Energetics of arousal episodes in hibernating arctic ground squirrels

Arctic ground squirrels overwintering in northern Alaska experience average soil temperature of −10°C. To examine energetic costs of arousing from hibernation under arctic compared to temperate conditions, captive ground squirrels were maintained in ambient temperatures (T _a) of 2, −5 and −12°C. Rates of oxygen consumption and carbon dioxide production were used to estimate metabolic rate and fuel use during the three phases of arousal episodes: rewarming, euthermia, and recooling. Respiratory quotient comparisons suggest exclusive use of lipid during rewarming and mixed fuel use during euthermia. Animals rewarming from torpor at T _a −12°C took longer, consumed more oxygen, and attained higher peak rates of oxygen consumption when compared to 2°C. T _a had no significant effect on cost or duration of the euthermic phase. Animals recooled faster at −12°C than at 2°C, but total oxygen consumption was not different. T _a had no significant effect on the total cost of arousal episodes when all three phases are included. Arousal episodes account for 86% of estimated costs of a complete hibernation cycle including torpor when at 2°C and only 23% at −12°C. Thus, due to the higher costs of steady-state metabolism during torpor, proportional metabolic costs of arousal episodes at T _a characteristic of the Arctic are diminished compared to relative costs of arousals in more temperate conditions.     

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     

Hibernation by a free-ranging subtropical bat (Nyctophilus bifax)

Knowledge about torpor in free-ranging subtropical bats is scarce and it is widely believed that low and stable ambient temperatures are necessary for prolonged torpor. We present temperature-telemetry data from free-ranging male (n = 4) and female (n = 4) subtropical vespertilionid bats, Nyctophilus bifax (~10 g), exposed to pronounced daily fluctuations of ambient temperature. All bats used torpor on every day in winter and both males and females exhibited multi-day torpor bouts of up to 5.4 days. Although females were larger than males, patterns of torpor were similar in both sexes. Torpor use was correlated with prevailing weather conditions and, on days when bats remained torpid, maximum ambient temperature was significantly lower than on days when bats aroused. Moreover, the duration of interbout normothermic periods at night increased with increasing average nightly ambient temperature. Skin temperature of torpid bats varied by 10.2 ± 3.6°C day⁻¹ (n = 8, N = 47) and daily minimum skin temperature was positively correlated with the daily minimum ambient temperature. Our study shows that prolonged torpor is an important component of the winter ecology of a subtropical bat and that torpor and activity patterns of N. bifax predominantly reflect prevailing weather conditions.     

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     

Seasonal changes in energetics and torpor patterns in the subtropical blossom-bat Syconycteris australis (Megachiroptera)

Little is known about how animals from tropical and subtropical climates adjust their energy expenditure to cope with seasonal changes of climate and food availability. To provide such information, we studied the thermal physiology, torpor patterns and energetics of the nocturnal blossom-bat (Syconycteris australis 18 g) from a subtropical habitat in both summer and winter. In both seasons, S. australis frequently entered daily torpor at ambient temperatures between 12 and 25°C when food and water were withheld. Unlike patterns observed in temperate animals, mean minimum metabolic rates during torpor were lower in summer (0.47 ± 0.07 ml O₂ g⁻¹ h⁻¹) than in winter (0.75 ± 0.11 ml O₂ g⁻¹ h⁻¹). Body temperatures during torpor were regulated at 19.3 ± 1.0°C in summer and at 23.4 ± 2.0°C in winter. Torpor bout duration was significantly longer in summer (7.3 ± 0.6 h) than in winter (5.5 ± 0.3 h), but in both seasons, bout duration was not affected by ambient temperature. Consequently, average daily metabolic rates were also significantly lower in summer than in winter. Body temperatures and metabolic rates in normothermic bats did not change with season. Our findings on seasonal changes of torpor in this bat from the subtropics are opposite to those made for many species from cold climates which generally show deeper and longer torpor in winter and are often entirely homeothermic in summer. More pronounced torpor in subtropical S. australis in summer may be due to low or unpredictable nectar availability, short nights which limit the time available for foraging, and long days without access to food. Thus, the reversed seasonal response of this subtropical bat in comparison to temperate species may be an appropriate response to ecological constraints. Received: 6 May 1997 / Accepted: 19 October 1997     

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     

Warming up for dinner: torpor and arousal in hibernating Natterer’s bats (Myotis nattereri) studied by radio telemetry

