Facultative hypothermic responses in an Afrotropical arid-zone passerine,the red-headed finch (<Emphasis Type="Italic">Amadina erythrocephala</Emphasis>)

We investigated thermoregulation and facultative hypothermic responses to food deprivation in the red-headed finch (Amadina erythrocephala), a 22-g passerine endemic to the arid regions of southern Africa. We predicted that, like most other passerines investigated, A. erythrocephala exhibits shallow rest-phase hypothermia, but not torpor. We observed significant reductions in rest-phase energy expenditure and body temperature (Tb) in response to restricted feeding. The maximum extent of Tb reduction (ca. 5 degrees C) and energy savings (ca. 10%) were consistent with those reported for a number of other passerine species. The lowest Tb we observed in a bird able to arouse spontaneously was 34.8 degrees C. The parameters of facultative hypothermic responses in A. erythrocephala were indicative of shallow rest-phase hypothermia, but not torpor. The limited available data on hypothermic responses in passerines suggest that many species do not possess the capacity for torpor. In passerines, torpor appears to be restricted to a few nectarivores and aerial insectarivores, and may have evolved independently of the torpor observed in non-passerine taxa such as the Trochiliformes and Caprimulgidae. The basal metabolic rate (BMR) of A. erythrocephala was 30-46% lower than predicted by various allometric equations, but was similar to the predicted BMR for a 22-g desert bird.     

Thermoregulation in African Green Pigeons (Treron calvus) and a re-analysis of insular effects on basal metabolic rate and heterothermy in columbid birds

Columbid birds represent a useful model taxon for examining adaptation in metabolic and thermal traits, including the effects of insularity. To test predictions concerning the role of insularity and low predation risk as factors selecting for the use of torpor, and the evolution of low basal metabolic rate in island species, we examined thermoregulation under laboratory and semi-natural conditions in a mainland species, the African Green Pigeon (Treron calvus). Under laboratory conditions, rest-phase body temperature (T _b) was significantly and positively correlated with air temperature (T _a) between 0 and 35 °C, and the relationship between resting metabolic rate (RMR) and T _a differed from typical endothermic patterns. The minimum RMR, which we interpret as basal metabolic rate (BMR), was 0.825 ± 0.090 W. Green pigeons responded to food restriction by significantly decreasing rest-phase T _b, but the reductions were small (at most ~5 °C below normothermic values), with a minimum T _b of 33.1 °C recorded in a food-deprived bird. We found no evidence of the large reductions in T _b and metabolic rate and the lethargic state characteristic of torpor. The absence of torpor in T. calvus lends support to the idea that species restricted to islands that are free of predators are more likely to use torpor than mainland species that face the risk of predation during the rest-phase. We also analysed interspecific variation in columbid BMR in a phylogenetically informed framework and verified the conclusions of an earlier study which found that BMR is significantly lower in island species compared to those that occur on mainlands.     

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

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

Torpor in dark times: patterns of heterothermy are associated with the lunar cycle in a nocturnal bird

Many studies have shown that endotherms become more heterothermic when the costs of thermoregulation are high and/or when limited energy availability constrains thermoregulatory capacity. However, the roles of many ecological variables, including constraints on foraging opportunities and/or success, remain largely unknown. To test the prediction that thermoregulatory patterns should be related to foraging opportunities in a heterothermic endotherm, we examined the relationship between the lunar cycle and heterothermy in Freckled Nightjars (Caprimulgus tristigma), which are visually orienting, nocturnal insectivores that are dependent on ambient light to forage. This model system provides an opportunity to assess whether variation in foraging opportunities influences the expression of heterothermy. The nightjars were active and foraged for insects when moonlight was available but became inactive and heterothermic in the absence of moonlight. Lunar illumination was a much stronger predictor of the magnitude of heterothermic responses than was air temperature (T(a)). Our data suggest that heterothermy was strongly related to variation in foraging opportunities associated with the lunar cycle, even though food abundance appeared to remain relatively high throughout the study period. Patterns of thermoregulation in this population of Freckled Nightjars provide novel insights into the environmental and ecological determinants of heterothermy, with the lunar cycle, and not T(a), being the strongest predictor of torpor use.     

