Shallow Torpor Research Articles

Similarities and differences regarding acute anorexia nervosa and semi-starvation: does behavioral thermoregulation play a central role?

To clarify the association between acute anorexia nervosa (AN) and semi-starvation (SS) by focusing on similarities and differences in physiology, mood, and behavior. A comparison of published literature between these two groups. Both groups show similar hormonal and metabolic changes in response to caloric restriction and extreme weight loss (~25%). Associated changes result in a reduced body temperature (Tcore-low). Maintenance of body temperature within a specific range is crucial to survival. However, both groups cannot activate autonomic strategies to maintain their Tcore-low, such as increasing metabolic rate, constricting skin blood vessels, or shivering. Furthermore, Tcore-low increases the individuals' "coldness sensations" throughout the body, hence the frequent reports from ANs and SSs of "feeling cold." To eliminate these uncomfortable "coldness sensations" and, importantly, to maintain Tcore-low, ANs, and SSs "select" different thermoregulatory behavioral strategies. It is proposed that the primary differences between AN and SS, based on genetics, now manifest due to the "selection" of different thermo-regulatory behaviors. AN patients (ANs) "select" hyperactive behavior (HyAc), which increases internal metabolic heat and thus assists with maintaining Tcore-low; in harmony with hyperactive behavior is a lively mood. Also related to this elevated arousal pattern, ANs experience disrupted sleep. In contrast, SS individuals "select" a passive thermo-behavioral strategy, "shallow torpor," which includes reduced activity, resulting in energy conservation. In addition, this inactivity aids in the retention of generated metabolic heat. Corresponding to this lethargic behavior, SS individuals display a listless mood and increased sleep. Initial similarities between the two are attributable to physiological changes related to extreme weight loss. Differences are most likely attributable to genetically programmed "selection" of alternate thermoregulatory strategies, primarily to maintain Tcore-low. However, if acute AN is prolonged and evolves into a chronic condition, AN will more closely align with starvation and more precisely reflect SS symptomology.

Saving energy via short and shallow torpor bouts

Maintaining a high and stable body temperature as observed in most endothermic mammals and birds is energetically costly and many heterothermic species reduce their metabolic demands during energetic bottlenecks through the use of torpor. With the increasing number of heterotherms revealed in a diversity of habitats, it becomes apparent that triggers and patterns of torpor use are more variable than previously thought. Here, we report the previously overlooked use of, shallow rest-time torpor (body temperature >30 °C) in African lesser bushbabies, Galago moholi. Body core temperature of three adult male bushbabies recorded over five months showed a clear bimodal distribution with an average active modal temperature of 39.2 °C and a resting modal body temperature of 36.7 °C. Shallow torpor was observed in two out of three males (n = 29 torpor bouts) between June and August (austral winter), with body temperatures dropping to an overall minimum of 30.7 °C and calculated energy savings of up to 10%. We suggest that shallow torpor may be an ecologically important, yet mostly overlooked energy-saving strategy employed by heterothermic mammals. Our data emphasise that torpor threshold temperatures need to be used with care if we aim to fully understand the level of physiological plasticity displayed by heterothermic species.

