Long-term telemetry of heart rates and energy metabolic rate during the diurnal cycle in normothermic and torpid African blue-naped mousebirds (Urocolius macrourus)

Colies are one of the phylogenetically oldest groups among the modern birds; the earliest finds are from about 35 million years ago. In states of energy deficiency they can undergo torpor during the night when metabolic rate and body temperature are decreased drastically to save energy (up to 90%). Here, we report the first measurements of heart rate (HR) by long-term telemetry, in seven individuals of blue-naped mousebirds (Urocolius macrourus); simultaneously and continuously metabolic rate (MR) was determined. HR at night was about 20% below the range of expected values (246/310 bpm). Mean oxygen pulse (O2 output/stroke) in normothermic birds was in a range of 0.019-0.020 ml O2/stroke; during torpor nights this value decreased significantly to 0.0086. Mean cardiac output ranged from 724 to 1214 ml blood/kg per min; in torpid birds this value fell to 400 ml blood/kg per min. Cardiac regulation of metabolic demand within an activity phase (day or night) is mainly achieved by chronotropy. Inotropy contributes at most 25% to the differences in MR between day and night (ca. 40%). Entry into torpor is brought about mainly by changes in HR (decrease from 240 to 90 bpm); after torpor levels have been reached, there is an increase in HR (to 200 bpm) and a sharp decrease (-53%) in stroke volume. This regulation by inotropy is also characteristic of arousal from torpor.     

Heterothermy in an Australian passerine, the Dusky Woodswallow (Artamus cyanopterus)

Information regarding passerine heterothermy and torpor is scant, although many species are small and must cope with a fluctuating food supply and presumably would benefit from energy savings afforded by torpor. We studied whether insectivorous Dusky Woodswallows (Artamus cyanopterus; ∼35 g) enter spontaneous torpor (food ad libitum) when held outdoors as a pair in autumn/winter. Woodswallows displayed pronounced and regular daily fluctuations in body temperature (T _b) over the entire study period. The mean T _b ranged from ∼39°C to 40°C (photophase, day time) and ∼33°C to 36°C (scotophase, night time). However, on 88% of bird nights, nocturnal T _b minima fell to < 35°C. The lowest T _b observed in air was 29.2°C. However, when a bird fell into water its T _b dropped further to ∼22°C; this T _b was regulated for several hours and the bird survived. Our observations suggest that heterothermy is a normal part of the daily thermal regime for woodswallows to minimise energy expenditure. Spontaneous nocturnal torpor in captive woodswallows suggests that torpor in the wild may be more pronounced than recorded here because free-living birds are likely challenged by both low food availability and adverse weather.     

Daily torpor in the gray mouse lemur (Microcebus murinus) in Madagascar: energetic consequences and biological significance

Patterns and energetic consequences of spontaneous daily torpor were measured in the gray mouse lemur (Microcebus murinus) under natural conditions of ambient temperature and photoperiod in a dry deciduous forest in western Madagascar. Over a period of two consecutive dry seasons, oxygen consumption (VO₂) and body temperature (T _b) were measured on ten individuals kept in outdoor enclosures. In all animals, spontaneous daily torpor occurred on a daily basis with torpor bouts lasting from 3.6 to 17.6 h, with a mean torpor bout duration of 9.3 h. On average, body temperatures in torpor were 17.3±4.9°C with a recorded minimum value of 7.8°C. Torpor was not restricted to the mouse lemurs’ diurnal resting phase: entries occurred throughout the night and arousals mainly around midday, coinciding with the daily ambient temperature maximum. Arousal from torpor was a two-phase process with a first passive, exogenous heating where the T _b of animals increased from the torpor T _b minimum to a mean value of 27.1°C before the second, endogenous heat production commenced to further raise T _b to normothermic values. Metabolic rate during torpor (28.6±13.2 ml O₂ h^–1) was significantly reduced by about 76% compared to resting metabolic rate (132.6±50.5 ml O₂ h^–1). On average, for all M. murinus individuals measured, hypometabolism during daily torpor reduced daily energy expenditure by about 38%. In conclusion, all these energy-conserving mechanisms of the nocturnal mouse lemurs, with passive exogenous heating during arousal from torpor, low minimum torpor T _bs, and extended torpor bouts into the activity phase, comprise an important and highly adapted mechanism to minimize energetic costs in response to unfavorable environmental conditions and may play a crucial role for individual fitness. Received: 8 July 1999 / Accepted: 3 December 1999     

