Roost Temperature Research Articles

Tricolored Bat (Perimyotis subflavus) microsite use throughout hibernation

Abstract White-nose syndrome (WNS) has caused dramatic population declines in several bat species, including the Tricolored Bat (Perimyotis subflavus). Several studies have documented bats using colder roosting temperatures after infection; however, this strategy may have costs such as increased freezing risks or greater predation risks and it is unknown when during hibernation bats begin to utilize these colder temperatures. Our aim was to examine Tricolored Bat roost locations in a WNS-positive site in relation to roost microclimate and other environmental conditions throughout the hibernation season. We conducted monthly censuses of tricolored bats across 2 hibernation seasons (November to March 2020 to 2021 and October to March 2021 to 2022) in a WNS-positive hibernaculum in northwestern South Carolina and recorded skin and adjacent wall temperature, tunnel section, and distance from the entrance for each bat species. We continuously measured hibernacula temperature and relative humidity during both hibernation seasons. Most bats roosted in the back part of the tunnel where temperatures were warmer and more stable, and vapor pressure deficit (VPD) and human disturbance were low. However, >20% of bats roosted in the front, where roost temperatures were significantly colder and VPD was higher but more variable; human disturbance was also higher in this section. The proportion of bats in each tunnel section did not vary among months and we did not find evidence of significant movement to the front section of the tunnel as hibernation progressed based on marked bats; however, bats in the front section roosted higher on the wall suggesting that they may be avoiding human disturbance or predators. Our results support the notion that no optimum hibernation temperature exists for tricolored bats and that high VPD and disturbance are likely important factors driving microsite use. Protection of Tricolored Bat hibernacula that offer a range of microclimates or a network of sites in close proximity that offer different microclimates may be helpful for recovery of this species.

Optimally warm roost temperatures during lactation do not improve body condition in a long-lived bat.

Lactation is the most energetically demanding time in the life of female mammals. To maximize lifetime reproductive success, females of long-lived species, such as bats, face a trade-off between investing in current and future reproduction. However, it is unclear whether global warming could influence this trade-off through shifts in the energy budget: warmer temperatures may reduce thermoregulatory costs, leaving mothers with more energy available for maternal care or for improving their own body condition (BC), which may increase survival and ensure future reproduction. Here, we investigated whether lactating Bechstein's bats (Myotis bechsteinii) allocate the energy saved in optimally warm roosts into their own BC. We analysed a 14-year dataset on the individual BC of 237 females marked with radio-frequency identification tags from four wild maternity colonies. In two of the colonies, the temperature in the roosts, in which the females raised their offspring, was artificially kept in the bats' thermoneutral zone to reduce their thermoregulation costs. We found that BC shortly after the lactation period did not differ between mothers from heated and non-heated colonies. Our results suggest that mothers do not invest the energy saved in warmer roosts in their own BC, consistent with an increased investment in maternal care.

Occupancy of urban roosts by spectacled flying‐foxes (Pteropus conspicillatus) is not affected by diurnal microclimate

AbstractOne of the most significant changes to Earth's climate in recent decades has been an increase in the frequency, intensity and duration of heatwaves. During heatwaves, animal's thermal window can be exceeded, and in extreme cases, mass mortality events have been observed. In 2018, a heatwave in north‐eastern Australia resulted in the death of approximately one‐third of the spectacled flying‐fox (Pteropus conspicillatus) population at urban roosts in Cairns. The species has now been listed as endangered with future heatwaves considered the greatest threat to its survival. In this study, we investigated long‐term climatic trends for Cairns, paying particular attention to the frequency of extreme heat events from 1943 to 2022. We then characterized the microclimate of urban flying‐fox roosts during the Austral summers of 2021/2022 and 2022/2023 across Cairns to assess the long‐term feasibility of urban spectacled flying‐fox roosts. From the long‐term climate records, we observed an overall increase in Cairns' average annual temperature of 1.3°C from 1943 to 2022 and an increase in the number of excessively hot days per decade, from 16 in the first decade (1943–1952) to 67 in the last (2013–2022). We regularly detected maximum roost temperatures of 30–35°C during our study, with excessively hot days (>35°C) recorded more frequently than expected compared to Cairns's maximum temperatures from the last decade (2013–2023). We detected only 1 day where roost temperatures exceeded 40°C and no period that replicated the 2018 heatwave conditions. Furthermore, we found a significant negative relationship between roost ambient temperature and humidity, where the hottest days also coincided with those with the lowest humidity. Importantly, we found no difference in microclimate between roosts that were occupied and unoccupied by flying‐foxes during our study, suggesting that other environmental or behavioural factors are more influential for roost selection than the roosting microclimate. Ensuring the long‐term conservation of spectacled flying‐foxes under a changing climate will require the management of urban roosts to increase their thermal resistance to heatwaves, and more research is needed to identify the variables modulating this aspect.

