Contrasting effects of climate change on the timing of reproduction and reproductive success of a temperate insectivorous bat

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1 bs_bs_bannerjournal of Zoology Contrasting effects of climate change on the timing of reproduction and reproductive success of a temperate insectivorous bat R. K. Lučan 1, M. Weiser 2 & V. Hanák 1 1 Department of Zoology, Faculty of Science, Charles University, Prague, Czech Republic 2 Department of Botany, Faculty of Science, Charles University, Prague, Czech Republic Journal of Zoology. Print ISSN Keywords chiroptera; phenology; reproductive output; temperate bats. Correspondence Radek K. Lučan, Department of Zoology, Faculty of Science, Charles University, Viničná 7, Praha 2, CZ 12844, Czech Republic. rlucan@centrum.cz Editor: Virginia Hayssen Received 31 October 2012; revised 10 January 2013; accepted 18 January 2013 doi: /jzo Abstract We used long-term datasets to analyse (1) the patterns of covariation between basic climatic variables (temperature and rainfall) and the timing of reproduction and reproductive success; and (2) long-term trends in both reproductive parameters of a maternity colony of Daubenton s bats Myotis daubentonii in South Bohemia, Czech Republic. The mean April temperature was the best predictor of the timing of reproduction. The higher the April temperature, the earlier the first neonates appeared. The mean date of first parturition was June 4, but it advanced significantly by c. 11 days between 1970 and Similarly, the mean April temperature increased over the study period by c. 2.7 C. Between 1999 and 2012, the mean reproductive success (proportion of reproductive females) was 74%, but varied between 33% (2009) and 93% (2006). It was negatively related to May July precipitation. Thus, reproductive success was lower in years with increased rainfall. Given the published evidence that advancement in parturition is positively related to survival of juvenile bats rising spring temperatures may have a beneficial influence on the population dynamics of Daubenton s bats. However, increased incidence of climatic extremes, such as excessive summer rainfall, may largely buffer this effect. Consequently, populations of temperate insectivorous bats may experience increasing environmental stress under continuing climate change. Introduction Recent climatic change is characterized by positive trends in global temperatures and a growing incidence of climatic extremes (IPCC, 2007). Warming of climate influences the timing of life-cycle events of a wide spectrum of organisms. Flowering and leaf unfolding, the timing of insect emergence and migratory bird arrival have advanced in response to climate change (Sparks & Menzel, 2002; Walther et al., 2002; Cotton, 2003). While the impact of climate on phenology has mostly been studied in plants where long-term data from botanical gardens are available (Ahas et al., 2002), information on other groups of organisms, such as mammals, remains scarce (Isaac, 2009). Temperate insectivorous bats have enormous value as bioindicators of climate change (Jones et al., 2009), because availability of their main diet, aerial insects, largely depends on ambient temperature and precipitation (Racey & Swift, 1985; Arlettaz et al., 2001; Fukui et al., 2006; Ciechanowski et al., 2007). Variation in temperature and rainfall during the reproductive period is a main factor affecting the timing of reproduction and reproductive success in insectivorous bats (Grindal et al., 1992; Ransome & McOwat, 1994; Burles et al., 2009; Sherwin, Montgomery & Lundy, 2012), although most data are from incidental observations or studies lasting typically 4 years (Racey & Swift, 1981; Grindal et al., 1992; Lewis, 1993; Burles et al., 2009). In general, frequent low temperatures are a primary factor prolonging gestation and delaying fledging of juveniles (Racey & Swift, 1981), whereas high rainfall negatively influences reproductive success (Grindal et al., 1992). In contrast, Adams (2010) reported decreased reproductive success in response to decreased precipitation in a bat assemblage living in a semiarid area in Colorado, US. To meet increased energetic demands connected with reproduction, female bats can increase food intake or enter torpor (Speakman & Thomas, 2003). While the former mechanism is commonly used (Barclay, 1989; Rydell, 1989; Wilkinson & Barclay, 1997), the latter is rather avoided because lowering body temperature negatively affects the rate of foetal development (Racey, 1973; Racey & Swift, 1981; Racey & Speakman, 1987; Dietz & Kalko, 2006). For example, female Lasiurus cinereus prolonged their foraging bouts at least by 73% between early lactation and fledging (Barclay, 1989). However, in many species such as Daubenton s bat Myotis daubentonii, increase in flight and foraging activity from pregnancy to lactation has not been recorded and the key factor enabling the increased energy demands of lactation to be met is the timing of reproduction to coincide with peak abundance Journal of Zoology 290 (2013) The Zoological Society of London 151