Is ‘nocturnal hypothermia’ a valid physiological concept in small birds?: a study on Bronze Mannikins Spermestes cucullatus

The thermoregulatory capacity and metabolic responses to light–dark cycles under various mild food-deprivation treatments were measured in Bronze Mannikins Spermestes cucullatus (10–11 g). We measured the response of minimum oxygen consumption to ambient temperature in order to determine the basal metabolic rate (BMR), thermal conductance and limits of thermoneutrality of the Mannikins. In addition, we measured oxygen consumption in response to light–dark cycles and three mild food-deprivation treatments. Bronze Mannikins have a low BMR (1.67 mlO₂/g/h) that is c. 50–60% of that predicted from phylogenetically independent allometric curves for all birds. A low BMR resulted in amplitudes of metabolism between the active and rest phases that were double those predicted allometrically from body mass. The reduced nocturnal metabolic rate did not represent torpor. Typically, Mannikins would need to reduce their metabolic rate during the rest phase to c. 17% of BMR to attain the average torpor metabolic rate of other birds. The data are, however, consistent with those of other group-living Afrotropical birds that benefit energetically from group huddling in environments in which moderate seasonality is accompanied by unpredictable climates – and thus unpredictable energy inputs in time and space. When food-deprived and placed under moderate cold stress (20 °C), Mannikins decreased their rest-phase metabolic rates to the same magnitude as several small Holarctic birds. We suggest that, in the context of the progress made to quantify and define proximate heterothermic responses in endotherms, such as torpor and hibernation, the term nocturnal hypothermia often applied to moderate nocturnal reductions in metabolic rate is vague, misleading and inappropriate.     

Reproductive activity influences thermoregulation and torpor in pouched mice, Saccostomus campestris

The Afrotropical pouched mouse Saccostomus campestris displays sexual disparity in the use of daily torpor; males reluctantly enter torpor. We tested the hypothesis that males may compensate for a limited heterothermic capacity with lower basal and resting metabolic rates relative to females. We also investigated the association between gonadal activity (testosterone) and the propensity for daily torpor. Body temperature and oxygen consumption were measured at various ambient temperatures and were compared between sexes under ad libitum and restricted-diet treatments. Whereas no significant sex differences were observed in body temperature and oxygen consumption under ad libitum treatment, there were pronounced differences in heterothermic responses under food restriction. Females employed torpor more frequently and also had lower minimum torpor body temperatures (ca. 25 degrees C) than males (ca. 29 degrees C). Testosterone inhibited torpor in males, whereas the majority of saline-treated animals employed torpor under both ad libitum and restricted-diet treatments. This study demonstrated that the limited capacity of male S. campestris to enter torpor is a consequence of reproductive activity and that opportunistic breeding and the absence of seasonal testes regression compromises the capacity to conserve energy through daily torpor.     

Radiant heat affects thermoregulation and energy expenditure during rewarming from torpor

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

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

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

Role of Akt signaling pathway regulation in the speckled mousebird (Colius striatus) during torpor displays tissue specific responses

Pronounced heterothermic responses are relatively rare among birds. Along with taxa such as hummingbirds and caprimulgids, the order Coliiformes (mousebirds) is known to possess the physiological capacity for torpor. During torpor, body temperature is greatly reduced and a bird becomes unresponsive to external stimuli until ambient temperatures return to more favorable conditions. Under such conditions, these birds are forced to rely only on their internal fuel storage for energy and show great reduction in metabolic rates by decreasing energy-expensive processes. This study investigated the role of the key insulin-Akt signaling kinase pathway involved in regulating energy metabolism and protein translation in the liver, kidney, heart, skeletal muscle, and brain of the speckled mousebird (Colius striatus). The degree of phosphorylation of well-conserved target residues with important regulatory function was examined in both the euthermic control and torpid birds. The results demonstrated marked differences in responses between the tissues with decreases in RPS6 S235/236 phosphorylation in the kidney (0.52 fold of euthermic) and muscle (0.29 fold of euthermic) as well as decreases in GS3K3β S9 in muscle (0.60 fold of euthermic) and GSK3α S21 (0.71 fold of euthermic) phosphorylation in kidney during torpor, suggesting a downregulation of this pathway. Interestingly, the liver demonstrated an increase in RPS6 S235/236 (2.89 fold increase) and P70S6K T412 (1.44 fold increase) phosphorylation in the torpor group suggesting that protein translation is maintained in this tissue. This study demonstrates that avian torpor is a complex phenomenon and alterations in this signaling pathway follow a tissue specific pattern.     