Thermally unstable roosts influence winter torpor patterns in a threatened bat species

Abstract Many hibernating bats in thermally stable, subterranean roosts have experienced precipitous declines from white-nose syndrome (WNS). However, some WNS-affected species also use thermally unstable roosts during winter that may impact their torpor patterns and WNS susceptibility. From November to March 2017–19, we used temperature-sensitive transmitters to document winter torpor patterns of tricolored bats (Perimyotis subflavus) using thermally unstable roosts in the upper Coastal Plain of South Carolina. Daily mean roost temperature was 12.9 ± 4.9°C SD in bridges and 11.0 ± 4.6°C in accessible cavities with daily fluctuations of 4.8 ± 2°C in bridges and 4.0 ± 1.9°C in accessible cavities and maximum fluctuations of 13.8 and 10.5°C, respectively. Mean torpor bout duration was 2.7 ± 2.8 days and was negatively related to ambient temperature and positively related to precipitation. Bats maintained non-random arousal patterns focused near dusk and were active on 33.6% of tracked days. Fifty-one percent of arousals contained passive rewarming. Normothermic bout duration, general activity and activity away from the roost were positively related to ambient temperature, and activity away from the roost was negatively related to barometric pressure. Our results suggest ambient weather conditions influence winter torpor patterns of tricolored bats using thermally unstable roosts. Short torpor bout durations and potential nighttime foraging during winter by tricolored bats in thermally unstable roosts contrasts with behaviors of tricolored bats in thermally stable roosts. Therefore, tricolored bat using thermally unstable roosts may be less susceptible to WNS. More broadly, these results highlight the importance of understanding the effect of roost thermal stability on winter torpor patterns and the physiological flexibility of broadly distributed hibernating species.

Artificially raised roost temperatures lead to larger body sizes in wild bats

Climate warming has major consequences for animal populations, as ambient temperature profoundly influences all organisms' physiology, behavior, or both.1 Body size in many organisms has been found to change with increased ambient temperatures due to influences on metabolism and/or access to resources.2,3,4,5,6 Changes in body size, in turn, can affect the dynamics and persistence of populations.7 Notably, in some species, body size has increased over the last decades in response to warmer temperatures.3,8 This has primarily been attributed to higher food availability,3 but might also result from metabolic savings in warmer environments.9,10 Bechstein's bats (Myotis bechsteinii) grow to larger body sizes in warmer summers,11 which affects their demography as larger females reproduce earlier at the expense of a shorter life expectancy.12,13 However, it remains unclear whether larger body sizes in warmer summers were due to thermoregulatory benefits or due to increased food availability. To disentangle these effects, we artificially heated communal day roosts of wild maternity colonies over four reproductive seasons. We used generalized mixed models to analyze these experimental results along with 25 years of long-term data comprising a total of 741 juveniles. We found that individuals raised in heated roosts grew significantly larger than those raised in unheated conditions. This suggests that metabolic savings in warmer conditions lead to increased body size, potentially resulting in the decoupling of body growth from prey availability. Our study highlights a direct mechanism by which climate change may alter fitness-relevant traits, with potentially dire consequences for population persistence.

