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Torpor is an adaptation for energy conservation employed by many species of small-bodied endotherms. However, surprisingly little is known regarding proximate factors influencing day-to-day variation in torpor expression in the wild. We used open-flow respirometry to quantify torpor expression in nine little brown bats (Myotis lucifugus, LeConte 1831) at two ambient temperatures (7°C and 17°C) following either sham feeding or consumption of a high-protein meal (50% or 100% of the mass required to reach satiation for each individual). Food consumption significantly increased the time spent normothermic before torpor entry but did not affect either the rate of body cooling or torpid metabolic rate. Bats did not fully exploit potential energy savings by maximising their use of torpor. Instead they varied torpor expression such that total energy expenditure over the course of each 22-h trial was balanced against gross energy intake immediately before the trial, independent of ambient temperature. This was accomplished by adjusting the timing of entry into torpor (thus altering the time spent torpid), rather than by modulating torpid metabolic rate. However, pre-trial body mass was also a significant predictor of torpor expression, which suggests that energy reserves combine with recent foraging success to influence individuals’ decisions about depth and duration of their torpor bouts. We also present evidence that little brown bats use the heat generated through digestion (i.e. the heat increment of feeding) to substitute for active thermogenesis at sub-thermoneutral temperatures, thereby reducing the energetic costs of thermoregulation prior to torpor entry. "Funding was provided by Natural Sciences and Engineering Research Council (NSERC, Canada) Discovery grants to C.K.R.W. and K.L.C., and a NSERC Undergraduate Student Research Award to A.L.M. The University of Winnipeg Bat Lab and C.K.R.W. are also funded by the Canada Foundation for Innovation and the Manitoba Research and Innovation Fund." http://jeb.biologists.org/content/213/12/2165

Hibernation physiology and energy expenditure have been relatively well studied in large captive hibernators, especially rodents, but data from smaller, free-ranging hibernators are sparse. We examined variation in the hibernation patterns of free-ranging little brown bats (Myotis lucifugus) using temperature-sensitive radio-transmitters. First, we aimed to test the hypothesis that age, sex and body condition affect expression of torpor and energy expenditure during hibernation. Second, we examined skin temperature to assess whether qualitative differences in the thermal properties of the hibernacula of bats, compared with the burrows of hibernating rodents, might lead to different patterns of torpor and arousal for bats. We also evaluated the impact of carrying transmitters on body condition to help determine the potential impact of telemetry studies. We observed large variation in the duration of torpor bouts within and between individuals but detected no effect of age, sex or body condition on torpor expression or estimates of energy expenditure. We observed the use of shallow torpor in the midst of periodic arousals, which may represent a unique adaptation of bats for conservation of energy during the most costly phase of hibernation. There was no difference in the body condition of hibernating bats outfitted with transmitters compared with that of control bats captured from the same hibernaculum at the same time. This study provides new information on the energetics of hibernation in an underrepresented taxon and baseline data important for understanding how white-nose syndrome, a new disease devastating populations of hibernating bats in North America, may alter the expression of hibernation in affected bats. "This work was supported by grants from the Natural Sciences and Engineering Research Council (NSERC, Canada), the Canada Foundation for Innovation (CFI) and the Manitoba Research and Innovation Fund to C.K.R.W. and a NSERC Canada Graduate Scholarship, a Manitoba Graduate Scholarship and a Bat Conservation International Student Research Scholarship to K.A.J." http://jeb.biologists.org/content/215/12/2141

8 pages. White-nose syndrome (WNS) is a new disease of bats that has devastated populations in eastern North America. Infection with the fungus, Geomyces destructans, is thought to increase the time bats spend out of torpor during hibernation, leading to starvation. Little is known about hibernation in healthy, free-ranging bats and more data are needed to help predict consequences of WNS. Trade-offs presumably exist between the energetic benefits and physiological/ecological costs of torpor, leading to the prediction that the relative importance of spring energy reserves should affect an individual’s use of torpor and depletion of energy reserves during winter. Myotis lucifugus mate during fall and winter but females do not become pregnant until after spring emergence. Thus, female reproductive success depends on spring fat reserves while male reproductive success does not. Consequently, females should be ‘‘thrifty’’ in their use of fat compared to males. We measured body condition index (BCI; mass/forearm length) of 432 M. lucifugus in Manitoba, Canada during the winter of 2009/2010. Bats were captured during the fall mating period (n = 200), early hibernation (n = 125), and late hibernation (n = 128). Adult females entered hibernation with greater fat reserves and consumed those reserves more slowly than adult males and young of the year. Consequently, adult females may be more likely than males or young of the year to survive the disruption of energy balance associated with WNS, although surviving females may not have sufficient reserves to support reproduction. "This work was supported by the Natural Sciences and Engineering Research Council (NSERC, Canada) Discovery Grant to CKRW; the Canada Foundation for Innovation (CFI) Leader’s Opportunity Fund Grand to CKRW; the Manitoba Research and Innovation Fund Grant to CKRW; the NSERC Canada Graduate Scholarship to KAJ; the Manitoba Graduate Scholarship to KAJ; and the Bat Conservation International Student Research Scholarship to KAJ." http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0021061

Field studies of use of torpor by heterothermic endotherms suffer from the lack of a standardized threshold differentiating torpid body temperatures (T_b) from normothermic T_b's. This threshold can be more readily observed if metabolic rate (MR) is measured in the laboratory. I digitized figures from the literature that depicted simultaneous traces of MR and T_b from 32 respirometry runs for 14 mammal species. For each graph, I quantified the T_b measured when MR first began to drop at the onset of torpor (T_b-onset). I used a general linear model to quantify the effect of ambient temperature (T_a) and body mass (BM) on T_b-onset. For species lighter than 70 g, the model was highly significant and was described by the equation T_b-onset = (0.055 ± 0.014)BM + (0.071 ± 0.031)T_a + (31.823 ± 0.740). To be conservative, I recommend use of these model parameters minus 1 standard error, which modifies the equation to T_b-onset - 1 SE = (0.041)BM + (0.040)T_a + 31.083. This approach provides a standardized threshold for differentiating torpor from normothermia that is based on use of energy, the actual currency of interest for studies of torpor in the wild. Few laboratory studies have presented the time-course data required to quantify T_b-onset, so more data are needed to validate this relationship. https://www.journals.uchicago.edu/doi/10.1086/521085