Comparison of hibernation, estivation and daily torpor in the edible dormouse, Glis glis

Three major forms of dormancy in mammals have been classified: hibernation in endotherms is characterised by reduced metabolic rate (MR) and body temperature (T _b) near ambient temperature (T _a) over prolonged times in the winter. Estivation is a similar form of dormancy in a dry and hot environment during summertime. Daily torpor is defined as reduced MR and T _b lower than 32 °C, limited to a duration of less than 24 h. The edible dormouse (Glis glis) is capable for all three distinct forms of dormancy. During periods of food restriction and/or low T _a, daily torpor is displayed throughout the year, alternating with hibernation and estivation in winter and summer respectively. We recorded T _b, O₂-consumption and CO₂-production in unrestrained dormice at different T _a's for periods of up to several months. Cooling rate and rate of metabolic depression during entrance into the torpid state was identical in all three forms of dormancy. The same was true for thermal conductance, maximum heat production, duration of arousal and cost of an arousal. The only difference between hibernation and daily torpor was found in the bout duration. A daily torpor bout lasted 3–21 h, a hibernation bout 39–768 h. As a consequence of prolonged duration, MR, T _b and also the T _b − T _a gradient decreased to lower values during hibernation bouts when compared to daily torpor bouts. Our findings suggest that all three forms of dormancy are based on the same physiological mechanism of thermal and metabolic regulation. Accepted: 27 June 2000     

Cool birds: first evidence of energy-saving nocturnal torpor in free-living common swifts Apus apus resting in their nests

Daily torpor is a means of saving energy by controlled lowering of the metabolic rate (MR) during resting, usually coupled with a decrease in body temperature. We studied nocturnal daily torpor under natural conditions in free-living common swifts Apus apus resting in their nests as a family using two non-invasive approaches. First, we monitored nest temperature (T_nest) in up to 50 occupied nests per breeding season in 2010–2015. Drops in T_nest were the first indication of torpor. Among 16 673 observations, we detected 423 events of substantial drops in T_nest of on average 8.6°C. Second, we measured MR of the families inside nest-boxes prepared for calorimetric measurements during cold periods in the breeding seasons of 2017 and 2018. We measured oxygen consumption and carbon dioxide production using a mobile indirect respirometer and calculated the percentage reduction in MR. During six torpor events observed, MR was gradually reduced by on average 56% from the reference value followed by a decrease in T_nest of on average 7.6°C. By contrast, MR only decreased by about 33% on nights without torpor. Our field data gave an indication of daily torpor, which is used as a strategy for energy saving in free-living common swifts.     

Metabolic and cardiovascular adaptations in the western chipmunks, genusEutamias

Summary The metabolic and cardiac responses to temperature were studied in two species (four subspecies) of western chipmunks (genusEutamias), inhabiting boreal and alpine environments. A specially designed (Fig. 1) implantable biopential radiotransmitter was used to measure heart rate in unrestrained animals. The estimated basal metabolic rates (EBMR) were 1.78 (E. minimus borealis), 1.64 (E. m. oreocetes), 1.50 (E. m. operarius), and 1.69 ml O₂ g^–1 h^–1 (E. amoenus luteiventris), or 839, 752, 698, and 628 ml O₂ kg^–0.75 h^–1, respectively, for the four subspecies (Table 1). The two alpine species (E.m.or. andE.m.op.) had significantly lower EBMR than both of their boreal counterparts. The EBMR from all animals are 120–135% of the predicted values based on body weights of the animals. The thermal neutral zone for the four subspecies ranged from 23.5 to 32°C and the minimum thermal conductances were 0.113, 0.111, 0.112 and 0.112 ml O₂ g^–1 h^–1 °C^–1, respectively, or 54.4, 54.0, 50.4 and 52.1 ml O₂ kg^–0.75 h^–1 °C^–1, respectively (Fig. 2). No interspecific diffence in conductance was observed. These values are 72 to 85% of their weight specific values. The body temperature ranged between 35.0 and 39.5°C and was usually maintained between 36 and 38°C in all subspecies between ambient temperatures of 3 and 32°C. The estimated basal heart rates were 273, 296, 273 and 264 beats/min, respectively, for the four subspecies, 49–55% of their predicted weight specific values. The resultant oxygen pulses (metabolic rate/heart rate) were 5.49, 4.50, 4.48 and 5.56×10^–3 ml O₂/beat, respectively, which are 2 to 2.4 times their weight specific values (Table 2).The observed reduction of basal heart rate without the corresponding decreases of basal metabolic rate and body temperature indicate sufficient compensatory increases in stroke volume and/or A-V oxygen difference at rest. Such cardiovascular modifications provide extra reserves when demand for aerobic metabolism rises during bursts of activity typically observed in the western chipmunk.Abbreviations A-V arterio-venous - EBMR estimated basal metabolic rate (ml O₂ g^–1 h^–1) - HR heart rate (beats/min) - MR metabolic rate (ml O₂ g^–1 h^–1) - OP oxygen pulse (ml O₂/heart beat) - T_a, T_b ambient and body temperature (°C)     

