Compared with the vast scientific literature on bird migration, we know virtually nothing about bat migration. Migratory patterns of bats are enigmatic due to the small size and cryptic behavior of bats. Our current knowledge on bat migration is mostly based on the recapturing of banded animals (Hutterer et al. 2005). Such recaptures, however, are extremely rare so that information on migratory connectivity is concentrated around a few areas where banding efforts have been intense. Currently, four European bat species are considered to be long-distance migrants, i.e. they travel seasonally up to 2,000 or 4,000 km (Hutterer et al. 2005): The parti-coloured bat Vespertilio murinus, the noctule bat Nyctalus noctula, Leisler’s bat N. leisleri, and Nathusius’ bat Pipistrellus nathusii. All of them are listed under Supplement II of the Convention on Migratory Species (CMS). In our research, we try to shed light on two aspects of the biology of migratory bats:
1) Migratory connectivity using stable isotopes, and
2) migratory physiology.
Stable hydrogen isotope ratios vary across continents according to ambient temperature and precipitation (Bowen et al. 2005 Oecologica). Since animals take up hydrogen from local food or surface water, they incorporate the specific stable isotope ratio into their tissues. Hair keratin is particularly relevant for geo-tracking animals, because keratin remains inert after hair growth. Since animals, including bats, moult before migration, they carry the isotopic signature of the breeding habitats to the wintering grounds. When captured during migration or in the hibernacula, this information can be used to connect breeding and wintering habitats. Recently, we have established this method for predicting the breeding provenance of European bats. For this, our collaborators have collected hair samples of sedentary bats throughout Europe (see map below) and then we measured the stable hydrogen isotope ratio in it. We were able to demonstrate that the stable hydrogen isotope in fur (deltaDh) is related to that of surface water (deltaDmap) (see inlet graph in below picture). For details check our article in PLoS ONE.
Sampling points from which we obtained hair samples of sedentary bats. The inlet picture shows the correlation between stable isotope ratios in hair keratin and surface water (Popa-Lisseanu et al. 2012 PLoS ONE). The regression can be used to predict the provenance of bats based on the stable hydrogen isotope ratio of hair.
In a first study, we estimated the breeding origin of bats killed at Geman wind power facilities. From each species, we analysed stable isotopes in the fur keratin from about a dozen of animals. We then developed probability maps to distinguish between bats originating from local and distant populations. We found that Pipistrellus nathusii most likely originated from distant populations such as Baltic Countries and Russia (see right graph A), whereas P. pipistrellus most likely came from local populations in Germany (see right graph B; Voigt et al. 2012 Biol Cons). This finding highlights the fact that bat fatalities at wind turbines may have negative effects on both local and distant populations.
Some migratory bats like Pipistrellus nathusii travel several thousands of kilometers from their breeding habitats in Northeastern Europe to their wintering habitats in France. Flapping flight is energetically expensive; although cost efficient when considering the distance travelled. But how do 5-g bats accomplish the strenuous task to fly across Europa twice a year? Obviously, bats are athletes on the wing, and we can learn much about exercise physiology of mammals by looking at the muscular physiology of bats. We are currently studying the relationship between metabolic rate and flight speed in bats, the oxidative fuel use and the flight velocity of free-ranging migratory bats to better understand the physiological performance of insectivorous bats during their annual journeys (funded by the Deutsche Forschungsgemeinschaft)s.