**1. Introduction**

When one thinks about caves, the first image that comes to mind is that of a dark place full of stalactites and stalagmites, with lots of bats hanging on the walls. Bats are mysterious and scary creatures for most people but extremely interesting and enigmatic animals for zoolo‐ gists. Not only their night activity, longevity, underground roosting, and active flight make them a fascinating species to study but the actual methods used to study them are also of interest [1]. Up to the 1990s, almost all bat research was closely associated with their roosts [2]; animals being captured at the roost entrances, measured, and marked. As bats have high roost fidelity, they can be here caught and recorded repeatedly [3]. Recent developments in

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ultrasound detectors and miniaturized telemetry, however, have significantly expanded the range of possible bat research topics to include subjects as time of foraging activity [1].

While microchiropteran bats are able to use a wide range of natural and mad‐made structures as roosts, roost availability and presence of an abundant food supply are often the main limit‐ ing factors for bats, particularly in temperate zone. Roost availability can influence species distribution, foraging behaviour, social and mating behaviours, population size, diversity, and even bat morphology or physiology [4]. While providing many benefits (e.g. protection against bad weather and predators, effective thermoregulation, higher probability of mating and rearing young, lower foraging costs or information transfer), roosts also represent a major evolutionary pressure regarding the survival and reproductive success of each individual bats.

Bats spend a significant proportion of their life hidden in roosts, though their requirements may differ through the year or even at different times of the day. As such, the diversity of bat roosts is very high, ranging from short‐term ephemeral to long‐term permanent sites. Almost half of the approximately 1200 species of living bats, including all European bats, use per‐ manent roost sites such as buildings, caves, mines, tunnels, tree hollows, or rock crevices [5]. Caves and similar underground spaces offer temperate bats long‐term roost sites with specific microclimatic conditions that fulfil two crucial factors: a relatively stable above‐freezing tem‐ perature (close to the mean annual surface temperature for the area) and high humidity [6].

In this review, we focus on the ecology of temperate zone bats roosting in caves of the Moravian Karst, Czech Republic (**Figure 1**), habitats that supply many of the bats' needs and that can be used year‐round. In doing so we summarize the results of our research on various aspects of bat ecology over winter, including variation in flight activity at the cave entrance, factors affecting site selection within hibernacula, and level of bat movement activity inside the cave. In addition, we summarize present knowledge on white‐nose syndrome (WNS). The

**Figure 1.** Map of Europe with the location of the Czech Republic indicated. The circle within the inset map indicates the location of the Moravian Karst.

study of these factors, along with a general understanding of bat hibernation, are essential prerequisites to understanding the impact of disturbance on hibernating bat populations and for providing focus to future conservation efforts [7].

## **2. Variability in cave use by bats (flight activity at the cave entrance)**

ultrasound detectors and miniaturized telemetry, however, have significantly expanded the range of possible bat research topics to include subjects as time of foraging activity [1].

While microchiropteran bats are able to use a wide range of natural and mad‐made structures as roosts, roost availability and presence of an abundant food supply are often the main limit‐ ing factors for bats, particularly in temperate zone. Roost availability can influence species distribution, foraging behaviour, social and mating behaviours, population size, diversity, and even bat morphology or physiology [4]. While providing many benefits (e.g. protection against bad weather and predators, effective thermoregulation, higher probability of mating and rearing young, lower foraging costs or information transfer), roosts also represent a major evolutionary pressure regarding the survival and reproductive success of each individual bats. Bats spend a significant proportion of their life hidden in roosts, though their requirements may differ through the year or even at different times of the day. As such, the diversity of bat roosts is very high, ranging from short‐term ephemeral to long‐term permanent sites. Almost half of the approximately 1200 species of living bats, including all European bats, use per‐ manent roost sites such as buildings, caves, mines, tunnels, tree hollows, or rock crevices [5]. Caves and similar underground spaces offer temperate bats long‐term roost sites with specific microclimatic conditions that fulfil two crucial factors: a relatively stable above‐freezing tem‐ perature (close to the mean annual surface temperature for the area) and high humidity [6]. In this review, we focus on the ecology of temperate zone bats roosting in caves of the Moravian Karst, Czech Republic (**Figure 1**), habitats that supply many of the bats' needs and that can be used year‐round. In doing so we summarize the results of our research on various aspects of bat ecology over winter, including variation in flight activity at the cave entrance, factors affecting site selection within hibernacula, and level of bat movement activity inside the cave. In addition, we summarize present knowledge on white‐nose syndrome (WNS). The

**Figure 1.** Map of Europe with the location of the Czech Republic indicated. The circle within the inset map indicates the

location of the Moravian Karst.