Daily torpor and energetics in a tropical mammal, the northern blossom-bat Macroglossus minimus (Megachiroptera)

Little is known about torpor in the tropics or torpor in megachiropteran species. We investigated thermoregulation, energetics and patterns of torpor in the northern blossom-bat Macroglossus minimus (16 g) to test whether physiological variables may explain why its range is limited to tropical regions. Normothermic bats showed a large variation in body temperature (T _b) (33 to 37 °C) over a wide range of ambient temperatures (T _as) and a relatively low basal metabolic rate (1.29 ml O₂ g⁻¹h⁻¹). Bats entered torpor frequently in the laboratory at T _as between 14 and 25 °C. Entry into torpor always occurred when lights were switched on in the morning, independent of T _a. MRs during torpor were reduced to about 20–40% of normothermic bats and T _bs were regulated at a minimum of 23.1 ± 1.4 °C. The duration of torpor bouts increased with decreasing T _a in non-thermoregulating bats, but generally terminated after 8 h in thermoregulating torpid bats. Both the mean minimum T _b and MR of torpid M. minimus were higher than that predicted for a 16-g daily heterotherm and the T _b was also about 5 °C higher than that of the common blossom-bat Syconycteris australis, which has a more subtropical distribution. These observations suggest that variables associated with torpor are affected by T _a and that the restriction to tropical areas in M. minimus to some extent may be due to their ability to enter only very shallow daily torpor. Accepted: 22 September 1997     

The effect of temperature on the pattern of torpor in a marsupial hibernator

Physiological variables of torpor are strongly temperature dependent in placental hibernators. This study investigated how changes in air temperature affect the duration of torpor bouts, metabolic rate, body temperature and weight loss of the marsupial hibernator Burramys parvus (50 g) in comparison to a control group held at a constant air temperature of 2°C. The duration of torpor bouts was longest (14.0±1.0 days) and metabolic rate was lowest (0.033±0.001 ml O₂·g^-1·h^-1) at2°C. At higher air temperatures torpor bouts were significantly shorter and the metabolic rate was higher. When air temperature was reduced to 0°C, torpor bouts also shortened to 6.4±2.9 days, metabolic rate increased to about eight-fold the values at 2°C, and body temperature was maintained at the regulated minimum of 2.1±0.2°C. Because air temperature had such a strong effect on hibernation, and in particular energy expenditure, a change in climate would most likely increase winter mortality of this endangered species.Abbreviationst STP standard temperature and pressure - T _a air temperature - T _b body temperature - VO₂ rate of oxygen consumption     

Time-dependent thresholds for torpor initiation in the rufous hummingbird (Selasphorus rufus)