The frequency and function of arousals during hibernation in free-living mammals are little known. We used temperature-sensitive radio transmitters to measure patterns of torpor, arousal and activity in wild Natterer’s bats Myotis nattereri during hibernation. Duration of torpor bouts ranged from 0.06 to 20.4 days with individual means ranging from 0.9 to 8.9 days. Arousals from torpor occurred most commonly coincident with the time (relative to sunset) typical for bats emerging from summer roosts to forage. Bats with lower body condition indices had a shorter average duration of their torpor bouts. We found a non-linear relationship between duration of torpor bout and ambient temperature: the longest average torpor bouts were at temperatures between 2 and 4°C with shorter bouts at lower and higher ambient temperatures. One individual was radio-tracked for ten nights, remained active for an average of 297 min each night and was active for longer on warmer nights. Our results suggest that vespertilionid bats use relatively short torpor bouts during hibernation in a location with a maritime climate. We hypothesise that Natterer’s bats time arousals to maximise opportunities for potential foraging during winter although winter feeding is not the sole determinant of arousal as bats still arouse at times when foraging is unlikely.     

Enhanced anti-tumor activity of interferon-alpha in SOCS1-deficient mice is mediated by CD4<Superscript>+</Superscript> and CD8<Superscript>+</Superscript> T cells

Interferon-alpha (IFN-α) is an immunomodulatory cytokine that is used clinically for the treatment of melanoma in the adjuvant setting. The cellular actions of IFN-α are regulated by the suppressors of cytokine signaling (SOCS) family of proteins. We hypothesized that the anti-tumor activity of exogenous IFN-α would be enhanced in SOCS1-deficient mice. SOCS1-deficient (SOCS1^−/−) or control (SOCS1^+/+) mice on an IFN-γ^−/− C57BL/6 background bearing intraperitoneal (i.p.) JB/MS murine melanoma cells were treated for 30 days with i.p. injections of IFN-A/D or PBS (vehicle). Log-rank Kaplan-Meier survival curves were used to evaluate survival. Tumor-bearing control SOCS1^+/+ mice receiving IFN-A/D had significantly enhanced survival versus PBS–treated mice (P = 0.0048). The anti-tumor effects of IFN-A/D therapy were significantly enhanced in tumor-bearing SOCS1^−/− mice; 75% of these mice survived tumor challenge, whereas PBS-treated SOCS1^−/− mice all died at 13-16 days (P = 0.00038). Antibody (Ab) depletion of CD8⁺ T cells abrogated the anti-tumor effects of IFN-A/D in SOCS1^−/− mice as compared with mice receiving a control antibody (P = 0.0021). CD4⁺ T-cell depletion from SOCS1^−/− mice also inhibited the effects of IFN-A/D (P = 0.0003). IFN-A/D did not alter expression of CD80 or CD86 on splenocytes of SOCS1^+/+ or SOCS1^−/− mice, or the proportion of T regulatory cells or myeloid-derived suppressor cells in SOCS1^+/+ or SOCS1^−/− mice. An analysis of T-cell function did reveal increased proliferation of SOCS1-deficient splenocytes at baseline and in response to mitogenic stimuli. These data suggest that modulation of SOCS1 function in T-cell subsets could enhance the anti-tumor effects of IFN-α in the setting of melanoma.     

Dietary cholesterol enhances torpor in a rodent hibernator

Dietary cholesterol can affect both body lipid composition and steroid hormone concentration. We investigated whether a diet rich in cholesterol influences torpor patterns of hibernating chipmunks (Tamias amoenus) and, if so, whether these changes are better explained by diet-induced changes in body lipid composition or the concentration of testosterone, which at high levels inhibits torpor. Two groups of chipmunks were maintained either on a cholesterol diet (rodent chow containing 10% cholesterol) or a control diet (rodent chow) during pre-hibernation fattening and throughout the hibernation season. Torpid chipmunks on the cholesterol diet had significantly lower minimum body temperatures (−0.2 ± 0.2 vs +0.6 ± 0.2 °C), lower metabolic rates (0.029 ± 0.002 ml O₂ g⁻¹h⁻¹ vs 0.035 ± 0.001 ml O₂ g⁻¹h⁻¹), and longer torpor bouts at −1 °C (6.8 ± 0.5 vs 4.1 ± 1.0 days) than chipmunks on the control diet. Dietary cholesterol resulted in a significant increase in blood plasma cholesterol (sevenfold), liver cholesterol content (6.9-fold) and liver triglyceride content (3.5-fold) in comparison to controls. In contrast, dietary cholesterol had no detectable effect on the concentration of plasma testosterone, which was very low in both groups. Since torpor was deeper and longer in animals on the cholesterol diet our study suggests that torpor patterns of chipmunks were either directly affected by the dietary cholesterol or via changes in body lipid composition. Accepted: 22 January 1997     

