Temperature-sensitive Radio Transmitters Research Articles

Constant-temperature predictions underestimate growth of a fungal amphibian pathogen under individual host thermal profiles

Ectotherm body temperatures fluctuate with environmental variability and host behavior, which may influence host-pathogen interactions. Fungal pathogens are a major threat to ectotherms and may be highly responsive to the fluctuating thermal profiles of individual hosts, especially cool-loving fungi exposed to high host temperatures. However, most studies estimate pathogen thermal performance based on averages of host or surrogate environmental temperatures, potentially missing effects of short-term host temperature shifts such as daily or hourly heat spikes. We recorded individual thermal profiles of Australian rainforest frogs using temperature-sensitive radio-transmitters. We then reproduced a subset of individual thermal profiles in growth chambers containing cultures of the near-global amphibian pathogen Batrachochytrium dendrobatidis (Bd) to investigate how realistic host temperature profiles affect Bd growth. We focused on thermal profiles that exceed the thermal optimum of Bd because the effects of realistic heat spikes on Bd growth are unresolved. Our laboratory incubation experiment revealed that Bd growth varied in response to relatively small differences in heat spike characteristics of individual frog thermal profiles, such as a single degree or a few hours, highlighting the importance of individual host behaviors in predicting population-level disease dynamics. The fungus also grew better than predicted under the most extreme and unpredictable frog temperature profile, recovering from two days of extreme (nearly 32 °C) heat spikes without negative effects on overall growth, suggesting we are underestimating the growth potential of the pathogen in nature. Combined with the previous finding that Bd reduces host heat tolerance, our study suggests that this pathogen may carry a competitive edge over hosts in the face of anthropogenic climate change.

Winter torpor and activity patterns of a fishing bat (Myotis macropus) in a mild climate

AbstractSmall insectivorous bats often enter a state of torpor, a controlled, reversible decrease in body temperature and metabolic rate. Torpor provides substantial energy savings and is used more extensively during periods of low temperature and reduced prey availability. We studied torpor use and activity of a small (10.1 ± 0.4 g) fishing bat, Myotis macropus, during winter in a mild climate in Australia. We predicted that the thermal stability of water would make foraging opportunities in winter more productive and consistent in a riparian habitat compared to a woodland habitat, and therefore, fishing bats would use torpor less than expected during winter compared to other bats. Using temperature-sensitive radio transmitters, we recorded the skin temperature of 12 adult (6 M, 6 F) bats over 161 bat-days (13.4 ± 5.4 days per bat) during Austral winter (late May to August), when daily air temperature averaged 6.2–18.2°C. Bats used torpor every day, with bouts lasting a median of 21.3 h and up to 144.6 h. Multiday torpor bouts were more common in females than males. Arousals occurred just after sunset and lasted 3.5 ± 2.9 h. Arousals tended to be longer in males than females and to occur on warmer evenings, suggesting some winter foraging and perhaps male harem territoriality or other mating-related activity was occurring. The extensive use of torpor by M. macropus during relatively mild winter conditions when food is likely available suggests torpor might function to minimize the risks of mortality caused by activity and to increase body condition for the upcoming breeding season.

