Torpid Animals Research Articles

Heme oxygenase expression and Nrf2 signaling during hibernation in ground squirrelsThis article is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease.

Mammalian hibernation is composed of long periods of deep torpor interspersed with brief periods of arousal in which the animals, fueled by high rates of oxygen-based thermogenesis in brown adipose tissue and skeletal muscle, power themselves back to euthermic (~37 degrees C) body temperatures. Strong antioxidant defences are important both for long-term cytoprotection during torpor and for coping with high rates of reactive oxygen species generated during arousal. The present study shows that the antioxidant enzyme heme oxygenase 1 (HO1) is strongly upregulated in selected organs of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) during hibernation. Compared with euthermic controls, HO1 mRNA transcript levels were 1.4- to 3.8-fold higher in 5 organs of hibernating squirrels, whereas levels of the constitutive isozyme HO2 were unchanged. Similarly, HO1 protein levels increased by 1.5- to 2.0-fold in liver, kidney, heart, and brain during torpor. Strong increases in the levels of the Nrf2 transcription factor and its heterodimeric partner protein, MafG, in several tissues indicated the mechanism of activation of hibernation-responsive HO1 gene expression. Furthermore, subcellular distribution studies with liver showed increased nuclear translocation of both Nrf2 and MafG in torpid animals. The data are consistent with the suggestion that Nrf2-mediated upregulation of HO1 expression provides enhanced antioxidant defence to counter oxidative stress in hibernating squirrels during torpor and (or) arousal.

Shotgun Proteomics Analysis of Hibernating Arctic Ground Squirrels

Mammalian hibernation involves complex mechanisms of metabolic reprogramming and tissue protection. Previous gene expression studies of hibernation have mainly focused on changes at the mRNA level. Large scale proteomics studies on hibernation have lagged behind largely because of the lack of an adequate protein database specific for hibernating species. We constructed a ground squirrel protein database for protein identification and used a label-free shotgun proteomics approach to analyze protein expression throughout the torpor-arousal cycle during hibernation in arctic ground squirrels (Urocitellus parryii). We identified more than 3,000 unique proteins from livers of arctic ground squirrels. Among them, 517 proteins showed significant differential expression comparing animals sampled after at least 8 days of continuous torpor (late torpid), within 5 h of a spontaneous arousal episode (early aroused), and 1-2 months after hibernation had ended (non-hibernating). Consistent with changes at the mRNA level shown in a previous study on the same tissue samples, proteins involved in glycolysis and fatty acid synthesis were significantly underexpressed at the protein level in both late torpid and early aroused animals compared with non-hibernating animals, whereas proteins involved in fatty acid catabolism were significantly overexpressed. On the other hand, when we compared late torpid and early aroused animals, there were discrepancies between mRNA and protein levels for a large number of genes. Proteins involved in protein translation and degradation, mRNA processing, and oxidative phosphorylation were significantly overexpressed in early aroused animals compared with late torpid animals, whereas no significant changes at the mRNA levels between these stages had been observed. Our results suggest that there is substantial post-transcriptional regulation of proteins during torpor-arousal cycles of hibernation.

Differential expression of microRNA species in organs of hibernating ground squirrels: A role in translational suppression during torpor

Mammalian hibernation includes long periods of profound torpor where the rates of all metabolic processes are strongly suppressed in a reversible manner. We hypothesized that microRNAs (miRNAs), small non-coding transcripts that bind to mRNA, could play a role in the global suppression of mRNA translation when animals enter torpor. Selected miRNA species (4-9 of the following: mir-1, mir-24, mir-15a, mir-16, mir-21, mir-122a, mir-143, mir-146 and mir-206) were evaluated in four organs of euthermic versus hibernating ground squirrels, Spermophilus tridecemlineatus using RT-PCR. Levels of mir-24 transcripts were significantly reduced in heart and skeletal muscle of torpid animals as were mir-122a levels in the muscle. Mir-1 and mir-21 both increased significantly in kidney during torpor by 2.0- and 1.3-fold, respectively. No changes were found for the four miRNA species analyzed in liver. Protein levels of Dicer, an enzyme involved in miRNA processing were also quantified in heart, kidney and liver. Dicer protein levels increased by 2.7-fold in heart during hibernation but decreased by 60% in kidney. These data are the first report that differential regulation of miRNA levels occurs during mammalian hibernation and they provide a mechanism for reversible gene silencing during torpor that can be rapidly reversed to allow renewed translation of mRNA when animals arouse back to euthermia.