Microhabitat of Myotis leibii summer roosts at the southwestern periphery of their range

Understanding microhabitat use is needed to make sound conservation decisions for at-risk, patchy-habitat specialists, such as rock-habitat specialists. Rock habitats offer unique microclimatic refugia for reptiles and mammals. Eastern small-footed bats ( Myotis leibii (Audubon and Bachman, 1842)) use rock roosts during the summer, but data on these summer roosts are lacking for this species classified as (critically) imperiled in several US states and Canadian provinces and globally endangered. Our goal was to characterize the structure and microclimate of Myotis leibii roosts at the southwestern periphery of their range. We predicted that Myotis leibii roost temperatures would be warmer and less variable than ambient temperatures and that solitary bats would use horizontal roosts cooler at night, whereas maternity group roosts would be vertical and warmer at night. During summers of 2019 and 2020, we recorded physical (e.g., width) and temperature attributes of 58 Myotis leibii roosts at 16 sites in the Ouachita Mountains. Crevice roosts of Myotis leibii had narrow dimensions like elsewhere in their range and roost temperatures (measured with iButtons) were warmer and more variable than ambient temperatures. Group roosts were larger and had more stable temperatures than solitary roosts. These findings may be useful for assessing population threats, monitoring roost suitability, identifying roost-rich areas that need protection, and even planning artificial roost structures where natural roosts are limited.

Temporal Variations among the Growth Parameters of the Forearms of Juvenile Greater Horseshoe Bats (Rhinolophus ferrumequinum) in the Nursery Roost at Stackpole, West Wales, U.K.

We addressed the question of why juvenile R. ferrumequinum that are born early (May to early June) in the nursery roost at Stackpole, West Wales, tend to have a higher forearm growth rate than individuals born later (late June to July). An appropriate nonlinear growth equation (either logistic, Gompertz, von-Bertalanffy, or asymptotic regression) was identified to fit the measurements of the forearms of juvenile R. ferrumequinum collected from marked individuals at the roost between 1994 and 2013. The logistic growth model provided the best line of fit to the changes in the forearm lengths, indicated by the lowest standard error. Over the 20-year study period, the mean values of the logistic growth parameters were asymptotic length (L∞) = 55.10 mm and growth constant (K) = 0.11 day-1. Multiple regression analysis indicated that the most significant predictors of the variance in L∞ and K (R2 = 58.7% and 54.3%, respectively) were the nursery roost temperatures at midnight, and the total foraging times of the adult bats during the night. If the juveniles were born early, when the roost temperature was colder, and the foraging times of the adults were shorter, then a lower value of K produced forearms that grew toward a longer L∞. If the juveniles were born late, when the roost temperature was warmer, and the foraging times of the adults were longer, then a higher value of K produced forearms that grew toward a shorter L∞. Reasons are proposed to explain the differences between the growth of early and late born individuals, including the effect of increased body mass associated with warmer roost temperatures and possible causes of longer foraging times.

SUN HATS FOR BAT BOXES: MITIGATING THE RISK OF OVERHEATING AT NORTHERN LATITUDES

At northern latitudes, bats often use roosting structures provided by people. Conventional thinking suggests that bat boxes should be entirely black to absorb heat and assist bats with thermoregulation. However, with long periods of summer daylight in subarctic Canada, we suspected that risk of overheating might occur in black boxes. We investigated whether roost temperatures exceeded 42°C, the upper limit of the thermoneutral zone in Little Brown Myotis (Myotis lucifugus), and whether replacing the black roof with a white one, akin to a sun hat, would alleviate the risk of overheating without compromising roost temperatures especially at night. We also investigated whether bats used the boxes. The internal temperature of black boxes exceeded 42°C on some days in 2 summers. Substituting the black roof with a white one consistently reduced the maximum daily roost temperature to below 42°C, but this was also accompanied by a slightly lower minimum roost temperature at night and a cooler roost temperature regime overall. The reduction in maximum daily roost temperature by the white roof was more pronounced on days with higher maximum daily ambient temperatures. At 5 sites with paired white-roof and black-roof boxes, 4 had bat use of both boxes and 1 had no use of either box. Bats used 10 of 19 (53%) black-roof boxes in 2019, and 16 of 21 (77%) in 2020. Replacing a black roof with a white one can mitigate risk of overheating for boxes in subarctic latitudes, and likely through a range of temperate latitudes. However, completely black boxes are still necessary to enhance the thermal environment for roosting bats through most of the summer. We suggest that providing bats with roosting options, such as paired black-roof and white-roof boxes immediately adjacent to one another, is a better habitat enhancement option than just single black boxes.

Changes in hibernating tricolored bat (Perimyotis subflavus) roosting behavior in response to white-nose syndrome.