Cold, cheap love songs

Songs help birds attract mates by advertising their fitness or territory to potential mates. Because the sexiest songs are assumed to be energetically expensive, or in some way related to a certain attractive attribute, potential mates can judge the singers to learn more about their qualities and potential as an eligible mate. In much the same way that birds use song, some bat species also use complex songs either to boast about their strength or to lure mates into their territory. The New Zealand lesser short-tailed bat (Mystacina tuberculata) is one of only two bat species where courting males congregate together – forming a lek – where they perform serenades within each other's earshot in the hope of attracting females. To be considered sexy, the quality of the bats’ songs should be costly or related to the beauty of their territory. Yet, if the songs were expensive to produce – somehow revealing the singer's inner strength – bats might hypothetically try to cut corners in other expensive areas of their lives, such as the body temperature they maintain. Kathleen Collier from the University of Auckland, New Zealand, along with collaborators Zenon Czenze from the University of New England, Australia, and Stuart Parsons from Queensland University of Technology sought to estimate the costs of singing for lek-breeding bats, using skin temperature, to estimate how much energy singing uses.To study this unique phenomenon of communal singing in bats, the researchers attached temperature-sensitive radio transmitters to the backs of 14 male New Zealand lesser short-tailed bats at their singing roosts. The researchers then recorded the bats’ skin temperatures while the mammals quietly went about their usual routine and also when they were singing. The researchers then used the skin temperatures collected by the radio transmitters to estimate the metabolic rate of the bats during both activities.Throughout 100 nights of observation, the researchers found that the bats were not significantly warmer or cooler when singing. And when the team compared the bats’ estimated metabolic rates, they also were not significantly different. The bats were not exerting themselves while singing out their hearts for a mate, implying that their singing may not be as much of an energetic sacrifice as was previously thought. However, it is possible that using skin temperature to estimate metabolic rate may have missed some other important energy consuming activities – such as panting to keep body temperature close to normal – and may have hidden the animals’ actual metabolic rates while singing. Yet, even though these data are just rough estimates of the bats’ actual energy use, they do allow us to compare between serenading and non-serenading behaviours, which had not been possible before.This study is also the first to have investigated the energetic costs of singing in a mammal and suggests that the costs may be less than previously thought. If these mating rituals are less energetically expensive, then what messages could these displays actually be sending to potential mates? In fact, are potential mates even getting what they thought they were investing in? Even though that isn't clear right now, it isn't deterring male New Zealand lesser short-tailed bats from getting together and serenading in the hope of attracting an eligible date.

Free-ranging bats alter thermoregulatory behavior in response to reproductive stage, roost type, and weather

Abstract Heterotherms vary their use of torpor and choice of refugia to deal with energetic stresses such as reproductive activity and extreme weather. We hypothesized that a temperate-region bat would vary its use of heterothermy in response to air temperature but use of torpor would also be influenced by reproductive stage and roost choice. To test this hypothesis, we collected data on skin temperatures of female Indiana bats (Myotis sodalis) carrying temperature-sensitive radiotransmitters during the summers of 2013–2015. We also measured internal temperatures and external characteristics of roosts used by these bats. We analyzed the influence of daytime air temperature, roost canopy closure, roost type, and bat reproductive stage, on daily heterothermy index and torpor characteristics of 17 bats during 103 full roost days (data collected consistently from when a bat entered its roost in the morning to when it emerged at night). Our data showed that Indiana bat heterothermy was influenced by reproductive stage, roost choice, and weather. Although they used torpor, pregnant bats were the least heterothermic (daily heterothermy index = 3.3 ± 0.6°C SE), followed by juvenile bats (5.6 ± 0.5°C), lactating bats (5.7 ± 0.5°C), and one postlactating bat (13.2 ± 1.6°C). Air temperature also influenced heterothermy of pregnant bats less than bats of other reproductive stages. Thermoregulatory strategies varied on a continuum from use of normothermia in warm roosts to use of long and deep bouts of torpor in cool roosts. The thermoregulatory strategy used seemed to be determined by potential reproductive costs of torpor and energetic consequences of weather. Because Indiana bats used different degrees of heterothermy throughout the summer maternity season, managers should offer maternity colonies an array of refugia to facilitate varying behaviors in response to weather and energetic demands.