Effects of light and temperature on the attachment and development of <Emphasis Type="BoldItalic">Gloiopeltis tenax</Emphasis> and <Emphasis Type="BoldItalic">Gloiopeltis furcata</Emphasis> tetraspores

Two 60-day experiments were conducted to study the influence of photon flux density (PFD) and temperature on the attachment and development of Gloiopeltis tenax and Gloiopeltis furcata tetraspores. In the first experiment, tetraspores of the two Gloiopeltis species were incubated at five temperature ranges (8°C, 12°C, 16°C, 20°C, 24°C) under a constant PFD of 80 μmol photons m⁻² s⁻¹ with a photoperiod of 12:12. In a second experiment, tetraspores were incubated under five PFD gradients (30, 55, 80, 105, 130 μmol photons m⁻² s⁻¹) at a constant temperature of 16°C with a photoperiod of 12:12. Maximum density of attached tetraspores was observed at 16°C for both species. Maximum per cent of spore germinating into disc was recorded at 12–16°C for G. tenax and 8–12°C for G. furcata. Maximum per cent of discs producing erect axes for G. tenax and G. furcata were recorded at 24°C and 20°C, respectively. Light had no significant effect on tetraspore attachment and developing into disc, but it affected the growth, sprouting and survival of its discs. Under 30–55 μmol photons m⁻² s⁻¹, the discs of the two species of Gloiopeltis did not form thallus until the end of the experiment. Optimum PFD range for G. tenax discs was 80–105 μmol photons m⁻² s⁻¹, whilst it was 80–130 μmol photons m⁻² s⁻¹ for G. furcata. Results presented in this study are expected to assist the progress of artificial seeding of Gloiopeltis.     

Metabolism and temperature regulation during daily torpor in the smallest primate, the pygmy mouse lemur (Microcebus myoxinus) in Madagascar

Thermoregulation, energetics and patterns of torpor in the pygmy mouse lemur, Microcebus myoxinus, were investigated under natural conditions of photoperiod and temperature in the Kirindy/CFPF Forest in western Madagascar. M. myoxinus entered torpor spontaneously during the cool dry season. Torpor only occurred on a daily basis and torpor bout duration was on average 9.6 h, and ranged from 4.6 h to 19.2 h. Metabolic rates during torpor were reduced to about 86% of the normothermic value. Minimum body temperature during daily torpor was 6.8 °C at an ambient temperature of 6.3 °C. Entry into torpor occurred randomly between 2000 and 0620 hours, whereas arousals from torpor were clustered around 1300 hours within a narrow time window of less than 4 h. Arousal from torpor was a two-step process with a first passive climb of body temperature to a mean of 27 °C, carried by the daily increase of ambient temperature when oxygen consumption remained more or less constant, followed by a second active increase of oxygen consumption to further raise the body temperature to normothermic values. In conclusion, daily body temperature rhythms in M. myoxinus further reduce the energetic costs of daily torpor seen in other species: they extend to unusually low body temperatures and consequently low metabolic rates in torpor, and they employ passive warming to reduce the energetic costs of arousal. Thus, these energy-conserving adaptations may represent an important energetic aid to the pygmy mouse lemur and help to promote their individual fitness. Accepted: 2 November 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     

Photosynthetic characteristics and physiological plasticity of an Aphanizomenon flos-aquae (Cyanobacteria, Nostocaceae) winter bloom in a deep oligo-mesotrophic lake (Lake Stechlin, Germany)

In winter of 2009/2010, Aphanizomenon flos-aquae bloomed in the ice and snow covered oligo-mesotrophic Lake Stechlin, Germany. The photosynthesis of the natural population was measured at eight temperatures in the range of 2–35°C, at nine different irradiance levels in the range of 0–1,320 μmol m⁻² s⁻¹ PAR at each applied temperature. The photoadaptation parameter (I _k) and the maximum photosynthetic rate (P _max) correlated positively with the temperature between 2 and 30°C, and there was a remarkable drop in both parameters at 35°C. The low I _k at low temperatures enabled the active photosynthesis of overwintering populations at low irradiance levels under ice and snow cover. The optimum of the photosynthesis was above 20°C at irradiances above 150 μmol m⁻² s⁻¹. At lower irradiance levels (7.5–30 μmol m⁻² s⁻¹), the photosynthesis was the most intensive in the temperature range of 2–5°C. The interaction between light and temperature allowed the proliferation of A. flos-aquae in Lake Stechlin resulting in winter water bloom in this oligo-mesotrophic lake. The applied 2°C is the lowest experimental temperature ever in the photosynthesis/growth studies of A. flos-aquae, and the results of the P–I and P–T measurements provide novel information about the tolerance and physiological plasticity of this species.     

