Natural hypometabolism during hibernation and daily torpor in mammals

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1 Respiratory Physiology & Neurobiology 141 (2004) Natural hypometabolism during hibernation and daily torpor in mammals Gerhard Heldmaier, Sylvia Ortmann 1, Ralf Elvert 2 Department of Biology, Philipps University, Karl von Frisch Strasse, D Marburg, Germany Accepted 19 March 2004 Abstract Daily torpor and hibernation are the most powerful measures of endotherms to reduce their energy expenditure. During entrance into these torpid states metabolic rate is suppressed to a fraction of euthermic metabolism, paralleled by reductions in ventilation and heart rate. Body temperature gradually decreases towards the level of ambient temperature. In deep torpor body temperature as well as metabolic rate are controlled at a hypothermic and hypometabolic level. Torpid states are terminated by an arousal where metabolic rate spontaneously returns to normal levels again and euthermic body temperature is established by a burst of heat production. In recent years some of the cellular mechanisms which contribute to hypometabolism have been disclosed. Transcription, translation, as well as protein synthesis are largely suppressed. Cell proliferation in highly proliferating epithelia like the intestine is suspended. ATP production from glucose is reduced and lipids serve as the major substrate for remaining energy requirements. All these changes are rapidly reverted to normometabolism during arousal. Hibernation and daily torpor are found in small mammals inhabiting temperate as well as tropical climates. It indicates that this behaviour is not primarily aimed for cold defense, instead points to a general role of hypometabolism, as a measure to cope with a timely limited or seasonal bottleneck of energy supply Elsevier B.V. All rights reserved. Keywords: Hibernation, torpor; Hypoxia, hypometabolism; Metabolism, reduction, hypoxia, temperature; Temperature, metabolism 1. Introduction Mammals and birds can maintain a constant body temperature of 37 C over a wide range of ambient Corresponding author. Tel.: ; fax: address: heldmaier@staff.uni-marburg.de (G. Heldmaier). 1 Present address: Institute for Zoo and Wildlife Research, D Berlin, Germany. 2 Present address: German Mouse Clinic at the GSF, D München, Munich, Germany. temperatures. This is the result of their high level of metabolic rate, generating large amounts of heat in the body, even at rest, i.e. they are tachymetabolic endotherms. Furthermore, in a cold environment they can enhance their rate of heat production, in proportion to the rate of heat loss, to maintain body temperature at a desired level. Thus, endothermy allows to maintain internal temperatures at the level of maximum physical performance. This is contrasted by ectotherms, like reptiles or invertebrates, which may experience a wide range of temperatures, often outside their range of maximum physical performance. A fur /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.resp

2 318 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) ther advantage of endotherms is that they can exploit environments where air temperatures remain below the freezing point of body fluids for extended periods of time, like birds in Antarctica and reindeer on Spitzbergen. However, these physiological and ecological advantages of endothermy are achieved only at the price of high energy costs. Endotherms require about eight times as much energy as ectotherms when compared at the same body size and temperature (Else and Hulbert, 1981). This difference becomes more than two orders of magnitude if they are compared in the cold where the metabolic rate of ectotherms is lowered due to thermodynamic temperature effects, whereas metabolic rate of endotherms is raised by thermoregulatory heat production. The life of endotherms, therefore, depends upon a continuous high supply of food and energy is wasted as heat, whereas ectotherms require much less food, and can invest a greater percentage of food into their own growth and reproduction. Despite the fact that mammals are considered as tachymetabolic endotherms, they may undergo periods of hypometabolism. A small degree of hypometabolism is regularly observed during the 24 hourly rhythm of activity and rest. During the resting phase metabolic rate of most mammals decreases by about 20% below the level of metabolic rate during the activity phase. This shallow circadian hypometabolism is often associated with a 0.5 