Winter thermoregulation in free-ranging pygmy falcons in the Kalahari Desert

Among birds, pronounced heterothermy tends to be used by small species belonging to phylogenetically older taxa and inhabiting environments with unpredictable and/or seasonally scarce food resources. Previous evidence suggests that a small arid-zone raptor, the pygmy falcon (Polihierax semitorquatus), enters torpor and decreases its body temperature (Tb) to below 31 °C on winter nights in the Kalahari Desert, where sub-zero night-time temperatures occur regularly. To confirm that this species has the capacity for heterothermy and to characterise its thermoregulatory patterns under natural conditions, we implanted temperature-sensitive radio transmitters into free-ranging falcons. Based on data from seven individuals, we found no evidence for the substantial reductions in Tb previously reported. The mean minimum rest-phase Tb during the study period was 37.89 ± 0.67 °C, we observed no instances of Tb < 36 °C, and air temperature did not significantly influence patterns of thermoregulation. Our results highlight intraspecific variability in heterothermy among birds and reveal that, contrary to previous suggestions, even in mid-winter pygmy falcons may not reduce Tb below normothermic levels.

Responding to the weather: energy budgeting by a small mammal in the wild.

Energy conservation is paramount for small mammals because of their small size, large surface area to volume ratio, and the resultant high heat loss to the environment. To survive on limited food resources and to fuel their expensive metabolism during activity, many small mammals employ daily torpor to reduce energy expenditure during the rest phase. We hypothesized that a small terrestrial semelparous marsupial, the brown antechinus Antechinus stuartii, would maximize activity when foraging conditions were favorable to gain fat reserves before their intense breeding period, but would increase torpor use when conditions were poor to conserve these fat reserves. Female antechinus were trapped and implanted with small temperature-sensitive radio transmitters to record body temperature and to quantify torpor expression and activity patterns in the wild. Most antechinus used torpor at least once per day over the entire study period. Total daily torpor use increased and mean daily body temperature decreased significantly with a reduction in minimum ambient temperature. Interestingly, antechinus employed less torpor on days with more rain and decreasing barometric pressure. In contrast to torpor expression, activity was directly related to ambient temperature and inversely related to barometric pressure. Our results reveal that antechinus use a flexible combination of physiology and behavior that can be adjusted to manage their energy budget according to weather variables.

Roost use and thermoregulation by female Australian long-eared bats (Nyctophilus geoffroyi and N. gouldi) during pregnancy and lactation

Small insectivorous bats commonly use torpor while day-roosting, even in summer. However, reproductive female bats are believed to benefit from avoiding torpor because a constant, elevated body temperature maximises the rate of offspring growth, which could increase offspring survival. We used temperature-sensitive radio-transmitters to locate roosts and document the thermal biology of pregnant and lactating females of Nyctophilus geoffroyi (9 g) and N. gouldi (11 g) at a woodland in a cool temperate climate. Unlike males, reproductive female Nyctophilus spp. roosted as small groups (&amp;lt;25) within insulated tree cavities. Roost switching occurred every 3.7 ± 1.5 (N. geoffroyi) or 1.7 ± 0.8 days (N. gouldi), and radio-tagged individuals roosted together and apart on different days. Skin temperature during roosting was most often between 32 and 36°C, and torpor was used infrequently. Male Nyctophilus have been shown in previous studies to use torpor daily during summer. These contrasting torpor patterns likely reflect the warmed cavities occupied by maternity colonies and the thermally unstable shallow crevices occupied by individual males. Our results support the hypothesis that availability of thermally suitable roosts will influence thermoregulatory patterns of reproductive females and hence the growth rates and survival of their offspring. Thus, it is important to conserve woodland habitat with trees in a range of decay stages to provide opportunities for selection and movement among roost trees by reproductive female bats.

The avian "hibernation" enigma: thermoregulatory patterns and roost choice of the common poorwill.

