Understanding factors that influence patterns of population dynamics is of fundamental importance in animal ecology and conservation biology. Among life-history traits, the probability of survival, particularly adult survival, has the largest impact on population changes in long-lived species (Prévot-Juillard, Lebreton & Pradel 1998), as is the case with many mammal and bird species. Among mammalian species of comparable body size, bats are generally considered long-lived (Tuttle & Stevenson 1982; Altringham 1996). Also of importance is the first-year survival of juveniles, as this often determines recruitment to reproductive age. Therefore, knowledge of survival rates is of special interest in the study of bat population dynamics.
The pipistrelle bat (Pipistrellus pipistrellus Schreber, 1774), with a body mass of about 5·5 g, is probably the smallest hibernating mammal of the northern hemisphere (Geiser & Ruf 1995). It is widely distributed and one of the most common bat species in Europe (e.g. Barratt et al. 1997). So-called mass hibernacula of P. pipistrellus, comprising up to several thousand individuals, are known to occur (Dumitresco & Orghidan 1963; Lustrat & Julien 1997). Recent findings suggest that P. pipistrellus should be split into two sibling species (Jones & van Parijs 1993; Barratt et al. 1997). According to spectral characteristics of their echolocation calls, they are tentatively referred to as the 45- and 55-kHz phonic type, respectively. The 45-kHz type will retain the name Pipistrellus pipistrellus (Jones & Barratt 1999). The discovery of this ‘new’ species complicates the interpretation of previous studies and calls for further research.
For endothermic animals of the temperate zones, winter represents a serious energetic challenge. Animals respond to food shortage and low temperatures by either migration, by morphological or behavioural adaptation, or by reducing metabolism, i.e. hibernation (Speakman & Rowland 1999). The overwhelming majority of temperate zone bat species hibernate to bypass the energetic bottleneck (Webb, Speakman & Racey 1996). Hibernation is clearly a strategy to promote over-winter survival. Nevertheless, bat populations are suspected to suffer from increased mortality in the cold season (Davis & Hitchcock 1965; Speakman & Rowland 1999). The critical period is early spring: although emergence from hibernacula coincides with increased food availability (Speakman & Racey 1989; Park, Jones & Ransome 2000), arousal at the end of hibernation may entail increased mortality risks because it is energetically costly (Thomas, Dorais & Bergeron 1990) and fat reserves are depleted in early spring, which may lead to starvation, particularly of lighter bats (e.g. Johnson, Brack & Rolley 1998). Consequently, we suspected seasonal patterns in survival probabilities of temperate bats, with reduced over-winter survival compared to summer/autumn.
Survival rates often differ between sexes and age-classes (Lebreton et al. 1992) and are influenced by environmental variables like temperature (North & Morgan 1979). Hibernators like bats are likely to be less affected by winter temperatures than, e.g. non-hibernating birds because they select hibernacula that are buffered against fluctuations of ambient temperature. Nevertheless, there is some evidence that winter mortality of hibernators may be related to weather factors (Armitage & Downhower 1974). Pipistrelle bats are known to select cold sites for hibernation, which may even freeze (Nagel & Nagel 1991; Webb et al. 1996), and may thus be more susceptible to varying winter conditions than many other temperate bats.
Little is known on sex-specific survival in temperate bats, but various patterns are suspected depending on the mating system (Davis 1966; Greenwood 1980; Stevenson & Tuttle 1981). Gerell & Lundberg (1990) have explained low male survival rates in the pipistrelle bat with reference to its mating system, which is a resource defence polygyny (Clutton-Brock 1989). Juvenile survival in mammals and birds is generally assumed to be low during some period following fledging or weaning, and to approach stability after becoming adult (Loery et al. 1987). Studies that have investigated age-specific or seasonal survival are sparse (but see Ransome 1995 for life history tactics of the greater horseshoe bat, Rhinolophus ferrumequinum Schreber).
Population studies are usually confined to female bats, who form easily observable maternity colonies, whereas males roost solitarily during summer and are therefore difficult to sample (e.g. Speakman et al. 1991). At hibernacula, however, sexes are mixed and thus can be studied comparatively.
Survival processes are usually not directly observable in wild animals. This applies particularly to bats. Therefore, survival studies frequently employ mark–recapture methods. Recent advances in capture–recapture methodology have enabled researchers to address specific biological hypotheses concerning variation of population parameters (Lebreton et al. 1992). This sophisticated modelling approach has still not been applied to bat population dynamics, apart from a recent study by Hoyle, Pople & Toop (2001). However, some earlier studies using simpler approaches provided rough estimates of annual and sex-specific survival rates (e.g. Davis 1966; Keen & Hitchcock 1980; Hitchcock, Keen & Kurta 1984; Boyd & Stebbings 1989; Gerell & Lundberg 1990).
In this study, we present a survival analysis based on live-recapture data of Pipistrellus pipistrellus, 45-kHz phonic type, sampled at a mass hibernaculum where about 5000 pipistrelles hibernate (Sendor, Kugelschafter & Simon 2000). Summer swarming, defined simply as nocturnal flight activity at hibernacula (Fenton 1969; Sendor et al. 2000), is a regular phenomenon at this location. The bats arrive at and leave the site on the same night, without using it as a day roost. Due to the almost year-round presence of bats, hibernacula are ideal locations to study population dynamics, permitting a study design that allows an examination of seasonal patterns. The aims of our study were to examine variation in pipistrelle bat survival by addressing the following working hypotheses: (i) male survival probabilities are expected to be lower than females (Gerell & Lundberg 1990); (ii) first-year survival of juveniles is expected to be lower than adult survival; (iii) autumnal survival rates are expected to exceed spring survival; and (iv) cold winters should reduce, mild winters should enhance, spring survival probabilities.
We will discuss the results obtained in the context of mortality-related aspects of life-history theory (Stearns 1992) and their consequences for population dynamics.