through 2 C decrease in core body temperature. In birds this response is even more exaggerated, and the shallow circadian hypometabolism may be as large as 30% and the shallow circadian hypothermia may be as large as 4 C (Aschoff and Pohl, 1970). In addition to this common response a large number of mammals and birds may enter much more extended periods of hypometabolism and hypothermia, like daily torpor, estivation, or hibernation (Lyman, 1982; Heldmaier and Ruf, 1992; Geiser and Ruf, 1995). During these states of hypometabolism metabolic rate can be lowered to a fraction of basal metabolic rate. In this study, the extent of hypometabolism and hypothermia during daily torpor and hibernation in mammals is compared, and the current knowledge about physiological and biochemical mechanisms of hypometabolism is reviewed. 2. Methods Alpine marmots (Marmota marmota), Common dormice (Glis glis), and Djungarian hamsters (Phodopus sungorus) were bred and raised at the Department of Biology at Philipps University Marburg. They were implanted with temperature transmitters into the abdominal cavity. For the study of hypometabolic states they were kept in metabolic cages, which were placed in climate chambers at constant ambient temperature and constant darkness. Animals could freely move about and were left undisturbed (IR-video observation). Body temperature, oxygen consumption, and carbon dioxide production were recorded continuously in 1 min intervals for periods of up to several weeks. For details of methods see Heldmaier and Ruf (1992), Ortmann and Heldmaier (2000) and Wilz and Heldmaier (2000). 3. Hibernation Marmots are strictly seasonal hibernators. Their hibernation season begins late in September or early October and lasts through the end of March or early April. The entire hibernation season consists of a sequence of hibernation bouts. Each bout is characterised by four different states, i.e. entrance into hibernation, maintenance of deep hibernation for several days, which is terminated by an arousal, and followed by a euthermic period of 1 2 days. An example for the hibernation bout of an Alpine marmot is given in Fig. 1. Total duration of this hibernation bout was 6 days, which is a relatively short bout duration, frequently observed early in the hibernation season. Mean torpor bout length of marmots is 11.7 days, but in midwinter individual hibernation bouts may last up to 20 days. Thus, the entire hibernation season of 6 7 months (mean value 193 days) consists out of a sequence of 15 through 20 hibernation bouts (Heldmaier et al., 1993a,b; Körtner and Heldmaier, 1995; Ruf and Arnold, 2000). Entrance into hibernation is characterised by a rapid reduction of metabolic rate, until the minimum metabolic rate in hibernation (0.014 mlo 2 g 1 h 1 ) is reached, which is only a fraction (1/25th) of euthermic resting metabolic rate (0.34 mlo 2 g 1 h 1 ) (Fig. 1). Metabolic reduction is paralleled by a rapid decrease of body temperature, which continues into

3 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) entry deep hibernation euthermia entry arousal T b metabolic rate (mlo2 g -1 h -1 ) temperature ( C) T a MR days 0 Fig. 1. Hibernation bout in an Alpine marmot (Marmota marmota). A continuous record of metabolic rate and body temperature reveals the development of hypometabolism and hypothermia during entrance into hibernation (1), maintenance of hypometabolism during deep hibernation (2), rapid rewarming during arousal (3), and a constant high level of metabolic rate and body temperature in the euthermic state (4). MR metabolic rate, T b body temperature, T a ambient temperature. Data redrawn from Ortmann and Heldmaier (2000). deep hibernation as an exponential decline. Marmot body temperatures always remained C above ambient temperature. During deep hibernation metabolic rate of marmots occasionally showed short bursts of activity (Fig. 1). They were paralleled by slight increases in body temperature, and may thus be interpreted as bursts of thermoregulatory heat production to reduce the development of hypothermia. Deep hibernation is terminated by an arousal, and marmots raise their body temperature to the euthermic level within about 3 h by a burst of maximum heat production. The time required for arousal from hibernation, the duration of the euthermic period and entrance into hibernation show only little variation. Thus, the duration of the hibernation bout depends largely upon the time spent in deep hibernation. Similar patterns of hibernation have been