor in those groups that could survive without the requirement of heterothermic periods for energy conservation. 4. Interestingly, while torpor in mammals occurs in the phylogenetically old groups, lending support to the hypothesis of an ancestral derivation of torpor, the opposite is the case for birds. Modern bird groups and ancestral mammal groups contain mainly small species that often rely on fluctuating food supply, whereas modern mammalian orders and ancient bird orders contain the largest species with low energy requirements for maintenance and thermoregulation. 5. It is, therefore, possible that not phylogenetic position but size and diet determine the occurrence of heterothermy. Moreover, because endothermy and torpor in birds has apparently evolved separately from that in mammals and because it is possible that daily torpor and hibernation represent two distinct torpor patterns that evolved separately, a convergent evolution of torpor in endotherms cannot be excluded.

Temperature selection and use of torpor by the marsupial Sminthopsis macroura

Many small mammals display daily torpor to minimize energy expenditure during the rest phase when faced with unfavorable environmental conditions. However, given a choice of thermal environments, it is not clear whether these daily heterotherms select ambient temperatures that minimize metabolic rates during torpor or ambient temperatures that minimize metabolic rates during normothermia. It is also not clear whether they prefer being normothermic or torpid. In this study, we investigated temperature selection, activity patterns, body temperature patterns, and the use of torpor in Sminthopsis macroura in a thermal gradient when food was freely available or restricted. Animals employed torpor regularly immediately after cessation of activity even when excess food was provided. Nevertheless, they selected high ambient temperature near the zone of thermoneutrality during both normothermia and torpor. Reduced food supply did not affect either temperature preference or total activity, but it did result in significantly prolonged torpor bouts. In addition, it lowered the daily minimum body temperature during torpor by about 2 degrees C. Our study shows that S. macroura selected a high ambient temperature to reduce energy costs during normothermia. Nevertheless, daily torpor was frequently employed at the high temperature, especially when food was restricted.

Thermal relations of metabolic rate reduction in a hibernating marsupial

We tested whether the reduction of metabolic rate (MR) in hibernating Cercartetus nanus (Marsupialia, 36 g) is better explained by the reduction of body temperature (Tb), the differential (delta T) between Tb and air temperature (Ta), or thermal conductance (C). Above the critical Ta during torpor (Ttc) of 4.8 +/- 0.7 degrees C where the Tb was not regulated, the steady-state MR was an exponential function of Tb (r2 = 0.92), and the overall Q10 was 3.3. However, larger Q10 values were observed at high Tb values during torpor, particularly within the thermoneutral zone (Q10 = 9.5), whereas low Q10 values were observed below Tb 20 degrees C (Q10 = 1.9). The delta T did not change over Ta 5-20 degrees C, although MR fell, and therefore the two variables were not correlated. Below the Ttc, Tb was regulated at 6.1 +/- 1.0 degrees C and MR increased proportionally to delta T. Our study suggests that MR in torpid C. nanus is largely determined by temperature effects and metabolic inhibition. In contrast, delta T explains MR only below the Ttc and C appears to affect MR only indirectly via changes of Tb, suggesting that delta T and C play only a secondary role in MR reduction during hibernation.