Black or white? Physiological implications of roost colour and choice in a microbat

Although roost choice in bats has been studied previously, little is known about how opposing roost colours affect the expression of torpor quantitatively. We quantified roost selection and thermoregulation in a captive Australian insectivorous bat, Nyctophilus gouldi (n=12) in winter when roosting in black and white coloured boxes using temperature-telemetry. We quantified how roost choice influences torpor expression when food was provided ad libitum or restricted in bats housed together in an outdoor aviary exposed to natural fluctuations of ambient temperature. Black box temperatures averaged 5.1 °C (maximum 7.5 °C) warmer than white boxes at their maximum daytime temperature. Bats fed ad libitum chose black boxes on most nights (92.9%) and on 100% of nights when food-restricted. All bats used torpor on all study days. However, bats fed ad libitum and roosting in black boxes used shorter torpor and spent more time normothermic/active at night than food-restricted bats and bats roosting in white boxes. Bats roosting in black boxes also rewarmed passively more often and to a higher skin temperature than those in white boxes. Our study suggests that N. gouldi fed ad libitum select warmer roosts in order to passively rewarm to a higher skin temperature and thus save energy required for active midday rewarming as well as to maintain a normothermic body temperature for longer periods at night. This study shows that colour should be considered when deploying bat boxes; black boxes are preferable for those bats that use passive rewarming, even in winter when food availability is reduced.     

Thermoregulation in free-ranging Nycteris thebaica (Nycteridae) during winter: No evidence of torpor

Bats are among the most heterothermic mammals, with nearly all species investigated under free-ranging conditions to date exhibiting some degree of daily torpor and/or hibernation. We investigated thermoregulation during late winter by seven Nycteris thebaica in a warm, semi-arid habitat in northern South Africa, using temperature-sensitive transmitters to measure skin temperature (T_skin). Unexpectedly, we found no evidence for any expression of daily torpor or hibernation based on a total of 86 days of data from 7 bats (one male and six females), despite air temperatures as low as ～10 °C. Instead, daytime T_skin was distributed unimodally with most values in the 33–35 °C range, and a minimum T_skin of 28.4 °C at a roost temperature of 24.6 °C. There are several possible reasons why N. thebaica may avoid torpor, including predation in roosts, and the long nightly foraging periods of this species compared to many others.     

Suppression of MAPKAPK2 during mammalian hibernation

Metabolic signaling coordinates the transition by hibernating mammals from euthermia into profound torpor. Organ-specific responses by activated p38 mitogen activated protein kinase (MAPK) are known to contribute to this transition. Therefore, we hypothesized that the MAPK-activated protein kinase-2 (MAPKAPK2), a downstream target of p38 MAPK, would also be active in establishing the torpid state. Kinetic parameters of MAPKAPK2 from skeletal muscle of Richardson’s ground squirrels, Spermophilus richardsonii, were analyzed using a fluorescence assay. MAPKAPK2 activity was 27.4 ± 1.27 pmol/min/mg in muscle from euthermic squirrels and decreased by ∼63% during cold torpor, while total protein levels were unchanged (as assessed by immunoblotting). In vitro treatment of MAPKAPK2 via stimulation of endogenous phosphatases and addition of commercial alkaline phosphatase decreased enzyme activity to only ∼3–5% of its original value in muscle extracts from both euthermic and hibernating squirrels suggesting that posttranslational modification suppresses MAPKAPK2 during the transition from euthermic to torpid states. Enzyme S_0.5 and n_H values for ATP and peptide substrates changed significantly between euthermia and torpor, and also between assays at 22 versus 10 °C but, kinetic parameters were actually closely conserved when values for the euthermic enzyme at 22 °C were directly compared with the hibernator enzyme at 10 °C. Arrhenius plots showed significantly different activation energies of 40.8 ± 0.7 and 54.3 ± 2.7 kJ/mol for the muscle enzyme from euthermic versus torpid animals, respectively but MAPKAPK2 from the two physiological states showed no difference in sensitivity to urea denaturation. Overall, the results show that total activity of MAPKAPK2 is in fact reduced, despite previous findings of p38 MAPK activation, and kinetic parameters are altered when ground squirrels enter torpor but protein stability is not apparently changed. The data suggest that MAPKAPK2 suppression may have a significant role in the differential regulation of muscle target proteins when ground squirrels enter torpor.     