found in all hibernators studied so far, in eutherian mammals, in marsupials and even in monotremes (Wang, 1989; Heldmaier et al., 1993a; Grigg and Beard, 2000; Nicol and Andersen, 2000). The repeated arousals and the euthermic periods are relatively short but nevertheless they are energetically very costly. In the marmot 72% of all energy reserves required for the entire hibernation season are consumed during arousals (17%) and the euthermic periods (57%) (Heldmaier et al., 1993a). Similar estimates were also obtained for other hibernators (Wang, 1978, 1989). The physiological reasons for these energetically costly arousals are still not known. However, it is clear that more energy can be saved during hibernation when the duration of deep hibernation is extended. Very long hibernation bouts of more than 30 days have been observed in bats and dormice, indicating that they use energy savings by hypometabolism rather efficiently. 4. Daily torpor Small mammals and hummingbirds may use their circadian resting period for a few hours of hypometabolism and hypothermia, instead of hibernation (Fig. 2). A bout of daily torpor follows the same sequence of events as observed in hibernation, i.e. entrance into torpor, maintenance of deep torpor, arousal, and return to the euthermic state. At 13 C ambient

4 320 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) metabolic rate (mlo2 g -1 h -1 ) 4 2 MR T b temperature ( C) time of day (h) Fig. 2. Daily torpor in the Djungarian hamster (Phodopus sungorus). During the noturnal activity period the hamster displayed an ultradian pattern of metabolic rate and body temperature which is largely associated with the ultradian pattern of locomotor activity. At the end of his nocturnal activity period he entered torpor, remained in deep torpor for the entire diurnal resting period, and spontaneously returned to euthermia prior to his nocturnal activity period. T a temperature the hamster shown in Fig. 2 (30.8 g body mass) had an average metabolic rate during the nocturnal activity period of 2.63 mlo 2 g 1 h 1 (minimum resting metabolic rate of 2.1 mlo 2 g 1 h 1 ). During the diurnal resting period he entered torpor and lowered its metabolic rate to about 0.5 mlo 2 g 1 h 1 within 40 min (average metabolic rate during the entire torpor phase mlo 2 g 1 h 1, lowest value mlo 2 g 1 h 1 ), i.e. metabolic rate is reduced to about 1/4th of the euthermic value. Body temperature decreased from 34.2 to 17.9 C towards the end of the torpor period. Deep torpor was maintained for about 9 h. It was terminated by an arousal, and within 30 min the hamster raised its body temperature to the euthermic level again. During daily torpor the extent of hypometabolism and hypothermia is usually less pronounced as compared to hypometabolism in hibernation (Geiser and Ruf, 1995). However, in some species, like dormice or elephant shrews, torpid metabolic rates can be as low as minimum metabolic rate in hibernation (Lovegrove et al., 1999; Wilz and Heldmaier, 2000). It has been discussed whether daily torpor, estivation and hibernation are based on different physiological adaptations. At present no clear qualitative differentiation is known. The physiological properties of daily torpor, estivation, and hibernation are very similar. All are characterised by major reductions of metabolic rate, heart rate, ventilation, body temperature, and in the torpid state thermoregulatory control is maintained. This suggests that they are based on a common paradigm of physiological inhibition. The classifications of hibernation, daily torpor, or estivation simply represent gradual differences in the timing, the duration, and the amplitude of physiological inhibition. Lowest values of metabolic rate and body temperature are usually reached in hibernation (Heldmaier and Ruf, 1992; Geiser and Ruf, 1995; Wilz and Heldmaier, 2000). 5. Initiation of hypometabolism during entrance into torpid states Entrance into hibernation is initiated by a depression of metabolic rate which anticipates the decrease in body temperature (Fig. 1). The same is true for entrance into daily torpor (Fig. 2). Minimum metabolic rates are reached relatively early in the torpid phase, and this point we have chosen to mark the end of entry into hibernation or torpor. Body temperature is not a suitable parameter to differentiate between entrance and deep hibernation, since it continues to decline exponentially towards the end of the hibernation bout, or may even be elevated again by bursts of heat production.