Coping with the stress: Expression of ATF4, ATF6, and downstream targets in organs of hibernating ground squirrels

Perturbation of the endoplasmic reticulum (ER) protein folding apparatus via any one of several environmental or metabolic stresses rapidly triggers a complex program of cellular responses that is termed the unfolded protein response (UPR). Stresses that trigger this response in mammals can include low temperature, hypoxia, ischemia, and oxidative stress. All of these can be natural features of mammalian hibernation, and hence the UPR might be integral to long term survival in a state of cold torpor. The present study analyzes changes in gene and/or protein expression of multiple markers of the UPR in tissues of euthermic (control) versus hibernating ground squirrels, Spermophilus tridecemlineatus. Immunoblot analysis of ATF4 protein expression revealed strong increases of 1.9- to 2.5-fold in brown adipose tissue, skeletal muscle, and brain during hibernation. However, transcript levels of atf4 were unchanged or lowered which suggests that ATF4 protein levels were regulated at the translational level. Subcellular localization studies showed that ATF4 translocated into the nucleus during hibernation, as did its cofactor, the phosphorylated form of CREB-1, which rose by 25- to 39-fold in nuclear extracts of brain and skeletal muscle of torpid animals. The responses of other proteins involved in the UPR including p-PERK, ATF6, GADD153, and GADD34 were also evaluated. The data suggest that ATF4 up-regulation may play an important role in coordinating gene expression responses that support the hibernating phenotype.

Mitochondrial metabolism during daily torpor in the dwarf Siberian hamster: role of active regulated changes and passive thermal effects

During daily torpor in the dwarf Siberian hamster, Phodopus sungorus, metabolic rate is reduced by 65% compared with the basal rate, but the mechanisms involved are contentious. We examined liver mitochondrial respiration to determine the possible role of active regulated changes and passive thermal effects in the reduction of metabolic rate. When assayed at 37 degrees C, state 3 (phosphorylating) respiration, but not state 4 (nonphosphorylating) respiration, was significantly lower during torpor compared with normothermia, suggesting that active regulated changes occur during daily torpor. Using top-down elasticity analysis, we determined that these active changes in torpor included a reduced substrate oxidation capacity and an increased proton conductance of the inner mitochondrial membrane. At 15 degrees C, mitochondrial respiration was at least 75% lower than at 37 degrees C, but there was no difference between normothermia and torpor. This implies that the active regulated changes are likely more important for reducing respiration at high temperatures (i.e., during entrance) and/or have effects other than reducing respiration at low temperatures. The decrease in respiration from 37 degrees C to 15 degrees C resulted predominantly from a considerable reduction of substrate oxidation capacity in both torpid and normothermic animals. Temperature-dependent changes in proton leak and phosphorylation kinetics depended on metabolic state; proton leakiness increased in torpid animals but decreased in normothermic animals, whereas phosphorylation activity decreased in torpid animals but increased in normothermic animals. Overall, we have shown that both active and passive changes to oxidative phosphorylation occur during daily torpor in this species, contributing to reduced metabolic rate.