Understanding animals' behavioral and physiological responses to pathogenic diseases is critical for management and conservation. One such disease, white‐nose syndrome (WNS), has greatly affected bat populations throughout eastern North America leading to significant population declines in several species. Although tricolored bat (Perimyotis subflavus) populations have experienced significant declines, little research has been conducted on their responses to the disease, particularly in the southeastern United States. Our objective was to document changes in tricolored bat roost site use after the appearance of WNS in a hibernaculum in the southeastern U.S. and relate these to microsite temperatures, ambient conditions, and population trends. We censused a tricolored bat hibernaculum in northwestern South Carolina, USA, once each year between February 26 and March 2, 2014–2021, and recorded species, section of the tunnel, distance from the entrance, and wall temperature next to each bat. The number of tricolored bats in the hibernaculum dropped by 90.3% during the first 3 years after the arrival of WNS. However, numbers stabilized and slightly increased from 2018 to 2021. Prior to the arrival of WNS, 95.6% of tricolored bats roosted in the back portion of the tunnel that was the warmest. After the arrival of WNS, we observed a significant increase in the proportion of bats using the front, colder portions of the tunnel, particularly during the period of population stabilization and increase. Roost temperatures of bats were also positively associated with February external temperatures. Our results suggest that greater use of the colder sections of the tunnel by tricolored bats could have led to increased survival due to slower growth rates of the fungus that causes WNS in colder temperatures or decreased energetic costs associated with colder hibernation temperatures. Thus, management actions that provide cold hibernacula may be an option for long‐term management of hibernacula, particularly in southern regions.

Energy saving in day-roosting female Myotis emarginatus during reproduction (Chiroptera: Vespertilionidae)

From May to June 2012, we studied the behaviour of day-roosting reproductive female Geoffroy’s bats in a maternity roost in eastern Burgenland (Austria) which is under surveillance from 2011 till to date. By using a remote-controlled infrared-illuminated video camera, we conducted six weekly sessions of direct observation and instantaneous scan sampling, each lasting 16 hours. Based on a total of 384 sampling sessions, we quantified the amount of time adult females spent in the activities resting, alert, grooming and relocating during pregnancy and lactation. Ambient and roost temperatures were recorded hourly, the numbers of individuals returning to the roost in the mornings were registered constantly by using an infrared light barrier. Over the entire study period, all bats arriving in the maternity roost in the morning formed immediately a single huddling cluster. As a rule, this cluster was large, multilayered, three-dimensional and tight. It did not change in size and form until the onset of pre-emergence activities. It consisted of an interior part in which about 50% of all bats roosted and the periphery consisting of those bats which had not succeeded in entering the interior. Over the entire diurnal stay in the roost, resting – which causes the least energy output – was the predominant behaviour of all roost mates. Significant differences were found, however, in the amount of time allocated to some activities by bats occupying different positions in the cluster. While bats in the interior of the cluster spent the estimated 90–95% of the entire day-roosting period resting, bats on the periphery spent only 57–73% resting. The average percentage of time allocated by peripheral bats to grooming decreased from 27% in the first week to 19–13% in the following weeks of pregnancy and stayed at 16% during the two weeks of lactation. The mean percentages of being alert and of relocating ranged between 7–10% and 4–7%, respectively. During the last two weeks of pregnancy and the two weeks of lactation, roost temperatures, daily colony size and reproductive states did not influence the huddling behaviour significantly. However, activities performer in the first and second week were probably influenced by unrest due to colony formation after the return from hibernation (week 1) and by cold ambient temperatures during the week 2. Our study supports the hypothesis that the short duration and notable timing of reproduction typical for Myotis emarginatus (Spitzenberger & Weiss 2020) is achieved by maximal energy saving through continuous huddling in a large, three-dimensional, multilayered and tight cluster over both the entire day-roosting and entire reproductive period, differences in the behaviours of bats located in the interior and on the periphery of the cluster and lack of social interactions between roost mates.