Behavioral thermoregulation by hypoxic rats.

To address whether a shift in hypothalamic thermal setpoint might be a significant factor in induction of hypoxic hypothermia, behavioral thermoregulation was examined in 7 female Sprague-Dawley rats implanted with radiotelethermometers for deep body temperature (Tb) measurement in a thermocline during normoxia (PO2 = 125 torr) and hypoxia (PO2 = 60 torr). Normoxic rats (TNox) selected a mean ambient temperature of 19.7 +/- 1.4 (SE) degrees C and maintained Tb at 37.0 +/- 0.2 degrees C. Hypoxic rats selected a significantly higher ambient temperature (THox = 28.6 +/- 2.2 degrees C) but maintained Tb significantly lower at 35.5 +/- 0.3 degrees C. Without a thermal gradient (ambient temperature = 25 degrees C), Tb during hypoxia was 35.4 +/- 0.4 degrees C. The maintenance of a lower body temperature during hypoxia through behavioral thermoregulation despite having warmer temperatures available supports the hypothesis that the thermoregulatory setpoint of hypoxic rats is shifted to promote thermoregulation at a lower Tb, effectively reducing oxygen demand when oxygen supply is limited.     

Arrhenius parameters of mitochondrial membrane respiratory enzymes in relation to thermoregulation in endotherms

The relationship between the body temperature (Tb), the Arrhenius critical temperature (T*), and the apparent activation energy above T* (Ea1), of liver and heart mitochondrial respiratory enzymes from eleven homeothermic and eight heterothermic species was determined using a linear regression analysis. An inverse relation was observed between T* and Ea1 during torpor and hibernation. In all thermoregulatory states, T* decreased with Tb and T* was equal to or below Tb. During torpor Ea1 increased in a linear manner as Tb was lowered. It appears that the above Arrhenius parameters are closely linked to the thermoregulatory state of endotherms and thus may represent an adaptation for function at low Tb's.     

Defining torpor in free-ranging bats: experimental evaluation of external temperature-sensitive radiotransmitters and the concept of active temperature

A variety of definitions involving body temperature (Tb), metabolic rate and behavior have been used to define torpor in mammals and birds. This problem is confounded in some studies of free-ranging animals that employ only skin temperature (Tsk), a measure that approximates but may not precisely reflect Tb. We assess the accuracy of Tsk in the context of a recent definition for torpor called active temperature. We compared the active temperatures of individual big brown bats (Eptesicus fuscus), which aggregate in cavities, with solitary, foliage-roosting hoary bats (Lasiurus cinereus). In captive big brown bats, we compared Tsk and core Tb at a range of ambient temperatures for clustered and solitary roosting animals, compared Tsk and Tb during arousal from torpor, and quantified the effect of flight on warming from torpor. Hoary bats had significantly lower active temperatures than big brown bats despite having the same normothermic Tsk. Tsk was significantly lower than Tb during normothermia but often greater than Tb during torpor. Flight increased the rate of warming from torpor. This effect was more pronounced for Tsk than Tb. This suggests that bats could rely on heat generated by flight muscles to complete the final stages of arousal. Using active temperature to define torpor may underestimate torpor due to ambient cooling of external transmitters or animals leaving roosts while still torpid. Conversely, active temperature may also overestimate shallow torpor use if it is recorded during active arousal when shivering and non-shivering thermogenesis warm external transmitters. Our findings illuminate the need for laboratory studies that quantify the relationship between metabolic rate and Tsk over a range of ambient temperatures.     