2 Climate change and reproduction of a temperate bat R. K. Lučan, M. Weiser and V. Hanák of insects (Henry et al., 2002; Dietz & Kalko, 2007). Furthermore, juvenile bats must learn to fly, echolocate and capture prey during the relatively short temperate summer so that they store sufficient fat reserves prior to hibernation. Consequently, it is advantageous for parturition to occur as early as possible as it was proven that juvenile bats born earlier in the summer have a significantly higher probability of surviving their first year than young born later (Ransome, 1989; Frick, Reynolds & Kunz, 2010). Bat species may differ in susceptibility to inclement weather because of different foraging or roosting strategies. In a study of reproduction of two insectivorous bats using natural roosts during years with contrasting weather, Burles et al. (2009) observed, that while adverse weather negatively influenced reproduction of M. lucifugus, adverse weather had a positive effect on M. keenii. They hypothesized that contrasting effects of weather may have arisen from different foraging strategies of the two species. By contrast, Syme, Fenton & Zigouris (2001) found no detrimental effect of an exceptionally cold summer on the timing of reproduction and reproductive success in a population of M. lucifugus roosting in buildings in the south east US, largely because insect prey remained abundant and available. The single difference they observed was a changed pattern of clustering behaviour of bats in roosts. They concluded that flexible roosting behaviour and an available food supply ameliorated the impact of bad weather on reproduction of this species. Last but not least, climate could play an important role in the aetiology of emerging diseases, such as the white-nose syndrome (WNS) devastating bat populations in North America (Flory et al., 2012). While separating combined effects of the disease and climate change on population dynamics in areas already affected with WNS may be difficult, a study conducted in a WNS-free area might help to show the pure effect of climate. Understanding how bats respond to shifts in climatic conditions is important for determining longterm impacts of global climate change on their populations (Sherwin et al., 2012). We used long-term data from a large maternity colony of Daubenton s bat from Southern Bohemia, Czech Republic, to analyse (1) the patterns of covariation between basic climatic variables (temperature and rainfall) and the timing of reproduction and reproductive success and (2) long-term trends in both reproductive parameters as related to climate. Daubenton s bat is a small (c. 8 g), heterothermic, insectivorous, vespertilionid bat that inhabits most of the western Palearctic (Horáček, Hanák & Gaisler, 2000). It is one of the most common species in Europe, and its abundance has markedly increased over the past decades (Kokurewicz, 1995). It is primarily a tree-dwelling bat species during the reproductive season, but frequently uses synanthropic roosts and occasionally even caves (Dietz, von Helversen & Nill, 2009). Daubenton s bat is a typical water-surface forager, capturing insects (mostly Dipterans) by hawking < 0.5 m above the water or gaffing them directly from the surface (Jones & Rayner, 1988). Unlike aerial hawking bats, its foraging activity is not constrained by low air temperatures and it was observed foraging in a temperature as low as -3.3 C (Ciechanowski et al., 2007; Dietz & Kalko, 2007). Furthermore, a detailed study on thermoregulatory behaviour of Daubenton s bats revealed a low dependency on torpor (Dietz & Hörig, 2011). Given this fact, we hypothesized that timing of reproduction should not be influenced by ambient temperature during pregnancy. However, because activity of insects as well as detection skills and flight performance of foraging bats may be limited by increased precipitation (Voigt et al., 2011), we predicted that increased rainfall during pregnancy and lactation may induce some females to forgo reproduction or lose newborn offspring, that is decrease reproductive success. Materials and methods Study area The study area is located in the northern part of the Třeboňsko basin, South Bohemia, Czech Republic (approximately 49 9 N, E). The region lies m a.s.l and represents a unique combination of well-preserved natural habitats (wetlands, peat bogs), semi-natural forests and agricultural landscapes (less than 30% of the area) with a low human population density. For this reason, it has been established as a Biosphere Reserve by the United Nations Educational, Scientific and Cultural Organization and is also protected by the Ramsar convention. The mean annual temperature reaches 6 7 C and the mean annual precipitation is mm (Tolasz et al., 2007). The studied roost is a small abandoned cellar-like building made of bricks, formerly used as a limekiln. The building is 5 m long, 4 m wide and 3 m high. The walls are about 1 m thick. Several crevices of various size are in the ceiling, the largest of them (entrance cm, depth 60 cm) has been used as the main roosting place of a maternity