Caves, crevices and cooling capacity: Roost microclimate predicts heat tolerance in bats

Abstract The microsites that animals occupy during the rest phase of their circadian activity cycle influence their physiology and behaviour, but relatively few studies have examined correlations between interspecific variation in thermal physiology and roost microclimate. Among bats, there is some evidence that species exposed to high roost temperatures (Troost) possess greater heat tolerance and evaporative cooling capacity, but the small number of species for which both thermal physiology and roost microclimate data exist mean that the generality of this pattern remains unclear. Here, we test the hypothesis that bat heat tolerance and evaporative cooling capacity have co‐evolved with roost preferences. We predicted that species occupying roosts poorly buffered from high outside environmental temperature exhibit higher heat tolerance and evaporative cooling capacity compared to species inhabiting buffered roosts in which Troost remains well below outside conditions. We used flow‐through respirometry to investigate thermoregulation at air temperatures (Ta) approaching and exceeding normothermic body temperature (Tb) among six species with broadly similar body mass but differing in roost microclimate (hot vs. cool roosts). We combined these data with empirical measurements of Troost for each study population. Hot‐roosting species tolerated Ta ~4°C higher than cool‐roosting bats before the onset of loss of coordinated locomotion and non‐regulated hyperthermia. The evaporative scope (i.e. ratio of maximum evaporative water loss [EWL] to minimum thermoneutral EWL) of hot‐roosting species (16.1 ± 2.4) was substantially higher than that of cool‐roosting species (5.9 ± 2.4). Maximum evaporative cooling capacities (i.e. evaporative heat loss/metabolic heat production) of hot‐roosting species were >2, while the corresponding values for cool‐roosting species were ≤1. The greater heat tolerance and higher evaporative cooling capacity of hot‐roosting species compared with those occupying cooler roosts reveal variation in bat evaporative cooling capacity correlated with roost microclimate, supporting the hypothesis that thermal physiology has co‐evolved with roost preference. A free Plain Language Summary can be found within the Supporting Information of this article.

Nightly torpor use in response to weather conditions and individual state in an insectivorous bat

Torpor is a well-known energy conservation strategy in many mammal and bird species. It is often employed when environmental conditions are unfavourable to maximize survival probabilities. However, torpor often carries with it the physiological costs of a low body temperature and of rewarming in addition to potential missed opportunities for foraging. Therefore, we hypothesised that decision making regarding when to use torpor should reflect the most important environmental conditions for species distributions, and thus how they may be impacted by ongoing climate change. We investigated how weather conditions affect nightly torpor patterns in the nocturnal insectivorous Australian eastern long-eared bat (Nyctophilus bifax). By measuring the skin temperature of 37 free-ranging individuals, we confirmed that torpor was used more frequently during the winter and at subtropical compared to tropical locations. Using mixed-effect models we show that lower ambient temperatures were the main driver of individual torpor use, probably due to lower roost temperatures and prey availability. However, increased rain, wind and humidity, and decreasing barometric pressure, as well as brighter moonlight, also led to more time spent torpid per night. We suggest that bats evaluate multiple environmental cues to make decisions regarding torpor use versus active foraging based upon their expectations of the energetic benefits, prey availability and relative predation risk. Interactions between some of these effects and body mass (whilst controlling for forearm length) indicate that individual variation in body size and/or state-dependent effects of energy reserves also partly determined the use of nightly torpor in these bats.

Winter roosting ecology of tricolored bats (Perimyotis subflavus) in trees and bridges

Abstract Tricolored bats (Perimyotis subflavus) that roost in subterranean hibernacula have experienced precipitous declines from white-nose syndrome (WNS); however, understudied populations also use during winter non-subterranean roosts such as tree cavities, bridges, and foliage. Our objectives were to determine winter roost use by tricolored bats in an area devoid of subterranean roosts, determine roost microclimates to relate them to growth requirements of the fungal causal agent of WNS, and determine habitat factors influencing winter tree selection. From November to March 2017–2019, we used radiotelemetry to track 15 bats to their day roosts in the upper Coastal Plain of South Carolina and recorded microclimates in accessible tree cavities and bridges. We also characterized habitat and tree characteristics of 24 used trees and 153 random, available trees and used discrete choice models to determine selection. Roost structures included I-beam bridges, cavities in live trees, and foliage. Bridges were warmer and less humid than cavities. Roost temperatures often were amenable to fungal growth (< 19.5°C) but fluctuated widely depending on ambient temperatures. Bats used bridges on colder days (8.7°C ± 5.0 SD) and trees on warmer days (11.3°C ± 5.4). Bats selected low-decay trees closer to streams in areas with high canopy closure and cavity abundance. Bats also appeared to favor hardwood forests and avoid pine forests. Our results suggest that access to multiple roost microclimates might be important for tricolored bats during winter, and forest management practices that retain live trees near streams and foster cavity formation in hardwood forests likely will benefit this species. Our results also suggest tricolored bats using bridge and tree roosts might be less susceptible to WNS than bats using subterranean hibernaculum roosts. Thus, forests in areas without subterranean hibernacula in the southeastern United States that support bats during winter might represent important refugia from WNS for multiple species.