Torpor and activity patterns in free-ranging sugar gliders Petaurus breviceps (Marsupialia)

Almost all studies on daily torpor in mammals have been conducted in the laboratory under constant environmental conditions. We investigated torpor and activity patterns in free-ranging sugar gliders (Petaurus breviceps, 100 g) using temperature telemetry and compared field data with published information obtained in the laboratory. Body and/or skin temperature and activity patterns of 12 sugar gliders were monitored from autumn to spring. Healthy sugar gliders were active between sunset and sunrise, but on cold or rainy nights activity was substantially reduced. Animals in poor condition occasionally foraged during the day. Eleven gliders were monitored for 8–171 days and all of these entered daily torpor. Torpor was observed on 103 days (17% of observation days), usually occurred on rainy or cold nights, and frequency of torpor changed with season. Torpor bouts lasted between 2 and 23 h (average 13 h) and the body temperature fell to a minimum of 10.4°C. Torpor was thus much deeper, longer and more frequent than in laboratory studies on the same species. Our study shows that cold or wet conditions curtail foraging in wild sugar gliders and that they employ daily torpor regularly during adverse weather. This suggests that minimisation of energy loss by the use of torpor in sugar gliders is pivotal for their survival in the wild. Received: 8 July 1999 / Accepted: 23 December 1999     

Physiological differences in the growth of <Emphasis Type="Italic">Sargassum horneri</Emphasis> between the germling and adult stages

The effects of temperature, irradiance, and daylength on Sargassum horneri growth were examined at the germling and adult stages to discern their physiological differences. Temperature–irradiance (10, 15, 20, 25, 30°C × 20, 40, 80 μmol photons m⁻²s⁻¹) and daylength (8, 12, 16, 24 h) experiments were carried out. The germlings and blades of S. horneri grew over a wide range of temperatures (10–25°C), irradiances (20–80 μmol photons m⁻²s⁻¹), and daylengths (8–24 h). At the optimal growth conditions, the relative growth rates (RGR) of the germlings were 21% day⁻¹ (25°C, 20 μmol photons m⁻²s⁻¹) and 13% day⁻¹ (8 h daylength). In contrast, the RGRs of the blade weights were 4% day⁻¹ (15°C, 20 μmol photons m⁻²s⁻¹) and 5% day⁻¹ (12 h daylength). Negative growth rates were found at 20 μmol photons m⁻²s⁻¹ of 20°C and 25°C treatments after 12 days. This phenomenon coincides with the necrosis of S. horneri blades in field populations. In conclusion, we found physiological differences between S. horneri germlings and adults with respect to daylength and temperature optima. The growth of S. horneri germlings could be enhanced at 25°C, 20 μmol photons m⁻²s⁻¹, and 8 h daylength for construction of Sargassum beds and restoration of barren areas.     

Early development of germlings of <Emphasis Type="Italic">Sargassum thunbergii</Emphasis> (Fucales,Phaeophyta) under laboratory conditions

Morphology and culture studies on germlings of Sargassum thunbergii (Mertens et Roth) Kuntze were carried out under controlled laboratory conditions. Growth characteristics of these germlings grown under different temperatures (from 10 to 25°C), irradiances (from 9 to 88 μmol photons m⁻² s⁻¹), and under blue and white light conditions are described. The development of embryonic germlings follows the classic “8 nuclei 1 egg” type described for Sargassaceae. Fertilized eggs spent 5–6 h developing into multicellular germlings with abundant rhizoids after fertilization. Under conditions of 20°C, 44 μmol photons m⁻² s⁻¹ and photoperiod of 12 h, young germlings with one or two leaflets reached 2–3 mm in length after 8 weeks. Temperature variations (10, 15, 20, 25°C) under 88 μmol photons m⁻² s⁻¹ significantly influenced the growth rate within the first week, although this effect became less obvious after 8 weeks, especially at 15 and 20°C. Variation in germling growth was highly significant under different irradiances (9, 18, 44, 88 μmol photons m⁻² s⁻¹) at 25°C. Low temperature (10°C) reduced germling growth. Growth of germlings cultured under blue light was lower than in white light. Optimal growth of these germlings occurred at 25°C and 44 μmol photons m⁻² s⁻¹.     