Summary Three models for torpor initiation were tested in rufous hummingbirds (Selasphorus rufus) during moult, when these birds appear to avoid the use of torpor. In model 1, the level of energy reserves at which torpor is initiated (the threshold) remains constant throughout the night. In model 2, the threshold declines throughout the night, at a constant rate equivalent to the rate at which energy reserves are depleted during torpor. In model 3, the threshold declines at a rate equivalent to the rate of energy reserve depletion during torpor for most of the night, but at a higher rate (corresponding to the rate of energy expenditure during normothermia) during the final 2 h of the night, when these birds are usually normothermic. Model 1 predicts the most frequent and longest bouts of torpor, whereas model 3 predicts the fewest and shortest bouts. To determine the thresholds for each of 12 birds, food supply was manipulated to induce entry into torpor at different times on successive nights. Threshold slopes matched the predictions of model 3 most closely. Calculations comparing observed incidence of torpor with the predictions of model 1 show that the actual, time-dependent threshold for torpor initiation resulted in a 72% reduction in the number of torpor bouts compared with the number of torpor bouts that should have been initiated by a constant threshold. The advantage of a time-dependent threshold is that, although torpor is initiated when needed to prevent energy reserves from falling below a critical level, the amount of time spent in torpor can be minimized. This may be especially important to rufous hummingbirds during the spring moult, because lowered metabolic rates during torpor probably result in decreased rates of feather replacement during the moult and may thus have consequences for thermoregulation, territorial defence, and timing of the spring migration.     

Seasonal energetics and torpor use in North American flying squirrels

Seasonal cold temperatures require mammals to use morphological, behavioural, or physiological traits to survive periods of extreme cold and food shortage. Torpor is a physiological state that minimizes energy requirements by decreasing resting metabolic rate (MR) and body temperature (T_b). Many rodent species are capable of torpor, however, evidence in northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans, respectively) has remained anecdotal. We experimentally attempted to induce torpor in wild-caught flying squirrels by lowering ambient temperature (T_a) and measuring MR using open-flow respirometry. We also studied seasonal differences in MR and T_b at various T_a. Both MR and T_b provided evidence for torpor in flying squirrels, but only infrequent, shallow torpor. MR decreased infrequently and any decreases were rarely sustained for longer than one hour. We found a significant positive relationship between T_a and T_b only in G. volans, which suggests that G. volans is more susceptible to low T_a compared with G. sabrinus, possibly due to their small body size. We observed no substantive seasonal or interspecific differences in the relation between MR and T_a, with the exception that northern flying squirrels expended more energy at cold T_a during warm season trials than other species-season combinations. The infrequency of torpor use in our experiments suggests that other energy-saving strategies, such as social thermoregulation, may limit the reliance on torpor in this lineage.     

Seasonal torpor and normothermic energy metabolism in the Eastern chipmunk (Tamias striatus)

To assess the changes in thermoregulatory characteristics that accompany the seasonal expression of torpor we measured seasonal differences in body mass adjustments, body temperature (T _b) and metabolic rate (MR) in both summer- and winter-acclimated individuals from a species of food-storing hibernator, the Eastern chipmunk (Tamias striatus). Torpor occurred only in the winter and was associated with lower normothermic T _b, during inter-bout arousal periods than in the summer. Chipmunks increased body mass before the initiation of torpor in winter, and steadily lost mass as the hibernation season progressed. Torpor expression was correlated to initial mass gain, with the individuals who showed the largest mass increase in the fall showing the highest degree of torpor. Acclimation to winter-like conditions produced a decline in normothermic MR at all ambient temperatures examined. The findings indicate that torpor expression is accompanied by a decrease in T _b and MR during normothermy, indicating that a conservation of energy metabolism occurs, not only in torpor, but also during the inter-bout arousal periods.     

Body temperature and metabolic rate during natural hypothermia in endotherms

During daily torpor and hibernation metabolic rate is reduced to a fraction of the euthermic metabolic rate. This reduction is commonly explained by temperature effects on biochemical reactions, as described by Q ₁₀ effects or Arrhenius plots. This study shows that the degree of metabolic suppression during hypothermia can alternatively be explained by active downregulation of metabolic rate and thermoregulatory control of heat production. Heat regulation is fully adequate to predict changes in metabolic rate, and Q ₁₀ effects are not required to explain the reduction of energy requirements during hibernation and torpor.Abbreviations BMR basal metabolic rate - BW body weight - C thermal conductance - C_HL thermal conductance as derived from HL - C_HP thermal conductance as derived from HP - HL heat loss - HP heat production - MR metabolic rate - RQ respiratory quotient - T_a ambient temperature - T_b body temperature     