Hummingbirds use insulated nests to save energy for the clutch

All wild animals need to perform a balancing act with the energy they take in and the energy they burn. To save energy, hummingbirds can use torpor, a process where they decrease their body temperature and metabolism. However, mothers only use torpor in an emergency, as their eggs and chicks are sensitive to low temperatures. Erich Eberts, from Loyola Marymount University, USA, and an international team of colleagues from various US and Canadian universities set out to determine how female Allen’s hummingbirds (Selasphorus sasin) manage their energy budget at night-time while incubating chicks and, importantly, whether their nests provide energy savings.From January to May in 2017 and 2018, the team located more than 60 Allen’s hummingbird nests with eggs or chicks in the suburbs around Loyola Marymount University in Los Angeles, CA, USA. The researchers then monitored 56 of the nests until the chicks fledged (40 nests), were killed by predators (12 nests) or failed for some other unknown reason (4 nests). To measure the temperatures of the hummingbird mothers, the researchers placed thermal cameras 0.5-1 m away from nests, programmed to capture one image every minute, successfully collecting thermal images on 108 nights from 14 of the 56 nests they were monitoring. The researchers then used the thermal images to determine the mother’s body temperature, and the temperature of a nearby branch or leaf to measure the local environmental temperature.For their analysis, the team then defined the thermal states of the hummingbirds as either normal body temperature, deep torpor (low body temperature), shallow torpor (moderately low body temperature) or the transition to or from torpor. Then, in a separate series of theoretical models, the researchers estimated the energetic costs of the hummingbirds under a range of conditions, with either torpid or normal body temperatures for 0, 2, 6, 10 or 12 h, and either nesting or completely exposed to the surrounding air.The researchers found that on 103 nights (of the total 108), the hummingbird mothers maintained normal body temperatures, and failed to enter torpor to conserve energy. However, one hummingbird did enter deep torpor on two nights in February, decreasing its skin temperature to 14°C, and two other hummingbirds used shallow torpor on three nights, decreasing skin temperatures to approximately 19°C. However, the chicks of the mothers that sporadically used torpor still developed successfully and took flight when they were old enough. The researchers concluded that hummingbird mothers rarely enter torpor to protect their chicks from nocturnal low temperatures, although they do resort to using torpor in exceptional circumstances.Nevertheless, the team discovered that the temperatures within the nest remained approximately 7.5°C higher than the surrounding air, thanks to the insulation they provided, which allowed the roosting mothers to conserve energy. In one of the teams’ energetic models, a nesting mother maintaining a normal body temperature for 12 h could save around 24% of her energy thanks to her nest compared with a mother exposed to the environment. Yet, if nesting hummingbirds used torpor for 6 h, they could save 41% of the energy they would expend maintaining a normal body temperature in the open.Overall, Eberts and colleagues successfully used a non-invasive method to determine that mother Allen’s hummingbirds almost always forgo torpor when nesting. The mothers prioritize the health of their chicks, instead of saving energy for themselves. In addition, hummingbird mothers can depend on their well-constructed, insulated nests to provide a warm home and save energy when raising their young.

Free-living Allen’s hummingbirds (Selasphorus sasin) rarely use torpor while nesting

For reproducing animals, maintaining energy balance despite thermoregulatory challenges is important for surviving and successfully raising offspring. This is especially apparent in small endotherms that exhibit high mass-specific metabolic rates and live in unpredictable environments. Many of these animals use torpor, substantially reducing their metabolic rate and often body temperature to cope with high energetic demands during non-foraging periods. In birds, when the incubating parent uses torpor, the lowered temperatures that thermally sensitive offspring experience could delay development or increase mortality risk. We used thermal imaging to noninvasively explore how nesting female hummingbirds sustain their own energy balance while effectively incubating their eggs and brooding their chicks. We located 67 active Allen’s hummingbird (Selasphorus sasin) nests in Los Angeles, California and recorded nightly time-lapse thermal images at 14 of these nests for 108 nights using thermal cameras. We found that nesting females usually avoided entering torpor, with one bird entering deep torpor on two nights (2% of nights), and two other birds possibly using shallow torpor on three nights (3% of nights). We also modeled nightly energetic requirements of a bird experiencing nest temperatures vs. ambient temperature and using torpor or remaining normothermic, using data from similarly-sized broad-billed hummingbirds. Overall, we suggest that the warm environment of the nest, and possibly shallow torpor, help brooding female hummingbirds reduce their own energy requirements while prioritizing the energetic demands of their offspring.

A heterothermic spectrum in hummingbirds.

Many endotherms use torpor, saving energy by a controlled reduction of their body temperature and metabolic rate. Some species (e.g. arctic ground squirrels, hummingbirds) enter deep torpor, dropping their body temperature by 23-37°C, while others can only enter shallow torpor (e.g. pigeons, 3-10°C reduction). However, deep torpor in mammals can increase predation risk (unless animals are in burrows or caves), inhibit immune function and result in sleep deprivation, so even for species that can enter deep torpor, facultative shallow torpor might help balance energy savings with these potential costs. Deep torpor occurs in three avian orders, but the trade-offs of deep torpor in birds are unknown. Although the literature hints that some bird species (mousebirds and perhaps hummingbirds) can use both shallow and deep torpor, little empirical evidence of such an avian heterothermy spectrum within species exists. We infrared imaged three hummingbird species that are known to use deep torpor, under natural temperature and light cycles, to test whether they were also capable of shallow torpor. All three species used both deep and shallow torpor, often on the same night. Depending on the species, they used shallow torpor for 5-35% of the night. The presence of a heterothermic spectrum in these bird species indicates a capacity for fine-scale physiological and genetic regulation of avian torpid metabolism.

