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▪ Abstract  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 (T b ) for MR reduction, perhaps with the exception of very small species and at high torpor T b , where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to T b effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of T b are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and T b reduction at torpor onset. Over the ambient temperature (T a ) range where torpid heterotherms are thermo-conforming, the T b -T a differential is more or less constant despite a decline of MR with T a ; however, in thermo-regulating torpid individuals, the T b -T a differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower T b . 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.

Mammalian and avian torpor is highly effective in reducing energy expenditure. However, the extent of energy savings achieved and thus long-term survival appear to differ between species capable of multiday hibernation and species restricted to daily heterothermy, which could, however, be due to thermal effects. We tested how long-term survival on stored body fat (i.e. time to lean body mass), crucial for overcoming adverse periods, is related to the pattern of torpor expressed under different ambient temperatures (T a : 7 °C typical of hibernation, 15 and 22 °C typical of daily torpor) in the small marsupial hibernator the pygmy-possum ( Cercartetus nanus ). Possums expressed torpor at all T a s and survived without food for 310 days on average at T a 7 °C, 195 days at T a 15 °C, and 127 days at T a 22 °C. At T a 7 and 15 °C, torpor bout duration (TBD) increased from < 1–3 to ~ 5–16 days over 2 months, whereas at T a 22 °C, TBD remained at < 1 to ~ 2 days. At all T a s daily energy use was substantially lower and TBD and survival times of possums much longer (3–12 months) than in daily heterotherms (~ 10 days). Such pronounced differences in torpor patterns and survival times even under similar thermal conditions provide strong support for the concept that torpor in hibernators and daily heterotherms are physiologically distinct and have evolved for different ecological purposes.

Torpor, and its differential expression, is essential to the survival of many mammals and birds. Physiological characteristics of torpor appear to vary between those species that express strict daily heterothermy and those capable of multiday hibernation, but comparisons are complicated by the temperature-dependence of variables. Previous reviews have compared these different torpor strategies by measuring the depth and duration of torpor in multiple species. However, direct comparison of multiple physiological parameters under similar thermal conditions are lacking. Here, we quantified three physiological variables; body temperature, metabolic rate (MR) and heart rate (HR) of two small heterothermic bats (daily heterotherm Syconycteris australis, and hibernator Nyctophilus gouldi ) under comparable thermal conditions and torpor bout durations. When normothermic and resting both MR and HR were similar for the two species. However, during torpor the minimum HR was more than fivefold higher, and minimum MR was 6.5-fold higher for the daily heterotherm than for the hibernator at the same subcutaneous T b (16 ± 0.5 °C). The data show that the degree of heterothermy defined using T b is not necessarily a precise proxy for physiological capacity during torpor in these bats and is unlikely to reveal accurate energy budgets. Our study provides evidence supporting a distinction between daily torpor in a daily heterotherm and short term torpor in a hibernator, at least within the Chiroptera with regard to these physiological variables. This exists even when individuals display the same degree of T b reduction, which has clear implications for the modelling of their energy expenditure.

The proportion of organisms exposed to warm conditions is predicted to increase during global warming. To better understand how bats might respond to climate change, we aimed to obtain the first data on how use of torpor, a crucial survival strategy of small bats, is affected by temperature in the tropics. Over two mild winters, tropical free-ranging bats (Nyctophilus bifax, 10 g, n = 13) used torpor on 95% of study days and were torpid for 33.5±18.8% of 113 days measured. Torpor duration was temperature-dependent and an increase in ambient temperature by the predicted 2°C for the 21(st) century would decrease the time in torpor to 21.8%. However, comparisons among Nyctophilus populations show that regional phenotypic plasticity attenuates temperature effects on torpor patterns. Our data suggest that heterothermy is important for energy budgeting of bats even under warm conditions and that flexible torpor use will enhance bats' chance of survival during climate change.

MicroRNAs (miRNAs) regulate many physiological processes through post-transcriptional control of gene expression and are a major part of the small noncoding RNAs (snRNA). As hibernators can survive at low body temperatures (Tb) for many months without suffering tissue damage, understanding the mechanisms that enable them to do so are of medical interest. Because the brain integrates peripheral physiology and white adipose tissue (WAT) is the primary energy source during hibernation, we hypothesized that both of these organs play a crucial role in hibernation, and thus, their activity would be relatively increased during hibernation. We carried out the first genomic analysis of small RNAs, specifically miRNAs, in the brain and WAT of a hibernating bat (Myotis ricketti) by comparing deeply torpid with euthermic individual bats using high-throughput sequencing (Solexa) and qPCR validation of expression levels. A total of 196 miRNAs (including 77 novel bat-specific miRNAs) were identified, and of these, 49 miRNAs showed significant differences in expression during hibernation, including 33 in the brain and 25 in WAT (P≤0.01 &│logFC│≥1). Stem-loop qPCR confirmed the miRNA expression patterns identified by Solexa sequencing. Moreover, 31 miRNAs showed tissue- or state-specific expression, and six miRNAs with counts >100 were specifically expressed in the brain. Putative target gene prediction combined with KEGG pathway and GO annotation showed that many essential processes of both organs are significantly correlated with differentially expressed miRNAs during bat hibernation. This is especially evident with down-regulated miRNAs, indicating that many physiological pathways are altered during hibernation. Thus, our novel findings of miRNAs and Interspersed Elements in a hibernating bat suggest that brain and WAT are active with respect to the miRNA expression activity during hibernation.