Energetics and torpor of a South American “living fossil”, the microbiotheriid<Emphasis Type="Italic"> Dromiciops gliroides</Emphasis>

We examined the energetics of the living fossil microbiotheriid Dromiciops gliroides, a nocturnal and rare small marsupial, endemic to the northern portion of the temperate forest of southern South America. We investigated the effects of changes at ambient temperature and food restriction on the energetics and patterns of torpor. We determined whether they exhibit shallow daily torpor or deep prolonged torpor like some Australian marsupials. Thermal conductance was 92.5% of the expected value for a similarly sized eutherian and basal metabolic rate was 82.9 and 58.6% of the predicted value for standard metatherians and eutherians, respectively. Euthermic D. gliroides showed daily fluctuations in body temperature, being significantly higher during the night. Dromiciops gliroides entered torpor and aroused spontaneously. The duration of torpor bouts increased in response to decreasing ambient temperature; torpor bout duration ranged from 10 h at 20 °C to 120 h at 12.5 °C. This study is the first record of deep torpor or hibernation for a South American mammal. Torpor in this species as well as in marsupials in general appears to be an opportunistic response to unpredictable biotic and abiotic conditions.Abbreviations VO₂ metabolic rate - Tb body temperature - Ta ambient temperature - BMR basal metabolic rate - C thermal conductance - T_m temperature differentialCommunicated by I.D. Hume     

The effect of He−O2 exposure on metabolic rate,thermoregulation and thermal conductance during normothermia and daily torpor

Recently it was proposed that the low metabolic rate during torpor may be better explained by the reduction of thermal conductance than the drop of body temperature or metabolic inhibition. We tested this hypothesis by simultaneously measuring body temperature and metabolic rate as a function of ambient temperature in both torpid and normothermic stripe-faced dunnarts, Sminthopsis macroura (Marsupialia; approx. 25 g body mass), exposed to either air or He–O₂ (21% oxygen in helium) atmospheres. He–O₂ exposure increases the thermal conductance of homeothermic mammals by about twofold in comparison to an air atmosphere without apparent side-effects. Normothermic S. macroura exposed to He–O₂ increased resting metabolic rate by about twofold in comparison to that in air because of the twofold increase in apparent thermal conductance. Torpid S. macroura exposed to He–O₂ at ambient temperatures above the set-point for body temperature showed a completely different metabolic response. In contrast to normothermic individuals, torpid individuals significantly decreased or maintained a similar metabolic rate as those in air although the apparent thermal conductance in He–O₂ was slightly raised. Moreover, the metabolic rate during torpor was only a fraction of that of normothermic individuals although the apparent thermal conductance differed only marginally between normothermia and torpor. Our study shows that a low thermal conductance is not the reason for the low metabolic rates during torpor. It suggests that interrelations between metabolic rate and body temperature of torpid endotherms above the set-point for body temperature differ fundamentally from those of normothermic and homeothermic endotherms.Abbreviations T _a ambient temperature - T _b body temperature - BMR basal metabolic rate - C apparent thermal conductance - He–O ₂ 21% oxygen in helium - MR metabolic rate - MSe mean square-error - RMR festing metabolic rate - TMR metabolic rate during torpor - T difference T _b-T _a - TNZ thermoneutral zone - T _set set-point for body temperature - O ₂ rate of oxygen consumption     

Torpor, thermal biology, and energetics in Australian long-eared bats (Nyctophilus)