Shallow torpor keeps hummingbirds on their toes when conserving energy

Keeping warm is costly, especially when paired with one of the world's fastest lifestyles. During the day, hummingbirds zip about feeding on sugar-rich nectar to fuel their high-octane existence, but at night, the aeronautic acrobats must conserve energy, slipping into a miniature form of hibernation, known as torpor; many drop their body temperature to around 10°C and some get even chillier. But animals that use torpor are in danger from predators, so some minimise the risk by entering a shallower version from which they can emerge more rapidly. However, it wasn't clear to Anusha Shankar (Cornell University, USA), Catherine Graham (Swiss Federal Research Institute) and Donald Powers (George Fox University, USA) whether birds are also capable of using both shallow and deep types of torpor, either to avoid incurring the costs of deep torpor or as an intermediate step along the way. To find out, Shankar with Powers, Isabelle Cisneros and Sarah Thompson, also from George Fox University, headed to the Chiricahua Mountains of Arizona, USA – home to many migrating hummingbirds during spring and summer – to find out how the energetic birds deal with cold nights.‘We studied three species of hummingbird: tiny black-chinned hummingbirds [3 g], the blue-throated mountain-gem [8 g] and Rivoli's hummingbird [7 g]’, says Shankar, who trapped the wild birds by carefully dropping a net over them as they feasted at a feeder during the hour before sunset. Then, the team gently placed each hummingbird in a chamber positioned outside on a table next to a window, allowing the bird to settle before recording the temperature of its eye every 10 min using an infrared camera located in the window to find out how the animal's body temperature varied as it conserved energy overnight.Monitoring the air temperature, which ranged from 3 to 23°C, the team even recorded one particularly cold night when there was frost and the mercury fell to −1°C. They then analysed the body temperature of the birds and could see that they were quite warm as they settled down for the evening, with eye temperatures stabilising between 29 and 31°C. However, as the night progressed and the air temperature fell, the birds’ body temperature began to decline also, with all of the tiny black-chinned hummingbirds (Archilochus alexandri) dropping into deep torpor for approximately half of the night, with body temperatures below 20°C. However, the larger blue-throated mountain-gems (Lampornis clemenciae) maintained their warm evening body temperature for two-thirds of the night, only allowing their bodies to decline slightly below 30°C as they fell into shallow torpor for a quarter of the night; they rarely fell into deep torpor. Meanwhile, the Rivoli's hummingbirds (Eugenes fulgens) seemed to take an equally nonchalant approach to conserving energy. Although half of the birds resorted to deep torpor, with body temperature around 10°C for ∼20% of the night, 10 of the 12 birds dipped into shallow torpor for about a third of the night.So, hummingbirds are capable of selecting how torpid they become, suggesting that the costs associated with falling into deep torpor may sometimes outweigh the energy conservation benefits. ‘For example, torpid birds in trees might be more conspicuous than a mammal concealed in a burrow that can use deep torpor all the time’, says Shankar, explaining that hummingbirds in shallow torpor can warm up and become fully alert within a matter of seconds to evade predators, in contrast with hummingbirds that have descended into deep torpor, which may take up to half an hour to recover. ‘This makes shallow torpor more effective than deep torpor in some situations’, Shankar concludes.

Considerations of varied thermoregulatory expressions in migration theory

Optimal migration theory has been used for three decades to generate predictions of stopover behavior and understand migration ecology. Yet, to date, there have been no attempts to understand the impacts of thermoregulation on migration theory predictions of stopover behavior. Though most migrants are homeothermic, a diverse group of migrants from bats to hummingbirds and warblers make use of some degree of heterothermy. We consider how thermoregulation influences stopover fuel deposition rates, and thus alters optimal migration theory predictions of stopover behavior using a hypothetical migratory bat as a model organism. We update the analytical models of optimal migration theory by considering scenarios of fixed metabolic rate (the current assumption of optimal migration theory) and three different mass‐specific metabolic rates including homeothermy, shallow torpor heterothermy and deep torpor heterothermy. Our results predict that heterotherms will make shorter stopovers, have a decreased departure fuel load, and reduce the overall time and energy costs associated with stopovers relative to homeotherms, highlighting that thermoregulation can drastically influence stopover behavior and ultimately play a critical role in population level patterns of migration.