Torpor and basking in a small arid zone marsupial

The high energetic cost associated with endothermic rewarming from torpor is widely seen as a major disadvantage of torpor. We tested the hypothesis that small arid zone marsupials, which have limited access to energy in the form of food but ample access to solar radiation, employ basking to facilitate arousal from torpor and reduce the costs of rewarming. We investigated torpor patterns and basking behaviour in free-ranging fat-tailed dunnarts Sminthopsis crassicaudata (10 g) in autumn and winter using small, internal temperature-sensitive transmitters. Torpid animals emerged from their resting sites in cracking soil at approximately 1000 h with body temperatures as low as 14.6 degrees C and positioned themselves in the sun throughout the rewarming process. On average, torpor duration in autumn was shorter, and basking was less pronounced in autumn than in winter. These are the first observations of basking during rewarming in S. crassicaudata and only the second direct evidence of basking in a torpid mammal for the reduction of energetic costs during arousal from torpor and normothermia. Our findings suggest that although overlooked in the past, basking may be widely distributed amongst heterothermic mammals. Therefore, the energetic benefits from torpor use in wild animals may currently be underestimated.

Antioxidant defense in hibernation: Cloning and expression of peroxiredoxins from hibernating ground squirrels, Spermophilus tridecemlineatus

Mammalian hibernation is characterized by prolonged torpor bouts interspersed by brief arousal periods. Adequate antioxidant defenses are needed both to sustain cell viability over weeks of deep torpor and to defend against high rates of oxyradical formation associated with massive oxygen-based thermogenesis during arousal. The present study shows that up-regulation of peroxiredoxins contributes to antioxidant defense during torpor in thirteen-lined ground squirrels, Spermophilus tridecemlineatus. Expression levels of three isozymes of the 2-Cys peroxiredoxin (Prdx) family were quantified by Western blotting, the results showing 4.0- and 12.9-fold increases in Prdx1 protein in brown adipose tissue (BAT) and heart, respectively, during hibernation compared with euthermia. Comparable increases in Prdx2 were 2.4- and 3.7-fold whereas Prdx3 rose by 3.1-fold in heart of torpid animals. Total 2-Cys peroxiredoxin enzymatic activity also rose during hibernation by 1.5-fold in heart and 3.5-fold in BAT. Furthermore, RT-PCR showed that prdx2 mRNA levels increased by 1.7- and 3.7-fold in BAT and heart, respectively, during hibernation. A partial nucleotide sequence of prdx2 from ground squirrels was obtained by PCR amplification, the deduced amino acid sequence showing 96–97% identity with Prdx2 from other mammals. Some unique amino acid substitutions were identified that might contribute to stabilizing Prdx2 conformation at the near 0 °C body temperatures during torpor.

Rapid and reversible changes in intrahippocampal connectivity during the course of hibernation in European hamsters

The hippocampal formation is a highly plastic brain structure that undergoes structural remodeling in response to internal and external challenges such as metabolic imbalance and repeated stress. We investigated whether the extreme alterations in metabolic status that occur during the course of hibernation in European hamsters cause structural changes in the dendritic arborizations of the CA3 pyramidal neurons and their main excitatory afferents, the mossy fiber terminals (MFT), that originate in the dentate gyrus. We report that apical, but not basal, dendritic trees of Golgi-impregnated CA3 principal neurons are significantly shorter, less branched, and less spiny in hypothermic hamsters compared with active animals. After the induction of arousal from torpor, within 2 h, the apical dendritic lengths, branching patterns, and spine density estimations returned to levels found in active, euthermic hamsters. The ultrastructure of MFT in hibernating hamsters showed a significant reduction in synaptic vesicle density and in the percentage of MFT area covered by spine profiles. Awakened hamsters showed restoration of MFT morphology to that seen in active animals. MFT of torpid animals also showed a significant increase in the percentage area of mitochondrial profiles that remained higher 3 h after induced arousal from hibernation compared with euthermic controls. Thus, the torpid/awakening cycle of the hibernating European hamster causes a rapid and reversible morphological reorganization of intrahippocampal subregions involved in information processing. The reported reductions in morphological connectivity between the dentate gyrus and the CA3 subregions could underlie the cessation of exploratory activity and spatial navigation skills during hibernation.