colony of up to 200 Daubenton s bats for more than 45 years (Lučan & Hanák, 2011a). The number of bats using the roost varied with the reproductive cycle with a maximum occurring during the late pregnancy and in the post-lactation period (Lučan, 2009). Owing to easy access to the roosting place of the colony, neonates and juveniles could be observed from a short distance (<0.5 m), and therefore their age could be estimated. Reproductive data We recorded dates of first neonates in 20 of the 43 years between 1970 and 2012 (1970, 1971, 1973, 1974, 1976, 1978, 1980, 1981, , 2004, 2005 and ) based on direct observation in the roost or by estimating the age of juveniles using the criteria of Krátký (1981). Inspections in the roost were made at 3 14-day intervals (mean 6 3 days) from 20 May to 15 June. To minimize disturbance to the bat colony, we inspected the roost in first 2 h after evening departure of adults so that only juveniles were usually present. Female Daubenton s bats usually stay in the roost with their neonates during the first day after parturition, but from on the second day, they leave juveniles in their day roost when they forage. After the departure of their mothers, neonates aggregate into 152 Journal of Zoology 290 (2013) The Zoological Society of London

3 R. K. Lučan, M. Weiser and V. Hanák Climate change and reproduction of a temperate bat Table 1 Descriptive statistics used in analyses of reproductive success in the studied colony of Daubenton s bats Myotis daubentonii Year Emergence count Bats captured Proportion of bats captured Adult females captured Reproductive success small groups on the ceiling of the roost to sustain high body temperature and are easily observed. Although our data did not allow us to analyse variation in the timing of birth within a single season, most crevice- and cavity-dwelling bat species have well-synchronized parturition within a single colony (e.g. Hoying & Kunz, 1998; Harbusch & Racey, 2006). In accord with published information, we observed that the majority of pregnant female Daubenton s bats gave birth within a day period every season. To obtain data on reproductive success, we sampled the whole colony once a year in the post-lactation period (typically in first 2 weeks in August, i.e. when the number of bats in the roost was highest) between 1999 and We used a mist net stretched over the entrance to the roost together with a hand net to capture as many bats as possible. On average, we managed to sample % (range %) of all bats present in the roost based on emergence counts made 1 7 days prior the sampling (Table 1). Upon capture, we recorded the sex, age and reproductive state of each bat. In adult female Daubenton s bats, we examined the nipples and environs for signs of suckling during the lactation period. Such signs were clearly visible as only some 3 5 weeks had passed since the weaning of juveniles. Female Daubenton s bats with enlarged nipples and no surrounding fur were assessed as reproductive in that given year. Females with slightly enlarged but furred nipples and no chin spot (Richardson, 1994) were assessed as adult, but either not reproductive or they may have lost their offspring prior to weaning. Females with a dark chin spot and no signs of lactation in the past were assessed as 1-year-old nulliparous. Nulliparous female Daubenton s bats made up to 10% of non-juvenile bats in the study roost and were not evaluated in calculations of reproductive success. We enumerated the overall reproductive success of the colony as the proportion of reproductive females to all adult females (Burles et al., 2009). Climate data We transformed calendar dates of first parturitions onto Julian dates beginning from April 1. Data on mean monthly temperatures and rainfall were obtained from a weather station in České Budějovice (c. 25 km away, same climatic area). Statistical analyses Pearson s correlations were used to evaluate (1) temporal trends in reproductive traits and best explanatory climatic variables; and the (2) relation between timing of parturitions and reprodutive success. The relationship between climatic parameters and reproductive traits was analysed using generalized linear models (GLM) as implemented in R statistics software (version ; R Development Core Team, 2010) with Akaike s information criterion (AIC) for model building. This method compares the parsimony of alternative models by comparing the robustness of particular model with the number of variables included (Burnham & Anderson, 2002). From the models set, we excluded those for which Shapiro Wilk test of residuals (procedure shapiro.test) indicated non-normality (P 0.1), or for which we found heteroscedascity. Heteroscedascity was measured as Spearman s rank correlation coefficient of model residuals and its fitted values. Models having this criterion higher than 0.1 were excluded. Individual models were scored using AICc (procedure aictab from the AICcmodavg package, ver.1.26; Mazerolle, 2012), as this criterion is more appropriate for our data-limited case than standard AIC (Hurvich & Tsai, 1991). Candidate models were chosen using Akaike weights (Burnham & Anderson, 2002). Models with Akaike weights 10% of the one with the highest weights were included in the final confidence set. Model averaging using Akaike weights (see p. 150 in Burnham & Anderson, 2002) was further carried out when more than one model had DAIC c < 2. In analyses of timing of parturition, we applied GLMs with Gaussian error structure given the distribution of the data. As dependent variables, we analysed the Julian date of first parturitions. As predictor variables, we used mean monthly temperatures and precipitation during the pregnancy period Journal of Zoology 290 (2013) The Zoological Society of London 153