52 Cave Investigation

The ecology and behaviour of temperate zone bats are fundamentally affected by seasonal changes in day length and other associated climatic variables [8], the effect of which become more pronounced at increasing latitudes. In order to remain nocturnal, therefore, bats must display behavioural flexibility in circadian and circannual activity patterns. We have been investigating nightly and seasonal changes in bat flight activity at the entrance of a natural karstic cave (Kateřinská cave, Czech Republic), an important hibernaculum monitored for hibernating bats since 1970 [9, 10]. Activity was recorded using a double infrared‐light (IR) automatic logging system that allows discrimination between bats leaving the cave and those entering. Recently, automatic loggers capable of collecting large quantities of data over long periods are increasingly being used to monitor activity at European hibernacula, e.g. in the Netherlands, Denmark and Germany (e.g. [11–13]). The use of such IR automatic loggers has been shown to provide a reliable index of activity levels [14, 15] and, unlike netting, they have the advantage of not disturbing or interfering with the bats' normal activity. Their main draw‐ back, however, is that they are unable to distinguish between individual bats or bat species [16, 14]. If the study is focused on the activity of the bat assemblage as whole, however, this is a minor problem. Connection of an IR logging system to a camera can help in later species identification, though the use of flashlight will affect natural bat behaviour. Note, however, that some authors (e.g. [15]) state that species identification using this method can be unreli‐ able. Ultrasound bat detectors can also be connected to IR logging systems and these have been used to monitor activity of a single species (e.g. the lesser horseshoe bat *Rhinolophus hipposideros* [17], the greater horseshoe bat *Rhinolophus ferrumequinum* [18]) or overall activity of all species in a locality (e.g. [14, 15, 19]). Unfortunately, this method is not very reliable at distinguishing echolocation calls of individual *Myotis* species [20].

The level of bat activity (at the cave entrance) varies seasonally and five periods have been defined (**Figure 2**), all showing a non‐random temporal distribution with flight activity con‐ centrated around a specific time [21]. In each case, activity level is influenced by a range of climatic factors, the effect and contribution of which change nightly and over the year [22].

(1) *Hibernation period (mid‐November–early March).* Bats show very low or almost no activity and departures from the cave are very rare. Interruption of lethargy in these bats is most often caused by (i) changes in ambient conditions outside cave, (ii) changes in the physiologi‐ cal state of the hibernating bat (e.g. dehydration), or (iii) direct disturbance [23, 24]. During hibernation, average temperature and daily temperature range (i.e. the difference between daily maximum and minimum temperatures) are key factors predicting the general level of flight activity [16, 18, 22]. As temperature increases, so the percentage of nights with bat activ‐ ity also increases. Similarly, an increase in temperature fluctuation during the day will also

**Figure 2.** Out‐flight (negative values) and in‐flight (positive values) medians for each defined period as monitored by double IR‐light barrier between March 2000 and November 2002. Explanation: HI, hibernation period (mid‐November– beginning of March); LH, late hibernation (March–mid‐April); DE, departure (and transition) period (mid‐April until beginning of June); SU, summer period (mid‐June–end of July); and SW, swarming period (late July–mid‐November).

result in bat arousals and increased flight activity. Note, however, that bat activity at the cave entrance has been recorded at temperatures as low as −13.2°C (cf. [25]). Daily recordings were positive at maximum daily temperature exceeding 6.2°C, when some bat species are able to forage [16]. The activity within defined temperature groups [22] was significantly lower dur‐ ing deep hibernation period than during late hibernation (**Figure 3**). Opinions on the level of activity desynchronization at sunset and loss of nocturnality during hibernation differ and the results of research are inconsistent, some supporting desynchronization and others not (e.g. [18, 26–28]). Our own data [22] clearly indicate that activity at the cave entrance is syn‐ chronized with sunset, even in winter, and that a concentration of activity occurs between 3 and 3.5 h after sunset. No change in activity patterns has been recorded following the emer‐ gence of white‐nose syndrome (WNS) in Europe, suggesting that the hibernation behaviour model described, including changes in activity, could represent a behavioural adaptation that has prevented fatal impact of the disease observed in North America [29].

(2) *Late hibernation period (March–mid‐April)*, with intensive departure activity during the first quarter of the night. Movement activity inside the cave is relatively high and the bats are

**Figure 3.** Activity levels (median values ± interquartile range) in individual temperature group during hibernation (15 November–4 March) and late hibernation (5 March–14 April). The percentage of nights on which activity occurred in individual temperature groups is indicated by the grey area for the hibernation period and the dashed line denotes late hibernation. *Source*: [22].

probably already preparing themselves for departure from the hibernaculum [30]. Flight activ‐ ity is positively affected by average daily temperature, and negatively so by minimum tem‐ perature during the preceding day. Bats react very quickly to temperature changes from day to day, with activity decreasing or increasing if temperatures drop or rise by more than 2°C. Such rapid changes in activity level become feasible as the bats move towards the hibernacu‐ lum entrance, enabling them to register fluctuations in ambient temperature [19, 30] and, as a consequence, potential changes in insect abundance. Bats are capable of foraging at very low temperatures, e.g. Daubenton's bat *Myotis daubentonii* at temperatures as low as −3.3°C [31]. In some species, the activity increases during late hibernation period, presumably, as food availability is already higher and foraging effectively compensates for any energy loss [32].