Evidence for a reduced transcriptional state during hibernation in ground squirrels

During mammalian hibernation, metabolic rate can be reduced to <5% of the euthermic rate as a result of coordinated suppression of multiple energy expensive metabolic processes. Gene transcription is one of these and the present study examines mechanisms of transcriptional control that could contribute to lowering the rate of gene expression in torpor. Histone deacetylases (HDAC) have been linked to gene silencing and measured HDAC activity was 1.82-fold higher in skeletal muscle of hibernating thirteen-lined ground squirrels, Spermophilus tridecemlineatus, compared with euthermic controls. Western blotting also showed that HDAC1 and HDAC4 protein levels were 1.21-and 1.48-fold higher, respectively, in muscle from torpid animals. Histone H3 was also evaluated by Western blotting. Total histone H3 was unchanged but two forms of covalently modified histone H3 that are associated with active transcription (phosphorylated Ser 10 and acetylated Lys 23) were significantly reduced by 38–39% in muscle during hibernation. Finally, RNA polymerase II activity was measured using a PCR-based approach; activity in muscle from hibernating squirrels was only 57% of the euthermic value. These data support an overall decrease in transcriptional activity in skeletal muscle of hibernating animals that is accomplished by multiple molecular mechanisms.

Energy Metabolism and Evaporative Water Loss in the European Free‐Tailed Bat and Hemprich’s Long‐Eared Bat (Microchiroptera): Species Sympatric in the Negev Desert

We compared the thermoregulatory abilities of two insectivorous bat species, Tadarida teniotis (mean body mass 32 g) and Otonycteris hemprichii (mean body mass 25 g), that are of different phylogenetic origins and zoogeographic distributions but are sympatric in the Negev Desert. At night, both were normothermic. By day, both were torpid when exposed to ambient temperatures (T(a)) below 25 degrees Celsius, with concomitant adjustments in metabolic rate (MR). Otonycteris hemprichii entered torpor at higher T(a) than T. teniotis, and, when torpid, their body temperatures (T(b)) were 1 degrees -2 degrees Celsius and 5 degrees -8 degrees Celsius above T(a), respectively; MR was correspondingly reduced. At night, the lower critical temperature of T. teniotis was 31.5 degrees Celsius, and that of O. hemprichii was 33 degrees Celsius. Mean nocturnal thermoneutral MR of T. teniotis was 37% greater than that of O. hemprichii. At high T(a), evaporative water loss (EWL) increased markedly in both species, but it was significantly higher in T. teniotis above 38 degrees Celsius. In both species, the dry heat transfer coefficient (thermal conductance) followed the expected pattern for small mammals, by day and by night. Total EWL was notably low in normothermic and torpid animals of both species, much lower than values reported for other bats, indicating efficient water conservation mechanisms in the study species. Comparing thermoregulatory abilities suggests that O. hemprichii is better adapted to hot, arid environments than T. teniotis, which may explain its wider desert distribution. By both standard and phylogenetically informed ANCOVA, we found no differences in basal metabolic rate (BMR) between desert and nondesert species of insectivorous bats, substantiating previous studies suggesting that low BMR is a characteristic common to insectivorous bats in general.

