A new study has found that soprano pipistrelle bats exposed to low-intensity broadband radiofrequency noise fly in random directions when released at night, an effect that persisted for more than two hours after the exposure ended. The work, published May 28 in Science, adds to a growing body of research on how human-generated electromagnetic fields affect animal navigation.
Bats migrate seasonally across hundreds of kilometers and are known to use the Earth’s magnetic field as one component of their navigation. Prior research by Oliver Lindecke and colleagues had already shown that soprano pipistrelles calibrate their internal compass at sunset — using the position of the setting sun to set their magnetic bearing before flying at night. That finding raised an obvious follow-up: what happens if something interferes with that calibration window?
Radiofrequency (RF) electromagnetic fields, spanning roughly 3 kHz to 300 MHz, are generated by radars, radio transmitters, household electronics, and mobile base stations. How anthropogenic electromagnetic noise affects living systems remains poorly understood, particularly when effects occur at sublethal levels. Earlier work had shown disruption of magnetic orientation in European robins and some invertebrates, but evidence for bats which navigate using a combination of echolocation, solar cues, and magnetic sensing — was limited.
Lindecke and colleagues exposed soprano pipistrelle bats (Pipistrellus pygmaeus) to weak broadband radiofrequency noise spanning 0 to 300 MHz for approximately 30 minutes while the bats were observing the sunset, then tested their flight orientation later in the night. The study captured 86 migratory bats along the Baltic Sea coastline during four autumn migration seasons. A control group of 79 bats received no exposure. After the exposure period, bats were released inside an enclosed round field laboratory at night, and their departure flight directions were recorded.
Because soprano pipistrelles migrating toward their wintering grounds are known to calibrate environmental cues at sunset for navigation later at night, the researchers predicted the noise would disrupt the bats’ ability to sense the magnetic field during that crucial calibration window — but would only affect bats exposed during that specific period.
Exposed bats departed in random directions. Control bats, by contrast, oriented consistently in the expected southerly migratory direction. Most surprisingly, further tests showed that the bats’ orientation was disrupted regardless of whether the exposure occurred during the sunset calibration period or afterward, once the sun had set.
The duration of the effect was also unexpected. The researchers noted that electromagnetic noise exposure disrupted orientation for several hours beyond the exposure period itself. This “carryover effect” had not been predicted under existing models, which assume that disruption to the magnetic sense ends once the RF noise is no longer present.
Richard Holland, Professor in Animal Behaviour at Bangor University, said: “This finding was quite surprising. Our intention was to see how the noise would affect the magnetic sensing system of bats, but the results suggest that the impact of this electromagnetic noise is more complicated than that. Although it is known that electromagnetic noise in this range disrupts the magnetic sense, it was not previously assumed to have a significant impact on migrating animals, because it is more prevalent in cities than rural areas. It was assumed that because animals would move rapidly through it, they would not be affected for very long, if at all. However, our findings indicate that even brief exposure can have effects that last beyond the period of exposure, and independently of other cues.”
Will Schneider, a research fellow at Bangor and co-author of the study, offered two possible explanations for the carryover: “It may be that the effect is on their interpretation of the magnetic field — for example, because the electromagnetic noise makes it look unusual, the bats decide to ignore it. On the other hand, it might be that it introduces some sort of stressor, that makes the bats decide not to migrate that night, which is why they head off randomly, instead of in the migratory direction.” Schneider also noted that “current exposure standards are designed exclusively for humans, leaving wildlife vulnerable even within the confines of these guidelines.”
The study was conducted under controlled laboratory conditions using an enclosed round field arena. It does not establish what exposure levels or durations bats encounter in natural settings, nor does it track whether disoriented bats recover their bearings over longer time periods. The precise mechanism behind the carryover effect remains unknown. The paper does not conclude that electromagnetic noise is causing population-level declines in bat species, and the findings, while suggestive, are from a single species tested over four autumn migration seasons in one coastal location in Latvia.
This study sits within an active research area. Prior work has shown that broadband electromagnetic noise common in urban environments disrupts magnetic compass orientation in night-migratory European robins, and that analysis of more than two million captured landbirds from 152 species found an association between geomagnetic disturbances and increased vagrancy in North American migratory birds. RF effects have also been documented in American cockroaches and the marine crustacean Gondogeneia antarctica. Proximity to radars has been associated with reduced activity and foraging in bats in previous studies.