Compared to mammals, there are relatively few studies examining heterothermy in birds. In 13 bird families known to contain heterothermic species, the common poorwill (Phalaenoptilus nuttallii) is the only species that ostensibly hibernates. We used temperature-sensitive radio-transmitters to collect roost and skin temperature (Tskin) data, and winter roost preferences for free-ranging poorwills in southern Arizona. Further, to determine the effect of passive rewarming on torpor bout duration and active rewarming (i.e., the use of metabolic heat to increase Tskin), we experimentally shaded seven birds during winter to prevent them from passively rewarming via solar radiation. Poorwills selected winter roosts that were open to the south or southwest, facilitating passive solar warming in the late afternoon. Shaded birds actively rewarmed following at least 3days of continuous torpor. Average torpor bout duration by shaded birds was 122h and ranged from 91 to 164h. Active rewarming by shaded birds occurred on significantly warmer days than those when poorwills remained torpid. One shaded bird remained inactive for 45days, during which it spontaneously rewarmed actively on eight occasions. Our findings show that during winter poorwills exhibit physiological patterns and active rewarming similar to hibernating mammals.

Measurement of bovine body and scrotal temperature using implanted temperature sensitive radio transmitters, data loggers and infrared thermography

Synchronous and continuous measurement of body (BT) and scrotal temperature (ST) without adverse welfare or behavioural interference is essential for understanding thermoregulation of the bull testis. This study compared three technologies for their efficacy for long-term measurement of the relationship between BT and ST by means of (1) temperature sensitive radio transmitters (RT), (2) data loggers (DL) and (3) infrared imaging (IRI). After an initial pilot study on two bulls to establish a surgical protocol, RTs and DLs were implanted into the flank and mid-scrotum of six Wagyu bulls for between 29 and 49days. RT frequencies were scanned every 15min, whilst DLs logged every 30min. Infrared imaging of the body (flank) and scrotum of each bull was recorded hourly for one 24-h period and compared to RT and DL data. After a series of subsequent heat stress studies, bulls were castrated and testicular tissue samples processed for evidence of histopathology. Radio transmitters were less reliable than DLs; RTs lost >11% of data, whilst 11 of the 12 DLs had 0% data loss. IRI was only interpretable in 35.8% of images recorded. Pearson correlations between DL and RT were strong for both BT (r>0.94, P<0.001) and ST (r>0.80, P<0.001). Surgery produced temporary minor inflammation and scrotal hematoma in two animals post-surgery. Whilst scar tissue was observed at all surgical sutured sites when bulls were castrated, there was no evidence of testicular adhesion and normal active spermatogenesis was observed in six of the eight implanted testicles. There was no significant correlation of IRI with either DL or RT. We conclude that DLs provided to be a reliable continuous source of data for synchronous measurement of BT and ST.

Thermoregulation by captive and free-ranging Egyptian rousette bats (Rousettus aegyptiacus) in South Africa

Many bats, including some small-bodied tropical and subtropical Pteropodidae, use torpor to offset energetic constraints. We tested the hypothesis that medium-sized (110–160 g) cave-roosting Egyptian rousette bats (Rousettus aegyptiacus) at the southern extent of their range are able to employ torpor. We measured daytime body temperatures (Tb) of 9 wild individuals using implanted temperature-sensitive radiotransmitters. The bats roosted in a cave on Table Mountain, Cape Town, South Africa (a typically cool and wet area). Daily mean cave temperature (Tc) ranged between 7°C and 12°C (mean 9.3°C). All wild individuals exhibited a circadian cycle in Tb, with an average of 37.7°C upon return from foraging, decreasing to 35.5°C by mid-day. Before emergence for feeding, Tb increased to about 37°C. No individual allowed Tb to drop below 34°C indicating (assuming a threshold temperature of 30°C) that individuals in this population do not use torpor. Bats were active throughout the day within the roost and metabolic heat production may have contributed to the maintenance of rest-phase Tb. Ten individuals implanted with temperature-sensitive passive integrated transponder tags were held in captivity at temperatures of 25–30°C and subjected to food deprivation for 2–3 days. The lowest Tb recorded for any captive individual was 33.4°C despite losing an average of 10% of their initial body mass. Despite exposure to cool Ta in the cave, and often cold, wet, and windy conditions while foraging, or substantial food restriction in captive bats, R. aegyptiacus did not resort to using torpor.