Mitochondrial Metabolism in Hibernation: Metabolic Suppression, Temperature Effects, and Substrate Preferences

We compared liver and skeletal muscle mitochondrial function among activity states to characterize regulated reversible metabolic suppression in the mammalian hibernator Spermophilus tridecemlineatus. At 37 degrees C, succinate oxidation was 70% lower in the liver mitochondria from torpid animals than in those from summer-active animals or in animals arousing from torpor. Respiration was very sensitive to temperature (Q(10) 5.8-9.8), and when measured at 25 degrees or 5 degrees C there was no difference among the three states. Liver mitochondria from summer-active animals oxidized pyruvate and beta -hydroxybutyrate at higher rates than those from torpid animals, and flux through complex 4 of the electron transport chain was about three- and fivefold higher than flux through complexes 2-4 and complexes 1-4, respectively. In the hibernating and arousing animals there was no difference in flux through complexes 2-4 and complex 4, suggesting a downregulation of cytochrome c oxidase in liver mitochondria during the hibernation season. Muscle mitochondrial respiration did not differ between the torpid and summer-active states in any of the parameters measured. The data support a regulated, reversible decrease of liver (but not muscle) mitochondrial oxidative phosphorylation in hibernating ground squirrels.

Metabolic rate and body temperature reduction during hibernation and daily torpor.

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

The effect of temperature on adrenergic receptors of alveolar type II cells of a heterothermic marsupial

Fat-tailed dunnarts, Sminthopsis crassicaudata, survive dramatic changes in body temperature during torpor without experiencing surfactant dysfunction. Adrenergic factors regulate surfactant secretion through β 2-adrenergic receptors on alveolar type II cells. Temperature has no effect on the secretory response of dunnart type II cells to adrenergic stimulation. We hypothesise that during torpor, dunnart type II cells up-regulate the number of adrenergic receptors present on type II cells to enable stimulation at lower concentrations of agonist. Here, we isolated type II cells from warm-active (35 °C) and torpid (15 °C) dunnarts and examined the effects of an in vitro temperature change on the number and activity of adrenergic receptors. Torpor did not affect the β-adrenergic receptor number. However, we observed a significant decrease in adrenergic receptor number when cells from warm-active animals were incubated at 15 °C and when cells from torpid animals were incubated at 37 °C. cAMP production was significantly higher in type II cells from torpid dunnarts than warm-active dunnarts and this may contribute, in part, to the temperature insensitivity we have previously observed in the adrenergic regulation of surfactant secretion.

Normalization of aortic function during arousal episodes in the hibernating ground squirrel

Hypothermia is commonly used to restrict organ damage during preservation of tissue, but does not offer complete protection. Organ damage after reperfusion/rewarming is amongst others caused by an impairment of vascular properties, particularly endothelium-dependent vasodilatation. We hypothesized that hibernating small animals, which frequently cycle through periods of deep cooling (torpor) and full rewarming (arousal), employ specific mechanisms to preserve vascular function after cooling and reperfusion. Therefore we measured contraction of aortic tissue of hibernating European ground squirrels after 24 h and 7 days of torpor, arousal (1.5 h) and in non-hibernating animals. To assess the role of nitric oxide (NO), experiments were performed in the absence and presence of the NO-synthesis inhibitor, L-NMMA (10 −4 M). Maximum contraction to phenylephrine and angiotensin II was doubled in 7-days torpid animals without a shift in EC 50, compared to the other 3 groups. Maximum contraction to KCl was doubled in 7-days torpid animals compared to the arousal group and non-hibernating animals. Relaxation to acetylcholine (ACh) and sodium nitrite in phenylephrine precontracted rings did not differ between groups. In the presence of L-NMMA, the maximum of concentration-response curves for all three vasoconstrictors was increased by about 30% in the arousal group, but unaffected in other groups. L-NMMA completely inhibited ACh-induced relaxation in 24-h torpid animals and non-hibernating animals, but only partially in 7-days torpid animals and in the arousal group. From this we conclude that vascular adaptation proceeds during torpor. Further, increased contractility of aortic tissue during long torpor returns to normal within 1.5 hours of arousal, which is associated with an increased basal NO synthesis. In addition, involvement of NO in agonist-mediated relaxation differs between the various stages of hibernation.Thus, hibernating animals have effectively developed mechanisms to preserve vascular function after cooling and rewarming.

MRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii).