result in bat arousals and increased flight activity. Note, however, that bat activity at the cave entrance has been recorded at temperatures as low as −13.2°C (cf. [25]). Daily recordings were positive at maximum daily temperature exceeding 6.2°C, when some bat species are able to forage [16]. The activity within defined temperature groups [22] was significantly lower dur‐ ing deep hibernation period than during late hibernation (**Figure 3**). Opinions on the level of activity desynchronization at sunset and loss of nocturnality during hibernation differ and the results of research are inconsistent, some supporting desynchronization and others not (e.g. [18, 26–28]). Our own data [22] clearly indicate that activity at the cave entrance is syn‐ chronized with sunset, even in winter, and that a concentration of activity occurs between 3 and 3.5 h after sunset. No change in activity patterns has been recorded following the emer‐ gence of white‐nose syndrome (WNS) in Europe, suggesting that the hibernation behaviour model described, including changes in activity, could represent a behavioural adaptation that

**Figure 2.** Out‐flight (negative values) and in‐flight (positive values) medians for each defined period as monitored by double IR‐light barrier between March 2000 and November 2002. Explanation: HI, hibernation period (mid‐November– beginning of March); LH, late hibernation (March–mid‐April); DE, departure (and transition) period (mid‐April until beginning of June); SU, summer period (mid‐June–end of July); and SW, swarming period (late July–mid‐November).

**-30**

**LH DE SU SW HI**

**Period**

**33**

**-241**

**272**

**-2**

**3**

(2) *Late hibernation period (March–mid‐April)*, with intensive departure activity during the first quarter of the night. Movement activity inside the cave is relatively high and the bats are

has prevented fatal impact of the disease observed in North America [29].

**-55**

**-300**

**-200**

**-100**

**0**

**Median bat passes per night**

**100**

**200**

**300**

54 Cave Investigation

**44**

**-109**

**86**

*Median OUT Median IN*

(3) S*pring migration (mid‐April–early June)*, a period of relatively high activity. At this time, the cave may serve as a transitional roost during the spring migration and, from around May, as a temporary roost for males as females already start to form summer colonies. Emergence activity in the first quarter of the night is high, and a small number of bats may re‐enter the cave in the last part of the night. Average daily temperature and average daily atmospheric pressure at this time has a significant positive influence on overall flight activity. The degree of variability in activity explained by such climatic factors is the lowest during this period, however, suggesting that either temperature is no longer a limiting factor, or that endogenous rhythms have a strong influence on departure from the hibernaculum [11, 22]. However, the use of underground roosts including caves in the spring may be species specific; it may differ by region, and can also depend on roost structure [6, 18, 33].

(4) *Summer period (mid‐June–end of July)*. During this period, the cave is used only sporadically (**Figure 2**), though the bats visiting the roost stay the whole night, i.e. they enter before mid‐ night and leave after midnight. This type of activity suggests that, during this period, the cave may be being used as a night roost between peaks in foraging activity or as a transitional day roost [11, 18]. At this time, the cave entrance is visited almost exclusively by males [34, 35] as adult females occupy maternity roosts during lactation and return to these between foraging bouts, night roosts being used sporadically and for brief periods [36, 37].

Flight activity at the night roost entrance is influenced by fluctuation in ambient temperature, rather than any absolute temperature threshold, the higher the difference between maximum and minimum daily temperature, the higher the activity level. This corresponds with a model proposing that activity changes in temperate insectivorous bats reflect changes in insect activ‐ ity [8], i.e. if day‐insect abundance is high due to warmer nights, bat foraging activity may continue overnight with no visits registered at the cave entrance (low activity). On the other hand, when nights are cooler and the daily temperatures range is higher, bats will tend to spend more time in the night roost. Foraging activity is highest at dusk and just before dawn, after which the bats return to the day roost [36]. This model is also supported by the influence of rainfall, with flight activity at the cave entrance increasing as rainfall increases whether the nights are warm or cold.

(5) *Autumn migration or swarming period (late July–mid‐November)*. This period is typified by very high general activity and an increasing number of bats entering the cave. With the break‐ up of the summer breeding colonies, activity at the cave entrance gradually increases as adult females and juveniles arrive [9, 38], often in small groups of 2–12. The majority of bats does not roost in the cave and probably arrive after the first foraging period; hence, peak activ‐ ity tends to occur around midnight. Activity around the cave entrances in autumn probably enables juveniles to recognize potential hibernacula and to meet individuals of the opposite sex, which live separately during summer (e.g. [15]). Activity level is positively related to average daily temperature, atmospheric pressure and rainfall. Thus, when nights are warm and insect activity is high (high atmospheric pressure), the bats will quickly catch enough prey and will search for the cave entrances (swarming sites) in order to mate or obtain shelter it be raining [14, 22].