Food availability, ambient temperatures (T a), and prevailing weather conditions have long been presumed to influence torpor use. To a large extent, this is based on measurements in the laboratory of animals placed on restricted diets and kept at low T a. Information on the determinants of torpor employment in the field is limited. We assessed winter torpor by insectivorous, free-ranging Australian owlet-nightjars (Aegotheles cristatus; 22 birds, 834 bird-days over six winters). Birds in three habitats were investigated to test whether torpor use is affected by annual T a, rainfall, and arthropod abundance. Owlet-nightjars entered daily torpor regularly at all sites. Torpor frequency, depth and bout duration were greatest during two periods with lower arthropod abundance, providing rare evidence of the link between food availability and torpor patterns of wild birds. Temporal organization of torpor was similar among sites, and nocturnal torpor was more frequent than previously reported. Our findings quantitatively demonstrate that reduced food resources affect torpor usage independently from T a, and support the view that food availability is a primary ecological determinant of torpor use in the wild.

The seasonal changes in thermal physiology and torpor expression of many heterothermic mammals are controlled by photoperiod. As function at low body temperatures during torpor requires changes of tissue lipid composition, we tested for the first time whether and how fatty acids are affected by photoperiod acclimation in hamsters, Phodopus sungorus, a strongly photoperiodic species. We also examined changes in fatty acid composition in relation to those in morphology and thermal biology. Hamsters in short photoperiod had smaller reproductive organs and most had a reduced body mass in comparison to those in long photoperiod. Pelage colour of hamsters under short photoperiod was almost white while that of long photoperiod hamsters was grey-brown and black. Short photoperiod acclimation resulted in regular (28% of days) torpor use, whereas all hamsters in long photoperiod remained normothermic. The composition of total fatty acids differed between acclimation groups for brown adipose tissue (5 of 8 fatty acids), heart muscle (4 of 7 fatty acids) and leg muscle (3 of 11 fatty acids). Importantly, 54% of all fatty acids detected were correlated (r(2) = 0.60 to 0.87) with the minimum surface temperature of individuals, but the responses of tissues differed. While some of the compositional changes of fatty acids were consistent with a 'homeoviscous' response, this was not the case for all, including the sums of saturated and unsaturated fatty acids, which did not differ between acclimation groups. Our data identify a possible nexus between photoperiod acclimation, morphology, reproductive biology, thermal biology and fatty acid composition. They suggest that some of the changes in thermal physiology are linked to the composition of tissue and organ fatty acids.

Increased winter survival by reducing energy expenditure in adult animals is often viewed as the primary function of torpor. However, torpor has many other functions that ultimately increase the survival of heterothermic mammals and birds. In this review, we summarize new findings revealing that animals use torpor to cope with the conditions during and after natural disasters, including fires, storms, and heat waves. Furthermore, we suggest that torpor, which also prolongs longevity and was likely crucial for survival of mammals during the time of the dinosaur extinctions, will be advantageous in a changing world. Climate change is assumed to lead to an increase in the occurrence and intensity of climatic disasters, such as those listed above and also abnormal floods, droughts, and extreme temperatures. The opportunistic use of torpor, found in many heterothermic species, will likely enhance survival of these challenges, because these species can reduce energy and foraging requirements. However, many strictly seasonal hibernators will likely face the negative consequences of the predicted increase in temperature, such as range contraction. Overall, available data suggest that opportunistic heterotherms with their flexible energy requirements have an adaptive advantage over homeotherms in response to unpredictable conditions.

Radiotelemetry is used to quantify behavioral, ecological, and physiological variables of animals. Because of technological limitations, relative transmitter size generally increases with decreasing body mass of the study animal, and the recommended transmitter mass of <5% of body mass often prohibits work on small mammals. We compared burst running speed, important for predator avoidance, in 2 small marsupials, Sminthopsis crassicaudata (fat-tailed dunnart) and Planigale gilesi (Giles' planigale), without and with implanted transmitters. In both species maximum running speed was not affected by the transmitters, whose mass ranged from 6.4% to 14.1% of body mass. Further, relative transmitter mass was not correlated with maximum running speed. Consequently, transmitters well above 5% of body mass need not affect locomotor performance of small terrestrial mammals.

Given their relatively small body size, high thermoregulatory costs, and low metabolic rate, we tested the hypothesis that red squirrels (Tamiasciurus hudsonicus) would employ bouts of daily torpor to save energy during winter. We collected data on body temperature (Tb) using surgically implanted data loggers for squirrels in the Cypress Hills region of Saskatchewan, where extended periods of cold snowy weather make foraging difficult and should lead to high levels of energy expenditure. Based on over 8000 measurements from 4 animals over 3 winters, we found no evidence for torpor use. However, Tb was lowest in January and highest in September and May, and mean monthly Tb was correlated with mean monthly ambient temperature (Ta). Given that taxonomically related species can and do use torpor, it remains to be determined what makes heterothermy in this species costly to the extent that its use is precluded.