Do small precocial birds enter torpor to conserve energy during development?

Precocial birds hatch feathered and mobile, but when they become fully endothermic soon after hatching, their heat loss is high and they may become energy depleted. These chicks could benefit from using energy-conserving torpor, which is characterised by controlled reductions of metabolism and body temperature (Tb). We investigated at what age the precocial king quail Coturnix chinensis can defend a high Tb under a mild thermal challenge and whether they can express torpor soon after achieving endothermy to overcome energetic and thermal challenges. Measurements of surface temperature (Ts) using an infrared thermometer showed that king quail chicks are partially endothermic at 2-10 days, but can defend high Tb at a body mass of ∼13 g. Two chicks expressed shallow nocturnal torpor at 14 and 17 days for 4-5 h with a reduction of metabolism by >40% and another approached the torpor threshold. Although chicks were able to rewarm endogenously from the first torpor bout, metabolism and Ts decreased again by the end of the night, but they rewarmed passively when removed from the chamber. The total metabolic rate increased with body mass. All chicks measured showed a greater reduction of nocturnal metabolism than previously reported in quails. Our data show that shallow torpor can be expressed during the early postnatal phase of quails, when thermoregulatory efficiency is still developing, but heat loss is high. We suggest that torpor may be a common strategy for overcoming challenging conditions during development in small precocial and not only altricial birds.

Disease recovery in bats affected by white-nose syndrome.

Processes associated with recovery of survivors are understudied components of wildlife infectious diseases. White-nose syndrome (WNS) in bats provides an opportunity to study recovery of disease survivors, understand implications of recovery for individual energetics, and assess the role of survivors in pathogen transmission. We documented temporal patterns of recovery from WNS in little brown bats (Myotis lucifugus) following hibernation to test the hypotheses that: (1) recovery of wing structure from WNS matches a rapid time scale (i.e. approximately 30 days) suggested by data from free-ranging bats; (2) torpor expression plays a role in recovery; (3) wing physiological function returns to normal alongside structural recovery; and (4) pathogen loads decline quickly during recovery. We collected naturally infected bats at the end of hibernation, brought them into captivity, and quantified recovery over 40 days by monitoring body mass, wing damage, thermoregulation, histopathology of wing biopsies, skin surface lipids and fungal load. Most metrics returned to normal within 30 days, although wing damage was still detectable at the end of the study. Torpor expression declined overall throughout the study, but bats expressed relatively shallow torpor bouts - with a plateau in minimum skin temperature - during intensive healing between approximately days 8 and 15. Pathogen loads were nearly undetectable after the first week of the study, but some bats were still detectably infected at day 40. Our results suggest that healing bats face a severe energetic imbalance during early recovery from direct costs of healing and reduced foraging efficiency. Management of WNS should not rely solely on actions during winter, but should also aim to support energy balance of recovering bats during spring and summer.

Shallow Torpor Expression in Free-Ranging Common Hamsters With and Without Food Supplements

Energy expenditure during winter can be reduced by expressing torpor, which is characterized by decreased metabolic rate and body temperature. In addition to deep, multiday torpor bouts alternating with short arousal episodes during the hibernation period, common hamsters can also enter shallow torpor bouts (STBs), lasting for less than 24 h at a minimum body temperature between 30-20 °C. Food supplements provided shortly before winter have been shown to shorten hibernation in males, but did not affect hibernation duration in females. In the presented study, we analyzed the expression of STBs and compared supplemented to unsupplemented common hamsters. Body temperature during winter was recorded using subcutaneously implanted data loggers (iButtons). The results revealed that supplemented males showed more STBs and thus spent more time in shallow torpor than unsupplemented individuals. The duration of STBs, however, was shorter in supplemented males and both mean and minimum body temperature were significantly higher compared to unsupplemented males. In females, shallow torpor expression did not differ between individuals with and without food supplements. STBs were mainly expressed before the onset of the first deep, multiday torpor bout, but the number of STBs was not related to that of deep torpor bouts. These results indicate that in males, shallow torpor combined with feeding on food stores could be the more appropriate overwintering strategy when sufficient external energy reserves are available. Females generally cache more food than males and are therefore assumed to be less affected by the additional food stores. These results underline the flexibility of the species in the use of heterothermy and could enable adequate and rapid responses to changes in food availability.