Barbastelles in a Production Landscape: Where Do They Roost?

Extensive areas of old forests have declined all over the temperate regions of Europe mainly due to extensive forestry. This is likely to have negative impact on bats that roost in trees, such as the western barbastelle Barbastella barbastellus. We investigated its selection of summer roosts in a commercially used landscape in southern Sweden. We captured and radio-tracked 14 bats and found 17 occupied roosts. Nine of the roosts, including two used by a maternity colony (ca. 30 females), were located between overlapping boards on the gables of barns. The remaining eight roosts, all used by single individuals, were under lose bark on thin trees (DBH = 0.2–0.35 m). All recorded roosts had entrances pointing downwards, were adjacent to deciduous trees providing protective darkness, and were in areas without artificial lighting. In the barns, the bats avoided the northern aspect, which is the lightest (sun sets in the NW and rises in the NE). Roost temperatures did not differ between tree- and barn roosts. Average ambient light intensity on emergence and return was 13.3 lux (SD = 10.1 lux). Roosts in trees and barns shared common physical characteristics, yet despite this both maternity roosts were located in barns, perhaps because such roosts had more space than available tree roosts. Our results suggest that in areas deprived of large trees and extensive old forest, barbastelle shows flexibility in roost selection, although they consistently avoid artificial lights of all kinds. An abundance of potential roosts in trees and buildings and absence of light pollution are therefore key elements in a holistic conservation program for this species.

Usage of buildings in the life cycle of two endangered Rhinolophus species in the Mediterranean region: implications for roost protection

Synanthropic roosting may allow cave-dwelling bats to cope with habitat fragmentation provided that suitable buildings are sustainably protected. This study on Asinara Island, Sardinia, focuses on roost requirements in synanthropy at different life stages of two endangered bat species, Rhinolophus hipposideros and Rhinolophus ferrumequinum. We rated the roost potential of 532 buildings and compared it with actual roost usage. Microclimate was compared across different roost types and between species, and bat composition and behaviour in nurseries of R. hipposideros were related to roost structure and microclimate. The two species occupied 25% of structures rated as “high potential”, versus 5% and 0% rated as “intermediate” and “low potential”. Concerning microclimate, R. hipposideros preferred warmer and drier day roosts, with higher temperature fluctuations during summer, while winter, and night, roost microclimate was comparable between species. In larger, warmer, and drier nurseries, colony size and proportion of reproductive females were higher and parturition started earlier. Before parturition, roost temperatures were inversely correlated to clustering, supporting its thermoregulatory function. Mothers spent in total 50% of the night inside the nursery caring the pup. Roost microclimate, size, and location close to foraging areas may thus promote breeding success. We conclude that a structure-based rating of roost potential supplemented by species-specific microclimatic requirements constitutes a promising predictor of roost usage. Moreover, adequate buildings support the complete life cycle of R. hipposideros in the absence of suitable caves. Buildings thus deserve increased protection measures in fragmented Mediterranean landscapes to ensure sustainable bat conservation.

Winter habitats of bats in Texas.

Few studies have described winter microclimate selection by bats in the southern United States. This is of particular importance as the cold-adapted fungus, Pseudogymnoascus destructans, which causes the fatal bat disease white-nose syndrome (WNS), continues to spread into southern United States. To better understand the suitability of winter bat habitats for the growth of P. destructans in this region, we collected roost temperature and vapor pressure deficit from 97 hibernacula in six ecoregions in Texas during winter 2016-17 and 2017-18. We also measured skin temperature of Rafinesque's big-eared bats (Corynorhinus townsendii), Townsend's big-eared bats (C. townsendii), big-brown bats (Eptesicus fuscus), southeastern myotis (Myotis austroriparius), cave myotis (M. velifer), tri-colored bats (Perimyotis subflavus), and Mexican free-tailed bats (Tadarida brasiliensis) during hibernation to study their use of torpor in these habitats. We found that temperatures within hibernacula were strongly correlated with external air temperatures and were often within the optimal range of temperatures for P. destructans growth. Hibernacula and skin temperatures differed among species, with Rafinesque's big-eared bats, southeastern myotis, and Mexican free-tailed bats occupying warmer microclimates and having higher torpid skin temperatures. For species that were broadly distributed throughout Texas, hibernacula and skin temperatures differed within species by ecoregion; Tri-colored bats and cave myotis in colder, northern regions occupied colder microclimates within hibernacula and exhibited colder skin temperatures, than individuals of the same species in warmer, southern regions. These data illustrate the variability in microclimates used as hibernacula by bats in Texas and suggest similar variation in susceptibility to WNS in the state. Thus, monitoring microclimates at winter roosts may help predict where WNS may develop, and where management efforts would be most effective.