The energetic cost of arousal from torpor in the marsupial Sminthopsis macroura : benefits of summer ambient temperature cycles

The costs of arousal from induced torpor were measured in the striped-faced dunnart (Sminthopsis macroura; ca. 25 g) under two experimental ambient temperature cycles. The sinusoidal-type temperature cycles were designed to evaluate the effects of passive, ambient temperature heating during arousal from torpor in these insectivorous marsupials. It was hypothesised that diel ambient temperature cycles may offer significant energy savings during arousal in animals that employ daily torpor in summer as a response to unpredictable food availability. The cost of arousal in animals in which passive, exogenous heating occurred was significantly lower than that in animals not exposed to an ambient temperature cycle. The total cost of all three phases of torpor (entry, maintenance and arousal) was almost halved when animals were exposed to an ambient heating cycle from 15 °C to 25 °C over a 24-h period. In all animals, irrespective of the experimental ambient temperature cycle employed, the minimum torpor body temperature was 17–18 °C. The body temperature (T_b) of animals exposed to exogenous heating increased from the torpor T_b minimum to a mean value of 22.59 °C before endogenous heat production commenced. This relatively small increase in T_b of ca. 5 °C through `free' passive heating was sufficient to account for the significant ca. three-fold decrease in the cost of arousal and may represent an important energetic aid to free-ranging animals. Accepted: 4 October 1998     

Comparative physiology of Australian quolls (Dasyurus; Marsupialia)

Quolls (Dasyurus) are medium-sized carnivorous dasyurid marsupials. Tiger (3,840 g) and eastern quolls (780 g) are mesic zone species, northern quolls (516 g) are tropical zone, and chuditch (1,385 g) were once widespread through the Australian arid zone. We found that standard physiological variables of these quolls are consistent with allometric expectations for marsupials. Nevertheless, inter-specific patterns amongst the quolls are consistent with their different environments. The lower T _b of northern quolls (34°C) may provide scope for adaptive hyperthermia in the tropics, and they use torpor for energy/water conservation, whereas the larger mesic species (eastern and tiger quolls) do not appear to. Thermolability varied from little in eastern (0.035°C °C⁻¹) and tiger quolls (0.051°C oC⁻¹) to substantial in northern quolls (0.100°C oC⁻¹) and chuditch (0.146°C oC⁻¹), reflecting body mass and environment. Basal metabolic rate was higher for eastern quolls (0.662 ± 0.033 ml O₂ g⁻¹ h⁻¹), presumably reflecting their naturally cool environment. Respiratory ventilation closely matched metabolic demand, except at high ambient temperatures where quolls hyperventilated to facilitate evaporative heat loss; tiger and eastern quolls also salivated. A higher evaporative water loss for eastern quolls (1.43 ± 0.212 mg H₂O g⁻¹ h⁻¹) presumably reflects their more mesic distribution. The point of relative water economy was low for tiger (−1.3°C), eastern (−12.5°C) and northern (+3.3) quolls, and highest for the chuditch (+22.6°C). We suggest that these differences in water economy reflect lower expired air temperatures and hence lower respiratory evaporative water loss for the arid-zone chuditch relative to tropical and mesic quolls.     

Torpor and thermal energetics in a tiny Australian vespertilionid,the little forest bat (<Emphasis Type="Italic">Vespadelus vulturnus</Emphasis>)

Data on thermal energetics for vespertilionid bats are under-represented in the literature relative to their abundance, as are data for bats of very small body mass. Therefore, we studied torpor use and thermal energetics in one of the smallest (4 g) Australian vespertilionids, Vespadelus vulturnus. We used open-flow respirometry to quantify temporal patterns of torpor use, upper and lower critical temperatures (T _uc and T _lc) of the thermoneutral zone (TNZ), basal metabolic rate (BMR), resting metabolic rate (RMR), torpid metabolic rate (TMR), and wet thermal conductance (C _wet) over a range of ambient temperatures (T _a). We also measured body temperature (T _b) during torpor and normothermia. Bats showed a high proclivity for torpor and typically aroused only for brief periods. The TNZ ranged from 27.6°C to 33.3°C. Within the TNZ T _b was 33.3±0.4°C and BMR was 1.02±0.29 mlO₂ g⁻¹ h⁻¹ (5.60±1.65 mW g⁻¹) at a mean body mass of 4.0±0.69 g, which is 55 % of that predicted for a 4 g bat. Minimum TMR of torpid bats was 0.014±0.006 mlO₂ g⁻¹ h⁻¹ (0.079±0.032 mW g⁻¹) at T _a=4.6±0.4°C and T _b=7.5±1.9. T _lc and C _wet of normothermic bats were both lower than that predicted for a 4 g bat, which indicates that V. vulturnus is adapted to minimising heat loss at low T _a. Our findings support the hypothesis that vespertilionid bats have evolved energy-conserving physiological traits, such as low BMR and proclivity for torpor.