Seasonal energetics and torpor use in North American flying squirrels

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

Seasonal body mass fluctuations of captive Tawny Frogmouths (Podargus strigoides) are consistent with seasonal heterothermy

Prewinter fat accumulation along with the use of shallow torpor permits the survival of free-ranging Australian Tawny Frogmouths (Podargus strigoides) during winter, when prey availability is low and metabolic costs to maintain high body temperatures increase. Captive studies of these animals currently provide no direct support for this ability. We investigated mass fluctuations of 18 captive birds over 8 years at a North American zoo facility to determine whether or not body mass gains were correlated with seasonality, and subsequently provide indirect evidence for regular torpor use. We found that mass fluctuations were correlated with season, as predicted. The lowest body masses occurred during summer, and the greatest mass occurred during late fall and early winter, based on the seasons of the Northern Hemisphere. We conclude that body fat accumulation by captive Tawny Frogmouths occurs in preparation for winter when, under natural conditions, they regularly use torpor, and that Tawny Frogmouths are capable of altering physiological processes to match the environments in which they live, no matter their Southern Hemisphere origins.

Energy Homeostasis in Monotremes.

In 1803, the French anatomist Étienne Geoffroy Saint-Hilaire decided that the newly described echidna and platypus should be placed in a separate order, the monotremes, intermediate between reptiles and mammals. The first physiological observations showed monotremes had low body temperatures and metabolic rates, and the consensus was that they were at a stage of physiological development intermediate between “higher mammals” and “lower vertebrates.” Subsequent studies demonstrated that platypuses and echidnas are capable of close thermoregulation in the cold although less so under hot conditions. Because the short-beaked echidna Tachyglossus aculeatus, may show very large daily variations in body temperature, as well as seasonal hibernation, it has been suggested that it may provide a useful model of protoendotherm physiology. Such analysis is complicated by the very significant differences in thermal relations between echidnas from different climates. In all areas female echidnas regulate Tb within 1°C during egg incubation. The lactation period is considered to be the most energetically expensive time for most female mammals but lactating echidnas showed no measurable difference in field metabolic rate from non-lactating females, while the lactation period is more than 200 days for Kangaroo Island echidnas but only 150 days in Tasmania. In areas with mild winters echidnas show reduced activity and shallow torpor in autumn and early winter, but in areas with cold winters echidnas enter true hibernation with Tb falling as low as 4.5°C. Monotremes do not possess brown adipose tissue and maximum rates of rewarming from hibernation in echidnas were only half those of marmots of the same mass. Although echidnas show very large seasonal variations in fat stores associated with hibernation there is no relationship between plasma leptin and adiposity. Leptin levels are lowest during post-reproductive fattening, supporting suggestions that in evolutionary terms the anorectic effects of leptin preceded the adiposity signal. BMR of platypuses is twice that of echidnas although maximum metabolism is similar. High levels of thyroid hormones in platypuses may be driving metabolism limited by low body temperature. Monotremes show a mosaic of plesiomorphic and derived features but can still inform our understanding of the evolution of endothermy.

The bear circadian clock doesn't 'sleep' during winter dormancy.

BackgroundMost biological functions are synchronized to the environmental light:dark cycle via a circadian timekeeping system. Bears exhibit shallow torpor combined with metabolic suppression during winter dormancy. We sought to confirm that free-running circadian rhythms of body temperature (Tb) and activity were expressed in torpid grizzly (brown) bears and that they were functionally responsive to environmental light. We also measured activity and ambient light exposures in denning wild bears to determine if rhythms were evident and what the photic conditions of their natural dens were. Lastly, we used cultured skin fibroblasts obtained from captive torpid bears to assess molecular clock operation in peripheral tissues. Circadian parameters were estimated using robust wavelet transforms and maximum entropy spectral analyses.ResultsCaptive grizzly bears housed in constant darkness during winter dormancy expressed circadian rhythms of activity and Tb. The rhythm period of juvenile bears was significantly shorter than that of adult bears. However, the period of activity rhythms in adult captive bears was virtually identical to that of adult wild denning bears as was the strength of the activity rhythms. Similar to what has been found in other mammals, a single light exposure during the bear’s active period delayed subsequent activity onsets whereas these were advanced when light was applied during the bear’s inactive period. Lastly, in vitro studies confirmed the expression of molecular circadian rhythms with a period comparable to the bear’s own behavioral rhythms.ConclusionsBased on these findings we conclude that the circadian system is functional in torpid bears and their peripheral tissues even when housed in constant darkness, is responsive to phase-shifting effects of light, and therefore, is a normal facet of torpid bear physiology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0173-x) contains supplementary material, which is available to authorized users.