Migrating silver-haired bats fine-tune torpor to save energy

Ultra-distance marathon runners are almost a race apart. Covering thousands of miles over weeks and even months, these extreme human athletes require mental stamina and physical endurance. Yet, many migratory species take these distances in their stride. ‘Some studies suggest that silver-haired bats spend their winters in southern USA and their summers in the Canadian boreal forest’, says Dylan Baloun, currently a graduate student at the University of Saskatchewan, Canada. But the bats that undertake these odysseys are at a natural disadvantage. ‘They are nocturnal, so daytime stopovers are mandatory’, says Baloun, who explains that stopovers can be extremely costly for some species – totalling ∼70% of the entire migration cost for some birds. But, the mini mammals have a nifty energy-saving trick up their sleeves. ‘Bats use torpor’, says Baloun, explaining that they drop their body temperature while they rest to conserve energy. However, no one had directly measured how much energy the torpid mammals conserve while hanging out during the day and how the roost temperature affects their energy saving. So, Baloun and PI Chris Guglielmo from the University of Western Ontario, Canada, packed up their lab and headed out to the Long Point Bird Observatory to catch bats in the act.‘It is difficult to bring and maintain state-of-the-art physiological equipment into the field, even with a mobile laboratory trailer to maintain a stable climate’, says Baloun, who ventured out into the near-by woods at night to retrieve silver-haired bats during their spring and autumn migrations. Back in the temporary lab, the duo weighed and scanned the animals using magnetic resonance to measure how much fat and muscle the animals were carrying. Then, after attaching a thermometer to each bat to track its body temperature, they snuggled the individual animals inside soft cloth bags where they could slip into torpor and slumber away at roost temperatures of 10, 17.0 or 25°C. Finally, when the bats were preparing to depart again during the following dusk, the duo repeated the body measurements before allowing the animals to continue on their way.Calculating how much fat and muscle the bats lost during their daytime stopover and converting the loss into the amount of energy consumed during their slumber, the duo could see that the animals used essentially the same amount of energy over the course of the day, regardless of the temperature in their roost. The team then calculated the total amount of torpor experienced by each animal and it was clear that the bats were fine tuning how cold they became and for how long, with the result that they all consumed similar amounts of energy during their stopover regardless of the air temperature. And when the duo calculated how much energy the bats consumed during their break, they were impressed to see that it was as low as 15–20% of the total migration energy cost. ‘By entering a state of torpor, bats save energy for future migratory and potentially reproductive costs’, says Baloun.However, the team also realised that some of the bats were conserving energy and using torpor in slightly different ways, with bats that carried more weight using less torpor at the warmer temperatures, while the heavier females consumed more energy during spring stopovers than the lighter males. ‘Body mass was a major determinant in how bats used torpor’, says Baloun.So, migrating bats have the ability to dramatically reduce their energy costs during enforced daytime stopovers and the study adds another piece to the puzzle of how these tiny mammals pull off extreme feats of endurance, at which most of us can only marvel.