5 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) ,10 intermittent ventilation in hibernation 40 metabolic rate (mlo 2 h -1 ) T b MR O 2 Analyser (V) 0,05 0, time (hours) body temperature ( C) T a time (hours) Fig. 3. Entrance into hibernation in a common dormouse. Metabolic rate was measured in a small cuvette and at high flow rates to obtain a short lag time and high resolution. Metabolic depression precedes the development of hypothermia. Towards the end of entrance into hibernation the ventilation pattern becomes intermittent. The inset shows an enlarged section of the O 2 analyser output (volts, V), illustrating intermittent ventilation with extended periods ( 30 min) of apnoea. A more detailed analysis of entrance into hibernation was made in the common dormouse (Fig. 3). They hibernated in small sleeping boxes that permitted measurement of metabolic rate with a short time constant and high resolution, and allowed direct comparison of changes in body temperature and metabolic rate. The results show that the transition into hypometabolism is initiated by a rapid depression of metabolic rate which clearly precedes the development of hypothermia. Dormice show intermittent ventilation during hibernation, which can be seen as bursts of oxygen consumption in this record. The transition from continuous to intermittent ventilation is already established during entrance into hibernation, before minimum metabolic rate is reached. These examples indicate that hibernation and daily torpor are initiated by an active depression of metabolic rate below the level of BMR. This, however, does not mean that endothermic control over metabolic rate is suspended. In squirrels central thermosensitivity is maintained during entrance into hibernation, but the threshold for thermoregulatory responses is gradually shifted from the euthermic to the torpid level (Heller et al., 1977). If core temperature decreases too fast and drops below the sliding set-point of body temperature hibernators are able to stimulate thermoregulatory heat production, although they are in a transient state of metabolic depression. Such bursts of heat production decelerate the development of hypothermia, or even slightly increase body temperature again. This indicates that hibernating endotherms deliberately enter hypometabolism but maintain thermal and metabolic homeostasis during entrance into the torpid state. Body temperature decreases during entrance into hibernation, and this hypothermia may additionally contribute to the depression of metabolic rate. Hypoxia as well as hypercapnia have been suggested as factors involved in metabolic depression. However, there is no evidence so far that a lack of oxygen or an accumulation of carbon dioxide are a suitable stimulus for entrance into hibernation in physiological conditions. During entrance into hibernation carbon dioxide may accumulate in body fluids, which relates to the control of a constant ph at low temperature (Malan et al., 1973; Malan, 1988; Elvert

6 322 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) and Heldmaier, 2000). Metabolic depression during entrance into hibernation is initiated in the normoxic state, and even during prolonged bouts of deep hibernation normoxia of body fluids is maintained (Tähti and Soivio, 1973; Thomas et al., 1990; Szewczak and Jackson, 1992b). Hibernation of endotherms can, therefore, be characterised as a voluntary, normoxic hypometabolism. This is in contrast to hibernation in ectotherms. In hibernating turtles, frogs, or invertebrates metabolic depression is closely related with hypoxia, and may even be initiated by hypoxia (Hand and Hardewig, 1996; Boutilier, 2001). Turtles hibernate in lakes submerged for several months, where they tolerate prolonged periods of hypoxia, and may even be forced to tolerate anoxic conditions when the lakes are frozen (Jackson, 2002; Reese et al., 2003). The physiological mechanisms of metabolic depression during hibernation in endotherms, as well as the biochemical pathways involved, are only partly known. Theoretically, metabolic depression can be achieved either by a reduction of ATP synthesis, or by an inhibition of processes consuming ATP. There is some evidence that liver and muscle glycolysis is inhibited during entrance into torpid states (Hochachka, 1986; Storey, 1997; Heldmaier et al., 1999), and substrate utilisation is shifted towards lipolysis (Andrews et al., 1998). Furthermore, mitochondrial respiration is reversibly depressed in deep hibernation (Martin et al., 1999). In summary these changes suggest a lowered rate of ATP synthesis. Transcription, translation, and biosynthesis of proteins and other polymers are energetically demanding processes, and consume considerable amounts of ATP. The biosynthesis of polymers is largely depressed during hibernation (van Breukelen and Martin, 2001, 2002). This is in line with the observation that growth of epithelial cells in rapidly proliferating epithelia like the intestine are largely halted during hibernation (Carey, 1990; Carey et al., 2003). Buttgereit and Brand (1995) have shown that there is a hierarchy of ATP consuming processes in cells, and a shortage of energy supply will first inhibit pathways of macromolecule biosynthesis, followed by sodium cycling and then calcium cycling across the plasma membrane. It has been