4 Climate change and reproduction of a temperate bat R. K. Lučan, M. Weiser and V. Hanák (April-June), cumulative precipitation for this period and a standardized precipitation. We derived standardized precipitation for a given month by subtracting the mean precipitation of a particular month (calculated for the whole research period) from the actual value and dividing the difference by sample standard deviation. A positive value of this variable indicates excessive precipitation, while a negative one means deficient precipitation. Based on preliminary inspection of the dataset, we did not expect unimodal response to temperature or precipitation, and therefore we constructed our models using linear effects of predictors. Further, we restricted the procedure of model building to models containing two predictors and their interaction at maximum, and models that included both cumulative and monthly precipitation were not built. Also, we did not build models containing both cumulative precipitation and standardized precipitation as these variables were correlated (Pearson s r = 0.96). This resulted in a set of 91 models that was further reduced using criteria for residuals and by model averaging to choose the highest-ranking set of models. Effect of climatic variables on reproductive success was analysed using GLMs with binomial distribution of errors and a logit link function. As total size of the colony may be an important factor determining reproductive success, all built models included this parameter as a fixed predictor. Because of a low number of replicates, we decided to use a conservative approach and build models that only included one other linear predictor. As predictor variables, we used mean monthly temperatures and precipitation in April July, cumulative precipitation for periods April June, April July, May June, May July, June July, and sums of standardized precipitation for the same time periods. Altogether, we fitted 18 models. After the application of the criteria for model residuals, model set was further reduced to 13 models. From those, only eight models indicated reasonable fit with overdispersion lower than 4.0, as measured with Pearson s c 2 divided by the residual degrees of freedom (calculated using procedure c_hat, package AICCmodavg). Because of the overdispersion, AIC c was scaled to quasi-aic c (QAIC c) using the lowest overdispersion parameter of the remaining model set (ĉ = ; procedures c_hat and aictab). Results Figure 1 Changes in the timing of parturition of Daubenton s bats Myotis daubentonii (black dots, solid line), and mean April temperatures (open squares, dashed line) in South Bohemia, Czech Republic during the period Our results clearly demonstrate the profound effect of variation in temperature and rainfall on the timing of reproduction and reproductive success of the study population of Daubenton s bats. In total, we sampled 1497 Daubenton s bats ( per sample), of which 647 were adult females (46 15 females per sample). The remaining bats were either juveniles (n = 569, per sample), adult males (n = 151, 11 5) or nulliparous females (n = 130, 9 4). The total number of adult females captured was highly correlated (r = 0.92, n = 14, P = ) with the size of the colony in a given year but, the proportion of female Daubenton s bats to the overall colony size remained relatively stable (r =-0.36, n = 14, P = 0.201) indicating that our sample was representative. Overall size of the colony increased (r = 0.71, n = 14, P = 0.005) over the study period (Table 1). Timing of reproduction and climatic variables The mean date of first parturition was 4 June, but it varied between 24 May (in 2008) and 18 June (in 1999). It was negatively correlated with the year of observation (r =-0.58, n = 20, P = 0.008) and, on average, advanced by c. 11 days between 1970 and 2012 (Fig. 1). The highest-ranking model for the timing of reproduction (weight = 0.19; total number of models = 91; Table 2) included April temperature (Pearson s r =-0.68). The first five models had DAIC c < 2; hence, they were equally plausible. However, averaging of the subset of first five models gave highest Akaike weight (w = 0.35) to the first model. Thus, parturitions occurred earlier when April temperature was higher (Fig. 2). Using the linear regression equation, we estimated that an increase in the mean April temperature by 1 C resulted in a 2.8-day advancement in the time of first