Electromagnetic pollution disrupts the orientation of soprano pipistrelle bats

Little is known about the effects of human-made electromagnetic fields on wild animals. In a study published in the journal Science, an international team led by biologist Dr Oliver Lindecke of the University of Oldenburg reports that weak broadband radio-frequency fields, ranging from long- to ultra-high-frequency bands, disrupt the orientation of soprano pipistrelle bats. "Our findings suggest that electromagnetic noise may have a greater impact on animal behaviour than previously assumed," says Lindecke.

In 2014 it was discovered that weak electromagnetic noise of the kind produced by household devices or radio antennas can disrupt the magnetic compass of migratory birds. How electromagnetic noise affects the navigational behaviour of other animals, and of mammals in particular, had so far received little attention.

The animals that use the Earth's magnetic field to navigate on their migrations include the soprano pipistrelle (Pipistrellus pygmaeus). Lindecke, together with colleagues from Bangor University (UK) and the University of Latvia, established this in 2023. Now this team has investigated how these animals respond to weak, broadband electromagnetic noise in the frequency range from 10 kilohertz to 300 megahertz.

The team's earlier studies had shown that the soprano pipistrelle use the point at which the sun sets to recalibrate their magnetic compass at sunset to determine their flight direction later in the night. In the present study, which spanned four autumn migration seasons from 2021 to 2024, the team first exposed 34 animals to weak electromagnetic noise for 30 minutes while they observed the sunset. Some hours later, the researchers released the bats individually at night in a field laboratory and recorded their departure bearing. This bearing allows conclusions about the intended migratory direction, as the animals choose their preferred bearing as soon as they take off. The experiments were carried out on the Latvian Baltic coast, an important migration area for these animals.

Electromagnetic pollution has a complex effect on animal behaviour

The team had initially assumed that electromagnetic noise at sunset would prevent the animals from recalibrating their compass system. In fact, the bats exposed to the noise subsequently chose their departure direction at random, whereas animals in an undisturbed control group departed in a preferred direction.

In further experiments, the researchers exposed 28 bats to electromagnetic noise only after sunset. Here too, the bats were disoriented when later tested, even though the animals had been exposed to the noise only briefly and after their compass calibration.

"This effect surprised us greatly," says Lindecke. "Studies of migratory birds suggest that their magnetic compass works again immediately once the electromagnetic noise is no longer present. We had expected the bats' compass to be calibrated and to remain unaffected by the brief noise."

The team concludes from the field experiments that electromagnetic noise may affect animal behaviour in more complex ways than previously thought. Until now it was assumed that electromagnetic noise of the kind typical of cities, if it had any effect at all, would affect migrating animals only directly, that is, only when the animals came close to it during migration. "Our findings indicate, however, that even brief exposure can have serious effects that outlast the electromagnetic noise itself," says Richard Holland, Professor of Animal Behaviour at Bangor University.

Migratory behaviour of wild animals could be further affected in future

In the researchers' view, the experiment also shows that the bats' orientation system, possibly their magnetic sense, responds to electromagnetic noise differently from that of birds. This could be a further indication that the magnetic sense works differently in mammals than has so far been described for some migratory bird species. An after-effect of this kind would not be expected on the basis of the established models that researchers use to describe the light-dependent magnetic compass of birds, since this compass relies on very short-lived quantum-physical processes.

Lindecke stresses that the experiments do not yet allow a conclusive judgement as to whether additional natural orientation cues in the environment would mitigate the observed effects in free flight. The researchers expect, however, that increasing urbanisation and the growing global spread of wireless technology will raise electromagnetic pollution and thereby further disrupt the migratory behaviour of wildlife in the future. "The applicable limits are intended to protect us humans, but they do not take wildlife into account," says Lindecke. "Our results show that wild animals can be affected well below these thresholds."

The study was funded in part by the Collaborative Research Centre (CRC) "Magnetoreception and Navigation in Vertebrates" at the University of Oldenburg, which has been supported by the German Research Foundation (DFG) since 2018. Lindecke has been a fellow of the CRC since 2021 and has led a subproject focusing on migratory bats since 2023.