estimated that transcription, translation and the synthesis of macromolecules may be held responsible for about 20 30% of cellular energy expenditure (Buttgereit and Brand, 1995; Wieser and Krumschnabel, 2001). Their inhibition may contribute to metabolic rate reduction in hibernation or daily torpor, but total reduction of metabolic rate is much larger, suggesting that further unknown pathways are involved. Furthermore, the endocrine or neural signalling which initiates metabolic depression during entrance into hibernation is still a mystery, and is certainly one of the most challenging questions in hibernation research. It is not clear how normoxic hypometabolism of endotherms relates to hypoxic hypometabolism of ectotherms (Hochachka, 1986; Storey and Storey, 1990). Metabolic rates during hypoxic hypometabolism of some ectotherms like frogs or turtles are even less than in hibernating endotherms (Donohoe et al., 1998). During hypoxic hypometabolism it was found that translation is inhibited, and mitochondrial respiration is largely suppressed (Hofmann and Hand, 1994; Boutilier, 2001). This indicates at least some similarity of biochemical changes in normoxic and hypoxic hypometabolism, although the initial stimulus for the entrance into hypometabolism may be different. 6. Control of metabolic rate in hibernation In deep hibernation metabolic rate is maintained at a fraction of basal metabolic rate for several days, or up to several weeks. The low level of metabolic rate depends upon active metabolic inhibition during entrance into hibernation, and on the thermodynamic effect of low body temperature which contributes to the deceleration of biochemical reactions. During hibernation body temperature gradually approaches ambient temperature in the hibernaculum. There is a range of ambient and body temperature where hibernators show longest torpor bouts and lowest metabolic rates, which can be considered as optimum temperature for hibernation. Different species of hibernators do have different temperature ranges where they can hibernate and a more narrow temperature range where they prefer to hibernate. Marmots can hibernate at body temperatures between 2.5 and 18 C, but prefer to hibernate at body temperatures between 5 and 10 C(Ortmann and Heldmaier, 2000; Ruf and Arnold, 2000). Other hibernating rodents usually prefer slightly lower body temperatures during hibernation, ranging between 1 and 6 C, and may

7 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) metabolic rate (mlo2 g -1 h -1 ) body temperature ( C) Fig. 4. Metabolic rate as related to body temperature in hibernating marmots. Data converted from Ortmann and Heldmaier (2000) (mean ± S.E.M.). even tolerate supercooling at 3 C(Barnes, 1989). Dormice and Malagasy lemurs may hibernate with body temperatures above 20 C (Dausmann et al., 2000; Elvert and Heldmaier, 2000) and bears prefer to hibernate with body temperatures above 30 C(Hock, 1960). In order to analyse the effect of body temperature on metabolic rate in hibernation marmots were exposed to ambient temperatures from 0 C to about 18 C. Each temperature was kept constant for days or weeks, to record complete hibernation bouts. At temperatures below 1 C and above 20 C marmots failed to enter hibernation spontaneously (Ortmann and Heldmaier, 2000). Hibernating marmots maintained a constant minimum metabolic rate in hibernation at body temperatures between 7 and 18 C (corresponding ambient temperatures 4 and 15 C) (Fig. 4). This is not consistent with the simple concept that low body temperature is the cause of low metabolic rate in hibernation, because it predicts that metabolic rate at 18 C should be three times greater than at 7 C. The constant metabolic rate over this range of body temperatures suggests that metabolic rate in deep hibernation is temperature compensated. Similar temperature ranges of constant metabolic rate have also been found in ground squirrels and dormice (Heldmaier et al., 1993b; Buck and Barnes, 2000; Wilz and Heldmaier, 2000), suggesting that temperature compensation may be a general property of downregulated metabolism in hibernation. At body temperatures below 7 C the marmots increased their metabolic rate (Fig. 4). This was caused by thermoregulatory heat production. At ambient temperature below 4 C marmots cannot maintain their desired high level of body temperature with minimum metabolic rate in hibernation. They raise their metabolic heat production to compensate for the increasing heat loss. This allows them to continue hibernation with a body temperature of about 5 C even when ambient temperature decreases to 1 C. If metabolic rate in hibernation would be the result of aq 10 effect one would expect lowest metabolic rates at lowest body temperatures. However, this is not the case. In contrast hibernation at ambient and body temperatures below the preferred hibernation levels require additional energy for thermoregulation, which accelerates the combustion of body lipid stores, and reduces the time which can be spent in hibernation. Burrow temperatures in the hibernacula of marmots are about 14 C early in the hibernation season, but decrease to zero or even below in January and February. This causes a critical thermal and energetic load to marmots towards the end of their hibernation season (Arnold et al., 1991; Ruf and Arnold, 2000).