Physiological and behavioural responses of a small heterothermic mammal to fire stimuli

The predicted increase of the frequency and intensity of wildfires as a result of climate change could have a devastating impact on many species and ecosystems. However, the particular physiological and behavioural adaptions of animals to survive fires are poorly understood. We aimed to provide the first quantitative data on physiological and behavioural mechanisms used by a small heterothermic marsupial mammal, the fat-tailed dunnart (Sminthopsis crassicaudata), that may be crucial for survival during and immediately after a fire. Specifically, we aimed to determine (i) whether captive torpid animals are able to respond to fire stimuli and (ii) which energy saving mechanisms are used in response to fires. The initial response of torpid dunnarts to smoke exposure was to arouse immediately and therefore express shorter and shallower torpor bouts. Dunnarts also increased activity after smoke exposure when food was provided, but not when food was withheld. A charcoal/ash substrate, imitating post-fire conditions, resulted in a decrease in torpor use and activity, but only when food was available. Our novel data suggests that heterothermic mammals are able to respond to fire stimuli, such as smoke, to arouse from torpor as an initial response to fire and adjust torpor use and activity levels according to food availability modulated by fire cues.

How to spend the summer? Free-living dormice (Glis glis) can hibernate for 11 months in non-reproductive years.

Edible dormice are arboreal rodents adapted to yearly fluctuations in seed production of European beech, a major food source for this species. In years of low beech seed abundance, dormice skip reproduction and non-reproductive dormice fed ad libitum in captivity can display summer dormancy in addition to winter hibernation. To test whether summer dormancy, that is, a very early onset of hibernation, actually occurs in free-living dormice, we monitored core body temperature (Tb) over ~12 months in 17 animals during a year of beech seeding failure in the Vienna Woods. We found that 8 out of 17 dormice indeed re-entered hibernation as early as in June/July, with five of them having extreme hibernation durations of 11 months or more (total range: 7.8–11.4 months). Thus, we show for the first time that a free-living mammal relying on natural food resources can continuously hibernate for >11 months. Early onset of hibernation was associated with high body mass in the spring, but the distribution of hibernation onset was bimodal with prolonged hibernation starting either early (prior to July 28) or late (after August 30). This could not be explained by differences in body mass alone. Animals with a late hibernation onset continued to maintain high nocturnal Tb’s throughout summer but used short, shallow torpor bouts (mean duration 7.44 ± 0.9 h), as well as occasional multiday torpor for up to 161 h.Electronic supplementary materialThe online version of this article (doi:10.1007/s00360-015-0929-1) contains supplementary material, which is available to authorized users.

Temperatures below leaf litter during winter prescribed burns: implications for litter-roosting bats

Some bat species, including eastern red bats (Lasiurus borealis), roost for short periods beneath leaf litter on the forest floor during winter in the south-eastern USA, a region subjected to frequent fire. The variability in fuel consumption, the heterogeneous nature of burns, and the effects of litter and duff moisture on forest-floor temperatures during winter burning could influence potential survival for bats beneath the leaf litter if they are unable to escape on-coming flames. We measured temperatures below leaf litter in 64 south-slope plots during nine controlled burns in the Ouachita Mountains of Arkansas to determine the probability of survival. Maximum temperature recorded under leaf litter at each plot averaged 292°C (±20 s.e.) and ranged from 10 to 717°C. Only three (5%) of the plots experienced temperatures that were deemed survivable (<60°C sustained for 60 s) during burns on warmer winter days (air temperatures 15.0–26.1°C). Temperatures below the leaf litter measured just before the arrival of fire (average = 19.6°C) suggested that if bats were roosting in plots they would have been in shallow torpor, which would have enabled faster escape from approaching flames. Burning during the warmer periods of winter (e.g. ≥15°C) and during afternoons could potentially improve survival by bats roosting under leaf litter by reducing arousal and escape times.