Hibernation and torpor in tropical and subtropical bats in relation to energetics, extinctions, and the evolution of endothermy

Torpor, the most effective means of energy conservation available to endotherms, is still widely viewed as a specific adaptation in a few high-latitude, cold-climate endotherms with no adaptive function in warm regions. Nevertheless, a growing number of diverse terrestrial mammals and birds from low latitudes (0-30°), including species from tropical and subtropical regions, are heterothermic and employ torpor. Use of torpor is especially important for bats because they are small, expend large amounts of energy when active, rely on a fluctuating food supply, and have only a limited capacity for storage of fat. Patterns of torpor in tropical/subtropical bats are highly variable, but short bouts of torpor with relatively high body temperatures (T(b)) are most common. Hibernation (a sequence of multiday bouts of torpor) has been reported for free-ranging subtropical tree-dwelling vespertilionids, cave-dwelling hipposiderids, and house-dwelling molossids. The observed range of minimum T(b) is ～6-30 °C, and the reduction of energy expenditure through the use of torpor, in comparison to normothermic values, ranges from 50 to 99%. Overall, torpor in the tropics/subtropics has been reported for 10 out of the currently recognized 18 bat families, which contain 1079 species, or 96.7% of all bats. Although it is unlikely that all of these are heterothermic, the large majority probably will be. Frequent use of torpor, including hibernation in diverse groups of tropical/subtropical bats, suggests that heterothermy is an ancestral chiropteran trait. Although data especially from the field are still scarce, it is likely that torpor, highly effective in reducing requirements for energy and water even under warm conditions, plays a crucial role in the long-term survival of the majority of small tropical and subtropical bats. Discovering how bats achieve this provides numerous opportunities for exiting new research.     

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 by an Australian murine rodent, the ash-grey mouse (Pseudomys albocinereus)

We examine here the thermal physiology of the ash-grey mouse, as there is a paucity of data to explain how Australian rodents meet thermoregulatory demands. Most ash-grey mice remained normothermic over a range of ambient temperatures (10°C to 30°C), although they became hyperthermic at high ambient temperatures. One individual entered torpor at ambient temperatures of 20°C and 25°C, with minimal body temperatures of 24.5°C and 28.4°C respectively, before spontaneously arousing. This is the first evidence of torpor use by an Australian murine rodent. Our data suggest that although ash-grey mice have the physiological ability to use torpor, it is used rarely, presumably due to other behavioural and physiological adaptations. Their higher-than-expected basal metabolic rate (1.56±0.25mLO(2)g(-1)h(-1)) indicates that ash-grey mice do not have a frugal approach to energy expenditure. Other standard physiological variables were typical of a generalised rodent. A readily-available omnivorous diet, nocturnal activity, semi-fossorial habit and social behaviour presumably allow a high energy lifestyle. A reluctance to use torpor, despite an apparent physiological ability to do so, supports the idea that the use of torpor reflects a net balance between the costs and benefits of a heterothermic thermoregulatory strategy.     

Torpor in free-ranging tawny frogmouths (Podargus strigoides).

Several small caprimulgiform birds (<80 g) are known to enter torpor, apparently to cope with a fluctuating supply of insect prey. Since the large Australian tawny frogmouth (Podargus strigoides; 381-556 g) is also insectivorous, we investigated its thermoregulatory behaviour and thermal biology to determine whether this species is also heterothermic. In an open woodland at approximately 1,000 m altitude, we equipped eight free-ranging birds with external temperature-sensitive radio transmitters attached to an elastic harness to measure skin temperature (T(skin)). Core body temperature (T(b)) was measured in three of these birds fitted with an additional intraperitoneal transmitter. T(skin) was closely correlated with T(b), although T(skin) was usually several degrees below T(b). During the three coldest months of the year (June-August), shallow torpor with T(b) as low as 29.1 degrees C occurred frequently, whereas during spring and summer, torpor was not recorded. Torpor occurred either during the night and/or during the first half of the day. Night torpor bouts were initiated after a short activity period around dusk and lasted on average for about 7 h. Torpid birds always aroused before sunrise to either commence a second short foraging period or to fly directly to a day roost tree. After birds roosted, T(b) fell again around sunrise, and birds occasionally entered a second dawn torpor bout; however, in most cases, T(b) increased rapidly not long after entry, most likely due to passive heating by the sun. We conclude that despite their large body size and energetically conservative hunting strategy, tawny frogmouths, like several related caprimulgiform species, frequently enter shallow torpor when low T(a) demands high energetic costs for normothermic thermoregulation and likely reduces insect availability.     