8 324 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) At body temperatures above 18 C hibernating marmots may also raise their metabolic rate, and above 21 C no distinct hibernation bouts were observed. The reason for this increase in metabolic rate is not known. It could be a temperature effect, indicating that marmots were not able to maintain minimum hibernation metabolism at this high level of body temperature. This hypothesis is supported from observations during entrance into hibernation, where minimum levels of metabolic rate were never reached before body temperature had decreased below 25 C. In deep hibernation all physiological functions related to metabolic rate are reduced to a similar extent. Heart rate of dormice decreases from about 450 min 1 in euthermia at 5 C ambient temperature to about 10 min 1 in hibernation at the same temperature. However, the oxygen pulse, i.e. the amount of oxygen consumed per heart beat, remains unchanged and varied between and mlo 2 in both states (Elvert and Heldmaier, 2000). Ventilation rate decreased in parallel with heart rate, but changed its pattern from continuous to intermittent ventilation (Fig. 3). This is a widespread ventilation pattern in hibernators, which only occurs during metabolic depression in hibernation (Milsom, 1992). In dormice the periods of apnoea lasted up to 60 min. The intermittent pattern of ventilation was maintained through the entire temperature range at minimum metabolic rate. When metabolic rate increased for thermoregulation, close to 0 C, the dormice returned to a periodic breathing pattern again. It has been suggested that the basic brain stem generates a pattern of evenly spaced breaths, but this is converted to an episodic breathing by a supramedullary input during metabolic depression (Milsom, 1992). Blood gases change drastically during the bursts of intermittent ventilation, which stimulates the thresholds for initiation and termination of a burst of breaths. The oxygen consumption per breath of a euthermic dormouse ranges between 0.04 and 0.07 ml. In hibernation this value drops to about ml. This indicates that the oxygen supply by ventilation by far exceeds the oxygen needs of the animal, i.e. they apparently hyperventilate with regard to oxygen supply, even during low frequency intermittent ventilation. There is also evidence that hibernators may obtain oxygen by diffusion during apnoea in deep torpor, e.g. through the trachea, and therefore, need only very little additional ventilation for the uptake of oxygen (Thomas et al., 1990; Szewczak and Jackson, 1992b; Wilz et al., 2000). The expiration of carbon dioxide is influenced by temperature effects and the role of the bicarbonate buffer system in deep hibernation. At low body temperature an increase of ph is expected due to temperature effects on the dissociation of water, and the solubility of carbon dioxide in body fluids increases. In hibernation the blood pco 2 is maintained constant at about 40 mmhg, causing an apparent acidosis of body fluids, which could contribute to hypometabolism (Malan et al., 1973; Malan, 1988; Szewczak and Jackson, 1992a). In dormice we observed that the RQ remained constant at during torpor at moderately high ambient and body temperatures, however, at low temperatures the RQ decreased to about 0.2 during entrance into torpor in parallel with the decrease of body temperature, indicating retention of carbon dioxide in body fluids (Elvert and Heldmaier, 2000). The large storage capacity for carbon dioxide, in combination with the low metabolic rate, allow extended periods of apnoea in hibernation at low body temperature. 7. Comparative physiology of hypometabolism Hibernation and daily torpor have been observed in a large number of small mammals. This behaviour is not limited to one or few mammalian orders, instead in most recent mammalian orders a few species, genera or families are known as hibernators or torpidators (Fig. 5). It is most frequently observed in mammalian orders with species of small size, but is largely lacking in orders with large species, like elephants and horses. Hares and tree-shrews are the only orders with small species where the evidence for torpid states is lacking. However, it is not clear whether they never become torpid, or if it simply has not been detected yet. One of the most easily measured properties of torpid states is the development of hypothermia, which however, is not a decisive criterion of hypometabolism. Small mammals living in well insulated nests at high ambient temperature will show only small changes of body temperature. These may be overlooked and