The Resistance of a North American Bat Species (Eptesicus fuscus) to White-Nose Syndrome (WNS)

White-nose Syndrome (WNS) is the primary cause of over-winter mortality for little brown (Myotis lucifugus), northern (Myotis septentrionalis), and tricolored (Perimyotis subflavus) bats, and is due to cutaneous infection with the fungus Pseudogymnoascus (Geomyces) destructans (Pd). Cutaneous infection with P. destructans disrupts torpor patterns, which is thought to lead to a premature depletion of body fat reserve. Field studies were conducted at 3 WNS-affected hibernation sites to determine if big brown bats (Eptesicus fuscus) are resistant to Pd. Radio telemetry studies were conducted during 2 winters to determine the torpor patterns of 23 free-ranging E. fuscus hibernating at a site where Pd occurs. The body fat contents of free-ranging E. fuscus and M. lucifugus during hibernation at 2 different WNS-affected sites were also determined. The numbers of bats hibernating at the same site was determined during both: a) 4–7 years prior to the arrival of Pd, and, b) 2–3 years after it first appeared at this site. The torpor bouts of big brown bats hibernating at a WNS-affected site were not significantly different in length from those previously reported for this species. The mean body fat content of E. fuscus in February was nearly twice that of M. lucifugus hibernating at the same WNS-affected sites during this month. The number of M. lucifugus hibernating at one site decreased by 99.6% after P. destructans first appeared, whereas the number of E. fuscus hibernating there actually increased by 43% during the same period. None of the E. fuscus collected during this study had any visible fungal growth or lesions on their skin, whereas virtually all the M. lucifugus collected had visible fungal growth on their wings, muzzle, and ears. These findings indicate that big brown bats are resistant to WNS.     

The tradeoff between torpor use and reproduction in little brown bats (Myotis lucifugus)

In mammals, reproduction, especially for females is energetically demanding. Therefore, during the reproductive period females could potentially adjust patterns of thermoregulation and foraging in concert to minimise the energetic constraints associated with pregnancy and lactation. We assessed the influence of pregnancy, lactation, and post-lactation on torpor use and foraging behaviour by female little brown bats, Myotis lucifugus. We measured thermoregulation by recording skin temperature and foraging by tracking bats which carried temperature-sensitive radio-tags. We found that individuals, regardless of reproductive condition, used torpor, but the patterns of torpor use varied significantly between reproductive (pregnant and lactating) females and post-lactating females. As we predicted, reproductive females entered torpor for shorter bouts than post-lactating females. Although all females used torpor frequently, pregnant females spent less time in torpor, and maintained higher skin temperatures than either lactating or post-lactating females. This result suggests that delayed offspring development which has been associated with torpor use during pregnancy, may pose a higher risk to an individual’s reproductive success than reduced milk production during lactation. Conversely, foraging behaviour of radio-tagged bats did not vary with reproductive condition, suggesting that even short, shallow bouts of torpor produce substantial energy savings, likely obviating the need to spend more time foraging. Our data clearly show that torpor use and reproduction are not mutually exclusive and that torpor use (no matter how short or shallow) is an important means of balancing the costs of reproduction for M. lucifugus.     

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.     

Plasticity in body temperature and metabolic capacity sustains winter activity in a small endotherm (Rattus fuscipes)

Small mammals that remain active throughout the year at a constant body temperature have a much greater energy and food requirement in winter. Lower body temperatures in winter may offset the increased energetic cost of remaining active in the cold, if cellular metabolism is not constrained by a negative thermodynamic effect. We aimed to determine whether variable body temperatures can be advantageous for small endotherms by testing the hypothesis that body temperature fluctuates seasonally in a wild rat (Rattus fuscipes); conferring an energy saving and reducing food requirements during resource restricted winter. Additionally we tested whether changes in body temperature affected tissue specific metabolic capacity. Winter acclimatized rats had significantly lower body temperatures and thicker fur than summer acclimatized rats. Mitochondrial oxygen consumption and the activity of enzymes that control oxidative (citrate synthase, cytochrome c-oxidase) and anaerobic (lactate dehydrogenase) metabolism were elevated in winter and were not negatively affected by the lower body temperature. Energy transfer modeling showed that lower body temperatures in winter combined with increased fur thickness to confer a 25 kJ day^? 1 energy saving, with up to 50% owing to reduced body temperature alone. We show that phenotypic plasticity at multiple levels of organization is an important component of the response of a small endotherm to winter. Mitochondrial function compensates for lower winter body temperatures, buffering metabolic heat production capacity.     