Unsuspected retreats: autumn transitional roosts and presumed winter hibernacula of little brown myotis in Colorado

Documentation of autumn and winter roosts of many species of hibernating bats are lacking from western North America. However, recent evidence suggests that rather than using caves and mines, many individuals and some species of bats may roost in inconspicuous rock crevices at these times of year. I investigated autumn use of rock crevices and other roosts by the little brown myotis (Myotis lucifugus) in the Rocky Mountains of western Colorado through radiotelemetry (n = 38). Objectives were to determine the types and characteristics of roosts, describe patterns of movements to these roosts from summer colonies, and contrast findings with results of surveys of bats in caves and abandoned mines in Colorado during autumn and winter. Forty-four autumn transitional roosts and presumed hibernacula were located in buildings, trees, and rock crevices. Bats used short-distance movements changing in elevation to autumn transitional roosts and presumed hibernacula rather than major latitudinal migrations. Roost type and distance from capture site to roosts had the highest variable importance at the landscape scale. Microclimate comparisons showed that buildings provided warmer minimum average temperatures, which may benefit juvenile bats early in the transition season. Tree roost temperatures during autumn would allow bats to conserve energy by using daily torpor and passive rewarming to assist with afternoon arousals. Rock crevice roosts in talus were found to be suitable for hibernation by exhibiting the coolest average temperatures and maintaining the highest relative humidity levels. Autumn access and spring egress to high-elevation talus sites used by these bats were not obstructed by winter snow pack. These rock crevices also provided temperature and humidity levels that would support the persistence and growth of Pseudogymnoascus destructans (Pd), the causal agent of white-nose syndrome. However, bats in this study appeared to roost alone, which could inhibit the bat-to-bat spread of Pd. Surveys of caves and mines within and surrounding the study area revealed few hibernating little brown myotis, suggesting that most individuals in Colorado may instead utilize rock crevices as roosts during winter.

An experimental test of effects of ambient temperature and roost quality on aggregation by little brown bats (Myotis lucifugus)

Environmental factors, such as ambient temperature (Ta) or roost/nest quality, can influence social behaviour of small-bodied endotherms because individuals may aggregate for social thermoregulation when Ta is low or select the warmest possible sites for roosting. Female temperate bats form maternity colonies in spring to communally raise pups and exploit social thermoregulation. They also select roosts with warm microclimates because low roost temperature (Troost) delays juvenile development. We studied captive female little brown bats (Myotis lucifugus) to test the hypothesis that variation in Ta and Troost influence social group size. First, we predicted that female bats would preferentially select artificially heated roosts over unheated roosts. Second, we predicted that, as Ta decreased, group size would increase because bats would rely more heavily on social thermoregulation. Third, we predicted that experimentally increasing Troost (i.e., roost quality) above Ta would result in larger group sizes due to greater aggregation in high quality roosts. We captured 34 females from a maternity colony and housed them in a flight-tent provisioned with four bat boxes. Each box was outfitted with a heating pad and thermostat. Over the course of eight-days we heated each roost box in sequence to near thermoneutral Troost for two days. Bats preferentially selected heated roosts over unheated roosts but, contrary to our prediction, group size decreased when Troost was much greater than Ta (i.e., when the benefits of a warm roost should have been highest). Our results suggest that social thermoregulation and the availability of warm roosts influence aggregation in bats and have implications for the potential of summer habitat protection and enhancement to help bat populations in the face of threats like white-nose syndrome.

Roost characteristics as indicators for heterothermic behavior of forest‐dwelling bats

AbstractMany forest‐dwelling bats spend their diurnal inactivity period in tree cavities. During this time bats can save energy through heterothermy. A heterothermic response (torpor) is characterized by a lowered body temperature, reduced metabolic rate, and reduction of other physiological processes, and can be influenced by the microclimatic conditions of roost cavities. The thermal and physical characteristics of roosts used by the sympatric, ecologically, and morphologically similar bat species Myotis bechsteinii, M. nattereri, and Plecotus auritus were compared. These three species differ in their heterothermic behavior, with the lowest skin temperatures observed for P. auritus. Therefore, we hypothesized that roosts occupied by the three species should differ in roost characteristics and microclimatic conditions, whereby P. auritus should select colder and thermally less stable roosts. The results showed that horizontal depth of the cavity, diameter of the roost tree, and microclimatic conditions within roosts differed among species. Roosts of P. auritus had the lowest horizontal depth, lowest thermal stability, and lowest mean minimum roost temperatures. Height of the roost, diameter of the roost tree, and vertical depth were also shown to influence microclimatic conditions. With increasing diameter of the tree and increasing horizontal depth, mean minimum roost temperature increased and thermal stability improved. Furthermore, with ascending height above ground insulation and mean roost temperatures increased. Our results imply that species such as P. auritus, which use pronounced torpor as a primary energy saving strategy, prefer colder cavities that support their heterothermic strategy.