The role of polyunsaturated fatty acids in the expression of torpor by mammals: a review

Heterothermic mammals increase the proportion of polyunsaturated fatty acids (PUFA) in their body fats prior to entering torpor. Because PUFA have low melting points, it is thought that they play an important role in maintaining the fluidity of depot fats and membrane phospholipids at low body temperatures. However, PUFA are more prone to autoxidation when exposed to reactive oxygen species (ROS) during torpor and during the periodic arousals that characterize hibernation. A lack of PUFA or an excess of PUFA may constrain the use of torpor by heterothermic mammals. We performed a mixed model meta-analysis of 17 controlled-feeding studies to test the effect of dietary PUFA on the depth and expression of torpor by daily heterotherms and hibernators. We also reviewed the literature on the PUFA content of the diet and depot fats of heterothermic mammals to address two principal topics: (1) Do low dietary levels of PUFA reduce the expression of torpor under laboratory conditions and, if so, are free-ranging animals constrained by a lack of PUFA? (2) Do high dietary levels of PUFA result in a reduction in the use, depth, and duration of torpor and, if so, do free-ranging animals seek to optimize rather than maximize PUFA intake? Low-PUFA diets consistently increase the lower setpoint for body temperature and minimum metabolic rate for both hibernators and daily heterotherms. Above the lower setpoint, low-PUFA diets usually increase body temperature and metabolic rate and decrease the duration of torpor bouts and this effect is similar for hibernators and daily heterotherms. Free-ranging rodent hibernators have dietary PUFA intakes that are far higher than those of the low-PUFA diets offered in controlled-feeding experiments, so these hibernators may never experience the constraints associated with a lack of PUFA. Diets of free-ranging insectivorous bats and echidnas have PUFA levels that are less than half as high as those offered in experimental low-PUFA diets, yet they exhibit deep and extended bouts of torpor. We argue that alternate mechanisms exist for maintaining the fluidity of body fats and that high-PUFA intake may not be a prerequisite for deep and extended bouts of torpor. Four studies indicate that animals that were fed high-PUFA diets are reluctant to enter torpor and show shallower and shorter torpor bouts. Although authors attribute this response to autoxidation, these animals did not have a higher PUFA content in their depot fats than animals where PUFA was shown to enhance torpor. We suggest that these contradictory results indicate inter-specific or inter-individual variation in the ability to control ROS and limit autoxidation of PUFA. High dietary levels of PUFA will constrain the expression of torpor only when the oxidative challenge exceeds the capacity of the antioxidant defence system. Studies of diet selection indicate that insectivorous species with low dietary PUFA levels seek to maximize PUFA intake. However, herbivorous species that have access to plants and plant parts of high-PUFA content do not appear to maximize PUFA intake. These data suggest that animals attempt to optimize rather than maximize PUFA intake. The effect of PUFA should be viewed in the light of a cost-benefit trade-off, where the benefit of high-PUFA intake is an easier access to low body temperatures and the cost is increased risk of autoxidation.     