Surface Activity and Body Temperature of Eastern Hognose Snakes (Heterodon platirhinos) at Cape Cod National Seashore, Massachusetts USA

Abstract Snakes, as ectotherms, rely heavily on environmental temperature to drive physiological processes. Activity in terrestrial snakes is influenced heavily by environmental temperature, but climatic, temporal, and ecological factors play a role as well. Understanding when and under what conditions a species is most likely to be active on the surface can be important when dealing with a cryptic species and a species that occurs at low densities. Surface activity, body temperature (Tb), and microclimatic data were collected during a study of 17 Heterodon platirhinos in an early-successional sand-dune ecosystem from 2009 to 2011 using temperature-sensitive radiotransmitters. Mean Tb (27.9°C, SE = 0.24) and Tb range (5.5–39.5°C) were similar to those reported in other studies of H. platirhinos. Females were generally more active than males, except in September when males may be searching for mates. Mixed-effects logistic regression was used to assess the relationship between snake surface activity and se...

Surgical implantation of temperature‐sensitive transmitters and data‐loggers to record body temperature in koalas (Phascolarctos cinereus)

Under predicted climate change scenarios, koala distribution in Australia is expected to be adversely affected. Recent studies have attempted to identify suitable habitat, based on models of bioclimatic regions, but to more accurately reflect the thermal tolerance and behavioural adaptations of the various regional populations, the koala's response to periods of heat stress will need to be investigated at the individual animal level. To explore the safety and suitability of temperature-sensitive intra-abdominal implants for monitoring core body temperature in the koala. A temperature-sensitive radio transmitter and thermal iButton data-logger, waxed together as a package, were surgically implanted into the abdominal cavity of four captive koalas. In one animal the implant was tethered and in the other three, it was left free-floating. After 3months, the implants were removed and all four koalas recovered without complications. The tethering of the package in the one koala resulted in minor inflammation and adhesion, so this practice was subsequently abandoned. The free-floating deployments were complication-free and revealed a diurnal body temperature rhythm, with daily ranges of 0.4-2.8°C. The minimum recorded body temperature was 34.2°C and the maximum was 37.7°C. The difference in the readings obtained from the transmitters and iButtons never exceeded 0.3°C. The suitability of the surgical approach was confirmed, from both the animal welfare and data collection points of view.

Using pairs of physiological models to estimate temporal variation in amphibian body temperature

Physical models are often used to estimate ectotherm body temperatures, but designing accurate models for amphibians is difficult because they can vary in cutaneous resistance to evaporative water loss. To account for this variability, a recently published technique requires a pair of agar models that mimic amphibians with 0% and 100% resistance to evaporative water loss; the temperatures of these models define the lower and upper boundaries of possible amphibian body temperatures for the location in which they are placed. The goal of our study was to develop a method for using these pairs of models to estimate parameters describing the distributions of body temperatures of frogs under field conditions. We radiotracked green-eyed treefrogs (Litoria serrata) and collected semi-continuous thermal data using both temperature-sensitive radiotransmitters with an automated datalogging receiver, and pairs of agar models placed in frog locations, and we collected discrete thermal data using a non-contact infrared thermometer when frogs were located. We first examined the accuracy of temperature-sensitive transmitters in estimating frog body temperatures by comparing transmitter data with direct temperature measurements taken simultaneously for the same individuals. We then compared parameters (mean, minimum, maximum, standard deviation) characterizing the distributions of temperatures of individual frogs estimated from data collected using each of the three methods. We found strong relationships between thermal parameters estimated from data collected using automated radiotelemetry and both types of thermal models. These relationships were stronger for data collected using automated radiotelemetry and impermeable thermal models, suggesting that in the field, L. serrata has a relatively high resistance to evaporative water loss. Our results demonstrate that placing pairs of thermal models in frog locations can provide accurate estimates of the distributions of temperatures experienced by individual frogs, and that comparing temperatures from model pairs to direct measurements collected simultaneously on frogs can be used to broadly characterize the skin resistance of a species, and to select which model type is most appropriate for estimating temperature distributions for that species.