All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

Reversible depression of oxygen consumption in isolated liver mitochondria during hibernation.

The biochemical mechanisms by which hibernators cool as they enter torpor are not fully understood. In order to examine whether rates of substrate oxidation vary as a function of hibernation, liver mitochondria were isolated from telemetered ground squirrels (Spermophilus lateralis) in five phases of their annual hibernation cycle: summer active, and torpid, interbout aroused, entrance, and arousing hibernators. Rates of state 3 and state 4 respiration were measured in vitro at 25 degrees C. Relative to mitochondria from summer-active animals, rates of state 3 respiration were significantly depressed in mitochondria from torpid animals yet fully restored during interbout arousals. These findings indicate that a depression of ADP-dependent respiration in liver mitochondria occurs during torpor and is reversed during the interbout arousals to euthermia. Because this inhibition was determined to be temporally independent of entrance and arousal, it is unlikely that active suppression of state 3 respiration causes entrance into torpor by facilitating metabolic depression. In contrast to the observed depression of state 3 respiration in torpid animals, state 4 respiration did not differ significantly among any of the five groups, suggesting that alterations in proton leak are not contributing appreciably to downregulation of respiration in hibernation.

Alterations in the surface properties of lung surfactant in the torpid marsupial Sminthopsis crassicaudata.

Torpor changes the composition of pulmonary surfactant (PS) in the dunnart Sminthopsis crassicaudata [C. Langman, S. Orgeig, and C. B. Daniels. Am. J. Physiol. 271 (Regulatory Integrative Comp. Physiol, 40): R437-R445, 1996]. Here we investigated the surface activity of PS in vitro. Five micrograms of phospholipid per centimeter squared surface area of whole lavage (from mice or from warm-active, 4-, or 8-h torpid dunnarts) were applied dropwise onto the sub-phase of a Wilhelmy-Langmuir balance at 20 degrees C and stabilized for 20 min. After 4 h of torpor, the adsorption rate increased, and equilibrium surface tension (STeq), minimal surface tension (STmin), and the %area compression required to achieve STmin decreased, compared with the warm-active group. After 8 h of torpor, STmin decreased [from 5.2 +/- 0.3 to 4.1 +/- 0.3 (SE) mN/m]; %area compression required to achieve STmin decreased (from 43.4 +/- 1.0 to 27.4 +/- 0.8); the rate of adsorption decreased; and STeq increased (from 26.3 +/- 0.5 to 38.6 +/- 1.3 mN/m). ST-area isotherms of warm-active dunnarts and mice at 20 degrees C had a shoulder on compression and a plateau on expansion. These disappeared on the isotherms of torpid dunnarts. Samples of whole lavage (from warm-active and 8-h torpor groups) containing 100 micrograms phospholipid/ml were studied by using a captive-bubble surfactometer at 37 degrees C. After 8 h of torpor, STmin increased (from 6.4 +/- 0.3 to 9.1 +/- 0.3 mN/m) and %area compression decreased in the 2nd (from 88.6 +/- 1.7 to 82.1 +/- 2.0) and 3rd (from 89.1 +/- 0.8 to 84.9 +/- 1.8) compression-expansion cycles, compared with warm-active dunnarts. ST-area isotherms of warm-active dunnarts at 37 degrees C did not have a shoulder on compression. This shoulder appeared on the isotherms of torpid dunnarts. In conclusion, there is a strong correlation between in vitro changes in surface activity and in vivo changes in lipid composition of PS during torpor, although static lung compliance remained unchanged (see Langman et al. cited above). Surfactant from torpid animals is more active at 20 degrees C and less active at 37 degrees C than that of warm-active animals, which may represent a respiratory adaptation to low body temperatures of torpid dunnarts.

Prey selection by insectivorous bats: are essential fatty acids important?