Torpor and hibernation in a basal placental mammal, the Lesser Hedgehog Tenrec Echinops telfairi

The patterns of heterothermy were measured in Lesser Hedgehog Tenrecs, Echinops telfairi, under semi-natural conditions in an outdoor enclosure during the austral mid-winter in southwestern Madagascar. The animals were implanted with miniaturized body temperature (Tb) loggers (iButtons) that measured body temperature every 42 min for 2 months (May and June). The tenrecs entered daily torpor on all 60 consecutive days of measurement, that is, on 100% of animal days, with body temperature closely tracking ambient temperature (Ta) during the ambient heating phase. The mean minimum daily Tb of the tenrecs was 18.44 +/- 0.50 degrees C (n = 174, N = 3), and never exceeded 25 degrees C whereas, apart from a few hibernation bouts in one animal, the mean maximum daily Tb was 30.73 +/- 0.15 degrees C (n = 167, N = 3). Thus during winter, tenrecs display the lowest normothermic Tb of all placental mammals. E. telfairi showed afternoon and early evening arousals, but entered torpor before midnight and remained in torpor for 12-18 h each day. One animal hibernated on two occasions for periods of 2-4 days. We consider E. telfairi to be a protoendotherm, and discuss the relevance and potential of these data for testing models on the evolution of endothermy.     

The effect of oxygen and adenosine on lizard thermoregulation

A regulated decrease in internal body temperature (Tb) appears to play a protective role against metabolic disruptions such as exposure to ambient hypoxia. This study examined the possibility that Tb depression is initiated when low internal oxygen levels trigger the release of adenosine, a neural modulator known to influence thermoregulation. We measured selected Tb of Anolis sagrei in a thermal gradient under varied ambient oxygen conditions and following the administration of the adenosine receptor antagonist 8-cyclopentyltheophylline (CPT). The average decrease in Tb observed following exposure to hypoxia (<10% O2) and following exhaustive exercise were 5 degrees and 3 degrees C, respectively, suggesting a role of oxygen availability on initiation of regulated hypothermia. When A. sagrei were run to exhaustion and recovered in hyperoxic (>95% O2) conditions, exercise-induced Tb depression was abolished. Administration of CPT similarly abolished decreased Tb due to both exercise and hypoxia. Trials using Dipsosaurus dorsalis indicate that elevated ambient oxygen during exercise does not influence blood pH or lactate accumulation, suggesting that these factors do not initiate changes in thermoregulatory setpoint following exhaustive exercise. We suggest that when oxygen is limiting, a decrease in arterial oxygen may trigger the release of adenosine, thereby altering the thermoregulatory setpoint.     

Hypothermia versus torpor in response to cold stress in the native Australian mouse Pseudomys hermannsburgensis and the introduced house mouse Mus musculus

This study compared torpor as a response to food deprivation and low ambient temperature for the introduced house mouse (Mus musculus) and the Australian endemic sandy inland mouse (Pseudomys hermannsburgensis). The house mouse (mass 13.0+/-0.48 g) had a normothermic body temperature of 34.0+/-0.20 degrees C at ambient temperatures from 5 degrees C to 30 degrees C and a basal metabolic rate at 30 degrees C of 2.29+/-0.07 mL O2 g(-1) h(-1). It used torpor with spontaneous arousal at low ambient temperatures; body temperature during torpor was 20.5+/-3.30 degrees C at 15 degrees C. The sandy inland mouse (mass 11.7+/-0.16 g) had a normothermic T(b) of 33.0+/-0.38 degrees C between T(a) of 5 degrees C to 30 degrees C, and a BMR of 1.45+/-0.26 mL O2 g(-1) h(-1) at 30 degrees C. They became hypothermic at low T(a) (T(b) about 17.3 degrees C at T(a)=15 degrees C), but did not spontaneously arouse. They did, however, survive and become normothermic if returned to room temperature (23 degrees C). We conclude that this is hypothermia, not torpor. Consequently, house mice (Subfamily Murinae) appear to use torpor as an energy conservation strategy whereas sandy inland mice (Subfamily Conilurinae) do not, but can survive hypothermia. This may reflect a general phylogenetic pattern of metabolic reduction in rodents. On the other hand, this may be related to differences in the social structure of house mice (solitary) and sandy inland mice (communal).