Quantifying torpor in small mammals non-invasively using infrared thermocouples

Measurements of torpor use are pivotal for many research areas concerning the thermal biology of endotherms. Here, I used infrared thermocouples to non-invasively examine torpor patterns in the small marsupial fat-tailed dunnart (Sminthopsis crassicaudata). Sensors were installed inside the nesting chambers to continuously monitor fur temperature in undisturbed animals. Firstly, to verify the measurements, fur temperature was monitored simultaneously with body temperature using internal radio transmitters (n=6). Secondly, I conducted a food restriction study to demonstrate the reliability of the method within a physiological experiment (n=8). Based on the correspondence of simultaneously measured fur and body temperature during torpor bouts, I was able to confirm that infrared thermocouples provide reliable temporal information on torpor patterns. Furthermore, torpor use was successfully monitored over a 20-day food restriction study. The method can easily be adapted to suit other small mammal or bird species and presents a useful, inexpensive approach for examining torpor patterns remotely and non-invasively in the laboratory.     

Hybrid cooling vest for cooling between exercise bouts in the heat: Effects and practical considerations

While continuous cooling strategies may induce some ergonomic problems to occupational workers, cooling between work bouts may be an alternative for cooling them down in hot environments. The purpose of this study was to assess the effects of wearing a newly designed hybrid cooling vest (HCV) between two bouts of exercise. Inside a climatic chamber set at an air temperature of 37 °C and a relative humidity of 60%, twelve male participants underwent two bouts of intermittent exercise interspersed with a 30 min between-bout recovery session, during which HCV or a passive rest without any cooling (PAS) was administered. The results indicated that thermoregulatory, physiological, and perceptual strains were significantly lower in HCV than those in PAS during the recovery session (p≤0.022), which were accompanied with a large effect of cooling (Cohen's d=0.84–2.11). For the second exercise bout, the exercise time following HCV (22.13±12.27 min) was significantly longer than that following PAS (11.04±3.40 min, p=0.005, d=1.23) During this period, core temperature T_c was significantly lower by 0.14±0.0.15 °C in HCV than that in PAS. The heart rate drift over time was declined by 2±2 bpm min⁻¹ (p=0.001, d=1.00) and the rise in physiological strain index was reduced by 0.11±0.12 unit min⁻¹ (p=0.010, d=0.96) following the use of HCV. These findings suggested that using HCV could accelerate between-bout recovery and improve subsequent exercise performance by the enlarged body core temperature margin and blunted cardiovascular drift.     

Direct effects of incubation temperature on morphology,thermoregulatory behaviour and locomotor performance in jacky dragons (Amphibolurus muricatus)

Incubation temperature is one of the most studied factors driving phenotypic plasticity in oviparous reptiles. We examined how incubation temperature influenced hatchling morphology, thermal preference and temperature-dependent running speed in the small Australian agamid lizard Amphibolurus muricatus. Hatchlings incubated at 32 °C grew more slowly than those incubated at 25 and 28 °C during their first month after hatching, and tended to be smaller at one month. These differences were no longer significant by three months of age due to selective mortality of the smallest hatchlings. The cooler incubation treatments (25 °C and 28 °C) produced lizards that had deeper and wider heads. Hatchlings from 28 °C had cooler and more stable temperature preferences, and also had lower body temperatures during a 2-h thermoregulatory behaviour trial. Locomotor performance was enhanced at higher body temperatures, but incubation temperature had no measurable effect either independently or in interaction with body temperature. Our study demonstrates that incubation temperature has direct effects on morphology and thermoregulatory behaviour that appears to be independent of any size-dependent effects. We postulate a mechanistic link between these two effects.     

Hibernation in pygmy possums (Marsupialia: Burramyidae)

The minimum body temperatures and the longest torpor bouts recorded for pygmy possums (family Burramyidae) are compared with hibernating representatives from the insectivores, rodents and Australian bats. The lowest body temperatures of pygmy possums are around 6 degrees C which is within the range of the other mammalian groups. The torpor bouts of the pygmy possums last up to two weeks and the durations are similar to those of eutherian hibernators.     

Tropical bats counter heat by combining torpor with adaptive hyperthermia

Many tropical mammals are vulnerable to heat because their water budget limits the use of evaporative cooling for heat compensation. Further increasing temperatures and aridity might consequently exceed their thermoregulatory capacities. Here, we describe two novel modes of torpor, a response usually associated with cold or resource bottlenecks, as efficient mechanisms to counter heat. We conducted a field study on the Malagasy bat Macronycteris commersoni resting in foliage during the hot season, unprotected from environmental extremes. On warm days, the bats alternated between remarkably short micro-torpor bouts and normal resting metabolism within a few minutes. On hot days, the bats extended their torpor bouts over the hottest time of the day while tolerating body temperatures up to 42.9°C. Adaptive hyperthermia combined with lowered metabolic heat production from torpor allows higher heat storage from the environment, negates the need for evaporative cooling and thus increases heat tolerance. However, it is a high-risk response as the torpid bats cannot defend body temperature if ambient temperature increases above a critical/lethal threshold. Torpor coupled with hyperthermia and micro-torpor bouts broaden our understanding of the basic principles of thermal physiology and demonstrate how mammals can perform near their upper thermal limits in an increasingly warmer world.     