Previous studies have suggested that Australian long-eared bats (Nyctophilus) differ from northern-hemisphere bats with respect to their thermal physiology and patterns of torpor. To determine whether this is a general trait of Australian bats, we characterised the temporal organisation of torpor and quantified metabolic rates and body temperatures of normothermic and torpid Australian bats (Nyctophilus geoffroyi, 7 g and N. gouldi, 10 g) over a range of air temperatures and in different seasons. The basal metabolic rate of normothermic bats was 1.36 ± 0.17 ml g⁻¹ h⁻¹ (N. geoffroyi) and 1.22 ± 0.13 ml g⁻¹ h⁻¹ (N. gouldi), about 65% of that predicted by allometric equations, and the corresponding body temperature was about 36 °C. Below an air temperature of about 25 °C bats usually remained normothermic for only brief periods and typically entered torpor. Arousal from torpor usually occurred shortly after the beginning of the dark phase and torpor re-entry occurred almost always during the dark phase after normothermic periods of only 111 ± 48 min (N. geoffroyi) and 115 ± 66 min (N. gouldi). At air temperatures below 10 °C, bats remained torpid for more than 1 day. Bats that were measured overnight had steady-state torpor metabolic rates representing only 2.7% (N. geoffroyi) and 4.2% (N. gouldi) of the basal metabolic rate, and their body temperatures fell to minima of 1.4 and 2.3 °C, respectively. In contrast, bats measured entirely during the day, as in previous studies, had torpor metabolic rates that were up to ten times higher than those measured overnight. The steady-state torpor metabolic rate of thermoconforming torpid bats showed an exponential relationship with body temperature (r ² = 0.94), suggesting that temperature effects are important for reduction of metabolic rate below basal levels. However, the 75% reduction of metabolic rate between basal metabolic rate and torpor metabolic rate at a body temperature of 29.3 °C suggests that metabolic inhibition also plays an important role. Torpor metabolic rate showed little or no seasonal change. Our study suggests that Australian Nyctophilus bats have a low basal metabolic rate and that their patterns of torpor are similar to those measured in bats from the northern hemisphere. The low basal metabolic rate and the high proclivity of these bats for using torpor suggest that they are constrained by limited energy availability and that heterothermy plays a key role in their natural biology. Accepted: 22 November 1999     

Warm-up rates during arousal from torpor in heterothermic mammals: physiological correlates and a comparison with heterothermic insects

Summary This study examines the relationship between warm-up rate, body mass, metabolic rate, thermal conductance and normothermic body temperature in heterothermic mammals during arousal from torpor. Predictions based on the assumption that the energetic cost of arousal has been minimised are tested using data for 35 species. The observation that across-species warm-up rate correlates negatively with body mass is confirmed using a comparative technique which removes confounding effects due to the non-independence of species data due to shared common ancestry. Mean warm-up rate during arousal correlates negatively with basal metabolic rate and positively with the temperature difference through which the animal warms, having controlled for other factors. These results suggest that selection has operated to minimise the overall energetic, cost of warm-up. In contrast, peak warm-up rate during arousal correlates positively with peak metabolic rate during arousal, and negatively with thermal conductance, when body mass has been taken into account. These results suggest that peak warm-up rate is more sensitive to the fundamental processes of heat generation and loss. Although heterothermic marsupials have lower normothermic body temperatures and basal metabolic rates, marsupials and heterothermic eutherian mammals do not differ systematically in warm-up rate. Pre-flight warm-up rates in one group of endothermic insects, the bees, are significantly higher than predictions based on rates of arousal of a mammal of the same body mass.Abbreviations BMR basal metabolic rate - ICM independent comparisons method - MWR mean warm-up rate - PMR peak metabolic rate - PWR peak·warm-up rate - Tb_activity body temperature during activity - Tb_torpor body temperature during torpor - T _arousal increase in body temperature during arousal     

Thermoregulation, energy metabolism, and torpor in blossom-bats, Syconycteris australis (Megachiroptera)

Since little information is available on torpor in bats of the suborder Megachiroptera, we investigated whether the small (18 g) blossom-bat Syconycteris australis displays torpor in the laboratory. Bats entered daily torpor when food and water were withheld for one night and the air temperature (T_a) was below about 26°. Torpor began shortly after lights went on in the morning and lasted for a maximum of 12 hours. During torpor at T_a18°, metabolic rates fell to a minimum of about 15% of that in resting individuals at the same T_a, and to about 40% of the basal metabolic rate. The body temperature (T_b) during torpor was metabolically defended at or above about T_b 18°. Individuals that did not enter torpor in the morning reduced their T_b from about 34.5°, observed in resting individuals that had been fed during the previous night, to values between 30.2 and 32.8°, and the resting metabolic rate fell by about 25%. The ability to undergo short periods of torpor may explain why the distribution range of S. australis extends much further south than that of other small Australian megachiropteran bats.     