Magnitude of food overabundance affects expression of daily torpor

Many small mammal species use torpor as a strategy for reducing energy expenditure in winter. Some rodent hibernators also hoard food to provide reserves of energy, and individuals with large hoards express less torpor than those with smaller reserves. These facts imply that animals can recognize levels of food availability, but where food is very plentiful, it is unclear whether torpor expression is affected by temporal changes in the extent of food overabundance. Moreover, the relationship between daily torpor and excess food availability has not been clearly established. The large Japanese field mouse Apodemus speciosus caches food for use as a winter energy resource and exhibits daily torpor under artificial winter conditions. The present study examined whether individuals exposed to different magnitudes of overabundant food exhibited differences in expression of daily torpor, and secondly whether torpor expression varied in response to changes in the overall quantity of overabundant food. It was observed that while absolute quantities of overabundant food did not appear to affect daily torpor expression, the mice did respond to changes in food availability, even when food remained overabundant. This suggests that the mice respond to fluctuations in food availability, even where these changes do not place any constraint on energy budgets. Thus recognition of changing food availability cannot be a purely physiological response to shortage or plenty, and may contribute to predictions of future energy availability. The expression of torpor was inhibited in response to increasing food availability, and the mice used shallower torpor when food availability increased to superabundance. These findings suggest that daily torpor may be regulated not only physiologically in response to energy constraints but also psychologically, via recognition of food availability.

Winter and Summer Torpor in a Free-Ranging Subtropical Desert Bat: The Fishing Myotis (Myotis vivesi)

Use of torpor likely favors the survival of subtropical bats in harsh environments. The fishing myotis (Myotis vivesi) is a species endemic to desert islands in the Gulf of California, where summers are extremely hot and winters are cold and windy. We explored thermoregulating abilities of M. vivesi measuring skin temperature (Tskin) on free-ranging individuals in winter 2010 and 2011, and in summer 2010. We also measured ambient (Ta) and roost (Troost) temperatures during the study, and we obtained data for wind speed at night time during winter periods. We found that all bats entered torpor in both winters and that at least three individuals hibernated for several days, which had not been reported previously for bats in subtropical deserts. In summer, three individuals entered short bouts of shallow torpor in early mornings. Roosts were slightly warmer than Ta in winter at nightime, and in summer they never reached temperatures > 38.7°C, even at Ta ≈ 45°C. Roost occupancy in winter was higher during win...

Effects of unsaturated fatty acids on torpor frequency and diet selection in Djungarian hamsters (Phodopus sungorus).

Essential polyunsaturated fatty acids (PUFA) have been shown to play a beneficial role in hibernating mammals. High amounts of dietary PUFA led to an earlier hibernation onset, deeper and longer hibernation bouts and a higher proportion of hibernating animals in several species. In contrast, the relevance of dietary PUFA for daily heterotherms exhibiting only brief and shallow torpor bouts is less well studied. Therefore, diets differing in PUFA composition were used to examine the effects on the frequency of spontaneous daily torpor in Djungarian hamsters (Phodopus sungorus). In contrast to earlier studies, we were interested in whether the ratio of n-6 to n-3 PUFA affects torpor expression, and in comparison with a diet rich in monounsaturated fatty acids (MUFA). Although we found a positive effect on torpor frequency in hamsters fed a diet rich in n-6 PUFA compared with the groups fed diets either rich in n-3 PUFA or MUFA, the latter two groups did not show unusually low torpor frequencies. The results of the additional diet choice experiment indicated that hamsters in short photoperiod select food with only a slight excess of n-6 PUFA compared with n-3 PUFA (ratio of 1 to 1.5). However, there was no significant difference in torpor frequency between the diet choice group and hamsters fed on standard chow with a sevenfold excess of n-6 PUFA. In summary, the present data strongly indicate that the dietary composition of unsaturated fatty acids plays a minor role in the occurrence of spontaneous daily torpor in Djungarian hamsters.