Digestive capacities allow the Mexican long-nosed bat (Leptonycteris nivalis) to live in cold environments

Digestive capabilities of nectar-feeding vertebrates to assimilate sugars affect their ability to acquire and store energy and could determine the minimal temperatures at which these animals can survive. Here, we described the sugar digestive capability of Leptonycteris nivalis and related it with its capacity to live in cold environments. We measured the enzymatic activity, food intake rate and changes in body mass of bats feeding at four different sucrose concentrations (from 5 to 35% wt./vol.). Additionally, we used a mathematical model to predict food intake and compared it with the food intake of bats. L. nivalis was able to obtain ~ 111.3 kJ of energy regardless of the sugar concentration of their food. Also, bats gained ~ 2.57 g of mass during the experimental trials and this gain was independent of sugar concentration. The affinity (1 / K_m) of sucrase (EC 3.2.1.48) was one order of magnitude higher relative to that reported for its sister species Leptonycteris yerbabuenae (0.250 and 0.0189 mmol^? 1 L, respectively), allowing this species to have a higher energy intake rate. We propose that the high ability to acquire energy conferred L. nivalis the faculty to invade cold environments, avoiding in this way the ecological competition with its sympatric species L. yerbabuenae.     

Transient yellow colouration of the bat Artibeus jamaicensis coincides with pollen consumption

Atypical colouration of the fur is not commonly recorded in bats. Here we report a transient yellow colouration attributed to dietary components in Artibeus jamaicensis in 2004 and 2005 at two localities of Yucatan, Mexico. Change in colouration was recorded in January when 62% of A. jamaicensis captured (n = 50) appeared yellow. All faecal samples collected from atypically coloured individuals consisted mainly of Ceiba pentandra pollen, which was also recovered from the fur. Carotenoid pigments contained in pollen ingested during peak Ceiba flowering appear to be incorporated into the hairs of A. jamaicensis. Further investigations are required to understand how pigment is transferred between the pollen and the hairs of A. jamaicensis.     

Staying cold through dinner: cold-climate bats rewarm with conspecifics but not sunset during hibernation

For temperate endotherms (i.e., mammals and birds) energy costs are highest during winter but food availability is lowest and many mammals depend on hibernation as a result. Hibernation is made up of energy-saving torpor bouts [periods of controlled reduction in body temperature (T _b)], which are interrupted by brief periodic arousals to normothermic T _b. What triggers these arousals in free-ranging hibernators is not well understood. Some temperate bats with intermittent access to flying insects during winter synchronize arousals with sunset, which suggests that, in some species, feeding opportunities influence arousal timing. We tested whether hibernating bats from a cold climate without access to food during winter also maintain a circadian rhythm for arousals or whether cues from conspecifics in the same cluster are more important. We used temperature telemetry to monitor skin temperature (T _sk) of free-ranging little brown bats (Myotis lucifugus) hibernating in central Manitoba, Canada, where temperatures from 22 October to 22 March were too cold for flying insects. We found no evidence bats synchronized arousals with photoperiod but they did arouse synchronously with other bats in the same cluster. Thus, in the northern part of their range where flying insects are almost never available during winter, little brown bats exhibit no circadian pattern to arousals. Warming synchronously with others could reduce the energetic costs of arousal for individuals or could reflect disturbance of torpid bats by cluster-mates.     

Hibernation patterns of Turkish hamsters: influence of sex and ambient temperature

Turkish hamsters (Mesocricetus brandti) are a model organism for studies of hibernation, yet a detailed account of their torpor characteristics has not been undertaken. This study employed continuous telemetric monitoring of body temperature (T _b) in hibernating male and female Turkish hamsters at ambient temperatures (T _as) of 5 and 13 °C to precisely characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (T _b > 20 °C), followed by deep torpor bouts lasting 4–6 days at T _a = 5 °C and 2–3 days at T _a = 13 °C. Females at T _a = 5 °C had longer bouts than males, but maintained higher torpor T _b; there were no sex differences at T _a = 13 °C. Neither body mass loss nor food intake differed between the two T _as. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both T _as generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.     