parturition. The mean April temperature showed inter-annual variation by up to 7.3 C and increased over the study period by c. 2.7 C (r = 0.56, n = 20, P = 0.011). Reproductive success and climatic variables The mean proportion of reproductive female Daubenton s bats (i.e. the reproductive success) was 74%, but it varied between 33% (2009) and 93% (2006). No temporal trend was apparent in the reproductive success (r =-0.45, n = 14, P = 0.111) over the study period. The highest-ranking model for the reproductive success (weight = 0.40; total number of models = 18; Table 3) included May July standardized precipitation (r =-0.75). All the other models in a candidate set (n = 7) included monthly precipitation, cumulative precipitation or standardized precipitation. Hence, the reproductive success decreased with increasing 154 Journal of Zoology 290 (2013) The Zoological Society of London

5 R. K. Lučan, M. Weiser and V. Hanák Climate change and reproduction of a temperate bat Table 2 Candidate set of 13 highest-ranking models estimated according to AIC c that best explain the variation in the timing of reproduction Model Residual deviance d.f. F Model significance (p) K AIC c DAIC c Weights April T < April T + SP April T + April P April T CP June T CP April T + May P April T PA April T + May T April T + March P April T + March T April T March P June T + CP April T June P AIC c, Akaike information criterion; CP, April June cumulative precipitation; d.f., residual degrees of freedom; K, number of parameters in the model; P, precipitation; SP, standardized precipitation for April June; T, temperature. long-term data that could hardly be acquired from tree roosts. Thus, colony has been studied in detail since the late 1960s (Lučan, 2006; Lučan & Hanák, 2011a,b). Despite some degree of disturbance because of the research activity, we did not observe any negative effects on bats. Indeed, the number of bats in the roost steadily increased over the last decades and it represents the largest maternity colony of Daubenton s bats in the Czech Republic. Therefore, we strongly believe that the observed trends are representative of other central European populations of this species. Figure 2 Effect of the mean April temperature on the timing of parturition of Daubenton s bats Myotis daubentonii in South Bohemia, Czech Republic in the period (dashed outline: 95% confidence interval). rainfall (Fig. 3). The May July standardized precipitation had a significant positive trend (r = 0.74, n = 14, P = 0.003) over the study period. We found no significant correlation (r = 0.50, n = 12, P = 0.099) between the timing of parturition and the reproductive success indicating that factors driving both reproductive traits were different. Discussion Representativity of our data Daubenton s bat is a typical representative of tree-dwelling bats that form fission fusion societies characterized by simultaneous occupancy of multiple day-roosts and frequent roostswitching (Kerth & König, 1999; Kapfer et al., 2008). These attributes make this group extremely hard to study. Roosting tradition adopted by the studied colony (cf. building roost with stable occupancy) provided unique conditions to gather a Effect of spring temperature Our long-term data provided evidence that spring temperature significantly influenced the timing of parturition in Daubenton s bats. Previous studies showed that weather plays an important role in the timing of reproduction in temperate insectivorous bats. Low spring ambient temperatures may prolong the gestation through its influence on the frequency of using body torpor by pregnant female Daubenton s bats (Racey & Swift, 1981), it may negatively affect activity of flying insects (Hoying & Kunz, 1998; Ciechanowski et al., 2007) or both. However, Daubenton s bats are able to forage even at very low temperatures (down to -3.3 C), most probably because of their foraging strategy and predominant prey, aquatic insects, which may be less affected by low temperatures than some other groups (Ciechanowski et al., 2007; Dietz & Kalko, 2007). Thus we predicted no influence of spring temperatures on reproductive timing. Contrary to our predictions, the timing of parturition was strongly affected by April temperatures. Because April is the month when most bats leave their hibernacula, we hypothesize that rather than having a direct effect on bats prey availability or thermoregulation (i.e. use of torpor), an increased spring temperature may shorten hibernation and, consequently, advance the onset of pregnancy or, alternatively, shorten the gestation period in females. Journal of Zoology 290 (2013) The Zoological Society of London 155