The influence of reproductive condition and concurrent environmental factors on torpor and foraging patterns in female big brown bats (Eptesicus fuscus).

Unlike many other mammals, bats in temperate regions employ short bouts of torpor throughout the reproductive period to maintain a positive energy balance. In addition to decreasing energy expenditure during the day, they typically alter foraging patterns as well. It is well known that various environmental conditions influence both torpor and foraging patterns, but studies of these factors often have focussed on one element in isolation thus it is not known how the two behaviours are collectively influencing temperate bats. The objective of our study was to assess how reproductive condition and environmental factors concurrently affect energy balance in female big brown bats (Eptesicus fuscus). We equipped pregnant and lactating bats in southwest Saskatchewan, Canada with temperature-sensitive radio-transmitters. While transmitters were active, skin temperature data were collected and foraging patterns were determined using triangulation. Of the various environmental and physiological parameters used to model torpor characteristics, roost type was the most important factor. Bats roosting in trees used deeper and longer torpor bouts than those roosting in buildings. Lactating bats had a tendency to forage for longer durations than pregnant bats, and often made more foraging trips. When taken together, we found that foraging duration and torpor duration were not directly related during pregnancy, but exhibited an inverse relationship during lactation. This provides support for the hypothesis that there are physiological trade-offs for reproductive bats and suggests that how bats compensate is not entirely predictable based on current environmental conditions.

Effect of wind on the accuracy of externally attached, temperature-sensitive radiotransmitters

Recording and interpreting thermoregulatory patterns is important in determining many aspects of animal ecology and behavior. Externally attached radiotransmitters are a relatively non-invasive method to continuously monitor skin temperature as a proxy for core body temperature. However, the effects of ambient conditions, such as wind or rain, on the accuracy of externally attached radiotransmitters are often overlooked. We investigated the effect of wind speed on the accuracy of radiotransmitters. We found that the accuracy of radiotransmitter readings was correlated with wind speed, with higher wind speeds resulting in underestimations of skin temperature. These effects may result in misinterpretations of recorded data, and ultimately may lead to inaccuracies in describing behavior. © 2013 The Wildlife Society.

Summer heterothermy in Rafinesque’s big-eared bats (Corynorhinus rafinesquii) roosting in tree cavities in bottomland hardwood forests

Many small mammals are heterothermic endotherms capable of maintaining an elevated core body temperature or reducing their thermoregulatory set point to enter a state of torpor. Torpor can confer substantial energy savings, but also incurs ecological costs, such as hindering allocation of energy towards reproduction. We placed temperature-sensitive radio transmitters on 44 adult Rafinesque's big-eared bats (Corynorhinus rafinesquii) and deployed microclimate dataloggers inside 34day roosts to compare the use of torpor by different sex and reproductive classes of bats during the summer. We collected 324 bat-days of skin-temperature data from 36 females and 4 males. Reproductive females employed fewer torpor bouts per day than non-reproductive females and males (P<0.0001), and pregnant and lactating females had higher average (P<0.0001) and minimum (P<0.0001) skin temperatures than non-reproductive females. Pregnant females spent less time torpid (P<0.0001) than non-reproductive females, but lactating females used relatively deep, long torpor bouts. Microclimates varied inside tree species with different configurations of entrances to the roost cavity (P<0.0001). Bats spent more time torpid when roosting in water tupelo (Nyssa aquatica) trees possessing only a basal entrance to the cavity (P=0.001). Of the tree species used as roosts, water tupelo cavities exhibited the least variable daytime and nighttime temperatures. These data demonstrate that use of summer torpor is not uniform among sex and reproductive classes in Rafinesque's big-eared bat, and variation in microclimate among tree roosts due to species and structural characteristics facilitates the use of different thermoregulatory strategies in these bats.