Temperature regulation and metabolism of an Australian bat, Chalinolobus gouldii (Chiroptera: Vespertilionidae) when euthermic and torpid

The thermal and metabolic physiology of Chalinolobus gouldii, an Australian vespertilionid bat, was studied in the laboratory using flow-through respirometry. Chalinolobus gouldii exhibits a clear pattern of euthermic thermoregulation, typical of endotherms with respect to body temperature and rate of oxygen consumption. The basal metabolic rate of euthermic Chalinolobus gouldii is approximately 86% of that predicted for a 17.5-g mammal and falls into the range of mass-specific basal metabolic rates ascribed to vespertilionid bats. However, like most vespertilionid bats, Chalinolobus gouldii displays extreme thermolability. It is able to enter into torpor and spontaneously arouse at ambient temperatures as low as 5 °C. Torpid bats thermoconform at moderate ambient temperature, with body temperature ≈ ambient temperature, and have a low rate of oxygen consumption determined primarily by Q ₁₀ effects. At low ambient temperature (< 10 °C), torpid C. gouldii begin to regulate their body temperature by increased metabolic heat production; they tend to maintain a higher body temperature at low ambient temperature than do many northern hemisphere hibernating bats. Use of torpor leads to significant energy savings. The evaporative water loss of euthermic bats is relatively high, which seems unusual for a bat whose range includes extremely arid areas of Australia, and is reduced during torpor. The thermal conductance of euthermic C. gouldii is less than that predicted for a mammal of its size. The thermal conductance is considerably lower for torpid bats at intermediate body temperature and ambient temperature, but increases to euthermic values for torpid bats when thermoregulating at low ambient temperature. Accepted: 22 August 1996     

Metabolic Hormone FGF21 Is Induced in Ground Squirrels during Hibernation but Its Overexpression Is Not Sufficient to Cause Torpor

Hibernation is a natural adaptation that allows certain mammals to survive physiological extremes that are lethal to humans. Near freezing body temperatures, heart rates of 3–10 beats per minute, absence of food consumption, and depressed metabolism are characteristic of hibernation torpor bouts that are periodically interrupted by brief interbout arousals (IBAs). The molecular basis of torpor induction is unknown, however starved mice overexpressing the metabolic hormone fibroblast growth factor 21 (FGF21) promote fat utilization, reduce body temperature, and readily enter torpor–all hallmarks of mammalian hibernation. In this study we cloned FGF21 from the naturally hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and found that levels of FGF21 mRNA in liver and FGF21 protein in serum are elevated during hibernation torpor bouts and significantly elevated during IBAs compared to summer active animals. The effects of artificially elevating circulating FGF21 concentrations 50 to 100-fold via adenoviral-mediated overexpression were examined at three different times of the year. This is the first time that a transgenic approach has been used in a natural hibernator to examine mechanistic aspects of hibernation. Surgically implanted transmitters measured various metrics of the hibernation phenotype over a 7-day period including changes in motor activity, heart rate and core body temperature. In April fed-state animals, FGF21 overexpression decreased blood insulin and free fatty acid concentrations, effects similar to those seen in obese mice. However, elevated FGF21 concentrations did not cause torpor in these fed-state animals nor did they cause torpor or affect metabolic parameters in fasted-state animals in March/April, August or October. We conclude that FGF21 is strongly regulated during torpor and IBA but that its overexpression is not sufficient to cause torpor in naturally hibernating ground squirrels.     

Global analysis of circulating metabolites in hibernating ground squirrels

Hibernation in mammals involves major alterations in nutrition and metabolism that would be expected to affect levels of circulating molecules. To gain insight into these changes we conducted a non-targeted LC–MS based metabolomic analysis of plasma using hibernating ground squirrels in late torpor (LT, T_b ~ 5 °C) or during an interbout arousal period (IBA, T_b ~ 5 °C) and non-hibernating squirrels in spring (T_b ~ 37 °C). Several metabolites varied and allowed differentiation between hibernators and spring squirrels, and between torpid and euthermic squirrels. Methionine and the short-chain carnitine esters of propionate and butyryate/isobutyrate were reduced in LT compared with the euthermic groups. Pantothenic acid and several lysophosphatidylcholines were elevated in LT relative to the euthermic groups, whereas lysophosphatidylethanolamines were elevated during IBA compared to LT and spring animals. Two regulatory lipids varied among the groups: sphingosine 1-phosphate was lower in LT vs. euthermic groups, whereas cholesterol sulfate was elevated in IBA compared to spring squirrels. Levels of long-chain fatty acids (LCFA) and total NEFA tended to be elevated in hibernators relative to spring squirrels. Three long-chain acylcarnitines were reduced in LT relative to IBA; free carnitine was also lower in LT vs. IBA. Our results identified several biochemical changes not previously observed in the seasonal hibernation cycle, including some that may provide insight into the metabolic limitations of mammalian torpor.