9 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) Fig. 5. Occurrence of hibernation ( ) and daily torpor ( ) in mammals. Mammalian orders are labelled by symbols when at least one species has been reported to show either daily torpor or hibernation. In some groups like marsupials, chiroptera, and rodents a large number of species show either one or both of these hypometabolic behaviours. Phylogenetic tree according to Carroll (1997). one would need metabolic rate measurements instead to prove hypometabolism. The same is true for large mammals, which have a low thermal conductance and high thermal inertia due to their body size, and may show only small changes in body temperature during short periods of hypometabolism. An example for this is the recent discovery of nocturnal hypometabolism in overwintering red deer, which provided the first evidence for hypometabolism in artiodactyla (Arnold et al., 2004). This indicates that the ability for hypometabolism is widespread amongst mammals, and may even be considered as a basic property of mammalian physiology. Since hibernation and torpor is also found in a number of bird species, it is most likely that this behaviour is based on ancient traits of vertebrate physiology. Mammals cover a wide range of body size, ranging from small Etruscan shrews (2 g) through 5 t elephants. If we include aquatic mammals the maximum body mass of mammals is shifted to 50 t blue whales. Metabolic rate is allometrically related to body size, and in an interspecific comparison basal metabolic rate scales to a body mass exponent of 0.29 (Fig. 6) (BMR = 5.01M ; where BMR is in mlo 2 g 1 h 1 and M in g body mass) (Heldmaier, 2003)). There are slightly different allometric relations for basal metabolic rate in different mammalian orders, but a comparison across all mammals, in-

10 326 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) mouse hamster squirrel rat Euthermia BMR = 3.4 M metabolic rate (ml O 2 g -1 h -1 ) 0,1 bat mouse Daily Torpor marsupial dormouse squirrel rabbit fox marmot hedgehog marmot dog sheep bear chimpanzee man pig cattle horse elephant HibMR = 0.03 Hibernation 0, body mass (g) Fig. 6. Minimum metabolic rate in hibernation ( ) and daily torpor ( ) as related to body size in mammals. To illustrate the extent of metabolic depression basal metabolic rate of euthermic mammals is included (, Heldmaier, 2003). Data for hibernation at 2 7 C ambient temperature, and for daily torpor at C ambient temperature from Heldmaier and Ruf (1992), Geiser and Ruf (1995), Ortmann and Heldmaier (2000) and Buck and Barnes (2000). cluding more than 250 species, revealed a body mass exponent of 0.29 rather than the exponent of 0.25 originally suggested by Kleiber (1932) (Hayssen and Lacy, 1985; Lovegrove, 2003; Heldmaier, 2003). The physiological significance and control of basal metabolic rate is not known, and the same is true for its ecological significance since most animals live most of their time at higher metabolic rates. However, basal metabolic rate is operationally well defined as the minimum metabolic rate required for maintenance of all physiological functions in a resting euthermic mammal. During hibernation metabolic rate is lowered to a fraction of this basal metabolic rate (Fig. 6). Hibernators can live for weeks at this low level of metabolic rate and arouse again without loss of physiological functions. Minimum metabolic rate in hibernation can be considered as a pilot-light which maintains basic functions of cellular integrity, and may also ignite the metabolic machinery for return to tachymetabolism in euthermia. Minimum metabolic rate in hibernation does not show an allometric relation with body size, instead all hibernators have a constant metabolic rate of about 0.03 mlo 2 g 1 h 1 (Fig. 6). Due to the allometric increase of euthermic metabolic rate, the amplitude of metabolic rate reduction in hibernation is largest in small hibernators, where it may be reduced to more than 1/100th of the euthermic value. This amplitude decreases with increasing body mass, due to the reduction of basal metabolic rate with increasing body mass. If the two regression lines are extrapolated beyond the range of data they meet at a body mass of 15 t, i.e. in a theoretical mammals of this size basal metabolic rate equals minimum metabolic rate in hibernation. Therefore, energy savings by hibernation are most effective in small mammals, and its significance decreases with increasing body size. It is in accordance with the observation that hibernation is most frequently found in small mammals, and only rare examples for hibernation exist in larger sized mammals (marmot, bear). This is even more obvious for the use of daily torpor, where metabolic rate reduction is less pronounced, and which is most frequently found in mammals below 200 g body mass.