Frequent Arousals from Winter Torpor in Rafinesque’s Big-Eared Bat (Corynorhinus rafinesquii)

Extensive use of torpor is a common winter survival strategy among bats; however, data comparing various torpor behaviors among species are scarce. Winter torpor behaviors are likely to vary among species with different physiologies and species inhabiting different regional climates. Understanding these differences may be important in identifying differing susceptibilities of species to white-nose syndrome (WNS) in North America. We fitted 24 Rafinesque’s big-eared bats (Corynorhinus rafinesquii) with temperature-sensitive radio-transmitters, and monitored 128 PIT-tagged big-eared bats, during the winter months of 2010 to 2012. We tested the hypothesis that Rafinesque’s big-eared bats use torpor less often than values reported for other North American cave-hibernators. Additionally, we tested the hypothesis that Rafinesque’s big-eared bats arouse on winter nights more suitable for nocturnal foraging. Radio-tagged bats used short (2.4 d ± 0.3 (SE)), shallow (13.9°C ± 0.6) torpor bouts and switched roosts every 4.1 d ± 0.6. Probability of arousal from torpor increased linearly with ambient temperature at sunset (P<0.0001), and 83% (n = 86) of arousals occurred within 1 hr of sunset. Activity of PIT-tagged bats at an artificial maternity/hibernaculum roost between November and March was positively correlated with ambient temperature at sunset (P<0.0001), with males more active at the roost than females. These data show Rafinesque’s big-eared bat is a shallow hibernator and is relatively active during winter. We hypothesize that winter activity patterns provide Corynorhinus species with an ecological and physiological defense against the fungus causing WNS, and that these bats may be better suited to withstand fungal infection than other cave-hibernating bat species in eastern North America.

Hibernation energetics of free-ranging little brown bats

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 under-represented 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.

BATS USE SHALLOW TORPOR TO MINIMISE COSTS

White-nose syndrome is a devastating fungal disease that is currently decimating the bat populations of North America. Specifically targeting hibernating bats, the disease appears to cause its victims to arouse from torpor more frequently, preventing them from conserving energy and leading to starvation. Craig Willis and Kristin Jonasson from the University of Winnipeg, Canada, explain that it is difficult to assess the energetic impact of this disease without realistic models of how hibernating bats use energy. So, the duo decided to monitor the skin temperatures of hibernating little brown bats to find out whether the hibernating animals optimise their use of torpor during hibernation and how well temperature measurements can be used to estimate the animals' energy consumption rates (p. 2141).Fitting temperature-sensitive radio transmitters to 22 bats hibernating in a small cave in central Manitoba, Canada, the duo monitored the bats' body temperatures for 3 months. Recording the lengthy periods when the bats became torpid (13.1 days on average), depressing their body temperatures, and the brief arousals when the animals returned their body temperatures to normal, Willis and Jonasson found that the duration of torpor varied significantly from animal to animal. They also found that the bats were torpid for an impressive 99.6% of the hibernation period.Next, the pair analysed the temperature recordings to find out whether the bats' age, sex or body condition affected their hibernation. They did not. However, when the scientists analysed the arousal periods, they noticed something unusual. Occasionally the bats dropped their body temperatures slightly during arousal and entered a period of shallow torpor. The duo suspects that the bats use these bouts – which they refer to as heterothermic arousal – to minimise the over all metabolic costs of hibernation. Finally, the team measured the masses of the tiny animals to estimate their energy use over the course of the hibernation, and found that their calculated predictions of energy use agreed well with the bats' mass loss, although the calculations slightly overestimated the energy consumption.Having assessed the mammal's hibernation energetics, Willis says, ‘Our findings will be important for understanding the energetic and survival implications for bats suffering from white-nose syndrome.’

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 20.4days with individual means ranging from 0.9 to 8.9days. 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 297min 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.