6 Climate change and reproduction of a temperate bat R. K. Lučan, M. Weiser and V. Hanák Table 3 Candidate set of seven highest-ranking models estimated according to AIC c that best explain the variation in reproductive success Model Deviance d.f. F Model significance (p) K QAIC c DQAIC c Weights May July SP May July CP July P June Jyly SP April July SP June July CP April July CP QAIC c, quasi-akaike information criterion; CP, cumulative precipitation; d.f., residual degrees of freedom; K, number of parameters in the model; P, precipitation; SP, standardized precipitation. Daubenton s bats observed in the past decades (Kokurewicz, 1995) may have been partially controlled by long-term climatic trends. Figure 3 Effect of May July rainfall on the reproductive success of Daubenton s bats Myotis daubentonii in South Bohemia, Czech Republic in the period (dashed outline: 95% confidence interval). Data on rainfall are standardized with positive values indicating excess rainfall and negative values indicating deficient rainfall. Ovulation and onset of pregnancy in hibernating bats is triggered by the increase in ambient temperature and arousal from hibernation (Heideman, 2000). Our long-term observation in the study roost revealed an almost 1-month interannual variation in the date when the first bats arrived from their hibernacula (Lučan & Hanák, 2011a), which corresponds with variation in the timing of parturitions. Accordingly, the observed inter-annual variation by up to 7.3 C in the mean April temperature during 20 years of the study suggests corresponding variation in the onset of spring activity of Daubenton s bats. Further, our data showed an overall increase in spring temperature over the study period followed by a concomitant advance of first births. Increase in spring temperatures resulting in overall prolongation of the growing season and advancement in reproductive timing during the past three to four decades was reported by many researchers (e.g. Ahas et al., 2002; Sparks & Menzel, 2002; Walther et al., 2002). Frick et al. (2010) reported a positive influence of early parturition on the first-year survival in M. lucifugus. As Daubenton s bat is a Palearctic ecological equivalent of M. lucifugus (Gaisler & Zukal, 2004), climate may have an analogous effect on its life-history traits. Further, positive population trends of Effect of rainfall Increased rainfall during pregnancy and lactation may delay parturition and decrease reproductive success in M. lucifugus (Burles et al., 2009), as well as in M. yumanensis (Grindal et al., 1992). While rainfall and reproductive timing were not related in our study, we showed that increased May July precipitation adversely affected reproductive success. Increased rainfall may restrict foraging activity and prey detectability by reproductive female Daubenton s bats and, consequently, induce some of them to forgo breeding or desert their pups (Grindal et al., 1992; Jones et al., 2009). Furthermore, high precipitation may increase mortality of juveniles during the onset of flight, when they are particularly vulnerable (Heideman, 2000). The negative influence of precipitation was mostly evident during years when the precipitation showed a particular increase. For example, the lowest reproductive success over the study period (when only 33% of adult females reproduced) was observed in 2009, when the May July rainfall was 73% higher than the long-term average, while the reproductive success was much higher in years with average or below-average precipitation. Precipitation extremes (both positive and negative) may have different effects on bats relative to the climatic regime of the ecosystem in which they live. While increased precipitation negatively affected reproduction of bats in relatively humid areas of temperate forests (Grindal et al., 1992; Burles et al., 2009; this study), decreased precipitation, on the other hand, imposed a serious negative effect on the reproductive output of bats living in arid areas (Adams, 2010). In both ecosystems, however, long-term precipitation followed trends predicted for the 21st century climate change scenarios (IPCC, 2007) with a tendency to increase in mid- to high latitudes (this study), but to decrease in lower latitudes (Adams, 2010). Last, but not least, timing of reproduction and reproductive success were not correlated. Hence, the two reproductive traits are driven by different climatic variables, and these variables have contrasting effects on overall reproductive output. 156 Journal of Zoology 290 (2013) The Zoological Society of London

7 R. K. Lučan, M. Weiser and V. Hanák Climate change and reproduction of a temperate bat Although rising spring temperatures may have a beneficial influence on the population dynamics through a positive effect on juvenile survival, increasing incidence of climatic extremes, such as excessive summer precipitation, may counter this effect by reducing reproductive success. Consequently, with predictions of continuing climate change, populations of temperate insectivorous bats will likely experience increasing environmental stress. Acknowledgement We thank to Jaroslav Červený, Jiří Gaisler, Magdalena Lučanová, Jaroslav Závora and numerous students from the Faculty of Science, University of South Bohemia for their help with field research. Comments of two anonymous reviewers greatly improved the paper. Special thanks go to Prof. Paul A. Racey for his useful comments on the paper and his improvement on its language. 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