11 G. Heldmaier et al. / Respiratory Physiology & Neurobiology 141 (2004) Comparison of energy savings by hibernation and daily torpor During hibernation small mammals can save up to 98% of their energy requirements as compared to the euthermic state (Wang, 1978, 1989). These are maximum values of energy savings, which can only be achieved by very small mammals, hibernating at temperature and humidity conditions which allow the use of very long torpor bouts with only short arousal episodes. Average values of energy savings by hibernation will be in the order of about 90%. During preparation for hibernation most small mammals accumulate lipid stores which amount to 20 30% of their body weight, e.g. a 300 g hibernator like a squirrel has a maximum lipid store of about 90 g. Its daily resting energy requirements in euthermia would be about 138 kj, requiring the combustion of 3.5 g lipids per day (BMR = 5.01M ; 2010 mlo 2 consumed for the oxidation of 1 g lipid (Heldmaier, 2003)). If energy requirements can be reduced by 90% in hibernation its lipid store would support the maintenance of a hibernation season for 257 days (90/0.35). Energy savings by daily torpor are less pronounced. For example a bout of daily torpor may last up to 12 h per day and during this period metabolic rate is reduced to about 1/4th of the active metabolic rate, which would reduce total daily energy requirements in the case of maximum torpor durations only by 40%. However, long term energy budgets in torpid hamsters revealed that they actually save up to 67% of their energy requirements as compared to hamsters which remained active all the time (Ruf and Heldmaier, 1992). The greater energy savings by daily torpor can be explained by the activity behaviour of hamsters. Feeding activity in the cold, outside the well insulated nest, causes a major portion of their daily energy expenses. Torpid hamsters can reduce this activity because they need less food. This reduces their energy expenditure for foraging, digestion, resorption and processing of food compounds. Therefore, two factors contribute to the reduction of energy requirements in torpidators, first the immediate reduction of metabolic rate in torpor, and secondly, this effect is enhanced by the reduction of energy requirements for feeding related activities. Similar relations have also been found for the use of torpor in lemurs (Ortmann et al., 1997; Schmid et al., 2000; Heldmaier and Klingenspor, 2002), indicating that daily torpor is a surprisingly efficient strategy for reduction of total energy requirements by 60 70%. Nevertheless, hibernation is a more effective strategy for energy savings, and it appears questionable why small mammals use daily torpor instead. Hibernation requires long term seasonal preparations, which include the accumulation of energy reserves as internal lipid stores or external food, and the selection of thermally stable burrows, caves or other hibernacula. During hibernation all territorial, social and reproductive activities are reduced or even suspended and hibernators retreat into their well protected hibernacula. Therefore, hibernation can only be organised as an obligatory behaviour, and is part of a fixed seasonal timing program. In contrast daily torpor allows to maintain territorial and social activities. The incidence and duration of daily torpor can be varied from day to day. This allows torpidators to adjust their energy requirements day per day, in response to food availability, temperature and other energetic challenges. All torpidators show a high variability in the incidence of daily torpor, and reductions in food supply are a very potent stimulus for daily torpor (Heldmaier and Steinlechner, 1981; Ruf and Heldmaier, 1992). In essence daily torpor allows the reduction of daily energy requirements by 60%, but at the same time territorial and social activities can be maintained, which makes daily torpor an attractive alternative to hibernation for seasonal reductions of energy requirements. Hibernation and daily torpor have been most frequently observed in small mammals living in cold environments. In the torpid state they undergo massive changes in their behaviour. An animal that was actively running around, may be found motionless a few hours later, cold to the touch, and without any respiratory movements or other visible signs of life. Since hypothermia is the most obvious and easily measured parameter, it was tempting to conclude that hibernation and torpor are strategies to cope with the cold. However, torpid states may also be entered in warm environments, as is known for diurnal torpor in bats during summer, which may even occur when they are resting in the woodwork of roofs during a hot summer day. Similarly, nocturnal torpor of hummingbirds may be displayed at rather moderate ambient temperatures in their tropical environment. Bears hibernate in cold dens, but maintain a high body temperature

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