Although body temperatures of torpid animals fall well below the melting point of many fats, body fats must remain fluid to be metabolized. High levels of dietary essential fatty acids (EFAs) may facilitate the use of torpor, as an increase in the proportion of EFAs in body fat decreases the melting point of body fats. The purpose of our study was to determine the importance of EFAs in the diets of four reproductive classes (males and nonreproductive pregnant, and lactating females) of three species of insectivorous bats: Myotis yumanensis, M. lucifugus, and M. californicus. Males are expected to use torpor frequently and, therefore, should select a diet high in EFAs. We ranked insect orders found in bat feces (diet) and light-suction-trap samples (availability) on the basis of their linoleic acid (a common EFA) content. We used these ranks to obtain estimates of the linoleic acid content of bats' diets, i.e., "dietary fat scores," and of available insects' "trap sample fat scores," respectively. Dietary fat scores did not differ significantly among reproductive classes. As predicted, the average fat score of males was high relative to the fat score of available insects. However, fat scores of male M. lucifugus and M. yumanensis were significantly greater than those of male M. californicus. Thus, EFAs do not appear to be as important in the diet of the latter species, suggesting that torpor may not be employed regularly.

Thermoregulatory Responses of Cold-Acclimated Fat Mice (Steatomys pratensis)

Ten wild-caught fat mice ( Steatomys pratensis ) were acclimated under short photoperiod (10L:14D) for 50 days at 15°C followed by an additional 50 days at 10°C. All the fat mice became torpid at least once during the study, and there was no significant effect of gender or ambient temperature on the number of animals that displayed torpor. There also was no significant effect of gender or ambient temperature on the body temperature of torpid mice at ambient temperatures of 10 or 15°C, although the minimum body temperature recorded (13–15°C) was ca. 5°C lower than that reported in previous studies. The oxygen consumption of torpid animals was slightly higher at 10°C than at 15°C, presumably to prevent a further decline in body temperature. However, the mean duration of torpor was longer at 10°C (19.5 ± 1.5 h, n = 4) than at 15°C (14.5 ± 4.1 h, n = 4), which enabled fat mice to save 50.4 ± 9.5% of the daily energy expenditure of euthermic animals at 10°C, compared to savings of only 39.9 ± 4.5% at 15°C. Therefore, it appears that S. pratensis is capable of modifying the duration of torpor to offset an increase in the cost of torpor at lower ambient temperatures.

The degree of dietary fatty acid unsaturation affects torpor patterns and lipid composition of a hibernator.

Diets rich in unsaturated and polyunsaturated fatty acids have a positive effect on mammalian torpor, whereas diets rich in saturated fatty acids have a negative effect. To determine whether the number of double bonds in dietary fatty acids are responsible for these alterations in torpor patterns, we investigated the effect of adding to the normal diet 5% pure fatty acids of identical chain length (C18) but a different number of double bonds (0, 1, or 2) on the pattern of hibernation of the yellow-pine chipmunk, Eutamias amoenus. The response of torpor bouts to a lowering of air temperature and the mean duration of torpor bouts at an air temperature of 0.5 degree C (stearic acid C18:0, 4.5 +/- 0.8 days, oleic acid C18:1, 8.6 +/- 0.5 days; linoleic acid C18:2, 8.5 +/- 0.7 days) differed among animals that were maintained on the three experimental diets. The mean minimum body temperatures (C18:0, +2.3 +/- 0.3 degrees C; C18:1, +0.3 +/- 0.2 degree C; C18:2, -0.2 +/- 0.2 degrees C), which torpid individuals defended by an increase in metabolic rate, and the metabolic rate of torpid animals also differed among diet groups. Moreover, diet-induced differences were observed in the composition of total lipid fatty acids from depot fat and the phospholipid fatty acids of cardiac mitochondria. For depot fat 7 of 13 and for heart mitochondria 7 of 14 of the identified fatty acids differed significantly among the three diet groups. Significant differences among diet groups were also observed for the sum of saturated, unsaturated and polyunsaturated fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)