Some bat species seek shelter during the day in the attics of large, mostly historic buildings within human settlements, even though they forage for insects at night in the dark surrounding countryside.
Photo: PantheraLeo1359531, Source: Wikimedia Commons, License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.de)
This requires daily commuting between brightly lit districts and dark foraging grounds. A team at the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) has shown that greater mouse-eared bats (Myotis myotis) - Germany's largest bats - prefer to use sparsely lit or completely dark areas when commuting through residential areas. In the journal "Science of the Total Environment", the team advocates for the preservation and expansion of dark corridors in towns and villages to protect nocturnal animals.
The scientific team, led by first author Dr Daniel Lewanzik and project head Prof Christian Voigt from the Leibniz-IZW, equipped greater mouse-eared bats in three villages in the German federal states of Baden-Württemberg and Thuringia with miniaturized, high-resolution GPS loggers. After a few days, they removed the loggers and analysed the bats' daily routes as they commuted between their daytime roosts and foraging grounds. The scientists reconstructed a total of 38 commuting routes and correlated them with environmental variables such as the number of street lights and the type of light they emitted, the vegetation, impervious surfaces and the proximity to water bodies. "Our analyses show that the bats did not choose random routes through the settlement areas, but used dark corridors as far away from street lights as possible", explains Lewanzik, who is now a conservationist at the Federal Agency for Nature Conservation (BfN). The bats exhibited this avoidance behaviour towards street lights at all three locations studied.
Landscape and vegetation influence commuting behaviour
Analyses of the correlation between the bats' movements and landscape features provided further clues about the commuting routes of the greater mouse-eared bat through the settlement areas: In all three study areas, the bats frequently flew along vegetation near water bodies. "They were probably not directly seeking proximity to the water, but rather using the rivers, streams or lakes as natural dark corridors", explains Lewanzik. In the two larger settlements, bats preferred areas with natural, non-sealed surfaces; areas with an impervious surface level of 50 percent or more were hardly flown over at all. "Dense development and heavy lighting of roads, pavements and even green spaces leave the greater mouse-eared bats with only narrow corridors suitable for commuting between their roosts and their hunting grounds", summarises Voigt.
There are historical reasons why this species of bat still regularly seeks its daytime roost in buildings, sometimes even in the centre of a town or village: local mouse-eared bat colonies develop a very loyal to their roosts over decades, sometimes using the attics of old churches or castles for many generations, sometimes even for centuries. However, the expansion of street lighting has only increased significantly in the last century. In addition, new sub-urban districts have sprung up on the outskirts of towns or villages over the past 50 years. This has also led to a further increase in light pollution.
Recommendations for urban and regional planning
Advancing urbanisation poses a challenge for many wildlife species, including light-sensitive bats. If dark corridors become increasingly brightly lit, bats that live in urban or sub-urban areas may be forced to abandon their colonies, according to the authors, yet alternative roosts are scarce. Colonies of bats may also decline and eventually collapse if cities grow without taking dark corridors into account. In urban and regional planning, care should therefore be taken to ensure that dark green corridors connecting natural areas are preserved or even expanded. Artificial light, especially in green spaces and parks, but also beyond, should be limited to what is absolutely necessary. In this way, the negative impact of urbanisation on biodiversity can be mitigated.
The project on which this publication is based was funded by the German Federal Ministry of Research, Technology and Space (BMFTR) under grant number 01|O2104A.
Lewanzik D, Melber M, Scholz C, Schüll I, Zebele M, Brandt M, Schumann A, Düsing K, van den Bogaert V, Greving H, Thomas J, Hensle E, Voigt CC (2025): Urban sprawl and light pollution disrupt commuting corridors of urban-roosting bats. Science of the Total Environment 1008, 181019.
