Population dynamics of the pipistrelle bat: effects of sex, age and winter weather on seasonal survival

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Abstract
Summary: Life‐history theory assumes increased mortality at certain stages such as hibernation. However, seasonal variation of survival rates of hibernating mammals has rarely been estimated. In this study, apparent survival of pipistrelle bats (Pipistrellus pipistrellus) hibernating and performing summer swarming at a large hibernaculum (Marburg Castle, Hesse, Germany), was modelled using seasonal (summer/winter) capture–recapture data for the years 1996–2000. The spring survival interval includes the period of arousal at the end of hibernation and therefore validly measures survival associated with hibernation. In five summers and four winters, 15 839 bats were captured and released (13 082 individuals) and 3403 recaptures recorded. Analysis was complicated by transience and trap‐dependence. Recapture rates varied seasonally and by group. The autumnal survival estimates were negatively biased due to transience effects that could not be taken into account. Survival could be modelled using two age‐classes, with reduced first‐year juvenile survival. The age effect persisted over the first autumn and spring. There was virtually no evidence for sex‐specific survival rates; male and female survival were found to be almost equal. In the best‐fitting models, survival rates varied over time and differed among sexes and age‐classes by a constant amount. Between years, there was only a small variation in spring survival, which could not be explained by winter severity. Adult spring survival was surprisingly high, averaging 0·892 (= 0·028). No evidence for increased mortality during hibernation could be found. This contradicted the expectation of reduced over‐winter survival due to depleted fat reserves at the end of hibernation. Thus, hibernation does apparently not entail a survival cost for the pipistrelle bat. Rough estimates of annual adult survival averaged 0·799 ( = 0·051), which considerably exceeds previous estimates; annual juvenile survival was estimated at 0·527 ( = 0·095). Hence, previous studies have substantially underestimated pipistrelle bat survival. Possible consequences of these findings for various aspects of life histories are discussed.

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