Reduction of metabolic rate and thermoregulation during daily torpor

Physiological mechanisms causing reduction of metabolic rate during torpor in heterothermic endotherms are controversial. The original view that metabolic rate is reduced below the basal metabolic rate because the lowered body temperature reduces tissue metabolism has been challenged by a recent hypothesis which claims that metabolic rate during torpor is actively downregulated and is a function of the differential between body temperature and ambient temperature, rather than body temperature per se. In the present study, both the steady-state metabolic rate and body temperature of torpid stripe-faced dunnarts, Sminthopsis macroura (Dasyuridae: Marsupialia), showed two clearly different phases in response to change of air temperature. At air temperatures between 14 and 30°C, metabolic rate and body temperature decreased with air temperature, and metabolic rate showed an exponential relationship with body temperature (r ²=0.74). The Q ₁₀ for metabolic rate was between 2 and 3 over the body temperature range of 16 to 32°C. The difference between body temperature and air temperature over this temperature range did not change significantly, and the metabolic rate was not related to the difference between body temperature and air temperature (P=0.35). However, the apparent conductance decreased with air temperature. At air temperatures below 14°C, metabolic rate increased linearly with the decrease of air temperature (r ²=0.58) and body temperature was maintained above 16°C, largely independent of air temperature. Over this air temperature range, metabolic rate was positively correlated with the difference between body temperature and air temperature (r ²=0.61). Nevertheless, the Q ₁₀ for metabolic rate between normothermic and torpid thermoregulating animals at the same air temperature was also in the range of 2–3. These results suggest that over the air temperature range in which body temperature of S. macroura was not metabolically defended, metabolic rate during daily torpor was largely a function of body temperature. At air temperatures below 14°C, at which the torpid animals showed an increase of metabolic rate to regulate body temperature, the negative relationship between metabolic rate and air temperature was a function of the differential between body temperature and air temperature as during normothermia. However, even in thermoregulating animals, the reduction of metabolic rate from normothermia to torpor at a given air temperature can also be explained by temperature effects.Abbreviations BM body mass - BMR basal metabolic rate - C apparent conductance - MR metabolic rate - RMR resting metabolic rate - RQ respiratory quotient - T _a air temperature - T _b body temperature - T _lc lower critical temperature - T _tc critical air temperature during torpor - TMR metabolic rate during torpor - TNZ thermoneutral zone - T difference between body temperature and air temperature - VO₂ rate of oxygen consumption     

The “minimal boundary curve for endothermy” as a predictor of heterothermy in mammals and birds: a review

According to the concept of the “minimal boundary curve for endothermy”, mammals and birds with a basal metabolic rate (BMR) that falls below the curve are obligate heterotherms and must enter torpor. We examined the reliability of the boundary curve (on a double log plot transformed to a line) for predicting torpor as a function of body mass and BMR for birds and several groups of mammals. The boundary line correctly predicted heterothermy in 87.5% of marsupials (n = 64), 94% of bats (n = 85) and 82.3% of rodents (n = 157). Our analysis shows that the boundary line is not a reliable predictor for use of torpor. A discriminate analysis using body mass and BMR had a similar predictive power as the boundary line. However, there are sufficient exceptions to both methods of analysis to suggest that the relationship between body mass, BMR and heterothermy is not a causal one. Some homeothermic birds (e.g. silvereyes) and rodents (e.g. hopping mice) fall below the boundary line, and there are many examples of heterothermic species that fall above the boundary line. For marsupials and bats, but not for rodents, there was a highly significant phylogenetic pattern for heterothermy, suggesting that taxonomic affiliation is the biggest determinant of heterothermy for these mammalian groups. For rodents, heterothermic species had lower BMRs than homeothermic species. Low BMR and use of torpor both contribute to reducing energy expenditure and both physiological traits appear to be a response to the same selective pressure of fluctuating food supply, increasing fitness in endothermic species that are constrained by limited energy availability. Both the minimal boundary line and discriminate analysis were of little value for predicting the use of daily torpor or hibernation in heterotherms, presumably as both daily torpor and hibernation are precisely controlled processes, not an inability to thermoregulate.